Potential Assessment of Electrified Heavy-Duty Trailers Based on the Methods Developed for EU Legislation (VECTO Trailer)

Abstract

Since 1 January 2024, newly produced heavy-duty trailers are subject to the assessment of their performance regarding CO₂ and fuel consumption according to Implementing Regulation (EU) 2022/1362. The method is based on the already established approach for the CO₂ and energy consumption evaluation of trucks and buses, i.e., applying a combination of component testing and vehicle simulation using the software VECTO (Vehicle Energy Consumption calculation TOol). For the evaluation of trailers, generic conventional towing vehicles in combination with the specific CO₂ and fuel consumption-relevant properties of the trailer, such as mass, aerodynamics, rolling resistance etc., are simulated in the “VECTO Trailer” software. The corresponding results are used in the European HDV CO₂ standards with which manufacturers must comply to avoid penalty payments (2030: −10% for semitrailers and −7.5% for trailers compared with the baseline year 2025). Methodology and legislation are currently being extended to also cover the effects of electrified trailers (trailers with an electrified axle and/or electrically supplied auxiliaries) on CO₂, electrical energy consumption, and electric range extension (special use case in combination with a battery-electric towing vehicle). This publication gives an overview of the developed regulatory framework and methods to be implemented in a future extension of VECTO Trailer as well as a comparison of different e-trailer configurations and usage scenarios regarding their impact on CO₂, energy consumption, and electric range by applying the developed methods in a preliminary potential analysis. Results from this analysis indicate that e-trailers that use small batteries (5–50 kWh) to power electric refrigeration units achieve a CO₂ reduction of 5–10%, depending primarily on battery capacity. In contrast, e-trailers designed for propulsion support with larger batteries (50–500 kWh) and e-axle(s) (50–500 kW) demonstrate a reduction potential of up to 40%, largely determined by battery capacity and e-axle rating. Despite their reduction potential, market acceptance of e-trailers remains uncertain as the higher number of trailers compared with towing vehicles could lead to slow adoption, especially of the more expensive configurations.

1. Introduction

In order to align with the decarbonization efforts of the heavy-duty transport sector in the EU, newly produced trucks have been subject to a standardized procedure regarding the determination of energy consumption and CO₂ emissions since January 2019. The approach used is based on specific component measurements assessing the energy demand and efficiencies/losses of the relevant vehicle components and a subsequent simulation of the energy consumption and CO₂ emissions of the entire vehicle with the simulation software “VECTO” (v4.3.3) (Vehicle Energy Consumption calculation TOol). The legal framework and obligations of manufacturers and type approval authorities are laid down in Regulation (EU) 2017/2400 [1]. The simulated values are the basis for the CO₂ standards laid down in Regulation (EU) 2019/1242 [2], which states manufacturers must comply with a certain CO₂ reduction compared with the baseline assessed from 1 July 2019 to 30 June 2020. The overall reduction targets are −45% in 2030, −65% in 2035 and −90% from 2040 on.

Regulation (EU) 2017/2400 currently covers different vehicle categories (trucks, buses, vans) as well as propulsion systems (conventional, hybrid electric, battery electric, and fuel cell electric) but only standardized bodies and trailers. These standardized bodies and trailers represent typical configurations (mass, tires, rolling resistance, aero resistance, cargo volume) and are used because the specific body/trailer is not known at the time of motor vehicle certification with VECTO. However, the impact of trailers (referring to both trailers and semitrailers) was deemed significant enough by the European Commission to establish a separate standardized method regarding the determination of their influence on CO₂ emissions and fuel consumption laid down in Implementing Regulation (EU) 2022/1362 [3], which has been applicable since 1 January 2024. Furthermore, trailers are also considered separately in the HDV CO₂ standards (reduction targets from 2030 on: −10% for semitrailers and −7.5% for trailers compared with the baseline year 2025). The method for trailers is in its very principle similar to the previously described VECTO approach but uses generic conventional towing vehicles representing the average state of vehicle technology of new vehicles in the “baseline” year (1 July 2019–30 June 2020) and considers the specific CO₂- and fuel consumption-relevant characteristics of the trailer, like mass, aerodynamics, rolling resistance, etc. A special tool variant of the VECTO software family (“VECTO Trailer”) is applied for this purpose.

Since 2023, the European Commission has started work on extending Implementing Regulation (EU) 2022/1362. The most important topics in this context are electrified trailers (e-trailers) and advanced aerodynamics. In European legislation, the term “e-trailer” specifically refers to trailers that contribute to vehicle propulsion. However, in the context of this paper, the term is used more broadly to also include “passive” e-trailers that supply electrical power for auxiliary functions such as operating electric refrigeration units. To support this regulatory extension, the European Commission assigned a consortium of IDIADA Automotive Technology and Graz University of Technology’s Institute of Thermodynamics and Sustainable Propulsion Systems (ITnA) to develop methods for the technical annexes and a related simulation approach. The work is being conducted in close cooperation with the Commission and stakeholders, including industry and member states. For the subject of e-trailers, ITnA is in the project lead.

Due to the emerging nature of e-trailer technology, the current state of published research is still limited, with most studies concentrating on specific applications, optimization strategies, or individual operational scenarios. For instance, Bank et al. [4] investigate the benefits of predictive energy management strategies for electrified refrigerated trailers, demonstrating how route-aware strategies can minimize fuel use and emissions. Surcel et al. [5] evaluate the fuel and CO₂ savings of a trailer designed for long hauling and retrofitted with an electric axle, reporting tangible efficiency gains under real-world conditions. Zhang et al. [6] perform a multi-objective optimization of e-trailer component configurations to balance energy consumption, TCO (Total Cost of Ownership), and freight capacity losses under standard and real-world Chinese driving cycles. Chen et al. [7] explore the stability and regenerative braking strategies for electric tractor–semitrailers in scenarios involving simultaneous braking and turning. Additionally, Boonstra et al. [8] use MATLAB Simulink to assess the potential fuel savings of conventional trucks operating with active e-trailers, tailored to the traffic profile of the Rotterdam port area. Despite these contributions, there is currently no publication that provides a generalized assessment of the energy-saving potential in typical European applications for the wide range of e-trailer configurations currently being designed.

This paper provides a brief overview of the existing VECTO Trailer approach, which is described to give context and support understanding of the results presented. The core contribution of this work is the development of a comprehensive methodology to incorporate e-trailers into the regulatory assessment framework. This includes the identification and modeling of relevant e-trailer functions, the development of methods intended for future integration into the VECTO Trailer software, and a comparative analysis of various e-trailer configurations and usage scenarios. The potential analysis focuses on the impact of e-trailers on CO₂ emissions, energy consumption, and electric range extension—the latter being specifically relevant when used in conjunction with battery-electric towing vehicles (BEVs).

2. Implementing Regulation (EU) 2022/1362 and VECTO Trailer

Implementing Regulation (EU) 2022/1362 introduces a standardized method to determine the individual performance of heavy-duty trailers regarding their influence on the CO₂ emissions and fuel consumption of the vehicle combination. The basic approach, consisting of component test procedures and vehicle longitudinal dynamics simulation, as it is already used for motor vehicles, was already the subject of several experimental studies evaluating its accuracy in terms of CO₂ and fuel consumption compared with real-world measurements [9,10]. The VECTO Trailer approach itself is schematically illustrated in Figure 1.

The formula in Figure 1 describes the driving resistances and additional power demands relevant in VECTO:

• P_e: Mechanical power demand on the ICE of the towing vehicle [W]
• P_air: Air resistance [W]
• P_Roll: Rolling resistance [W]
• P_Acc: Acceleration resistance [W]
• P_Grd: Gradient resistance [W]
• P_Loss: Power losses in the powertrain [W]
• P_Aux: Power demand due to auxiliary systems [W]

Regarding the trailer, the vehicle manufacturer needs to provide some basic information to categorize the trailer (like number of axles, body type, etc.) as well as to determine the main trailer parameters as considered by the Regulation. These main parameters are as follows:

• Curb weight of the trailer: In the case of refrigerated trailers, this also includes the TRU (Transport Refrigeration Unit).
• Main external dimensions: These are used to calculate the air drag of the basic configuration of generic towing vehicle and trailer.
• Standard or specific aerodynamic devices: The former have fixed air drag reductions compared with the basic configuration, depending on the trailer type, while the effect of the latter is to be assessed using a certified CFD (Computational Fluid Dynamics) method.
• Axle and tire features: These include the declaration of whether an axle is liftable and/or steered as well as the rolling resistance, determined on a drum test rig, as set out in Regulation (EU) 2020/740 [11].

The input data is then used in the longitudinal-dynamics software “VECTO Trailer”, which calculates the CO₂ emissions and fuel consumption of the vehicle combination for a set of mission profiles (Long-haul (LH), Regional Delivery (RD), and Urban Delivery (UD)) in two payload conditions each. The mission profiles were defined during the development phase of VECTO for motor vehicles to depict representative speed, gradient, and standstill times typical for the respective driving scenarios in Europe [12]. In addition to the individual consumption values, the tool also computes a weighted result by applying a certain weighting for each mission profile and payload combination defined in the HDV CO₂ standards depending on the specific trailer type. In order to facilitate the interpretation of the result in terms of performance and energy efficiency of the trailer, an efficiency ratio is also calculated, which represents the ratio of CO₂ emissions of the generic towing vehicle with the trailer under consideration and the CO₂ emissions of the generic towing vehicle with a reference trailer (standard market specifications, approx. year of manufacture 2020, defined per trailer type).

The vehicle owner receives all information via a so-called “Customer Information File” (CIF) together with the vehicle documents where the weighted result for CO₂ emissions is the relevant metric for the CO₂ standards. In future, this CO₂ shall also be taken into account in toll rates on motorways.

3. Current Developments in VECTO Trailer

A major extension of Regulation (EU) 2022/1362 is currently under elaboration, with one of the main topics being the consideration of e-trailers. The related work on the technical regulation as well as the elaboration of methods on how to consider the various e-trailer functions in the simulation approach were finished in spring 2025. However, this did not include the actual extension of the simulation software, which means that although the regulatory framework is already set (which mainly specifies which parameters have to be provided and how they shall be determined), there might still be changes happening in terms of how specific functions are handled in the official VECTO Trailer tool compared with what is described in this paper. The official tool and the associated application date for the revised regulation are not expected to be available before late 2026.

3.1. E-Trailer Working Principles and Consideration in a Regulatory Simulation Approach

This section highlights the main relevant e-trailer functions identified in the context of CO₂ savings and energy consumption and their respective consideration in the simulation approach. When elaborating the approach, it was crucial to ensure that the methodology is compatible with the intended use in the vehicle approval process and aligned with the context of the European HDV CO₂ standards. To meet these objectives, several key constraints had to be considered:

• Robustness of the approach

  ○

  Inputs to the tool need to be generally describable in the technical annexes.

  ○

  Parameterization effort shall be as low as possible, as a simulation needs to be carried out individually for each vehicle as part of the vehicle approval.

  ○

  Generic data, assumptions, and model functions cannot be individually adapted for specific vehicles.

• There is no possibility to incorporate manufacturer-specific control algorithms into the simulations, e.g., via software or hardware in-the-loop methods (see conclusions from Task 5 in the project “Further development and update of VECTO with new technologies” [13]).
• At the time of calculation, no data on the specific use of the vehicle (e.g., charging behavior by the vehicle operator) is available.
• For the evaluation of the performance e-trailer in the context of the HDV CO₂ standards, the results need to be consolidated into very few parameters.

Given these boundary conditions, modeling approaches typically used in vehicle development are not suitable. Instead, an alternative approach was developed to fulfill the specific regulatory requirements.

Figure 2 gives an overview of the relevant e-trailer features to be covered by the first amendment of Regulation (EU) 2022/1362.

The left part of the figure deals with the TRUs, and although this topic is not exclusive to e-trailers, its operation needs to be considered at least in a basic way to depict the CO₂ reduction potential, especially of “passive” e-trailers (e-trailers that are designed to supply an electrical consumer on board—usually a TRU—and which are not allowed to contribute to vehicle propulsion) in a meaningful way. For this purpose, typical generic TRU consumption values were defined considering data from the literature [14,15,16,17] as well as data from bilateral contacts with industry during the stakeholder process. The values elaborated and agreed to in the working group (referring to a three-axle semitrailer with 85 m³ cargo volume) are as follows:

• Fuel consumption of 2.5 [L/h] for a Diesel–electric TRU (current standard TRU technology).
• Electrical energy consumption of 5 [kW] for pure electric TRU.

For the purpose of this paper (and to understand the results presented in Section 4) it is not necessary to go into more detail on the TRU topic. However, it should be mentioned that the final methods in the regulation will be more complex, differentiating between several general “refrigeration energy supply types” (e.g., ICE (internal combustion engine)-driven compressor, electric-driven compressor, etc.) as well as, for example, the different sources from which the electrical energy can be supplied to electric-driven compressors. This includes supply via battery, e-axle, photovoltaic modules, electric supply from towing vehicle, separate fuel cell system(s), or an ICE-driven generator (running on Diesel or H₂), including any combinations of those.

The right side of Figure 2 shows the actual working principles of e-trailers that are to be considered by the regulatory assessment.

Generation of electrical energy during various driving conditions:

• Brake blending:

  ○

  Description: A feature of an e-trailer applying electric regenerative braking in driving conditions where a braking request is sent from the towing vehicle to the trailer.

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  Proposed generic modeling: The feature is considered by splitting the service brake power (total brake power minus endurance brake power on the towing vehicle) between the towing vehicle and trailer based on the vertical wheel force distribution. The amount of service brake actuation required by the trailer is then covered as much as possible by the e-axle, considering the torque limitations of the e-components.

• Free recuperation:

  ○

  Description: A feature that applies electric regenerative braking in driving conditions where the trailer is pushing but no braking request is made by the towing vehicle. The feature has the primary goal of using braking energy for electric recuperation that would otherwise be dissipated by the endurance brake on the towing vehicle. For this feature to work properly, the control algorithm on the e-trailer must be able to estimate the driving status of the entire vehicle very well—and, in particular, whether the endurance brake on the towing vehicle is currently active. Information of this kind is not yet included as standard in the communication protocols between the towing vehicle and trailer. In reality, active “free recuperation” results in a complex interaction between the brake control of the e-trailer and the tractor unit.

  ○

  Proposed generic modeling: Such iterative controls cannot be modeled in detail in VECTO Trailer due to it being a “backwards” calculation model in its core. Hence, the feature is proposed to be considered in a simplified way by shifting a fixed percentage of the endurance brake power to the e-trailer, considering the full load capabilities of the e-components as well as an optional power limit, which can be defined by the manufacturer.

• Dynamo mode:

  ○

  Description: An operation mode of an e-trailer in which the e-axle applies electric braking to supply the vehicle with electrical energy (e.g., for supply of a refrigeration unit) and/or to increase the state of charge (SOC) of the battery predominantly or exclusively in driving situations where the vehicle is pulling the trailer. This e-trailer operation mode induces additional CO₂ emissions on an ICE towing vehicle. Nevertheless, this mode is necessary to ensure cooling safety of refrigerated trailers with electric TRUs in case there is not sufficient electrical energy available from the battery (plug-in charged from the grid) and from CO₂-free regenerative electric braking.

  ○

  The relevance of the dynamo mode lies in the fact that, for each kWh of electrical energy generated, the additional CO₂ emissions emitted by the ICE on the towing vehicle are significantly lower compared with those produced by a Diesel–electric generator on conventional refrigerated trailers. As a rough approximation, it can be said that around 10% of the total emissions from a classic articulated truck with a refrigerated trailer come from a typical Diesel–electric TRU. If the electricity for the electric compressor is not generated by the small Diesel generator on the refrigerated trailer, but by the e-trailer in dynamo mode, the attributable CO₂ emissions are reduced by half, mainly due to the significantly higher efficiency of the Diesel ICE on the towing vehicle.

  ○

  Proposed generic modeling: In the model, this mode is essentially taken into account by applying a generically defined braking power (e.g., 20 kW, speed-dependent, with full load limitations from component and/or manufacturer side considered).

Storage of electrical energy includes the modeling of the energy storage system (battery or super capacitors). Here the inputs into VECTO Trailer (e.g., capacity, current limitations, internal resistance) as well as corresponding component test procedures are taken from Regulation (EU) 2017/2400.

Use of electrical energy

• Propulsion support:

  ○

  Description: An operation mode of an e-trailer in which the overall propulsion of the vehicle is supported by the application of motive power by the trailer. A key boundary condition in this respect is that, in accordance with the provisions currently discussed in the related UN Regulation No. 13 [18] working group, an e-trailer in propulsion mode must not push the towing vehicle. The working group is tasked with establishing boundary conditions, limitations, and provisions for the application of specific e-trailer functionalities. As this work is still in progress, no citable version of the corresponding regulation is currently available including these e-trailer provisions. Beyond the prerequisite to not push the towing vehicle, many different control strategies may be followed by manufacturers linked to basic design layout features (maximum power of the e-axle and battery capacity) as well as basic control features (like direct sensing of the coupling force and/or indirect estimation of the driving state using, e.g., speed and acceleration sensors).

  ○

  Proposed generic modeling: The proposed generic modeling of this mode follows the main principle that the trailer provides as much of its own propulsion as possible up to the point where the trailer still is not pushing the towing vehicle. This is handled in the longitudinal dynamics simulation by calculating the force at the coupling point and controlling the e-axle actuation accordingly, taking into account the torque and speed limitations of the components. In real-life use, the effect of e-trailer propulsion on overall energy consumption is also determined by the limits of the electrical energy required for propulsion (i.e., essentially by the battery capacity). This effect is modeled by simulating the vehicle combination in two different modes (battery full and battery empty, see explanation in Section 4.1).

• Energy supply to TRU: Supplying an electrically driven TRU is the key function of passive e-trailers currently available on the market. However, active e-trailers can of course also be configured as reefers.
• BEV range extension: For e-trailers with propulsion capabilities, a dedicated application scenario is to serve as a range extender for BEVs (battery-electric towing vehicles) by taking over part of the propulsion work. In this respect, VECTO Trailer is also intended to simulate active e-trailers in combination with a generic BEV towing vehicle, providing an estimate of the extended electric range enabled by the e-trailer.

3.2. Type Classification as Discussed in UN Regulation No. 13

Figure 3 shows an overview of different e-trailer concepts currently discussed in the related UN Regulation No. 13 working group. The illustration reflects the state of discussions as of late 2024 and is intended solely to provide a high-level overview and visualization of the general e-trailer concepts. While this classification does not directly influence the methods developed for Implementing Regulation (EU) 2022/1362, it is used in the following sections to help contextualize the described methods and results within the broader conceptual framework.

Type 1

A type 1 e-trailer solely generates electrical energy without contributing to propulsion. This concept is viable only if a major electric consumer is on board the trailer, which, within the scope of Implementing Regulation (EU) 2022/1362, could be only an electric TRU. The battery on the e-trailer serves both as a buffer and as a storage unit for electrical energy from the grid to power the TRU.

A type 1 concept, as defined by UN Regulation No. 13, does not require a specific data interface between the towing vehicle and the trailer. This means that the towing vehicle typically neither detects the presence of an e-trailer nor recognizes which modes are active on the trailer. To ensure that the towing vehicle’s control is not adversely affected, UN Regulation No. 13 proposes limits on electric braking for type 1 trailers at 20 kW.

Regarding the strategy for regenerative electric braking, the dynamo mode is primarily or exclusively engaged when the towing vehicle is pulling the trailer. This increases propulsion demand and, consequently, fuel consumption for the towing vehicle. Nevertheless, as described in Section 3.1, there is a CO₂ advantage in this operating mode compared with a conventional Diesel–electric TRU. The absence of electric regenerative braking during actual braking, unlike in a typical Hybrid Electric Vehicle (HEV), may result from type approval constraints or hardware limitations.

Type 2

A type 2 e-trailer would build on the functions of type 1 e-trailers but add two main functions:

• It can assist in propulsion in certain conditions.
• It can include more sophisticated controls regarding the generation of electrical energy (free recuperation).

This type also would not require any data communication between the towing vehicle and trailer. Thus, as for type 1, to avoid unsafely interfering with the towing vehicle operation, the propulsion as well as the recuperation function also would be limited to 20 kW. However, type 2 e-trailers will not be implemented, in accordance with the latest information in UN Regulation 13.

Type 3

A type 3 e-trailer is capable of both electric braking and electric propulsion assistance without a predefined low-threshold maximum value. To enable this, the following prerequisites must be met:

• An ISO 11992-2:2014 [19] interface for connection of the e-trailer with the EBS (Electronic Braking System).
• Sensors on the trailer to detect the driving state, which can be done directly and/or indirectly, as described in the previous section.

Nevertheless, the e-trailer is also operated on its own regarding its controls for propulsion support and regenerative braking functions. With regard to propulsion support, UN Regulation No. 13 stipulates that the trailer must not push the towing vehicle above 15 km/h.

There is a wide range of specifications for type 3 trailers conceivable, the key parameters being as follows:

• Power ratings of the e-axle(s).
• The capacity of the battery for the “plug-in” effect.
• The controls (sensors and algorithms).

A type 3 trailer configuration remains beneficial even without a refrigeration unit by providing propulsion assistance, delivering zero-CO₂ propulsion using energy from the grid and, to a lesser extent, energy from regenerative braking. Another application is serving as a range extender for a BEV, assisting with propulsion and reducing the towing vehicle’s energy demand.

Type 4

A type 4 e-trailer builds on the functions of type 3 e-trailers but includes communication and controls allowing the towing vehicle to optimize the complete vehicle performance. To make this possible, the following functions/requirements must be met:

• A future ISO 11992-2:2026 interface for exchange of torque requests.
• Algorithms on the towing vehicle to optimize the complete vehicle combination like

  ○

  ICE towing vehicle and e-trailer with energy optimization as for an HEV.

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  BEV towing vehicle and e-trailer with efficiency and SOC optimizations (e.g., shifting of electric braking between towing vehicle and trailer based on the actual demands).

The above-mentioned communication standard is not yet available and is currently scheduled for release in 2026. Similarly, towing vehicles with such algorithms are still to be developed and are not expected in the coming years.

4. E-Trailer Reduction Potential Estimation

This section compares the CO₂ reduction potential and details the underlying methodology over a variety of different e-trailer configurations.

4.1. Basic Approach for Modeling the Different Operating Modes of an E-Trailer

Hybrid powertrain configurations, i.e., vehicles that have both an internal combustion engine (in the context of this paper, specifically on the towing vehicle) and an electric powertrain (in the context of this paper, specifically on the trailer) and that also can be charged from the grid (“plug-in”) have various states of operating behavior depending on the level of SOC. To represent this variability, it is proposed to apply the approach already established in VECTO for modeling plug-in hybrid towing vehicles. The approach consists of simulating the vehicle in two archetypical modes—charge-depleting mode (CD) and charge-sustaining mode (CS)—and then weighting the results for the two modes together to produce a representative average figure for the mix in real operation.

Charge-depleting mode (CD)

The charge depleting mode represents the operation of the vehicle with a full battery, i.e., the electric powertrain is primarily supplied with electrical energy charged from the grid. For an e-trailer, this means that the following functions (each, if available) are to be considered as follows in the CD mode simulation:

• Propulsion support function = ON
• Regenerative braking = ON
• Electric refrigeration unit = ON
• Dynamo mode = OFF

To derive a representative value for energy consumption over a given mission, a special approach in the simulation of the CD mode is applied:

• The SOC is “artificially” kept constant in the simulation, in the middle of the usable SOC range (“center SOC”). This is done to guarantee each vehicle being able to drive the complete cycle independent of its electric-storage capacity. This ensures the comparability of the results for electrical energy consumption and ranges between all possible vehicle configurations. Otherwise, vehicles with a small battery would not be able to run the full cycle. Thus, the cycle specification implicitly depicted in the result would not be representative. Furthermore, the constant SOC in the middle of the usable SOC range is representing the best guess for average real-world usage since there is no explicit correlation between actual SOC level and distance in the VECTO mission profiles available (i.e., location of typical charging points in the mission).
• The electrical energy consumed in the cycle is accumulated in a separate counter independent of the virtually constant SOC.

As a result of the CD mode—and in the specific context of a towing vehicle with an e-trailer—kilometer-related values are obtained for fuel consumption or energy consumption of the towing vehicle and for electrical energy consumption of the trailer.

Charge-sustaining mode (CS)

In the CS mode, the SOC variation between the start and end of the mission is defined to be zero. This reflects a vehicle operation where no electrical energy is available from the grid. However, electric braking provides a certain amount of electrical energy that can be used accordingly (for supply of the refrigeration unit and/or propulsion support).

The control strategy in the vehicle (in the concrete case here, on the e-trailer, which in the case of type 1 to type 3 is decoupled from the control system of the towing vehicle) aims to optimize the operating modes of the powertrain (propulsion, recuperation, braking) in such a way that a minimum fuel consumption is achieved. This also requires knowledge or assumptions about the driving cycle.

For the VECTO Trailer tool, the aim was to find a generally valid algorithm that optimizes the operation of an e-trailer over a mission and that precisely provides a balanced SOC. This required the definition of a general control strategy that is used for all configurations to ensure comparability of the results between different e-trailers. The proposed approach is based on the simulation of the entire mission in each of three states (A, B, C) and on combining the results in postprocessing using the criterion of balanced SOC in the weighting. As for the CD mode simulations, the SOC is virtually kept constant in the simulation of all states.

Table 1. Simulation states for charge-sustaining mode.

CS State ID	State of the e-Trailer Feature (If Present on the Trailer)				Remarks
	Electric Refrigeration Unit	Regenerative Braking	Propulsion Support Function	Dynamo Mode
A	ON	ON	OFF	ON
B	ON	ON	OFF	OFF	This state is identical to CD mode for a passive e-trailer.
C	ON	ON	ON	OFF	This state is identical to CD mode for an active e-trailer.

describes these three states. The supply to the electric refrigeration unit must always be guaranteed, and regenerative braking always makes sense, so these two functions are always “ON” in all states. States A and B both have the propulsion support function “OFF” and differ in dynamo mode (state A: ON, state B: OFF). State C is defined by propulsion support “ON” and dynamo mode “OFF”. The fourth possible combination of propulsion support function and dynamo mode, each “ON”, makes no sense from an energy point of view.

From a simplified view on the actual operation of e-trailers, these states can be activated as follows, considering the SOC and incorporating hysteresis elements:

Type 1 e-trailers: Only states A and B are relevant. If the SOC level is sufficient, the vehicle is operated in state B. Below a certain threshold value, it switches to state A.

Type 3 e-trailers without e-TRU: Only states B and C are relevant. If the SOC level is sufficient, the vehicle is operated in state C. Below a certain threshold value, it switches to state B.

Type 3 e-trailers with e-TRU: This is the most general case, and all three states A, B, and C are relevant. If the SOC level is sufficient, the vehicle is operated in state C. As the SOC decreases, the system switches successively to B and finally to A. In the actual operation of a vehicle in this most general case, the operation strategy has the task of maximizing the use of electrical energy for propulsion support without having to switch to dynamo mode unnecessarily. This can be achieved only through either smart controllers with the aid of route planning tools or selective switching of functions by a trained driver.

For the algorithm in VECTO Trailer, it is assumed that the mission profile is known and the distribution of the three states is optimized by the operation strategy. The related results can be calculated by the following postprocessing, which performs a case distinction based on the energy balance in the e-trailer battery in the simulations for state B (

Table 2. Postprocessing of states for consolidated CS mode results.

Case Determined Based on State B	Interpretation	Applied Weighting for Consolidated CS Mode Results
Energy balance in battery < 0	Not enough regenerative electrical energy available to cover the demand of the refrigeration system. Thus, additionally phases in dynamo mode are required.	Weighting of B and A so that the combined energy balance is zero.
Energy balance in battery > 0	Regenerative electrical energy available to be used for vehicle propulsion support.	Weighting of B and C so that the combined energy balance is zero. For trailers without propulsion support, B is the final result. *
Energy balance in battery = zero	Regenerative electrical energy available exactly covers the demand of the refrigeration system.	B is the final result (also emerges as a boundary case of the two cases mentioned above).

* In this case, the distribution of braking power on the trailer between the electric and service brakes from the simulation would not be correct, but this is irrelevant for the energy consumption or CO₂ emissions of the towing vehicle.

).

As already mentioned, the approach presented here represents an optimization of the result with regard to the distribution of states A to C. This approach also corresponds to the basic principle followed for parallel HEV in the VECTO tool for towing vehicles, where the parameter of the control strategy used is optimized. In the assessment of hybrid vehicles of different configurations (e-axle assessment, battery capacity, etc.) by VECTO, a fair and comparable result can be achieved only through these boundary conditions.

Vehicles in real operation will generally not manage to achieve the optimum result due to various influences, such as inaccurate estimation of the driving conditions up to the end of the mission. However, real e-trailers have further possibilities to improve energy consumption compared with the algorithms proposed for VECTO Trailers, e.g., by differentiating the operating points of the e-axle(s) within the various functional modes (propulsion support, dynamo mode, and regenerative braking).

For vehicles without a “plug-in” function, only the results from this mode (and not those from charge-depleting mode) are relevant.

Weighting of results from the two operation modes

For trailers with a “plug-in” function, a weighting of the results of the CD mode and the CS mode needs to be carried out to generate a single representative CO₂ value per mission profile and payload combination. This combined value is required for the subsequent calculation of the final CO₂ figure reported in the CoC (Certificate of Conformity) and for the HDV CO₂ standards. The weighting should reflect a typical usage pattern of the vehicle. For this purpose, a so-called utility factor (UF) is defined, which indicates the share of the total daily distance traveled in charge-depleting mode (Equation (1)).

UF = Daily distance CD/Daily distance Total

(1)

• UF: Utility factor [-]
• Daily distance CD: Daily distance driven in CD mode [km]
• Daily distance Total: Total daily driven distance [km]

The values for “total daily driven distance” are defined generically in VECTO for each mission profile (480 km for LH, 320 km for RD, and 240 km for UD).

The value for “daily distance that can be driven in CD mode” is calculated by Equation (2).

Daily distance CD = E_el,usable/EC_el,CD

(2)

• E_el,usable: Total usable electrical energy during the daily mission [kWh]
• EC_el,CD: Kilometer-specific electrical energy consumption as simulated for the CD mode [kWh/km]

The “Total usable electrical energy during the daily mission” is calculated starting from the certified capacity of the battery or the battery subsystems. This base capacity is reduced by the following effects:

• Usable SOC range: Since real-world SOC usage is typically limited to reduce battery aging and is difficult to verify, e.g., by a type approval authority, a generic default of 80% usable SOC is proposed, derived from discussions with e-trailer manufacturers. Only lower values may be indicated in the VECTO Trailer input; values above 80% are not permitted.
• Battery aging deterioration: In line with the Commission’s approach, VECTO should reflect battery status at midlife, requiring consideration of capacity deterioration. While UN GTR No. 22 is developing methods to assess battery degradation, these currently apply only to towing vehicles. As no definitions or verification methods exist yet for e-trailers, a generic 10% capacity reduction is proposed for modeling. However, since e-trailers have recently been added to the GTR’s scope, specific values may be used in VECTO Trailer once relevant methods are finalized and adopted.

These adjustments result in the overall usable battery capacity. Based on this, the electrical energy actually available in the mission for the CD mode is calculated, taking into account the following influences:

• Assumptions on the typical charging pattern:

  ○

  The battery is assumed to be fully charged at the start of the daily trip. Electric cooling systems on reefers require overnight grid connection, making 100% battery charge at trip start a reasonable assumption. Differentiating between reefer and non-reefer e-trailers would introduce complexity and unjustified differences in comparisons given the currently available data limitations.

  ○

  No intermediate charging is considered during the mission. During the mission, especially in long-haul operations, interim charging often could take place only at public charging infrastructure. There, it is assumed that charging the towing vehicle has priority.

• A key operational factor for refrigerated e-trailers is maintaining a battery reserve to ensure cooling during standstill periods without grid access—critical for trailers without an ICE genset as emergency backup. This reserve triggers a switch from battery to dynamo mode, which functions only while the vehicle is moving and is ineffective during standstills like traffic jams or on parking spaces without e-infrastructure. As reported by manufacturers, operators can choose SOC-based switchover thresholds to manage this reserve. If these operational patterns are not considered, CO₂ savings, especially from type 1 e-trailers, may be systematically overestimated. As real-world data on dynamo mode contributions were not yet available, a default model was proposed as follows:

Cooling safety capacity = 20% of the usable SOC but a minimum of 5 kWh (or the usable capacity if smaller) and a maximum of 10 kWh.

The minimum of 5 kWh approximately corresponds to the smallest battery capacities reported for type 1 e-trailers during bilateral discussions. The upper limit is required for e-trailers with large battery capacities (e.g., type 3) to prevent unrealistically high safety reserves that would distort the real reduction potential.

With these definitions, the UF can be calculated. The final results for a specific mission and payload combination weighted for the CD and CS modes then are determined based on Equation (3):

RES _weighted = RES _CD × UF + RES _CS × (1 − UF)

(3)

• RES _weighted: Weighted result of the two operation modes
• RES _CD: Result in charge-depleting mode
• RES _CS: Result in charge-sustaining mode
• UF: Utility factor [-]

The term “result” in this context applies to kilometer-specific values for fuel consumption, CO₂ emissions, and electrical energy consumption.

Additional Results for E-Trailers with “Propulsion Support” Feature in Combination with a BEV

As mentioned previously, type 3 and type 4 e-trailers may also serve as a range extender for BEV towing vehicles by taking over part of the propulsion work. For those, the additional range of the BEV (ΔER_BEV) can be calculated as follows:

ΔER_BEV = ER_e-trailer × (1 − EC_{el, BEV + e-trailer}/EC_{el, BEV + ref-trailer})

(4)

• ΔER_BEV: Additional range of BEV due to e-trailer operation [km]
• ER_e-trailer: Electric range of the e-trailer calculated based on the electrical energy consumption in CD mode and the e-trailer’s usable battery capacity [km]
• EC_{el, BEV + e-trailer}: Electrical energy consumption of the BEV in CD mode considering the e-trailer operation [kWh/km]
• EC_{el, BEV + ref-trailer}: Electric energy consumption of the BEV in combination with the reference trailer according to Regulation (EU) 2022/1362 [kWh/km]

The validity of Equation (4) requires that the electric range of the BEV towing vehicle, when combined with the e-trailer, exceeds the standalone electric range of the trailer itself. This condition, however, is generally fulfilled for practical scenarios. Even if this is not the case, the towing vehicle could be interim-charged during the mission, allowing the range gain from the e-trailer to be fully exploited.

4.2. Modeling Framework for E-Trailer Potential Assessment

The potential analysis focused exclusively on three-axle semitrailers, as this vehicle type is currently the sole focus of manufacturers. Both reefer and non-reefer configurations were examined for all general e-trailer types (types 1 to 4) to obtain a complete picture. For reefers, the TRU technology was defined to be “pure electric”.

Regarding the modeling, a simplified approach was employed for the assessment due to the absence of a VECTO Trailer tool version capable of simulating e-trailers.

The initial step involved creating reference simulation models in the “Declaration mode” of the VECTO tool for towing vehicles. This mode, also used for official certification, ensures consistent simulation parameters to produce representative driving conditions (velocity and acceleration traces). Generic towing vehicle models were defined to provide input for these simulations, as summarized in

Table 3. Main vehicle specifications of the used towing vehicles.

		ICE Diesel	BEV
ICE rated power	kW	350	-
EM power (peak/continuous)	kW	-	360/540
Powertrain configuration	-	conventional AMT 12 gears	E2 AT 3 gears
Battery nominal energy content	kWh	-	550
Vehicle curb mass (tractor only)	kg	7747	9906
RRC (steer/drive)	-	C/C	A/B
CdxA	m2	5.63	4.77
Retarder	-	Yes	No

. These models include the following:

• A generic Diesel ICE truck, which depicts the average state of vehicle technology of new vehicles in the “baseline” year (1 July 2019–30 June 2020) and is used in the official VECTO Trailer tool.
• A BEV with a virtual vehicle configuration based on 2024 vehicle and component technology as a basis for comparison.

The efficiencies/specifications of the powertrain components of the ICE truck are detailed in [20] and refer to the 4 × 2 articulated truck “5-LH”. The parameterization of the BEV is based on data from studies carried out by ITnA in cooperation with Ricardo for the European Commission (report is currently unpublished) and vehicle data available at ITnA from work on VECTO model development.

The defined towing vehicles were paired with reference trailers specified in Implementing Regulation (EU) 2022/1362, corresponding to the e-trailer types analyzed (curtain-sided or refrigerated three-axle semitrailers). Model adjustments were made depending on the specific operating mode and features of the e-trailer to reflect realistic driving and gear-shifting behavior.

• Passive e-trailer concepts:

  ○

  Consideration of the actual e-trailer mass and a reduction in the full-load capability of the towing vehicle for the operating mode in which the e-trailer runs in dynamo mode (CS mode state A). This includes an additional load of approximately 25 kW on the power unit (ICE or EM (electric machine)) of the truck, accounting for the 20 kW to be generated at the e-axle and the additional powertrain losses.

• Active e-trailer concepts: Due to the propulsion feature, a more detailed differentiation had to be made depending on the mode of operation:

  ○

  CS mode without propulsion support is modeled in the same way as described above.

  ○

  CS mode with propulsion support (state C) and CD mode:

  ○

  For combinations with conventional towing vehicles, a P4 hybrid configuration (featuring electric motors at the wheel hubs) was defined with the e-axle power rating adjusted to approximate the intended propulsion contribution of the e-trailer. The P4 e-axle rating was iteratively adjusted by incorporating e-trailer operation in subsequent steps until the trailer’s modeled e-axle average power matched the reference simulation’s e-axle usage. This approach was necessary because e-axle(s) on the towing vehicle have fewer control restrictions than e-axle(s) on trailers, resulting in different e-axle utilization at identical power ratings. For combination with BEVs, a similar method was applied by scaling the towing vehicle’s electric motor full-load capacity to reflect the propulsion support from the e-trailer.

The time-resolved simulation outputs—such as driving resistances, velocity, acceleration, and braking power—served as inputs for the subsequent modeling of all relevant e-trailer operating modes as detailed in Section 4.1. For each mission and payload combination, the e-trailer’s operating point was determined based on its mode of operation, the available features, and the driving state of the vehicle combination from the reference simulation. The modeling of individual features followed the methodology described in Section 3.1, with e-trailer operation depending on the following boundary driving states:

• Positive total traction force for the vehicle combination:

  ○

  The e-trailer may operate in “propulsion support” mode (CD or CS mode state C), “dynamo mode” (CS mode state A), or be passively towed (CS mode state B).

• Negative total traction force for the vehicle combination with braking of the towing vehicle:

  ○

  The e-trailer may apply the feature “dynamo mode” (CS mode state A), “free recuperation” (all modes), and/or “brake blending” (all modes). If the trailer’s total braking demand exceeds the braking power applied by the dynamo mode, additional regenerative braking can occur. Consequently, the three braking functions are applied sequentially until the total braking demand is covered electrically (considering the full-load limits of the electric machine(s)).

• Negative total traction force for the vehicle combination without braking of the towing vehicle:

  ○

  This could be the case, for example, when the vehicle decelerates solely due to the driving resistances. In this case, the e-trailer may be operated in “dynamo mode” (CS mode state A) or remain inactive (every other case where dynamo mode is not active).

• Total traction force for the vehicle combination is zero:

  ○

  In this case, the e-trailer may operate in “dynamo mode” (CS mode state A).

Based on the resulting operation of the e-trailer, the new traction force required from the towing vehicle is calculated. Considering the powertrain losses of the towing vehicle, a new operating point for the ICE or for the EM of the BEV towing vehicle is determined. Fuel or energy consumption is then derived using map interpolation. This process is repeated for each time step, and the final weighted results are computed using the postprocessing methodology outlined in Section 4.1 and Section Additional Results for E-Trailers with “Propulsion Support” Feature in Combination with a BEV. The results obtained using this methodology were compared with manufacturer-reported data from prototype testing campaigns, demonstrating a good level of agreement. However, it should be noted that these reported values could not be independently verified and only partially covered the range of trailer configurations assessed, as described in the following section. To address this limitation, an experimental study is planned by the Joint Research Centre (JRC) of the European Commission for 2025–2026. This study aims to validate the proposed e-trailer methodology by assessing its ability to accurately reflect real-world performance and reduction potential through measurements.

Analyzed E-Trailer Concepts

For the actual analysis, the focus was set on covering the announced variants by manufacturers across a broad range, i.e., nominal battery capacity and e-axle rating for the different e-trailer types.

Certain configurations were evaluated for different regenerative braking functions and/or generic usage patterns to assess the sensibility of the potential for a given trailer on the generic assumptions. These generic assumptions, as well as the sensitivity scenarios, were defined as follows:

• Each trailer starts the daily mission fully charged to 100% of its usable SOC, with no interim charging available during the mission.
• Assumption of a generic usable battery SOC of 80% with regard to the installed nominal capacity as well as a generic battery deterioration for half of its lifetime of 10%.
• Assumption of fixed average endurance brake shares per mission profile (53% for LH, 47% for RD, 40% for UD) that are based on real-world test data provided by Knorr Bremse and analysis performed by ITnA (around 18.000 km in 45 trips with various vehicles and drivers).
• Scenarios regarding regenerative braking features:

  ○

  Minimum scenario: neither brake blending nor free recuperation is available (worst case scenario regarding available CO₂ free electrical energy).

  ○

  Average scenario: only free recuperation is available, assuming that 50% of the endurance brake power is shifted to the e-axle of the trailer.

  ○

  Maximum scenario: both brake blending as well as free recuperation are available and for the latter, assuming that 75% of the endurance brake power is shifted to the e-axle of the trailer.

• Type 4 e-trailers were modeled differently with regard to electric braking, in line with the feature that for this concept the towing vehicle control system can optimize the energy consumption of the entire vehicle combination, including the trailer, during operation.
• Generic usage pattern in case of an electric TRU defined by the amount of kWh reserved for cooling safety to trigger the activation of the dynamo mode:

  ○

  Pessimistic scenario: 25 kWh reserved (no plug-in effect)

  ○

  Average scenario: 10 kWh reserved (2 h of average cooling)

  ○

  Optimistic scenario: 5 kWh reserved (1 h of average cooling)

As for the non-e-axle-related trailer features (e.g., rolling resistance, aero features), the specifications were adopted unchanged from the reference trailers in order to isolate the “e-axle” effect in the analysis.

Table 4. Evaluated e-trailer configurations with a conventional towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Plug-In	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern
Type 1	Pure electric	20	No	6	minimum	not relevant
Type 1	Pure electric	20	No	6	average	not relevant
Type 1	Pure electric	20	No	6	maximum	not relevant
Type 1	Pure electric	20	Yes	25	average	pessimistic
Type 1	Pure electric	20	Yes	25	average	average
Type 1	Pure electric	20	Yes	25	average	optimistic
Type 1	Pure electric	20	Yes	55	average	average
Type 2	Pure electric	20	Yes	25	average	average
Type 2	None	20	Yes	25	average	average
Type 3	None	100 peak	Yes	60	average	average
Type 3	None	100 peak	Yes	200	average	average
Type 3	None	500 peak	Yes	250	average	average
Type 3	None	500 peak	Yes	500	minimum	average
Type 3	None	500 peak	Yes	500	average	average
Type 3	None	500 peak	Yes	500	maximum	average
Type 3	Pure electric	100 peak	Yes	60	average	average
Type 3	Pure electric	100 peak	Yes	200	average	average
Type 3	Pure electric	500 peak	Yes	250	average	average
Type 3	Pure electric	500 peak	Yes	500	average	average
Type 3	Pure electric	500 peak	Yes	580	average	average
Type 4	Pure electric	100 peak	Yes	60	fully optim.	average
Type 4	Pure electric	100 peak	Yes	200	fully optim.	average
Type 4	Pure electric	500 peak	Yes	250	fully optim.	average
Type 4	Pure electric	500 peak	Yes	500	fully optim.	average
Type 4	Pure electric	500 peak	Yes	560	fully optim.	average

shows an overview of the evaluated e-trailer configurations.

Table 5. Evaluated e-trailer configurations with a battery electric towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Plug-In	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern
Type 1	Pure electr.	20	No	6	none	not relevant
Type 1	Pure electr.	20	Yes	55	none	average
Type 3	Pure electr.	100 peak	Yes	130	none	average
Type 3	Pure electr.	500 peak	Yes	350	none	average
Type 3	None	100 peak	Yes	130	none	average
Type 3	None	500 peak	Yes	350	none	average
Type 4	Pure electr.	100 peak	Yes	130	fully optim.	average
Type 4	Pure electr.	500 peak	Yes	350	fully optim.	average

summarizes the e-trailer configurations simulated with the BEV towing vehicle. Regarding the e-axle rating and battery capacity, the impact of both small and large component sizes was examined, consistent with the assessments for conventional towing vehicles. Specifically, no dedicated regenerative braking functions were considered for type 1 to type 3 trailers, as energy recuperation on the BEV is assumed to be more efficient than on the e-trailer. Additionally, since a BEV typically does not apply a purely loss-making endurance brake, the free recuperation feature on the e-trailer becomes obsolete. However, for type 4 trailers, the brake blending function is taken into account for rare instances where the towing vehicle employs its service brake in addition to its electric axle. Type 2 trailers were not evaluated here as the assessment with BEV was performed after evaluating the combination with conventional towing vehicles, which showed that these types have no practical use case (see Section 4.3).

4.3. Reduction Potential of E-Trailer Concepts with Diesel ICE Towing Vehicle

This section examines the weighted CO₂ reduction potential of e-trailers when paired with a conventional towing vehicle, which aligns with the relevant metric used under the CO₂ standards. The weighting refers to the mission profile and payload mix as applied according to Implementing Regulation (EU) 2022/1362 for three-axle semitrailers. Specifically, the distribution is 63% LH with representative payload, 27% LH with low payload, 7% RD with representative payload, and 3% RD with low payload. While the UD cycle was also examined—showing consistently higher reduction rates, mainly due to increased energy regeneration—it is excluded from the weighting and not discussed further, to maintain focus on results with official regulatory relevance.

The basis for comparison is the generic reference trailer. For configurations with a TRU, the reference is the generic reference reefer but including the proposed generic Diesel consumption of a Diesel–electric refrigeration unit. For configurations without a TRU, the reference chosen is the generic reference curtainsider, as it represents the most common body type for three-axle semitrailers.

Type 1

Table 6. Reduction potential of type 1 e-trailers with conventional towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern	CO2/Energy Reduction Potential
Type 1	Pure electr.	20	6	minimum	not relevant	4.9%
Type 1	Pure electr.	20	6	average	not relevant	5.7%
Type 1	Pure electr.	20	6	maximum	not relevant	5.9%
Type 1	Pure electr.	20	25	average	pessimistic	5.4%
Type 1	Pure electr.	20	25	average	average	6.9%
Type 1	Pure electr.	20	25	average	optimistic	7.9%
Type 1	Pure electr.	20	55	average	average	10.1%

summarizes the potentials for the examined type 1 configurations. The following conclusions are drawn:

• The maximum potential of the technology is approx. 10% for the case with a battery that is large enough to cover the entire electrical energy demand of the TRU by the plug-in effect.
• The minimum potential of the technology is approx. 5% in the configuration that the entire electrical energy requirement of the TRU has to be covered by the dynamo mode.

  ○

  Those results are in line with manufacturer estimates.

• In the sensitivity analysis, the assumptions on the generic usage pattern (available plug-in energy) proved to be more important than characteristics and assumptions on regenerative braking, as the full load capacity of the e-axle (20 kW) limits the additional gain from optimized regeneration.

Type 2

Table 7. Reduction potential of type 2 e-trailers with conventional towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern	CO2/Energy Reduction Potential
Type 2	Pure electr.	20	25	average	average	6.5%
Type 2	None.	20	25	average	average	2.6%

summarizes the results for a type 2 e-trailer. The nominal battery size assumed here is 25 kWh, analogous to the current typical configuration for type 1. The conclusions are as follows:

• With this battery size, the reduction potential of a type 2 trailer with TRU is no higher than that of a type 1 trailer (calculated with 6.5% for type 2 vs. 6.9% for type 1). This is because the limit for the potential is the overall available CO₂-free energy (from plug-in and electric regenerative braking), which is the same in both cases. The fact that somewhat less potential was determined for type 2 is due to the limitations of the assumed operating strategy.
• The type 2 configuration without TRU and with the assumed small battery size has only approx. 2.5% CO₂ reduction potential. The effect results from the slight reduction in the propulsion demand on the towing vehicle due to the small plug-in effect and the small recuperation capabilities of the e-axle.
• The approx. 4% points more potential of the configuration with TRU is due to the CO₂ savings achieved by the dynamo mode of the e-axle compared with the CO₂ emitted by the Diesel unit of the reference trailer. These additional savings therefore occur in addition to the available CO₂-free electrical energy.

The analysis shows that such a configuration (i.e., propulsion support feature with small dimensioning of e-axle and battery) does not make sense. Such e-trailer configurations have not been announced by any manufacturer. The analysis was carried out because, at the time of the calculations, such e-trailers were still listed as a separate “type” in the UN Regulation No. 13 scheme.

Type 3

Table 8. Reduction potential of type 3 e-trailers with conventional towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern	CO2/Energy Reduction Potential
Type 3	None	100 peak	60	average	average	6.4%
Type 3	None	100 peak	200	average	average	18.4%
Type 3	None	500 peak	250	average	average	20.6%
Type 3	None	500 peak	500	minimum	average	33.3%
Type 3	None	500 peak	500	average	average	34.4%
Type 3	None	500 peak	500	maximum	average	34.9%
Type 3	Pure electric	100 peak	60	average	average	10.5%
Type 3	Pure electric	100 peak	200	average	average	22.0%
Type 3	Pure electric	500 peak	250	average	average	21.6%
Type 3	Pure electric	500 peak	500	average	average	38.3%
Type 3	Pure electric	500 peak	580	average	average	43.4%

shows the results for type 3 e-trailers. The conclusions for the trailer configuration without TRU are as follows:

• Considering the announced “high-end” configurations with some 500 kW e-axle rating and 500 kWh installed battery capacity, a maximum CO₂ reduction of around 35% can be expected.
• The sensitivity analysis showed that the installed battery capacity has the greatest impact on the overall reduction potential. High e-axle ratings are beneficial only when paired with a battery capable of supplying sufficient energy over extended portions of the cycle.
• Features and assumptions related to regenerative braking affect the reduction potential by approximately 2% points.

The analyses for a type 3 e-trailer with TRU furthermore reveal the following:

• The trends are generally similar to configurations without TRU, but with a higher overall CO₂ reduction potential. This is because, with the same battery and EM specifications, more CO₂ is replaced per kWh through electric TRU supply compared with propulsion support.
• The maximum potential is reached with a battery capable of fully supporting both propulsion and TRU supply across all relevant cycles, requiring approximately 580 kWh of installed capacity. This corresponds to a CO₂ reduction of around 43%.

Type 4

Table 9. Reduction potential of type 4 e-trailers with conventional towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern	CO2/Energy Reduction Potential
Type 4	Pure electric	100 peak	60	fully optim.	average	11.6%
Type 4	Pure electric	100 peak	200	fully optim.	average	22.9%
Type 4	Pure electric	500 peak	250	fully optim.	average	25.6%
Type 4	Pure electric	500 peak	500	fully optim.	average	40.1%
Type 4	Pure electric	500 peak	560	fully optim.	average	43.4%

shows the results for type 4 e-trailers. Compared with the corresponding type 3 trailers with the same specifications, the following differences can be observed:

• The maximum potential is identical to that of type 3 but can be achieved with a smaller battery.
• With a fully sized e-axle and a medium-sized battery, an additional reduction potential of approximately 4% points can be expected.
• There is hardly any additional potential type 4 vs. type 3 with small e-axle ratings, since in this case the small e-axle can utilize nearly its full rating even without an integrated control.
• In urban missions (which are not included in the weighted reduction potentials shown in Table 9), the differences in potential between type 4 and type 3 trailers become more pronounced. The maximum CO₂ reduction potential reaches approximately 50% for type 3 trailers with a TRU and around 60% for type 4 trailers with a TRU.

Type 4 configurations without a TRU were not analyzed separately, as the key findings from above are also applicable. However, the differences between type 3 and type 4 trailers are expected to be even smaller in this case, primarily due to the already higher utility factor (UF) levels observed across both configurations.

4.4. Reduction Potential of E-Trailer Concepts with BEV Towing Vehicle

This section discusses the weighted results calculated for e-trailers in combination with a battery electric towing vehicle. As a reference for the comparison in the case of the trailer with TRU, a reefer with an electric TRU and a nominal battery capacity of 55 kWh was defined. The 55 kWh corresponds to the capacity needed to cover the electrical energy demand for refrigeration in all VECTO missions. This approach is based on the assumption that a fully electric towing vehicle will likely be coupled with a TRU configuration that operates without direct CO₂ emissions.

The calculation results are presented in

Table 10. Electrical energy consumption and range extension of e-trailers with battery electric towing vehicle.

E-Trailer Type	Refrigeration Supply Type	Max Rating of e-Axle(s) [kW]	Nominal Battery Capacity [kWh]	Regenerative Braking Functions	Generic Usage Pattern	BEV Range Extension [km]	Eff. Ratio kWh/km Based
Type 1	Pure electr.	20	6	none	not relevant	−25	1.005
Type 1	Pure electr.	20	55	none	average	+0	1.001
Type 3	Pure electr.	100 peak	130	none	average	+60	1.014
Type 3	Pure electr.	500 peak	350	none	average	+161	1.084
Type 3	None	100 peak	130	none	average	+78	1.015
Type 3	None	500 peak	350	none	average	+185	1.068
Type 4	Pure electr.	100 peak	130	fully optim.	average	+60	1.013
Type 4	Pure electr.	500 peak	350	fully optim.	average	+162	1.079

. Two key aspects are discussed here: first, the estimated range extension of the BEV achieved through the e-trailer, and second, the impact on the total energy consumption of the vehicle combination. To assess the latter, an efficiency ratio is defined as the sum of the electrical energy consumption of both the BEV and the e-trailer divided by the electrical energy consumption of the BEV with the reference trailer.

The key findings can be summarized as follows:

For the investigation of the effect of coupling a BEV with a type 1 e-trailer, the two boundary battery configurations, i.e., 6 and 55 kWh, were analyzed. With the smaller battery, the electric cooling unit is supplied via dynamo mode. The additional drag of the trailer slightly increases the energy consumption of the BEV and results in a loss of range of 25 km. This combination makes no sense from an energy point of view, as it would be much more efficient to supply the cooling unit directly by connecting it to the HV (High-Voltage) battery of the towing vehicle (“ePTO”). In the case of coupling with a type 1 e-trailer with a large battery, there is no reduction in the BEV range due to the omission of the dynamo mode. However, there is also no benefit in equipping an e-axle on the trailer compared with the supply via ePTO mentioned above. This leads to the conclusion that type 1 e-trailers are not meaningful when used with BEVs.

For type 3 e-trailers, range extensions of approximately 60 to 180 kilometers were calculated depending on the e-axle rating and battery capacity. This range extension comes at the cost of increased overall energy consumption of the vehicle combination by approximately 2 to 8 percent due to the higher total mass. In this context, it needs to be kept in mind that an increase in the battery capacity of the BEV towing vehicle is of course also associated with an increase in energy consumption per kilometer. The greatest range extension is achieved by trailers without a TRU, as more of the trailer’s battery capacity can be dedicated to propulsion support. This results in an additional range extension of around 20 kilometers compared with trailers with a TRU and a similar electric component configuration.

For type 4 e-trailers, only very little additional potential was calculated compared with type 3 e-trailers with identical components. This can be explained by the fact that in most driving situations all available kinetic energy can already be recuperated by the BEV. Further optimization potential (e.g., by optimizing the load points between the e-axles of the BEV and the e-trailer) was not investigated here.

5. Summary and Outlook

For trailers, as with other vehicles, electrification could be a key way to reduce emissions and enhance efficiency. It supports propulsion—cutting CO₂ from ICE towing vehicles or extending BEV range—and powers refrigeration. Electricity is supplied via either grid-charged batteries or electric braking. Braking can be regenerative or, in the case of refrigerated trailers, non-regenerative during driving (dynamo mode). This mode is still CO₂-efficient, generating electricity with about 50% fewer emissions than typical small Diesel gensets.

Two main types of e-trailers are expected to enter the market in the coming years. Type 1 trailers use an e-axle—limited to 20 kW by UN Regulation No. 13—exclusively to power electric refrigeration units with battery capacities typically ranging from 5 to 50 kWh. Type 3 trailers, designed primarily to support propulsion, are also constrained by UN Regulation No. 13, which prohibits them from pushing the towing vehicle. These trailers may feature significantly larger e-components, with e-axle power ratings between 50 and 500 kW and battery capacities from 50 to 500 kWh. Once ISO 11992-2:2026 is finalized and compatible towing vehicles become available, type 4 trailers may emerge. Unlike type 3, these would allow coordinated control by the towing vehicle, enabling advanced functions such as optimized regenerative braking, though they are not expected before the end of the decade.

This paper assesses the CO₂ reduction potential of e-trailers in the context of three-axle semitrailers operated with a generic ICE towing vehicle and compared with a reference trailer as defined in Implementing Regulation (EU) 2022/1362. Under a weighted mission profile and payload mix, type 1 e-trailers show a CO₂ reduction potential of 5–10%, primarily influenced by battery capacity. Type 3 e-trailers offer up to a 40% reduction, mainly depending on the combination of battery capacity and e-axle rating.

The results obtained in this paper were compared with manufacturer-reported data from prototype testing campaigns, demonstrating a good level of agreement. However, it should be noted that these reported values could not be independently verified and only partially covered the range of trailer configurations assessed. To address this limitation, an experimental study is planned by the Joint Research Centre (JRC) of the European Commission for 2025–2026. This study aims to validate the proposed e-trailer methodology by assessing its ability to accurately reflect real-world performance and reduction potential through measurements

The market introduction of e-trailers is currently hindered, at least in part, by the absence of specific provisions in technical regulations being in force (e.g., UN Regulation No. 13 on approval of brake systems or UN Regulation No. 100 on safety requirements for electric vehicles), which makes obtaining type approvals difficult or impossible. These topics are currently being worked on intensively in the relevant international working groups.

Market acceptance of e-trailers remains uncertain, particularly given that fleets typically have more trailers than towing vehicles. This imbalance may slow adoption, especially for higher-cost configurations such as those with propulsion capabilities, large batteries, and high-power e-axles, which are likely to be used only in specific heavy-duty applications where the e-trailer is in constant operation. However, pressure for adoption is expected to grow once e-trailers are included in the EU HDV CO₂ emission certification scheme, likely around 2027, as part of the broader EU CO₂ standards.