Hybrid Electric Vehicle Emission Characteristics at Various Ambient Temperatures

Abstract

To quantify how ambient temperature affects hybrid electric vehicle emissions, a diesel hybrid electric vehicle was tested under three different temperatures by a chassis dynamometer in this work. The results show that the total NOx emission factors follow the sequence of −10 °C > 25 °C > 40 °C and the particle number (PN) emission factors follow the sequence of 40 °C > 25 °C > −10 °C. Both NOx and PN instantaneous emission peaks corresponding to engine-starts can be found in each entire test cycle. The drastic temperature change of upstream SCR and DPF as a result of more frequent engine stop-and-goes may induce NOx and PN peaks. More attention should be paid to exhaust temperature management for hybrid electric vehicles to tackle this emission issue, especially for low-temperature condition.

1. Introduction

With the increase in global environmental pollution and climate change, many countries have adopted strategies to achieve net zero targets by 2050 which have been proposed at the COP 26 summit of UNFCC [1,2]. In China, carbon peak by 2030 and carbon neutrality by 2060 have been pledged [3]. According to the data from the Chinese Ministry of Ecology and Environment, vehicular emissions have become one of the major air pollution sources in many large cities [4]. Previous investigations forecast that the vehicle inventory will continue to increase to 360–540 million by 2030 [5,6,7]. In recent years, due to multiple power sources, hybrid electric vehicles (HEVs) are regarded as one of the effective and promising ways of mitigating the issues of the energy crisis and environmental pollution.

Hitherto, there are numerous publications that focus on hybrid electric vehicles from different aspects, including energy management strategies [8,9,10], battery and engine size optimization [1,11,12], and hybrid drivetrain design [13,14,15]. Meanwhile, hybrid electric vehicles still have an internal combustion engine and their emission issues have attracted extensive attention. Wang et al. [16] assessed the characteristics of instantaneous particle number (PN) emissions from four hybrid electric vehicles under real-world driving conditions. The results showed that high PN emissions were affected by the energy output strategy and state of charge condition. Yang et al. [17] investigated real driving PN emissions from two China-6 compliant hybrid electric vehicles. The results suggested that the power management strategy should be amended to deal with the PN issue. Jeong et al. [18] studied fuel consumption and emission according to HEVs urban driving in Korea. Minimizing the engine preheating time during cold starts and utilizing the stored energy in the motor would enhance the fuel efficiency and emission reduction rate. Suarez-Bertoa et al. [19] analyzed the unregulated emissions from light-duty hybrid electric vehicles and found that NH₃, ethanol and acetaldehyde emissions were in the same range as gasoline vehicles were.

Ambient temperature is a key parameter that affects hybrid electric vehicle emissions. Li et al. [20] addressed real-world particle and NOx emissions from HEVs under cold- weather conditions and found that winter significantly increased pollutant emissions. Ehrenberger et al. [21] also observed higher PN and NOx emissions at low temperature. These studies obtained some quantitative results, but the correlations between ambient temperature and hybrid electric vehicle emissions are still unclear, especially for diesel hybrid electric vehicles.

To quantify how ambient temperature affects hybrid electric vehicle emissions, a diesel HEV was tested under three different temperatures. The results in this work are helpful in optimizing the hybrid electric vehicle emission strategy and improving the future policies.

2. Materials and Methods

2.1. Test Vehicle and Laboratory

A well-maintained China-6 diesel hybrid electric vehicle was tested in a laboratory, as shown in Figure 1. Standard China-6 reference fuel was fed to the tested vehicle. Detailed vehicle information is illustrated in

Table 1. Vehicle information.

Parameter	Value
Fuel	diesel
Curb weight (kg)	3400
Transmission	Automatic
Engine	4-cylinder, turbocharged, intercooled
Displacement (L)	2.29
Engine rated power (kW)	103
Engine rated speed (r/min)	3200
Aftertreatment	DOC + DPF + SCR
Motor rated power (kW)	35
Motor rated speed (r/min)	2090
Battery nominal voltage (V)	386.4
Battery capacity (Ah)	40
Vehicle manufacture	Shaanxi heavy duty automobile Co., Ltd., Xi’an, China

. An ECDM-4L-4WD chassis dynamometer (MAHA, Haldenwang, Germany) was used to simulate the road load. A climate chamber (IMTECH SFTP, Leipzig, Germany) was used to control the ambient temperature, which could simulate temperatures from −35 °C to 60 °C. The CVS (constant-volume sampling) system (HORIBA, Kyoto, Japan) was adapted to dilute the exhaust gas. NOx emission was detected by a multicomponent emission analyzer (HORIBA, Kyoto, Japan). Particle number concentration was measured with an AVL 489 (AVL, Graz, Austria) based on the condensation particle counter (CPC) method. Detailed properties of the measurement devices can be found in our previous work [3,7].

2.2. Test Procedure and Data Processing

Tests were conducted at three different ambient temperatures: −10 °C, 25 °C and 40 °C. The tested vehicle was soaked in an ambient chamber for about 8 h before each test. A real driving test work condition was chosen and applied to chassis dynamometer procedure, as shown in Figure 1b. The chassis dynamometer resistance was in accordance with the ambient temperature in each test. It should be noted that the vehicle’s initial state of charge (SOC) was kept at around 25% in each test, as illustrated in Figure 2. In addition, the test vehicle underwent DPF regenerative pretreatment in order to ensure consistent initial conditions of the diesel particle filter (DPF) before the each test.

According to the China 6 regulation, the NOx and PN emission factors were calculated by the moving average window (MAW) method. Detailed specification can be found in previous work [22]. The NOx and PN emission factors of different driving conditions and the entire real driving emission test trip can be calculated using Equation (1). The window duration was determined using Equation (2).

$$e_p={\sum }_{t_1}^{t_2}{\displaystyle \frac{{\dot{m}}_p}{W\left(t_{2,i}\right)-W\left(t_{1,i}\right)}}$$

(1)

$$W\left(t_{2,i}-\Delta t\right)-W\left(t\right)<W_{ref}\le W\left(t_{2,i}\right)-W\left(t_{1,i}\right)$$

(2)

where e_p is the pollutant emission factor, g/kWh; ${\dot{m}}_p$ is the instantaneous pollutant emission rate, ppm for NOx and #/cm³ for PN; W(t) is the accumulative engine work at the tth second, kWh; W_ref is the world harmonized transient cycle (WHTC) reference work, kWh; Δt is the data sampling period, equal to 1 s or less; and t_1,i and t_2,i are the start and end times of the ith window, s.

The instantaneous NOx concentration (ppm) and particle number concentration (#/cm³) were obtained from measurement apparatus.

3. Results and Discussion

3.1. NOx Emission and Characteristic

Figure 3 illustrates NOx emission factors under different ambient temperatures and driving phases. Generally, the total NOx emission factors present a tendency of decreasing with ambient temperatures. And the total NOx emission factors follow the sequence of −10 °C> 25 °C > 40 °C. The decreasing oxygen concentration at high ambient temperature will hinder the chemical reaction of NOx formation [23]. In addition, the intake air humidity is higher at 40 °C than that of −10 °C, which is detrimental to the formation of NOx. In each test, urban NOx emission is much higher than that of the other specific phase, which is due to the presence of NOx of instantaneous NOx emission peaks during the engine cold-start period, as shown in Figure 4. In comparison with the instantaneous NOx emission at 25 °C and 40 °C, the higher instantaneous NOx emission peaks at −10 °C occur under urban and rural phases, which leads to higher NOx emission factor in these specific phases.

In order to further analyze why instantaneous NOx emission peaks are found, the relationship between instantaneous NOx emission and engine speed is investigated, as shown in Figure 5. It is clearly noticed from Figure 5 that the instantaneous NOx emission exhibits a peak value corresponding to engine re-start. Figure 6 presents the upstream selective catalytic reduction (SCR) temperature at different ambient temperatures. In the cold start period, the upstream SCR temperature is below the urea injection temperature which lead to SCR without activation. In the other period, the instantaneous NOx emission corresponds to the upstream SCR temperature valley which means the engine is off. If the upstream SCR temperature is below the lower limit of urea injection temperature due to a period of engine off, the deNOx efficiency is low for a lack of SCR reductant when the engine is restarted, and instantaneous NOx emission peaks occur. On the contrary, the instantaneous NOx emission peaks will not appear. Thus, it is deducted that the occurrence of the instantaneous NOx emission peaks is associated with the upstream SCR temperature fluctuation when the engine is off.

3.2. PN Emission Characteristic

The PN emission factors and instantaneous emissions are presented in Figure 7 and Figure 8, respectively. In general, the PN emission factors increase with rising ambient temperature and follow the sequence of 40 °C > 25 °C > −10 °C. With the rise in intake air temperature, a shortage of oxygen in the cylinder will promote the generation of particulates. For the specific phase emission, the PN emission factor at −10 °C exhibits slightly different trends compared to that at the other two temperatures. In particular, the PN emission factor at −10 °C follows the sequence of urban > rural > highway and the others follow the sequence of urban > highway > rural. It is clearly noticed from Figure 8 that the instantaneous PN emission is higher in the cold-start phase in each test, which leads to higher PN emission in the urban phase. Especially, the instantaneous PN emissions at −10 °C sharply increase at around 1000 s.

Figure 9 shows the instantaneous PN emission versus speed under different ambient temperatures. In the entire test cycle, the PN peaks corresponding to engine-starts can still be found at the ambient temperature. Yang et al. [17] showed that more frequent engine stop-and-goes could be the underlying reasons for the PN emissions.

In addition, the relationship between instantaneous PN emission and upstream DPF temperature is determined, as shown in Figure 10. Beyond the engine cold-start period, the occurrence time of the PN peak corresponds to the upstream DPF temperature steep rise, which is caused by the engine-starts. Especially for the instantaneous PN emissions at around 1000 s under −10 °C, DPF regeneration could be induced in a drastic temperature change. In this process, DPF regeneration can eliminate the inner soot, which was captured in the DPF pores and channels previously, thus leading to higher pore diameter and lower filtration efficiency. That’s why a slightly higher emission in the urban phase under −10 °C is observed.

It is worth noting that the immense change in upstream SCR and DPF temperature is related to the more frequent engine stop-and-goes. Thus, exhaust temperature management plays a key role in tackling the emission issue of HEV, especially for low-temperature conditions.

4. Conclusions

This work mainly investigated the impact of various ambient temperatures on hybrid electric vehicle emission. The conclusions from the experimental measurement are summarized as follows:

(1)

The total NOx emission factors follow the sequence of −10 °C> 25 °C > 40 °C and the PN emission factors follow the sequence of 40 °C> 25 °C > −10 °C. The shortage of oxygen in the cylinder at high ambient temperature will promote the generation of PN and hinder the chemical reaction of NOx formation.

(2)

Both NOx and PN instantaneous emission peaks corresponding to engine-starts can be found in each entire test cycle. The drastic temperature change of upstream SCR and DPF as a result of more frequent engine stop-and-goes may induce the NOx and PN peak occurrence.

(3)

Especially for the low-temperature condition, more attention should be paid to exhaust temperature management for HEVs to tackle the emission issue.

Our works shows a strong influence of various ambient temperatures on hybrid electric vehicle emission. The results presented in this research could be used for hybrid electric vehicle emission strategy optimization and to formulate future policies. Meanwhile, we acknowledge several limitations to this work. CO₂ emissions were not considered and only three temperatures were analyzed. But the main findings could be generalized to these conditions. Future work could test more vehicles from different manufacturers to verify these conclusions.