Parallel Hybrid ( Boosted ) Range Extender Powertrain

The parallel hybrid powertrain developed by Punch Powertrain provides also a nice solution for range extender and boosted range extender vehicles. While most range extender solutions like the GM Volt or the Jaguar XJ Limo Green Hybrid Concept have a series hybrid topology Punch‘s uses a parallel hybrid topology. A parallel topology offers a higher efficiency when the vehicle is driven in hybrid mode. This is even more the case by applying the strategy developed at Punch for this powertrain. This strategy can narrow down the combined engine and CVT operation to peak efficiency. This parallel topology does not only offer the possibility to extend the EV-range of the vehicle, it also allows extending the power in cases the vehicle is used in high power situations its EV powertrain is not designed for. By applying this powertrain in vehicles used for urban and suburban deliveries the electric powertrain can be sized for the majority of its use, i.e. low to medium speed driving using low to medium power. The engine can kick in when more power is required, e.g. high speed driving or accelerating with high payloads. Consequently this concept allows using a lower power rating for the electric powertrain (motor/generator and battery). Due to its parallel topology also one electric machine is required. These cost advantages come with the necessity to use a CVT transmission. Most parts of this transmission are already made in high volume while the other parts are carried over from the other hybrid powertrains from Punch. Consequently this CVT transmission can be produced at low cost. As such Punch Powertrain offers a valid alternative to equip electric vehicles with a cost and fuel efficient range extender or boosted range extender powertrain. Copyright 2010 EVS25. Keywords— parallel HEV, series HEV, PHEV, simulation, vehicle performance


Introduction
Powertrain's hybrid powertrain as presented at EVS-24 is a parallel hybrid powertrain for charge sustaining and plug-in hybrid powertrains.Following the vehicle test in the summer of 2009 Punch concluded that the switched reluctance electric motor/generator was capable of driving the vehicle in electric mode up to fairly high speeds.This implies that the electric motor/generator can also be used for EV applications.
The many announcements of EVs with range extender, as production vehicle or as concept/demonstrator vehicle, made Punch Powertrain consider whether a range extender option can be created with its parallel powertrain configuration.Some conceptual thinking revealed that applying a more powerful motor/generator, a battery pack matching the EV-range and a smaller engine allowing continuous operation with a depleted battery matches the needs of the application.
This paper focuses on the comparison of the series hybrid range extender topology and the parallel hybrid range extender topology based on Punch Powertrain's developments.Both energy efficiency as well as a cost comparison are reported.

The Parallel Powertrains Developed by Punch Powertrain
Punch Powertrain has already presented its parallel hybrid powertrain development at different occasions [1,2,3].Since mid 2009 the powertrain is built into a demonstrator vehicle (see Figure 1) for functional tests and calibration for driveability.When the driveability is realized an identical powertrain is planned to go on a chassis dynamometer test bench for strategy validation.As the hybrid powertrain is built into the demonstrator no engine downsizing is applied.Hence this hybrid powertrain has more power than its conventional counterpart.This hybrid configuration is the power hybrid.The typical engine downsizing for fuel economy would be replacing the engine by an engine 25 to 30% smaller.In most cases a 3-cylinder engine can be used.Depending on the battery size and the strategy this economy hybrid can be used as charge sustaining or as plug-in.The plug-in option will change the blending of engine power and electric power until the battery is depleted to a certain level.The (boosted) range extender concept, subject of this paper, even requires a smaller engine, in most cases a 2cylinder engine.The different powertrains are depicted in Figure 2.

The Powertrains
The range extender powertrains under consideration can be seen as EV powertrains.In the case of the parallel hybrid range extender, a conventional powertrain with an engine and transmission is mechanically connected to EV powertrain transmission.For the series hybrid range extender a genset comprising an engine and generator is linked to the vehicle DC-bus.
Figure 4: The parallel and series hybrid configurations

The Powertrain Component Sizing
The powertrain components need to be sized according to the vehicle.For this study a C-segment electric vehicle from a prior study with the following characteristics was used: To support a constant speed of at least 120 km/h, the electric traction motor needs to be capable of minimum 25 kW continuous mechanical output power as this is required at this speed.The battery is able to support this speed for less than one hour.Both the series hybrid range extender and the parallel hybrid range extender require this motor for traction.The electric motor efficiency map is derived from the current switched reluctance motor at Punch Powertrain.

Series Hybrid Range Extender Add-On
To sustain the constant speed of 120 km/h the genset needs to deliver about 28 kW electric power and an engine capable of delivering 31 kW mechanical power.The electric power rating is continuous because this power needs to be available for periods longer than the 20 to 30 s that peak power can be provided by electric machines.The selected engine is a 1l engine for which detailed fuel consumption data is available.The generator uses the same efficiency map as the traction motor.
The engine and generator are matched to find the optimal operating points over the engine/generator speed range.

Parallel hybrid range extender
Series hybrid range extender range from 8.3 kWe to 28 kWe as depicted in Figure 6 (SpFC and Pout respectively).This allows a fairly efficient load following strategy within this power range.When the required traction power is outside this range it is more efficient to use the batteries as buffer while operating the genset within the range or shutting it off.Very often the series hybrid is described as a powertrain allowing the engine to operate in its sweet point.This principle was applied in an alternative strategy by using the genset at a fixed point when battery state-of-charge is low.The fixed point is the genset power resulting in the same state of charge at the end of the cycle as in the beginning.This power setting is within the power range with specific fuel consumption below 300 g/kWh.With this strategy the battery is more intensively used as a buffer.

Parallel Hybrid Range Extender Add-On
For the parallel hybrid range extender add on the same 1l engine is used and a CVT transmission as used in the parallel hybrid powertrain from Punch.The hybrid strategy applied during range extension is the same as the used by the "standard" hybrid powertrain.Its strategy principles have been reported in earlier publications, as mentioned above.

Simulations
Due to the very early stages of development of this range extender concept Punch Powertrain decided to implement the series hybrid range extender in its basic simulation tool that was also used in the early development stages of the hybrid project.This tool uses a backwards calculation of power through the powertrain.The parallel hybrid range extender is also simulated with this tool.Since both powertrains perform equally in EV mode the simulations are restricted to range extending.

Drive Cycles
Different drive cycles were used in the simulation.First of all, the NEDC cycle because this cycle is still the reference for fuel consumption.Additionally the Artemis light duty cycle also called CADC is used.It is developed in the Artemis project, a project in the European 5 th Framework Programme, and a likely successor of the NEDC-cycle.
Two cycles recorded in real traffic were also simulated, the MOL-cycle developed by VITO as well as an additional recording of this cycle made in the DECADEproject (also a project in the European 5 th Framework Programme).This last cycle is called MOL Decade.The original MOL-cycle is fairly aggressive.The MOL Decade is recorded in different circumstances, like different routing, road works, busier traffic and lowered speed limits.This results in a less aggressive cycle.Table 2   This parameter and its calculation method was originally been developed by van de Weijer [4].The speed profile of the CADC, MOL and MOL-Decade cycles is given above.

Simulation Results
Before comparing the simulation results, it needs to be mentioned that the absolute levels of the fuel consumption should not be compared with actual vehicles.Furthermore, these differences should be placed in the context of a range extender vehicle that depending on the application may use its engine infrequently, in some cases never.

Load Following Series Hybrid Strategy
The simulations were run with an initial state of charge of 30%.If possible parameters influencing the EV-driving and the use of the genset were changed to reach the same final SoC.If the final SoC could not be made identical to the initial by adopting the strategy, the difference in SoC over the cycle was corrected using the total fuel consumption and electric charge generated during the cycle.

Diff.[%]
NEDC For all cycles the series hybrid range extender has a higher fuel consumption than the parallel hybrid range extender.Differences range from 6 to 9%.It needs mentioning that the simulation tool does not allow drawing conclusions if the difference in fuel consumption is small.In this case all results are considered to be significantly different.
To illustrate the operation of the genset two graphs were taken from the simulation of the MOL-Decade cycle.The first graph in Figure 8 is from city traffic with speeds up to 50 km/h.This part of the cycle shows accelerations followed by decelerations.So there is no part at constant speed.The generator is only activated during acceleration.
The second graph in Figure 8 shows a part of the extra urban part.Also here most part of the cycle exists of accelerations followed by decelerations.One exception is the part between 1908s and 1923s where speed is about constant.Here the genset generates about 10 kW to maintain the speed.
The power to maintain a speed is also illustrated in Figure 9.This figure is taken from the NEDC cycle.To maintain a speed of 70 km/h the genset generates 9 kW.At 50 km/h the required power drops below the minimum generated power and the genset is switched off.

Constant Load Series Hybrid Strategy
The simulations were run with an initial state of charge of 30%.The genset was set to operate at a fixed power whenever the SoC was below a given level.This power level of the genset was changed to reach the same final SoC.By adapting the SoC level under which the genset is active the power level of the genset could be lowered if it was on the high side and vice versa.This may also result in more efficient genset operation.Both parameters were changed until the lowest fuel consumption was reached with a final SoC equaling the initial SoC.
Table 4 shows the results of these simulations.Only two cycles, CADC and MOL, show a significant difference.In these cases the fuel consumption for the constant load series hybrid is higher.This is most probably caused by extra conversion losses in the battery that are not compensated by a more efficient genset operation.

Cost Comparison
When looking at the subsystems used in both configurations the following overview can be made.5 one can see that comparing the cost of both hybrid configurations comes down to comparing the cost of the standard automotive CVT with the cost of the generator plus the single ratio reduction gearbox.The current experience shows that the cost difference will be too small to be decisive if other factors come into consideration.

Vehicle integration
One such factor in the decision to go for the series or the parallel configuration is vehicle integration.The parallel hybrid topology requires all main components except the battery to be built into the engine compartment.Therefore vehicles with this type of range extender will resemble more the standard vehicle.More or less standard engines are the preferred fuel converter because they match the transmission.
The series hybrid topology allows more freedom in accommodating the genset in the vehicle.The fuel converter does not have to be a standard engine like in the parallel topology.Other fuel converters like gas turbines can be considered if their efficiency and time to get up to speed allows a flexible operation.

Boosted Range Extender
The parallel hybrid configuration allows both the engine and electric traction power to simultaneously drive the wheels.This feature can be turned into a benefit in vehicles that very seldom require full power.Examples are mail distribution vehicles in cities and suburbs.During normal operation these vehicles never leave the area where they are used and high speeds do not occur.Only when these vehicles need to travel over longer distance, e.g. for maintenance or moving to another operating area, they may require the high power.
For these applications the parallel hybrid range extender allows reducing the electric motor to the requirements of the normal operation.Whenever more power is required the engine can kick in and the powertrain changes from EV to HEV operation.The benefit is a downsizing that can be applied on the electric drive, hence a cost saving.

Conclusions
This study has compared a parallel range extender powertrain developed by Punch Powertrain with a series hybrid range extender powertrain with similar performance.For the series hybrid powertrain two different optimizations were applied, one focusing on load following with an optimized genset performance and one with a constant load requiring more buffering by the battery.Simulations have shown that the parallel range extender hybrid powertrain is more fuel efficient than a series hybrid powertrain.The load following strategy of the series hybrid is more efficient than the constant load strategy.
Other factors are also considered.The costs of both powertrains are comparable.The vehicle integration of the series hybrid range extender allows more flexibility because the genset can be put in a more convenient place.Finally the parallel hybrid range extender allows opting for a boosted range extender depending on vehicle use.
As a result one can conclude that the parallel hybrid range extender developed by Punch Powertrain is a valid alternative for the series hybrid range extender.

Figure 1 :
Figure 1: The hybrid powertrain in front of the Smart Forfour demonstrator

Figure 2 :
Figure 2: Hybrid configurations being developed by Punch Powertrain How the different configurations have an effect on powertrain length and hence space in the engine compartment is shown in Figure 3.

Figure 3 :
Figure 3: Pictures of the 3 Powertrain Concepts

Figure 5
depicts the maximum engine torque T eng,max as well as the nominal generator torque T gen,nom .Additionally the engine torque corresponding with the most efficient engine operation (T eng, opt ) as well as genset operation as a system (T sys,opt ) are shown

Figure 5 :
Figure 5: Engine and generator performanceThis results in genset system performance with a specific fuel consumption below 300 g/kWe in a power Figure 6: Genset performance

Figure 7 :
Figure 7: Non-type approval drive cycles The RPA-parameter is the relative positive acceleration.It represents the amount of energy per kg vehicle mass required to perform the acceleration of the drive cycle.

Figure 8 :Figure 9 :
Figure 8: Genset power versus vehicle speed in MOL-Decade cycle.Generated Power vs Vehicle Speed

Table 1 :
Vehicle Properties

Table 2 :
gives an overview of the cycle characteristics.Overview of cycles used in the simulation

Table 3 :
Simulation results, parallel hybrid vs. load following series hybrid

Table 4 :
Simulation results, load following series hybrid vs. constant load series hybrid When comparing the results of the constant load series hybrid with the parallel hybrid the difference have further increased.The constant load series hybrid consumes 8 to 16% more fuel.