Auxiliary power units for range extended electric vehicles N Powell, M Little, J Reeve, J Baxter Ricardo UK Ltd, UK S Robinson, A Herbert Jaguar Land Rover, UK A Mason, P Strange Tata Motors European Technical Centre, UK D Charters MIRA Ltd, UK S Benjamin, S Aleksandrova Coventry University, UK
ABSTRACT This paper describes research carried out into auxiliary power units (APU) for range extended electric vehicles (RE-EV), as part of the Low Carbon Vehicle Technology Project (LCVTP) (see acknowledgement). APU requirements are specified and compared to the attributes of a variety of prime power sources. It is concluded that for many applications adaptation of a volume production 4-stroke gasoline engine will be the most suitable choice for a RE-EV, until production volumes make a bespoke engine viable. The paper explores potential engine modifications and generator integration. The development of a 55kW APU, using a volume production two cylinder engine and a close coupled design permanent magnet generator are described including its installation into a technology validator vehicle. 1.
In 2005, passenger cars accounted for 13% of UK carbon dioxide (CO2) emissions (1), and thus offer significant potential for overall CO2 reduction. The NAIGT consensus roadmap provides a pathway to achieve future fleet average CO2 targets (2), and plug-in hybrid vehicles feature as a step in this roadmap towards mass market electric vehicles (EV). Electric power provides an energy efficient method of powering a vehicle for the majority of journeys, 93% of which are less than 40km (3). However, in the near to medium term most customers are likely to expect a low carbon vehicle to be able to also complete longer journeys when required. Current battery technology means that a large, heavy and expensive battery is required to provide acceptable range. For example, to provide a 500km range, for a C/D-segment vehicle, would require an approximately 70kWhr battery. With current battery pack technology costing approximately $800 per kWh, and even the most optimistic forecasts suggesting costs will only fall to approximately $300 per kWh by 2020 (4), this equates to a battery cost of approximately $50,000 at todays costs. Therefore replacing some of the battery capacity with an on-board charging source, or Auxiliary Power Unit
____________________________________________ © The author(s) and/or their employer(s), 2012
(APU), to facilitate longer journeys using the existing fuel infrastructure makes economic sense. An APU maintains all, or a proportion of, the EV functionality, once the battery state of charge (SOC) falls below a threshold level in a Range Extended Electric Vehicle (RE-EV). An APU in a RE-EV thus provides the functionality to complete longer journeys, at lower total vehicle cost than a pure EV with a large battery, as illustrated in Figure 1. A RE-EV is also less likely to require a bespoke vehicle body, as the battery is smaller than an EV. The APU also allows flexibility in vehicle architecture and engine operating strategy, due to the absence of a mechanical connection between the APU and the wheels. 20000
APU / Transmission Cost
Motor/Generator & Power Electronics Cost 10000
Based on: EV Battery Size = 30kWh RE-EV Battery Size = 13kWh
Figure 1: Example of powertrain cost advantage of a RE-EV compared to an EV This paper describes the requirements for an APU and discusses some of the many possible solutions explored during the collaborative Low Carbon Vehicle Technology Project. 2.
2.1 Power requirement An APU may be sized from an emergency “get-you-home” device, of relatively low power, to a device that provides the full functionality of the vehicle once the battery has been depleted. Customer expectations of vehicle functionality will take time to change, so most APU interest is in units which provide the later capability. Sustained high speed cruise condition or hill gradient are the most important conditions for determining the required APU power. In a pure RE-EV vehicle there is no mechanical connection between the engine and the wheels, and therefore, as shown in Figure 2, the efficiency of energy conversion from the mechanical to electrical in the APU, and from electrical to mechanical in the motor and associated power electronics, must be considered when calculating the APU power.
RE-EV Power Transmission Losses Power to Wheels ~68% Flywheel power
Generator Losses 10%
Storage Losses 5%
Power electronics Losses 5%
Motor Losses 10%
Power electronics Losses 5%
Drivetrain Losses 2%
Conventional Arctitecture Transmission Losses
Power to Wheels ~95% Flywheel power
Drivetrain Losses 5%
Figure 2: A comparison of the typical energy conversion efficiencies in a RE-EV and in a conventional drivetrain architecture Assuming the efficiency of the RE-EV energy conversion chain, is approximately 70%, Figure 3 shows that the APU power required for a large passenger car to sustain a motorway cruising speed is between 30 and 50kW. Furthermore the APU must supply power for other parasitic loads, for example HVAC which typically require an additional 2-3kW. It is noted that due to the inherent inefficiencies of an electrical driveline, a clutched direct drive to the vehicle driveline, for higher speed operation, is an attractive option until there is a breakthrough in power electronics efficiency. This could also serve to reduce the size of the generator and associated cost of the power electronics. Power requirement for 1955kg car (Crr=0.01 & CdA=1.001) 80 70 60
Total power f or hill climb (4%) assuming 68% ef f iciency
Total power f or hill climb (4%) assuming 100% ef f iciency
Total power assuming 68% ef f iciency
20 Total power assuming 100% ef f iciency
10 0 0
Figure 3: APU power requirement for a large passenger car to maintain sustained cruising speed
2.2 Duty Cycle Unlike a conventional vehicle architecture, there is no requirement for the engine to operate in the relatively inefficient low speed and low load region of the operating map. Operation at a single most efficient speed and load point is possible, although operation along a line of best efficiency versus power out is more likely, for NVH and emissions reasons. The RE-EV architecture means the maximum engine speed can be reduced, typically to around 4000rev/min, in order to minimise friction losses and to reduce NVH. Furthermore, there is no risk of over-revving due to driver abuse or a missed gearchange. There is also no need for the engine to idle, although it will typically be operated at a lower load and at a lower speed than peak operating efficiency at start-up, in order to achieve catalyst light off. Furthermore, the engine does not need to respond to transients as rapidly as in a conventional vehicle. 2.3 Legislative Current European legislation specifies a weighted average approach for plug-in hybrid vehicles, between SOC-depleting and SOC-sustaining conditions, to the calculation of CO2 emissions (5). No account is taken of CO2 used to generate the grid-supplied electricity. Figure 4 shows that for a typical RE-EV this results in a relatively low declared CO2 emissions figure. It is noted that toxic emissions standards must be achieved in SOC-sustaining mode only. The weighted values of CO2 shall be calculated as below:
M1= CO2 in g/km with a fully charged electrical energy/power storage device (Condition A SOC-depleting) M2 = CO2 in g/km with an electrical energy/power storage device in minimum state of charge (Condition B SOC-sustaining) De = Electric range Dav = 25km (assumed average distance between opportunities to recharge the battery) Example: For a vehicle that has test-results: Condition A (SOC-depleting) CO2 = 0 g/km (test can be completed in EV-mode) Condition B (SOC-sustaining) CO2= 150 g/km Electric-range = 40km Gives, Weighted CO2 = (40 * 0 + 25 * 150) / (40 + 25) = 57 g/km
Figure 4: Example calculation of CO2 emissions for a plug-in hybrid vehicle under UNECE 101 2.4 Efficiency For an ‘emergency’ APU, efficiency is less important, than light weight and compactness as the APU is hardly ever used and thus represents dead weight in the vehicle for most of its life. However, for the 30-50kW APU, primarily considered in this paper, efficiency is more important.
The RE-EV architecture affects overall efficiency in three ways. Firstly, the first part of any journey may be completed in (an energy efficient) electric only mode. Secondly, because the engine is not directly connected to the wheels, the engine may be operated over a limited part of the conventional engine map only. However, as shown in figure 2, there are inefficiencies associated with the energy conversions inherent in a RE-EV architecture. In practice, for most real driving conditions, the efficiency loss, due to the energy conversion process, will be larger than the efficiency gained by operation of the engine at optimum efficiency conditions.
Fuel Consumption (litres/100km)
Therefore, for shorter journeys which may be completed in pure electric mode, the RE-EV architecture provides an energy efficient mode. However, substantial total CO2 emissions come from longer journeys, at higher speeds, and it is also likely that the customer will be well aware of the fuel consumption achieved on longer journeys. Figure 5 compares the overall fuel consumption of a RE-EV with a conventional gasoline vehicle, versus journey length. In a RE-EV, it is assumed that the first part of any journey is carried out in electric only mode. Once the battery is depleted, the APU provides power, but in a less efficient way than a conventional vehicle, because of the energy conversion processes. At a certain journey length, the overall fuel used by a RE-EV will therefore be greater than the conventional vehicle. 12
220km RE-EV uses more fuel than a conventional architecture
10 8 6
Average journey 240g/kWh APU Instantaneous 240g/kWh APU
Average journey 220g/kWh APU Instantaneous 220g/kWh APU
Conventional architecture 260g/kWh 0
Journey length (km)
Figure 5: Comparison of the total fuel used versus journey length for a REEV and a conventional gasoline vehicle (Example is for a large passenger car, at a steady state 100kph, and shows that a RE-EV with a 240g/kWh APU uses more fuel than an engine in a conventional vehicle for journeys over 150km) A target may be proposed such that a RE-EV should use no more fuel than a conventional gasoline vehicle over a “reasonably” long journey. This target can be cascaded to a BSFC target for the APU. For the example shown in Figure 5, to achieve a target 220 km journey length using less fuel in a RE-EV than in a conventional vehicle, requires that the RE-EV engine BSFC (mech) to be less than 220g/kWhr.
2.5 NVH In an electrified vehicle with an auxiliary power unit (RE-EV) the operation of the APU is not expected to be within the direct control of the driver and hence its noise signature should be reduced to a level where it is not subjectively prominent to the vehicle occupants upon start-up and in operation. The levels of masking noise during zero-emission driving are low and therefore the required levels of acoustic refinement of the APU are very high, in order to be subjectively acceptable in a charge sustaining mode. Even with APU noise minimised, subjective analysis carried out on the expectations of the driver to changes in APU speed and load suggested that at least some relationship between engine speed and vehicle speed is desirable, and counterintuitive NVH responses must be avoided. 2.6 Package and weight In order to understand the trade-off between APU weight and efficiency, the energy required by a passenger car performing multiple NEDC cycles was calculated. By assuming an approximate inverse relationship between APU fuel consumption and weight, the amount of fuel used by a RE-EV completing different journey lengths, initially in all electric mode until the battery is depleted, and then in charge sustaining mode, is calculated and shown in figure 6. It is shown that for charge sustaining conditions that APU efficiency is much more important than APU weight.
Figure 6: Example of the relationship between CO2 emissions and APU mass for a large passenger RE-EV There are many systems to package in a RE-EV, including the battery, generator, motor and power electronics, gearbox, engine, intake and exhaust, fuel and cooling systems, generator. Therefore packaging and orientation of the APU is important, and will depend upon the specific application.
2.7 Other requirements For best efficiency, it is preferable to start an APU at a higher speed and load than conventional idle. The toxic emissions and afterteatment requirements must be carefully considered at this condition, prior to catalyst light-off, because of the relatively high gas flows. Emissions results have been presented in separate papers (6). Other requirements for APU design include consideration of the effect of vehicle induced vibration on the bearing surfaces of a static engine, due to potential long periods of operation in pure EV mode. Furthermore an APU has potentially longer periods of no use compared to a conventional engine and a regular period of operation is likely to be required in order to lubricate running surfaces and to mitigate partially the effects of fuel ageing (7). 3.
There are a variety of prime power source options for an APU. Some of these, together with a summary of their applicability to APU application are summarised in Table 1. Table 1: A summary of APU prime power source options APU Options 4 stroke gasoline
Stirling Diesel Fuel cell Wankel rotary Motorcycle engine
Attributes Proven for APU application with a mixture of power density and efficiency attributes at relatively low cost in volume production which are well matched to APU requirements (8) Compact and with multifuel capability, but current technology is less efficient than conventional gasoline and the high speed of turbines demands either high-speed generator technology or gearing Relatively unsuited to APU use due to low power density, though possible application for waste heat recovery Heavier and more expensive than gasoline due to emissions control requirements, and with inferior NVH Potential longer-term solution but needs development of hydrogen infrastructure and on-board storage (9) Low vibration and with some potential in the under 10 kW class where efficiency is less important Powerful and frequently light in weight with high-rpm bias. Wide variety available, but power characteristics, oil consumption, durability, emissions and integrated ancillaries would typically need more development than a passenger car engine
Analysis carried out by the authors suggests that a conventional gasoline engine architecture, optimised for APU duty cycles, is well suited as an APU power source in the short to medium term. It is also noted that anticipated production volumes, at least in the near to medium term of RE-EVs are likely to be relatively small. In order, to achieve an acceptable cost, it is likely that modification of an existing volume production engine may be a preferable route over a bespoke engine, unless packaging demands dictate otherwise. The following sections, describe work carried out within Low Carbon Vehicle Technology Project (LCVTP) to investigate adaptation of a volume production engine for APU application.
ENGINE TECHNOLOGY FOR APU APPLICATION
The differences described in Section 2.2, between an APU and a conventional duty cycle, present different design freedoms on the engine. Of particular interest is the application of Atkinson or Miller cycle, typically through late inlet valve closing (IVC), to achieve efficiency targets. Furthermore, the lower engine speed range of an APU allows optimisation of the engine breathing, through for example more restrictive intake ports to generate more charge motion, in order to improve thermal efficiency by faster burn. A validated Ricardo WAVE model was built representative of a small turbo-charged two cylinder gasoline engine in order to investigate how modifications to the base engine design could be applied to optimise efficiency within these different design freedoms. The baseline model was simulated at full load and part load between 2000 and 3000 rev/min to select the most efficient operating point between 25 and 30 kW. Various strategies were then evaluated to investigate BSFC benefits from modifications such as conversion from PFI to GDI, valve timing optimisation and friction reduction. The results, shown in Table 2, suggest that the target BSFC described in section 2.4 is achievable and show particular benefits from the application of Miller cycle, through optimised late IVC. Table 2: Simulated improvement to base engine BSFC WAVE Model Details PFI GDI Miller Valve Timing Optimised Valve Timing Optimised model w. larger turbine and reduced friction
BSFC (g/kWhr) 234 232 230 221 218
Application of these improvements to a Ricardo vehicle simulation model demonstrated an improvement in fuel consumption over the NEDC cycle of 5.6 % over the baseline model. Testing was also carried out on a modified Tata Nano engine, at MIRA, in order to investigate modifications to the breathing and the application of Atkinson cycle. The geometric compression ratio of the base engine was increased to between 14 and 15 and sweeps performed to determine optimum late IVC. BSFC benefits of approximately 8% at APU rated power were demonstrated, although it should be noted that the engine swept volume would need to be increased by approximately 10% to maintain the rated power. 5.
APU TECHNOLOGY DEMONSTRATOR
As described in Section 3, forecasts suggest that in the near to medium term the market for APUs is relatively small, and may not justify the investment in the production of dedicated engines. Adaptation of existing volume-production engines could be a more cost effective approach. A Fiat TwinAir engine (10) was therefore selected to investigate some of the challenges of integrating a generator to a volume production engine, to create an APU for installation into a technology demonstrator vehicle based on a Land Rover Freelander 2 vehicle.
Thermal efficiency considerations, for a 30-50kW engine, suggest most future APUs will be 2 or 3 cylinder, so the Fiat TwinAir engine was selected as a good example of a modern compact engine and to explore the challenges of fitting a generator to a two-cylinder engine. The challenges included integration of a generator to a turbo-charged engine and high crankshaft torsional damper (TV) due to low cylinder count. A TwinAir engine was procured as a ‘black-box’ by removal from a Fiat 500 vehicle. A number of starter-generator integration options were considered. Four high level coupling solutions are possible: Direct coupling (e.g. bolting) of the generator to the engine crankshaft In-line compliant coupling Offset axes between engine and generator, with step-up in speed for example via belt, chain or gear drive Integration of the generator and the engine crankshaft For this research, direct coupling of the generator to the engine crankshaft, in place of the standard flywheel, was selected for further study. With this arrangement the need for one or two generator bearings was removed, as was the requirement for a compliant coupling which minimises the axial length of the APU. A number of generator options were also researched, including AC induction, permanent magnet and reluctance machines each offering a different balance of attributes. An outer rotor permanent magnet design was selected to go to hardware. The outer rotor design maximises the torque density, at relatively low speeds whilst also helping to minimise the axial length of the machine to assist packaging. The 55kW at 4000 rev/min generator, shown in Figure 7, was designed by Ricardo and made to print by project partner Zytek. Bolting the generator rotor directly to the crankshaft imposes different loads on the engine crankshaft. Careful analysis of the crank dynamics was carried out, in order to check crank loads and to specify the airgap between the generator stator and rotor. Validation testing of the machine has been completed and has confirmed the thermal management approach which allows the machine to operate on either the (high temperature) engine cooling circuit, or the (low temperature) power electronics cooling circuit.
Figure 7: Bespoke 55kW APU motor-generator designed by Ricardo and built to print by Zytek, shown fitted to a Fiat Twin - Air engine
The APU has been fitted into a technology demonstrator RE-EV, based on a Land Rover Freelander 2 vehicle, as shown in Figure 8. NVH was a particular consideration for this installation. A time domain transfer path analysis (TPA) highlighted exhaust orifice noise as one aspect of the APU noise signature that could not be sufficiently masked by the wind and road noise when using a carryover exhaust muffler. A validated acoustic WAVE model was therefore used to optimise the Figure 8: Installation of the APU in exhaust system for the reduced a Technology Demonstrator Vehicle operating range (1500-4000rev/min) and longer exhaust system run; this optimisation reduced both the orifice noise level and the exhaust backpressure. The re-designed exhaust geometry incorporates a new Helmholtz resonator (HHR) and alternative centre muffler. The TPA showed that the revised design offers acceptable exhaust noise in the RE-EV application. In addition the TPA simulation was used to evaluate the implications of different mounting strategies on structure borne noise and vibration. The challenges and solutions for delivering acceptable NVH from 2 and 3 cylinder engines are discussed further in (11). The life cycle benefits of the RE-EV technology developed during the LCVTP, including the APU, have been demonstrated (12). Further results from the technology demonstration vehicle will be presented in future papers. 6.
This paper has discussed the requirements for a 30-50kW APU for RE-EV in the short to medium term. The adaptation of an existing volume production gasoline engine has been explored to satisfy these requirements, while RE-EV production volumes make a bespoke engine less commercially attractive. A method of specifying the target BSFC for an APU engine has been presented, and methods of achieving this demonstrated, including the adoption of Atkinson or Miller cycle, through late IVC. Adaption of a Fiat TwinAir engine into an APU, and its installation into a technology demonstrator vehicle has been described. Although, this is not an optimised example, it has shown the practicality of fitting a compact outer rotor permanent magnet generator directly onto the crankshaft of a 2 cylinder volume production engine to create a compact APU solution. 7. (1) (2) (3)
REFERENCE LIST KING, J., 2008, The King review of low-carbon cars. NEW AUTOMOTIVE INNOVATION AND GROWTH TEAM (NAIGT), 2009. An independent report on the future of the automotive industry in the UK. DEPARTMENT FOR TRANSPORT, 2009. Low carbon transport: A greener future, DfT analysis (2002/2006 average), http://www.official-documents.gov.uk/document/cm76/7682/7682.pdf
(6) (7) (8) (9) (10) (11) (12)
ELEMENT ENERGY, 2012. Cost and performance of EV batteries final report for the committee on climate change. UNECE Regulation No. 101, 2010. Uniform provisions concerning the approval of passenger cars powered by an internal combustion engine only, or powered by a hybrid electric powertrain. UNECE Regulation No.83, 2010. Uniform provisions concerning the approval of vehicles with regard to the emission of pollutants according to engine fuel requirements. ALEKSANDROVA, S., BENJAMIN, S., 2011. Using Wave for reducing emissions for REEV application, European Ricardo Software User Conference, 5th April 2011. ABBOUD, E., CORY, D., General Motors, 2011. Comprehensive overview of human interface for an extended range electric vehicle, SAE 2011-01-1023 Grebe U.D., Nitz, L.T., May 2011. Voltec – The Propulsion System for Chevrolet Volt and Opel Ampera. MTZ Worldwide, pp4-11 OWEN, N. (ed), 2009. Fuel cells and hydrogen in a sustainable energy economy, Final report from the Roads2HyCom Project, 8 April 2009. Available at: www.roads2hy.com, [Last accessed 2nd August 2012] SACCO, D., MASTRANGELO, G., MICELLI, D., 2010. TwinAir: The Extreme Downsized Engine Solution for Future Urban Mobility. Aachener Kolloquium Fahrzeug und Motorentechnik MAUNDER, M., POGGI, M., TATE, S., STRONG, G., CARDEN, P., Delivering competitive NVH from low CO2 powertrain solutions, ATZlive Automotive acoustics conference, Zurich, 2011. PATTERSON, J., 2012. Life cycle CO2 footprint of an LCVTP Vehicle, LCVTP Final Dissemination Event, University of Warwick, UK. http://www2.warwick.ac.uk/fac/sci/wmg/research/lcvtp
*The Low Carbon Vehicle Technology Project (LCVTP) is a collaborative research project between leading automotive companies and research partners, revolutionising the way vehicles are powered and manufactured. The project partners include Jaguar Land Rover, Tata Motors European Technical Centre, Ricardo, MIRA LTD., Zytek, WMG and Coventry University. The project includes 15 automotive technology development work-streams that will deliver technological and socio-economic outputs that will benefit the West Midlands Region. The £19 million project is funded by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).