Analysis of four-stroke, Wankel, and microturbine based range extenders for electric vehicles
Highlights
► VSP correlates well with the engine use, regenerative braking and boost setting. ► Wankel engine vehicle is the most efficient in urban driving. ► Over-expanded engine vehicle is the most efficient in annual combined use. ► The higher the annual urban commuting driving the lower is energy consumption. ► Over-expanded solution has 5.7% WTW less energy usage and 8.8% less CO2 emissions.
Introduction
Kyoto protocol, a global trend to diminish emissions from all sectors, including transportation, is under effect. The 2011 growing political instability in the Middle East has further raised concerns over oil supply security in the midterm. These facts are awakening the international community for the importance of new paradigms of land mobility.
In this framework, the automotive industry is experiencing a major shift in paradigm, with energetic and environmental sustainability being put more and more on the forefront of OEMs research efforts [1]. The gradual electrification of the vehicle is one of the strategies adopted both by industry and the policy makers all around the developed world in the scope of this paradigm shift. Vehicle electrification enables the improvement of urban air quality (no local emissions), the diversification of primary energy sources (electricity can be generated from a wider range of sources, not necessarily with fossil origin), and allows the use of technologies that may improve energy-efficiency (such as regenerative braking and low consumption electric driven components). Furthermore, the promotion of electric mobility initiatives within regions where electric energy production displays low Greenhouse Gas (GHG) emissions (namely from renewable sources) can provide a significant contribution to the overall reduction in primary energy consumption and GHG emissions [2], [3], [4], [5] in order to comply with the evermore stringent current and future international agreements and policies on GHG emissions, such as the Climate and Energy Package issued by the European Commission [6]. This is particularly the case in some European countries (like Portugal, where the data for the present study has been gathered) where electricity production has a strong bias towards renewable sources.
Even OEMs themselves need to globally reduce the emissions level of their global model portfolio down to values that are very difficult to achieve without implementing hybrid or electric powertrain technologies at least to a part of their portfolio. This is the case of the Corporate Average Fuel Economy program (CAFE) ruled in the USA by the Environmental Protection Agency (EPA) and the Department of Transportation (DOT) [7].
The road towards total vehicle electrification still poses some big challenges. Currently, the main hurdle resides in electrical storage technology [8]: compared with liquid fuels, they display much lower specific energy, energy density and refuelling/recharging rate. For instance, in a wide survey on Electric/Hybrid Vehicles performed by the authors [9] it was concluded (Fig. 1) that, on average, Battery-only Electric Vehicles (BEVs) have a consumption around 0.12 Wh/km for each kg of vehicle mass. For a 1500 kg vehicle (roughly the curb weight of the Nissan Leaf [10] and the Opel Ampera [11]) this yields consumptions around 180 Wh/km, which would require a Lithium-Ion battery pack with a volume around 1.17 L/km, a weight around 1.8 kg/km and a cost that could reach four digit figures (in USD) for each 100 km of range. Now considering an ICE-based vehicle with consumption around 7 L/100 km, this makes the BEV energy storage around 17 times bigger and 35 times heavier than that of conventional vehicles for the same available range. Concerning recharging times from a house socket (16 A, 220 V) of 3.5 kW, those 18 kW h that provide a 100 km range would require nearly 6 h of recharging. As long as the current limitations in storage technology are not overcome, they will strongly limit the market appeal for Battery-only Electric Vehicles (BEV) in mainstream applications other than short range urban commuting.
A good solution for the aforementioned limitations is the downsizing of the battery pack for the typical daily range complemented by the generation of electric power on-board. In fact, 50% of the existing vehicles have a daily mileage of less than 35 km for PT [12] and 48 km for US [13], much lower than the 150 km average range of Battery-only EVs recorded by a wide survey on EVs made by the authors [9]. Solar panels have low energy density, while fuel cell vehicles still lack refuelling networks and are still a developing technology (hardly any mass market vehicles are available as of 2011) [9]. Therefore, the best option seems to be the use of an internal combustion engine (ICE) working as a “Range Extender” (RE) unit. It will be off as long as there is sufficient energy in electric storage for pure electric driving (Charge Depletion mode, CD) and will be activated whenever the State of Charge (SOC) drops below a certain level (Charge Sustaining mode, CS). This strategy enables this Extended Range Electric Vehicle (EREV) to retain a very low overall local emissions level (the RE will only work occasionally, when the SOC is too low), to have a small, light and affordable battery pack and simultaneously to be capable of occasional long trips without the need for an additional vehicle or for electricity recharging stops.
Recently it has been proved that the EREV makes good use of electric power for vehicle propulsion [14], [15]. While the BEV seems best suited for short range urban driving and the full Hybrid Electric Vehicles (HEVs) are the best option for frequent long highway trips while still performing well in urban driving, the EREV electric mobility concept seems best suited for small to medium daily ranges combined with occasional long trips. This way the EREV will work basically as a plug-in EV (PHEV) most of the time, with all the corresponding advantages that are typical of electric mobility, namely no local emissions, high energy efficiency (especially in urban driving) and low cost of operation. Naturally, the EREV also have some disadvantages in relation to BEVs, as the added complexity of the RE system and a possible negative impact on the perceived (not necessarily real) environmental sustainability of the vehicle due to fossil fuel use. The current study does not deal with Total Cost of Ownership comparison between EREVs and BEVs, but of course the trend-offs will depend on the cost due to the alteration of the battery pack and the added maintenance needs of the specific RE solution deployed.
Two basic approaches can be used when designing or choosing a RE: either (a) focusing on maximizing efficiency and minimizing emissions, in-line with electric mobility quest for sustainability or (b) favouring compactness and simplicity at the expense of added emissions and lower efficiency (as the system will be seldom used).
Approach (a) can be accomplished with the use of conventional Spark Ignition (SI) piston engines working under high efficiency cycles such as the over-expanded (Miller) Cycle. This cycle, also known as Atkinson Cycle, is used by the Toyota Prius [16] and other HEVs and owes its high efficiency to an expansion stroke that is considerably longer than the compression stroke, enabling the conversion of a higher fraction of the energy of combustion into useful work. This can be accomplished by changing intake valve timings so that the effective compression stroke is reduced in conjunction with an increase of the compression ratio [17].
A study on engines used in EREVs [18] proved that engine efficiency was preponderant for the reduction of vehicle consumption. A survey made by the team identified a total of 35 EREV models from several car manufacturers [9], most of them using SI engines. Although many of these engines use the over-expanded cycle they work over a broad range of load and speed, instead of working at their maximum efficiency set point, failing to maximize the efficiency potential. Concerning the Opel Ampera [11], [19] RE engine, it works at variable speed and mostly at Wide Open Throttle (WOT). The engine operation may vary, namely when a more aggressive charging is needed or in charge sustaining mode at high speed, when the engine complements the electric motor torque. According to the manufacturer, although the engine efficiency was a priority it was not set to work at a fixed condition in order to have a smooth, quiet and pleasant driving [18].
Previous theoretical and experimental work by the authors [20], [21] has proved that, when over-expansion is combined with optimal valve timings and effective compression ratios, efficiencies which are higher than those of diesel engines can be obtained in significant portions of the speed/load map. Nevertheless, this working philosophy has the disadvantage of displaying low power densities. This drop in power density due to over expansion is less intense when using the Late Intake Valve Closure (LIVC) strategy instead of Early Intake Valve Closure (EIVC) as it is beneficial for volumetric efficiency, especially at high speeds [21].
Some range extenders based on approach (b) have been proposed recently, namely those based on Wankel engines and microturbines. These engines display very high power densities but display poor efficiencies. Recently some REs have been presented as modules to the market. AVL presented a 15 kW Wankel and a 2 cylinder unit with minimum fuel consumption of 260 g/kW h [22]. FEV presented a 20 kW Wankel-based RE [23] while LOTUS presented a 1.2 L 3 cylinder engine [24] with two different power levels (15 kW @ 1500 rpm and 35 kW @ 3500 rpm). Capstone also presented a 30 kW Microturbine RE with very low weight and volume (91 kg, 56 × 15 × 28 cm) [25] but with low efficiency (260–350 g/kW h). Some concept EREVs based on these engines have been presented recently, such as the Audi A1 e-tron using a 15 kW Wankel engine [26] or the Jaguar C-X75 Concept super car with a twin Microturbine RE (2 × 70 kW) from Bladon Jets [27]. Although compact and silent, Wankel engines are not efficient due to their unflexible geometry, heat losses, combustion chamber sealing and high oil consumption during the warm-up phase. Also the extreme compactness of the Microturbine system, that does not even need a cooling system, has as main drawbacks a high acquisition cost and a typically low efficiency, around 25% for ambient temperature, even after incorporating a heat regenerator [25].
Energy conversion phenomena of plug-in electric vehicles (PHEVs) operating according to different control strategy modes and according to different driving cycles was analyzed by simulation and analytical analysis (e.g. [28], [29]). These studies only cover one range extender philosophy, with conventional gasoline engine, and aim at comparison of different energy management strategies in charge depleting mode and in a Well-To-Wheel context. The specific study of different RE can be found in Varnhagen et al. [30] Top of FormBottom of Form for rotary and Wankel engine options in different driving cycles and in a Well-To-Wheel context.
A thorough comparison of several RE philosophies in terms of energy efficiency and emissions covering over-expanded, Wankel and micro-turbine has not been made until now. The Life Cycle Analysis (LCA) is a useful tool to assess all these systems not only on a Tank-to-Wheel (TTW) basis but also on a broader Well-to-Wheel (WTW) basis. For the specific case of EREV assessment, it is also vital to perform this analysis using driving scenarios that can be representative of the typical EREV use – a combination of frequent short to medium urban driving cycles and occasional long trips. The authors’ research group has background not only in using the LCA tool in the scope of electric mobility studies [2], [11], [31], [32], but also have collected extensive data on real driving cycles under different scenarios [33], [34], [35].
The purpose of this paper is to compare the energy efficiency and CO2 emissions of four different range extender engine solutions deployed in the same baseline vehicle under a combination of driving scenarios that aims to be representative of typical EREV driving. The scenarios vary from very frequent short urban commuting, mostly under charge depleting regime (RE switched off, no fossil fuel use, only battery power), to occasional long business or holiday trips, where the EREV will work under charge sustaining regime most of the time (RE switched on, low battery SOC).
In order to perform this analysis, the road vehicle simulation software ADVISOR [36] was used in TTW stage. This software, developed by NREL, uses a combined backward-forward approach that enables the software to model advanced batteries and power train components while maintaining a relatively fast simulation speed. It has been demonstrated as a reliable tool for studying energy consumption and vehicle performance and for testing energy-related control schemes [30]. It has been widely validated and used in the research community [30], [37], [38], [39], [40], [41].
To make the assessment more representative of what actually happens under real driving, the driving scenarios were shaped and combined using real driving cycle data collected by the authors, incorporating additional information such as road gradients and higher Vehicle Specific Powers (VSP) [42]. These are required in order to simulate the high power demand situations that occur under some driving conditions and that cannot be replicated using “standard” speed-time driving cycles. There is a recent study regarding VSP effect on electrical consumption and gasoline consumption of a PHEV retrofitted Toyota Prius [42], aiming at developing a model to characterize and explain variation in battery current, fuel use and emissions.
The combination of these scenarios should illustrate well the great potential and flexibility of EREVs, namely the absence of fossil fuel usage during most of its lifetime while keeping the capability for occasional long trips. What will be the merit of each RE philosophy under these scenarios in terms of improved energy efficiency and reduced CO2 emissions is the aim of the present work.
Section snippets
Range extender philosophies to be assessed
A standard imaginary EREV with a range extender based on a “conventional” piston engine and with dimensions, weight, and electric range similar to that of the Opel Ampera (Chevrolet Volt [11], [19]) will be used as the baseline for the comparisons. The fuel tank will be kept unchanged, while the weight released by each RE system will be compensated by the total battery weight. Using this baseline model, the following range extenders will be compared:
- (i)
Conventional SI piston engine (Baseline,
Tank-to-Wheel
ADVISOR results for the fuel utilization stage were obtained for the four vehicle options and seven driving cycles. In CD only battery power is used. In CS all systems can be used; however in this mode the battery has only 30% of its energy capacity and available power. Although capable of using battery and engine power in CS, both combined powers may not be enough to match the power available in CD mode (with battery at full charge). Table 6 shows the vehicle overall efficiencies in both CD
Conclusion
A baseline plug-in hybrid gasoline vehicle was used to simulate different Range Extender (RE) solutions: over-expanded cycle engine with Eco-Boost operation modes, Wankel engine and a micro-turbine. These RE possibilities were studied in terms of their energy consumption and CO2 emissions under different real driving scenarios and in a WTW context. The vehicle curb weight was kept constant by introducing battery modules.
The vehicle with the lightest engine (Wankel) and biggest battery pack was
Acknowledgements
The author João Ribau is supported by a PhD grant financed by the FCT – Fundação para a Ciência e Tecnologia (SFRH/BD/60373/2009). The author Francisco Brito is supported by a Post-doctoral grant financed by FCT (SFRH/BPD/51048/2010).
The authors would like to acknowledge FCT and the MIT-Portugal Program through the projects Power demand estimation and power system impacts resulting of fleet penetration of electric/plug-in vehicles (MIT-Pt/SES-GI/0008/2008) and MOBI-MPP – Assessment and
Glossary
- BEV
- Battery-Only Electric Vehicle
- BSFC
- Brake Specific Fuel Consumption
- CAFE
- Corporate Average Fuel Economy (US)
- CD
- charge depleting
- CS
- charge sustaining
- DOT
- Department of Transportation (US)
- EPA
- Environmental Protection Agency (US)
- ER
- extended range
- EREV
- Extended Range Electric Vehicle
- EV
- electric vehicle
- GHG
- Greenhouse Gas
- HWEFT
- Highway Fuel Economy Cycle
- ICE
- internal combustion engine
- NEDC
- new European driving cycle
- OBD
- OBD (On-Board Diagnostic)
- RE
- range extender
- SI
- spark ignition engine
- SOC
- battery’s state-of-charge
- TTW
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