Optimisation of a Low-Cost Production Automotive Engine for Range Extender Application in an Electric Vehicle

  • Ashwini Agarwal

Student thesis: Doctoral ThesisPhD


Electrification of vehicles is the direction the automotive industry has embarked on to address the well documented problems of air quality and climate change. Advantages of electric vehicles include zero tail pipe emissions, low energy cost and city-friendly driveability. However, a fundamental drawback is their limited range and high recharge time required. The range extended electric vehicle (REEV) is a solution that enables the driver to be range anxiety free, seeks to achieve a practical compromise between the onboard battery size and the single charge driving range. The range extender or auxiliary power unit (APU) consists of an onboard fuel convertor, usually an internal combustion engine (ICE) that converts fuel such as gasoline into electrical power while the vehicle is in operation. This enables the traction battery storage capacity to be reduced whilst still maintaining an acceptable driving range. The ICE can be optimised for a limited number of steady state points which offer significant improvement in fuel economy as well as emissions. Thermal management, a critical part in a vehicle is governed by two constraints – packaging and cooling performance. The packaging space in the vehicle limits the cooling system size. It becomes even more challenging in a hybrid system since extra components are in operation compared to a conventional car. Further, to mimimise fuel consumption of the APU, it is important that the engine and generator are operated at their maximum efficiency in addition to optimising the complete system to reduce any parasitic losses in the auxiliary systems. However, they have conflicting temperature requirements to achieve their own optimal efficiency. Cost is one of the most significant factors for such a powertrain as the range extender is an additional system to a vehicle that already includes an expensive fully capable electric system. Therefore, the APU must maximise its cost advantage over the proportion of the battery pack that it is effectively replacing. With the above as the background, this research work elaborates on the considerable experimental work undertaken to optimise a very low-cost 624cc production automotive engine of c. 25kW for running as an APU in critical speed/load ranges. The engine optimisation process only included changes that were possible in the normal volume-production process. This included introduction of a new engine management system and electronic throttle, bespoke engine inlet and exhaust manifold development, engine calibration for improved fuel economy and well as introduction of an electric water pump. The optimisation achieved a best engine BSFC of 245g/kWh at 2500rpm with 92% reduction in engine inlet manifold volume. On commissioning of the APU, the total dry weight of the APU was measured to be 81.5kg as against an initial target of 80kg. Experimental analysis showed the ESFC was below the target best ESFC of <270g/kWh under optimal temperature conditions of the engine, M/G and inverter unit in separate coolant circuits for the engine and generator. The APU produced a peak power of 22.78kW at 5100rpm. Best ESFC of 260g/kWh was measured at 2500rpm, and the ESFC remained below 270g/kWh across 2000 to 3500rpm at full load. The specific performance of the APU at peak power was 270W/kg which was within the target of 250 to 313W/kg. The experimental analysis also demonstrated that it is possible to operate the APU in a combined engine-generator coolant loop, with marginal drop of 4% in power and 2% increase in ESFC. However, the combined loop provides greater flexibility of package installation and simplifies vehicle integration, with reduction in parasitic losses. It also reduces the overall package cost. This research contributes in the area of optimising an industry first low-cost production, low-cylinder count engine for range extender application as well as thermal management of an APU by providing an insight on the performance of the engine and generator in a single coolant loop with changing coolant and oil temperatures. This is considered to be the novelty of this research since literature review revealed that though researchers had written about potential cost savings on combining of coolant circuits, there was no evidence that it had actually been implemented.
Date of Award16 Sept 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorChris Brace (Supervisor), Sam Akehurst (Supervisor) & Andrew Lewis (Supervisor)


  • Range extender
  • Electric vehicle
  • Internal combustion engines

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