Thermal Management of a Low Cost Range Extender for Electric Vehicles

Abstract

Range extenders are a solution to partly overcome the limitations of current battery technology and are gaining popularity despite their complexity, due to the potential for reduced tailpipe emissions and fuel consumption. The range extender or Auxiliary Power Unit (APU) consists of an on board fuel convertor 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. One of the key requirements of an APU is to provide maximum electrical power. In order to do so it is important that the engine as well as the generator are operated at their maximum efficiency in addition to optimising the complete system to reduce any parasitic losses in auxiliary systems. The conflicting requirements of running the engine at a high temperature (∼90°C) and the generator to run as cold as possible (∼50°C) has consequences on vehicle system integration such as the need to use separate coolant loops, radiators, pumps etc. Employing a common cooling loop can reduce parasitic loads and simplify vehicle integration, but requires operating the engine and/or the generator under sub-optimum thermal conditions. The paper discusses the development of a thermal management system using a single coolant loop for the APU. The APU was tested on a bespoke rig first using two independent cooling loops to characterise the APU performance. The benefits of employing a single coolant loop are then weighed against the compromise in performance observed as a result of operating the engine and/or generator under suboptimum thermal conditions in a single loop. In the single coolant loop, there is a drop in APU power of around 4% and an ESFC penalty of circa 2% at full load across the operating regime. However there is a sizeable saving in parasitic losses, simplified package installation with reduction in overall package cost.