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Rising fuel prices and changes to CO2 and fuel economy legislation have prompted an interest in the electrification of vehicles since this can significantly improve vehicle tailpipe CO2 emissions over homologation test cycles. To this end plug-in hybrid electric vehicles (PHEVs) and range extended electric vehicles (REEVs) have been introduced to the market. The operation of the engines in these vehicles differs from conventional vehicles in several key ways. This study was conducted to better understand how the engine design and control strategy of these vehicles affects the temperature and operating regimes experienced by engine crankcase lubricants. A Toyota Prius Plug-in PHEV and GM Volt REEV were tested on a chassis dynamometer over several legislated and pseudo ‘real world’ drive cycles to determine the operating strategy and behaviour of the powertrain. The lubricant and coolant temperatures were monitored, together with other key control parameters.

Engine downsizing is a key approach employed by many vehicle manufacturers to help meet fleet average CO2 emissions targets. With gasoline engines in particular reducing engine swept volume while increasing specific output via technologies such as turbocharging, direct injection (DI) and variable valve timing can significantly reduce frictional and pumping losses in engine operating areas commonly encountered in legislative drive cycles. These engines have increased susceptibility to abnormal combustion phenomena such as knock due to the high brake mean effective pressures which they generate. This ultimately limits fuel efficiency benefits by demanding use of a lower geometric compression ratio and sub-optimal late combustion phasing at the higher specific loads experienced by these engines.

Most major regional automotive markets have stringent legislative targets for vehicle greenhouse gas emissions or fuel economy enforced by fiscal penalties. Large improvements in vehicle efficiency on mandated test cycles have already taken place in some markets through the widespread adoption of technologies such as downsizing or dieselization. There is now increased focus on approaches which give smaller but significant incremental efficiency benefits such as reducing parasitic losses due to engine friction. Fuel economy improvements which achieve this through the development of advanced engine lubricants are very attractive to vehicle manufacturers due to their favorable cost-benefit ratio. For an engine with components which operate predominantly in the hydrodynamic lubrication regime, the most significant lubricant parameter which can be changed to improve the tribological performance of the system is the lubricant viscosity.