Extending the knock limit of a turbocharged gasoline engine via turboexpansion

Turboexpansion is a concept which is aimed at reducing the fuel consumption of pressure-charged gasoline engines by providing over-cooled air to the engine in order to extended the knock limit (relative to a conventional charge-cooled system). An extension to the knock limit allows several possible shifts in the compromise adopted in pressure-charged gasoline engines, including increased specific output, an increase in trapped compression ratio or a reduction in the octane appetite of the engine. All are considered important in the drive to reduce the CO2 output of passenger cars fitted with gasoline engines against the backdrop of the ACEA commitment of 140g/km of CO2 per km by 2008. The present work reports on initial results from running a single-cylinder engine under conditions of varying temperature at fixed charge-air densities to assess the worth, in combustion terms, of pursuing reduced charge-air temperature to extend the knock limit. To enable this, a charge-air conditioning rig has been constructed based on three positive-displacement superchargers and heat exchangers. This device is capable of providing close control of the charge air required for this testing programme. It has been shown that reducing the charge air inlet temperature, at a fixed density, provides an extension to the knock limit of up to 3.5° of spark advance at high compression ratio for a temperature reduction from 40 to 20°C at 2000rpm. © Lotus Engineering.