Extending The Knock Limit Of A Turbocharged Gasoline Engine Via Turboe

ABSTRACT

Turboexpansion is a concept which is aimed at reducing the fuel

consumption of pressure-charged gasoline engines by providing overcooled

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 pressurecharged

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 singlecylinder

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 chargeair

conditioning rig has been constructed based on three positivedisplacement

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.