Surrogate fuels for premixed combustion in compression ignition engines

Abstract

Simple surrogate fuels are needed to model practical fuels, which are complex mixtures of hydrocarbons. The surrogate fuel should match the combustion and emissions behaviour of the target fuel as much as possible. This paper presents experimental results using a wide range of fuels in both the gasoline and diesel auto-ignition range, but of different volatilities and compositions, in a single cylinder diesel engine. Premixed combustion in a compression ignition engine is defined, in this paper, to occur when the injection event is clearly separated from the combustion and the engine-out smoke is very low – below 0.05 FSN (filter smoke number). Under such circumstances, if the combustion phasing is matched for two fuels at a given operating condition and injection timing, the emissions are also comparable regardless of the differences in composition and volatility. For the experimental conditions considered, combustion phasing at a given operating condition and injection timing depends only on the octane index (OI), OI =  (1-K)RON +  KMON, where RON and MON are research and motor octane numbers and K is an empirical constant that depends on operating conditions. A mixture of iso-octane, n-heptane and toluene can be found to match the RON and MON of any practical gasoline and will be a very good surrogate for the gasoline since it will have the same OI. If the compression ratio is greater than 14, practical diesel fuels, with DCN (derived cetane number) between 40 and 60, will have comparable ignition delays ton-heptane, which is an adequate surrogate for such fuels. However, premixed combustion can be attained only at much lower loads at a given speed with diesel fuels compared to gasolines.