A collaborative study to evaluate the maximum achievable thermal efficiency of a spark ignited engine has been conducted using a range of gasoline and advanced biogasoline fuels, including isomers of butanol. Engines used in this study included an Atkinson cycle 1.5l and prototype 1.8l lean boosted engine. This study was the first stage of an objective to establish practical and implementable routes to minimising well-to-wheels CO2 emissions from liquid hydrocarbon fuels in these engines.
A fuel matrix was designed to assess the effects of variations in volatility, octane and bio-component level and type.
Performance and emissions tests were conducted over a range of engine operating conditions. Thermal efficiency was mapped at stoichiometric and lean conditions, and the limit of lean combustion was established for the different fuels. A maximum steady-state thermal efficiency of 42.9% was achieved on the lean boosted engine.
Both the Atkinson cycle engine and the lean boosted engine appeared relatively insensitive to the various properties of the test gasoline formulations used other than octane.
The butanol blends offered measurable gains in thermal efficiency in line with the relative octane lift over the base gasoline. These fuels, together with a high volatility fuel, were amongst the lowest NOx emitters. Importantly, the butanol blends allowed the greatest volume of crude derived gasoline displacement at fixed fuel oxygen content.
The results obtained enabled a simulation of the tank-to-wheels CO2 contribution of the various fuels in a hybrid version of the lean boosted prototype engine.