Alcohol-Based Fuels in High Performance Engines

The paper discusses the use of alcohol fuels in high performance pressure-charged engines such as are typical of the type being developed under the ‘downsizing’ banner. To illustrate this it reports modifications to a supercharged high-speed sports car engine to run on an ethanol-based fuel (ethanol containing 15% gasoline by volume, or ‘E85’). The ability for engines to be able to run on alcohol fuels may become very important in the future from both a global warming viewpoint and that of security of energy supply. Additionally, low-carbon-number alcohol fuels such as ethanol and methanol are attractive alternative fuels because, unlike gaseous fuels, they can be stored relatively easily and the amount of energy that can be contained in the vehicle fuel tank is relatively high (although still less than when using gasoline). These fuels also have a much higher octane rating than gasoline which makes them attractive as a fuel for pressure-charged engines, which are frequently knock-limited in their ignition advance curves.

The modifications made to the engine include the reconfiguration of the fuel system to investigate making best use of the high latent heat of vaporisation of alcohol by injection of a proportion of the fuel mass before the supercharger - the so-called ‘wet compressor’ technique. With this configuration, engine performance data on E85 with optimised engine management settings are presented and compared to the original gasolinefuelled performance. Discussion is made of the nature of the evaporative effect versus oxygen displacement, its effect on supercharger drive power and of the improvement of the spark advance curve as a result of the increase in octane rating of the fuel. To illustrate this response, curves when varying the percentage of fuel delivered upstream of the supercharger are presented. Some discussion of issues to be addressed on the fitment of such an engine to a vehicle is also made, together with how the wet compressor technique might be adopted with minimal impact on evaporative emissions. Conclusions as to the attractiveness of this approach in maximising ethanol-fuelled engine performance are drawn based on the results presented and the vehicle issues listed. There is also some discussion as to how to best configure an ‘omnivorous’ engine to make best use of a range of liquid fuels as they are introduced to the marketplace, with the best efficiency on each.