A study of mechanical variable valve operation with gasoline–alcohol fuels in a spark ignition engine

This work involved study of the effects of gasoline–ethanol and gasoline–butanol blends on the combustion, fuel economy and engine-out emissions of a single cylinder research engine equipped with a mechanical variable valvetrain on the inlet and variable valve timing on the exhaust. Gasoline or iso-octane were splash blended with varying amounts of ethanol or 1-butanol and studied under a range of part-load engine conditions. During warm idle operation, high ethanol content fuels allowed significant improvement in tolerance to internally recycled burned gases, primarily associated with increased burning velocities of such blends when near to stoichiometric fuelling levels. In turn this allowed higher valve lifts to be used, with reduced throttling locally at the inlet valves, further small fuel savings and reductions in engine-out emissions of NOx. Conversely, the use of 1-butanol had a negligible effect on residual gas tolerance, regardless of blend volume. At moderate speeds and loads, where throttling losses were less, it was apparent that the valvetrain could still be used to attain additional thermal efficiency improvements including reduced compression losses and further expansion work for all fuels. However, a trade-off with increased pumping losses during the exhaust stroke was apparent, with the throttling moved from the inlet to the exhaust valves at the most retarded valve timings studied. For all fuel blends, it was extremely interesting to note that variable valve timing alone offered the greatest NOx reduction potential at moderate loads, insinuating the ability to operate variable valve timing with and without early intake valve closing may offer one viable path to meeting future engine emissions targets.