Lean air-fuel mixtures supplemented with hydrogen for S.I. engines: A possible way to reduce specific fuel consumption?

Abstract

From a theoretical viewpoint, a lean or ultra-lean mixture improves thermal efficiency of internal combustion engines when operating at partial loads. This is confirmed is current practice with compression-ignited motors. However, spark-ignited engines actually reach the lowest specific fuel consumptions at air-fuel ratios not very far from the stoichiometric when the fuel is gasoline, since with leaner mixtures combustion becomes too slow and erratic. Stratified-charge engines allow leaner mixtures, but they require combustion chamber designs that penalize efficiency. The presence of hydrogen in the mixture increases combustion speed and reduces misfiring. As a result, the lowest specific fuel consumptions are obtained with leaner mixtures as a whole and are reduced in amounts, as the ratio of hydrogen to the total amount of fuel supplied is increased. In this work, a new system of hydrogen enrichment is presented: a small amount of hydrogen is stratified around the spark-plug in an open combustion chamber, in order to combine stratified-charge and hydrogen-enrichment benefits. An experimental prototype is described in detail and the first experimental results are reported.