Mixture preparation in petrol engines

The aim of this work is to study the aspects of mixture preparation in petrol engines which are of interest to the mechanical engineers and researchers. The work reviewed the characteristics and behaviour of fuel and air flow in the intake manifolds of petrol engines and emphasised on investigation of fuel distribution between cylinders in a carburation engine and a multi-point injection engine. Characteristics of the flows of liquid fuel film, fuel droplets, fuel evaporation and process of mixing of fuel and air were analysed. The presence and behaviour of a large amount of liquid fuel film and fuel droplets in the intake manifolds was confirmed to be a main cause of inhomogeneity of mixture supplied to the cylinders. Atomising fuel spray and heating inlet manifolds are good methods for improving fuel evaporation and mixing of fuel and air in the intake manifolds to achieve homogeneous mixture for good operations of the engines. Fuel distribution between cylinders of a carburation engine and a multi-point injection engine was investigated, using the method of exhaust gas analysis. The results showed that in the multi-point injection engine fuel distribution between the cylinders is much better than in the carburation engine although the injection engine is equipped with a pneumatically controlled fuel injection pump (delta A/F ratio of 0.35 to 0.93 in the injection engine in contrast of delta A/F ratio of 0.76 to 1.7 in the carburation engine at normal running conditions). Injection engines equipped with an electronically controlled fuel injection system would have much better improvement in fuel distribution than the injection engine used for the present study. Heating the intake manifold in the carburation engine (by increasing coolant temperature) indicated a good improvement in fuel distribution between the cylinders while heating inlet air was not an effective way. In the injection engine, fuel distribution between the cylinders was seen to be not significantly affected by heating.