Development of fuel metering techniques for spark ignition engines

This chapter starts by providing an overview of fuel-metering techniques developed for spark ignition (SI) engines, following by a case study on liquid versus gaseous phase liquified petroleum gas (LPG) port injection in a single-cylinder SI engine. Fuel-metering approaches, from the least to the most modern, include carburettor, throttle body injection (TBI), manifold port injection (MPI or sometime called PFI, port fuel injection), and gasoline direct injection (GDI). Based on the mixture formation process, GDI engines can be categorised into air-guided, wall-guided, and spray-guided methods while based on the ignition modes they can be homogeneous spark ignition - (HCI), stratified spark ignition (SPI) and homogeneous charge compression ignition (HCCI) engines. The comparative study shows significant variations in cylinder to cylinder mixture distribution for both liquid and gaseous LPG phases. The experimental data confirms the power and efficiency advantages of liquid phase injection over gaseous phase injection. Analysis shows the charge state at the end of compression is the major contributor to the performance difference. Secondary differences such as the mixture homogeneity also have an important impact. The lower temperature combustion with liquid phase LPG also delivers substantial NOx emission reduction. Suggestions are made for overcoming the deficiency of gaseous phase injection which may be preferred as its application avoids the high cost of the high pressure in-tank LPG pump needed for liquid phase injection.