Direct Injection (DI) versus Indirect Injection (IDI) Engines

The DI and IDI engines have been in use for many years. The IDI engines were mostly employed in high speed small engine applications. Now, after the development of high speed direct injection (HSDI) engines the IDI engines due to their poor fuel economy are being phased out of production. The combustion in IDI engines takes place in two stages; a rich mixture burns in the pre-chamber where all the fuel is injected and the partially burned rich fuel- air mixture from the pre-chamber is transported to the main chamber where due to presence of excess air combustion is completed. The jet of high pressurepartially burned gases from the pre-chamber enters the main chamber generating high turbulence that causes rapid mixing and most of the fuel burns as lean mixture.

A comparison of NO_x emissions with DI and IDI engines is shown on Fig. 3.4. At light loads, most of NO may form in the pre-chamber. But, at higher loads additional NO formation would occur in the main chamber. Although temperatures are higher in the pre-chamber than the main chamber, but except at light loads mixture is overall rich and hence, the lower formation of NO. In the DI engines, at the end of premixed combustion higher peak pressures and temperatures are obtained compared to the IDI engines and NO is formed in near stoichiometric mixtures during mixing controlled phase and post combustion gases. Due to these factors overall, the indirect injection engines emit lower NOx. In the DI engines due to low turbulence levels some of CO formed in the rich spray regions may not find the required oxygen for complete combustion while thetemperatures are still high. It results in .higher CO emissions than the IDI engines even though more excess air is present in the DI engines.