Effect of Standard Tuning Parameters on Mixture Homogeneity and Combustion Characteristics in a Hydrogen Direct Injection Engine

Dihydrogen, as a zero CO₂ fuel, is a strong candidate for internal combustion engine to limit global warming. This study shows the impact of standard tuning parameters on mixture homogeneity and combustion characteristics. A 2.2L Diesel engine on which the head was reworked to allow side mounted direct injector and central mounted spark plug was selected. The discussed tests were made at low engine speed and partial load. A spark advance sweep at different air-fuel ratios (λ) was conducted. The exponential relation between λ and NO_x emissions is highly marked and extremely low NO_x emissions up to 1.7 g/kWh at minimum spark advance for maximum brake torque can be measured. A λ sweep was performed at different starts of injection (SOI). The results show that, depending on the engine speed, a later SOI might lead to lower NO_x emissions. For a λ setpoint of 1.8, at 1500 rpm, late SOI leads to 30% higher NO_x emissions where at 2500 rpm these emissions are 26% lower. This assessment is explained by the cycle-to-cycle variations. Finally, the second λ sweep, where the rail pressure (P_rail) was varied, reveals that maximizing the P_rail does not necessarily lower down NO_x emissions. At 2500 rpm of engine speed, for richer mixtures than λ = 1.8, the higher P_rail leads to higher NO_x emissions. Having a lower P_rail might lead to a slightly leaner mixture near the spark plug at spark timing. This effect extends the ignition delay thus lower the NO_x emissions.