Fuel Injection Timing and Injection Pressure

Effect of fuel injection timing on NO_x emissions from CI engines is quite similar to that of spark timing in SI engines. Typical effect of injection timing on NO_x , HC, smoke emissions and fuel consumption is shown on Fig 3.6. With retarded injection timing, as expected the NO_x emissions decrease sharply. On the other hand an increase in smoke results with retarded injection timing If the injection timing is retarded too much, HC emissions in naturally aspirated engines also may increase sharply. An increase in injection pressure results in higher NO_x and HC, but yields lower smoke and PM emissions. Effect of injection pressure on PM emissions is shown on Fig. 3.7.

With high injection pressures of 1500 bars and higher, small nozzle orifices of about 0.15 mm dia. are used. The high injection pressure and small nozzle holes give very good fuel atomization with very small fuel droplets which vaporize rapidly resulting in low soot emissions