contd...

maximum injection pressures were limited to about 700 bars. Since then, the injection pressures have increased to a value exceeding 2000 bars. Modern electronic fuel injection systems employing unit injectors, common rail systems etc. provide use of very high injection pressures.

The beneficial effects of high injection pressure are;

• Improved fuel atomization producing finer fuel droplets.
• The smaller fuel droplets evaporate at a faster rate resulting in rapid fuel-air mixing.
• A shorter injection duration
• With shorter injection duration injection timing may be retarded. Fuel may now be injected closer to TDC in hotter air giving shorter ignition delay, resulting in  emission benefits
• Higher spray penetration and better air utilization.

The effect of peak injection pressure on PM-NO_x trade off is shown on Fig. 6.3. The width of band on this figure relates to the contribution of lubricating oil to the particulate emissions. With increase in peak injection pressure, the PM-NO_x trade-off curve moves closer to origin indicating reduction both in the PM and NO_x , although the reductions in PM at a given NO_x level are more obvious as seen in Fig 6.3 .

Injection Rate Shaping and Multiple Injection

NO_x formation is influenced by

       (i)        duration of ignition delay,
       (ii)       amount of fuel injected during delay period, and
       (iii)      the rate of mixture preparation within the combustion chamber.

The shape of ideal rate of injection curve during ignition delay and the main injection period depends on engine load and speed.  General principles of fuel injection scheduling are;

• rate of fuel injection within the delay period must be kept small to reduce the amount of fuel burned during ‘pre-mixed’ combustion phase, and
• during the main injection period, rate of injection should be increased steeply to inject fuel within a short period when the temperature and pressure in the combustion chamber are high for rapid combustion of the injected fuel.