contd...
The engine compression ratio is reduced to13.5 - 14:1 so that ignition delay is long to allow formation of a homogeneous mixture. In split/multiple injection approach, the first injection is made at least 50 to 60 º btdc. Cooled EGR is used to retard start of combustion of the premixed charge so that a higher engine power is obtained before knock combustion results. The low temperature autoignition is obtained near tdc just prior to main injection. The main fuel injection is made slightly after tdc. This strategy shows improved combustion efficiency without excessive CO and HC emissions.
| Figure 7.11 |
Schematic of multiple injection strategy for HCCI operation of a diesel engine |
Late In-Cylinder Injection:
If the injection is made very close to or after tdc the fuel encounters a gradual decrease in gas temperature and pressure. It results in a long ignition delay and allows more time for mixture formation and favourable conditions for HCCI combustion are obtained. As most of the fuel now burns as premixed, a drastic reduction in soot formation is observed and high rates of EGR can be used to obtain very low NOx.
The late injection strategy has the advantage in that the start of combustion is better related to the injection timing and hence the combustion process can be controlled by injection timing. Also, very little changes are necessary in the injection equipment.
Nissan Motor Corporation developed an HCCI diesel engine using late injection strategy with low compression ratio. They called it MK (modulated Kinetics) combustion system. Late fuel injection and low compression ratio were combined with 30 – 45 % EGR to prolong ignition delay beyond end of injection. Injection timing was around 3-6 degrees btdc. Injection duration was reduced by use of high injection pressures and larger nozzle orifices such that the injection duration was 4-10º CA shorter than the delay period. The fraction of fuel that burns premixed increases substantially giving very low soot formation. Use of EGR and low compression ratio gave low NOx emissions. NOx obtained were less than 1/10 th and PM emissions less than half at 6 bar bmep and 2000 rpm compared to conventional diesel engine.
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