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contd....
The pilot injection is a form of multiple-injection with two injection pulses. In the pilot injection, a small quantity of fuel (about 10% of fuel per cycle) is injected 3 to 10 crank angle degrees before the main injection event. The pilot-injected fuel has more time to undergo precombustion reactions .When the main fuel injection is made, the combustion begins soon after, resulting in a short delay period. As the pilot fuel quantity injected during delay period is rather small, the peak rate of combustion pressure rise and peak pressures are reduced. Typical pilot injection strategy and cylinder pressure are shown on Fig. 6.4.
| Figure 6.4 |
Pilot injection, injector needle lift and combustion pressure traces in a naturally aspirated diesel engine at low loads. |
With pilot injection, the injection timing of main fuel can be retarded to give low NOx emissions without adversely affecting the engine power and fuel efficiency. Also, with pilot injection less fuel burns as pre-mixed and a larger fraction of fuel burns in diffusion combustion mode. In the normal engine, NOx reduction is obtained by retarding the injection timing which results in higher smoke and PM emissions, and higher BSFC. Results with pilot injection and conventional engine with retarded injection are compared in Table 6.2. At the same NOx emission levels of 5.5 g/kWh, with pilot injection typically 23 % lower PM, 50% lower HC and 4 % lower BSFC were obtained.
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