Numerical Simulations Of Knock Combustion In Pilot Ignited Directly Injected Natural Gas Engine

In the high-pressure direct-injection (HPDI) of natural gas in a compression ignitionengine, the natural gas supplies over90percent of energy with performance and efficiencycomparable to that of a conventional diesel engine. It has the potential to reduce demand forpetroleum derived fuels and significantly reduce the level of pollutants and greenhouse gasesemitted form heavy duty transport vehicles. In the HPDI engine, natural gas is injecteddirectly into the engine cylinder at the end of the compression process. A few crank angledegrees earlier, a small amount of pilot diesel is injected into the engine cylinder. The dieselpilot autoignites and then ignites the natural gas. So most of the combustion is non-premixedas in a conventional diesel engine. But significant knock would occur under some conditions.Knock caused by rapid energy release would produce large combustion noise and lead todamage to the engine.Numerical simulation by CONVERGE revealed knock process in the engine. Theresults were processed by Fieldview software. A knock index KI is defined as the average ofthe local PPmaxvalue at8different locations. The results show the knock characteristics inthis engine and the effect of natural gas injection timing, pilot diesel amount, the initialtemperature on knock. Conclusions are as follows:1. The fluctuations (an indication of knock) are due to a rapid premixed burning phaseof combustion rather than spontaneous ignition of end gas as occurs in spark-ignited engines.The variation of pressure fluctuation magnitude at different locations is different.2. The pilot injection timing and its amount keep unchangeable. Vary the natural gasinjection timing. The relative injection timings are3°CA,5°CA and8°CA. The knockintensity indexes are0.77MPa,0.16MPa and0.36MPa. So the knock intensity doesn’t increase as the relative injection timing shortenes.3. The effect on knock of diesel mass was studied. The natural gas injection timing, itsamount and pilot mass keep unchangeable. The diesel pilot energy is2.9%,5.6%and8.2%of the total energy respectively. The knock intensity indexes are0.30MPa,0.53MPa and0.36MPa respectively. Knock intensity decreases as the diesel amount reduces.4. Change the initial temperature in the cylinder while keep the fuel injectionparameters constant. The knock intensity indexes are0.30MPa,0.77MPa and0.86MPa withrespect to330K,340K and350K. Higher initial temperature would lead to higher knockintensity.5. Different from the phenomenon in diesel engine that longer ignition delay leads tohigher pressure rise rate during premixed combustion, knock intensity isn’t correlated withthe ignition delay.