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Based on modified natural gas direct injection engine, we studied the impacts of ignition timing and jet timing on natural gas engine start process in this paper. The results shows that: when the first jet ignition occurs in the first compression stroke, the engine reaches idle speed 400rpm fastest; as the jet timing is delayed, emissions during engine start is gradually reduced, but when the jet late, HC surge occurs, the emissions deteriorates; with the ignition advance angle increasing, the engine speed growth accelerates, the peak moves forward; with the ignition advance angle increasing, HC emissions peak increases, the peak moves forward.

In order to convert the high pressure common rail system engine to CNG/Diesel dual fuel engine, an electronically controlled system is developed. The system includes a CNG fuel supply system, the CNG electronic control unit (ECU) and its matching harness etc. During starting and idle load conditions, the engine runs under pure diesel mode, when the speed and load reached a certain set point, the diesel ECU reduces the pilot diesel quantity, meanwhile, the CNG ECU increases the natural gas quantity, then the engine runs under dual fuel mode. The engine experiment data show that in different conditions, the highest substitution rate is 90% and the average substitution rate is 83%; the average savings ratio of operation cost per hour is 26%.

The distribution of temperature and NOX concentration field under different crank angle of a CNG direct-injection engine converted from a four-valve gasoline vehicle engine are calculated by the computation model of the CNG direct-injection engine work process using CFD software. The results show that the temperature field spreads around the center of the spark plug in initial combustion period. The position of maximum temperature in combustion chamber moves from the spark plug to the wall and temperature tends to uniform distribution as the piston goes down along with the spread of flame front and the completion reaction after the flame. Temperature distribution is vital to the generation of NOX which changes along with high-temperature zone. The change principle of quantity of NOX in the cylinder is that at first, the quantity continues increasing, then declines, and finally remains stable. Besides this, the generation amount of NOX increase along with the speed.