Improvement of Combustion of CNG Engine using CNG Direct Injection and Gas-jet Ignition Method

Lean combustion is a useful mean to improve performance of a CNG engine. However, conventional premixed fuel injection method limits the lean combustible range. Lean combustion for CNG has problems such as misfire and THC emission. This study proposes the gas-jet ignition method to extend lean combustible limit of CNG engines. Results show that CNG engine can be operated at equivalence ratio ranges from 0.3 and 0.8 by applying two-stage injection combined with gas-jet ignition. First-injection timing is one of important parameter that affects THC emissions. Analyses of in-cylinder pressure histories and CFD calculation explain that combustion stability and THC emission are affected by fuel distribution. The distribution is controlled by the first and second injection timing. The second injection achieves good ignitability. The THC emission is caused by over-lean mixture in the cylinder in the case of early timing of first injection. Other reason of THC emission is that, in the case of late injection timing, first flame has the difficulty in catching the combustible mixture just after ignition. This type of mixture distribution causes misfires or cyclic variation, leading to THC emission. Further attempt was made to improve the flame development by using CNG-hydrogen blend fuel. Hydrogen addition improves flame development and exhaust gas emission when applied with the gas-jet ignition method.