Research on control of ignition and THC formation in CNG engines by the application of gas-jet direct-ignition technique

Increasing world energy demand and recent climate change due to global warming have led us to search for sustainable energy sources with the lowest possible greenhouse-gas emissions. Our main energy, which sources from fossil fuel, is not sustainable and its combustion produces high content of harmful emissions. Hydrogen is always thought to be an ideal fuel because it is clean, renewable, and has abundant energy sources. Apparently, before hydrogen can be realized as the main energy source, a few major problems, such as the realtime production, safe and convenient storage, efficient combustion of hydrogen gas, and high production cost, need to be addressed. While waiting for the hydrogen technology to mature, the use of other types of gaseous fuel to combat the greenhouse-gas emissions deem necessary. Natural gas is not a renewable fuel but it has abundant resources and the lowest average specific CO2 emission among the non-renewable fossil fuel energy resources. Gas engines are typically utilized for electric power generation but are becoming popular in transportation sector. Recent statistical data obtained from NGV Global shows the worldwide growth of NGVs is increasing exponentially. A CNG engine is usually operated in the lean mode where equivalence ratio is between 0.7 and 1.0 by employing premixed-type ignition technique. The CNG lean-burn approach has the advantage of high thermal efficiency, low NOX emission, and lower fuel consumption compared to stoichiometric combustion. However, CNG lean combustion has problems such as poor ignitability and poor flame propagation which cause high cyclic variation, misfires and high THC emission. Using the lean-burn approach, the ignitability of the first flame core relies on local fuel-air mixture concentration near the ignition position. Too rich or too lean local mixture will cause the first flame core to quench before it begins to propagate to other parts of the combustion chamber. It causes poor combustion quality, misfires and higher cycle-to-cycle variations. In this study, a gas-jet direct-ignition method was applied to improve lean CNG engines operation. The gas-jet ignition method employs late injection timing technique which is very near to the ignition timing. The injected fuel reaches the ignition point with low jet velocity to ensure ignitability, while at the same time creating enough combustible mixture to support v flame core development. Through experiments, it was found that the gas-jet ignition method was able to operate in ultra lean mode at equivalence ratio less than 0.3. To enable engine operation at equivalence ratio between 0.3 and 0.8, gas-jet ignition with two-stage injection method has to be implemented. The second gas injection followed by ignition similar to the gas-jet ignition, ensures ignitability. The first injection is delivered early similar to the premixed-type ignition method to create non-heterogeneous mixture to sustain flame development from the kernel initiated by the gas-jet ignition. In the application of gas-jet ignition with two-stage injection using a real engine, the ignitability, combustibility and THC formation were investigated by varying the first injection fuel delivery timing. Moreover, the effect of hydrogen addition to CNG fuel on the ignitability, combustibility and THC formation were also investigated. The fuel-air mixture distribution inside the combustion chamber prior to ignition timing was calculated using CFD software to provide hints on the combustion cyclic variability and THC formation found during the engine tests. Furthermore, flame of the gas-jet ignition with two-stage injection combustion was observed using a constant volume chamber. It was found that the gas-jet ignition and gas-jet ignition with two-stage injection methods are effective to extend lean combustible ranges of CNG engines. The first injection timing in the two-stage injection method is a key factor to the better engine performance because it affects flame development after ignition. The combustion cyclic variation and THC emission are sensitive to mixture distribution in clearance space of the combustion chamber around spark position just after ignition. Such mixture distribution is highly dependent on the first fuel delivery. At lean equivalence ratio, early injection produces well-dispersed bulk mixture leaner than CNG lean combustible limit, thus slows flame development. Late injection tends to produce rich mixture in the clearance space preventing initial flame development. Both cause relatively high THC emission. When hydrogen was added to CNG, it was found to effectively improve flame propagation, resulting in low THC emission free from cyclic variation. Hydrogen flame also supports CNG combustion with lean mixture.