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In order to further study the effects of air and EGR dilution on the fuel economy improvement of natural gas engines under the premise of meeting EU6 legislation, a comparison between stoichiometric operation with EGR and lean burn operation with and without EGR has been conducted at 1600rpm 50% and 75% load. The conversion efficiencies of the catalysts for both NOx and CH4 emissions are assumed at 90% for lean burn operation. Experiment results indicate that under the condition of meeting both NOx and CH4 predetermined engine-out emissions limits for EU6 legislation, lean operation with a small fraction of EGR dilution enables more advanced combustion phasing compared to pure lean operation, which results in much better fuel economy, thus further improvement compared to stoichiometric operation is achieved.

Liquefied natural gas (LNG) engine could provide both reduced operating cost and reduction of greenhouse gas (GHG) emissions. Stoichiometric operation with EGR and the three-way catalyst has become a potential approach for commercial LNG engines to meet the Euro VI emissions legislation. In the current study, numerical investigations on the knocking tendency of several combustion chambers with different geometries and corresponding performances were conducted using CONVERGE CFD code with G-equation flame propagation model coupled with a reduced natural gas chemical kinetic mechanism. The results showed that the CFD modeling approach could predict the knock phenomenon in LNG engines reasonably well under different thermodynamic and flow field conditions.

A yc6112ZLQ turbocharged 6 cylinder engine with intercooler was converted to operate in dual fuel mode with compressed natural gas (CNG) and pilot diesel. The influence of the CNG ratio, pilot diesel injection advance (ADC) and intake temperature after intercooler on the combustion process, emissions and engine performance was investigated. The results show that the combustion process of dual-fuel engines is faster than diesel engine. Both the ignition timing of the pilot fuel and the excess air ratio of total fuel λ dominate the combustion characteristics of duel-fuel engines. With the increase of CNG ratio, the pressure and temperature in cylinder decrease at rated mode, but increase at torque and low speed modes. With advanced the pilot injection timing or increased the intake temperature, the cylinder pressure and temperature increase.