Natural Gas SI Engine Research Articles

Pollutant formation modelling in natural gas SI engines using a level set based flamelet model

The level set flamelet model for turbulent premixed combustion was recently extended to improve the modelling of the spark ignition process, to incorporate the influence of unsteady flame kernel development on turbulent flame propagation, to embrace flame—wall interaction, and to comprise pollutant prediction models. The formation of nitrogen oxides (NO x) is predicted using the extended Zeldovich mechanism. The concentrations of the involved intermediate species are determined using a chemical equilibrium assumption. The formation of engine-out unburnt hydrocarbons (UHCs) and carbon monoxides (COs) is directly linked to incomplete combustion due to flame quenching at the time the flame hits the wall. In this paper, the new model is used for the analysis of combustion in a homogeneous charged natural gas SI engine operating on lean conditions. The investigated engine operating (PAM) points differ in the applied equivalence ratio, ignition timing, and rate of exhaust gas recirculation. The influence of the load on the peak pressures, peak pressure locations, and pollutant formations is discussed in detail. A good agreement with experimental data was obtained among all engine operating points.

Optimization of a natural gas SI engine employing EGR strategy using a two-zone combustion model

Natural gas has been recently used as an alternative to conventional fuels in order to satisfy some environmental and economical concerns. In this study, a natural gas spark-ignition engine employing cooled exhaust gas recirculation (EGR) strategy in a high pressure inlet condition was optimized. Both engine compression ratio and start of combustion timing were optimized in order to obtain the lowest fuel consumption accompanied with high power and low emissions. That was achieved numerically by developing a computer simulation of the four-stroke spark-ignition natural gas engine. A two-zone combustion model was developed to simulate the in-cylinder conditions during combustion. A kinetic model based on the extended Zeldovich mechanism was also developed in order to predict NO emission. In addition, a knocking model was incorporated with the two-zone combustion model in order to predict any auto-ignition that might occur. It was found that the value of the compression ratio at which the minimum fuel consumption occurs varies with the engine speed. A minimum fuel consumption of about 200g/kWh was achieved at an engine speed of 1500rpm, inlet conditions of 200kPa and 333K, and a compression ratio of about 12. Also, it was found that cooled EGR can significantly reduce NO emission at high compression ratio conditions. NO emission decreased by about 28% when EGR was increased from 20% at compression ratio of 10 to 27% at compression of 12 at the same engine speed of 3000rpm.

Quantitative imaging of equivalence ratios in a natural gas SI engine flow bench using acetone fluorescence

Although compressed natural gas (CNG) is a gaseous fuel, the mixing process is quite different from air-liquid fuel mixing. The aim of this work is to understand the effect of the fuel feeding system on mixture homogeneity. Planar laser-induced fluorescence has been used to produce quantitative equivalence ratio maps in the intake manifold. Fluorescence results from excitation of doped acetone in natural gas. Its emission is proportional to the fuel mass. Collected images were post processed to obtain the equivalence ratio. This work shows the difference between continuous injection at low speed and sequential injection. In the first part, we present the behaviour of the injection jet in the intake manifold. The second part displays a smaller section of the duct upstream of the intake valve. The study shows clearly the stratification effect obtained with continuous injection at low speed. A very homogenous mixture is observed for sequential injection with fuel trapped for a cycle and aspirated in the next cycle.