An Investigation on the Use of EGR on Natural Gas SI Engines
Natural gas, which consists mainly of methane (CH4), has the highest hydrogen to carbon ratio compared to other hydrocarbon fuels such as petrol and diesel. This makes the natural gas one of the cleanest available fuel, which could be used as a fuel for cars and buses to emit significantly lower emissions.
The use of lean burn (i.e. the amount of the air used to burn the natural gas is more than theoretically required for complete combustion) has been dominant in engine industry for the last few decades. This technique gave acceptable fuel consumption with a reasonable cost. Unfortunately, this technique produces high emissions of nitrogen oxides (NOx) and hydrocarbon (HC).
New techniques must be developed in order for the engines to meet the stricter governmental emission standards such as 2010 Federal emission standards. One of these techniques is the use of Exhaust Gas Recirculation (EGR). In this technique, a part of the exhaust gases is returned back to the engine intake. This will lead to a decrease in in-cylinder temperature, and hence, NOx emissions. In addition, using EGR permits the use of a three-way-catalyst (TWC) for further emission reduction.
In this research, the use of a stoichiometric air-fuel mixture with EGR was investigated in a natural gas spark ignition engine using both experimental and computer simulation methods. The effect of varying the percentage of EGR dilution in the inlet mixture on engine performance and NO emissions was experimentally investigated and analyzed at an engine speed of 1500 rpm and at different inlet pressures and compression ratios. The percentage of EGR dilution was changed from zero to the misfire limit in most of the studies. The measured in-cylinder pressure was analyzed in order to identify abnormal combustion occurrence such as surface ignition and engine knock in addition to engine misfire. Also, the measured in-cylinder pressure was used to analyze the combustion process and determine the heat release rate, and the total combustion duration at different operating conditions. Furthermore, the in-cylinder pressure was used to assess the engine stability by calculating the coefficient of variation in indicated mean effective pressure at different operating conditions. The effect of change of spark timing on engine performance and NO emissions was also investigated at an EGR dilution of 8%.