Experimental Investigation And Numerical Simulation On Natural Gas Combustion And Emissions In A Passenger Car Engine Equipped With An Electronic Fuel Injection System

Natural Gas (NG) has been widely used as clean fuel for passenger cars etc ground vehicle due to its advantages for providing higher efficiency and lower pollutant emissions. In this study, an experimental investigation on NG combustion and the influence of ignition timing and excessive air ratio on mixing, combustion performance and emissions was carried out on a four-cylinder SI type car engine. Meanwhile, the lean-burn combustion and the impact of air/fuel ratio ignition timing were explored and the lean-burn limit was measured.Relevant experiments covered an engine speed range from 1200 rpm to 3200 rpm with different load levels and engine performances including power, fuel economy and emissions were examined with necessary ignition timings. The results showed that for each specified operating condition there exists an optimal ignition time for the optimum BMEP (Brake Mean Effective Pressure) BSFC (Brake Specific Fuel Consumption). The ideal ignition timing for measured operating points depends on engine speed, torque and air/fuel ratio. It has also been found that ignition timing does not make obvious influence on the CO emissions, but significant effect on NOx and HC emissions. With regard to species in emission contents, there can be upt to 87% CH4 with left other ingredients including low-carbon saturated and unsaturated hydrocarbons, formaldehyde and other unconventional hydrocarbon emissions.Considering the impact of ignition timing on combustion, various load and speed conditions were studied, for example four different torque values for 1200 rpm. With the advance of ignition timing, cylinder pressure and heat release rate increased and the peak pressure moved close to the top dead center (TDC), while the flames development took longer and the rapid burning got shorter, and total burning time had the lowest value at the MBT (Maximum Brake Torque) ignition timing. Experimental results for lean-burn show that lower NOx and HC emissions can be achieved when there is no too lot loss on power with appropriate ignition advance. However, as the lean burn limit reached, the combustion became very instability and HC emissions increased dramatically.The impact of air/fuel ratio on NG combustion performance was conducted under 1200rpm, 2000rpm and 2800rpm with four load levels for each speed point. The results show that the maximum BMEP can be obtained with air/fuel ratio at 0.9 ~ 1.0 and the lowest BSFC at air/fuel ratio of 1.0 ~ 1.1. It has also been found that air/fuel ratio gave significant influence on CO, HC, and NOx emissions and alcohol, aldehydes, acids and other unregulated emissions increased apparently with the increase of air/fuel ratio.Finally, numerical simulation using AVL BOOST and AVL FIRE was conducted for revealing the mechanism of mixing and combustion with NG under IC engine environment. The impact of compression ratio etc operating parameters on the power performance and fuel economy was explored.