Influence Of Mixture Field On The Performance Of A Lean Burn Natural Gas Engine

Natural gas has been widely used to substitute the traditional fossil fuels such as the gasoline and diesel. Natural gas is cheap and has abundant resources and clean nature of combustion. Spark ignition natural engine include equivalent burning natural gas engine and lean burn natural gas engine. Compared with other natural gas engines, the lean burn natural gas engine is better in higher thermal efficiency and lower NOx emission. But in order to get lower NOx emmisions, the lean burn natural gas engine need to burn leaner mixture. However, this makes the engine’s cyclical variation rise and combustion stability decline. The lean burn natural gas engine loses its advantage of higher thermal efficiency. After the mixture is layered reasonably in the lean burn natural gas engine, the engine can run under higher excess air ratio. In this way, the engine can gurantee both thermal efficiency and better control of NOx emission.A turbocharged, multi-point and six-cylinder natural gas engine with electronic control is chosen in this research. Measurement and control platform and numerical simulation platform based on STAR- CD are established. This research is based on operating condition of 1450 r/min, 25% load and aims to do experiments and numerical simulation about the influence of the mixture concentration distribution on the lean burn natural gas engine. In the procee of research, the change of gas injection timing, injection direction and the number of natural gas nozzle holes influence the mixture concentration distribution in the cylinder. The results are as follows:1) The axial concentration of mixture has been formed before the intake bottom dead center(BDC). During the compression stroke, the radial stratication of mixture is weaken. In the intake stroke, the natural gas first entering into the cylinder is deposited on the bottom of the combustion chamber, but the gas coming into the cylinder later gathers in the upper part of the cylinder.2) Within a certain range, the delay of the gas injection timing causes that the mixture concentration is richer on the top of the cylinder, and also leads to a higher concentration of natural gas at the spart plug electrode zones. This type of mixture distribution is conductive to the formation of combustion flame kernel. The ignition delay period is reduced; CA10 and CA50 are advanced; the fuel economy is improved; the amount of NOx emissions are increased. However, if the gas injection timing is delayed overly, the fuel consumption rate will rise. This is mainly due to the incomplete combustion in the lean areas and the compression negative work. The mixture concentration is leaner in the upper part of the cylinder as the advance of the injection timing.This type of mixture distribution obstructs the formation of flame kernel. The ignition delay period is extended; CA10 and CA50 are delayed; the NOx emissions are decreased and the amount of fuel consumption is increased.3) The time of natural gas entering into the cylinder changes with the different direction of gas injection and then different entery time influences the mixture concentration distribution. If the gas is injected upward, the time for natural gas entering in the cylinder will be delayed; if the gas is injected downward, the time for natural gas residenting in the intake port is shorter. Thus, at the same injection timing, it is better to choose upward direction. Thus, at the same injection timing, if the gas is injected upward, the concentration of natural gas at the spart plug zones is higher; CA10 and CA50 are forward; the fuel economy is better and the amount of NOx emissions is higher.4) With the consideration of NOx emissions and fuel economy, the engine achieves the best state with respective optimum parameters in these two injection directions under the engine operating condition. For the upward injection direction, the gas injection timing is set to 335°CA BTDC and the ignition timing is set to 27°CA BTDC. At this condition, the engine works best. While for the downward injection direction, the engine works best when the gas injection timing is set at 295°CA BTDC and the ignition timing is set at 23°CA BTDC.5) Different numbers of natural gas nozzle holes influence the combustion and emission of the engine. When the number of the hole is three, the engine pressure is maximum and the heat release rate is highest, while the effects of employing two holes is weaker and using one hole is the weakest. CA10 and CA50 appear foreward tendency with the nozzle hole number increases. In addition, the amount of NOx emission is the highest when using three holes; when employing single hole, the amount is lowest. When the number of the hole is one, the fuel consumption rate is slightly higher than other two cases.