Discrimination of Lean Limit of LPG on Ignition Engine

Discrimination of the Lean Limit of LPG on Ignition Engines Wang Zhensuo, Deng Baoqing, Li Liguang2, Xiao Zongcheng, Wang Huiping1 (1. School of Automotive Engineering, Jilin University, Changchun, Jilin 130025; 2. School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030) Ming: Torque fluctuation , Speed ​​fluctuation, HC emission value and HC rise rate are all affected by air-fuel ratio, and their changing trends have good consistency; with the change of air-fuel ratio, the change of speed fluctuation is not obvious, which is affected by engine structure parameters and operation Because of the influence of parameters, it is difficult to determine the HC emission value used to determine the lean limit. Therefore, it is more reasonable to consider the torque fluctuation and the HC rise rate as the lean limit.

Lean combustion technology has the advantages of improving economy and reducing harmful emissions. It has been popularized and applied in ignition engines by combining advanced technologies such as direct injection and variable valve timing. The key to lean combustion technology is lean limit. The determination of the air-fuel ratio and the realization method of controlling the air-fuel ratio within the lean limit. In foreign countries, many researchers have conducted in-depth analysis and experimental research on the rarefied limit. Quader proposed the definitions of combustion lean limit, ignition lean limit and misfire lean limit; eywood pointed out that the coefficient of variation of the average indicated effective pressure can be used to judge the misfire lean limit and the HC emission value to judge the lean limit * has practical significance and so on. There are few domestic researches on this aspect, so the research on the discrimination of rarefied limit is very necessary. In this paper, combined with the application research of LPG lean combustion, the relevant tests of LPG lean limit discrimination are carried out, and an attempt is made to find a simple and accurate lean limit discrimination method suitable for the engine bench test.

1 Definition of Lean Limit and Analysis of its Criterion 11 Definition of Lean Limit According to Quader’s formulation, there are three definitions of lean burn limit, namely, burning lean limit, ignition lean limit and misfire lean limit.

Lean combustion limit: refers to the lean air-fuel ratio that guarantees flame propagation, and is a function of the temperature and pressure of the mixture. The influence of temperature on the combustion lean limit can be calculated by the following formula: Ignition lean limit: refers to the lean air-fuel ratio at which the external energy can make the mixture reach the lowest ignition temperature. It is related to the fuel, the speed of the mixture, and the temperature and pressure of the mixture when the spark plug is ignited. It is also related to the parameters of the ignition system (spark duration, energy release rate, electrode material and spark plug gap).

Misfire Lean Limit: refers to the air-fuel ratio when the mixture is not ignited, not completely burned, or because the flame propagation speed is too low and the flame is extinguished before the combustion is completed. Quader has qualitatively analyzed the influence of ignition and flame propagation on the misfire lean limit.

In practical applications, the engine lean limit can be divided into stable operating lean limit and applicable lean limit. When the unburnt gas mixture fund project of the ignition engine: the key teacher fund project of the Ministry of Education; the national technology innovation fund project (Guojimaoji (1998) No. 345) is too thin, the flame expansion period, the rapid combustion duration and The fluctuations during the cycle of the combustion process will increase, and after reaching a certain point, the engine operating conditions deteriorate and become unstable, and the hydrocarbon emissions increase rapidly. The occurrence point of these phenomena defines the lean limit of the stable operation of the engine, which is also called lean operation. limit. The applicable lean limit refers to the comprehensive consideration of the engine’s output power and emissions, the increase in the throttle opening allows the engine’s power drop to be compensated, and the lean limit to reduce emissions and improve fuel economy is achieved.

1.2 Discrimination of lean limit When the engine runs beyond the lean limit, a series of problems such as combustion fluctuation and emission deterioration will occur. The lean limit discrimination is to determine whether the engine is operating within the lean limit by detecting whether these phenomena occur. The discrimination method is divided into two categories, one is to install a sensor in the combustion chamber to directly measure the combustion change of the engine, and the other is to indirectly determine the combustion change by detecting the output variable.

In the early research process, there are many ways to judge the lean limit. The misfire rate, the fluctuation of the crank angle when the cylinder pressure rise rate is maximum, the fluctuation of the cylinder pressure map area, etc. have all been used as the method of judging the lean limit. Each scheme can make different judgment results according to different judgment conditions. For example, Quader regards 0.5% misfire rate as the limit of the misfire lean limit. He believes that the vehicle’s drivability at this time is already very bad2; while Ryan et al. believe that power loss and HC emissions rise sharply when the misfire rate is 5%, which should be taken as The judgment limit of the rarefied limit. SAEPaper Wang Zhensuo. Study on lean burn of LPG engine. Changchun: College of Automotive Engineering, Jilin University, 2000. Wang Zhensuo. LPG is used for performance optimization and lean combustion of small ignition engines. Journal of Jilin University (Engineering Science Edition) 2002, Li Xinghu. Study on combustion fluctuation of propane engine. Journal of Internal Combustion Engine, 1999 (1)

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