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Recently natural gas has attracted public attention as clean fuel for motor vehicles. We first developed a heavy-duty compressed natural gas (CNG) engine for city busses and manufactured many CNG-fueled engines. Both medium- and heavy-duty CNG engines achieved very low exhaust emissions. However, the fuel consumption of these engines for example the city-bus application are higher than that of a diesel engine. For this reason, these CNG engines always operate under the part-load conditions. Therefore, we developed a direct-injected CNG engine. Under a part-load condition, the engine is operated on the stratified-charged natural gas that is directly injected into the combustion chamber. It is the most important that the air/fuel ratio of the mixture stratified near the spark plug must be controlled to achieve the stable mixture condition.

Injection process simulation methods were developed for both the unit injector (UI) system and the pump-line nozzle (PLN) system consisting of an in-line injection pump, fuel line, and nozzle. Simulation results agreed well with measured ones. With regard to the shape of injection rate and the peak injection pressure change at various engine speeds, the injection characteristics of the UI system are better than those of the PLN system. Simulation results showed that similar injection characteristics can also be obtained with the PLN system by using a concave cam with a carefully designed cam profile for a sleeve-controlled in-line injection pump and by changing the prestroke according to the operating conditions. Engine test results demonstrated the possibility of improving the trade-off between NOx and fuel consumption by shaping the injection rate. The shape of injection rate plays an important role in diesel combustion(1,2)*, affecting exhaust emissions and also combustion noise.

The injection nozzle inclination angle affects the flow characteristics in nozzle holes. Stroboscopic photographs of instantaneous spray plumes show that the length of each spray plume is different. Test results show that the fuel quantities injected from the holes are remarkably less when the nozzle hole spray angle relative to the injection nozzle axis is smaller compared with others with the same hole diameter. Hence, the authors analyzed the flow characteristics in injection nozzles using a computational fluid dynamic technique. Calculation results show good qualitative agreement with experimental results. INJECTION NOZZLES are normally installed in a two-valve cylinder head with an inclination angle. As shown in Fig. 1, the spray angle of each nozzle hole is different in order to maintain the same impingement height against the piston cavity for each spray.

To improve both the durability and reliability of the main bearing, and also to reduce friction, a technique was developed to measure transient sliding friction of main bearings in a firing engine. The movement of bearing pin was also measured at same time to analyze the relationship between friction characteristics and oil film thickness. Test results are as follows: Main bearing friction is fairly constant over one working cycle, an increase is observed in a short time. When oil film thickness becomes minimums, engine load has some influence on the average main bearing friction value, but affects peak value substaintially. With an increase of engine speed, both average and peak values increase.