TAILIEUCHUNG - Tài liệu Diezel 1410 P9

Combustion. Engine efficiency is a comparison of the amount of power developed by an engine to the energy input as measured by the heating value of the fuel consumed | 9 ENGINE PERFORMANCE AND OPERATION A. COMBUSTION AND EFFICIENCY 9A1. Combustion. Engine efficiency is a comparison of the amount of power developed by an engine to the energy input as measured by the heating value of the fuel consumed. In order to understand the various factors responsible for differences in engine efficiency it is necessary to have some knowledge of the combustion process which takes place in the engine. In the diesel engine ignition of the fuel is accomplished by the heat of compression alone. To support combustion air is required. Approximately 14 pounds of air are required for the combustion of 1 pound of fuel oil. However to insure complete combustion of the fuel an excess amount of air is always supplied to the cylinders. The ratio of the amount of air supplied to the quantity of fuel injected during each power stroke is called the air-fuel ratio and is an important factor in the operation of any internalcombustion engine. When the engine is operating at light loads there is a large excess of air present and even when the engine is overloaded there is an excess of air over the minimum required for complete combustion. The injected fuel must be divided into small particles usually by mechanical atomization as it is sprayed or injected into the combustion chamber. It is imperative that each of the small particles be completely surrounded by sufficient air to effect complete combustion of the fuel. To accomplish this the air in the cylinder must be in motion with good fuel atomization combined with penetration and distribution. In mechanical injection engines this is accomplished by forcing scavenging air into the cylinder with a whirling motion to create the necessary turbulence. This is usually done in the 2-cycle engine by shaping the intake air ports or by casting them so that 1. The fuel must enter the cylinder at the proper time. That is the fuel injection valve must open and close in correct relation to the position of the piston. 2. The

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