128 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Air is heated by compression. Fuel injected, combustion begins. More fuel sprays into chamber. Combustion continues and pressure is formed while piston moves down. Spark occurs, ball of flame forms around electrodes. Flame front spreads smoothly while piston nears top dead center (TDC). Piston has reached TDC and flame is “shooting” through chamber, forming heat. Combustion pressure is maximum and piston is a few degrees down in cylinder. B A Figure 6-5. A—The operation of a four-stroke cycle gasoline engine. Note that a spark plug provides the heat needed to ignite the fuel. B—The operation of a diesel engine. Note that the heat of compression ignites the fuel as the fuel is sprayed into the combustion chamber (or precombustion chamber). When burned with pure oxygen, hydrogen produces heat and water. When burned in an engine using air instead of oxygen, water and a very small amount of pol- lution (NO X ) are released. However, this is only a fraction of the pollutants formed by the combustion of most other fuels. Note: Some gasoline blends contain some alcohol. However, vehicles that burn a gasoline blend that contains alcohol are not considered to have an alcohol fuel system. An alcohol fuel system burns pure alcohol. LPG, alcohol, and hydrogen fuel systems are not commonly used. Ignition Classification Two methods are commonly used to ignite (light) the air-fuel mixture in the engine combustion chamber— electric spark and compression. A spark ignition engine, Figure 6-5A, uses an electric ignition and a spark plug to start the combustion of the fuel. Gasoline-, LPG-, and alcohol-fueled engines use this method of ignition. A compression ignition engine, Figure 6-5B, uses the heat generated by the high compression pressure to heat the air and ignite the fuel. Diesel engines are compression ignition engines. Fuel Metering A gasoline engine can be classified by how fuel is metered into the engine. There are two basic types of fuel metering for gasoline engines—fuel injection and carbure- tion. Diesel engines are fuel injected. A fuel injected gasoline engine sprays fuel into air- stream in the intake manifold, either at the port or in the throttle body. An engine with this type of fuel metering is more properly termed a gasoline injected engine because a diesel engine also injects fuel into the engine. Computer- controlled gasoline injection is the most common type found on a modern car or light truck because it can closely match the fuel fed into the engine with engine load. Gasoline injection may be throttle body injection or multiport injec- tion. In addition, the injection may be indirect, where fuel is injected into the airstream, or direct, where fuel is injected directly into the cylinder, Figure 6-6. A carbureted engine uses engine vacuum to draw fuel out of the carburetor and into the engine intake manifold. Carbureted engines have been phased out for the more effi- cient computer-controlled gasoline injection. Combustion Chamber Classifications The design of the engine’s combustion chamber can also be used to classify an engine. Its shape, number of valves per cylinder, port configuration, number of spark plugs, etc., can all be used to indicate how an engine is con- structed. You should understand these differences if you are going to be a “top-notch” engine technician.