Chapter 6 Engine Types and Classifi cations 127 Copyright by Goodheart-Willcox Co., Inc. Water jackets Coolant Cooling fins A B Figure 6-4. A—An air cooling system has fins that transfer engine heat to the surrounding air. B—A liquid cooling sys- tem has pockets around the cylinders to hold coolant, which collects heat. harmonic balancer to prevent vibration. A small metal pad is added to the flywheel or balancer to counteract a longer stroke machined on the rod journals. Cooling System Classification There are two types of systems providing cooling for the engine—liquid cooling and air cooling. See Figure 6-4. Almost all vehicles now have a liquid-cooled engine. Liquid Cooling System A liquid cooling system surrounds the cylinders with coolant (a water-antifreeze solution). The coolant carries combustion heat out of the cylinder head and engine block to prevent engine damage. The liquid cooling system is very efficient because it will let the engine warm up quickly and can closely control engine operating temperature. This increases engine perfor- mance and reduces exhaust emissions. Air Cooling System An air cooling system circulates air over cooling fins on the cylinders and cylinder heads. This removes heat from the cylinders and heads to prevent overheating. Air-cooled engines are not commonly used in mod- ern cars and light trucks. The air cooling system cannot maintain as constant of an engine temperature as a liquid cooling system. This reduces engine efficiency and increases exhaust emissions. In order to comply with strict exhaust emission regulations, most vehicle manufacturers have phased out air-cooled engines. Fuel and Ignition System Classifications An automotive engine can also be classified by the type of fuel it burns, how it ignites the fuel, and how fuel is metered into the engine. These classifications are discussed in this section. Fuel Type A gasoline engine burns gasoline, which is normally metered into the intake manifold or combustion cham- bers. A throttle valve is used to control the airflow into the engine. This controls engine speed and power. The piston compresses the air-fuel mixture. A spark plug ignites the compressed air-fuel mixture, Figure 6-5A. A diesel engine burns diesel oil (fuel), which is a thicker fraction of crude oil. Diesel fuel is injected directly into the engine combustion chamber or a precombustion chamber. A precombustion chamber may be used to house the injector and glow plug. Only air flows through the intake manifold and a throttle valve is not used to control airflow and engine speed, Figure 6-5B. The amount of fuel injected into the com- bustion chambers controls engine speed. The compression ratio on a diesel is very high, generally between 17:1 and 20:1, which produces a very high pressure in the cylinder. When fuel is injected into the cylinder, it ignites and burns from the heat of high compression. A spark plug is not needed. A liquefied petroleum gas (LPG) fuel system burns a very light fraction of crude oil. The LPG is stored in a high- pressure tank. At high pressure, the petroleum is in liquid form, not gas. A special converter is used to change the liquid into a gas and a metering system meters the gas into the engine intake manifold. LPG is mainly propane and butane, but contains small amounts of other gases. It has combustion qualities equal to or better than high-octane gasoline. LPG is a very good fuel. It produces good power, economy, and low exhaust pollution levels. LPG has operating characteristics almost identical to those of gasoline. An alcohol fuel system is similar to a gasoline fuel system, but twice as much fuel must be metered into the engine. Also, for maximum efficiency, the compression ratio is usually higher than in a gasoline engine. A hydrogen fuel system uses hydrogen gas. This is a very promising fuel system because it produces no pollu- tion. Most systems in development use hydrogen as the fuel to power a fuel cell. However, since hydrogen gas is very explosive, it poses danger when stored in large quantities, such as at a distribution center or in a motor vehicle. One of the best sources of hydrogen is water. Each mol- ecule of water is made up of two atoms of hydrogen and one atom of oxygen. One simple method used to separate the hydrogen is a process called electrolysis. An electric current is sent through water to release the oxygen and hydrogen.