Today¡¯s automotive engines can be classified in several ways depending on the following design features.

- Operational cycles. Most technicians will generally come in contact with only four-stroke cycle engines. However, a few older cars have used and some cars in the future will use a two-stroke engine.

- Number of cylinders. Current engine designs include 3-, 4-, 5-, 6-, 8-, and 12-cylinder engines.

- Cylinder arrangement. An engine can be flat (opposed), in-line, or V-type. Other more complicated designs have also been used.

- Valve train type. Engine valve trains can be either the overhead camshaft (OHC) type or the camshaft in-block overhead valve (OHV) type. Some engines separate camshafts for the intake and exhaust valves. These are based on the OHC design and are called dual overhead camshaft (DOHC) engines. V-type DOHC engines have four camshafts ¨C two on each side.

- Ignition type. There are twp types of ignition systems: spark and compression. Gasoline engines use a spark ignition system. In a spark ignition system, the air/fuel mixture is ignited by an electrical spark. Diesel engines, or compression ignition engines, have no spark plugs. An automotive diesel engine relies on the heat generated as air is compressed to ignite the air/fuel mixture for the power stroke.

- Cooling systems. There are both air-cooled and liquid-cooled engines in use. Nearly all of today¡¯s engines have liquid-cooling systems.

- Fuel type. Several types of fuel currently used in automobile engines include gasoline, natural gas, diesel, and propane. The most commonly used in gasoline.