Petrol Fuel Engine

Fuel may be metered, or sent, to the cylinders by either a carburettor or an injection system. Therefore, reciprocating engines are also classified as carburetted or as fuel-injected engines. Because combustion depends upon both air and fuel, the power of an engine is limited by the amount of air reaching the cylinders. To increase power, an engine may be supercharged or turbocharged. A supercharger is an engine-driven pump, and a turbocharger is an exhaust-driven pump. Both pumps force extra air into the cylinders, increasing the engine power. The air needed to burn 1 unit of petrol weighs about 15 times as much as the petrol.

Engines are also classified by the number and arrangement of cylinders. The most common types include in-line, V, radial, and horizontal opposed. Radial engines have an odd number of cylinders, such as 3, 5, 7, or 9. Most other engines have an even number of cylinders--4, 6, 8, or 12.

The two most common valve arrangements are :
(1) L-head
(2) I-head

An L-head, or underhead, valve engine has the intake and exhaust valves side by side in the cylinder block. The intake valve admits the air-fuel mixture into the cylinder and the exhaust valve lets out the exhaust gases. An I-head, or overhead, valve engine has the two valves side by side in the cylinder head, the cylinder block's top cover. In some cars, each cylinder has four valves--two intake valves and two exhaust valves

The burning fuel-air mixture in a cylinder produces gas temperatures of about 2500 °C. Therefore, the metal parts of the engine must be cooled or they would melt. Most automotive petrol engines are liquid cooled. A liquid, usually water, is circulated around the cylinders to cool the metal. The heated liquid is then pumped through a radiator. A fan driven by the engine or by an electric motor draws air through the radiator to cool the liquid.

Most aircraft reciprocating engines are air cooled to reduce weight. Air is not as effective a coolant as liquids, so the outsides of the cylinders have many metal fins. These fins conduct heat out of the cylinder and offer a large surface area for the air to sweep over, thus ensuring effective cooling.

As a piston moves from the bottom to the top of a cylinder, it compresses the air and petrol mixture. A number, called the compression ratio, tells how much the mixture is compressed. A high-compression engine may have a compression ratio of 10 to 1. Such an engine compresses the mixture to a tenth of its original volume. A low-compression engine may have a ratio of 8 to 1.

High-compression engines burn petrol more efficiently than do low-compression engines. But high-compression engines require high-octane petrol. Until the 1970's, the octane level of petrol depended on the amount of lead additives--the more lead, the higher the octane. In the mid-1970's, manufacturers began to equip cars with devices called catalytic converters that reduce the pollutants in car exhausts. Lead was found to interfere with the effectiveness of catalytic converters. Cars with catalytic converters had to use low-octane petrol because high-octane lead-free petrol was costly to produce. As a result, the car industry reduced compression ratios so that engines could burn lower octane lead-free fuels efficiently.

Most reciprocating petrol engines operate on either a two-stroke or a four-stroke cycle. Cycle means the steps that must be repeated for each combustion of the fuel-air mixture in the cylinders. Stroke means the up-and-down or back-and-forth movements of the pistons. A four-stroke cycle engine has intake, compression, power, and exhaust strokes. A two-stroke cycle engine combines the exhaust and intake steps near the end of the power stroke. Although two-stroke cycle engines are less fuel-efficient than four-stroke cycle engines, they are simpler and cheaper to build.

A two-stroke cycle engine is used where low cost is important, as in a power lawn mower. It delivers more power for a given weight and size than does a four-stroke cycle engine. Each cylinder in a two-stroke cycle engine produces a power stroke for every turn of the crankshaft. But in a four-stroke cycle engine, a cylinder produces a power stroke on every other turn.