7) HOW PETROL ENGINES WORK

by engr. AFAN BAHADUR KHAN


Petrol engine is an engine that uses petrol as a fuel. Inside the engine, the combustion (burning) of fuel mixed with air produces hot gases that expand against parts of the engine and cause them to move. For this reason, petrol engines are called internal-combustion engines. The motion inside the engine is transferred outside it to turn wheels and propellers or to operate machines. In this way, a petrol engine turns heat energy into mechanical work. The rate at which a petrol engine produces work is usually measured in horsepower or watts .

Petrol engines are compact and light in weight for the power they produce. This makes them one of the most important types of engines for vehicles. Nearly all cars, lawn mowers, motorcycles, motor scooters, snowmobiles, and small tractors have petrol engines. So do many trucks, buses, aeroplanes, and small boats. Petrol engines may also be used as portable power plants--for example, to supply the power to run pumps and other machinery on farms.


KINDS OF PETROL ENGINES


There are two main types of petrol engines, reciprocating engines and rotary engines. Reciprocating engines have pistons that move up and down or back and forth. A part called a crankshaft changes this reciprocating motion into rotary motion. A rotary engine, also known as a Wankel engine, uses devices called rotors instead of pistons. The rotors produce rotary motion directly. This article discusses reciprocating engines, the more common type.

Reciprocating petrol engines are classified in a number of ways. These include :

(1) number of piston strokes per cycle,
(2) type of compression,
(3) by the way they are cooled,
(4) valve arrangement,
(5) cylinder arrangement, and
(6) way they are supplied with air and fuel.


CLASSIFICATION BASED ON CYCLES


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.


CLASSIFICATION BASED ON COMPRESSION


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


CLASSIFICATION BASED ON COOLING

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.



CLASSIFICATION BASED ON VALVE ARRANGEMENT

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.


CLASSIFICATION BASED ON CYLINDER ARRANGEMENT

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.


CLASSIFICATION BASED ON SUPPLY OF AIR AND FUEL


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.


WORKING OF A PETROL ENGINE




The working of an internal combustion engine is divided into four stages called four strokes of the engine and hence the engine is called a four stroke engine.

The intake stroke :


When the engine starts, the piston moves downwards in the cylinder, because of which a region of low pressure is created in the cylinder, above the piston. At this moment, the intake valve opens and the fuel mixture(petrol vapour and air mixture) is sucked into the cylinder from the carburettor.








The compression stroke :


When the sufficient amount of the fuel mixture (petrol vapour and air mixture) has entered the cylinder, the intake valve gets closed. The piston is then forced to move upwards which compresses the fuel-mixture to about one-eighth of its original volume. Higher the compression ratio, more will be the efficiency of the engine.






The power stroke :


Before the piston completes its upward movement, compressing the petrol vapour and air mixture, the spark plug produces a little electric spark inside the cylinder and this spark sets fire to the petrol-air mixture. The petrol vapour burns quickly in a little explosion, producing a large volume of gases and enormous heat. The heat thus produced expands the gases rapidly. The pressure of rapidly expanding hot gases pushes the piston downward with a great force. The piston pushes the piston rod and the piston rod pushes the crank shaft. The crank shaft is joined to the wheels of a car. When the crank shaft turns, the wheels rotate and move the car.





The exhaust stroke:


When the piston has been pushed to the bottom of the cylinder by the hot expanding gases in the power stroke, then the exhaust valve opens. After that, due to the momentum gained by the wheels, the piston is pushed upwards. The upward movement of the piston, expels the spent gases through the exhaust valve into the atmosphere, carrying away the unused heat. The exhaust valve then closes, the intake valve opens up, and the above four strokes of the engine are repeated again and again.