Petrol Engine

PETROL ENGINE Petrol engine is an engine that uses petrol as a fuel. Inside the combustion of fuel Produces hot gases that expand against parts of the engine the engine and cause them to move. For this reason, petrol engines are called internal-combustion engines 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. Nearly all cars, lawn mowers, motorcycles, motor scooters, snowmobiles, and small tractors have petrol engines. So do many trucks, buses, aeroplanes, and small boats.

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.

1) 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 is sucked into the cylinder from the carburetor.

2) Compression stroke:

When the sufficient amount of the fuel 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.

3) 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.

4) 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.

PARTS OF A PETROL ENGINE 1. CYLINDER BLOCK Cylinder block is a rigid frame that holds the cylinders in proper alignment. If the engine is liquid cooled, the block is jacketed, so that it can be surrounded by the liquid, or has passages for the liquid around each cylinder. Most cylinder blocks are made of cast iron or aluminium.

2. CYLINDERS Cylinders are rigid tubes that serve as a bearing for the pistons that move up and down inside them. They have highly polished surfaces. This permits a close fit between piston and cylinder and prevents gases from leaking past the piston

3. CYLINDER HEAD

Cylinder head is a casting bolted to the top of the cylinder block. The cylinder head, together with the upper end of the cylinder and the top of the piston, form the combustion chamber where the fuel-air mixture burns. A cylinder head and block may be one unit.

4. CRANKCASE Crankcase is a rigid frame that holds the crankshaft and the crankshaft bearings.

5. PISTON AND CONNECTING RODS There is a piston fitted in the cylinder. This piston is connected to a connecting rod which in turn is connected to the crank shaft. When the fuel-air mixture burns, the expanding gases exert a force on the piston. This force is then transmitted through a connecting rod to the crankshaft

6. CRANK SHAFT Crankshaft changes the reciprocating motion of the pistons into rotary motion. The crankshaft has a number of cranks, or throws. These cranks are displaced at angles to each other.

7. FLYWHEEL Flywheel stores energy during a piston's power stroke and releases it during other strokes. This helps to keep the crankshaft turning at a constant speed

8. VALVES In a four-stroke cycle engine, each cylinder has one or two intake valves, to let the air-fuel mixture into the combustion chamber, and one or two exhaust valves, to let the burned gases escape. These are called poppet valves, because they pop up and down as they open and close. The opening in the cylinder block or head uncovered by the valve is called the port.

9. CAMSHAFT Camshaft opens and closes the valves at the proper point in the engine cycle. It runs the length of the engine and has one cam at each intake and exhaust valve. In a four-stroke cycle engine, the camshaft is geared to the crankshaft so that it runs at half the crankshaft's speed.

10. FUEL SYSTEM

Fuel system includes: (1) a storage tank for petrol, (2) fuel lines to carry the petrol to the carburetor, (3) a carburetor to mix the petrol with air, and (4) an intake manifold to distribute the fuel-air mixture to the cylinders. The fuel system also includes a filter to clean dirt out of the fuel and an air cleaner to take dirt out of the air that is mixed with the petrol.

11. EXHAUST SYSTEM Exhaust system consists of (1) an exhaust manifold to collect the burned gases from the cylinders, (2) an exhaust pipe to carry the burned gases, and (3) a silencer to silence the noise of the exhaust gases.

12.LUBRICATION SYSTEM Lubrication system provides oil as a film between the moving parts of the engine to prevent wear from friction and to keep the engine cool. The two common types of four-stroke cycle engine lubrication systems are the wet sump and the dry sump. In the wet-sump engine, the oil supply is contained within the engine, in the bottom of the crankcase. In the dry-sump engine, the oil supply is contained in a separate oil tank.