Petrol Fuel Engine: September 2009

Car Starters

STARTERS

Starter is a device that sets an engine in motion. Starters are used to start the engines of many vehicles, including cars, trucks, locomotives.

Car starters consist of three main parts:

(1) a motor
(2) an overrunning clutch, and
(3) a solenoid.

The motor is operated by electricity supplied by the car's battery. The overrunning clutch connects the motor to the car's engine and enables it to turn the engine's crankshaft. The solenoid acts as a switch between the battery and the starter motor. It also controls the action of the overrunning clutch.

When the driver turns the key in the car's ignition, current flows from the battery to the solenoid. The electricity produces a magnetic field around the solenoid, pulling in a plunger. When the plunger is in this position, it connects the car battery to the starter motor. As a result, an electric current flows through a cylindrical coil, called an armature, in the starter motor. This makes the armature begin to rotate.

At the same time that the plunger connects the battery with the starter motor, it forces a shift lever to push the overrunning clutch onto the drive shaft of the motor. The overrunning clutch moves the notched teeth of the pinion gear into mesh with those on the flywheel, making it spin. The flywheel turns the crankshaft, which moves the pistons and sparks the engine, thus starting the car

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.
Exhaust stroke

PARTS OF A PETROL ENGINE

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. In automotive engines, the cylinder block and crankcase form a single unit. Most cylinder blocks are made of cast iron or aluminium.

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. The cylinders in most car engines are part of the block. Some engines have a cylinder sleeve made of specially hardened steel or cast iron pressed into the cylinder block.

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.

CRANKCASE

Crankcase is a rigid frame that holds the crankshaft and the crankshaft bearings. In small engines, all or part of the crankcase may be a part of the cylinder block.

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. The piston has two to six rings to prevent the gases from escaping and to keep lubricating oil from getting into the combustion chamber.

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. For example, in a six-cylinder, in-line, four-stroke cycle engine, the cranks are displaced at 120° angles to each other. As a result, the engine delivers three equally spaced power strokes in each revolution of the crankshaft to assure smooth operation.

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

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. In many two-stroke cycle engines, the movement of the piston takes the place of separate valves. As the piston moves, it covers and uncovers the ports.

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 (lobe) 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. The camshaft may be located in the head of an overhead valve engine, or in the crankcase.

FUEL SYSTEM

Fuel system includes : (1) a storage tank for petrol, (2) fuel lines to carry the petrol to the carburettor, (3) a carburettor 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. In addition, the system may include a governor to limit the engine's speed.

EXHAUST SYSTEM

Exhaust system consists of one or more parts. It may include (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.

IGNITION SYSTEM

Ignition system is the electrical circuit necessary to set fire to, or ignite, the fuel-air mixture in the different cylinders at different times. In a car a storage battery provides electric current, which is increased in voltage by an induction coil. The high-voltage current is carried through a distributor, which delivers the electricity to each cylinder at about the moment the piston reaches the top of the compression stroke. There the electric current jumps a gap between two terminals and sets fire to the petrol-air mixture. The terminals are encased in insulating material and called a spark plug.

Some car engines have an electronic ignition system. These systems use electronic parts, such as capacitors and transistors, to produce the ignition voltage and to control it. Electronic ignition systems may use a distributor to deliver the electricity to each cylinder, or the electricity may be delivered directly to the cylinders. Electronic systems require less maintenance than do ordinary systems, and they provide better engine performance.

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.

Some two-stroke cycle engines, such as those used on lawn mowers, motorcycles, and boats, have no separate lubrication system. Users of these engines mix a small amount of lubricating oil with the petrol. Larger heavy-duty two-stroke cycle engines have lubrication systems similar to those on four-stroke cycle engines.

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.

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 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 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 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 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.

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