Automotive Mechanics

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Transcript of Automotive Mechanics

Engine Mechanic s


Have you ever opened the hood of your car and wondered what was going on in there? A car engine can look like a big confusing jumble of metal, tubes and wires to the uninitiated. You might want to know what's going on simply out of curiosity. Or perhaps you are buying a new car, and you hear things like "3.0 liter V-6" and "dual overhead cams" and "tuned port fuel injection." What does all of that mean? In this article, we'll discuss the basic idea behind an engine and then go into detail about how all the pieces fit together, what can go wrong and how to increase performance. The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine -- combustion takes place internally.

Combustion of gases like gasoline (mixture of carbon and hydrogen) at high temperature (3316C), which produces pressure and causes power. Some unburnt gasoline or partially burnt gasoline are ejected to the air through tailpipe. Gravity, atmospheric pressure & vaccum cause the fuel to enter the cylinder and causes the generation of power. Electricity is required to crank the engine & start it. Electricity operates the fuel system and provide spark at the spark plug. Electric ignition system supplies the spark that ignite the compressed air- fuel mixture in the engine cylinder. To reduce the amount of wiring in the vehicle, the metal engine, chessis and body serve as return circuit. Engine is controlled in two ways: Mechanical and Electrical. Many automotive vehicles have electronic engine control (EEC).It usually controls the fuel system and ignition system. 2

This allow engine to operate as fuel efficient as possible. Many other components in the car may be controlled by electronic engine control (EEC).such as charging system, transmission, suspension, brake, air conditioning, drivers information system etc.

Internal Combustion Engine

Working of an engine

An engine is a mechanic that converts heat energy into mechanical energy. The heat from burning the fuel produces power which moves the vehicle. Sometimes the engine is called the power plant. Automotive engines are internal combustion engine, because the fuel that is burnt internally i.e. inside the engine. They have piston that moves up and down in cylinder. These are called piston engine. The internal combustion engine was invented by Jean Joseph Etienne Lenoir (Belgian Born). Lenoir made the first internal combustion engine that provides a reliable and continuous source of power, which was the gas engine using coal gas, in 1860, in France. The creation of the rocket made by the Chinese people is considered to be the simplest kind of internal combustion engine. Also previous pioneers who also worked on the internal combustion engine but failed, also helped in the development of the internal combustion engine. The first practical internal combustion engine based heavily on experience from the production of steam engines. The engine had a horizontal cylinder; slide valves were used to draw in the fuel-air mixture; and it was double acting, the mixture being fed into the cylinder alternately at either end of the piston. Once it is in the cylinder the mixture was ignited by electric sparks generated at spark plugs by a coil and a battery. This ignition system, a primitive ancestor of modern electric ignition, was unreliable. Because the first internal combustion engine was unreliable, many later pioneers made improvements of the first internal combustion engine. As a result many new engines were made. Such engines were the two and four stroke engine and the petrol engine. Siegfried Marcus in Austria in 1864 was able to create an engine that uses petrol as a fuel. The first internal combustion engine is the basic form for modern car engines. The invention of the internal combustion engine made some of mans most cherished dreams become reality: the aircraft, the motor car, the submarine, the tank and many other inventions before they could be born in there practical form. Nowadays the internal combustion engine is for effective and economical than ever, with reduced gas emissions and lower fuel consumption.

Piston engines are of two types Spark ignition engine (Petrol engine) Combustion ignition engine (Diesel engine) Diesel Engines vs. Gasoline Engines

The difference between them: The type of fuel used.


The way fuel gets into the cylinders. The way fuel is ignited.

The spark engine usually runs on a liquid fuel such as gasoline or an alcohol blend. The fuel must be volatile so that it vaporizes quickly. The fuel vapor mixes with air before entering the cylinders. This forms the highly combustible air fuel mixture that burns easily. The mixture then enters the cylinder and is compressed. Heat from an electric spark produced by the ignition system sets fires to, or ignites the air fuel mixture. As the mixture burns (Combustion) high temperature and pressure are produced in the cylinder. This high pressure applied to the top of the piston, forces it to move down the cylinder. The motion is carried by gears and shafts to the wheels that drive the car. The wheels turn and car moves. In the compression ignition engine, the fuel mixes with the air after it enters the engine cylinder. Compressing the air much raises its temperature to 538C or higher. A light oil called diesel fuel is then sprayed or injected into the hot air or heat of compression ignites the fuel. The method of ignition by heat of compression gives the diesel engine the name compression ignition engine.


Reciprocating to Rotary MotionThe reciprocating motion of the piston must be changed to rotary motion to turn the drive wheels. A connecting rod and a crank on the crankshaft make this conversion. The load on the piston due to combustion of fuel in the combustion chamber is transmitted to crankshaft through the connecting rod. One end of connecting rod known as small end and is connected to the piston through gudgeon pin while the other end known as big end and is connected to crankshaft through crank pin. Hence the rotation of the crankshaft leads to the rotate the wheels.( DOR=Direction of rotation.)


Engine Operation

Four Stroke Engine Animation

As their name implies, four-stroke internal combustion engines have four basic steps that repeat with every two revolutions of the engine: (1) Intake stroke (2) Compression stroke (3) Power stroke and (4) Exhaust stroke 1. Intake stroke: The first stroke of the internal combustion engine is also known as the suction stroke because the piston moves to the maximum volume position (downward direction in the cylinder). The inlet valve opens as a result of piston movement, and the vaporized fuel mixture enters the combustion chamber. The inlet valve closes at the end of this stroke. 2. Compression stroke: In this stroke, both valves are closed and the piston starts its movement to the minimum volume position (upward direction in the cylinder) and compresses the fuel mixture. During the compression process, pressure, temperature and the density of the fuel mixture increases. 3. Power stroke: When the piston reaches the minimum volume position, the spark plug ignites the fuel mixture and burns. The fuel produces power that is transmitted to the crank shaft mechanism. 4. Exhaust stroke: In the end of the power stroke, the exhaust valve opens. During this stroke, the piston starts its movement in the minimum volume position. The open exhaust valve allows the exhaust gases to escape the cylinder. At the end of this stroke, the exhaust valve closes, the inlet valve opens, and the sequence repeats in the next cycle. Fourstroke engines require two revolutions.


Basic Engine Construction

Valve ActionIn many engines cylinder has two valves. One is an intake valve, the other is the exhaust valve. The valve is the series of parts that open and close the valves. The action start at the camshaft. The crankshaft drives the camshaft through gears, sprockets and chain, or sprockets and a toothed timing belt. Most camshafts have a cam for each valve in the engine. Each cam is a round collar with a high spot or lob. The camshaft mounts overhead, on top of the cylinder head. The bucket tappet sits top of the valve stem. Underneath the bucket is the valve spring that holds the tappet up against the cam. When the rotating cam brings the cam lobe down against the top of the bucket tappet, the lobe pushes the tappet down. This compresses the spring and pushes the valve down off its seat. The valve opens. As the cam continues to rotate ,the lobe moves away from the tappet. The spring pushes the tappet and valve up until the valve seats.


Multiple-cylinder enginesA single-cylinder four-stroke piston engine spends three- quarters of its running time exhausting burned gas, drawing in fresh mixture and com- pressing it.

On only one of the four strokesthe power strokeis any energy produced and this makes the output of a singlecylinder four-stroke engine very uneven. This can be smoothed out if more cylinders, with their pistons driving a common crank- shaft, are used. A twin-cylinder four-stroke, for instance, will produce one power stroke for each revolution of the crank- shaft, instead of every other revolution as on a single-cylinder engine. If the engine has four cylinders it produces one power stroke for each half-turn of the crankshaft and at no time is the crankshaft free-wheeling on one of the three passive strokes. Even better results can be obtained using six cylinders, as the power strokes can be made to overlap, so that the crankshaft receives a fresh impulse before the previous