Reading Materials

Passage 1

Kinds of Engines

The engine is the heart of an automobile.The purpose of an automotive engine is to convert fuel into the energy that moves the automobile.Currently the easiest way to create motion from fuel is to burn the fuel inside an engine.Therefore, an automotive engine is an internal combustion engine, which burns fuel within the cylinders and converts the expanding force of the combustion into rotary force used to drive the automobile.

There are several types of internal combustion engines classified as reciprocating or rotary engine; spark ignition or compression ignition engine; and alternative-fuel engine or hybrid-electric engine.

Reciprocating Engine

A reciprocating engine consists of(1)compression of air or pre-compressed air(or airfuel mixture in the case of certain types of engines)within the cylinder of the engine by the action of a piston, (2)addition of heat energy into the compressed air by directly combusting the fuel in the compressed air, followed by(3)expansion of the hot pressurized combustion products in the cylinder against the piston connected to the load to produce useful work.The auto engine and the diesel engine are examples of a reciprocating engine.The compression ratio of an auto engine is lower and the combustion process is initiated by a spark plug while in a diesel engine, the compression ratio is significantly higher and the fuel is ignited by the heat of compression.

Spark Ignition Engines

Next, there are two types of spark ignition engines:the four-stroke cycle and the twostroke cycle.In the four-stroke engine, four strokes of the piston are required to complete a cycle:(1)intake stroke where the piston moves with the intake valve open and the exhaust valve closed such that a mixture of air with atomized and vaporized fuel is taken into the cylinder, (2)compression stroke, in which the air/fuel mixture is compressed with both valves closed followed by ignition of the air/fuel charge by a timed spark, (3)power or expansion stroke with both valves closed, and finally(4)the exhaust stroke in which the pistons moves with the exhaust valve open and thus completing the cycle.The cylinder walls are cooled by circulating a cooling medium through the cylinder jackets.

In the two-stroke engine, the intake and exhaust strokes are eliminated by using precompressed intake charge to displace the exhaust gases.The two-stroke engine has the advantage of a high power to weight ratio because the engine has a power stroke each revolution.This advantage, however, is offset by the loss of a portion of the intake charge with the exhaust gases, resulting in lower efficiencies.The two-stroke engine has thus limited applications such as in small boat engines, lawnmower engines where low cost and weight are more important than efficiency.

Compression Ignition Engines

With a sufficiently high compression ratio and a suitable fuel, auto ignition occurs in a reciprocating engine.The engine is similar to the spark ignited engines described above except that during the compression stroke, only air is taken into the piston and compressed to ignition conditions and then, the fuel is atomized directly into the combustion chamber at a controlled rate.

The core of the engine is the cylinder, with the piston moving up and down inside the cylinder.Most cars have more than one cylinder(four, six and eight cylinders are common).In a multi-cylinder engine, the cylinders usually are arranged in one of three ways:inline, V or flat.Different configurations have different advantages and disadvantages in terms of smoothness, manufacturing cost and shape characteristics.These advantages and disadvantages make them more suitable for certain vehicles.

Let's look at some key engine parts in more detail:

Spark plug

The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur.The spark must happen at just the right moment for things to work properly.

Valves

The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust.Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

Piston

A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston Rings

Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder.The rings serve two purposes:They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.

They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.Most cars that“burn oil”and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.