Combustions engines may be divided into types according to the duration of the cycle on which they operate, in term of piston stroke. There are two main types of internal-combustion engines—gasoline engines and diesel engines. The gasoline engine, found in most cars, is a spark-ignitionengine. It uses electricity and spark plugs to ignite the fuel and air mixture in the engine's cylinders. The diesel engine is a compression-ignitionengine. It compresses the air in the cylinders, causing the temperature of the air to rise. Fuel injected into the hot, compressed air immediately ignites. Most of the locomotives all over the world are diesel powered. Why do we call this compression- ignition engine by the name “diesel”? Simply because a man whose name was Rudolf Diesel patented his engine design in 1892 and operated the first successful model in 1897. Later, he founded a factory to make his engines.
Now let’s note what essential parts the engine must have. The essential parts and the job that each performs are as follows:
1 A piston sliding in a cylinder. The piston has two jobs: first, to compress the air change, second, to receive the pressure of the gases while they are burning and expanding.
2 A cylinder head which closes the top end of the cylinder so as to make a confined space in which to compress the air and to confine the gases while they are burning and expanding.
3 Valves or ports to admit the air and to discharge the spent or exhaust gases.
4 Connecting rod to transmit force in either direction from the piston to the crankshaft.
5 Crankshaft and the “main” bearing which support the crankshaft and permit it to rotate.
6 A supporting structure to hold the cylinders, crankshaft, and main bearing in firm relation to each other. This structure is usually made up of two parts, called frame and bedplate.
7 Fuel-injection pump to force the oil into the cylinder; also the fuel injection nozzle to break up the oil into a fine spray as it enters the cylinder.
8 Camshaft, driven by the crankshaft, to operate the fuel-injection pump and also to open the valves (in engines which use valves).
9 Flywheel, to store up surplus energy on the power stroke and to return that energy when piston is being pushed upward on the compression stroke.
10 Throttle, to regulate the amount of fuel supplied at each stroke, and thus control the engine speed and power.
During the combustion process, the stored chemical energy in the fuel is converted to thermal, or heat, energy. The temperature in each cylinder rises as high as 4500 "F (2480 °C) and creates pressures of 1,500 pounds per square inch. The pressure pushes against the tops of the pistons, forcing them to the other end of their cylinders. The pistons are connected by a rod or other mechanism to a crankshaft that they turn. In this way, a diesel engine supplies rotary power to drive vehicles and other machines.
To ignite the fuel, the compressed air must have a certain temperature. The degree to which the temperature of the air rises depends on the amount of work done by the piston in compressing it. This work is measured as a ratio between the volume of uncompressed air and the volume of the air after compression. The compression ratio necessary to ignite the fuel depends on the size of the engine's cylinders. In large cylinders, the compression ratio is about 13 to 1. For small cylinders, it may be as high as 22 to 1.
Near the end of the piston's compression stroke, the fuel is injected into the cylinder. In order for the fuel and air to mix well, the fuel is injected under high pressure as a spray. Combustion usually starts just before the piston ends its compression stroke. The power of diesel engines can be increased by supercharging, or forcing air under pressure into the cylinder. See Fuel injection.
Diesel engines have a high thermal efficiency, or ability to convert the stored chemical energy in the fuel into mechanical energy, or work. In the spark-ignition engine, the amount of power produced is determined by a throttle that regulates how much air and fuel enter the cylinder. The throttle may operate in a partly closed position much of the time, thereby reducing the engine's air intake and efficiency. Diesel engines do not require a throttle, making them more efficient than gasoline engines.
How a four-cycle diesel engine works
A cycle begins with the intake stroke when the piston moves down and draws air into the cylinder. The piston rises and compresses the air. During the compression stroke, the air temperature rises to about 900 °F (480 °C). When fuel is injected into the cylinder, it mixes with the hot air and burns explosively. Cases produced by this combustion push the piston down for the power stroke. During the exhaust stroke, the piston moves up and forces the burned gases out of the cylinder.
Exercise 3
Answer the following questions according to the text.
1 What two distinct classes are internal-combustion engines divided into?
2 What is the difference between gasoline engine and diesel engine?
3 Why do we call this compression- ignition engine by the name “diesel”?
4 What are the main parts of diesel engine?
5 When was the first successful model of diesel engine built?
6 What uses electricity and spark plugs to ignite the fuel and air mixture?
7 When did Rudolf Diesel patent his engine design?
8 What can you say about pistons?
9 When is the stored chemical energy in the fuel converted to thermal, or heat, energy?
10 How are the pistons connected to a crankshaft that they turn?
11 What is the compression ratio in large cylinders?
12 When does combustion usually start?
13 How do we call the ability to convert the stored chemical energy in the fuel into mechanical energy, or work.?
14 What is injected into the cylinder, and then mixes with the hot air and burns explosively?
