Engine Principles of Operation

ENGINE PRINCIPLES OF OPERATION

Definition of Technical Terms

Before discussing the principles of operating gasoline and diesel engines, it is imperative to know and be familiar with the meaning of the technical terms that will often be mentioned in the discussion. Some of the technical terms are:

1. Cycle – this is a series of events repeated in the same regular order. Events of instance that are completed in a cycle are Intake, Compression, Power & Exhaust.
2. Stroke – this refers to the distance traveled by the piston from top to bottom or bottom to top.
3. Top Dead Center – this refers to the topmost part reached by the piston during its upward motion. This is sometimes called upper dead center (UDC).
4. Bottom Dead Center this refers to the lowermost part reached by the piston during its downward motion. This is sometimes called lower dead center (LDC).
5. Combustion – this is a chemical reaction in which certain elements of the fuel combine with oxygen, causing an increase in temperature of the gases. There are actually three factors to effect combustion or burning namely Oxygen, Fuel and Temperature.
6. Atmospheric pressure – this refers to the weight of air at sea level which is generally equivalent to 14.7 pounds per square inch (psi).
7. Revolution – this refers to one complete rotation at an axis as in the crankshaft and camshaft equivalent to 360 degrees.
8. Compression Ratio – this refers to the ratio of the volume of fuel charge (gas) or compressed air (diesel) at the beginning of compression stroke to the volume at the end of compression stroke (piston is at TDC).
9. Vacuum – This is a space devoid of matter.
10. Suction – This is the drawing in of fuel-air mixture or air into the engine cylinder due to the downward movement of the piston and the vacuum created in the combustion chamber.
11. Scavenging – This refers to the process of removing burned gases inside the cylinder by means of air that enters the engine during intake stroke.
12. Air-fuel ratio The ratio of air and fuel by weight (usually in pounds) in a given mixture.
13. Turbulence – The swirling motion of air/fuel charge in the combustion chamber to effect perfect combustion.
14. Piston displacement – This is the volume the piston displaces as it moves from BDC to TDC.
15. High fuel efficiency – This refers to pushing large amount of air into the cylinders without increasing displacement hereby resulting in low compression ratio yet a high expansion ratio.
16. OTTO cycle engines These are engines where the four (4) piston strokes are almost of the same or equal duration. As a result, the compression ratio is equivalent to the expansion ratio.
OPERATION OF FOUR-STROKE DIESEL ENGINE

It is recorded that the first commercially successful compression-ignition engine was developed by Diesel. This is the reason why compression-ignition engines are called diesel engines. In view, however, of the improvement of technology and the differences in so many respects of the compression ignition of present day automobiles, it would be safer to say four-stroke and two-stroke compression-ignition engines although diesel engines is still acceptable in the business community.

Let us look at the operation of 4-stroke in a typical diesel engine.

Intake Stroke – In this stroke, only air is admitted into the engine cylinder through the intake manifold and intake valve. The timing of valve action and movement of the piston is the same as the four-stroke gasoline engine. It will be recalled that mixture of gasoline and air is introduced into the gasoline engine cylinder.
Compression Stroke – The air admitted during the suction stroke is compressed to high pressure (above 500 psi) and temperature (about 1,000 F) higher than the ignition temperature of the finely atomized fuel. In the case of the gasoline engine, low compression ratio which means lower pressure and temperature because increasing the temperature may start ignition of the fuel-air mixture. From bottom dead center, the piston goes up to top dead center with both valves closed.
Power Stroke – Ignition of the injected fuel charger does not happen at an instant. It is actually a gradual ignition and combustion. Similarly, turbulence is necessary for efficient combustion, this followed by the expansion of air and gases that drives the piston down with both valves closed.
Exhaust Stroke – The events taking place in a diesel engine during exhaust are similar to those in gasoline engines. Piston goes up with the exhaust valve open releasing burned gases and removing everything inside the cylinder and combustion prior to intake. The cycle is repeated in the same regular order.

Two-Stroke Diesel Engine

In a two-stroke compression-ignition (diesel) engine, the compression and power strokes are identical with those of a four-stroke engine and the scavenging procedure dose not differ from that of a spark-ignition engine. In may phases, two-stroke compression ignition engines are built along the same lines as two-stroke gas engines. Almost all two-stroke cycle engines operate similarly. Variations are mostly in two (2) aspects, that is, the method of producing the scavenge air and the method of admitting it into the engine cylinder.

Considering that these engines have one feature of admitting scavenge air which will be used in part to discharge burned gases, let us study the three (3) different methods of producing scavenge air.

By an enclosed crankcase with the back of the engine piston used as scavenge pump.
By a special built-in scavenge pump using the engine piston.
By using a separate scavenge pump, either driven from the engine crankshaft or using outside power.

Of all the three, the third method is the most widely used in both large and small engines for it has been proven the best. The first method although simple is less satisfactory as the crankcase pump has low volumetric efficiency which means less supply of air than is theoretically necessary. The second method is an attempt to improve the first one in respect to volumetric efficiency but also has some draw backs especially in designing large engines.
The removing burned gases and the admitting of fresh charge (scavenging) may be divided into two main groups. These are:

1. Return-flow scavenging In this scheme, the exhaust and scavenging parts are both opened and closed by the piston. The scavenge air is directed towards the other end of the cylinder by a slant of the scavenge parts or by the shape of the piston head pushing the burned gases before it. Return flow schemes may be subdivided into four kinds depending on the shape and relation position of the exhaust and scavenge parts. These are:

Cross-scavenging which is the simplest but also the least efficient.
Full-loop scavenging with exhaust parts above the scavenge parts.
Tangential loop scavenging.
Combination of loop and cross scavenging.

2. Uniflow Scavenging – In this system, burned gases are discharged from one end of the cylinder while admitting scavenge air is done from the other and which gives a straight flow of the scavenge air. This decreases the tendency to form turbulences and reduces the mixing of burned gases with scavenge air thus increasing the scavenge efficiency. The three main schemes of uniflow scavenging are:

Port and poppet valve scavenging, one piston
Port scavenging with opposed pistons.
Scavenging controlled by a sleeve valve.