INTERNAL COMBUSTION ENGINE CLASSIFICATION

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Internal combustion engines can be classified according to different criteria as follows:

1. According to ignition System.

a) Compression Ignition Engine (C. I. Engine)

In this type of engine, the heat of the compressed air itself ignites the fuel. No other means of ignition are required, e.g. Diesel Engine.

In a Compression Ignition Engine, e.g. Diesel Engine, a piston reciprocates in a cylinder. At downward stroke of piston, air enters the cylinder. At upward stroke of piston air is compressed. Due to compression pressure and temperature of air becomes quite high (over 35 bar and 500*C respectively). Finely atomised fuel oils sprayed into such compressed air ignite spontaneously and produce power.

b) Spark Ignition Engine (S. I. Engine)

In this type of engine (Otto engine), the fuel is ignited by the spark produced by a high-tension electrical circuit. In spark ignition Engine, liquid gasoline is sprayed or drawn through a nozzle or jet into the air stream going to the working cylinder. A combination of mild heating and reduction of pressure partially vapourises the gasoline. Proportionate mixing of air and gasoline vapour is done in carburetor. Mixture enters the cylinder where at a suitable time, an electric spark ignites the mixture, which burns then quickly and produces power.

Spark Ignition Engine Versus Compression Ignition Engines Similarities.

1. Both are Internal Combustion Engines.
2. Both run on liquid fuels.

Dissimilarities.

S. I. Engine.	C. I. Engine.
1.  Ignition system required	1. Not required.
2.  Draws air and fuel into the system.	2.  Draws only air into the cylinder.
3. Compresses air and fuel together.	3.  Compresses air only.
4. Fuel is mixed with air at before compression starts.	4. Fuel is mixed with air at the end of compression.
5. As too much compression of air and fuel mixture causes pre ignition and detonation permissible compression ratio is not high(about 7).	5. Only air can be compresses without pre-ignition and detonation, so compression ratio can be high (about 16).
6. Efficiency, being proportional to compression, is limited due to less compression ratios.	6. Higher efficiencies can be obtained due to possible higher compression ratio.
7. Uses highly volatile liquid fuels so that it can mix with air at low temperature.	7. Uses less volatile liquid fuels
8. Fuel used is costly.	8. Cheaper fuel can be used
9. More fuel is used for same power.	9. Less fuel consumption.
10. Lighter in weight.	10. Heavier and stronger engines due to higher pressures involved.
11. Initial cost less.	11. Initial cost high.
12. Smooth operation.	12. Certain roughness in operation encountered, especially in high-speed engines at light loads.

2. According to Operating Cycles.

(a) OTTO CYCLE (Constant Volume Combustion Cycle).

It is the ideal air standard cycle for Petrol engine, the gas engine and the high-speed oil engine. The engines based on this cycle have high thermal efficiency but noisiness results particularly at higher power due to higher pressures in the cylinders.

(b) DIESEL CYCLE (Constant Pressure Combustion Cycle).

It is the ideal Air standard cycle for Diesel Engine, especially suitable for low speed Diesel Engine but not for high speed Diesel Engine. (The thermal efficiency is lower than Otto cycle engines but engines run smoothly due to lower pressures in the cylinder.

(c) DUAL COMBUSTION CYCLE (Constant Pressure and Constant Volume Combustion Cycle).

Modern Diesel Engines do not operate purely on constant pressure combustion cycle but some part of combustion process takes place at constant volume while the rest is completed at constant pressure.

In general, this cycle resembles Constant volume combustion Cycle more than constant pressure combustion cycle. It is suitable for modern Medium and High Speed Diesel Engines. The thermal efficiency is more than Diesel Cycle but less than Otto cycle. Also noise level is in between the two. This is a more practical engine.

3. According to Strokes/Cycle.

In an engine, the following events form a cycle:

a)      Filling the engine cylinder with fresh air.

b)      Compressing the air so much that injected fuel ignited readily by coming in contact with hot air and burns efficiently.

c)      Combustion of fuel.

d)     Expansion of hot gases.

e)      Emptying the products of combustion from the cylinder.

Depending on how many strokes of piston are required in completing this cycle, the engines can be divided into two classes:

1. Four Stroke Engine

An engine, which needs 4 strokes of the piston (2 in and 2 out) to complete one cycle, is called Four-stroke engine.

2. Two Stroke Engine

An engine that needs only 2 strokes of the piston (1 in and 1 out) to complete one cycle is called Two-stroke engine.

4. According to Piston Action:

(a)    Single Acting Engine

One end of the cylinder and one face of the piston are used to develop power. The working face is at the end, which is away from crankshaft. Generally, single acting vertical engines develop power on the down stroke.

(b) Double Acting Engine

Both ends of the cylinder and both faces of the piston are used to develop power on the upward as well as on the downward stroke.

 c) Opposed Piston Engines.

Two pistons travel in opposite directions. The combustion space is in the middle of the cylinder between the pistons. There are two crankshafts. The upper pistons drive one, the lower pistons the other. Each piston is single acting.

5. According to Piston Connection.

(a)    Trunk Piston Type.

The piston is connected directly to the upper end of the connecting rod. A horizontal pin (Gudgeon Pin) within piston is encircled by the upper end of the connecting rod. This construction is quite common, especially in small and medium size engines.

   

      (b) Cross Head Type.

The piston fastens to a vertical piston rod whose lower end is attached to a ‘cross head’, which slides up and down in guides. The crosshead carries a crosshead pin, which is encircled by the upper end of the connecting rod. This more complicated construction is common in double acting engines and large slow speed single acting engines.