IC Engines

IC Engines - Introduction

IC Engines Introduction. The engines in which the combustion of fuel takes place inside the engine cylinder are called internal combustion engines (briefly written as IC engines). The working pressure and temperature inside the cylinder of an IC engine is very high. The efficiency of IC engines is about 35-40 percent.

Sequence of Operations in IC Engine

Each stroke in IC engines forms a sequence of operations in one cycle of IC Engines i.e suction stroke, compression stroke, expansion stroke, and exhaust stroke.

Strictly speaking, when an engine is working continuously, we may consider a cycle starting from any stroke. We know that when the engine returns back to the stroke where it started, we say that one cycle has completed. The following sequence of operation in a cycle is widely used.
  1. Suction stroke. In this stroke, the fuel vapor in correct proportion, is supplied to the engine cylinder.
  2. Compression stroke. In this stroke, the fuel vapor is compressed in the engine cylinder.
  3. Expansion or working stroke. In this stroke, the fuel vapor is fired just before the compression is complete. It results in the sudden rise of pressure, due to expansion of the combustion products in the engine cylinder. This sudden rise of pressure pushes the piston with a great force and rotates the crankshaft. The crankshaft, in turn, drives the machine connected to it.
  4. Exhaust stroke. In this stroke, the burnt gases (or combustion products) are exhausted from the engine cylinder, so as to make space available for the fresh fuel vapor.

Two Stroke vs Four Stroke Engines

Two Stroke vs Four Stroke Engines - Advantages and Disadvantages of Two Stroke over Four Stroke Cycle Engines.

In a two stroke engine, the working cycle is completed in two strokes of the piston or one revolution of the crankshaft. In a four stroke engine, the working cycle is completed in four strokes of the piston or two revolutions of the crankshaft.

The following are the advantages and disadvantages of two stroke over four stroke cycle engines :

Advantages
  1. A two stroke cycle engine gives twice the number of power strokes than the four stroke cycle engine at the same engine speed. Theoretically, a two stroke cycle engine should develop twice the power as that of a four stroke cycle engine.
  2. For the same power developed, a two stroke cycle engine is lighter, less bulky and occupies less floor area.
  3. A two stroke cycle engine has a lighter flywheel and gives higher mechanical efficiency than a tour stroke cycle engine.
Disadvantages 
  1. The thermal efficiency of a two stroke cycle engine is less than that of a four stroke cycle engine, because a two stroke engine has less compression ratio than that of a four stroke cycle engine.
  2. The overall efficiency of a two stroke cycle engine is also less than that of a four stroke cycle engine.
  3. The consumption of lubricating oil is large in a two stroke cycle engine because of high operating temperature.

Valve Timing Diagram of Petrol Engine

Valve Timing Diagram for a Four Stroke Cycle Petrol Engine - The petrol engines are also known as spark ignition engines. The valve timing diagram for a four stroke cycle petrol engine is shown in Figure below:


 
The following particulars are important for a four stroke cycle petrol engine regarding valve timing diagram :

(a) The inlet valve opens (IVO) at 10° — 20° before top dead center (TDC) and closes 30° — 40° after bottom dead center (BDC).
(b) The compression of charge starts at 30° — 40° after BDC and ends at 20° — 30° before TDC.
(c) The ignition (IGN) of charge takes place at 20°— 30° before TDC.
(d) The expansion starts at 20° — 30° before TDC and ends at 30° — 50° before BDC.
(e) The exhaust valve opens (EVO) at 30° — 50° before BDC and closes at 10° —15° after TDC.

Notes:

(i) The inlet valve of a four stroke I. C. engine remains open for 230°.
(ii) The charge is compressed when both the valves (i.e. inlet valve and exhaust valve) are closed.
(iii) The charge is ignited with the help of a spark plug.
(iv) The pressure inside the engine cylinder is above the atmospheric pressure during the exhaust stroke.

Valve Timing Diagram of Diesel Engine

Valve Timing Diagram for a Four Stroke Cycle Diesel Engine - The diesel engines are also known as compression ignition engines. The valve timing diagram for a four stroke cycle diesel engine is shown in Figure below:


The following particulars are important for a four stroke cycle diesel engine regarding valve timing diagram:

(a) The inlet valve opens at 10° — 20° before TDC and closes at 25° — 40° after BDC.

(b) The fuel valve opens at 10° — 15° before TDC and closes at 15°— 20° after TDC.

(c) The compression starts at 25° — 40° after BDC and ends at 10°— 15° before TDC.

(d) The expansion starts at 10° — 15° after TDC and ends at 30° — 50° before BDC.

(e) The exhaust valve opens at 30° — 50° before BDC and closes at 10° —15° after TDC.

Note: In diesel engines, the fuel is injected in the form of very fine spray into the engine cylinder, which gets ignited due to high temperature of the compressed air.

Comparison of Petrol and Diesel Engines

Petrol vs Diesel engines: A comparison - Basic difference between a petrol engine and a diesel engine based on working, pressures, combustion, compression ratios, speed, efficiency, maintenance, and running costs. The following points are important for the comparison of petrol and diesel engines:

SN
Petrol Engines
Diesel Engines
1.
A petrol engine draws a mixture of petrol and air during suction stroke.
A diesel engine draws only air during suction stroke.
2.
The carburetor is employed to mix air and petrol in the required proportion and to supply it to the engine during suction stroke.
The injector or atomiser is employed to inject the fuel at the end of compression stroke.
3.
The pressure at the end of compression is about 10 bar.
The pressure at the end of compression is about 35 bar.
4.
The charge (i.e. petrol and air mixture) is ignited with the help of a spark plug.
The fuel is injected in the form of fine spray. The temperature of the compressed air (about 600° C at a pressure of about 35 bar) is sufficiently high to ignite the fuel.
5.
The combustion of fuel takes place approximately at constant volume. In other words, it works on Otto cycle.
The combustion of fuel takes place approximately at constant pressure. In other words, it works on Diesel cycle.
6.
A petrol engine has compression ratio approximately from 6 to 10.
A diesel engine has compression ratio approximately from 15 to 25.
7.
The starting is easy due to low compression ration.
The starting is little difficult due to high compression ratio.
8.
As the compression ratio is low, the petrol engines are lighter and cheaper.
As the compression ratio is high, the diesel engines are heavier and costlier.
9.
The running cost of petrol engines is high because of higher cost of petrol.
The running cost of diesel engines is low because of the lower cost of diesel.
10.
The maintenance cost is less.
The maintenance cost is more.
11.
The thermal efficiency is upto about 26%.
The thermal efficiency is upto about 40%.
12.
Overheating trouble is more due to low thermal efficiency.
Overheating trouble is less due to high thermal efficiency.
13.
These are high speed engines.
These are relatively low speed engines.
14.
The petrol engines are generally employed in light duty vehicles such as scooters, motorcycles, cars. These are also used in aeroplanes.
The diesel engines are generally employed in heavy duty vehicles such as buses, trucks and earth moving machines etc.

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