In this article we will discuss about:- 1. Classification of IC Engines 2. Working of an IC Engine 3. Parts 4. Combustion 5. Performance.
Internal combustion engines or IC engines are extensively used in automobiles, locomotives, marine application, power generation etc. Here the working media is hot and high pressure products of combustion of air and gasoline/diesel fuel. The combustion occurs internally within a cylinder and hence the name. The hot high pressure gases push down a piston in the cylinder producing power.
- Classification of IC Engines
- Working of an IC Engine
- Parts of Internal Combustion (IC) Engine
- Combustion in IC engines
- Performance of Internal Combustion (IC) Engine
1. Classification of IC Engines:
IC engines can be classified based on:
(a) Type of fuel used—petrol, diesel, gas
(b) Type of mechanical cycle—2 stroke, 4 stroke
(c) Type of thermodynamic cycle used. Otto cycle (for gasoline engine and gas engine), diesel, dual cycle (for diesel engines)
(d) Type of ignition used. Spark ignition or SI engines/Compression Ignition or CI engines
(e) According to inlet conditions. Naturally aspirated or supercharged
(f) Type of cooling—air cooled, water cooled
(g) Number of cylinders—Single cylinder or multi cylinder
(h) According to axis of cylinders—Horizontal, Vertical or Radial
(i) According to fuel injection in Diesel engine—Air injection or solid injection.
(j) According to use—Automobile engines, Marine engines, Locomotive engines, Land engines.
The working of a 4-stroke SI engine is shown in Fig. 24.2. The cycle is completed in four strokes of the piston. The working has been summarized in the table given below. The position when the piston is at the topmost position is called top dead center position (TDC)/IDC inner dead centre in case of horizontal engine. During one cycle there are four strokes and the crank moves through two revolutions.
3. Parts of Internal Combustion (IC) Engine:
Main parts of the internal combustion engine are:
It is the main body of the engine. It has finely machined bore in which piston with rings closely fits and is able to reciprocate. In the cylinder combustion takes place. It should be able to withstand high temperature and pressure. It is necessary to cool the engine to avoid damage.
2. Cylinder Head:
The cylinder head closes the top side of the cylinder. It has inlet and exhaust valves and the spark plug to initiate combustion, in case of petrol engine. In case of diesel engines it has fuel injector instead of the spark plug.
The piston is able to reciprocate i.e., move up and down in the cylinder. On the upward stroke i.e., while raising towards the cylinder head it compresses the charge of air/fuel. Also during and after combustion it pushes down the piston. For sealing purpose there are piston rings on the piston.
4. Connecting Rod:
The movement/force on the piston is passed to the crankshaft through the connecting rod that is attached to the piston by piston pin or gudgeon pin. The end of the connecting rod connecting the piston side is called the small end of the connecting rod.
5. Crank Shaft:
The big end of the connecting rod is attached to the crankshaft through crank that converts the reciprocating motion of the piston to rotary motion.
The big end of the connecting rod is connected to the crank by means of a crank pin while the other end is connected to the crank shaft. Cranks may be centre crank in between two bearings or over hung crank on one side of the bearing.
7. Inlet and Exhaust Valve:
These allow fresh charge and let out spent gases respectively at proper times in the cycle.
It stores energy during power stroke and when required distributes energy during other strokes i.e. suction, compression and exhaust.
In case of gasoline engines—either carburettor or Multi Point Fuel Injection System (MPFI) to supply correct air fuel ratio.
10. Cam Shaft:
It is connected to the crank shaft and runs at half the speed of the crank shaft.
11. Cam Follower, Push Rods and Rocker Arm:
The valves are operated by means of these mechanisms.
4. Combustion in IC engines:
Combustion is relatively fast exothermic chemical reaction of carbon and hydrogen in the fuel with the oxygen present in the air producing heat energy. When fuel and oxygen react chemically and the heat evolved is sufficient to sustain the reaction—combustion is said to take place. Combustion will sustain if heat released is sufficient and reaction rates are high. However combustion is a complex phenomenon requiring deep understanding.
The energy of combustion is used for operating the internal combustion engines.
Basic conditions necessary for combustion in IC engine are:
(a) The presence of a combustible mixture
(b) Means of initiating combustion
(c) Stabilisation and propagation of flame in the combustion chamber.
The combustible elements in fuels are generally carbon and hydrogen with small amount of sulphur and other additives. Both the SI and CI engines are designed to meet the above given requirements. In IC engine, combustion generally takes place when the fuel is in gaseous state. It is necessary to have equipment in IC engine to achieve the gaseous state e.g. in SI engine we have a carburettor or multi-point fuel injection (MPFI) system.
Gasoline that may contain more than 40 constituents can be approximated to Octane fuel (C8H18) and its combustion in the presence oxygen in the air is given as-
Thus for 114 kg petrol we need 400 kg of oxygen for complete reaction.
Or for 1 kg of petrol O2 required is 400/114 kg
And air having 23.2% O2 by weight would be = 400/114 x 23.3 = 15.12
The ratio of Fuel to Air of 1:15.12 is called Stoichiometric ratio (stoichiometric ratio is chemically correct ratio of reactants required for complete reaction)
Thus for complete combustion of 1 kg of gasoline 15.12 kg air having 23.2% O2 proportion by weight is required. This can be easily found by substituting the molecular weights in the above equation as shown above.
In IC engines the ignition of hydrocarbon fuels is possible within certain range of air fuel mixtures. The flame propagates in IC engines if the temperature of burnt gases exceeds about 1200° C. Such temperatures are possible within Fuel to Air mixture ratios of 1 : 7 to 1 : 30. However practical limits of combustion are between 1 : 9 and 1: 21 for SI engines using carburettor. However chemically correct mixture of fuel to air ratio is around 1 : 15 i.e., for complete combustion of 1 kg of fuel about 15 kg of air is required.
5. Performance of Internal Combustion (IC) Engine:
These basic performance parameters are:
1. Power and Mechanical Efficiency:
The power obtained from an engine is most frequently called brake power (BP). Sometimes this is also called shaft power or merely delivered power. The total power actually developed on the pistons in the engine is called indicated power (IP).
A part of the indicated power developed by burning fuel and air does not appear as brake power but it is used in overcoming friction of the bearings, piston and other mechanical parts of the engine and also in indication of the fuel-air charge and delivering the exhaust gases. The power to perform these tasks is called the friction power (FP). These losses are sometimes called Mechanical Losses. The brake power is less than the indicated power by the amount of friction power consumed in the engine.
∴ IP = BP + Mechanical losses
= BP + FP
The friction power is difficult to determine experimentally because there is no direct method of measurement and there are variations under operating and test conditions for the engine.
The ratio of the power delivered by the engine (BP) to the total power developed within the engine (IP) is known the Mechanical Efficiency (ηm)-
2. Mean Effective Pressure and Torque:
We have already defined mean effective pressure and in general it is defined as that constant pressure when acting on the piston throughout the stroke, produces the work equal to that of the cycle.
Depending on the work considered, mean effective pressure will be called either Indicated Mean Effective Pressure – IMEP –(pmi) or Brake Mean Effective Pressure – BMEP – (pmb)
If A = Area of the piston in m2
L = Length of stroke in m
N = Speed of the engine rpm
IP = Indicated Power kW.
Then Pmi = Indicated mean effective pressure (bar).
The parameter – mean effective pressure – shows how well the engine is using its size (displacement) to produce work and is generally used for comparative purpose.
On the contrary torque cannot be used for comparative purpose as it will be higher for high power engines. Therefore, the main objective in design of the engine is that the mean effective pressure is higher.
3. Volumetric Efficiency:
We have a definition of volumetric efficiency of an engine as ratio of the actual weight of air inducted by the engine on the intake stroke to the theoretical weight of air that should have been inducted by filling the piston displacement volume with air at intake pressure and temperature.
In supercharge engines this efficiency is greater than one.
Where ma = Actual weight of air
mt = Theoretical weight of air
4. Air – Fuel Ratio:
This ratio shows the relative proportions of air and fuel in the combustion chamber.
5. Specific Fuel Consumption – Thermal Efficiency:
Let mf = Weight of fuel used in time t min.
Therefore, Specific fuel consumption is defined as the fuel consumption per hour per unit power. Its units will be kg/kW/Hour or kg/kWH. Depending on power under consideration this specific fuel consumption is Brake Specific Fuel Consumption – BSFC or Indicated Specific Fuel Consumption – ISFC.
This is the widely used (fuel consumption) parameter for comparative purposes.
As a revision, thermal efficiency is defined as the ratio of the output to the heat supplied by the burning of the fuel in cylinder. It gives us the fraction of the heat supplied which is converted into work. This is based on work developed i.e. indicated power (IP) or the shaft work available at the engine shaft (BP). Accordingly, the thermal efficiency may be either Indicated Thermal Efficiency or Brake Thermal Efficiency.
6. Heat Balance:
This gives us the distribution of heat supplied in the various ways and thus an indication of the working of the engine. General distribution of heat supplied in IC Engine.
7. Specific Power Output and Specific Weight:
Specific power output is defined by the ratio of the power produced by the engine to the displacement volume of the engine or it is the power produced per unit displacement of the piston.
Thus specific output depends on pm (bmep) and speed of the engine. By increasing the speed of the engine we can more work output but in that case the mechanical losses will be increased. In addition, increase in speed results in increase in mechanical stresses of the various engine parts. Similarly by increasing bmep, which requires better heat release and more load on engine, power output can be increased.
Specific weight is defined as the weight of the engine in kg to produce unit shaft power or BP developed. This gives the idea of the engine weight. This parameter plays an important role in the applications such as engine used in Automobiles and Aircrafts.
8. Exhaust Smoke and Emissions:
Smoke indicates that the combustion of the fuel is incomplete and naturally it limits the power output.
Exhaust emissions are oxides of nitrogen (NOx), unburnt hydrocarbons etc. and are the nuisance to the public and environment in general.
Exhaust emissions are, nowadays, considered as the engine performance parameters.