In this article we will discuss about the IC engine:- 1. Notes on Internal Combustion Engine 2. Basic Operation of IC Engine 3. Classification 4. Construction 5. Supercharging 6. Firing Order 7. Methods of Starting IC Engines 8. Governing.
Contents:
- Notes on Internal Combustion Engine
- Basic Operation of IC Engine
- Classification of IC Engines
- Construction of IC Engines
- Supercharging of IC Engines
- Firing Order of IC Engine
- Methods of Starting IC Engines
- Governing of IC Engines
1. Notes on Internal Combustion Engine:
The Internal Combustion Engines (I.C. Engines) are the machines in which the combustion of fuel occurs and the generated heat is utilized for obtaining the work in the expansion device which is called as a prime mover.
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In case of the steam engine or turbine, the combustion of fuel occurs and the heat of the burned fuel is utilized to generate steam which acts as a medium to drive the steam engine or the steam turbine. These engines are called external combustion engines.
The internal combustion engines are the prime movers which develop power after combustion of the fuel and coverts heat into mechanical work either by direct combustion of fuel in the engine cylinder or combustion of the fuel outside the cylinder.
The work is developed by using the pressure and forces of the hot gases produced due to combustion of fuel and then the gases are discharged from the engine. The internal combustion engines convert heat energy into mechanical power.
The heat energy is generated by direct combustion of fuel in the engine cylinder and part of heat is converted to actual work. This uses slider crank mechanism to convert the heat into actual work.
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There are two types of internal combustion engines:
(1) Reciprocating engines and
(2) Rotary engines or gas turbine.
2. Basic Operation of IC Engine:
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The reciprocating internal combustion engines are mainly using slider crank mechanism for generation of power. The piston is reciprocating in the cylinder and it is connected to the connecting rod and crankshaft. The slider crank mechanism converts reciprocating motion into rotary motion.
The fuel is burned in the engine cylinder either by self-ignition or by external aid such as spark plug. The air and fuel mixture is supplied to the engine cylinder. The combustion of fuel occurs due to its reaction with the oxygen in the air. This produces hot gases in the engine cylinder which applies the force on the piston and the reciprocating motion of the piston is converted to rotary motion. Fig. 13-2 shows basis operation of I.C. engine.
3. Classification of IC Engines:
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There are no standard methods or ways of classifying I.C. Engines.
They may be classified in many ways such as following:
(1) Arrangement of Engine Cylinders:
(i) Horizontal engine
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(ii) Vertical engine
(iii) V engine
(iv) In-line engine
(v) Opposed cylinder engine
(vi) Opposed piston engine
(vii) Deltic engine
(viii) Y engine
(ix) Radial engine
(2) Working Cycle Employed:
(i) Four stroke cycle engine
(ii) Two stroke cycle engine
(3) Fuel Used:
(i) Petrol engine
(ii) Diesel engine
(iii) Gas engine
(iv) Bi-fuel engine
(4) Nature of Thermodynamic Cycle Used:
(i) Otto cycle engine
(ii) Diesel cycle engine
(iii) Dual combustion cycle engine.
(i) Low speed engine
(ii) Medium speed engine
(iii) High speed engine
(6) Method of Cooling:
(ii) Water cooled engine
(7) Field of Application:
(i) Stationary engine
(ii) Marine engine
(iii) Automobile engine
(iv) Motor-cycle engine
(v) Aero engine
(vi) Locomotive engine, etc.
(8) Method of Ignition:
(i) Compression ignition engine
(ii) Spark ignition engine.
4. Construction of IC Engines:
Fig. 13-15 shows the mechanical elements of the reciprocating I.C. engine. The piston which reciprocates in the cylinder is made a very close fit in the cylinder. In order to prevent the leakage of gas from one side of the piston to other side of it, piston rings are inserted in the circumferential grooves of the piston.
The cylinder is bored in the cylinder block may be bolted to the top of the crank case. The top of the cylinder is sealed down by bolting on to it the cylinder head. Generally gasket made of copper sheets and asbestos is inserted between the cylinder and cylinder head. The combustion space is provided in the top of cylinder head.
As the combustion takes place in the cylinder of an engine, it gets very hot. In order to preserve the engine materials and lubrication, the engine should be cooled. In fig. 13-16, the passages of cooling medium are shown cast in the walls of the cylinder block round the cylinder and also cast in the cylinder head round the combustion space.
The reciprocating motion of the piston is converted into the rotary motion of the crankshaft by means of connecting rod and crank. The connecting rod connects the piston and the crank, which is mounted on the crankshaft. The pin which connects the piston and the connecting rod is known as the gudgeon pin or piston pin.
The end of the connecting rod, which fits over the gudgeon pin, is called the small end of the connecting rod, while the other end which fits over the crank pin is called the big end of the connecting rod. Fig. 13-15 shows the sketch of I.C. engine.
The bottom of the engine is closed by means of a sump. This sump often contains the oil, which is pumped round the engine for lubrication.
A mechanical cycle for an internal combustion engine can be completed in one revolution (two stroke cycle) of the crankshaft or in two revolutions (four stroke cycle) of the crankshaft.
In an engine running on four stroke cycle principle, there are mechanically operated valves which control admission and exhaust to and from the engine cylinder. The opening and closing of the valves is controlled by means of cams which are fixed to the camshaft.
The camshaft is operated by means of a gear drive or chain drive from the crankshaft. The camshaft runs at half the speed of the crankshaft. The valves, which are known as poppet valves, are held down in closed position by means of valve springs.
In case of engine, working on two stroke cycle, the control of admission and exhaust is by means of ports, which are cut circumferentially in the cylinder walls. These ports are opened and closed by means of the moving piston. All these series of events form a complete cycle of events and these events take place in the same order in any cycle.
If all the events are completed in four strokes of the piston, then the engine is said to be working on Four Stroke Cycle principle. If on the other hand, these events are completed in two strokes of the piston then the engine is said to be working on Two Stroke Cycle principle. Fig. 13-6 shows the sketch of two stroke engine.
5. Supercharging of IC Engines:
Supercharging is the name given to the process of increasing the pressure of the air-fuel mixture entering a petrol engine to a pressure greater than that of atmospheric air and is achieved usually by means of a small compressor or blower called a supercharger. Superchargers also whirl the mixture into a more homogeneous state.
The power required to drive superchargers or blowers is taken from the engine through gears. Superchargers are used to increase the capacity of an existing engine or to bring the reduction in size of the engine for the same output or to restore the normal sea power of the engine when it is running at high altitudes.
It has been found that by supercharging the petrol engine to twice the atmospheric pressure the power is more than doubled and the same degree of supercharging in compression ignition engines increases their output by about 75%.
6. Firing Order of IC Engine:
The firing order of the engine is a sequence in which the multi-cylinder engines are fired. In the multi-cylinder engine, the engines are fired not in the normal sequence but they are fired in different sequences. The firing order is maintained for proper balancing of the engine and adjustment of the unbalanced forces. The firing order indicate the sequence in which the cylinders are fired. The number of cylinder is written in sequence.
The following table shows firing order of different engines:
7. Methods of Starting IC Engines:
Starting devices are commonly employed with compression ignition engines and especially with the engines fitted with turbulence combustion chambers. Petrol engines as well as gas engines and small units of low compression C.I. engines may be started by hand or by electric starters. Generally the instruction book supplied by the manufacturers must be carefully studied for the starting procedure.
With larger units in the low compression class of C.I. engines the following methods are used for starting:
(1) Blow lamp
(2) Electric heater in the combustion chamber
(3) Compressed air
(4) Special cartridge.
Fig. 13-91 shows the blow lamp method of starting. The blow lamp is applied to the hot bulb or uncooled part of the cylinder until the spot is hot enough to ignite the oil vapour. The engine is then barred over and when injection occurs the oil ignites and the engine commences to run.
The electric heater acts exactly as the blow lamp, but the heat is supplied by electrical means. A compression relief valve is also used with smaller units to facilitate the work of starting. The medium and high compression engines are invariably started by compressed air.
The air is used for a few cycles to store up energy in the flywheel and the oil is sprayed in. In multi-cylinder engines the cylinders are brought into action one or two at a time. For road vehicles with Diesel engines all cylinders but one are cut off and the engine is started by electric starter or hand. When the engine has run for a .few seconds the other cylinders are brought into action.
The compressed air may be provided by means of:
(i) Hand driven compressors for small units
(ii) Compressors driven by the engine itself
(iii) Separately driven compressors.
8. Governing of IC Engines:
(1) In petrol engines control is accomplished by means of a throttle valve which is placed in the intake manifold immediately following the carburetor. The amount of opening of the throttle valve determines how much mixture may enter the cylinder. In automobiles the throttle valve is operated by foot control or accelerator pedal.
The mean effective pressure in the cylinder will change and consequently the power developed in the engine cylinder. The position of the throttle valve may be controlled by the speed governor of the centrifugal type. For certain applications the throttle may be positioned as a function of load while the speed is held constant.
(2) In Diesel engines the fuel flow is regulated by centrifugal governor which actuates link rods which operate some device on the fuel pump for by-passing a portion of the fuel which would otherwise be injected into the engine cylinder. In plunger type injection pump, the governor changes the relative angular position of the plunger as shown in fig. 13-94. The governor operates on the rack.
Since the Diesel engine does not rely on fixed proportions of air and fuel for combustion, any fuel ratio may be used upto a point where the oxygen is all consumed.
(3) In gas engines there are three methods of governing-
(i) Hit and miss method of governing
(ii) Quality governing
(iii) Quantity governing.
(i) Hit and Miss Method of Governing:
Fig. 13-93 shows the arrangement for hit and miss type of governing.
In this arrangement when the speed rises above the permissible value, the governor sleeve rises and the lever attached to the sleeve lifts the pecker block so that the gas valve does not open; as a result only air is taken into the engine cylinder.
Thus for this cycle no power is developed in the engine cylinder due to the complete absence of the fuel and as a result the speed falls. The speed continues to decrease till the pecker block falls back into the position. Thus we see that explosions are missed intermittently, but every charge is of normal strength.
This is a simple way of governing gas engines but is not much used on other engines because this method has the disadvantage of having a comparatively large variation in speed owing extra scavenging which takes place immediately after the missed explosions. This is an efficient method from the point of view of economy in gas consumption.
(ii) Quality Governing:
In this method the strength of the mixture is changed. When the speed is high the air-fuel ratio is increased. This results in a lower pressure and hence a reduction of speed takes place.
Ignition is not always satisfactory with this type of governing. It is impossible to find the most suitable point for all mixture strengths. The thermal efficiency is also reduced.
(iii) Quantity Governing:
In this method the mixture strength remains the same, but the quantity of mixture admitted to the cylinder for compression. The compression ratio is unaltered. The air standard efficiency remains the same. There is less pressure created by the combustion of the fuel, less work is done during that cycle and consequently there is a reduction in speed.
The governor gives the admission valve a short, as a result it will open by a small amount and hence a small quantity of air-gas mixture will enter the engine cylinder. This is the light load condition. If the admission valve has a large stroke, then it will open a large amount and hence a large quantity of air gas mixture will enter the engine cylinder. This is the heavy load condition.
Since in this method there is an impulse in each cycle due to the explosion of a correct mixture, it has the advantage of giving a more even turning moment and closer limits of speed variation. This method of governing is preferred for spark ignition engines.
