In this article we will discuss about the compression ignition engines:- 1. Introduction to Compression Ignition Engines 2. Operation of C.I Engine 3. Fuel Pump and Injector 4. Normal Combustion 5. Combustion Chambers.
Contents:
- Introduction to Compression Ignition Engines
- Operation of C.I Engine
- Fuel Pump and Injector Used in C.I Engines
- Normal Combustion in C.I. Engine
- Combustion Chambers in C.I Engine
1. Introduction to Compression Ignition Engines:
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In these engines, ignition is performing by means of compression of the air to a very high temperature. The compression ignition engine utilizes high pressure fuel injection pump to inject the fuel inside the engine cylinder. The air is supplied to the engine cylinder during the suction stroke and this air is compressed to a high temperature due to the backward motion of piston.
The C.I. engine uses the self-ignition of fuel at very high temperature and pressure. The injection mechanism is activated and the fuel in injected inside the engine cylinder. The high pressure in the injection pump result in an automization of fuel in fine droplet form and the droplets of spread over the complete engine cylinder. The droplets of fuel are vaporised because of the high temperature and self-ignite, which result in the generation of high flame in the engine cylinder which burns the remaining part of fuel.
The oil engine is very often referred to as the Diesel engine after Rudolph Diesel. The basic mechanical elements of the oil engine are the same as those of the petrol engine. These engines can be either two stroke or four stroke like petrol engines, but the method of fuel introduction and its ignition are different from the petrol engines.
In oil engines, the high compression ratio is employed, as a result the temperature of the compressed air is very high and when the fuel is sprayed into this high temperature air, the fuel will begin to burn. These engines are called Compression Ignition (C.I.) engines. It will be noted that there is no ignition system similar to that used in the petrol engine.
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Thus the oil engine requires a fuel pump which will pump the fuel to a high pressure. Secondly, an injector nozzle is required protruding into the engine combustion space. The fuel is sprayed through this nozzle at the correct moment by the pump into engine cylinder in form of mist.
(1) The fuel injection is either by means of compressed air or by means of mechanical means.
(2) The combustion is regulated so that there is no appreciable rise in pressure during injection.
(3) There is no danger of pre-ignition as only air is compressed during the compression stroke.
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2. Operation of C.I Engine:
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The operation of the engine can be explained as below:
The air is admitted in the engine cylinder during suction from the inlet manifold. The air is compressed to a very high temperature and pressure during the compression stroke. At the end of the compression stroke, before piston reaches to top dead centre position the fuel is injected in the engine cylinder through the injection mechanism.
The injection mechanism consists of fuel injection pump and injector. The fuel injection pump compresses fuel to a very high pressure and supplies it to the injector at the given moment i.e at the end of the compression stroke before the TDC. The injector atomises the fuel into fine droplet form through the orifices of the injector. The atomised fuel droplet enter the engine cylinder at high speed and interact with the hot air.
Fig. 13-32 shows the view of C.I. engine. The fuel droplets are vaporized due heat transfer from hot air and turbulence is created in the engine cylinder which mixes the air and fuel droplets. Because of the compression stroke the pressure and temperature rises in the engine cylinder, this results into self-ignition of the fuel.
The self-ignition of the first set of fuel admitted in the engine cylinder rises the temperature of air and generates the flame which burns rest of fuel in the cylinder during the expansion stroke.
The compression ignition engine has the advantage of being able to burn the comparatively low cost fuel oils which are extracted from crude oil after the lower boiling range petrol and kerosene have been removed. At the same time some large modern engines can achieve brake thermal efficiencies as high as 42%. The Diesel engine has therefore, one of the higher thermal efficiencies of all power cycles that are commercially available.
There are many other criteria besides thermal efficiency that are used to evaluate the performance and suitability of an engine. For instance, a high specific power based on engine weight would be desirable in transport application, but this index of performance is really not too satisfactory since it is a function of engine speed. Increasing the engine speed to increase the power has the detrimental effect of increasing mechanical friction and inertia loads.
Fig. 13-33 shows the working of four strokes C.I. engine. A more meaningful index of performance which is frequently used to compare engine outputs is the Mean Effective Pressure (M.E.P.) which is defined as the work performed during the cycle divided by the piston displacement volume.
The brake means effective pressure of Diesel engines are currently running from 5 to 18.6 bar and have exceeded 27.6 bar experimentally. Diesel engines are used in a broad range of heavy duty applications because of its high efficiency and reliability.
These range from 30 MW versions for power generation and marine applications to 3 MW Diesel-Electric locomotives, which dramatically increased the thermal efficiency of locomotives from 10% to 35% when the conversion from steam was made.
Diesels are also used in a few low weight applications, including some automobiles. The smaller engines generally use engines employing Otto cycle because it results in a lighter and less expensive engine than the Diesel engines.
Fig. 13-34 shows the sketch of two stroke C.I. engines, which similar to two stroke S.I. engine except the difference that it has fuel injection system rather than ignition system. The air is admitted on the engine cylinder.
3. Fuel Pump and Injector Used in C.I Engines:
Fig. 13-35 and fig. 13-36 explain the construction and working of a fuel pump. Such types of fuel pumps are commonly employed in majority of C.I. engines.
The pump unit consists of a barrel and a plunger. The plunger is raised by a cam and returned by a spring. The plunger fits so tightly that it seals off without a gasket even at high pressures and at low engine speeds. On the downward stroke of the plunger the fuel enters from the suction chamber and fills the barrel.
With the upward stroke of the plunger the fuel is compressed until the top edge of the cylindrical part of the plunger covers the ports leading to the suction chamber, where upon the fuel is forced out through the delivery valve and delivered to the feed pipe for the injection nozzle. This continues until the bottom edge of the cylindrical part of the plunger, which is connected to the top part by a groove, uncovers the ports and so restores the connection to the suction chamber.
The amount of fuel is adjusted to suit the working requirements by means of a governor, which changes the angular position of the plunger in the barrel. The bottom edge of the cylindrical part of the plunger is helical, so the ports leading to the suction chamber are uncovered earlier or later according to how much the plunger has been rotated.
Fig. 13-36(a) shows the position of plunger in the barrel when no fuel is injected. Fig. 13-36(b) shows the beginning of the delivery of the oil to the injector and fig. 13-36(c) shows the end of the effective stroke of the pump. The position of the plunger in the barrel is controlled by means of a rack which is operated by the governor or regulator of the engine when the load changes. As long as the load on the engine remains constant, the position of the plunger in the barrel remains the same.
The atomizer or injector of the engine is an important part. The fuel is pumped by the fuel pump in short impulses along the fuel pipe into the injection nozzle, which sprays the fuel into the combustion chamber in mist form. Fig. 13-37 shows the multi-orifice nozzle of the atomizer. The needle valve spindle of the injector is spring loaded at the top with a definite pressure.
When the pressure exerted by the fuel overcomes the spring load, the needle valve is lifted off its seat and through the orifices the oil is injected into the combustion chamber in mist form. Fig. 13-37 shows the multi-orifice nozzle of the atomizer. The needle valve spindle of the injector is spring loaded at the top with a definite pressure.
When the pressure exerted by the fuel overcomes the spring load, the needle valve is lifted off its seat and through the orifices the oil is injected into the combustion chamber in the atomised state. The lift of the valve is restricted to a definite amount.
The air taken by the engine always contains impurities and abrasive foreign bodies, which cause premature wear and tear on the cylinders and pistons. Therefore the intake pipe is fitted with air filter.
Diesel engines are generally water cooled. The thermostat maintains the desired temperature around the engine block before the water is allowed to pass through the radiator – a circulation by-pass permitting flow through the engine when the thermostat is closed.
The minimum temperature gradient is desirable across the cylinder block. The flow of water must be so controlled that the temperatures will be high enough throughout the engine system to prevent cylinder wear from low temperature corrosion.
The Diesel engine is of a heavier construction due to higher compression ratio and hence higher pressures are associated with the oil engine operation. The oil engine cylinders are longer and hence the cylinder block is also longer because of this higher compression.
4. Normal Combustion in C.I. Engine:
The combustion in C.I. engine is mainly occurring due to self-ignition fuel supplied in the engine cylinder. In the C.I. engine, the air is admitted in the engine cylinder through the inlet manifold. The air is admitted during the suction stroke and the air is compressed to a very high temperature during the compression process.
At the end of the compression stroke and some degrees before TDC the injection mechanism is activated and the fuel is injected in the engine cylinder through the orifice of the injector. The temperature of air during compression is very high and the fuel droplets in the atomised form are injected in the hot air and mixes with air. The fuel is injected in the engine cylinder at the end of the compression stroke for certain given duration.
The first charge of fuel droplets injection in the engine cylinder is vaporized and mixes with air. The fuel droplets ignite due to self-ignition. Then the flame is generated and the subsequent fuel injected in the engine cylinder also self-ignite. The rate of injection control the rate of combustion of fuel.
The injection of the fuel is stopped after certain duration of injection. The remaining part of the fuel burns during the expansion stroke. The variation of pressure versus crank angle can be plotted. This is shown in Fig. 13-88 The P-θ diagram drawn without combustion of fuel is overlapped over the P-θ diagram with combustion of fuel.
The combustion of fuel starts few degrees after the injection process which is known as ignition delay period. The ignition delay period is the period required for vaporization of fuel droplets, mixing of fuel with air and chemical reaction. The point A shows the moment of injection of fuel and the point B shows the start of combustion process.
The combustion process start some degrees before TDC at the end of the compression stroke and the pressure rises to maximum just 2-3° degrees after TDC. The pressure starts reducing during the rest of the expansion stroke.
The combustion process can be divided into four phases:
(1) Ignition delay period
(2) Uncontrolled combustion phase
(3) Controlled combustion
(4) After burning phase.
Fig. 13-88 shows the four phases of normal combustion in C.I. engine. The point of injection of the fuel is around 20-30° before TDC during the compression stroke. The fuel is injected in the engine cylinder for a period 60-100°.
The first phase is ignition delay period of fuel which is the delay in the ignition of fuel after the injection of first charge in the engine cylinder. This is the time required for vaporization of fuel droplet after they are injected in the hot air and mixing of fuel with air. The time is required for chemical reaction to take place. The lower ignition delay period indicates better quality of fuel.
The second phase is the uncontrolled combustion or rapid combustion phase in which the point at which the combustion starts to the point of sudden rise of pressure. The first charge of fuel ignites in the engine cylinder due to self-ignition of fuel and generates a flame.
The fuel injected in the engine cylinder during the ignition delay period remains accumulated in the engine cylinder till the first charge burns. Then due to rise in temperature due to combustion of fuel, the accumulated fuel also ignites at faster rate. This results into sudden rise of pressure in the cylinder at the end of compression stroke.
During the third phase, which is controlled combustion phase the fuel ignites and burn as soon as it is injected in the engine cylinder. Therefore the pressure rise is proportional to the rate of combustion of fuel.
The last phase is after burning phase in which the combustion of fuel occurs during the expansion stroke after the injection of fuel is stopped. The remaining unburned fuel burns during rest of the expansion process.
5. Combustion Chambers in C.I Engine:
It is the space in which the combustion of fuel occurs and becomes the part of the clearance volume of the engine cylinder. In this space the fuel is injected in the engine cylinder and the major part of the combustion process occurs in the combustion chamber.
There are four types of combustion chamber used in the diesel engine:
(1) Piston Crown:
The piston crown can have the combustion chamber which is grooved in the crown of the piston. The piston top surface has the groove to act like a combustion chamber and the fuel injected from the injector is sprayed over the piston crown surface.
The combustion of injected fuel occurs at the combustion chamber. The first charge of fuel burns in the combustion chamber and then rest of the fuel burns in the remaining part of the cylinder. This type of combustion chamber increases the weight of piston. Fig. 13-38 shows piston crown type combustion chamber.
This is the combustion chamber in the cylinder head. The combustion chamber has been grooved in the cylinder head surface.
The injector is placed in the combustion chamber space and it is known as open type of combustion chamber. The fuel from the fuel injector is injected into the combustion chamber and it is sprayed over the combustion chamber. The fuel in form of droplet gets vaporized when it comes in contact with the hot surface.
Then it self-ignite in the combustion chamber. This occurs when the piston is at the TDC position. The combustion of first charge of fuel results into creation of high pressure gases and which in turn creates a turbulence. Fig. 13-39 shows the combustion chamber in cylinder head type system.
(3) Pre-Combustion Chamber:
The pre-combustion chamber is small combustion chamber attached to the cylinder head for major part of the combustion of fuel. The fuel is burned in the pre-combustion chamber because the fuel injector is placed in the pre-combustion chamber. The fuel is injected in the pre-combustion chamber at the end of compression stroke before TDC.
The fuel injected in the pre-combustion chamber is sprayed over the small combustion chamber and get vaporised. The fuel is self-ignited and large part of fuel burns into the pre-combustion chamber resulting into generation of very high pressure, which results into escape of gases from the pre-combustion chamber to rest of the engine cylinder.
The hot gases are forced out of the pre-combustion chamber and move to the cylinder and piston, this results into generation of very high pressure and temperature in the cylinder. The turbulence created by flow of high pressure gases results into mixing of air and fuel particles and rest of the fuel burns at very high speed. Fig. 13-40 shows the pre-combustion chamber.
(4) Energy Cell:
The energy cell type is the new construction in which the diesel engines have a cell connected to the cylinder head. The cell consists of major and minor cell. The fuel in injected into the major cell where the injector is installed and the first charge fuel gets vaporized and burns in the major cell and the fuel accumulated during the combustion process gets burned when the gases along with air fuel mixture comes out of the major cell and goes to minor cell. Fig. 13-41 shows energy cell type combustion chamber.
The turbulence is very high which results into combustion of the fuel. The energy cell also large part of fuel to be burned into the cell and then rest of fuel burns in the cylinder.