COMBUSTION AND COMBUSTION CHAMBERS
Introduction (Combustion in S.I Engines)
Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in the fuel with the oxygen in the air resulting in liberation of energy in the form of heat. Combustion is a very complicated phenomenon and has-been a subject of intensive research for many years.
Stageso of Combustion in SI Engine
In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air and residual gases is ignited by a single intense and high temperature spark between the spark plug electrodes (at the moment of discharge the temperature of electrodes exceeds 10,000°C), leaving behind a thin thread of flame. From this thin thread combustion spreads to the envelop of mixture immediately surrounding it at a rate which depends primarily upon the temperature of the flame front itself and to a secondary degree, upon both the temperature and the density of the surrounding envelope. In this manner there grows up, gradually at first, a small hollow nucleus of flame, much in the manner of a soap bubble. If the contents of the cylinder were at rest, this flame bubble would expand with steadily increasing speed until extended throughout the whole mass. In the actual engine cylinder, however, the mixture is not at rest. It is, in fact, in a highly turbulent condition the turbulence breaks the filament of flame into a ragged front, thus presenting a far greater surface area from which heat is radiated; hence its advance is speeded up enormously. The rate at which the flame front travels is dependent primarily on the degree of turbulence, but its general direction of/movement, that of radiating outward from the ignition point, is not much affected. According to Ricardo the combustion can be imagined as if developing in two stages, one the growth and development of a semi propagating nucleus of flame called ignition lag or preparation phase, and the other, the spread of the flame throughout the combustion chamber [see Fig. 9].
Figure: 9. Stages of combustion in SI engine
The former is a chemical process depending upon the nature of the fuel, upon temperature and pressure, the proportion of the exhaust gas, and also upon the temperature coefficient of the fuel, that is, the relationship between temperature and rate of acceleration of oxidation or burning. The second stage is a mechanical one pure and simple. The two stages are not entirely distinct, since the nature and velocity of combustion change gradually. The starting point of the second stage is where first measurable rise of pressure can be seen on the indicator diagram, i.e., the point where the line of combustion departs from the compression line. In Fig. 14.2(b), A shows the point of passage of spark - (say 28° before TDC), B the point at which the first rise of pressure can be detected (say, 8°before TDC) and C the attainment of peak pressure. Thus AB represents the first stage (about 20° crank angle rotation) and BC the second stage. Although the point C makes the completion of the flame travel, it does not follow that at this point the whole of the heat of the fuel has been liberated, for even after the passage of the flame, some further chemical adjustments due to reassociation, etc., and what is generally referred to as after burning, will to a greater or less degree continue throughout the expansion stroke. The first stage AB, by analogy with diesel engines is called ignition lag, which label is wrong in principle. In spark ignition there is practically no ignition lag and a nucleus of combustion arises instantaneously near the spark plug electrodes. But during the initial period flame front spreads very slowly and the fraction of burnt mixture is small so that an increase of pressure cannot be detected on the indicator diagram. The increase of pressure maybe just one per cent of maximum combustion pressure corresponding to burning of about 1.5per cent of the working mixture, and the volume occupied by the combustion products may be about 5 per cent of the combustion chamber space.
The stage II is the main stage of combustion. The end of second stage is taken as the moment at which maximum pressure is reached in the indicator diagram (see Fig. 9). However, combustion does not terminate at this point and after burning continues for a rather long time near the walls and behind the turbulent flame front. The combustion rate in the stage III reduces, due to surface of the flame front becoming smaller and reduction in turbulence. About 10 per cent or more of heat is evolved in the after-burning stage and hence the temperature of the gases continues to increase to point D in Fig.9. However, the pressure reduces because the decrease in pressure due to expansion of gases and transfer of heat to walls is more than the increase in pressure due to combustion.
Effect of Engine Variables on Flame Propagation
A study of the variables which affect the flame propagation velocity is important because the flame velocity influences the rate of pressure rise in the cylinder, and has bearing or certain types of abnormal combustion.
There are several factors which affect the flame speed, the most important being fuel-air ratio and turbulence.
1. Fuel-air ratio. The composition of the working mixture influences the rate of combustion and the amount of heat evolved. With hydrocarbon fuels the maximum flame velocities occur when mixture strength is 110% of stoichiometric (i.e., about 10% richer than stoichiometric). When the mixture is made leaner or is enriched and still more, the velocity of flame diminishes. Lean mixtures release less thermal energy resulting in lower flame temperature and flame speed. Very rich mixtures have incomplete combustion (some carbon only burns to CO and not to CO2) that results in production of less thermal energy and hence flame speed is again low.
2. Compression Ratio. A higher compression ratio increases the pressure and temperature of the working mixture and decreases the concentration of residual gases. These favorable conditions reduce the ignition lag of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion. Total ignition angle is reduced. Maximum pressure and indicated mean effective pressure are increased.. Lastly, use of a higher compression ratio increases the surface to volume ratio of the combustion chamber, thereby increasing the part of the mixture which after-burns in the third phase. The increase in compression ratio results in increase in temperature that increases the tendency of the engine to detonate.
3. Intake temperature and pressure. Increase in intake temperature and pressure increases the flame speed.
4. Engine load. With increase in engine load the cycle pressures increase. Hence the flame speed increases. In SI engines with decrease in load, throttling reduces power of an engine. Due to throttling the initial and final compression pressures decrease and the dilution of the working mixture due to residual gases increases. This makes the smooth development of self propagating nucleus of flame difficult and unsteady and prolongs the ignition lag. The difficulty can be overcome to a certain extent by enriching the mixture at low loads (0.8 to 0.9of stoichiometric) but still it is difficult to avoid after-burning during a substantial part of expansion stroke. In fact, poor combustion at low loads and the necessity of mixture enrichment are among the main disadvantages of spark ignition engines which cause wastage of fuel and discharges of a large amount of products of incomplete combustion like carbon monoxide and other poisonous substances.
5. Turbulence. Turbulence plays a very vital role in combustion phenomenon. The flame speed is very low in non-turbulent mixtures. A turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front (diffusion). These two factors cause the velocity of turbulent flame to increase practically in proportion to the turbulence velocity. The turbulence of the mixture is due to admission of fuel-air mixture through comparatively narrow sections of the intake pipe, valves, etc. in the suction stroke. The turbulence can be increased at the end of the compression by suitable design of combustion chamber that involves the geometry of cylinder head and piston crown. The degree of turbulence increases directly with the piston speed. If there is no turbulence the time occupied by each explosion would be so great as to make the high speed internal combustion engines impracticable. Insufficient turbulence lowers the efficiency due to incomplete combustion of the fuel. However, excessive turbulence is also undesirable.
6. Engine Speed. The higher the engine speed, the greater the turbulence inside the cylinder. For this reason the flame speed increases almost linearly with engine speed. Thus if the engine speed is doubled the time required, in milliseconds, for the flame to traverse the combustion space would be halved. Double the original speed arid hence half the original time would give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation, which is the main phase of combustion, will remain almost constant at all speeds. This is an important characteristic of spark ignition engines. However, the increase in engine speed would lead to ignition advance due to the first phase of combustion. This can be illustrated with a numerical example. Consider a petrol engine running at 1500rpm. Let us say for the first stage of combustion the ignition lag, the time required in terms of crank angle, is 8° of crank rotation, and for the second stage, the propagation of flame through the combustion space, 12oofcrank rotation is required. Thus the total ignition period is20°of crank rotation. Now if the engine speed is doubled from 1500 to 3000 rpm, the time required for the second stage will again be 12° of crank rotation (due to doubling of turbulence intensity time in milliseconds is halved and in terms of crank angle remains constant), but for the first stage time in milliseconds is constant and hence in terms of crank angle it will be doubled, i.e., it would be 16°.This would make the total ignition period of 16 + 12 = 28° crank rotation at 3000rpm compared to 8° + 12°= 20° at .1500 rpm. From this it follows that with increase in engine speed ignition must be advanced. This is done in practice by automatic ignition advance mechanism.
7. Engine size. Engines of similar design generally run at the same piston speed. This is achieved by smaller engines having larger rpm and larger engines having smaller rpm. Due to the same piston speed, the inlet velocity, the degree of turbulence, and flame speed are nearly same in similar engines regardless of the size. However, in small engines the flame travel is small and in large engines large. Therefore, if the engine size is doubled the time required (in milliseconds) for propagation of flame through combustion space will also be doubled. But with lower rpm of larger engines the time for flame propagation in terms of crank angle would be nearly same as in smaller engines. In other words the number of crank degrees required for flame travel will be about the same irrespective of engine size, provided the engines are similar.
Rate of Pressure Rise
The rate of pressure rise is a very important aspect of flame development from engine design and operation point of view. It considerably influences the maximum cylinder pressure, the power produced and the smooth running of the engine. The rate or pressure rise depends on the mass rate of combustion of the mixture in the cylinder. Fig. 10 shows pressure-crankangle diagrams for three different combustion rates. One is for a high, the second for the usual and the third for a low rate of combustion
Figure: 10. Relationship b/w pressure and crank angle for different rates of combustion
It is clear from the figure that with lower rates of combustion longer time is required for combustion that necessitates the initiation of burning at an earlier point on the compression stroke. With higher rates of burning the time required for combustion is smaller and the rate of pressure rise is higher. Also, the peak pressure produced is close to TDC, which is desirable because it produces greater force acting through a large portion of the power stroke. But peak pressure and hence peak temperature too close to TDC gives a long time for rapid heat loss from the cylinder. The higher rate of pressure rise causes rough running of the engine because of vibrations and jerks produced in crankshaft. If the rate of pressure rise is very high it results in abnormal combustion called detonation. In practice the engine is so designed that approximately one-half of the pressure rise takes place as the piston reaches TDC. This results in peak pressure and temperature 10° to 15° after TDC. In this way very small portion of the expansion stroke is-lost and the gain is smooth engine operation and saving an appreciable period of time during which loss of heat is rapid. In the old engines with low compression ratios of ‘5 to 6 a rate of pressure rise of 2 bar per crank degree used to be thought as optimum. Today with higher compression ratios of the order of8 to 9, a rate of pressure rise of 3 to 4 bar per crank degree may be employed if the engine mountings are sufficiently stiff and efficient.
Abnormal Combustion
In normal combustion the flame started by the spark travels across the combustion chamber in ‘a fairly even way. Under certain engine operating conditions abnormal combustion may take place that is detrimental to, life and performance of the engine. There are a variety of ways in which abnormal combustion can occur. The important abnormal combustions are ‘detonation or knock’, ‘preignition’, ‘run-on’, etc. Of these detonation or knock is most important because it puts a limit on the compression ratio at which an engine can be operated, which, in turn, controls the efficiency and to some extent, power output.