In SI engines the combustion chambers have homogeneous air fuel mixture that is compressed after induction stroke. Towards the end of the compression stroke the spark plug gives out high temperature spark to initiate the ignition process. The spark may have temperature as high as 10,000°C. The combustion spreads from around the spark plug and spreads rapidly across the combustible mixture inside the chamber.
The flame speed would depend on the temperature of the flame front and to some extent on the temperature and density of the air fuel mixture. The molecules in and around the spark plug starts burning, when an intense and high temperature spark is generated between the electrodes of homogenous mixture of vaporised fuel, air and residual gases.
As the charge near the spark burns, a flame is formed whose speed is extremely low because the reaction zone is yet to be established and heat loss is high since walls are cold. With the burning of adjacent layers, the flame travels the entire cylinder. We can broadly have two types of combustion in SI engines— Normal Combustion and Abnormal Combustion.
Effect of Engine Variables on Ignition Lag in SI Engines:
Since ignition lag is primarily chemical phenomenon the fuel quality—especially its self-ignition temperature has important bearing on ignition lag. The higher the self-ignition temperature of the fuel longer will be the ignition delay.
The mixture strength giving highest temperature would result in lower ignition delay. Maximum temperature occurs in slightly richer mixtures.
The air fuel charge gets consumed as the flame travels across the combustion chamber. Velocity of flame propagation is of great importance in SI engine combustion because it—
(i) Affects the rate of pressure rise in combustion chamber and
(ii) It can cause abnormal combustion in engine.
Factors on which flame velocity depends are:
It plays a vital role in combustion; it is one of the major factors deciding flame speed. The flame velocity increases in proportion to the turbulence velocity. Flame velocity is very low for non-turbulent fuel mixtures. In such cases of low turbulence or no-turbulence, time for each explosion would be so high that IC engine would become impracticable.
(b) Fuel-Air Ratio:
When the mixture is too lean or too rich the flame speed decreases because-
(a) Too lean mixture produces less energy and
(b) Too rich mixture results in incomplete combustion.
Therefore it has to be just correct for proper flame speed.
(c) Intake Temperature and Pressure:
With increase in intake temperature and pressure the flame speed increases.
(d) Engine Load:
There is increase in flame speed with increase in engine load. At higher loads with more throttling the working mixture is suited for combustion and hence smooth operation is obtained.
(e) Engine Size:
In small size engines the flame has to travel less distance and hence it takes less time for the flame travel.
(f) Engine Speed:
Flame speed increases almost linearly with engine speed. The engine speed higher would be the turbulence and greater would be the flame speed.
(g) Compression Ratio:
At high compression ratios both pressure and temperature of the air fuel mixture would increase. These two conditions favour the flame speed. Thus flame speed increases with increase in compression ratio.
(h) Residual Gases:
Increase of residual gases in the combustion chamber would reduce the flame speed. The presence of the burnt out gases would reduce the fresh charge in the combustion chamber and hence its capacity to produce energy.
Combustion quality is a concept given by T H Ma. It is how closely the actual cycle matches the theoretical Otto cycle. Good quality combustion almost makes fast burn cycles whereas poor combustion quality consists of late and delayed burn cycles.
For best results the combustion must be completed close to the TDC for greater efficiency.
Abnormal Combustion in SI Engines:
In normal combustion flame travels across the combustion chamber in a fairly even and uniform manner till entire charge gets consumed. However under certain operating conditions combustion in the cylinder can become uneven and sudden and may result in what is known as Abnormal Combustion. Abnormal combustion includes—pre-ignition, knock, run-on, ping, rumbling etc.
These are undesirable and harmful to the engine. Of these pre-ignition and knock or detonation are the most important. Detonation puts a limit on the compression ratio. For higher efficiency the compression ratio should be higher but in SI engines one cannot go beyond 10-11 due to onset of knocking.
In IC engines the ignition should occur when it is required to take place. Usually it is timed to take place at 20 – 300 before top dead centre position (BTDC) depending on various factors like engine rpm, ignition lag etc. At times the combustion can occur automatically without the spark due to the temperature at some places in the combustion chamber like exhaust valve or the spark plug etc. may be quite high to initiate combustion. The temperature for pre- ignition is in the range of 1100 – 12000°C. Such combustion is known as pre-ignition that is production of flame front without the spark.
Pre-ignition is auto ignition without the initiating spark. Pre-ignition may originate from hotspots. Due to formation of number of flame front from various hotspots and normal flame front if already present combustion is erratic. Pre-ignition is totally undesirable effect because it raises the temperature of the combustion chamber and this leads to production of more hotspots and the process multiplies thus causing severe damage to the combustion chamber and producing uneven amount of energy.
Pre-ignition is sensitive to (i) the type of fuel (ii) Spark plug temperature (iii) carbon deposit in the chamber (iv) engine speed.
(a) Effects of pre-ignition are same as that of over-advanced ignition timing i.e., as if the spark would occur much before the required ignition advance of 20-300 BTDC. This adds to negative work by the piston. Pre-ignition is also reduces the efficiency of engine because if pre-ignition occurs during the compression stroke of the engine then the energy produced because of pre-ignition will oppose the compression stroke and thus reducing the efficiency of the engine.
(b) Peak pressure and temperature in the chamber increases as result of pre-ignition.
(c) Higher temperature and pressure lead to detonation that would cause more hot spots and more pre-ignition. This becomes vicious circle.
Another abnormal combustion in SI engine is auto ignition. In auto ignition the pressure and temperatures especially during the end portion of the charge may reach such values where ignition could take place on its own. This phenomenon is known as auto ignition. Pre-ignition also plays important role in occurrence of auto ignition.
Knock or detonation is phenomenon is signified by audible sharp metallic sound during abnormal combustion. Detonation is accompanied by very high pressures and uneven combustion resulting in mechanical damage and loss of power.
Theories of Detonation:
There are possibly three theories that can explain the knock:
(i) Auto-Ignition of the end charge
(ii) Detonation or Shockwave theory
(iii) Chemical changes during Detonation
Consider a normal case in which the flame front is travelling across the chamber releasing the energy and increasing the pressure and temperature of the mixture ahead of it. The flame front travels say across the chamber.
Suppose due to high pressure and temperatures the charge ahead of the flame front reaches self-ignition temperature further fuel mixture ahead it. Normally such conditions are obtained in the end charge i.e., that portion of the charge that is located towards the end portion of the combustion chamber.
Following would occur:
(a) The ignition process will commence in the end portion due to self-ignition of the end charge if such conditions have been reached.
(b) In the meantime the flame front travelling from left to right would continue to consume the charge. Let us assume that the normal flame front is at B-B when auto-ignition conditions arise in the end charge.
(c) The combustion of the end portion at B would take place only after the ignition delay period.
(d) There are two possibilities-
(i) If the time taken by the flame front B-B is less than the ID period the entire end charge would get consumed. No detonation would occur, or
(ii) However if the end charge is not consumed before the combustion occurs in the end portion due to less ID there would be second flame front travelling from right to left. This would result in detonation or knocking.
During detonation very high local pressures of the order of 150-200 bar are reached. Detonation of even 5% of the charge can lead to very violent knocking.
In the Auto-ignition theory the combustion is supposed to be quite normal before the onset of ignition in the end charge. However in the Detonation theory it is assumed that there is Shockwave travelling at about twice the speed of sound which would compress the charge to almost instantaneous reactions.
When the combustion is normal products of combustion are CO2, CO and H2O. However if there is detonation— aldehydes and peroxides have been observed.
The Detonation is harmful due to the following effects:
(a) Mechanical Damage:
Severe knocking conditions may result in pressures of the order of 200 bar, vibrations of large amplitudes upto 5000 cycles per second and temperature of 2000-25000°C. Under such severe conditions there could be marked mechanical damage to the piston, valves and cylinder head. The damage may become cumulative and lead to eventual failure.
Due to high production of heat detonation becomes major factor, which causes pre-ignition.
(c) Noise and Vibration:
Whereas mild knock is not audible and is not much harmful—the more violent detonation causes lot of noise and results in high vibration. Due to vibration the engine runs rough.
(d) Power Output and Efficiency:
With the formation of more heat energy. Power output and efficiency also decreases which is one of the major drawbacks.
(e) Heat Transfer Rates:
Due vibration the heat transfer rates in a detonating engine are increased. There is a marked increase in loss of heat to the cooling water.
There are many factors that affect detonation. However temperature, time, density and composition are most important.
1. Temperature Factors:
Any of the process occurring in the cylinder that tends to increase the temperature of cylinder will lead to Detonation. The temperature of fuel mixture plays an important and self-ignition temperature of the intake charge play important role in detonation. Possibility of detonation increases if (i) the temperature of the charge is high and (ii) the self-ignition temperature of the fuel is low.
(a) Raising Compression Ratio:
Due to higher compression ratio the temperature and density increases. Higher temperature may lead to detonation.
In super-charging or turbo-charging both the pressure and also the temperature of intake charge increases. This may lead to detonation.
(c) Increasing the Load:
Due to this the cylinder temperature and combustion cylinder wall temperature increase. These leads to increment in end gas temperature that results in detonation.
2. Time Factors:
The detonation can be avoided based on following time related factors:
(i) Increase the flame propagation speed,
(ii) Reducing the flame travel time by compact design and
(iii) Increase the ignition delay period.
Following time related factors that cause detonation can be controlled:
(a) Low turbulence, lead to less flame speed and more time is required to travel. This can be avoided by increasing flame speed.
(b) Distance travelled by the flame front should be reduced. Larger the distance, more the time and more the probability hence this can be reduced by reducing the path travelled.
(c) Flame speed is sometimes slow because of the fact that enough fuel is not present. Hence, having rich Air- fuel mixture can reduce this defect.
3. Density Factors:
Due to increase in the density of the unburnt mixture (due to various causes) there is an increment in the probability of Detonation.
Following are the causes:
(a) Increasing the Load:
This lead to more opening of throttle valve thereby admitting more mixture enters and causes detonation.
(b) Supercharging the Engine:
By doing so, when the compression ratio is increased the mixture becomes highly dense, which leads to detonation.
(c) Increasing the Inlet Pressure:
As the inlet pressure increases overall pressure in the engine increases which leads to less ignition delay, more dense change and high probability of detonation.
(d) Advancing the Spark Timing:
This leads to burning of fuel in the dense form and causing detonation.
4. Composition Factors:
Quality of fuel and the fuel-air ratio are great importance for controlling knock.
Once the compression ratio and engine size is decided, we can reduce knocking by having a change in composition that can be attained by:
(a) Increasing Octane Rating or Fuel:
Ring compounds like Benzene and addition of tetra ethyl lead reduces tendency to knock.
(b) Having Rich or Lean Mixture for Combustion:
This leads to longer delay period and low temperature. Both these factors reduce possibility of knocking.
Methods to Reduce Knocking:
(a) Good combustion chamber design to reduce the flame travel by central location of the spark plug or having more than one spark plug by proper placement, avoiding pockets of stagnant charge, having turbulence to increase flame speed.
(b) Cooling of spark plug and exhaust valve that are major hot spots in most of the combustion chambers. Increasing the number of spark plugs with proper placement.
(c) Reducing the temperature of the last portion of the charge by increasing the surface to volume ratio as in the wedge shaped chambers to increase the heat transfer to the walls.
(d) Using of lower compression ratio and high octane number fuel.
(e) In general rich mixtures are less prone to detonation due to cooling effect.
(f) Increasing engine rpm also leads reduction in detonation.
Other Abnormal Combustions:
There are other abnormal combustion like – Rumble, wild ping etc. but are not of much importance.
Due to the presence of deposits on the combustion chamber ignition may start from hot spots before or after the normal ignition. This would lead heavy explosive burning of the charge. There is appreciable rise in pressure and a low pitch noise called Rumble—caused by multiple pre-ignitions is audible. Rumble is harmful and results in damage to the engine parts.
It is caused due to some burning part of the deposit—causing ignition now and then. It is somewhat similar to knocking. It can also be known as knock, which is sudden erratic, sharp and unpredictable. The combustion progress in the above has been shown in table.
3. Run-on Ignition:
Due to the presence of deposits on the surface at times there is combustion even when ignition is switched off.
Tendency to knocking depends quite significantly on the type and composition of the Hydrocarbon fuels. Resistance to knocking is of great significance in fuels. The fuels differ quite widely in their property of anti- knocking characteristic. Hydrocarbons have a tendency to knock when the engine operating conditions become severe. Detonation is undesirable and resistance to detonation or knocking is required for the fuels. This measure of fuel resistance to knock is known as Octane Rating.
The rating of fuels for their resistance to knocking is done by comparing them with standard reference fuel. The standard fuel is mixture of two primary fuels—
(i) Iso Octane C8H18 which has very high knock resistance properties and is assigned rating of 100 and
(ii) Normal heptane having very poor anti-knock characteristics and is assigned rating of 0. The resistance of a fuel to knock is indicated by Octane Number.
Octane number is the percentage of iso-octane fuel in a mixture of iso-octane and n-heptane by volume which will give the same knocking as the fuel under standard test conditions. Octane number gives a fair indication of the anti-knock properties of the fuel. Gasoline with Octane No. 83 means that this fuel has the same knock characteristics as mixture of 83 % is—octane and with 17% n-heptane under standard test conditions.
Octane rating tests are carried out in standard Co-operative Fuel Research (CFR) engine of 82.6 mm bore and 114.3 mm stroke engine. It is robust 4-stroke engine where compression ratio can be varied from 3 to 30. The engine has multi-bowl carburettor in which fuel to be tested as well as iso-octane and n-heptane can be stored separately.
The testing for Octane rating can be done under two different conditions:
(i) Firstly under normal conditions which is called Research Method and the corresponding Octane No. is called RON.
(ii) Secondly more severe conditions which are close to the actual operating conditions which is called Motoring Method. The Octane No. found by motoring method is called MON.
The details of testing under these two conditions are as tabulated below:
From RON and MON we can get the change that would occur under test condition and practical conditions. We define sensitivity as difference between RON and MON. Thus sensitivity can also be defined as the measure of the extent to which a gasoline is downgraded under severe conditions. Higher the sensitivity, poor will be its performance under severe conditions.
Sensitivity = RON-MON
This is the newest rating system usually called Road Octane Number. It is the mean of the RON and MON. The Octane number displayed on petrol pumps is generally Road Octane Number. Our premium grade petrol is having Octane No. of 91.
Road Octane Number = (RON + MON)/2
In many applications like aero engines high anti-knocking fuels are required. In such engines we require anti-knock property even higher than Iso-Octane. When Octane number increased beyond 100 with the additions of new additives like Tetra Ethyl Lead (TEL) and tetra methyl lead need was felt for other units to indicate knock resistance of such fuels. The discovery of TEL in 1923 lead to marked improvement of ON when blended with gasoline.
The anti-knock rating of fuel of ON greater than 100 is given by Performance Number. Performance Number is found by carrying out test in a standard super charged engine under prescribed conditions without knocking. Performance Number is the maximum power that can be produced in such test by using the fuel under test and also by using iso-octane which is assigned Performance Number of 100. Thus if power using the fuel be 1.5 kW and with iso-octane it is 1.2 kW then
Performance Number = (1.5/1.2) x 100 = 125
Where 100 represents Performance Number of iso-octane
Performance Number can also be defined as = KLMEP of the fuel/ KLMEP of iso-octane Where KLMEP stands for knock limited mean effective pressure
Another method of Performance Number is to represent it in terms of ml of TEL per US gallon
Performance Number = 100 = millilitres of TEL per US gallon of gasoline
Performance Number can also be used for measuring ON less than 100 as shown in Fig. 26.7 but it is generally used for greater values only.
There are other methods for SI engine rating. Like Ricardois toluene no. in which toluene and heptanes are reference fuels and scale is drawn.
For anti-knock capability of fuels above ON 100 we make use of Performance Number
(a) Performance Number = 100 + ml of TEL per US gallons of fuel
(b) Performance Number = (KLMEP )fuel /(KLMEP)iso-octane
The type of combustion chamber design has important bearing on the performance and anti-knock characteristics.
Thus a combustion chamber must fulfill the following conditions:
(a) Maximum power output
(b) The operation should be as smooth as possible.
(c) Pressure rise should be uniform and smooth.
(d) Resistance to Knocking/Detonation.
To fulfill the above conditions, the chamber must be designed considering the following factors:
(a) It should have high compression ratio: The higher the compression ratio the higher would be the power output and the efficiency.
(b) There should be no excess air i.e., there should be maximum utilisation of inducted air and there should be no dead pockets.
(c) Adequate turbulence for high flame speed.
(d) Flame travel should be minimum hence engine should be compact; also compactness gives small surface volume ratio which leads to less heat loss during combustion.
(e) Absence of exhaust gases i.e., proper scavenging.
(f) Spark plugs should be properly located to have a good flame start and travel.
(g) Absence of hotspots i.e., no deposits and good cooling arrangement.
(h) Pressure rise in the combustion chamber should be uniform and moderate
(i) Valve head should have right rise and right cooling arrangement.
(j) Minimum time to complete combustion.
(k) The thermal and volumetric efficiencies should be high,
(l) Fuel efficient and non-polluting.
As early as 1908 Ford Co had used T-head type engines for its automobiles. The main disadvantage was presence of two camshafts and long distance of flame travel that resulted in detonation. Subsequently L-head and I-head or overhead valve type engines were developed.
Ricardo turbulent combustion chamber was designed to obtain fast flame speed while reducing knocking. This designed also reduced effective flame travel by reducing distance between piston crown and head. It was designed to dissipate heat at a much faster rate and thus was improvement to T-head and I-head chambers.