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Normal combustion Stages of combustion Importance of flame speed (petrol engine) Delay period and its importance (diesel engine) Effects of engine variables Abnormal combustion : knock, surface ignition Diesel knock Fuel requirements Fuel rating Anti-knock additives

Combustion in IC Engines

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In a conventional SI engine, fuel and air are mixed together in the intake system, inducted through the intake valve into the cylinder where mixing with residual gas takes place, and then compressed during the compression stroke.

Under normal operating conditions, combustion is initiated towards the end of compression stroke at the spark plug by an electric discharge.

Following inflammation, a turbulent flame develops, propagates through the premixed air-fuel mixture (and burned gas mixture from the previous cycle) until it reaches combustion chamber walls, then it extinguishes.

Normal combustion in SI engines

IGNITION LIMITSIgnition of charge is only possible within certain limits of fuel-air ratio. Ignition limits correspond approximately to those mixture ratios, at lean and rich ends of scale, where heat released by spark is no longer sufficient to initiate combustion in neighbouring unburnt mixture. For hydrocarbons fuel the stoichiometric fuel air ratio is 1:15 and hence the fuel air ratio must be about 1:30 and 1:7

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Normal combustion in SI engines

Auto-ignition temperature - Diesel -210°C; petrol or gasoline- 246°C

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There are three stages of combustion in SI Engine 1. Ignition lag stage2. Flame propagation stage3. After burning stage

Ignition lag stage: There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called IGNITION LAG. Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self ignition temperature , get ignited and produce a self propagating nucleus of flame. The ignition lag is generally expressed in terms of crank angle (θ1). The period of ignition lag is shown by path ab.

Stages of combustion in SI engines

Flame propagation stage:Once the flame is formed at “b”, it should be self sustained and must be able to propagate through the mixture. This is possible when the rate of heat generation by burning is greater than heat lost by flame to surrounding. After the point “b”, the flame propagation is abnormally low at the beginning as heat lost is more than heat generated. Therefore pressure rise is also slow as mass of mixture burned is small. Therefore it is necessary to provide angle of advance 30 to 35 deg, if the peak pressure to be attained 5-10 deg after TDC. The time required for crank to rotate through an angle θ2 is known as combustion period during which propagation of flame takes place.

After burning:Combustion will not stop at point “c” but continue after attaining peak pressure and this combustion is known as after burning. This generally happens when the rich mixture is supplied to engine.

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Factors effecting flame speed in SI engines

The factors which affect the flame propagations are1. Air fuel ratio2. Compression ratio3. Load on engine4. Turbulence and engine speed5. Other factors

Rate of flame propagation affects the combustion process in SI engines. Higher combustion efficiency and fuel economy can be achieved by higher flame propagation velocities. Unfortunately flame velocities for most of fuel range between 10 to 30 m/second.

A : F ratio. The mixture strength influences the rate of combustion and amount of heat generated. The maximum flame speed for all hydrocarbon fuels occurs at nearly 10% rich mixture. Flame speed is reduced both for lean and as well as for very rich mixture. Lean mixture releases less heat resulting lower flame temperature and lower flame speed. Very rich mixture results incomplete combustion (C CO instead of C0) and also results in production of less heat and flame speed remains low. The effects of A: F ratio on p-v diagram and p-θ diagram are shown below

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The higher compression ratio increases the pressure and temperature of the mixture and also decreases the concentration of residual gases. All these factors reduce the ignition lag and help to speed up the second phase of combustion. Figure above shows the effect of compression ratio on pressure (indirectly on the speed of combustion) with respect to crank angle for same A: F ratio and same angle of advance. Higher compression ratio increases the surface to volume ratio and thereby increases the part of the mixture which after-burns in the third phase. Engine with high C R have higher flame speeds

2. Compression ratio:

3. Load on Engine. With increase in load, the cycle pressures increase and the flame speed also increases. In S.I. engine, the power developed by an engine is controlled by throttling. At lower load and higher throttle, the initial and final pressure of the mixture after compression decrease and mixture is also diluted by the more residual gases. This reduces the flame propagation and prolongs the ignition lag. This is the reason, the advance mechanism is also provided with change in load on the engine. This difficulty can be partly overcome by providing rich mixture at part loads but this definitely increases the chances of afterburning. The after burning is prolonged with richer mixture. In fact, poor combustion at part loads and necessity of providing richer mixture are the main disadvantages of S,I. engines which causes wastage of fuel and discharge of large amount of CO with exhaust gases.

4. Turbulence : Turbulence plays very important role in combustion of fuel as the flame speed is directly proportional to the turbulence of the mixture. This is because, the turbulence increases the mixing and heat transfer coefficient or heat transfer rate between the burned and unburned mixture. The turbulence of the mixture can be increased at the end of compression by suitable design of the combustion chamber (geometry of cylinder head and piston crown). Insufficient turbulence provides low flame velocity and incomplete combustion and reduces the power output. But excessive turbulence is also not desirable as it increases the combustion rapidly and leads to detonation. Excessive turbulence causes to cool the flame generated and flame propagation is reduced. Moderate turbulence is always desirable as it accelerates the chemical reaction, reduces ignition lag, increases flame propagation and even allows weak mixture to burn efficiently.

Factors effecting flame speed in SI engines

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5. Engine Speed

The turbulence of the mixture increases with an increase in engine speed. For this reason the flame speed almost increases linearly with engine speed. If the engine speed is doubled, flame to traverse the combustion chamber is halved. Double the original speed and half the original time give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation , which is main phase of combustion will remain almost constant at all speeds. This is an important characteristics of all petrol engines.

6 Engine Size

Engines of similar design generally run at the same piston speed. This is achieved by using small engines having larger RPM and larger engines having smaller RPM. Due to same piston speed, the inlet velocity, 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 for propagation of flame through combustion space is also doubled. But with lower RPM of large engines the time for flame propagation in terms of crank would be nearly same as in small 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.

7. Other Factors.

Among the other factors, the factors which increase the flame speed are supercharging of the engine, spark timing and residual gases left in the engine at the end of exhaust stroke. The air humidity also affects the flame velocity but its exact effect is not known. Anyhow, its effect is not large compared with A :F ratio and turbulence.

Factors effecting flame speed in SI engines

Start ofinjection

End ofinjecction

-10 TC-20 10 20 30 8

Combustion in CI EngineThe combustion process proceeds by the following stages:

Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the compression stroke. The liquid fuel atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure air.

Premixed combustion phase (bc) – combustion of the fuel which has mixed with the air to within the flammability limits (air at high-temperature and high pressure) during the ignition delay period occurs rapidly in a few crank angles.

Mixing controlled combustion phase (cd) – after premixed gas consumed, the burning rate is controlled by the rate at which mixture becomes available for burning. The rate of burning is controlled in this phase primarily by the fuel-air mixing process.

Late combustion phase (de) – heat release may proceed at a lower rate well into the expansion stroke (no additional fuel injected during this phase). Combustion of any unburned liquid fuel and soot is responsible for this.

Four Stages of Combustion in CI Engines

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Types of CI Engines

Direct injection:quiescent chamber

Direct injection:swirl in chamber Indirect injection: turbulent

and swirl pre-chamber

Orifice -plate

Glow plug

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compression ratio engine speed output injection timing quality of the fuel intake temperature intake pressure

Factors effecting delay period in CI engines

1.Compression Ratio.

The increase in the compression temperature of the air with increase in compression ratio evaluated at the end of the compression stroke is shown in Fig. It is also seen from the same figure that the minimum auto ignition temperature of a fuel decreases due to increased density of the compressed air. This results in a closer contact between the molecules of fuel and oxygen reducing the time of reaction. The increase in the compression temperature as well as the decrease in the minimum auto ignition temperature decrease the delay period. The maximum peak pressure during the combustion process is only marginally affected by the compression ratio (because delay period is shorter with higher compression ratio and hence the pressure rise is lower).

2.Engine Speed:

The delay period could be given either in terms of absolute time (in milliseconds) or in terms of crank angle degrees With increase in engine speed, the loss of heat during compression decreases, resulting in the rise of both the temperature and pressure of the compressed air thus reducing the delay period in milliseconds. However, in degrees of crank travel the delay period increases as the engine operates at a higher rpm. The fuel pump is geared to the engine, and hence the amount of fuel injected during the delay period depends on crank degrees and not on absolute time. Hence, at high speeds, there will be more fuel present in the cylinder to take part in the second stage of uncontrolled combustion resulting in high rate of pressure rise.

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3 Output

With an increase in engine output the air-fuel ratio decreases, operating temperatures increase and hence delay period decreases. The rate of pressure rise is unaffected but the peak pressure reached may be high.

4. Injection timing:The effect of injection advance on the pressure variation is shown in Fig. for three injection advance timings of 9°, 18°, and 30° before TDC. The injected quantity of fuel per cycle is constant. As the pressure and temperature at the beginning of injection are lower for higher ignition advance, the delay period increases with increase in injection advance. The optimum angle of injection advance depends on many factors but generally it is about 20°bTDC.

5. Quality of Fuel used:The physical and chemical properties of fuel play very important role in delay period. The most important property of fuel which is responsible for chemical delay is its selfignition temperature. Lower the self-ignition temperature, lower the delay period. The cetane number (CN) of the fuel is another important parameter which is responsible for the delay period. A fuel of higher cetane number gives lower delay period and provides smoother engine operation. The delay period for a fuel having CN = 50 is lowest and pressure rise is also smooth and maximum pressure rise is least as most of the fuel burns during controlled combustion. The other properties of fuel which affects the physical delay period are volatility, latent heat, viscosity and surface tension. The viscosity and surface tension are responsible for the better atomization whereas latent heat and viscosity are responsible for the rapid evaporation of fuel.

6. Intake TemperatureThe delay period is reduced either with increased temperature. However, preheating of charge for this purpose is not desirable because it reduces the density of charge and volumetric efficiency and power output.

7. Intake pressureIncrease in intake pressure or supercharging reduces the auto ignition temperature and hence reduces the delay period. The peak pressure will be higher since the compression pressure will increase with intake pressure.

Factors effecting delay period in CI engines

If the temperature of an air-fuel mixture is raised high enough, the mixture will self-ignite without the need of a spark plug or other external igniter. The temperature above which occurs is called the self-ignition temperature (SIT).

On the other hand, self-ignition is not desirable in an SI engine, where a spark plug is used to ignite the air-fuel at the proper time in the cycle.

When self-ignition does occur in an SI engine higher than desirable, pressure pulses are generated. These high pressure pulses can cause damage to the engine and quite often are in the audible frequency range. This phenomenon is often called knock or ping.

Abnormal Combustion

Pre-ignition is usually followed by knockingLoss of performance

When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur.

knocking is observed in diesel engines and detonation is observed in petrol engines.

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KnockingKnock is the autoignition of the portion of fuel, air and residual gas mixture ahead of the advancing flame, that produces a noise.

As the flame propagates across combustion chamber, end gas is compressed causing pressure, temperature and density to increase. Some of the end gas fuel-air mixture may undergo chemical reactions before normal combustion causing autoignition - end gases then burn very rapidly releasing energy at a rate 5 to 25 times in comparison to normal combustion. This causes high frequency pressure oscillations inside the cylinder that produce sharp metallic noise called knock.

Knock will not occur when the flame front consumes the end gas before these reactions have time to cause fuel-air mixture to autoignite. Knock will occur if the precombustion reactions produce autoignition before the flame front arrives.

Knocking and Surface Ignition

Surface IgnitionSurface ignition is ignition of the fuel-air charge by overheated valves or spark plugs, by glowing combustion chamber deposits or by any other hot spot in the engine combustion chamber - it is ignition by any source other than the spark plug.

It may occur before the spark plug ignites the charge (preignition) or after normal ignition (postignition).

It may produce a single flame or many flames.

Surface ignition may result in knock.

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Diesel KnockKnocking is violet gas vibration and audible sound produced by extreme pressure differentials leading to the very rapid rise during the early part of uncontrolled second phase of combustion. In C.I. engines the injection process takes place over a definite interval of time. Consequently, as the first few droplets injected are passing through the ignition lag period, additional droplets are being injected into the chamber. If the ignition delay is longer, the actual burning of the first few droplets is delayed and a greater quantity of fuel droplets gets accumulated in the chamber. When the actual burning commences, the additional fuel can cause too rapid a rate of pressure rise, as shown on pressure crank angle diagram below, resulting in Jamming of forces against the piston (as if struck by a hammer) and rough engine operation. If the ignition delay is quite long, so much fuel can accumulate that the rate of pressure rise is almost instantaneous. Such, a situation produces extreme pressure differentials and violent gas vibration known as knocking (diesel knock), and is evidenced by audible knock. The phenomenon is similar to that in the SI engine. However, in SI Engine knocking occurs near the end of combustion whereas in CI engine, knocking the occurs near the beginning of combustion.

Delay period is directly related to Knocking in CI engine. An extensive delay periodcan be due to following factors: A low compression ratio permitting only a marginal self ignition temperature to be reached. A low combustion pressure due to worn out piston, rings and bad valves Low cetane number of fuel Poorly atomized fuel spray preventing early combustion Coarse droplet formation due to malfunctioning of injector parts like spring Low intake temperature and pressure of air

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OCTANE NUMBER The fuel property that describes how well a fuel will or will not self-ignite is called the octane

number or just octane. The higher the octane number of a fuel, the less likely it will self-ignite. Engines with low compression ratios can use fuels with lower octane numbers, but high-

compression engines must use high-octane fuel to avoid self-ignition and knock.

The tendency to detonate depends on composition of fuel. Fuel differ widely in their ability to resist knock. It is defined as the percentage of Iso-octane by volume in a mixture of Iso-octane and n-heptane which exactly matches the knocking tendency of a given fuel, in a standard fuel under given standard operating conditions. The rating of a particular SI fuel is done by comparing its antiknock performance with that of standard reference fuel which is usually combination of Iso-octane and nheptane. Iso-octane (C8H18) which has a very high resistance to knock and therefore it is arbitrarily assigned a rating of 100 octane number. N-heptane (C7H16) which is very prone to knock and therefore given a zero value. For example: Octane number 80 means that the fuel has same knocking tendency as mixture of 80% iso-octane and 20% n-heptane (by volume basis). A fuel having an octane number of 110 means fuel has the same tendency to resist as a mixture of 10 cc of Tetra ethyl lead (TEL) in one U.S gallon of Iso-octane.

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CETANE NUMBER

In a CI engine, self-ignition of the air-fuel mixture is a necessitity. The correct fuel must be chosen which will self-ignite at the precise proper time in the engine cycle. The property that quantifies this is called the cetane number. The larger the cetane number, the shorter is the ID and the quicker the fuel will self-ignite in the combustion

chamber.

The cetane number is a numerical measure of the influence the diesel fuel has in determining the ignition delay. Higher the cetane rating of the fuel lesser is the propensity for diesel knock. The cetane number of a diesel fuel is a measure of its ignition quality. The cetane number of a fuel is the percentage by volume of cetane in a mixture of cetane [C16H34] and a -methylnapthalane [C10H7 CH3] that has same performance in the standard test engine as that of the fuel. Cetane is arbitrarily assigned a number 100 and originally a -methylnapthalane was given a number 0 but now reference fuels is heptamethylnonane (HMN) which is given a value of 15. HMN is used because it is more stable compound and has slightly better ignition quality.Cetane number 40 means a mixture containing 40 % cetane and 60 % of heptamethylnonane (HMN) by volume which gives same ignition delay as tested fuel. For high sped engine, cetane number of 50 is required, for medium speed engine about 30. High octane number implies low cetane number . In other words good SI engine fuel is bad CI engine fuel.

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Anti Knock AgentsThe knock resistance tendency of a fuel can be increased by adding anti-knock agents. The anti knock agents are substances which decreases the rate of preflame reaction by delaying the auto ignition of the end mixture in engine until flame generated by sparkplug. TEL [P (C2H5)4]is most powerful anti knock agents. TEL increase the efficiency of engine and increase the specific output of SI engine. Its use will not improve the performance of engine which is not knocking unless the spark advanced. CR is increased or higher inlet pressure is used to take advantage of an increase in octane number. The use of leaded gasoline. However is not perfect solution to problem. It leads to emission of lead into atmosphere which is known to be very hazardous.

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Why Turbulence and swirl is created?Turbulence is necessary to break the flame front into pieces so that each and every part of combustion chamber gets flame to ignite the homogeneous air fuel mixture. If there is uneven flame distribution then there is incomplete combustion which gives less torque and also causes pollutionIn case of CI engines fuel is injected in the highly compressed air. There is always possibility that the portion of air near the injector will get more fuel and thus again farthest point from injector will get less fuel supply. This again gives rise to unequal fuel distribution and incomplete combustion.But for CI engines arrangement is made such that air inside the cylinder is flown past the injector at last once for proper mixing.

There is difference between turbulence and swirl. Turbulence is random movement but swirl is orderly movement which is particularly used for CI engines.

How it is created? Development of CI engine involves various methods to produce swirl. Some of the common methods used are induction swirl, divided chamber etc.SI engines have different shapes of piston head which create turbulence.F head, I head, Ricardo head are examples for that.

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Swirl & SquishThe intake valve/port is usually placed to give the mixture a pronounced "swirl" (the term is preferable to "turbulence", which implies movement without overall pattern) above the rising piston, improving mixing and combustion. The shape of the piston top also affects the amount of swirl. Another design feature to promote turbulence for good fuel/air mixing is "squish", where the fuel/air mix is "squished" at high pressure by the rising piston

A piston ring is a split ring that fits into a groove on the outer diameter of a piston in an internal combustion engine

The three main functions of piston rings in reciprocating engines are :Sealing the combustion chamber so that there is no transfer of gases

from the combustion chamber to the crank.Supporting heat transfer from the piston to the cylinder wall.Regulating engine oil consumption.

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Piston rings are a major source of hint to identify if the engine is two stroke or four stroke. Three piston rings suggest that it is a four stroke engine while two piston rings suggest that it is a two stroke engine.

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Air–fuel equivalence ratio (λ) Air–fuel equivalence ratio, λ (lambda), is the ratio of actual AFR to stoichiometry for a given mixture.λ= 1.0 is at stoichiometry, rich mixtures λ < 1.0, andlean mixtures λ > 1.0.

Effect of variables on Knock – Density factors

Density factors that reduce the density of the charge also reduce the knocking tendency by providing lower energy release.

1. compression ratio: when CR ratio increases , P & T increases and an overall increase in density of charge raises the knocking tendency

2. Mass of inducted charge: A reduction in mass reduces T & ρ at the time of ignition. This decreases knocking tendency

3. Inlet temperature of mixture: Increase in inlet temperature of mixture makes compression temperature higher. Volumetric efficiency is also lowered.

4. Retarding spark timing: It will effect the power output and torque

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Effect of variables on Knock – time factors

Increasing the flame speed or the ignition lag will tend to reduce the tendency to knock..

1. Turbulence: increase of turbulence increases the flame speed and reduces time to reach auto-ignition. This reduces the knocking

2. Engine size: Flame requires more time to travel In CC of large engines. Hence large engines will have more tendency to knock

3.Eninee speed : Knocking tendency reduces at higher speeds4. spark plug locations : Central location minimizes flame travel distance.

Effect of variables on Knock –composition factors

1. Fuel air ratio : flame speeds are affected by air-fuel ratio.2. Octane value: Paraffin series have he maximum and aromatic series have minimum

tendency to knock