COMBUSTION IN SI ENGINES

QUESTIONS IN RECENT EXAMS:3. a) Explain the combustion phenomenon in an S.I engine.

b) Explain the effect of various engine variables on S.I engine knock.

3. a) Discuss the variables affecting flame speed in SI engines.

b) Discuss about the basic requirements of SI engine combustion chambers

3. a) What is delay period and what are the factors which affect delay period

b) Explain with simple sketches, combustion chambers for SI engines

3. a) What is abnormal combustion, explain with supportive figures the concept of abnormal combustion

b) What are fuel requirements for a SI engine and what are the additives generally added to prevent knock during combustion.

COMBUSTION PROCESS IN SI ENGINES

Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat.

Following conditions are necessary for combustion to take place

1. The presence of combustible mixture2. Some means to initiate mixture3. Stabilization and propagation of flame in

Combustion Chamber

TYPES OF COMBUSTION IN SI ENGINE

Combustion in SI engine may roughly divided into two general types:

Normal and Abnormal In Normal combustion the spark ignites the

compressed fuel/air mixture and a smooth burn travels through the combustion chamber . 

In Abnormal combustion , certain conditions deviates the combustion from its normal course leading to loss of performance and possible damage to the engine. This is also known as knocking combustion.

STAGES OF COMBUSTION IN SI ENGINES

A theoretical pressure-crank angle diagram, during the process of compression (a→b), combustion (b→c) and expansion (c→d) in an ideal four-stroke spark-ignition engine is shown in Fig

In an actual engine the combustion process in an SI engine consists of Three stages

1. 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).

Ignition lag is very small and lies between 0.00015 to 0.0002 seconds.

An ignition lag of 0.002 seconds corresponds to 35 deg crank rotation when the engine is running at 3000 RPM.

2. 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.

3.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.

The flame velocity decreases during this stage. The rate of combustion becomes low due to lower flame velocity and reduced flame front surface.

FLAME FRONT PROPAGATION

The two important factors which determine the rate of movement of the flame front across the combustion chamber are the reaction rate and the transposition rate.

The reaction rate is the result of a purely chemical combination process .

The transposition rate is due to the physical movement of the flame front relative to the cylinder wall and is also the result of the pressure differential between the burning gases and the un-burnt gases in the combustion chamber.

Figure. Shows the rate of flame propagation

In area I, (A→B), the flame front progresses relatively slowly due to a low transposition rate and low turbulence.

The transposition of the flame front is very little since there is a comparatively small mass of charge burned at the start.

The low reaction rate plays a dominant role resulting in a slow advance of the flame.

As the flame front leaves the quiescent zone and proceeds into more turbulent areas (area II) where it consumes a greater mass of mixture, it progresses more rapidly and at a constant rate (B→C).

The volume of unburned charge is very much less towards the end of flame travel and so the transposition rate again becomes negligible thereby reducing the flame speed (C→D)

FACTORS AFFCTING THE FLAME PROPAGATIONRate 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.

The factors which affect the flame propagations are1. Air fuel ratio2. Compression ratio3. Load on engine4. Turbulence 5.Engine speed6. Engine size7.Other factors

1. 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. Very rich mixture results incomplete combustion

Lean mixture releases less heat resulting lower flame temperature and lower flame speed.

2. Compression ratio: 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.

3.Engine Output: The cycle pressure increases when the engine

output is increased. With the increased throttle opening the cylinder

gets filled to a higher density. This results in increased flame speed.

When the output is decreased by throttling, the initial and final compression pressures decrease and the dilution of the working mixture increases.

The main disadvantages of SI engines are the poor combustion at low loads

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 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

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.

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.

5. Engine SpeedThe turbulence of the mixture increases with an increase in

engine speed. For this reason the flame speed almost increases linearly with engine speed.

6.Engine Size: The size of the engine does not have much effect on the

rate of flame propagation. In large engines the time required for complete combustion

is more because the flame has to travel a longer distance. This requires increased crank angle duration during the;

combustion. This is one of the reasons why large sized engines are

designed to operate at low speeds.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. .

ABNORMAL COMBUSTIONIn Normal combustion, the flame initiated by

the spark travels across the combustion chamber in a fairly uniform manner.

Under certain operating conditions the combustion deviates from its normal course leading to loss of performance and possible damage to the engine.

This type of combustion may be termed as an Abnormal combustion or Knocking combustion

The consequences of this Abnormal combustion process are the loss of power, recurring pre-ignition and mechanical damage to the engine

AUTO-IGNITIONHeat-release due to combustion increases the

temperature and consequently the pressure, of the burned part of the mixture above those of the unburned mixture

The burned part of the mixture will expand, and compress the unburned mixture adiabatically thereby increasing its pressure and temperature.

This process continues as the flame front advances through the mixture and the temperature and pressure of the unburned mixture are increased further.

If the temperature of the un-burnt mixture exceeds the self-ignition temperature of the fuel spontaneous ignition or auto ignition occurs at various pin-point knocking.

This phenomenon is called knocking. The process of auto-ignition leads towards engine knock.

PHENOMENON OF KNOCKING IN SI ENGINE

In the Normal combustion the flame travels across the combustion chamber from A towards D

The advancing flame front compresses the end charge BB'D farthest from the spark plug, thus raising its temperature

The temperature is also increased due to heat transfer from the hot advancing flame-front.

In spite of these factors if the temperature of the end charge had not reached its self-ignition temperature, the charge would not auto-ignite and the flame will advance further and consume the charge BB'D.

This is the normal combustion process which is illustrated by means of the pressure-time diagram, Fig.6(b).

However, if the end charge BB'D reaches its auto ignition temperature and the charge will auto ignite, leading to knocking combustion

In Fig.6(c), it is assumed that when flame has reached the position BB', the charge ahead of it has reached critical auto ignition temperature.

During the pre-flame reaction period if the flame front could move from BB' to only CC then the charge ahead of CC would auto-ignite.

Because of the auto-ignition, another flame front starts traveling in the opposite direction to the main flame front

When the two flame fronts collide, a severe pressure pulse is generated.

This disturbance can force the walls of the combustion chambers to vibrate

The pressure-time trace of such a situation is shown in Fig.l2.6(d).

The impact of knock on the engine components and structure can cause engine failure and in addition the noise from engine vibration is always objectionable

EFFECT OF DETONATION

The harmful effects of detonation are as follows:1. Noise and Roughness. Knocking produces a

loud pulsating noise and pressure waves. These waves which vibrates back and forth across the cylinder. The presence of vibratory motion causes crankshaft vibrations and the engine runs rough.

2. Mechanical Damage.High pressure waves generated during knocking can

increase rate of wear of parts of combustion chamber. Sever erosion of piston crown , cylinder head and pitting of inlet and outlet valves may result in complete wreckage of the engine.

3. Carbon deposits. Detonation results in increased carbon deposits.

4. Increase in heat transfer. Knocking is accompanied by an increase in the rate of heat transfer to the combustion chamber walls.

The minor reason is that the maximum temperature in a detonating engine is about 150°C higher than in a non-detonating engine, due to rapid completion of combustion

5. Decrease in power output and efficiency. Due to increase in the rate of heat transfer the power output as well as efficiency of a detonating engine decreases.

EFFECT OF ENGINE OPERATING VARIABLES ON THE ENGINE KNOCKING

DETONATION Any factor in the design or operation of an engine which tends to reduce the temperature of the unburned charge should reduce the possibility of knocking

The following parameters which are directly or indirectly connected with temperature, pressure and density factors on the possibility of knocking is discussed below.

Density Factors

a)Compression Ratio: Compression ratio of an engine is an important factor which determines both the pressure and temperature at the beginning of the combustion process.

Increase in compression ratio increases the pressure and temperature of the gases at the end of the compression stroke.

This decreases the ignition lag of the end gas and thereby increasing the tendency for knocking

b) Mass of Inducted Charge: A reduction in the mass of the inducted charge into the cylinder of an engine by throttling reduces both temperature and density of the charge at the time of ignition. This decreases the tendency of knocking.

c) Inlet Temperature of the Mixture:Increase in the inlet temperature of the mixture makes the compression temperature higher thereby, increasing the tendency of knocking

Hence, a lower inlet temperature is always preferable to reduce knocking.

d) Temperature of the Combustion Chamber Walls:

Temperature of the combustion chamber walls plays a predominant role in knocking.

In order to prevent knocking the hot spots in the combustion chamber should be avoided.

Since, the spark plug and exhaust valve are two hottest parts in the combustion chamber, the end gas should not be compressed against them

Time FactorsIncreasing the flame speed or increasing the

duration of the ignition lag or reducing the time of exposure of the unburned mixture to autoignition condition will tend to reduce knocking

The following factors, reduce the possibility of knocking:

a) Turbulence: Increasing turbulence increases the flame speed and reduces the time available for the end charge to attain autoignition conditions thereby decreasing the tendency to knock.

b) Engine Speed: An increase in engine speed increases the turbulence of the mixture considerably resulting in increased flame speed. Hence knocking tendency is reduced at higher speeds.

c)Flame Travel Distance: The knocking tendency is reduced by shortening the time required for the flame front to traverse the combustion chamber

d) Engine Size: The flame requires a longer time to travel across the combustion chamber of a larger engine. Therefore, a larger engine has a greater tendency for knocking than a smaller engine since there is more time for the end gas to auto ignite

e) Combustion Chamber Shape: Generally, the more compact the combustion chamber is, the shorter is the flame travel and the combustion time and hence better antiknock characteristics. Therefore, the combustion chambers are made as spherical as possible to minimize the length of the flame travel for a given volume

f) Location of Spark Plug: In order to have a minimum flame travel the spark plug is centrally located in the combustion chamber, resulting in minimum knocking tendency.

COMBUSTION CHAMBERS FOR SI ENGINES

The design of the combustion chamber for an SI engine has an important influence on the engine performance and its knocking tendencies.

The design involves the shape of the combustion chamber,

The location of spark plug andThe location of inlet and exhaust valves.

REQUIREMENTS OF AN SI ENGINE COMBUSTION CHAMBER I) Smooth engine operationThe aim of any engine design is to have a smooth

operation and a good economy. These can be achieved by the following:

a. Moderate Rate of Pressure RiseLimiting the rate of pressure rise as well as the

position of the peak pressure with respect to TDC affect smooth engine operation

b) Reducing the Possibility of KnockingReduction in the possibility of knocking in an engine

can be achieved by reducing the distance of the flame travel by centrally locating the spark plug .

Satisfactory cooling of the spark plug and of exhaust valve which are the source of hotspots in the majority of the combustion chambers.

II. High Power Output and Thermal EfficiencyThis can be achieved by considering the following factors:a) A high degree of turbulence is needed to achieve a high

flame front velocity. Turbulence is induced by inlet flow configuration or squishSquish is the rapid radial movement of the gas trapped in between the

piston and the cylinder head .Squish can be induced in spark-ignition engines by having a bowl in

piston or with a dome shaped cylinder head b. High Volumetric Efficiency More charge during the suction stroke, results in an increased power

output.This can be achieved by providing ample clearance around the valve

heads,Large diameter valves and straight passages with minimum pressure

drop.c. Improved anti-knock characteristicsImproved anti-knock characteristics permits the use of a higher

compression ratio resulting in increased output and efficiencyd. A Compact Combustion ChamberReduces heat loss during combustion and increases the thermal

efficiency.

TYPES COMBUSTION CHAMBERS

Different types combustion chambers have been developed   Some of them are

T-Head TypeL-Head TypeI-Head Type or Overhead ValveF-Head Type

 T-Head Type:The  T-head  combustion chambers were

used in the early stage of engine development.This configuration provides two valves on either 

side of the cylinder , requiring two camshafts.Since the distance across the combustion chamber

is very long, knocking tendency is high in this type of engines.

From the manufacturing point of view,providing two camshafts is a disadvantage.

L-HEAD TYPE A modification of the T-head type of combustion chamber is the L-head type which provides the two valves on the same side of the cylinder and the valves are operated by a single camshaft.

In this type , the air flow has to take two rigt angle turns to enter the cylinder. This causes a loss of velocity head and loss in turbulence level resulting in a slow combustion process.

I HEAD TYPE OR OVER HEAD VALVE

In which both the valves are located on the cylinder head.

The over head valve engine is superior to a side valve or an L-head engine at high compression ratios.

Some of the important characteristics of this type of valve arrangement are :

less heat lossLess flame travel length and hence greater freedom

from knockHigher volumetric efficiency from larger valves or

valve lifts

F-HEAD TYPE: The F-head type of valve arrangement is a

compromise between L-head and I-head types.Combustion chambers in which one valve is in the

cylinder head and the other in the cylinder block are known as F-head combustion chambers

The main disadvantage of this type is that the inlet valve and the exhaust valve are separately actuated by two cams mounted onto camshafts driven by the

crankshaft through gears.