Lecture 5

Abnormal Combustion

11/6/2016

11/6/2016

Abnormal Combustion

The Abnormal Combustion:- When the combustion gets deviated from the normal behavior resulting loss of performance or damage to the engine. It is happened when the charge inside the cylinder may be ignite before the flame front reach it.

The abnormal combustion: 1. Auto ignition:- During the combustion of charge , the flame front make

pressure and temperature of unburned mixture at the end gas very high, so that

its value be reach the self ignition condition so it burn.

2. Surface ignition:- if a spot inside the combustion chamber as spark plug,

valves, carbon deposit be at high temperature it may be do as igniter source for

the charge.

11/6/2016

Abnormal Combustion

Auto ignition:-

Surface ignition:-

11/6/2016

Abnormal Combustion

11/6/2016

Abnormal Combustion

Pre-ignition:

It is the combustion of the charge during the compression process

before the appearance of the timed spark due to hot spots.

11/6/2016

Effect of Pre-ignition:-

It increase the tendency of detonation in engines.

It increases the heat transfer to the cylinder walls because high

temperature gases remain in contact with the cylinder for a longer

period.

The load on crankshaft during compression is abnormally high,

this may cause crank failure.

Pre-ignition in single cylinder engine will result in a steady

reduction in speed and power output.

Abnormal Combustion

11/6/2016 Pre-Ignition Normal Ignition

Abnormal Combustion

Note:-

Pre ignition is not responsible for abnormally high cylinder pressure, but there

can be a slight pressure rise above the normal due to the ignition point and

therefore, the peak pressure creeping forward to the TDC position where

maximum pressure occurs.

Over head spark plug and exhaust valves which are the main causes of pre

ignition should be carefully avoided in the engines.

If pre ignition occurs at the same time as the

timed of spark plug fire, combustion will appear

as normal. And if the ignition switched off, the

engine would continue to operate at the same

speed as if it were controlled by the

conventional time spark, provide the self

ignition temperature continues to occur at the

same point.

11/6/2016

Abnormal Combustion

11/6/2016

Abnormal Combustion

Detonation:-

It’s a sudden combustion of the end gas ahead of the flame front due to

increase in charge pressure and temperature higher than self ignition

conditions of the charge causing a pressure wave leading to a vibration of

cylinder walls.

As the flame propagates away from the spark plug the pressure and

temperature of the unburned gas increases.

Under certain conditions the end-gas can auto ignite and burn very rapidly

producing shock waves.

The end-gas auto ignites after a certain induction time which is dictated by

the chemical kinetics of the fuel-air mixture.

If the flame burns all the fresh gas before auto ignition in the end-gas can

occur then knock is avoided.

11/6/2016

Abnormal Combustion

11/6/2016

The effect of detonation on engine:- 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. The presence of vibratory motion causes crankshaft vibrations and

the engine runs rough.

2. Mechanical Damage.

(a)High pressure waves generated during knocking can increase rate of wear

rate of parts of combustion chamber. Sever erosion of piston crown ( in a

manner similar to that of marine propeller blades by cavitations), cylinder

head and pitting of inlet and outlet inlet and outlet valves may result in

complete wreckage of the engine.

(b) Detonation is very dangerous in engines having high noise level. In small

engines the knocking noise is easily detected and the corrective measures can be

taken but in aero-engines it is difficult to detect knocking noise and hence

corrective measures cannot be taken. Hence severe detonation may persist for a

long time which may

ultimately result in complete wreckage of the piston.

Abnormal Combustion

11/6/2016

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 increase heat transfer is due to two reasons:

1. 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.

2. The major reason for increased heat transfer is the scouring away of

protective layer of inactive stagnant gas on the cylinder walls due to

pressure waves. The inactive layer of gas normally reduces the heat

transfer by protecting the combustion and piston crown from direct

contact with flame.

Abnormal Combustion

11/6/2016

Knock Damage

Abnormal Combustion

11/6/2016

Factors affecting auto ignition delay or engine detonation

Factors Affecting Engine Detonation

Engine operating conditions Design parameters Fuel properties

Combustion Champer shape

Compression ratio Spark advance

Air-Fuel ratio

Engine speed

Octane number

Volatility

Spark plug position and numbers

Relative Humidity

Cooling water temperature

Inlet temperature and pressure

Abnormal Combustion

11/6/2016

1. Fuel properties

a. Octane number

b. Volatility

Factors affecting auto ignition delay or engine detonation

The octane number determines whether or not a fuel will knock in a given engine

under given operating conditions.

By definition,

Normal Heptane (n-C7H16) → has an octane value of zero and

Isooctane (C8H18) → has a value of 100.

A fuel’s octane number is determined by measuring what blend of these two

hydrocarbons matches the test fuel’s knock resistance.

Note:

The higher the octane number, the higher the resistance to knock.

Alcohols such as ethanol and methanol have high knock resistance.

The volatility is the ability of fuel to evaporate at normal condition.

When the volatility of fuel increases the burning rate increase and the auto

ignition delay increase so that the detonation intensity decreases.

Abnormal Combustion

11/6/2016

2. Design factors

a. Compression ratio

Ignition

TDC CAD

Pressure

Motoring curve

TDC TDC

r1 > r2 > r3

r3

r1

r1 > r2 > r3

r2

P2 = P1 (r)k and T2 = T1 (r)

k-1

Abnormal Combustion

11/6/2016

b. Effect of combustion chamber shape:

o

u

V

V

o

b

V

V

1

1

1

A

B D

C

D

dor

L

X

A

B

C

D

The best combustion chamber which burn more than 90% of charge in minimum time at the beginning of combustion.

Abnormal Combustion

11/6/2016

c. Spark plug position and numbers

The best position of spark plug be in the center of the combustion chamber

or offset from the center to the exhaust valve.

It can be use more than one spark plug in the same cylinder but the fire

order

Abnormal Combustion

11/6/2016

3. Operating conditions a. Spark Time:

SA1

SA2

SA3

Ignition 1

TDC CAD

Pre

ssu

re

Ignition 2

Ignition 3

The time of ignition has an effect on the detonation intensity, so that:

Spark advance :- increase the pressure and temperature inside cylinder so detonation

intensity increases.

Spark retard :- decrease the pressure and temperature inside cylinder so detonation

intensity decreases.

Abnormal Combustion

11/6/2016

b. Fuel-Air ratio

At low engine speeds the flame velocity is slow and thus the burn time is long, this results in more time for auto ignition However at high engine speeds there is less heat loss so the unburned gas temperature is higher which promotes auto ignition These are competing effects, some engines show an increase in its probability to knock at high speeds while others don’t.

c. Engine speed

d. Inlet temperature and pressure

P2 = P1 (r)k T2 = T1 (r)k-1

Due to the increase of inlet temperature and pressure, the combustion chamber temperature increase and the end gas temperature increase and the detonation intensity increase.

Abnormal Combustion

11/6/2016

e. Cooling water temperature

f. Relative humidity

Due to the increase of the amount of cooling water, the heat transfer from the engine increase so that engine parts temperature decrease and detonation intensity decrease.

Due to the decrease of the cooling water temperature, the heat transfer from the engine increase so that engine parts temperature decrease and detonation intensity decrease.

Abnormal Combustion

11/6/2016

Knock detection and control strategy

Knock sensors can be classified in two broad categories:

• Ionic Current technique

• Pressure Transducer

Direct measurements Remote measurements.

I. Direct measurements

Abnormal Combustion

11/6/2016

• Today, knock detection is generally performed using an Accelerometer mounted on the engine block.

Problems related to Knock sensors •Noises originated from mechanical sources •Comparing the signal by already designed values

II. Remote measurements

Abnormal Combustion

11/6/2016

-Cylinder Fluid flow :-In

Three parameters are used to characterize large-scale in-cylinder fluid motion:

swirl, squish, and tumble.

Swirl is the rotational flow about the cylinder axis.

Swirl is used to:

i) Promote rapid combustion in SI engines.

ii) Rapidly mix fuel and air in gasoline direct injection engines.

iii) Rapidly mix fuel and air in CI engines.

The swirl is generated during air induction into the cylinder by either:

i) Tangentially directing the flow into the cylinder, or

ii) Pre-swirling the incoming flow by the use of helical ports.

11/6/2016

iii) Many engines have a wedge shape cylinder head cavity or a bowl in the piston

where the gas ends up at TC.

During the compression process as the piston approaches TC more of the air

enters the cavity and the air cylinder moment of inertia decreases and the angular

velocity (and thus the swirl) increases.

Squish is the radial flow occurring at the end of the compression stroke in which the compressed gases flow into

the piston or cylinder head cavity.

11/6/2016

Squish is the radial flow occurring at the end of the compression stroke in which

the compressed gases flow into the piston or cylinder head cavity.

As the piston reaches TC the squish motion generates a secondary flow called

tumble, where rotation occurs about a circumferential axis near the outer edge of

the cavity.