8-2103471 Abnormal Combustion

8/2/2019 8-2103471 Abnormal Combustion

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ABNORMAL COMBUSTION:KNOCK

ANDSURFACE IGNITION

2103471 Internal Combustion Engine


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ABNORMAL COMBUSTION:

KNOCK AND SURFACE-IGNITION

Abnormal combustion reveals itself in many ways.Of the various abnormal combustion processeswhich are important in practice, the two majorphenomena are knock and surface-ignition.

These abnormal combustion phenomena are ofconcern because:

1) when severe, they can cause major engine damage;and

2) even if not severe, they are regarded as anobjectionable source of noise by the engine or

vehicle operator.


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Description: Knock and Surface Ignition.

There are two primary abnormal combustionphenomena: knock and surface ignition.

Knock is the engine sound that results from

spontaneous ignition of the unburned fuel-air mixtureahead of the flame (the end gas).

Surface ignition is the ignition of the fuel-air mixtureby any hot surface, other than the spark discharge,prior to arrival of the flame.


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Normal combustionA combustion process which isinitiated solely by a timed spark

and in which the flame frontmoves completely across thecombustion chamber in a uniformmanner at a normal velocity.

Abnormal combustionA combustion process in which a flamefront may be started by hot combustion-

chamber surfaces either prior to or afterspark ignition, or a process in which somepart or all of the charge may be consumedat extremely high rates.

Spark knock*A knock which is recurrent andrepeatable in terms of audibility.It is controllable by the sparkadvance; advancing the spark

increases the knock intensity andretarding the spark reduces theintensity.

Surface ignitionHot spots-combustion-chamber deposits

Surface ignition is ignition of the fuel-aircharge by any hot surface other than the

spark discharge prior to the arrival of thenormal flame front. It may occur before thespark ignites the charge (preignition) or afternormal ignition (postignition).

Description: Knock and Surface Ignition


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Knock is the name given to the noise which istransmitted through the engine structure whenessentially spontaneous ignition of a portion of theend gasthe fuel, air, residual gas, mixture ahead ofthe propagating flameoccurs.

There is an extremely rapid release of most of thechemical energy in the end-gas, causing very highlocal pressures and the propagation of pressurewaves of substantial amplitude across thecombustion chamber.


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Surface Ignition is ignition of the fuel-air mixture bya hot spot on the combustion chamber walls such asan overheated valve or spark plug, or glowingcombustion-chamber deposit: i.e., by any meansother than the normal spark discharge.

Following surface ignition, a flame develops at each

surface-ignition location and starts to propagateacross the chamber in an analogous manner to whatoccurs with normal spark-ignition.


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Engine Knock Knock is most important at wide open throttle engine

operation.

It is an engine / fuel / vehicle problem, and limits thecompression ratio that can be used for a fuel withgiven knock resistance.

It is usually characterized in terms of the pressurewaves produced inside the cylinder when knockoccurs, and the vibration of the engine structure thatknock produces.


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Pressure variation in the cylinder during knockingcombustion for normal combustion, light knockand heavy knock, respectively.

Engine Knock


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Significant variations one cycle to the next, largely due toburn-rate variations.

Knock intensity (maximum amplitude of band-pass-filtered pressure signal) in one hundred individualconsecutive cycles. One cylinder of V-8 engine, 2400rev/min, wide-open throttle.

Engine knock


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Knock Fundamentals.

As the flame propagates away from the spark plug thepressure and temperature of the unburned gasincreases.

The end-gas autoignites after a certain induction time which is dictated by the chemical kinetics of the fuel-air mixture.

Knock originates in the extremely rapid release of thefuels chemical energy in the end-gas region ahead ofthe propagation flame.

shock P,T

time time

P,T end-gas

flame


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Knock Fundamentals.

Under certain conditions the end-gas can autoigniteand burn very rapidly.The local high pressures thatthis rapid energy release sets up generate strong

pressure waves which cause the engine structure tovibrate.

The end-gas temperature is the most critical

parameter in assessing whether this spontaneousignition process is likely to occur before the flameconsumes all the end gas.

If the flame burns all the fresh gas before autoignitionin the end-gas can occur then knock is avoided.

Therefore knock is a potential problem when the burn

time is long!


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Observation windowfor photography

Spark plug

Intake valve

Exhaust valve

Normal cycle

Knock cycle


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Photographs of knocking combustion identifyinglocation of autoignition sites.

Knock Fundamentals.


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Important Engine Variables

The end-gas temperature and the time availablebefore flame arrival are the two fundamental

variables that determine whether or not knock willoccur.

The effects of practical engine variables such ascompression ratio, spark advance, speed, inletpressure and temperature, coolant temperature,fuel/air ratio, on knock can be explained in terms ofthese two fundamental variables.


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Engine parameters that effect occurrence of knock are:i) Compression ratio at high compression ratios, even

before spark ignition, the fuel-air mixture is compressed to ahigh pressure and temperature which promotes autoignition

ii) Engine speed At low engine speeds the flame velocity isslow and thus the burn time is long, this results in more timefor autoignitionHowever at high engine speeds there is less heat loss sothe unburned gas temperature is higher which promotesautoignition

These are competing effects, some engines show anincrease in propensity to knock at high speeds while othersdont.



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Engine parameters that effect occurrence of knock are:

iii) Spark timing maximum compression from the piston

advance occurs at TC, increasing the spark advance makesthe end of combustion crank angle approach TC and thusget higher pressure and temperature in the unburned gas

just before burnout.



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Relation between spark advance, speed, and torque loss,for SI engine at WOT, showing typical knock limit thatavoids knock problems.

Effect of Spark Timing


I E i V i bl


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x

x

x

xx

x

x

X crank angle corresponding

to borderline knock

Spark advance set to 1% below MBT to avoid knock

1% below MBT

Knock Mitigation Using Spark Advance



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Knock limit as a function of compression ratio and RONfor moderate and high turbulence combustion chambers.



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SI ENGINE KNOCK

1. Knock is most critical at wide-open throttle because of itspersistence and potential for damage. Part-throttle knock is atransient phenomenon and is a nuisance only.

2. Whether or not knock occurs depends on engine/fuel/vehiclefactors, and ambient conditions. This makes it an extremelycomplex phenomenon.

3. To avoid knock, the engine compression ratio is limited tobetween 9 10.5. Significant efficiency gains are possible if the compression ratio could be raised.

4. The major difficulty in operating an engine closer to the knock limit is quantifying the on-the-road knock problem sufficientlyprecisely.

5. Knock sensors and a feedback control are increasingly used to

adjust spark timing so that the engine can operate close to itsknock limit.


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SI ENGINE KNOCK

The tendency to knock depends on engine design andoperating variables which influence end-gas temperature,pressure and time spent at high values of these two properties

before flame arrival. Thus, for example, the tendency to knock is decreased through

reductions in end-gas temperature that follow from decreasinginlet air temperature, and retarding the spark from MBTtiming.

However, knock is a phenomenon that is governed by bothengine and fuel factors; its presence or absence in an enginedepends primarily on the antiknock quality of the fuel.

Individual hydrocarbon compounds vary enormously in theirability to resist knock, depending on their molecular size and

structure.


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Knock Detection

1. Noise: human ear is remarkably sensitive

2. Mechanical vibration: accelerometer mounted on thehead or block

3. Cylinder pressure fluctuation: pressure transducer incylinder head

4. Light emission fluctuation: optical probe in cylinderhead


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Knocking cylinder-pressure signal

Signal processing of a typical Cylinder pressure signalfor knocking operation


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Fuel Factor

Individual hydrocarbons vary widely in their ability toresist knock.

Practical fuels are blends of a large number ofindividual hydrocarbon compounds from all thehydrocarbon series of classes: alkanes (paraffins),cyclanes (napththenes), alkenes (olefins) andaromatics.

Their tendency to knock depends on molecular sizeand structure. Since practical fuels are blends ofmany hydrocarbons, empirical measures are used tocharacterize a fuels resistance to knock underspecified conditions.


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Fuel Factor

It is important to consider a fuels knock resistanceunder different engine conditions, since a fuelsresponse to different levels of engine thermal loadingdepends on its composition.

This property is defined by the fuels octane number.It determines whether or not a fuel will knock in agiven engine under given operating conditions: thehigher the octane number, the higher the resistanceto knock.

F l K k S l


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Fuel Knock Scale

To provide a standard measure of a fuels ability to resistknock, a scale has been devised in which fuels areassigned an octane number ON.

The octane number determines whether or not a fuel willknock in a given engine under given operatingconditions.

By definition, normal heptane (n-C 7H16 ) has an octanevalue of zero and isooctane (C 8H18 ) has a value of 100.

The higher the octane number, the higher the resistanceto knock.

F l K k S l


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Fuel Knock Scale

Blends of these two hydrocarbons define the knockresistance of intermediate octane numbers: e.g., a blendof 10% n-heptane and 90% isooctane has an octanenumber of 90.

A fuels octane number is determined by measuringwhat blend of these two hydrocarbons matches the testfuels knock resistance.

Octane Number Measurement


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Octane Number MeasurementTwo methods have been developed to measure ON using a standardized

single-cylinder engine developed under the auspices of the Cooperative FuelResearch Committee in 1931.

The CFR engine is 4-stroke with 3.25 bore and 4.5 stroke, compression ratio

can be varied from 3 to 30.

Research Motor

Inlet temperature (oC) 52 149Speed (rpm) 600 900

Spark advance ( oBTC) 13 19-26 (varies with r )Coolant temperature ( oC) 100

Inlet pressure (atm) 1.0Humidity (kg water/kg dry air) 0.0036 - 0.0072

Note: In 1931 iso-octane was the most knock resistant HC, now there are

fuels that are more knock resistant than isooctane.



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Testing procedure: Run the CFR engine on the test fuel at both research and motor conditions. Slowly increase the compression ratio until a standard amount of knock

occurs as measured by a magnetostriction knock detector. At that compression ratio run the engines on blends of n-hepatane and

isooctane. ON is the % by volume of octane in the blend that produces the stand. knock

The antiknock index which is displayed at the fuel pump is the average ofthe research and motor octane numbers:


2

ON RON index Antiknock

+=

Note the motor octane number is always higher because it uses more severeoperating conditions: higher inlet temperature and more spark advance.

The automobile manufacturer will specify the minimum fuel ON that will resistknock throughout the engine s operating speed and load range.

Knock Characteristics of Various Fuels


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Knock Characteristics of Various FuelsFormula Name Critical r RON MON

CH4 Methane 12.6 120 120C3H8 Propane 12.2 112 97CH4O Methanol - 106 92C2H6O Ethanol - 107 89C8H18 Isooctane 7.3 100 100Blend of HCs Regular gasoline 91 83n-C 7H16 n-heptane 0 0

For fuels with antiknock quality better than octane, the octane number is:

ON = 100 + 28.28T / [1.0 + 0.736T+(1.0 + 1.472T - 0.035216T 2)1/2 ]

where T is milliliters of tetraethyl lead per U.S. gallon

Fuel Additives


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Fuel AdditivesChemical additives are used to raise the octane number of gasoline.

The most effective antiknock agents are lead alkyls;(i) Tetraethyl lead (TEL), (C 2H5)4Pb was introduced in 1923(ii) Tetramethyl lead (TML), (CH 3)4Pb was introduced in 1960

In 1959 a manganese antiknock compound known as MMT was introduced tosupplement TEL (used in Canada since 1978).

About 1970 low-lead and unleaded gasoline were introduced over toxicologicalconcerns with lead alkyls (TEL contains 64% by weight lead).

Alcohols such as ethanol and methanol have high knock resistance.

Since 1970 another alcohol methyl tertiary butyl ether (MTBE) has beenadded to gasoline to increase octane number. MTBE is formed by reactingmethanol and isobutylene (not used in Canada).

8-2103471 Abnormal Combustion

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