ABNORMAL COMBUSTION1. KNOCK OR DETONATION

2. SURFACE IGNITION:(a) PRE-IGNITION

(b)POST IGNITION

J.P. Subrahmanyam

KNOCKKNOCK is the term used to describe a pinging noise emitted from a SI engine undergoing abnormal combustion.

The noise is generated by shock waves produced in the cylinder when unburned gas auto-ignites.

Engine Damage From Severe Knock

Damage to the engine is caused by a combination of high temperature and high pressure.

Piston Piston crown

Cylinder head gasket Aluminum cylinder head

Knock Limited Parameters1. Knock Limited Compression ratio. The highest

compression ratio that gives border-line or trace knock.

2. Knock Limited Inlet Pressure. The highest inlet pressure that gives border-line or trace knock.

3. Knock Limited Indicated Mean Effective Pressure, or klimep, the imep at border-line or trace knock.

4. Octane number requirement. The minimum octane number of a fuel to give border-line or trace knock.

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 a high pressure and temperature which promotes auto-ignition

ii) Engine speed – 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, thus some engines show an increase in propensity to knock at high speeds while others do not.

Knock

iii) Spark timing – maximum compression from the piston advance occurs at TC, increasing the spark advance makes the end of combustion crank angle approach TC and thus get higher pressure and temperature in the unburned gas just before burnout.

iv) Mass of Inducted charge – reduction in the mass of the charge by throttling or reduction in the amount of supercharging will reduce knock.

v) Inlet Temperature – reduction in inlet temperature will reduce knock and will also improve the volumetric efficiency.

vi)Temperature of the cylinder walls particularly near the end charge – this will reduce heat transfer to the end charge.

Knock

x

x

x

xx

x

x

X crank angle corresponding to borderline knock

Spark advance is set to 1% below MBT to avoid Knock but it will affect power and torque

1% below MBT

Knock Mitigation Using Spark Advance

Effect of fuel-air ratio on knock limited inlet pressure, compression ratio and imep.

Knock DetectionA knock sensor is mounted on the engine block to

determine if an engine is knocking. The sensor consists of a piezoelectric accelerometer

that produces an analog electrical signal when the engine vibration at a specific frequency (6-15 kHz) is measured.

In-cylinder pressure transducers are very expensive and not widely used for knock detection.

On receiving a “knock” signal, the ECU temporarily retards the spark timing as a counter measure to prevent engine damage from knock.

Fuel Knock ScaleTo provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by which fuels are assigned an octane number ON.

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 number of zero and Isooctane – 2,2,4 tri-methyl pentane - (C8H18) has a number of 100.

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

Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90.

A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knocking tendency.

Octane Number MeasurementTwo methods have been developed to measure ON using a standardizedsingle-cylinder engine developed under the auspices of the Cooperative Fuel Research (CFR) 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 900Spark advance (oBTC) 13 19-26 (varies with r)Coolant temperature (oC) 100Inlet 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.

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 knock detector.• At that compression ratio run the engines on blends of n-heptane and isooctane.• ON is the % by volume of isooctane in the blend that produces the same knock.The antiknock index which is displayed at the fuel pump is the average of the research and motor octane numbers:

Octane Number Measurement

2

MONRONindexAntiknock

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

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

Knock Characteristics of Various Fuels

Formula 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-C7H16 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 - 0.035216T2)1/2]where T is milliliters of tetraethyl lead per U.S. gallon in isooctane.

Another option is to use “performance number” which is the ratio of the klimep to the klimep with isooctane as fuel at the same inlet pressure.

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

The most effective antiknock agents are lead alkyls;(i) Tetraethyl lead (TEL), (C2H5)4Pb was introduced in 1923(ii) Tetramethyl lead (TML), (CH3)4Pb was introduced in 1960

In 1959 a manganese antiknock compound known as MMT (methylcyclopentadienyl manganese tricarbonyl) was introduced to supplement TEL. But it had some major emission related problems and so it was discontinued.

In the US, around 1970, low-lead and unleaded gasoline were introduced over toxicological concerns with lead alkyls (TEL contains 64% by weight lead).

Alcohols such as ethanol and methanol have high knock resistance and have been used as anti-knock additives as well as gasoline fuel supplements.

Since 1970 another alcohol methyl tertiary butyl ether (MTBE) has been added to gasoline to increase octane number. MTBE is formed by reacting methanol and isobutylene. Another option is to used the ethyl ether (ETBE).

Surface IgnitionAny local hot spot on the combustion chamber wall like

burr or metallic projection hot carbonaceous deposit hot exhaust valve seatspark plug electrode

will result in ignition of the charge coming in contact.If ignition is before the spark, it is called pre-ignition.It can occur at any time and can lead to knocking and

severe engine damage. This is called knocking pre-ignition. It is the most serious form of abnormal combustion.

Alcohols are generally more prone to pre-ignition.

Pre-ignition tendencyThe Ricardo method uses an electrically heated

wire in the engine to measure pre-ignition tendency. The scale uses isooctane as 100 and cyclohexane as 0.

Some common fuel components:Paraffins 50-100Benzene 26Toluene 93Xylene >100Cyclopentane 70Di-isobutylene 64There is no direct correlation between anti-knock

ability (ON) and pre-ignition tendency

Other forms of abnormal combustion

1. Run-on. This is the tendency of an engine to continue running after the ignition is switched off.Caused by the spontaneous ignition of the fuel-air mixture and not due to surface ignition.

2. Runaway surface ignition. This occurs when pre-ignition is persistent and it can cause melting of pistons.

Engine Stability - Misfire

If the fuel-air mixture is leaned out with excess air, or is diluted with increasing amounts of residual gas or exhaust gas recycle, the burn time increases the cycle-by-cycle fluctuations in the combustion process increases.

Eventually a point is reached where engine operation becomes rough and unstable, this point defines the engine’s stable operating limit.

With little or no dilution completion of combustion occurs prior to the exhaust valve opening consistently cycle after cycle.

With increasing dilution, first, in a fraction of the cycles the burns are so slow that combustion is only just completed prior to the exhaust valve opening.

As dilution increases further, in some cycles combustion is not complete prior to the exhaust valve opening and the flame extinguishes before all the fuel is burned. Finally misfire cycles start to occur where the mixture is not ignited.

As the dilution is further increased the proportion of partial burns and misfires increase to a point where the engine no longer runs.