Combustion PhenomenaCombustion Phenomena

Since the gasoline powered internal combustion Since the gasoline powered internal combustion engine was invented, the quality of the fuel has engine was invented, the quality of the fuel has been a limiting factor in the output of power. been a limiting factor in the output of power.

Engine ‘Detonation’ has restricted compression Engine ‘Detonation’ has restricted compression ratios and spark advance, both of which are key ratios and spark advance, both of which are key elements in the production of power. elements in the production of power.

Detonation is also classified as ‘knock’, Detonation is also classified as ‘knock’, ‘pinging’, or ‘pre-ignition’. The truth is, each of ‘pinging’, or ‘pre-ignition’. The truth is, each of these describes a specific phenomena with its these describes a specific phenomena with its own characteristics. own characteristics.

Combustion PhenomenaCombustion Phenomena

We use the generic term We use the generic term ‘Detonation’ to describe these ‘Detonation’ to describe these phenomena.phenomena.

‘‘Normal Combustion’ is whenever Normal Combustion’ is whenever the combustion of the fuel and air the combustion of the fuel and air burns as designed inside the engine.burns as designed inside the engine.

Abnormal combustion is anytime the Abnormal combustion is anytime the fuel and air does not burned as fuel and air does not burned as designed inside the engine.designed inside the engine.

Combustion PhenomenaCombustion Phenomena

Normal combustionNormal combustion Combustion starts when the spark plug fires Combustion starts when the spark plug fires

before TDC on the compression stroke.before TDC on the compression stroke. Approximately 5 – 50 degrees before TDC.Approximately 5 – 50 degrees before TDC. Optimum Optimum IGNITION ADVANCEIGNITION ADVANCE depends on load, rpm, depends on load, rpm,

and engine design.and engine design. 4 valve engines with spark plug in the middle of the 4 valve engines with spark plug in the middle of the

combustion chamber need less advance.combustion chamber need less advance. 2 valve engines with large bores and spark plug all the 2 valve engines with large bores and spark plug all the

way to the side will need more.way to the side will need more. Actual ignition advance on a stock engine has a lot to do Actual ignition advance on a stock engine has a lot to do

with emissions.with emissions. Tuning of ignition advance is done on a dynamometer or Tuning of ignition advance is done on a dynamometer or

on the street using detonation detection equipment. on the street using detonation detection equipment. (electronic ignition)(electronic ignition)

Combustion PhenomenaCombustion Phenomena Spark: A luminous disruptive electrical Spark: A luminous disruptive electrical

discharge of very short duration between discharge of very short duration between two conductors separated by a gas.two conductors separated by a gas. Luminous: You can see it.Luminous: You can see it. Disruptive: Its energy disturbs what it passes Disruptive: Its energy disturbs what it passes

through. Atoms are ‘excited’.through. Atoms are ‘excited’. Electrical Discharge: Electrical Discharge: Free ElectronsFree Electrons are ejected are ejected

through space.through space. Very Short Duration: A lot quicker than it looks.Very Short Duration: A lot quicker than it looks. Two Conductors: In our case, a ‘spark plug’.Two Conductors: In our case, a ‘spark plug’. Gas: Cylinder pressure fights the spark. The Gas: Cylinder pressure fights the spark. The

higher the pressure in the cylinder, the harder it higher the pressure in the cylinder, the harder it is for the spark to jump the gap. is for the spark to jump the gap.

Example:Example: Super or turbo-charged engines run Super or turbo-charged engines run narrower spark plug gaps because high pressure in narrower spark plug gaps because high pressure in cylinder tends to make plugs misfire.cylinder tends to make plugs misfire.

Combustion PhenomenaCombustion Phenomena Activation energy: The amount of energy Activation energy: The amount of energy

needed to get a chemical reaction started.needed to get a chemical reaction started. Fuel molecules and oxygen atoms merely next Fuel molecules and oxygen atoms merely next

to each other don’t just spontaneously ignite.to each other don’t just spontaneously ignite. Spark excites fuel and oxygen so they will chemically Spark excites fuel and oxygen so they will chemically

combine, starts burning.combine, starts burning. Fuel and oxygen molecule must be properly lined up.Fuel and oxygen molecule must be properly lined up. Fuel molecule burns from one end like a zipper.Fuel molecule burns from one end like a zipper. Heat liberated starts next fuel/oxygen pair burning.Heat liberated starts next fuel/oxygen pair burning. Chain reaction. One reaction passes energy to the Chain reaction. One reaction passes energy to the

next.next. Heat liberated in the chemical reaction causes gases Heat liberated in the chemical reaction causes gases

to expand, this is what forces the piston down.to expand, this is what forces the piston down.

Combustion PhenomenaCombustion Phenomena

States of combustionStates of combustion

1.1. NormalNormal combustion combustion

2.2. AbnormalAbnormal combustion combustion

1.1. Pre-ignitionPre-ignition combustion combustion

2.2. Post-ignitionPost-ignition combustion combustion

Normal CombustionNormal Combustion Spark plug fires.Spark plug fires.

Spark plug duration may last 6 – 8 degrees of crank Spark plug duration may last 6 – 8 degrees of crank rotation.rotation.

A A KernelKernel of flame begins. of flame begins. Flame front expands radially from the kernel. Flame front expands radially from the kernel. Flame FrontFlame Front is the area where the actual chemical is the area where the actual chemical

reaction between fuel and air occurs. reaction between fuel and air occurs. Behind flame front are already reacted gases which are Behind flame front are already reacted gases which are

now inert. They make up the exhaust gases.now inert. They make up the exhaust gases. End GasesEnd Gases are the unburned gases in front of the frame are the unburned gases in front of the frame

front.front. As flame front progresses, pressure in cylinder increases. As flame front progresses, pressure in cylinder increases.

For best power, peak pressure occurs 8 - 15 degrees after For best power, peak pressure occurs 8 - 15 degrees after TDC. (800 – 1000 psi)TDC. (800 – 1000 psi)

Most pressure in the cylinder is gone by 90 degrees after TDC.Most pressure in the cylinder is gone by 90 degrees after TDC. Video: Video: Normal CombustionNormal Combustion Video: Video: BMW advertBMW advert

Pre-ignition CombustionPre-ignition Combustion Pre-ignition occurs whenever the flame front Pre-ignition occurs whenever the flame front

begins before the spark plug is fired.begins before the spark plug is fired. Hot spotsHot spots in the combustion chamber may in the combustion chamber may

light off the air and fuel prematurely.light off the air and fuel prematurely. Sharp edges in combustion chamber will glow.Sharp edges in combustion chamber will glow. Incandescent (glowing) carbon deposits.Incandescent (glowing) carbon deposits. Overheated spark plugs (wrong heat range).Overheated spark plugs (wrong heat range). Overheated due to high coolant temperature.Overheated due to high coolant temperature. Overheated exhaust valves.Overheated exhaust valves.

Pre-ignition CombustionPre-ignition Combustion Piston is on upward stroke toward TDC of the compression stroke.Piston is on upward stroke toward TDC of the compression stroke. Fuel/Air mixture is being compressedFuel/Air mixture is being compressed

This causes its temperature to rise.This causes its temperature to rise. If a hot spot is hot enough to begin burning fuel/air, it will start a If a hot spot is hot enough to begin burning fuel/air, it will start a

flame front.flame front. If unwanted flame front starts early enough, it can kick the piston If unwanted flame front starts early enough, it can kick the piston

back.back. This results in severe loss of power and damage to the engine.This results in severe loss of power and damage to the engine.

Unwanted flame front begins moving across combustion chamber.Unwanted flame front begins moving across combustion chamber. Meanwhile, spark plug ignites air/fuel at normal time and starts Meanwhile, spark plug ignites air/fuel at normal time and starts

another flame front.another flame front. 2 flame fronts race towards each other and collide.2 flame fronts race towards each other and collide.

Pressure in cylinder increases dramatically over normal.Pressure in cylinder increases dramatically over normal. Causes engine parts to vibrate wildly causing characteristic ‘ping’ Causes engine parts to vibrate wildly causing characteristic ‘ping’

sound.sound. Causes heat transfer to cylinder walls and combustion chamber to Causes heat transfer to cylinder walls and combustion chamber to

increase due to agitation of gases.increase due to agitation of gases. This can cause even more hot spots on the next cycle.This can cause even more hot spots on the next cycle.

Damage usually shows up as Damage usually shows up as pock markspock marks on the top of the piston. on the top of the piston. May get so bad, it eats a hole through the center of the piston.May get so bad, it eats a hole through the center of the piston.

Post-Ignition Post-Ignition CombustionCombustion

Piston is on upward stroke toward TDC of the compression Piston is on upward stroke toward TDC of the compression strokestroke

Fuel/Air mixture is being compressedFuel/Air mixture is being compressed This causes its temperature to rise.This causes its temperature to rise.

Spark plug ignites fuel/air mixture at normal timeSpark plug ignites fuel/air mixture at normal time Flame front develops from kernelFlame front develops from kernel Expands radially from spark plugExpands radially from spark plug End gases, in front of flame front, is being compressed.End gases, in front of flame front, is being compressed. This causes the temperature of the end gases to rise.This causes the temperature of the end gases to rise. When temperature of fuel and air in end gases reaches a When temperature of fuel and air in end gases reaches a

critical temperature, they spontaneously ignite.critical temperature, they spontaneously ignite. This happens almost all at once.This happens almost all at once.

Pressure in cylinder increases dramatically over normal.Pressure in cylinder increases dramatically over normal. Causes engine parts to vibrate wildly causing characteristic Causes engine parts to vibrate wildly causing characteristic

‘knock’ sound.‘knock’ sound. Causes heat transfer to cylinder walls and combustion chamber to Causes heat transfer to cylinder walls and combustion chamber to

increase due to agitation of gases.increase due to agitation of gases. This can cause even more hot spots on the next cycle.This can cause even more hot spots on the next cycle.

Video: Video: Post-Ignition CombustionPost-Ignition Combustion

Post-Ignition Post-Ignition CombustionCombustion

Post-ignition combustion is often called detonation.Post-ignition combustion is often called detonation. It is not always possible to determine whether the It is not always possible to determine whether the

abnormal combustion is pre-ignition or post-ignition so abnormal combustion is pre-ignition or post-ignition so generically they are both called detonation.generically they are both called detonation.

Post-ignition abnormal combustion can be caused by over Post-ignition abnormal combustion can be caused by over advanced ignition timing.advanced ignition timing. Causes flame front to start too early in compression stroke.Causes flame front to start too early in compression stroke. End gases end up being compressed too much by flame front.End gases end up being compressed too much by flame front. This causes temperature to rise abnormally high.This causes temperature to rise abnormally high. Reaches critical temperature at which it spontaneously Reaches critical temperature at which it spontaneously

ignites.ignites. Post-ignition abnormal combustion can be caused by fuel Post-ignition abnormal combustion can be caused by fuel

with an octane rating that is too low.with an octane rating that is too low. A fuel’s octane rating determines the critical temperature at A fuel’s octane rating determines the critical temperature at

which it will spontaneously ignite.which it will spontaneously ignite. Damage usually shows up as melting down the sides of the Damage usually shows up as melting down the sides of the

pistons with damage to the ring lands.pistons with damage to the ring lands. Post-ignition combustion can damage or destroy the Post-ignition combustion can damage or destroy the

engine is a few revolutions.engine is a few revolutions.

Preventing Abnormal Preventing Abnormal CombustionCombustion

Use high octane fuelUse high octane fuel Spark not overly advancedSpark not overly advanced Knock down all sharp edges on parts in the Knock down all sharp edges on parts in the

combustion chambercombustion chamber Use proper heat range spark plugsUse proper heat range spark plugs Use a ‘closed loop’ spark control system.Use a ‘closed loop’ spark control system.

Example: Example: J an S SafeguardJ an S Safeguard Modern engines have highly advanced closed Modern engines have highly advanced closed

loop ignition systems.loop ignition systems. Can detect knockCan detect knock Can retard spark in responseCan retard spark in response When it feels it is safe, it will re-advance sparkWhen it feels it is safe, it will re-advance spark Can richen up mixture to quench combustion, cool off.Can richen up mixture to quench combustion, cool off.