Combustion Phenomena Since the gasoline powered internal combustion engine was invented, the quality of the fuel has been a limiting factor in the output of power. Engine ‘Detonation’ has restricted compression ratios and spark advance, both of which are key elements in the production of power. Detonation is also classified as ‘knock’, ‘pinging’, or ‘pre-ignition’. The truth is, each of these describes a specific phenomena with its own characteristics.

Combustion Phenomena We use the generic term ‘Detonation’ to describe these phenomena. ‘Normal Combustion’ is whenever the combustion of the fuel and air burns as designed inside the engine. Abnormal combustion is anytime the fuel and air does not burned as designed inside the engine.

Combustion Phenomena Normal combustion Combustion starts when the spark plug fires before TDC on the compression stroke. Approximately 5 – 50 degrees before TDC. Optimum IGNITION ADVANCE depends on load, rpm, and engine design. 4 valve engines with spark plug in the middle of the combustion chamber need less advance. 2 valve engines with large bores and spark plug all the way to the side will need more. Actual ignition advance on a stock engine has a lot to do with emissions. Tuning of ignition advance is done on a dynamometer or on the street using detonation detection equipment. (electronic ignition)

Combustion Phenomena Spark: A luminous disruptive electrical discharge of very short duration between two conductors separated by a gas. Luminous: You can see it. Disruptive: Its energy disturbs what it passes through. Atoms are ‘excited’. Electrical Discharge: Free Electrons are ejected through space. Very Short Duration: A lot quicker than it looks. Two Conductors: In our case, a ‘spark plug’. Gas: Cylinder pressure fights the spark. The higher the pressure in the cylinder, the harder it is for the spark to jump the gap. Example: Super or turbo-charged engines run narrower spark plug gaps because high pressure in cylinder tends to make plugs misfire.

Combustion Phenomena Activation energy: The amount of energy needed to get a chemical reaction started. Fuel molecules and oxygen atoms merely next to each other don’t just spontaneously ignite. Spark excites fuel and oxygen so they will chemically combine, starts burning. Fuel and oxygen molecule must be properly lined up. Fuel molecule burns from one end like a zipper. Heat liberated starts next fuel/oxygen pair burning. Chain reaction. One reaction passes energy to the next. Heat liberated in the chemical reaction causes gases to expand, this is what forces the piston down.

Combustion Phenomena States of combustion Normal combustion Abnormal combustion Pre-ignition combustion Post-ignition combustion

Normal Combustion Spark plug fires. Video: Normal Combustion Spark plug duration may last 6 – 8 degrees of crank rotation. A Kernel of flame begins. Flame front expands radially from the kernel. Flame Front is the area where the actual chemical reaction between fuel and air occurs. Behind flame front are already reacted gases which are now inert. They make up the exhaust gases. End Gases are the unburned gases in front of the frame front. As flame front progresses, pressure in cylinder increases. For best power, peak pressure occurs 8 - 15 degrees after TDC. (800 – 1000 psi) Most pressure in the cylinder is gone by 90 degrees after TDC. Video: Normal Combustion Video: BMW advert

Pre-ignition Combustion Pre-ignition occurs whenever the flame front begins before the spark plug is fired. Hot spots in the combustion chamber may light off the air and fuel prematurely. Sharp edges in combustion chamber will glow. Incandescent (glowing) carbon deposits. Overheated spark plugs (wrong heat range). Overheated due to high coolant temperature. Overheated exhaust valves.

Pre-ignition Combustion Piston is on upward stroke toward TDC of the compression stroke. Fuel/Air mixture is being compressed This causes its temperature to rise. If a hot spot is hot enough to begin burning fuel/air, it will start a flame front. If unwanted flame front starts early enough, it can kick the piston back. This results in severe loss of power and damage to the engine. Unwanted flame front begins moving across combustion chamber. Meanwhile, spark plug ignites air/fuel at normal time and starts another flame front. 2 flame fronts race towards each other and collide. Pressure in cylinder increases dramatically over normal. Causes engine parts to vibrate wildly causing characteristic ‘ping’ sound. Causes heat transfer to cylinder walls and combustion chamber to increase due to agitation of gases. This can cause even more hot spots on the next cycle. Damage usually shows up as pock marks on the top of the piston. May get so bad, it eats a hole through the center of the piston.

Post-Ignition Combustion Piston is on upward stroke toward TDC of the compression stroke Fuel/Air mixture is being compressed This causes its temperature to rise. Spark plug ignites fuel/air mixture at normal time Flame front develops from kernel Expands radially from spark plug End gases, in front of flame front, is being compressed. This causes the temperature of the end gases to rise. When temperature of fuel and air in end gases reaches a critical temperature, they spontaneously ignite. This happens almost all at once. Pressure in cylinder increases dramatically over normal. Causes engine parts to vibrate wildly causing characteristic ‘knock’ sound. Causes heat transfer to cylinder walls and combustion chamber to increase due to agitation of gases. This can cause even more hot spots on the next cycle. Video: Post-Ignition Combustion

Post-Ignition Combustion Post-ignition combustion is often called detonation. It is not always possible to determine whether the abnormal combustion is pre-ignition or post-ignition so generically they are both called detonation. Post-ignition abnormal combustion can be caused by over advanced ignition timing. Causes flame front to start too early in compression stroke. End gases end up being compressed too much by flame front. This causes temperature to rise abnormally high. Reaches critical temperature at which it spontaneously ignites. Post-ignition abnormal combustion can be caused by fuel with an octane rating that is too low. A fuel’s octane rating determines the critical temperature at which it will spontaneously ignite. Damage usually shows up as melting down the sides of the pistons with damage to the ring lands. Post-ignition combustion can damage or destroy the engine is a few revolutions.

Preventing Abnormal Combustion Use high octane fuel Spark not overly advanced Knock down all sharp edges on parts in the combustion chamber Use proper heat range spark plugs Use a ‘closed loop’ spark control system. Example: J an S Safeguard Modern engines have highly advanced closed loop ignition systems. Can detect knock Can retard spark in response When it feels it is safe, it will re-advance spark Can richen up mixture to quench combustion, cool off.