Combustion Phenomena

1. 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.

2. 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.

3. 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)

4. 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.

5. 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.

6. Combustion Phenomena • States of combustion • Normal combustion • Abnormal combustion • Pre-ignition combustion • Post-ignition combustion

7. Normal Combustion • Spark plug fires. • 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

8. 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.

9. 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.

10. 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

11. 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.

12. 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.