Let's talk COMBUSTION!
Thread Starter
Safety Car
Joined: Jan 2000
Posts: 4,965
Likes: 0
From: Midlothian, VA
Let's talk COMBUSTION!
I have picked up a few ASE certification books and have really been digging into them. As a fellow car nut looking for that extra power we all would like to attain I thought that the best approach would be of one of learning the fundamentals and basics of what gives us that power we all crave, COMBUSTION. Here's a break-down of what I have learned thus far....(Part I, the rest to be continued tomorrow after some rest 
)
What is combustion? Combustion is the burning of air and fuel above the engine's piston that ultimately leads to a massive production of heat and pressure that forces the piston to BDC (Bottom Dead Center), thus rotating the crank. In order for fuel to be ignited in an engine the air and fuel must swirl together and mix very well to give us the rapid combustion/explosion the engine needs do drive the piston back down. In rapid combustion, we get the hydrogen from the fuel combines with oxygen in the air to from a steam and carbon in the gasoline combines with air to make the relatively inert CO2 (carbon dioxide). In a perfect environment combustion would from water and carbon dioxide, but as we all know, perfect or ideal environments don't exist. For instance, our air is comprised of much more that oxygen, this is why there are so many by products and pollutants produced when air is mixed with gasonline and burned. Incomplete combustion.
What is going on under "Normal Combustion"? Let's say that conditions are optimal (air/fuel ratio, compression, spark timing, etc) thus we have optimal fuel burning conditions. In an engine the spark plug will ignite the air/fuel mixture so that the flame originates at the spark plug and spreads away from the point of ignition quickly and evenly in all directions. But normal combustion is slow in comparsion to an explosion. An explosion happens in a fraction of the time it takes for a fuel such as gasoline to combust. So we can't have the fuel burn too slow as there wouldn't be enough heat generated nor pressure created to efficiently drive the piston down nor can the combustion be too fast that the heat and pressure generated is far too great and stresses and damages vital engine components.
Types of ignition We have two fundamental types of fuel ignition: Preignition and Postignition. They are in principle, somewhat the same, but they differ greatly in their outcomes. Preignition is exactly what it stands for. The air/fuel mixture is ignited prior to the spark plug's normal ignition rate. Generally, a hot spot in they cylinder (i.e. a hot carbon deposit) will cause the end of the unburnt fuel to ignite before the spark plug ignites. This causes the two flames to burn toward each other and smash into each other causing a small pressure spike which causes the engine components to vibrate and emit a high pitch frequency heard as a "pinging". Prolonged pinging can lead to the more serious "detonation" and engine damage. Preignition as I mentioned was due to a hot spot, generally. This is known as surface ignition and this is caused by:
1. Incadescent piece of crabon in the combustion chamber.
2. An overheated engine due to improper cooling.
3. An exhaust valve overheated by a lean fuel mixture ( clogged injector, clogged fuel pump, stuck EGR valve, vacuum leak in PCV system, etc)
4. Overheated spark plug (to high of a heat range)
5. Exhaust valve overheated by leakage (bad valve spring, sticking valve, etc.)
6. Sharp edges in combustion chamber from gasket material, threads on spark plug.
7. Very hot intake air temperature and dry atmospheric conditions.
The latter type of ignition is postignition and this has many examples. Postignition is just what it means as well. We have ignition of the last of the unburnt fuel after the ignition of the burnt fuel. This gives us Wild Knock and Detonation. Wild knock occurs when a piece of incadescent carbon in the combustion chamber bounces around in there and hovers into the air/fuel charge and ignities it after engine ignition. This can cause a pinging sound to come and go at random for no real reason. Now, we arrive at that dreaded term, detonation. By far the most severe scenario of postignition. Detonation occurs when the last of the unburnt fuel ignites so quickly that it in effect causes and explosion (remeber an explosion occurs in the fraction of the time it takes for normal combustion). Thus you have a tremendous increase in heat and pressure which collides with the beginning of the burnt fuel and amplifies the heat and pressure produced and causes great harm to your motor. The intense heat can soften your piston domes which can lead to fracture and holes in your pistons, melt spark plugs, damage and break connecting rods, etc. This tremendous force exerted on the piston before it reaches TDC (top dead center) forces the piston out of its normal upswing in the cylinder. We get a large shock suffered to the piston and causes great vibration which is heard as a loud knock in your motor. Usually detonation caused knocking will be heard when you push the gas pedal to WOT when the car is traveling at normal engine speeds of 20 - 40 MPH. The knock generally isn't heard at high speeds/rpms because there really isn't enough time for the end gas to ignite and cause detonation. Detonation can be thought of as a race between the normal combustion flame and the autoigntion of the end gas. If normal combustion goes faster than autoignition, normal combustion prevails and detonation is prevented.
Factors attributed to detonation:
1. A slow burning, lean fuel mixture
2. Gas with low octane
3. Carbon deposits increase the compression ratio in the combustion chamber
4. Engine Operating temperature is too high
5. Ignition timing is too advanced
6. Bad rings and/or valve seals allowing oil to be burned in the cylinders (I dunno where I've seen that :rolleyes: )
7. Air Cleaner passages clogged allowing too much hot exhaust manifold air to enter engine.
8. Excessive boost from a S/C or Turbo application.
How if fuel mixed with air? The fuel is mixed with air by a process of atomization. This is the same principle that occurs when we pull the trigger on that bottle of Z7 :D. In our case the fuel injector breaks up the liquid fuel stream into tiny droplets that are sprayed into the engines combustion chamber and allowing it to mix more evenly with air to increase the speed of combustion. The smaller the droplets the more surface area you can produce with the same amount of fuel, i.e. smaller droplets, the more oxygen the fuel can associate with and hence the faster combustion rate.
This is all for now, tomorrow I will pick up on Spark Timing, Spark Intensity/ Duration, Air-Fuel Ratio, Compression, etc. :D Night fellas!
[Modified by C5Noir, 2:19 AM 8/21/2001]
)What is combustion? Combustion is the burning of air and fuel above the engine's piston that ultimately leads to a massive production of heat and pressure that forces the piston to BDC (Bottom Dead Center), thus rotating the crank. In order for fuel to be ignited in an engine the air and fuel must swirl together and mix very well to give us the rapid combustion/explosion the engine needs do drive the piston back down. In rapid combustion, we get the hydrogen from the fuel combines with oxygen in the air to from a steam and carbon in the gasoline combines with air to make the relatively inert CO2 (carbon dioxide). In a perfect environment combustion would from water and carbon dioxide, but as we all know, perfect or ideal environments don't exist. For instance, our air is comprised of much more that oxygen, this is why there are so many by products and pollutants produced when air is mixed with gasonline and burned. Incomplete combustion.
What is going on under "Normal Combustion"? Let's say that conditions are optimal (air/fuel ratio, compression, spark timing, etc) thus we have optimal fuel burning conditions. In an engine the spark plug will ignite the air/fuel mixture so that the flame originates at the spark plug and spreads away from the point of ignition quickly and evenly in all directions. But normal combustion is slow in comparsion to an explosion. An explosion happens in a fraction of the time it takes for a fuel such as gasoline to combust. So we can't have the fuel burn too slow as there wouldn't be enough heat generated nor pressure created to efficiently drive the piston down nor can the combustion be too fast that the heat and pressure generated is far too great and stresses and damages vital engine components.
Types of ignition We have two fundamental types of fuel ignition: Preignition and Postignition. They are in principle, somewhat the same, but they differ greatly in their outcomes. Preignition is exactly what it stands for. The air/fuel mixture is ignited prior to the spark plug's normal ignition rate. Generally, a hot spot in they cylinder (i.e. a hot carbon deposit) will cause the end of the unburnt fuel to ignite before the spark plug ignites. This causes the two flames to burn toward each other and smash into each other causing a small pressure spike which causes the engine components to vibrate and emit a high pitch frequency heard as a "pinging". Prolonged pinging can lead to the more serious "detonation" and engine damage. Preignition as I mentioned was due to a hot spot, generally. This is known as surface ignition and this is caused by:
1. Incadescent piece of crabon in the combustion chamber.
2. An overheated engine due to improper cooling.
3. An exhaust valve overheated by a lean fuel mixture ( clogged injector, clogged fuel pump, stuck EGR valve, vacuum leak in PCV system, etc)
4. Overheated spark plug (to high of a heat range)
5. Exhaust valve overheated by leakage (bad valve spring, sticking valve, etc.)
6. Sharp edges in combustion chamber from gasket material, threads on spark plug.
7. Very hot intake air temperature and dry atmospheric conditions.
The latter type of ignition is postignition and this has many examples. Postignition is just what it means as well. We have ignition of the last of the unburnt fuel after the ignition of the burnt fuel. This gives us Wild Knock and Detonation. Wild knock occurs when a piece of incadescent carbon in the combustion chamber bounces around in there and hovers into the air/fuel charge and ignities it after engine ignition. This can cause a pinging sound to come and go at random for no real reason. Now, we arrive at that dreaded term, detonation. By far the most severe scenario of postignition. Detonation occurs when the last of the unburnt fuel ignites so quickly that it in effect causes and explosion (remeber an explosion occurs in the fraction of the time it takes for normal combustion). Thus you have a tremendous increase in heat and pressure which collides with the beginning of the burnt fuel and amplifies the heat and pressure produced and causes great harm to your motor. The intense heat can soften your piston domes which can lead to fracture and holes in your pistons, melt spark plugs, damage and break connecting rods, etc. This tremendous force exerted on the piston before it reaches TDC (top dead center) forces the piston out of its normal upswing in the cylinder. We get a large shock suffered to the piston and causes great vibration which is heard as a loud knock in your motor. Usually detonation caused knocking will be heard when you push the gas pedal to WOT when the car is traveling at normal engine speeds of 20 - 40 MPH. The knock generally isn't heard at high speeds/rpms because there really isn't enough time for the end gas to ignite and cause detonation. Detonation can be thought of as a race between the normal combustion flame and the autoigntion of the end gas. If normal combustion goes faster than autoignition, normal combustion prevails and detonation is prevented.
Factors attributed to detonation:
1. A slow burning, lean fuel mixture
2. Gas with low octane
3. Carbon deposits increase the compression ratio in the combustion chamber
4. Engine Operating temperature is too high
5. Ignition timing is too advanced
6. Bad rings and/or valve seals allowing oil to be burned in the cylinders (I dunno where I've seen that :rolleyes: )
7. Air Cleaner passages clogged allowing too much hot exhaust manifold air to enter engine.
8. Excessive boost from a S/C or Turbo application.
How if fuel mixed with air? The fuel is mixed with air by a process of atomization. This is the same principle that occurs when we pull the trigger on that bottle of Z7 :D. In our case the fuel injector breaks up the liquid fuel stream into tiny droplets that are sprayed into the engines combustion chamber and allowing it to mix more evenly with air to increase the speed of combustion. The smaller the droplets the more surface area you can produce with the same amount of fuel, i.e. smaller droplets, the more oxygen the fuel can associate with and hence the faster combustion rate.
This is all for now, tomorrow I will pick up on Spark Timing, Spark Intensity/ Duration, Air-Fuel Ratio, Compression, etc. :D Night fellas!
[Modified by C5Noir, 2:19 AM 8/21/2001]
Thread Starter
Safety Car
Joined: Jan 2000
Posts: 4,965
Likes: 0
From: Midlothian, VA
Re: Let's talk COMBUSTION! (z06bliss)
I guess my point is trying to help us all understand our engines better, why we are plagued with certain ailments (KR) and how we can begin to go about fixing them in scientific manner
