RACING: Varibles Regarding Flame Speeds & Their Effect.
Fuel/Air Ratio - Gasoline can be ignited in normally aspirated engines between limits of 10 to 1 (rich) and 20 to 1 (lean) air/fuel ratios. Most gasolines will burn quicker at ratios in the 12.5 to 13.5 to 1 range. Max power is obtained with a rich mixture of around 12 to 13.5 to 1.
Charge Density - The Charge density is affected by the pressure & temperature of the charge. The higher the charge density, the quicker it will burn.
Charge Homogeny - Is how evenly the air and fuel molecules are distributed in the charge. This is very important to successful ignition. If there is a bunch of fuel molecules with no air present between the spark plug electrodes at the time that the spark jumps, there will be no ignition. If the charge between the spark plug electrode is leaner than 20 to 1 or richer than 11 to 1, there is a small chance for ignition. Ideally, the molecules throughout the charge should be evenly spaced and distributed, this allows for a smooth rate of burn. If the charge is randomly mixed, there will be variations in flame front propagation rates which will not produce maximum power as these may advance or delay when PCP is achieved. This phenomenon is known as ignition probability.
Charge Turbulence - Due to the fact that the charge is in constant motion from the valve and port flow characteristics along with inertial effects and piston motion, the mixing of fuel and air molecules is dynamic. From one split second to the next, the actual mixture and molecular distribution changes. This can mean in some instances, that if a very short duration spark was initiated the mixture might not ignite, whereas only a half millisecond later, conditions might be perfect for ignition. For this reason, very short duration sparks are undesirable. A long duration spark or Multispark Ignition System will ensure the highest ignition probability.
Fuel Characteristics - Low compression engines run on low octane fuel because they have relatively low charge densities and the burn rate within these confines is predictable. A low compression engine switched to 114 octane race fuel will always lose power unless the ignition advance is increased to compensate for the slower burn rates. Even then, a low compression ratio engine may lose power with the timing optimized for high octane fuel.
A high compression engine operates with much higher charge densities and consequently faster burn rates. The high octane fuel permits these rapid burn rates because it has less tendency to detonate under these conditions. As a result, high compression engines cannot realize their full HP potential without high octane fuel.
Inert Effects - Inert effects concern 2 areas. Residual exhaust gasses left over from the exhaust stroke tend to dilute the fresh charge and slow down burn rates. Camshaft timing, port flow and exhaust backpressure will affect charge dilution. Nitrogen is the major constituent of air and is essentially inert in the combustion process. Its presence lowers the burn rate but since there is little that can be done about it, it is ignored.
The second inert effect concerns the relatively cold, metal engine parts in direct contact with the combustion gasses. Combustion will not easily take place in areas where the gas temperatures are well below the ignition temperature. This is often used to advantage on engines to reduce the tendency to knock.
On many engines, a squish or quench area is used to negate combustion in certain areas to avoid knock. By having a matched area where the piston and combustion chamber come in close proximity at top dead center (TDC), the gasses are kept cool enough so that they will not ignite until the piston has moved down the bore and cylinder volumes are increasing. This keeps the rate of pressure rise below the knock limit. People are dismayed when they install a thicker head gasket to lower the compression ratio and have knocking worse than before. This is because they have negated the designed-in quench effect. A large squish area also tends to promote increased chamber turbulence which is important for mixing and power at high rpm.
Combustion Chamber Shape and Spark Plug Location - Combustion chambers and spark plug location and the number of plugs will have an effect on the time required to complete the combustion process. A large open chamber such as a hemi type has a high surface to volume ratio, it will combust more slowly than a pentroof chamber simply because it has more cold, metal molecules in contact with the combustion gasses which tends to slow reaction rates. For this reason, these chambers will require that the spark be initiated sooner to achieve PCP at the correct time.
The slowest combusting chamber would be an open chamber with a large bore size and the spark plug at one edge of the chamber. The flame front has a long distance to cover to complete combustion. By placing the spark plug in the center of the chamber, you reduce the distance that the flame front needs to travel and will be able to reduce the spark advance needed to achieve maximum power. Another solution would be to add another spark plug to create two flame fronts which would also require much less time to combust.
4 valve engines with shallow pentroof chambers (TSCC) and a central spark plug location are fast, efficient combustion chambers requiring minimal igniton advance for maximum power.