RE: Combustion and Ignition
Combustion and Ignition Part One
Now that we all understand what is involved in pumping air through the motor, it is time to delve into the portion of the cycle that actually produces power. The combustion process is probably the least understood aspect of modern internal combustion engines. Rotary engines have not had the benefit of extensive combustion research as piston engines have.
The point of filling the chamber with combustible mixture is of course to set it on fire, and convert this heat energy to torque at the eccentric shaft, as efficiently as possible. This seems simple enough on the surface, but it is every bit as complex as the other cycles, and just as important
- The Big Bang!
The first step towards understanding the combustion process is the realization that the air fuel mixture does not explode when the spark plug fires. If this were to happen, the engine itself would explode within seconds. Normal combustion is characterized by the flame front travelling smoothly from the point of ignition to the outer edges of the combustion chamber. The best analogy I have heard, is that the flame travels much like dry grass burning in a field, with the starting point being equivalent to the spark plug location. (Please, no field burning experiments!)
- Peak Cylinder Pressure, Maximum Torque Transfer, and Ignition Timing.
As the spark plug fires, initiating combustion, the fuel burns, and the pressure in the chamber increases. It is this pressure, reacted through the rotor, that spins the eccentric shaft creating horsepower. At approximately 45 degrees after top dead center, the rotor has the greatest mechanical advantage over the eccentric shaft. This is the point of maximum torque transfer to the eccentric shaft. It is often stated that to maximize power output, peak cylinder pressure should occur at this point. This is a gross oversimplification, and does not consider the average torque transfer throughout the cycle.
This illustration (From "The Rotary Engine", by Kenichi Yamamoto) shows gas pressure and torque fluctuation throughout the expansion cycle. Note the torque reversal before TDC shown on the bottom chart. This illustrates the power required to compress the mixture as the chamber nears TDC. Maximum output will occur when the ratio of cylinder pressure after TDC, to cylinder pressure before top dead center is the greatest. If for instance we had the option of manipulating the cylinder pressure, we would want to place all of the pressure after TDC, centered at the point of maximum torque transfer, with none of the pressure ocuring before TDC.
In the real world we do not have this option because the fuel does not burn fast enough to complete the combustion process within such a narrow range. In order to achieve the greatest cylinder pressure during this period of high torque transfer, we must ignite the mixture well before TDC. At full throttle, ignition occurs at approximately 25 degrees before TDC. In that period between ignition, and TDC, the chamber volume is decreasing, compressing the mixture that has just been ignited. Compressing this burning mixture (Which is expanding.) requires a certain amount of power, but this power loss if offset by the increased pressure "Under the curve" after TDC.
If you look closely at the torque fluctuation chart, you can see that the rotor effectively transfers power to the eccentric shaft over a very narrow range. If you give this a bit of thought, you will realize that the power output of the engine can be increased by increasing flame speed. Increasing the flame speed will release the energy from the fuel in a shorter period of time. This will allow more effective use of this energy by timing the combustion process to more closely match the torque transfer curve of the rotor/eccentric shaft combination.
Mazda's description of ignition goes something like this: "The air/fuel is ignited when the mixture in the spark gap, electrically energized sufficiently to resist the heat loss in its vicinity, is heated and ionized exceeding it's ignition point." A more simple explanation is that the spark heats the fuel until it is hot enough to burn. The important part of Mazda's explanation is that "Enough heat must be generated to resist the heat loss in its vicinity." As the plug fires, heat from the spark will be absorbed by the air/fuel charge in the gap, the spark plug itself, and the relatively cool metal of the rotor housing. Or in Mazda's words, "Everything in its vicinity."
The heat absorption into the rotor housing, and more importantly the spark plug, increases the spark energy required to ignite the fuel. While this energy is easily produced by a good ignition system, it is important to understand that heat absorption greatly affects the ability to ignite the fuel.
- Flame Travel.
Once the mixture in the spark gap has been ignited, the flame will contine to spread by the same process of successively heating gasoline molecules to their ignition point. This heat transfer from one molecule to another is in effect, the definition of combustion.
Now that you understand what can be gained by increasing the rate of combustion, it is time to discuss the several factors that affect the combustion rate.
- 1-Air Fuel Ratio
Flame travel through a gasoline and air mixture will be fastest within a fairly narrow mixture range from 11.5:1 to 13.5:1. Flame speed will decrease rapidly outside of this range. This varies a bit from one fuel to the next, but the above stated range is a generally accepted estimation.
- 2-Exhaust Gas Dilution
The presence of exhaust gasses in the mixture will slow the flame travel considerably, because the exhaust gasses will not burn. The heat transfer that was described above will be slowed, because these gasses will absorb a portion of the heat needed to ignite the fuel molecules. This same situation occurs when a high percentage of water is present, such as on a very humid day.
At idle, the percentage of exhaust gasses in the mixture can exceed 70%. At high rpm, if the exhaust flow is insufficient, the result will be excessive exhaust gas dilution. In addition to slowing the flame travel, this exhaust gas dilution will also increase the occurence of misfires, and "weak fires." (Incomplete combustion.)
Note:Even at 100% volumetric efficiency, approximately 10% of the chamber volume will be made up of exhaust gasses from the previous cycle. (Rememebr the clearance volume discussed in Part Two of the exhaust cycle article?)
- 3-Charge Density
The density of the charge also has a substantial affect on the flame speed. Quite simply, a denser (Greater mass per unit volume.) charge will burn at a faster rate. Higher density will increase the speed of heat transfer from one molecule to the next because there will be less dead space between these molecules.
Higher charge density can be achieved by increasing volumetric efficiency, decreasing charge temperature, or increasing the compression ratio. All three of the above conditions will increase the power output, and thermal efficiency of the engine.
- 4-Mixture Distribution.
It is easy to assume that the air and fuel molecules are thoroughly mixed, but in the real world, the distribution is less than optimum. As was stated earlier, the fastest flame travel occurs within a fairly narrow range of mixture ratios. If the air/fuel ratio is excessively rich in some areas, and excessively lean in others, the flame will spread in an irregular manner. This condition slows the combustion process.
Poor mixture distribution causes problems in addition to slowing combustion. It is not only possible, but normal to have a mixture in the spark gap that is too rich or lean to fire efficiently. This condition is most likely at low engine speeds when there is insufficient velocity to keep the fuel droplets in suspension, but it happens at higher speeds as well. The result is at best, incomplete combustion, and at worse, a complete misfire.
Complete misfires happen more often than you may think. At 6,000 rpm, each chamber will complete 100 cycles per second! At this rate, you could have several misfires in a row, and not interpret it as such. At best you would sense that engine was running rough.
What is commonly referred to as atomization, is actually large globs of fuel dispersed through the air. It is far from being atomized. Still, it is the commonly accepted term, and has come to refer to the size of the fuel droplets. Since the fuel must be in the presence of oxygen to burn, you can see that only the molecules on the outer "skin" of these fuel droplets will be able to burn unless something is done to break them into smaller particles.
As the flame travels through the mixture, the heat will vaporize the fuel droplets, and allow the greater percentage of fuel to come in contact with oxygen so that it will burn. The problem here, is that this vaporization is the result of heat absortion. The result is decreased flame travel speed, as much of the heat energy is being used to vaporize the fuel. If the fuel is not sufficiently vaporized when it enters the chamber, the ignition system will not be able to generate enough energy to initiate combustion!
This may seem unlikely to some of you because gasoline is considered by most to be extremely volatile. The fact is that until it mixes with air in the correct ratio, (Approximately 10:1 to 20:1) it will not ignite.
The biggest contributor to poor atomization is low inlet velocity. While this is much more of a problem with carbureted engines, fuel injected engines will also suffer from poor atomization if inlet velocity is excessively low. In the case of modern emission controlled engines, the injectors are placed as close to the combustion chamber as possible to reduce fuel "drop out" as the mixture travels from the injector to the combustion chamber.
To help illustrate this point, consider the way that a fuel injected car runs when the injectors are dirty the atomization is poor!
- 6-Heat Absorption
The final consideration is the relatively cool metal parts that make up the combustion chamber. The combustion chamber is made up of the face of the rotor, and the inner surface of the rotor housing. Both of these surfaces are kept relatively cool by oil and water. These cool metal surfaces will absorb heat during the combustion process, slowing the flame travel. In addition to decreased flame speed, the air fuel mixture immediately adjacent to these surfaces will not burn!
These cool surfaces will absorb enough heat from the air fuel mixture to keep it from reaching its ignition temperature. The rotary has a tremendous amount of surface area as compared to a piston engine of the same displacement. This is referred to as the surface to volume ratio. The rotary engine has a very high surface to volume ratio, and its poor thermal efficiency is due to this.
- Cyclic Variations
The mixture distribution, and so the combustion efficiency, will vary a great deal from one cycle to the next. This is due to the random nature of both fluid flow, and combustion. It has often been stated in automotive literature that the spark energy required to ignite homogenous air fuel mixtures between 11.5:1, and 13.5:1 is very low. This statement is normally taken one step further to suggest that once this minimum spark energy requirement has been met, there is nothing to be gained by using high current ignition systems.
This may be true in simple laboratory tests, with homogenous mixtures, but it ignores the dynamic nature of internal combustion engines. The fact is, that not only does the mixture quality in the spark gap change from one cycle to another, it is also changing during the cycle. For this reason, long duration, or multiple sparks are very beneficial to combustion efficiency. This is especially true in rotary engines where the air fuel mixture is traveling past the spark plug at a high rate of speed when it fires.
With several articles under your belt, it should now be obvious that the rotary engine (Or any internal combustion engine for that matter.) is a very complex device. Nothing illustrates this more clearly than the fact that exhaust efficiency, which seems completely unrelated, has a considerable effect on combustion efficiency.
A successful performance engine is the result of optimizing many different variables from header lengths to spark plug selection, and something as simple as intake velocity will affect not only low speed power, but high speed power as well through its effect on mixture quality.
The key is to understand why one variable affects another.
Paul Yaw Yaw Power Products