Combustion in glow plug engines
Introduction
This is an attempt to in laymans terms explain how the combustion takes place in a glow plug engine. First I will try to explain some basic thermodynamic and basic combustion phenomena. To later move on to the combustion in the engine. I will only discuss those parameters that are of interest to these kinds of engines.
The Heat Engine
All heat engines are based on the fact that hot gas expands more than cold gas. This means that you take a "cold" gas compresses it. After it has been compressed you heat it up, usually by some sort combustion. After that the gas is expanded. The good part is that you get more work from expanding the warm gas then was used to compress the cold gas. You end up with positive work. This is the heat engine. This is how your car engine work and also how your glow plug engine works.
There are several ways of solving the 'heat adding part', but we will talk about how that happens in the glow engine. In the glow plug engine the gas you use is mixture of air, fuel and oil, that will compress, combust and expand, and hopefully give some positive work.
Next I will discuss some things about the fuels we use.
Fuels
Heat of vaporization
The fuels that is used are normally liquid. But for any thing to burn it has to be in a gaseous form. So the liquid fuel has to vaporise before it can combust. A little physics: When substance goes from liquid to gas heat has to be supplied (this is usually referred to as boiling). This heat will help cool the engine and the incoming air. This cold gas will make the compression easier.
This heat is called Heat of vaporization.
Lower heating value
When you burn a fuel it will let of heat. This is what we call heating value. It the table you can see that I called it 'lower heat value'. There is also a higher heating value, but when talking about piston engines, always and only look at the lower heating value. In case you look things up in tables, look only for the lower heating value when you compare numbers. Else you will be fooled.
Stoichiometric Air / Fuel ratio
To get the most from a burning mixture, it is best to burn the fuel completely as well as all the oxygen. When this balance is right, is what is called stoichiometry. How many kg of air needed for 1 kg of fuel.
Heating value of the Mixture
This is where it gets interesting! What we put into out engine is a mixture of air and fuel. So the heating values isn't that interesting, when in comes to performance in heat engines. What is interesting is how much heat you can put into the cylinder. Remember we put a mixture if fuel and air there not just fuel. By having a mixture with lots of heat you would also got more power. As you can see in the table below having methanol and gasoline is almost equal in mixing value. But nitromethane is in an totally different ballpark. Now you might realize why we put nitromethane in our fuel.
 Methanol Nitromethane Gasoline Stoichiometric A/F ratio 6.45:1 1.7:1 ~14.5:1 Lower Heating value [MJ/kg]{kWh/kg} 19.9 {5.5} 11.3 {3.1} 43 {12} Heat of vaporization [MJ/kg] 1.17 0.56 0.18 Heating value mixture[MJ/kg] 3.08 6.6 2.96
Table of interesting properties of fuels. I added gasoline, most of you deal with this fuel everyday (like in your car), as a comparison.
Methanol
The simplest alcohol is a liquid with low heat content compared to gasoline and it has also a significant higher heat of vaporization. Methanol also has a low ignition point. It will ignite at about 500°C. This is a fairly low temperature. It explains why a glow plug is working, our engine are ignited by surface ignition from the glow plug coil.
Nitromethane
Nitromethane is used as fuel but it's very special. One special thing is that is requires very little air to burn. This is nice when you blend it with other fuels. You will be able to get more heat into the cylinder. By adding more fuel you get more cooling from the added fuel to the mixture.
An other fact of Nitromethane is that it's can be a mono propellant. That means that nitromethane can burn in the complete absence of air. In other words you could run your engine on only nitromethane.
Nitromethane burns very quickly and has a high tendency to knock. That is one reason why it isn't good to run exclusively on Nitromethane.
Oil
This is where I am more uncertain there are a vast amount of different oils and these oils have different features. The fuel oil is essential to the 2 stroke engines, and in some cases it probably has an impact on combustion. This will be discussed later.
One suspicion I have is that huge amounts of Oil will slow the rate of combustion down.
Usual glow plug engine fuels
Usually the glow fuel can be divided into three different categories according to Nitromethane content: No Nitro (0-5%), Low Nitro (~15%) and High Nitro (~30%).
The question what is the difference between these fuels?
In the first case you are only combusting pure methanol. Maximal power is obtained by running 1:4 fuel to air ratio. This means running fuel rich, and you get lots of fuel cooling.
Secondly when running 15% Nitromethane, this makes it possible of running even richer with more heat being released in the cylinder. I also makes for a faster combustion.
Third 30% nitro. Here we add more fuel and get more cooling. We should also get a more rapid combustion. But this seems not to be the case. My theory is that the vast amount of oil that is present in the cylinder at this time will reduce the flame speed. To make the combustion slower. The reduced temperature of the large amount of fuel can also be a reason for the combustion not being so fast. But the Nitromethane will probably stabilize the combustion as there is plenty of highly reactive fuels around.
Combustion
Now its time to explain some for fundamental things regarding combustion. I will begin with the simple flames, then move on to turbulence and later progress into how this fits inside engines.
Flames and laminar flame speeds
First we start off by having a quiescent fluid. This means a mixture of air and fuel, all in gas phase. Everything in the container is still, no movement of the gases. But the gas is well mixed.
Then a spark will the shoot off on one side of the container. Now we would see a flame front propagating throu the fluid. What will be seen is a light emitting sheet travelling through the fluid concentrically from the spark onwards.
The speed of which the flame propagates in this case is call the laminar flame speed. For methanol it is about 40[cm/s] at room temperature and pressure. Methanol burn pretty fast, gasoline burns about 10-20% slower. But remember! The laminar flame speed is dependent on air / fuel ratio, pressure and temperature.
All in all if one puts numbers behind combustion duration and the speed that our engine have. We will see that things don't add up. If the gas burned with laminar flame velocity. The engine would not rev. Example follows:
Say that we run the engine at 15000 rpm. That is 250 combustion's per second. We will assume that the combustion duration is 80 ° Crank angle degree (CAD). That makes the entire combustion time per cycle to 0.8 ms. Say that the flame has to travel 1 cm this makes the flame velocity of about 1100 cm/s. This is about 30 times to slow in we where using the laminar flame velocity. So things must go faster.
Turbulence
First I will try to explain how turbulence affects our engine. I will not explain in detail what turbulence actually is, because can be a hot hot potato. When a fluid (gas or liquid) flow slowly, it usually flows with a laminar flow pattern. When the flow speed increased it's flow pattern is turbulent (irregular).
If you look at the smoke coming from a cigarette. The first part of the smoke it smooth, there are smooth flow lines, later on there will be a transition to a more irregular pattern. This is the transition from laminar to turbulent flow.
If a flame is ignited in a turbulent flowing environment, the burn rate increases considerable. It makes the flame go so fast as it will bridge up the gap between the laminar case to the turbulent case.
Turbulence make the flame go faster. There is also some evidence that strong turbulence will stabilize the combustion. If the mix is uneven, strong turbulence will stabilize this.
How turbulence is created
It is not very hard to imagine that the flow inside an engine that has 250 cycles per seconds should be turbulent. But there are still mechanisms that controls the burn rate in the engine.
Squish
You might have heard about the squish area. It is the outer part of the combustion camber. During the last part of the compression stroke the the gas that resides below the squish area will be forced towards the center of the combustion chamber. This high velocity movement will generate high intensity turbulence during combustion.
By changing the area of the squish band the amount of gas that is forced is changed and effectively changing the turbulence.