Hello Graham,

I would suggest you stay away from Tetra Ethyl Lead, since it is an Octane Booster, which is going in the wrong direction.

An Octane booster is used to DECREASE the ease of Auto-ignition.

A Cetane booster is used to INCREASE the ease of Auto-ignition.

This entire subject gets mucked up in modeler minds for several reasons.

1. We are using a hybrid engine, not a true diesel engine.
2. We are adjusting the compression to affect ignition and ignition timing.

Please let me explain.

Our Glow/Diesel model engines use a carburetor for air and fuel induction. The fuel/air mixture (FAM) is drawing into the engine via differential pressure (Vacuum). This FAM is atomized and then vaporized within the engine and then compressed.

In a normal glow engine the glow plug is the source of ignition, the physical characteristics of the glow plug determine ignition timing. I.e plug temp, number of coils of the wire, wire diameter. The Compression Ratio (CR) also determines the ignition point as well as the load on the engine. I.e the prop size and shape. These factors are quite limited in adjustment, so a major change in ignition timing is not as easy to achieve as with the model diesel engine.

In a model diesel engine the induction system works the same as in the glow engine, but now we have a contra-Piston or just a simple disk-plate that has been adjusted to meet the pressure/temperature needs of our fuel. Thus we can also adjust the ignition timing by adjusting the CR of the engine. You must increase the CR of a cold engine for startup. This is to help achieve the require CR to reach auto-ignition temperatures. As the engine reaches a stable temp we must decrease the CR, which affectively reduces the timing of the auto-ignition. The wonderful part of all of these matters is that fact that a model diesel engine can swing a huge heavy prop to create a great deal of low velocity thrust, much like today’s modern electric park flyer aircraft/motor combinations. It can also swing a small high speed prop by increasing the CR which advances the timing.

In a real (normal) diesel engine, fuel does not enter the engine via the induction system. There is no carburetor, only an air-valve, just a butterfly valve to admit air. A direct injection fuel pump injects the fuel into the cylinder at exactly the correct time to facilitate, complete vaporization of the fuel, which does take a few milliseconds to occur. The fuel is injected into the cylinder that has already compressed (only) the air to a very high temperature and pressure. When the fuel hits this very hot environment it goes from a liquid to a vapor and then mixes with the oxygen within the cylinder and undergoes auto-ignition. The timing of this engine can be adjusted by the fuel injection pump and this does change with RPM and varying load changes, often using centrifugal weighs within the injection pump system. Sometimes for startup, the timing can be altered manually in some engines.

The important part of this discussion is not a treaties on how diesel engines work but to show the fact that our model diesel engines are not truly diesel engines. In fact they are working in a very difficult environment. We are asking them to detonate, or you could say run in a pre-ignition mode continuously. Now if you ask your auto engine to run like this it would not last long. It would be constantly pinging. What we are doing is asking our little engines to ping, or auto-ignite every other stroke. We should consider that they would want a fuel that is easy to auto-ignite, so that the CR can be maintained at a lower ratio. Cetane boosters give the engine this edge. They don’t all work the same, but for a simple explanation you can consider that they allow the hydrocarbon chains in the fuel mixture to fracture at a lower pressure or lower CR.

Octane booster like Tetra Ethyl Lead are added to fuel to keep the fuel from falling apart, to help the fuel hold up to a higher CR, or allow an engine to be run at a more advanced ignition timing. In a gas engine we are compressing both the air and fuel (FAM) as the piston reaches top dead center we don’t want the mixture auto-ignite without control, we want it to wait until the sparkplug fires. This control is critical, so in WW II it was found that high levels of Tetra Ethyl Lead would delay this auto ignition and thus high-octane aviation fuel allowed the Allied Forces to produce very high power to weight aircraft engines. Even today Aviation Blue 100 low lead has a huge amount of this very harmful and dangerous fuel additive, which was remove from car fuel in the early 1980’s.

In summation: our hybrid model diesel engine are not really diesel engine in the normal sense, since they compress the Fuel Air Mixture together during the compression stroke and not direct injected. We also must consider that our engine must be lightweight and thus we are forced to use as low of a Cr as possible to achieve Auto-ignition. Thus we need either ether or some other substance mixed with our fuel to increase the Cetane of the fuel. Just the opposite of what most folks think when we have such discussions. Everyone is so use to hearing Octane they get confused. Stay away from the ketones like acetone, lacquer thinners, and other things like alcohols since they have very high Octane.

Keep in mind that you are trying to achieve a low ED (energy density) fuel that is easy to auto-ignite. This is not easy, but it can be done. NO Black Magic mixes are needed, no voodoo or special engines or equipment, just a bit of simple testing using a good engine with a small high speed electric started and perhaps an iron to preheat the engine. If the mixture is some sort of secret mixture then it is either dangerous, or the components are difficult to find. Model diesel engines have been around a very long time and there are few things that have not been completely tested in the past, but Amsoil Cetane Booster is a very concentrated high quality product that is relative new, so this combined with simple things like Coleman stove fuel (yes this is primarily naptha) may work quite well.

Give it a try, you can’t hurt your little engine if you work slowly and make small changes to the mixtures. We would all like to hear your test results.

I hope this is useful and interesting.

Kelly