Compression sensitivity and ether %
 

Has anyone done any tests to determine how the percentage of ether affects the compression setting sensitivity?

This is one issue that seems to cause concern for use of diesels in competition use. As I recall the fixed compression diesels used very high percentage of ether but would also tolerate a wide range of compression and still run with acceptable power (not as good as today's ether/kero blends in modern diesel).

The norm for diesel fuel used today is 30 - 33% ether. For competition applications one needs to be able to have consistent power and run times. The engine must also sound like it is consistent meaning if it is missing in maneuvers it pulls attention from the maneuver to the engine and you lose points.

It seems that part of the solution might be to run higher levels of ether. Is there any other additives that might also help broaden the acceptable compression range?

RE: Compression sensitivity and ether %
 

I'll let some of the more sophisticated folks directly answer your question. Also, I have never flown a PAW, so my experience is with other brands. Basically, to fly stunt, you want the engine at the lowest compression and leanest setting which gives you a consistent fast run. If it burps in level flight it is a little undercompressed. If it burps in maneuvers it is too lean, or you have a tank problem. If it slows down in maneuvers, it is overcompressed.

I have found that as I run engines over time, the right setting progressively moves toward a hair lower compression and a hair leaner. I would say once you get a needle setting you like, leave the needle alone. Because a diesel is fuel efficient, a tiny change of the needle makes much more difference than with a glow engine. So far as tanks go, I fly only uniflow tanks.

RE: Compression sensitivity and ether %
 

I was typing a response and closed the window by mistake. I'm going along with Jim on this one. Fuel delivery is important in my opinion. The only thing in the engine that you might fiddle with is a fine taper needle and other venturis or spraybars, but I think PAW parts should be good.

RE: Compression sensitivity and ether %
 

In looking at some old literature on model diesels one of the fuel formula's used turpintine with ether and oil and kero. Does anyone know if turpintine adds anything to the quality of the engine run? Why did they use it?

RE: Compression sensitivity and ether %
 

People have mixed all sorts of diesel fuel concoctions. Many ran well enough to suit them. It has sort of settled out that kerosene is the usual thing. I would think if something relatively cheap and available, like turpentine, would be an improvement, it would be in general use. Just speculating.

RE: Compression sensitivity and ether %
 

While turpentine is flammable it is most useful as a solvent. The are only two things I can think of that could have recommended it for model diesel fuel mix 50+ years ago. First, it would de-gum the castor oil in the engine and, second, since it doesn't mix with water probably helped keep moisture from being attracted to engine parts between flying sessions.

RE: Compression sensitivity and ether %
 

Hi Stuntguy13

My memory is a bit hazy on this one but some experiments were done in the 60's and published in Aeromodeller magazine regarding the use of higher proportions of ether in the fuel mix to overcome the absence of "dope" (amyl nitrate etc) and the conclusion again from memory was that compression could be reduced and that the engine would run cooler than normal. Does anybody remember the test article?

Ployd

RE: Compression sensitivity and ether %
 

Ployd, everything you mention sounds reasonable. The same things we saw in our etherless experiment thread. The only thing missing is the likely small loss of power by running a higher percentage of ether.

RE: Compression sensitivity and ether %
 

Ployd,

You are correct. Di-ethyl ether has a much lower flashpoint so lower compression can be used. As gkamysz said, kerosene is the power ingredient so you need to balance the mix.

BTW, if you have not read the etherless thread, it is worth your while to read at least some of it (it's approaching 30 pages now).

Additives such as amyl nitrate or nitrite makes the mix run a little better. Some of the old, very small engines recommended as high as 60% ether and one even recommended just ether and oil.

Most engines seem to run well in the 30%-40% range which allows good power with lower compression. One of the signs that your ether is evaporating in the container is that you start to need more compression to start and run the engine.

Some of us use adequate ether so our old, irreplaceable diesels will last longer.;)

George