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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> % Nitro
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% Nitro - 11/13/2006 3:18:49 AM   
Semi Retired Aviator



Posts: 477
Joined: 9/20/2003
From: Melbourne Victoria, AUSTRALIA
Status: offline 		Now this may seem like a dumb question, but sometimes the human brain gets in a loop and can't see a way out; a little like not being able to see the methanol for the nitro.

As a general rule, do high compression engines require more or less nitro than low compression engines?? What's the answer?

If an engine detonates when using, say, 15% nitro, that can be reduced/eliminated by reducing the compression by putting a shim under the head, but that doesn't mean that lower compression engines REQUIRE higher nitro. It just means that detonation will be less if you lower the compression for a given % nitro.

My understanding was that the lower compression, the higher nitro content required, but now I'm not so sure, e.g. YS 4 strokes are high compression, but they run best on 15-30% nitro, and 30% nitro is a high content. Detonation can be a problem if you're not careful.

I thought that nitro was similar to ether in diesel fuels, that is, it assists in getting combustion started, and is therefore most beneficial in a low compression engine. Diesels have high compression, and in theory, therefore, shouldn't require ether to assist combustion starting, but let's leave diesels out of this for now.

Somebody give me the facts, as I've seen a lot written that would suggest both sides of this argument/discussion are valid, including on these forums.



< Message edited by Semi Retired Aviator -- 11/13/2006 6:08:45 AM >

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RE: % Nitro - 11/13/2006 3:41:31 AM   
Jeff_edge540


 

Posts: 144
Joined: 9/24/2006
From: Austin, TX, USA
Status: offline 		Higher compression requires lower nitro & a cooler plug to prevent detonation.
Perfect example is the Hungarian engine company Moki. They spec 0% nitro for their engines (and a very cold "Type F" plug). Folks in the U.S. are placing additional shims under the head to lessen the compression in order to run higher (10-15%) nitro in those engines.

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RE: % Nitro - 11/13/2006 4:57:39 AM   
Semi Retired Aviator



Posts: 477
Joined: 9/20/2003
From: Melbourne Victoria, AUSTRALIA
Status: offline
OK Jeff, I hear what you say, and that is high compression engines require lower nitro, principally to avoid detonation, but does that mean lower compression engines REQUIRE higher nitro, or just that they may run better with higher nitro?? There is a difference. It follows that they can take higher nitro than a high compression engine and avoid detonation, but do they need it?

I know guys who have reduced their engines' compressions by using shims, but they also run 0% nitro, and the engines seem to run OK and deliver acceptable RPM, but I haven't tached the engines. Do lower compression engines REQUIRE a certain % nitro to deliver more power?

What is the point in reducing compression so you can run a higher % nitro? The end result is probably about the same; high compression + lower nitro = lower compression + higher nitro??

< Message edited by Semi Retired Aviator -- 11/13/2006 5:30:58 AM >

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RE: % Nitro - 11/13/2006 8:24:32 AM   
Ed Cregger



Posts: 7744
Joined: 1/31/2002
From: Ringgold, GA, USA
Status: offline 		Part of the problem is the semantics being employed.

No glow engine "requires" nitromethane to run properly. It may run better with some nitro, but that is purely subjective.


Ed Cregger


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RE: % Nitro - 11/13/2006 9:26:11 AM   
Semi Retired Aviator



Posts: 477
Joined: 9/20/2003
From: Melbourne Victoria, AUSTRALIA
Status: offline 		Semantics to confuse certainly wasn't my intention Ed. I accept your point that glow engines don't 'require' nitro, but I have mistakenly used 5% nitro in my YS engine, a high compression engine, and it runs like a diesel tractor missing on a couple of cylinders, no matter what I do to it. When I up the nitro to 15%, it sings along, and the manufacturer recommends up to 30%, so nitro obviously has an effect. I haven't tried, but I suggest it wouldn't run at all with no nitro content.

One of the points I made in my original post was that if detonation can be reduced in a high compression engine by reducing the nitro in the fuel, does that mean that in a low compression engine, a lot more nitro could/should be used.

Guys I fly with have intentionally reduced the compression of their engines in the belief that they can use less nitro in those engines (presumably to save $$), when it appears that exactly the opposite may/should be the reality.

I'm quite confused about the issue, and logic alone would tell me that a high compression engine would be less likely to have a need for nitro than a low compression engine.

I'd welcome any further input on the issue.



< Message edited by Semi Retired Aviator -- 11/13/2006 9:28:32 AM >

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RE: % Nitro - 11/13/2006 12:23:20 PM   
speedster 1919



Posts: 1467
Joined: 12/17/2004
From: Martinsville, IN, USA
Status: offline 		The YS engines are made purposely for 20% nitro and I don't think you can call them high compression compared to others. Nitro is an American thing and since we are the largest world market most engines are decompressioned for sale here. We are nitro pigs. It starts out 5% was a benefit , if 5% worked good the 15% should be great and I will blow my fellow flyer away. Thats why we have running issues with certain brands that don't like nitro .

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RE: % Nitro - 11/13/2006 12:35:01 PM   
B.L.E.


 

Posts: 1308
Joined: 8/27/2005
From: Austin, TX, USA
Status: offline 		For what it's worth, I have tried 0% nitro fuel in a Cox .049 and it worked but was down on power, in fact it was a real dog. High compression engines will tolerate nitro if you run them really rich. With the higher compression engines, the richness that the high nitro fuel needs kind of cancels out power benefits of the higher nitro, resulting in nearly the same or even less power.
I also tried some 30% heli fuel in a OS .26 four stroke engine and it didn't seem to be any more powerful than it was on 15% fuel.

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RE: % Nitro - 11/13/2006 12:37:44 PM   
speedster 1919



Posts: 1467
Joined: 12/17/2004
From: Martinsville, IN, USA
Status: offline 		If the EPA in America banned nitro production the average flyer here would be scambling to remove shims , shave heads and buying more high compression engines like MVVS,WEBRA,ROSSI ,SUPER TIGRE, FOX ,ECT..........To burn FAI (no nitro) fuel to get more power from no nitro.

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RE: % Nitro - 11/13/2006 1:17:33 PM   
Jezmo



Posts: 823
Joined: 6/26/2006
From: Spring, TX, USA
Status: offline 		There are many factors that come into play here. Some of these are combustion shape, squish band design, and port timing. You have already mentioned compression ratio and glow plug heat range. I was into pylon racing for a while and spent (read wasted) hundreds of hours in search of even the slightest increase in power. I have mills, lathes, drill press, heli-arc, blast cabinets etc at my shop and during the pylon days modified just about everything imaginable in the engines. I was surprised to find that many things I changed affected something else and that experience led to the understanding that most of the things you can change are interelated with each other.

I also found that no two engines will respond exactly the same when modified. After an engine would suffer failure and be unrepairable I would make the exact same changes to an identical engine only to have it sometimes run better (make more power) and sometimes not run as good. There are a lot of physics involved including, but not limited to, fluid dynamics and thermal dynamics. It was fun and very educational but these days I prefer just buying a good engine and putting it on a plane (or heli) and have it run consistent. That way I spend more time flying and less time tinkering. I guess that's what happens when you get old.

Have fun and happy flying.

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RE: % Nitro - 11/13/2006 1:37:27 PM   
Sport_Pilot



Posts: 7728
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline 		We like nitro here because it is relatively cheap, compared to other countries. The main reason we like it is because it broadens the needle. That is the peak may not occur at just one click off either way, that is if at peak the RPM drops substantially on either side of the needle. This is because nitro needs more fuel for a given amount of air, thus it will run with a much richer mixture. Not only that but nitro is a monopropellent. That is that it is possible to burn it without any air. Because of this the nitro funny cars run as rich as the ignition system can ignite it, it is limited only by the amount of raw fuel the combustion chamber can hold, and to the extent that the ignition system can ignite it. That is why they use two huge electronic magnetos. To us it means it is tolerant to the rich side, that is at peak power it will actually be richer than the ideal stochastic mixture at peak, so it will be more tolerant of both rich and lean runs, thus easier to tune. The second reason we use it is because with the right compression ratio it will have more power. The more nitro the lower the compression but more power, but also more consumption. Nitro is not added to help ignition, if fact it makes it more difficult especially with cooler glow plugs. A high compression engine will run well on low nitro, but it will have slightly less power, and harder to tune (especially without finer threaded needles). Of course if you can handle the tuning and don't need the extra power, then the lower nitro will be preferable because of lower cost and lower consumption.

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RE: % Nitro - 11/13/2006 2:12:36 PM   
Jeff_edge540


 

Posts: 144
Joined: 9/24/2006
From: Austin, TX, USA
Status: offline
quote:

ORIGINAL: Sport_Pilot

We like nitro here because it is relatively cheap, compared to other countries. The main reason we like it is because it broadens the needle. That is the peak may not occur at just one click off either way, that is if at peak the RPM drops substantially on either side of the needle. This is because nitro needs more fuel for a given amount of air, thus it will run with a much richer mixture. Not only that but nitro is a monopropellent. That is that it is possible to burn it without any air. Because of this the nitro funny cars run as rich as the ignition system can ignite it, it is limited only by the amount of raw fuel the combustion chamber can hold, and to the extent that the ignition system can ignite it. That is why they use two huge electronic magnetos. To us it means it is tolerant to the rich side, that is at peak power it will actually be richer than the ideal stochastic mixture at peak, so it will be more tolerant of both rich and lean runs, thus easier to tune. The second reason we use it is because with the right compression ratio it will have more power. The more nitro the lower the compression but more power, but also more consumption. Nitro is not added to help ignition, if fact it makes it more difficult especially with cooler glow plugs. A high compression engine will run well on low nitro, but it will have slightly less power, and harder to tune (especially without finer threaded needles). Of course if you can handle the tuning and don't need the extra power, then the lower nitro will be preferable because of lower cost and lower consumption.


BINGO! Well Stated.

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RE: % Nitro - 11/13/2006 2:17:15 PM   
downunder



Posts: 3198
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline 		To put it simply, no glow engine needs nitro. As far as the compression thing goes, methanol can handle very high compressions (much higher than any production engine has) but nitro can't handle much compression before it detonates. When you blend the two then you need a compression somewhere between the two extremes but even a very low compression engine will still run quite well on zero nitro although its power will be way down from what it could potentially make. I once ran an ST G51 at 6:1 compression on zero nitro (theoretically it could then have handled 100% nitro) and it still started first flick but the power was markedly down. OTOH with another engine I raised the compression to 13.5:1 and gained an extra 20% more HP.

So, as a general rule, the higher the compression the less nitro the engine will tolerate before nasty things happen, much the same as feeding too much nitro into an engine with lower compression. Think of it this way, add nitro and add shims, take away nitro and take away shims. With most engines though there's a limit to how many shims you can take away so some machining can be involved to get a decently high compression.


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RE: % Nitro - 11/13/2006 2:42:50 PM   
DarZeelon



Posts: 7104
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: offline
quote:

ORIGINAL: Jeff_edge540

...They spec 0% nitro for their engines (and a very cold "Type F" plug).



Jeff,


The OS F plug is intended for four-stroke engines and is a hot plug; not a very cold one.

It behaves colder than it actually is, since the glow-coil is partly shrouded.

The attribute that makes it good for large glow engines is that its coil remains lit, despite not being very deep in the fire, because of the shroud.

But this attribute should not be mixed with those of a cold plug.


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