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Nitro - hotter or colder run?

Old 08-24-2005, 12:40 PM
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DesignMan
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Default Nitro - hotter or colder run?

A rather knowledgable friend of mine contends that higher Nitro content makes an engine run cooler. His argument is that you are running more fuel through the engine, so it has better heat dissipation

Sounds fishy to me, as I would expect that more power output must cause more heat.

Has anyone ever done an EXPERIMENT to measure the head temperature with low and high Nitro fuels?

I am only interested in actual, measured results, not theoretical argument here.
Old 08-24-2005, 02:03 PM
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BarracudaHockey
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Default RE: Nitro - hotter or colder run?

Higher nitro runs cooler for exactly the reason stated.

You need to run a richer needle to run higher nitro, more fuel, more oil, more oil less heat.
Old 08-24-2005, 02:06 PM
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Default RE: Nitro - hotter or colder run?

Larry,


Read Brian Hampton's (downunder) fuel page, as shown [link=http://www.flyhi.org.il/fuel.htm]here[/link] (it is from an Israeli R/C web site).

It also appears in some other web sites.


It is possible, since nitro's latent heat of evaporation and fuel-air ratio causes so much nitro to flow through the engine, it can cool the engine much more than can methanol, at its fuel-air ratio.

The question is if you are willing to pay for it...

With high nitro fuel, you are going to use MUCH more fuel and pay much more for each gallon and be required to reduce the C/R to allow high nitro and .....
Old 08-24-2005, 02:27 PM
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Default RE: Nitro - hotter or colder run?

Some of my engines run very hot on high nitro fuel. I would hate to see how hot they would be on 5% instead of 70%.
Old 08-24-2005, 03:37 PM
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Default RE: Nitro - hotter or colder run?

Hi!
That's correct, using some nitro (5-15%) CAN make the engine run cooler than an engine that runs on just 0% nitro ...the reason for this is that the needle setting can be set richer and still the engine can be made to produce good power...but setting the needle leaner...taking out more power.... ofcourse will make the engine run hotter still.
Regards!
Jan K
Sweden
Old 08-24-2005, 03:44 PM
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Default RE: Nitro - hotter or colder run?

Does it make a difference if there is methanol in the fuel or not? The 70% nitro fuel has nitro, 20%oil and 10% propolene oxide (igniter) and no methanol. This is the standard fuel formula foe C/L Navy Carrier.
Old 08-24-2005, 03:55 PM
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Default RE: Nitro - hotter or colder run?

The Hampton information is great stuff, but still just numbers on paper. What I am after is real, measured temperatures with a real engine. Guess I'll have to do the real-life testing myself!

Thanks for the input, it certainly indicates to me that it could go either way, depending on the nitro content and engine in question.
Old 08-24-2005, 04:06 PM
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Default RE: Nitro - hotter or colder run?

Usually, you'll find that with the higher power output that results, running higher-nitro fuels makes your engine run hotter.

While people try to use the specific heat of combustion, or the specific heat of vaporization to make their case, they forget that the specific heat has to do with the mass of fuel, so while nitro has a lower specific heat per unit mass of fuel than does methanol, you also must run a LOT more nitro through the engine per unit mass of air than if you use methanol. So when you run a kilogram of air through the engine, you wind up with more heat being generated when you use nitro than when you use methanol.

Higher power means more energy, and more energy means more heat.
Old 08-24-2005, 04:22 PM
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Default RE: Nitro - hotter or colder run?

I agree that in all the situations where I just used higher nitro, heat increased. A K&B sportster engine and Fox engines are designed to run on low nitro and over heat and knock or sieze when high nitro is used. The reason for this is not higher combustion temperature or the amount of cooling gasses but the heat caused by preignition.
Preignition robs power as well as melts holes in pistons. The mixture expands before the piston reaches TDC and fights the rotation of the crank. Yes ignition should start in advance but maximum expansion which follows should occur after TDC.
Higher nitro with a cooler plug and reduced compression (both retard timing) will indeed lower temperature while retaining correct timing.
Old 08-24-2005, 06:14 PM
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Default RE: Nitro - hotter or colder run?

Thats strange because when I switched from 15 to 25 percent on my helis I had to back the needle valve out and the engine produced more power but the back plate was no where near as hot.
Old 08-24-2005, 08:06 PM
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Default RE: Nitro - hotter or colder run?

Some have confused heat and temperature. More heat does mean more power but you can make power with more mass flow instead of heat. More heat does not necessarily mean more temperature. For example for a pound of water heated 100 degrees that would be 100 BTU. But 101 pounds of water heated one degree has 1 BTU more heat. That is why more nitro CAN run cooler, because you have to run it richer and nitro fuel air mixture is more dense it can deliver more power with a lower temperature.
But the opposite can happen if you have a high compression engine, or run it too lean. Run nitro almost as lean as methanol and it will have a higher temp.

Another example is a water turbine, most will make more power but run at a much cooler temp. The water will heat up only a few degrees as it passes through the turbine. But thousands (sometimes hundreds of thousands) of gallons of much denser water passes through that turbine, thus it makes thousands and thousands of horsepower.
Old 08-24-2005, 11:48 PM
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Default RE: Nitro - hotter or colder run?

Yes, many years ago I made some rudimentary measurements using the glow plug
element itself to measure combustion chamber temperature. I can say that under
some conditions using more nitro can cool an engine down.
I will expand on this.
Let's assume that an engine is being run on low or no nitro fuel, the adjustment of
the needle valve will be found to be rather sensitive and if the needle is richened up
only a little the power will be found to fall off quite a bit, also when the combustion
chamber temp was measured I found that if the engine was richened up enough to
cause the chamber temp to decrease aprox 300F then it lost a lot of power. I have no
way of knowing how much though.
When the same engine was switched to 18% nitro(oil content the same as before)
I found that if it was adjusted for absolute max power that it was hotter than before
(by the very unscientific touching method)but the glow plug said the same temp
as before(I can't say what the acuracy of measurement was any more, it's been a
long time), but now the interesting part, if the engine was richened up so that the
chamber temp dropped by 300F it only lost a few percent of max power but it cooled
down very noticeably, if it was richened until the chamber temp dropped 500F it was
still putting out more power than 5% nitro but you could almost hold your hand on the
head.
This research was done using an old style baffle piston engine, I didn't have any
schnurle engines back then.
A factor that has not been taken into acount is the mass flow per unit time of
lubricating oil through the engine, I didn't consider it important back then.
For what it's worth I hope it helps you,
Jim
Old 08-25-2005, 05:05 AM
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Default RE: Nitro - hotter or colder run?

never measured temps but i do know that many engines designed for no/low nitro seem to run cooler and are much happier when a few % of nitro are added in the really hot weather. i ASSume this is because the hot, less dense air effectively lowers the compression ratio and with the low/no nitro you end up running the engine too lean in an attempt to get it to peak.


dave
Old 08-25-2005, 11:08 AM
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Default RE: Nitro - hotter or colder run?

ORIGINAL: DesignMan

What I am after is real, measured temperatures with a real engine.
Go to http://www.modeltechnics.com/glowplug/firepower.html and you'll find a considerable number of tests done on many engines. Compare the charts for the same engine on different fuels and there's not too much difference in head temperatures. The temp probe was fitted in a hole drilled to within .020" of the combustion chamber (if you don't want to read the fine print ).
Old 08-25-2005, 11:28 AM
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Default RE: Nitro - hotter or colder run?

Hi Barracuda!'
The problem for you might be that your engine was not set up for running on 25% nitro...
Did you add several head shims when running 25%nitro or...Not many engiens are set-up to run on so much nitro. When you changed to 15% nitro all of sudden your engine had the correct fuelmix for its compression ratio, and thus ran cooler and produced more power.
Regards!
Jan K
Sweden
Old 08-25-2005, 05:39 PM
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Default RE: Nitro - hotter or colder run?

Engines work much better in the cold weather with high nitro. I run 25% in the 25 OS FPs with no problems winter or summer. They neen all the help they can get to keep up with the FXs Of course they're a fairly low comp. engine. so they thrive on it.
Old 08-25-2005, 06:07 PM
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Default RE: Nitro - hotter or colder run?

An excellent site. However, I don't have much clue as to the nitro content of the various fuels. Did I miss that somewhere?
Old 08-25-2005, 09:12 PM
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Default RE: Nitro - hotter or colder run?

DesignMan
At the bottom of each test it says what fuel was used like Duraglo Str or Duraglo 16. These are obviously English fuels so unless someone familiar with these fuels says otherwise I'd assume Str means Straight (zero nitro) and the 16 would be 16% nitro and so on.

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