TalonLM

Is there any correlation to Nitromethane content and engine size? I.E. Should smaller engines use more or less Nitro than their larger cousins.

balsaeater

Best I can tell not really it more linked to RPM as the main ingredient followed buy other issues of HP , tuning ,cost ,cooling are all factors but for most modelers it is RPM , tuning and HP as the main issues

Nitromethane is a chemical liquid which has a small part carbon type molecule (carbon hydrogen mix or carbohydrate) to burn and nearly 50% part of the molecule is oxygen, which is used to burn the carbon molecules and the rest of the molecule is Nitrogen which heats up and gives more compression
The atmosphere must still supply some extra small amount of oxygen to get a complete clean burn but Nitromethane is capable in certain situations to burn although less efficiently without outside oxygen from the atmosphere or elsewhere
There are other ways to achieve similar results
One method is if the engine was put in a static base where weight of steel containers holding liquid air components are not an issue and the nitromethane could be replaced with LOX (liquid Oxygen) and NOX (liquid Nitrogen) and this would achieve the same purpose to supply extra air components into the engine than what the engine can bring in itself with carburetter or similar aspiration methods .

Using only bottled gases and hydro carbon fuels like Methanol Gasoline Ethanol,natural gas etc it would be possible to get fuel to burn without a carburetor or air from atmosphere but it's weight is too much for modelers
However tuning would be super easy and engines would rarely quit or flame out


Another method is similar to above in using nitroes oxide gas but again weight of bottle to store nitoeos oxide is usually too heavy for modelers

Mechanical methods for larger engines can achieve similar results with turbos or blowers(superchargers) which would force more air into the engine
However complexity and weight make this more likely in engines of 1000cc and above

Methanol the main ingredient modelers fuel has a molecule which is roughly 50% carbon type (carbohydrate)and 50% oxygen and can easily do the exact same job of suppling more horse power from burning more fuel as Nitromethane and at a far cheaper job with simply increasing the compression ratio of the engine
However this often makes the motor more difficult and sensitive to tune .

Also most modelers engines for the USA market often are relatively low compression ratio for medium to high nitro use
In order to increase the compression ratio it would require a machine shop to bring the typical compression ratio ~9.5 : 1 up to more powerful ~13 : 1

Motors made for European market can often be made with higher compression as the majority use low or no nitro due to excessive higher costs of nitro in Europe
Often four times more costly than USA

The addition of lots more nitromethane means the more mechanically challenged type person you are the easier and more wide band will be the tuning of the glow engine
This is because the engine would getting its oxygen from a fixed source the fuel itself at a steady rate rather than from the air which can change hourly in temperature and humidity and require constant tuning changes

Also using higher nitro will gain some extra power as a side benefit and importantly it will be more the power will tend not to be so peaky like zero nitro fuels and a noticeable gain in low end torque at the lower RPM such as is needed in 3d work

The higher the RPM is made to go the more the engine will find extracting oxygen from the atmosphere more difficult .
Then the inclusion of high nitromethane rates there will have more benefits to ease of tuning and the RPM will be increased in a feedback loop

As often the smaller motors will turn at higher RPM it would be easier to tune the engines with the Higher RPM with more Nitromethane in the fuel

So a OS90 VR DF 15cc engine (large motor )doing 24,000 rpm will often use 30% nitro and a more sedate OS40LA coul

wjvail

My Feedback: (3)

Outstanding response by balsaeater.

The short answer to your question is there is little or no corollary between engine size and nitro content. There are many boat guys running OPS and CMB .90's on 65% nitro and an OS LA 10 will run fine on 10%. Top fuel cars are 500+ci and run 85% nitro. You cannot look at an engine and say "It's a .40 and needs 15%". You cannot assume because an engine is a _____ it needs a certain amount of nitro.

With the above said, there is complete connection between engine size, fuel consumption and the cost of fuel considering nitro content. I'm certain the makers of the Moki 2.10 and the Super Tigre 3000 had fuel burn rates and the cost to feed these engines in mind when they designed them to run on low or no nitro fuels. In general, all glow engines would run better with 10 to 15% nitro if they were designed that way however, in the case of larger engines, they are often designed to run on lower nitro fuels to keep operating costs down. Since fuel consumption rates for smaller engines is much lower, they are often designed to run on higher nitro fuel.

A few things to remember...

Even in todays inflated world, Methanol is still only $3.00 a gallon.
Nitro is 20 - $40.00 a gallon.
The cost to feed a Moki 2.10 on no nitro is less than the cost to feed a YS .91 in a heli running 30%.
A gallon of 40% fuel in a Cox 1/2A will last about 250 flights. Flying 4 flights a weekend equates to about 1 1/2 years of flying on a gallon of fuel.
A .90 heli holds about 20oz and a gallon of fuel last about 5 flights.
Nitro is stupid expencive in other parts of the world and engines designed for use outside the US are almost always low nitro... Large or small. Think Webra, Rossi....

Bill

Jezmo

Excellent responses guys, I can't add a thing to that.

downunder

There is in some respects. It's well known that the little Cox engines (actually even their big brothers...if you call a .15 big ) respond very well to high nitro but I suspect that's more to do with design, use a lowish compression and high nitro to get a lot of power. But small engines also have a much larger interior surface area for their swept volume than bigger engines and this area both soaks up heat and gives a larger % of unburnable stagnant mixture. Nitro would help make up for this bit of inefficiency.

Compression by itself though isn't an indicator of how much nitro an engine needs to run. I have an old Fox 35 with 6:1 compression that starts instantly, hot or cold, using no nitro. It doesn't idle though...could be because it hasn't got a throttle .

TalonLM

Thanks for all the info. All good reading, but I guess the real reason for the question is I have recently acquired an O.S. 15 LA and have not been able to get it to run for longer than 2-3 seconds. I started with 15% Cool Power MV and am now trying SIG's 1/2A fuel which is 25%. The longest run was on the 15%. Glow plug (A3) is good and I've replaced it just to make sure and it appears to glow quite well. I live at approx 5800ft. and haven't had any problems with my O.S. .46 AX. My assumption was that since the 25% was designed for 1/2A it might do well in the little .15. Not so. WJVAIL and DOWNUNDER have done a great job in explaining why this is not necessarily so. Can anyone point me to a thread or give me some info on this little thorn? I was really looking forward to putting it in a HOB Laser Stick or maybe a Spacewalker.

Sport_Pilot

It has to do with the shorter distance the flame front has to travel. Those old large radial engines they had before the jets took over required 140 octane fuel, the compression ratio was not that high, in fact lower than many automobiles. But the large piston diameters meant that the flame front took longer to burn from the spark plugs and thus trapped more fuel to be compressed by the flame front. The same thing is happening with our model engines to a lesser degree. Because of this a very large engine of a given compression ratio will run best on lower nitro fuel, and a very small one will require high nitro to run best. It's not that obvious when you compare say a .40 engine to a .91 engine, but very obvious when comparing an .049 to a 2.0 sized engine.

jeffie8696

Have you checked for a compression loss? Head gasket or????? Not to point out the obvious.[)] I myself have several Vintage .15 size engines that run really well on high nitro. Even up to 20% with 20% castor. Not all engines are created equal as the above answers also indicate. I have a loop scavenged Thunder Tiger .15 that makes 15,000 on an 8X4 prop with 15% nitro (with all castor lube, my preference). And I have 3 Foxs that do just as well on the same combo. On the other end of the scale I have a K&B .45 that just LOVES 15% and makes Mongo power on it. I have also tried FAI fuel and it works well in my Magnum .46, just depends on the compression. And I can't get a Cox Pee Wee .020 to even start on less than 20% nitro. [sm=lol.gif]

TalonLM

Just thought I'd pass this on. I finally got her running thanks to the O.S. Support forum. here is the post.
[link=http://www.rcuniverse.com/forum/m_7411599/anchors_7439164/mpage_1/key_/anchor/tm.htm#7439164]O.S. 15 LA won't start[/link]

As I said in the post to O.S. this is only my second glow engine and every time I "play" withthese things it's a new experience. Hope this might help someone else out there.

mrbigg

My Feedback: (21)

balsa eater quote-"The addition of lots more nitromethane means the more mechanically challenged type person you are the easier and more wide band will be the tuning of the glow engine
This is because the engine would getting its oxygen from a fixed source the fuel itself at a steady rate rather than from the air which can change hourly in temperature and humidity and require constant tuning changes

Also using higher nitro will gain some extra power as a side benefit and importantly it will be more the power will tend not to be so peaky like zero nitro fuels and a noticeable gain in low end torque at the lower RPM such as is needed in 3d work"


We have a couple of guys at the field that have swithched to Byron 30% and use an OS F plug in all their engines and have nothing but good news to report. No deadsticks, easier tuning, etc... He quoted to me what you stated-"Oxygen from a fixed source, the fuel itself, at a steady rate rather than from the air which can change in temerature and humidity." I might have to try this myself.

downunder

The main reason an engine is easier to tune when using nitro is because of the air/fuel ratios. Petrol has a workable ratio of about 12-15:1 which is quite a narrow range between rich and lean so you have to be quite precise. Methanol has a workable range of around 4.5-6:1 which is easier to set than petrol but still a little sensitive. Nitro though has a workable range 0f 0.5-2.5:1 which is very broad (meaning you almost can't go wrong ). When you blend nitro with methanol then the workable range falls somewhere in between the figures for raw methanol and raw nitro. So the more you mix the easier it is to get an acceptable tune.

jeffie8696

Now there is a concept I can get my brain around. [)]