Just a general question on this. What effect does adding nitro ( within limits, of course ) have on the torque and hp curves . Not just the peak but where the peaks occur in the rpm range. Any and all info much appreciated !

Dave,


The torque and power curves of an engine are by far more dependent on the timing numbers, than on the nitro content of the fuel. In timing numbers I include intake bypass(es), exhaust port(s), shaft valve port and compression ratio.

In an engine with given timing numbers... Well, let us look at it this way:

With more nitro, the mass, the density and the viscosity of the intake charge are larger, since more fuel is needed for a given amount of air.
Also, these qualities are also true of the exhaust gasses.

These would both tend to make the engine breath more efficiently at slightly lower RPM levels, thus lowering the RPM, at which peak HP and peak torque appear.

In the case of four-stroke engines, the lower compression ratio needed for higher nitro, decreases the induction efficiency, since vacuum builds up more slowly, as the piston descends. This would also lower the peak HP and Peak torque RPM levels.
This has little effect, if any in a two-stroke engine, since there is no actual induction (sub-piston pressure pushes the fresh mixture into the cylinder).

All these effects are minor at most...

E, Ive been thinking of doing that very experiment, my most logical candidate for a fourstroke would be my Saito .72, my Saito .80 has really high compression but may be suitable. For a two stroke I would have to get some fuel, all I use is Fox 5% nitro with 20% lube and the only suitable candidate I have would be my OS LA .65 or Enya .50CX which is very loud. All the rest are Fox or European 0 to 5% nitro type engine. I shall theeenk upon it some more but keep it under the headache limit. Offer suggestions. For example, does a ST .51 run OK on 15% nitro?

Hello!
Adding nitro (5-15%) to most sport engines will raise the hp/torque curve at all rpm! That simple.
Regards!

Jan Karlsson
Sweden

That's a very good question E-Dave and one I've wondered about too.


Jan, I don't know whether or not it is that simple. Have you plotted this out in controlled testing?

It should be easy to test: run three or four different loading props on the same engine with low or no nitro, tach the peak, use the prop-power.xls spreadsheet to calculate the HP ratings for each prop, then repeat after "tipping the can". Then plot the numbers using Excel - you will end up with two power curves to compare.

As Jan says, it is a power booster no matter how you cut it. The more nitro, the more poop.

Agreed, more nitro=more power BUT when and where in the power curve ? Will the power curves become more peaky or flatten out ? Will peak hp and torque be produced at the same rpm or move up or down on the rpm scale ?
Hobbsy, as far as two strokes go, I was looking at the on-line manual for the Webra 50 which I have ( the one that came with the engine seems to have disappeared ). It shows a nitro range from 0% to 15% with a note that it will run well on FAI fuel. This might be a broad enough range to produce a bit of interesting and hopefully relevant data. Haven't checked but the Webra 61 may well be the same. If you have access to one of these engines, it might possibly be a good choice.
Thanks everyone !

Looking at Ron Warring's analysis of the TD 15 in the January 1962 Aeromodeller. Peak HP at 10% nitro was @ .35 BHP at 17,000 RPM, torque at that point was 20 inch-ozs. At 20% nitro, the peak PHP was .36 at 17,000 RPM, due to torque at that point slightly above 20 in-oz. At 50% nitro peak BHP was @ .45 at 18,000, torque at that point 25 in-oz. The torque curve at lower RPM's was progressively flatter with increase in nitro. I would guess that the increase in torque with more nitro contributed more to the increase in BHP than the increase in RPM's. Clearly the case in the first two instances because RPM at peak BHP did not increase.

Of course, this is a single example of an engine designed to run on high nitro. But it clearly contradicts Dar Zeelon's coments.

Jim

I'll have to contradict Dar's comments as well. The link below will take you to a number of graphs for different engines but the most telling one is the last one for a YS 120SF although the first one for a Hanno Special also shows how nitro affects the curves. The graphs are all done in German but post #4 has a translation for the terms. Note that torque, HP etc are in metrics.

http://clstunt.com/cgi-bin/dcforum/d...rum=DCForumID1

Increasing the nitro content has the effect of advancing the engine's timing; one of the means by which it increases power. Nitromethane has a lower flashpoint and faster burn rate than methanol. Increasing its proportion in the fuel mix allows the air/fuel charge to ignite at a lower temp and burn faster. Consider the compression stroke and ignition sequence:

piston is on its way up, fuel charge is being compressed, compression causes heating, there is residual heat in the glow plug from previous ignition, plus catalytic reaction of platinum element and methanol, until heat becomes great enough to ignite air/fuel charge. the sooner this happens before top dead center, the more ignition advance there is, and the faster the charge burns, the higher the combustion pressure, the more power is developed with each cycle. In addition to lower flashpoint, a hotter plug will contribute to ignition advance due to more retained heat.. This is true in 2- and 4- stroke engines, and the trend of higher nitro and/or hotter plug can continue---(along with increasing rpm as well) to the point where there is enough residual heat and a low enough flashpoint that the fuel ignites as soon as it enters the combustion chamber, resulting in preignition, or a high enough nitro content can burn fast enough in a fully compressed state that it virtually explodes, causing detonation. Both conditions are detrimental to power output and engine longevity, detonation being worse because it occurs when valves and ports are closed, and there is nowhere for the explosion to vent, so it usually holes a piston, bends a rod, or can even crack the cylinder wall. This is the reason high compression engines can't tolerate high nitro as well as lower compression ones. Preignition can also damage engines because the charge is burning too early and causing undue stress on the still rising piston/rod. If it is a 2-stroke and any of the port area is still uncovered, or a 4-stroke and intake valve is still even partly open, cylinder pressure won't climb enough to cause damage, but engine will blow-back, or backfire. There is, somewhere in this maelstrom, a happy medium for every engine, where nitro content and ignition heat and rpm combine to create the optimum power output for the engine. All we have to do is experiment our entire life to find it, just before we crash and tear it all to hell anyway.

Jim, Brian, K. Hodges,


My theoretical post was not based on any empirical measurements.

It was only theoretical and some variables were not entered into the 'equation'.

The coupe-de-gras was, I believe, exhaust gas temperature and intake charge temperature.


Methanol is more volatile than nitro and has a higher latent heat of evaporation, so the temperature of the intake charge becomes lower, thus increasing its viscosity and mass, maybe enough to counteract the nitro fuel's lower stoichiometric ratio and larger intake mass. So the 'warmer' intake charge with nitro fuel, may flow into the cylinder more easily.

The higher exhaust gas temperatures with nitro fuel will make the spent gasses flow more easily too, despite their larger mass. This also has the same effect.

So, the higher temperatures have the effect of increasing the RPM, at which peak torque and peak HP are achieved, more than the larger masses have of decreasing them.


Maybe these curves K. Hodges refers us to would change and things will not look exactly the same, if the compression ratio was also changed accordingly and be higher for methanol than for nitro.

But the question was simple and actual empirical data proves my theory was wrong. I stand corrected.

Dar Zeelon,I didn't intend to imply that you were wrong. There are a lot of variables that neither you or I factored into the "equation" and I'm sure your theoretical knowledge is better than mine. You are entirely correct regarding the viscosity of the fuel charge and methanols' greater volatility, etc. I think where our view is different (but not necessarily at odds) is you're looking more microscopically than I. Most of my experience with methanol and nitromethane is with racing engines, motorcycles, specifically, and how we played with different mix ratios to increase our power. The results we got translate directly to our little glow engines; I am not much good with the finer mathematics, I just know from experience (and more than a few trashed 3/4 liter engines), what happened when you dinked around with methanol and nitro ratios.

e-dave
Errrrrrrrr....practical tests show nitro adds a bit more power and shifts the point of max power to slightly higher in the rev range.

Thanks for taking the time to reply everyone. Learning quite a bit here. Keep it coming.