I agree, but an extra inch in diameter with 200 more RPM is a SIGNIFICANT increase. As with most 4 strokes, he was probably trying to stay in a selected RPM range.

Arch

According to Pe Reiver's prop power calculator, which has proven to be quite accurate, a 16x8 prop is 30% more load than a 15x8 prop. Assuming the same brand and prop load factor... Add in the 200 rpm extra on the 16x8 and the engine was making around 39% more power on the high nitro mix than the 5%.

Mark

If you really believe that the 25% extra nitro made a 39% increase in power, I have a bridge I wanna sell you.

CR

Hi Charley,
A few details that matter in all this ;
Methanol's energy 22Mj/Kg but requires 6.5Kg Air per 1Kg Methanol for combustion.
Nitromethane's energy is only 11Mj/Kg ,thats half that of methanol but only requires 1.7Kg Air per Kg Nitro.
Combustion in an IC engine has the amount of air fixed by the stroke/bore.
So for 1Kg of Air Methanol gives 22/6.5 =3.4 Kj and Nitro gives 11/1.7 =6.5 Kj ,thats nearly double that of Methanol = 100% increase.
Therefore a good rule of thumb is 10% Nitro = 10% more power etc.
Nitro also has a very high vaporization temp and so cools the engine more than methanol as it vaporises.

Brian

Thanks Brian. This is an excellent explanation that all modellers should know!

Atul

Brian,

Just curiosity. What are the numbers for gasoline?

Thanks,

VB

Hi,
I looked it up this morning.
Energy is very high @~44Mj/Kg ,but requires 14.5Kg Air per Kg Gas for combustion.
So per Kg of Air it gives 44/14.5 = 3Mj energy.
Also v low vaporization temp therefore very hot engine, which in addition causes more expansion of air and gas during intake.
This results in less being burned per stroke = less power.
Remember the '60' 2 strokes ,with long pipe set ups for the vertical pulls, you could hold the cylinder head at full throttle and it stayed cool.
You would not do that with a small gas 2 stroke.

Brian

Hello Brian,

I've changed the nitro content incrementally on a few 2-strokes for comparison purposes, using the same prop. At no time did I see the kind of power increases you're talking about. Sure would like to see some test data.

CR

I normally ise 20% Nitro (all synthetic oil) on my YS-110, and I was getting about 9200 RPM with a 15x8 prop on 20/20 fuel.

Today I tried 30% and comfortably got 9800/9900 RPM. That's a 600/700 RPM increase on the same prop (15x8).

I also tried a 14x12 prop and got about 9400 RPM. Not bad.

So there's seems to be a substantial difference between 20% and 30%. The engine also runs a lot cooler (it was already quite cool on 20% nitro; but with 30% its even better and runs smoother as well).

The difference seems a LOT more pronounced on 4-strokes than on 2-strokes. On my JETT 76L, the difference between FAI fuel and 15% nitro is about 1000 RPM at the top end. However, in a vertical climb, the 15% fuel ensures that there's no drop-off in RPM while with FAI fuel this seems not to be true (i.e. the engine seems to lose some power as the load increases due to the climb).

Atul

Hi Charley,Atul,
Combustion in simple glow 2 strokes is very inefficient.
To make fuel mix changes and get the engine to reflect the change in its performance you would have to change the plug,crank timing and pipe timing to specifically suit the new mix.Not at all practical.So with 2 strokes we get the best compromise of prop/fuel/plug and pipe length for the particular 2 stroke we are dealing with.
4 strokes are more efficient.
YS 4 strokes are super at converting methanol and nitro into thrust,they just love that 30% fuel.
I don't know how much increase in nitro they can respond to positively as they have timing limitations also.
The critical ignition timing available from spark ignition means that the ignition timing can be fine tuned to suit fuel mix with the CDi, but only within the other timing/design limitations. To get deeper into this is interesting but imo thats for YS and Co.
For us users its enough to know the fuel mix range our particular engine is designed to work with.
Top fuel engines are designed for 90% nitro. A YS might not even start on that, I have not tried ,,,,, yet LOL.

Brian

A 10% increase in rpm requires much more than a 10% increase in power.

A 10% increase in power may not be dramatically evident in flight. To increase airspeed
10% requires much more than 10% increase in power.

Hopefully Brian or some other egghead can fill in some rough numbers explaining the relationship between power and rpm.
Extra credit will be awarded for a colorful graph.

Hi,
I would not dignify that comment with a 'point' never mind a 'graph'.
Atul's rpm increase was 7% not 10%, about what one would expect.
My only aim here WAS to give Charley some insight as to why modellers all over the world spend so much money on Nitro,, .
Might you be suggesting that this all works other than described.
I think you might be trying to compare apples with oranges.

Brian

Nitro improves the idle and decrease the chances of dead sticks. I know this by experience.

VB

That is one thing a lot of people don't realize is that engines run cooler on higher nitro. If you have an engine that is marginally overheating with 10% nitro, you can increase the Nitro to 20% and it will make the engine run cooler. That is because it lowers the flash point and thus the fuel burns at a lower temp.

This also can help for people who live in very warm climates...

Arch

Sorry Serious Power, I think maybe we have missed each other's point. I have enjoyed following you calculations so far and was hoping for more. Egghead was not meant as an insult, just the opposite!

My experience with nitro mirrors your figures. I thought Charley may have been disappointed with his nitro results and I was hoping to keep us all on the same page by reminding that the relationship between power and rpm is not linear. Rather than attempt a clumsy explanation I brought up a graph. Picture=1000 words. My 10% figure was only an round number to point out the difference between a 10% power increase and a 10% rpm increase. It was not meant as a comparison to any results noted so far.

As has been pointed out, nitro provides benefits beyond increased power.

Ah thats ok Aerobear.
I'm not your typical 'egghead' at all. Usually having the facts helps. A little knoledge etc.
It's just that people are sometimes misled into believing that using high nitro levels is a waste of money.
These can then endure engine problems for years, in fact it becomes normal for them.
The message is that most engines designed for pattern use are set up to exploit nitro at fairly high levels, 15& -40%.
A couple of extra details ; Methanol's boiling pt =65'C and flash pt = 11'C.
;Nitromethane's boiling pt = 102'C and flash pt = 35'C.

Brian

Indeed true. Engines running higher nitro do tend to run a bit cooler but not because of lower flash point.

Higher nitro requires a richer setting than lower nitro does. Richer means not just more nitro but more MeOH and heat dispersing Oil.

Tuning the timing of the typical 2 stroke or 4 stroke by mixture adjustment isn't the optimal set-up. YS's CDI promises superior tuning for timing and hence running. This lesson has been learned by the few in Pattern who tried ignition on earlier non-ignition equipped engines. YS marketing knows that introducing a "sure" thing, particularly because their equipment is popular in Pattern, will generate super returns.

And even I who am a staunch 2 stroker will look into this one. I really like the handling of the 2 stroke gassies so this is a no brainer

MattK

Arch
[/quote]

YS marketing knows that introducing a "sure" thing, particularly because their equipment is popular in Pattern, will generate super returns.



MattK
[/quote]


And I'm ready to order!
Anyone got an address of a dealer in Japan?
PM me.

Brian: you make a very good point about 2-strokes. My JETT 76 reacts differently to 15% fuel depending on the muffler installed. With the non-tuned muffler, the engine runs fine but at about 700 RPM Less than with the tuned muffler. However, with the tuned muffler, the transition is not as smooth. I wonder if others have experienced differences with different mufflers on 2-strokes.

The YS-engines though do seem to love 30%. There's a big difference in the way the engine behaves between 20% and 30% fuel.

Atul

Hi Atul,
We are a little off topic now, but what the ,, .
Anyway it sounds like your tuned set up is a bit short .
If you extend it , you may loose a little off the top end but it should transition better, and it will hold on better under load in the verticals.
Guys the cooler running is a direct consequence of the higher vaporization temp (higher boiling point) of the Nitro.
I know it sounds wrong but that is the case.

Brian

Is this correct? Nitro higher vaporization temperature means higher absorption heat before forming vapors. Therefore, the cooling capacity is higher as the vaporization temperature increases. We know that liquids coold a lot better than vapors.

VB

Hi VB,
Thats it.
It requires heat/energy to vaporise the fuel,so as to burn it.Only the vaporised fuel is burned.
It goes something like this;
-Compression of fuel/air mix heats it up.
-Ignition occurs burning existing vapors which vaporises more of the fuel which then burns etc until the charge of fuel or oxygen is gone.
It's like burning the fuel off a layer at a time until all the layers are gone.There is a name for this that I can't remember, will check.
If you can imagine a miniature pot of fuel(representing the fuel from a single intake stroke) sitting on a mini bunsen burner you will get the idea of the heat energy used to vaporise (boil off) the fuel charge.
So with Nitro the boiling pt is higher and there is more of it (remember it uses less air but the air quantity is fixed in ic by stroke) thus more heat/energy used.
This is not quite the same as set richer, I'm assuming correct tuning.
The net effect is a slightly slower flame with each increase in nitro.Gas expansion still occurs slightly slower cooler but for longer.
Thus that soft wet buttery sound with high nitro fuel.
I hope this makes some sense.

Edit; That term that I could not remember to describe the combustion of fuels is 'Deflagration'.
Try to google; internal combustion and/or methanol and/or nitromethane and select the Wikipedia pages.

Brian

Brian,

You're addressing this subject with such obvious knowledge and enthusiasm, that I can't help thinking that you're on the verge of deserting the dark side and returning to i/c.

I knew if I waited long anough that you would see the light!

Best wishes

Bob

Hi Bob,
Ha Ha.
No not going back to IC any time soon, but if I was to it would be with one of these CDI's.
They certainly negate some of the electric pro's.
I think moving electrons is easier than converting molecules.

Hope you are well also

WOW . . great info here! ! Is this now "RCUniversity" ???????

Cheers, JB