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Engine Performance Curve

DKNguyen

Senior Heliman

N/A

I don't know much about engines and have a question about the performance curve on an engine.

I'll start by using the electric motor performance curve, which I do know something about as a starting point. So if you have a motor torque/power vs RPM curve for a motor you must be given the voltage the curves were taken at for there to be any meaning. And from this one curve you can work out how the motor will operate at other voltages because increasing/decreasing the voltage will shift the power curve up/down and shift the torque curve (a line really) to the right/left, making more more/less torque for a given speed and giving higher no-load speeds. Like this:
http://www.motortech.com/BULL_E-1.htm

Now...for an engine curve. What happens? I assume the curves are taken when the engine is at full throttle (right?). So what would the curve look like at half throttle? The power curve would be shifted down, but where would the peak be? Still at the same RPM? Or would it have moved to a lower or higher RPM? Does the torque curve also get shifted down (unlike an electric motor). I think it does because you are burning less fuel so less explosion = less torque. How does the peak of the torque curve change?

The attached picture is the dyno of a Zenoah G26. With the headspeed I wanted and the gearing I am stuck with I need to be running it at 12500-13000 RPM which is the power peak (at full throttle?) in the graph. But the power I need is between 800-1200W which is much lower. If I controlled the throttle to be my RPM at my power level, would I still be at the peak power for that throttle setting? If not, where would I be? Above the peak? Below the peak? And how close would I be relative to the peak power and peak torque at that throttle setting (since I don't want to run the engine too close to the peak torque so I don't slip off to the left side of the peak on the curve and get a snowballing effect where the engine stalls and dies).

Thanks.

07-25-2008 Over year old.
Jafa

Elite Veteran

Sydney, Australia

I am stuck with I need to be running it at 12500-13000 RPM which is the power peak (at full throttle?) in the graph.
But the power I need is between 800-1200W which is much lower.
Those two sentences contradict each other
I presume you meant the peak torque is at much lower RPM ?
but torque is not normally specified in W

The torque curve of your Zenoah is very flat
I would expect it to work well from 9,000 to 14,000 rpm
That's a huge range

Remember that power is just torque * rpm
Power drops off at higher RPM as the engine gets to the point
where it can't breath and produce torque

You can gear the heli so the head speed is just above
peak torque or peak power
When you load it up you want to fall back to the peak
and the governor will open the throttle
to give the full potential of the engine

Which gearing you prefer depends on your flying style

It's gearing and RPM - forget about the throttle

Remember also - the Zenoah is a well used component
the gearing is not a mistery - you are not breaking new ground
Forget this theory and run what everybody else does


Protos | Logo 400 & 500 | Sceadu Evo | Freya Evo | Trex600N | Avant FX

07-26-2008 Over year old.
DKNguyen

Senior Heliman

N/A

Remember also - the Zenoah is a well used component
the gearing is not a mistery - you are not breaking new ground
Forget this theory and run what everybody else does
THe thing is I don't know what everyone else is doing for the G26 otherwise it wouldn't be a problem. I am aiming for a headspeed of 1500 and the gearing on the heli is fixed at 8.25. But since the heli was designed to be nitro, I am wondering if the gearing ratio is high for a slower gas engine to get that headspeed.

The reason I ask about throttle is I don't know how if the power peak on an engine changes to a diffent RPM at a lower throttle setting (this happens with electric motors) and I'd like to stay in between the torque peak and power peak of the engine.

The operating RPM for an engine seem to be between max torque and max power. I'm unsure about how safe it is to operate an engine so close to it's maximum operating RPM (which I assume is somewhere near it's redline?)
I am stuck with I need to be running it at 12500-13000 RPM which is the power peak (at full throttle?) in the graph.
But the power I need is between 800-1200W which is much lower.
How does this contradict? If I was running at 13000RPM at full throttle I'd be drawing 2.6 HP. But if my load didn't require so much power/torque, I'd have to decrease the throttle to stay running at 13000RPM. For this decreased, throttle setting, would 13000RPM still be the power peak (like if you graphed out the same RPM/torque/power curves at half throttle instead of full throttle)? Or does it change to another RPM?

I ask because this happens with electric motors when you change the voltage (which I guess is analogous to changing the throttle).

I am also trying to understand how engine characteristics change with throttle so the theory is very much important to me. I hate trying to explain how a voltage affects a motor after someone is already knee deep into a project because any wrong concepts have been already been reinforced and I don't want to be one of those people with engines.

07-26-2008 Over year old.
Pinecone

Key Veteran

Maryalnd

The torque peak occurs at the point where the engine has the highest volumetric efficiency, it breathes the best. It is where the intake and exhaust tuning, port/valve timing, etc, all come together to work the best.

If you reduce the throttle opening, you reduce the overall volumetric efficency since you are now sucking against a greater restriction, but most of the tuning points are RPM dependant, so this will still be the highest torque for that throttle opening.

Think about this, if you put a smaller choke carb on the engine and redid the dyno run, the torque curve would just be lower. Now, there may be some reshaping of the curve due to better efficiencies as lower RPM and greater losses at higher RPM, but pretty much the peak should stay at the same RPM.

But anyway, you want to be running at the peak power RPM, based on full throttle, because you want to maintain a given headspeed, and since you don't have a transmission that can change gears, you gear for the full throttle power peak to ensure that when you NEED the power, it is there.

And if the gearing is wrong for the engine chosen, you have to change the gear ratio. Either with other heli off the shelf gears, or custom ones.

Terry
Blade CP Trex 450 SE
QJ EP8v2 EX Gaui Hurricane 550
Vibe 50 Bergen Intrepid Gasser

07-27-2008 Over year old.
DKNguyen

Senior Heliman

N/A

Ah, okay. So you want to be running at the the peak power RPM at full throttle, even if the power peak changes to other RPM at lower throttle so when you need it, you can draw as much power from the engine as it can put out.

07-27-2008 Over year old.
Jafa

Elite Veteran

Sydney, Australia

Use search

Ask JackHeli - he's had a G26 in a heli
http://www.runryder.com/helicopter/...ser+zenoah+26cc


Protos | Logo 400 & 500 | Sceadu Evo | Freya Evo | Trex600N | Avant FX

07-27-2008 Over year old.
Pinecone

Key Veteran

Maryalnd

Yeap, because when you are at less than full throttle, you don't care if you are off the power peak for that throttle opening, you would just have the throttle opened a little more to have enough power.

But when you hit full throttle, you want to be at the power peak, otherwise you will not be able to et full power.

Terry
Blade CP Trex 450 SE
QJ EP8v2 EX Gaui Hurricane 550
Vibe 50 Bergen Intrepid Gasser

07-27-2008 Over year old.
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