Engine Performance Curves

[dknguyen]

So we all know about how motor torque/efficiency/speed/power curves are given at a certain voltage, and how increasing the voltage will shift the shift the torque-speed curve right and shift power-speed curve up (while shifting it slightly to the right in order to keep the power peak centered on half no-load speed since the maximum speed has just now increased).

BUt what happens with an engine torque/power vs speed curve? I assume these curves are taken at full throttle (because its not labelled and that just makes sense). But at half throttle, how does the curve change? Sure, all the curves will get shifted down, but because of the air-gas mixture and the volume of air decreasing with increasing engine speed, but less air is required to mix with less gas at half throttle, I think the peaks of the curves will also change. But I am not sure how they will change.

The curves will all move down for sure because there is less fuel being burned. But I suspect the peaks will move to the right since there is now a larger air:fuel ratio so the fuel is being burned better and the the amount of air taken in is limited at higher speeds because the engine has less time to suck in air (which is what seems to limit the HP at higher speeds and thus define the power peak for a given amount of fuel burned/fixed throttle setting)

 

[Hero999]

Engine is that curve from?

A high performance motorbike?

No road car redlines at 14000rpm.

 

[dknguyen]

Nope, it's a heli engine.

 

[Hero999]

Which is a similar thing I supose - it needs to be both powerful and lightweight.

It's probably totally different to the curve that you'd expect from a typical car engine.

 

[rjvh]

Hi dknguyen

the torq curve gives you an idea how much efficient power is given

you see that the bleu line peaks later it will produce at that time more power but effectively not that efficient as that it did at lower rpm's

the torq curve is a set curve that is uniqe for the motor

unles you change the motor design (in case of a petrol engine change carborators or exhaust or stroke of the piston or spark timing or valve size or timing) it will not change that much in the shape of it

it's better to look to the torq curve than the power curve as the torq curve tels you how much power you can expect for that rpm

the power curve is more for marketing purpose as it gives you only an indication how much it can produce but how much it is contributing to the output of the engine depens on the combination with the torq curve

curves are normaly given at full trotle

half trotle will not change the shape of the torqe curve but the output will be less as the power curve will be lower

your diagram indicates that if you keep the engine inbetween 9000 and 13000 you will have a quite steady force if the laod is dependible you might consider gears to keep the engine running in this range

Robert-Jan

 

[dknguyen]

Hi dknguyen

the torq curve gives you an idea how much efficient power is given

you see that the bleu line peaks later it will produce at that time more power but effectively not that efficient as that it did at lower rpm's

the torq curve is a set curve that is uniqe for the motor

unles you change the motor design (in case of a petrol engine change carborators or exhaust or stroke of the piston or spark timing or valve size or timing) it will not change that much in the shape of it

it's better to look to the torq curve than the power curve as the torq curve tels you how much power you can expect for that rpm

the power curve is more for marketing purpose as it gives you only an indication how much it can produce but how much it is contributing to the output of the engine depens on the combination with the torq curve

curves are normaly given at full trotle

half trotle will not change the shape of the torqe curve but the output will be less as the power curve will be lower

your diagram indicates that if you keep the engine inbetween 9000 and 13000 you will have a quite steady force if the laod is dependible you might consider gears to keep the engine running in this range

Robert-Jan

THe load should be quite steady. There are gears, but it is only a gear and no transmission since it is a model. My estimated power is only 800-1200W which is much less than full throttle so I was wondering if the power and torque peaks moved to different RPMs and different throttles. If the peaks do not change, then I can just adjust the throttle to maintain my required RPM (and this maintain operation at the peak which happens to be my the required RPM to get my desired output speed for my gear ratio, which is fixed) since I am operating so much below the engine's capable power output.

THe reason I am not paying much attention to the torque curve is because I can't estimate the torque required to rotate rotor blades with varying angles of attack at a given speed. I can only estimate the power required to spin the blades at a given RPM (which indirectly gives me torque I guess). I just know that I am supposed to stay above the torque peak so there is a negative feedback effect during fixed throttle, rather than a positive feedback effect (where slowing down the engine reduces the torque which slows down the engine even more which reduces the torque even more, etc.).

 

[rjvh]

Hi dknguyen

if you tilt your rotor blades you will have more friction so if you don't adjust the trothel position (giving more feul) your rpm will go down

in your case you want that the moter runs between the 10000 and 11000 that's where you have you torq peak

if you don't have the option of adjust the trotle when the device is in operation than i would sugest to let adjust the motor on 13000 rpm and than varies the load still if the load is to much and the rpm drops down under the 8500 there will be not that much power left over

is yor trotlle controlable with a servo??

Robert-jan

 

[dknguyen]

Yes, the load does vary. Yes, I can adjust the throttle during operating. I was planning on adjusting the throttle (with a servo, yes) to keep the engine running at 13000RPM as the load varies on the rotor blades. It must be 13000 RPM becaues this is a fixed transmission with a gear ratio that I cannot change and it needs to be 13000RPM to keep the rotor spinning at the speed I want. My question is that if I did this (adjusted throttle to keep the speed at 13000RPM) would I still be operating at the power peak of the engine? Or does the power peak of the engine change to other RPM at other throttle settings?

I would operate at the torque peak for maximum fuel efficiency, but the speed there is too slow and my gearing is fixed. It would also require more throttle adjustements to operate at that point because the motor is operating on the threshold of stability where the motor no longer inherently tries to maintain it's current speed when the load changes (below the torque peak, slowing down the motor reduces torque slowing it down further into a snowball effect until it stalls in the absence of a throttle increase).

Sorry if I am repeating myself. It seems there's a difficulty between communication between us.

 

[rjvh]

the position of the peak will not change with other trotle settings the intensity does

so with half trotle and minimum of load it will have it's peak at 13000 rpm as the load increase and you trotle is fixed the rpm will go down

if you can make your trotle operatable wit a servo you can increase the trotle position at an increasing load so your enging keeps in the 13000 rpm

i would make the engine work like this that the basic function of flying (lift) is achieved at 11000 rpm making other manouvres or conditions(which i asume will take more energy) can be handeled by incraesing the trotle position

you will get a more agressive reacting device but it wil not realy sweat in its performance when sudenly a breeze picks up

Robert-Jan

 

[dknguyen]

I was also planning on using a tuned pipe (why? because I can even though I don't need maximum power from the engine. Improve efficiency is always nice though). But since I am just learning engines I am probably not up to the task of tuning the thing. However, noise is a very important factor to me (seeing as how I had to move away from electric to engine due to energy density requirements). read somewhere that if you use a tuned pipe that is way too long, it doesn't interfere with the engine (but neither does it improve power) but instead acts like a better-than-average muffler. The extra cost and weight of a fancy tuned pipe being used as a muffeler is not an issue for me and I only want to buy once so I was thinking of using it like this until I know enough to start trimming the pipe to tune it.

Can anyone confirm this? A long tuned pipe will not impede engine performance? (whereas a progressively shorter pipe will improve engine performance up to the optimum point, after which if it is any shorter it will start impeding performance and making it very finicky).

 

[rjvh]

my experiance only goes as for for motorbike engines and car engines

exaust pipes for 2strokes are a complex mater and normaly they improve a specific range of rpm that they realy work well and you hope that they don't screw up the other part of the power range

Robert-Jan