Engines from an Engineering POV

Torque RPM and HP. What is important in an engine with a fixed pitch prop?

Would I rather have a little less HP but have greater torque or would I prefer to have more HP and less Torque?

Is torque a valid basis for comparing engines output? Can I use torque as a basis to predict or evaluate aircraft performance?

Looking for some technical explaination from our resident braniacs.

If you stick with real aircraft engines, your choice is horsepower.

Right on! Just put an actual aircraft engine on and your choices are easy!

1. How much HP do I want?

and the inevitable:

2. How much do I want to spend?

- Peter

To be honest, this is for my own education that I ask and I had only real aircraft engines in mind. It is a little theoretical since most of our engines today are limited in diversity, but I want to know about comparing engines with regard to aircraft performance. This seemed like the place to ask given the knowledge base here.

It's not a worthwhile question

Chill31 said:

To be honest, this is for my own education that I ask and I had only real aircraft engines in mind. It is a little theoretical since most of our engines today are limited in diversity, but I want to know about comparing engines with regard to aircraft performance. This seemed like the place to ask given the knowledge base here.

Click to expand...

Sorry to be so abrupt, but given that our props are direct drive and rpm limited to a great degree by tip speeds, HP is a function of torque or torque is a function of HP. Either is true. HP = torque * rpm / 5252. In addition, the torque curve on Lycoming and Continental engines and similar is usually very flat.

Turbo or super charging does not alter this much because of the rpm range. Gearing the prop speed down does affect this, a lot, but that's nearly always a conversion engine.

Lastly, no matter what the torque, it is horsepower that overcomes drag. Torque is a force, but HP is the ability to do work. The work that is done is to overcome the force: "drag" over a distance and for a time. One HP is 33,000 foot-pounds per minute.

Let me add a bit more to clarify it.

More horsepower = more performance.

Constant speed prop improves performance.

hevansrv7a said:

Sorry to be so abrupt, but given that our props are direct drive and rpm limited to a great degree by tip speeds, HP is a function of torque or torque is a function of HP. Either is true. HP = torque * rpm / 5252. In addition, the torque curve on Lycoming and Continental engines and similar is usually very flat.

Turbo or super charging does not alter this much because of the rpm range. Gearing the prop speed down does affect this, a lot, but that's nearly always a conversion engine.

Lastly, no matter what the torque, it is horsepower that overcomes drag. Torque is a force, but HP is the ability to do work. The work that is done is to overcome the force: "drag" over a distance and for a time. One HP is 33,000 foot-pounds per minute.

Click to expand...

Turbocharging or supercharging can vastly alter the hp curves of any engine at any rpm. Double the MAP at any given rpm and you roughly double the HP. The Reno Supersport engines are a good example, outputting well in excess of 700hp from 540-550 cubic inch Lycos and Contis at just over 3000 rpm.

Clarifying

rv6ejguy said:

Turbocharging or supercharging can vastly alter the hp curves of any engine at any rpm. Double the MAP at any given rpm and you roughly double the HP. The Reno Supersport engines are a good example, outputting well in excess of 700hp from 540-550 cubic inch Lycos and Contis at just over 3000 rpm.

Click to expand...

When I said that turbo or super charging does not alter it much, I was talking about the relationship between torque and HP. It is a given that increased MP will increase HP. Just extend the (nearly flat) lines on the power chart that comes with the engine to visualize this.

However, for non-racing use, turbo or super charging is usually used just to maintain MP in the thinner air at higher altitudes. Eh?

RPM / Torque

To put it in a way my simple mind can understand, horse power is a function of torque and RPM. That is an engine producing 100 HP at 1,000 RPM would have twice the torque of an engine producing 100 HP at 2,000 RPM and visa-versa. As stated earlier, with our direct drive engines, with RPMs limited by prop tip speeds usually in the 2,700 RPM range plus or minus s some small amount, a higher horse power engine, will of necessity have more torque. An engineering professor from years ago, explained it: ?Horse power is how much dirt you?re moving per minute, torque is how much dirt you?re moving per shovel full.? In other words, if you?re moving the shovel twice as fast, you only need half as much dirt in the shovel to get the same amount of work done.

The main advantage of a constant speed prop, (there are others) is the fact that you can get full rated RPM, and thus HP at any airspeed. You get your biggest constant speed boost on take-off when a fixed pitch set up for cruise would not allow the engine to turn at full rated RPM.

At least that?s how I get my mind around it.

hevansrv7a said:

When I said that turbo or super charging does not alter it much, I was talking about the relationship between torque and HP. It is a given that increased MP will increase HP. Just extend the (nearly flat) lines on the power chart that comes with the engine to visualize this.

However, for non-racing use, turbo or super charging is usually used just to maintain MP in the thinner air at higher altitudes. Eh?

Click to expand...

Agreed, the relationship between torque, hp and rpm is purely mathematical.

Hp is what matters as far a motivating your machine.

We see how very mild turbocharging vastly improves the performance of the Subaru 2.5L engines used in aircraft- going from a somewhat lame duck in atmo form to Vne busting speeds in turbo form using only 35 inches or so. 4 valve, large bore to stroke engines in particular, gain hp from boost at over a proportional ratio due to scavenging effects at valve overlap and pumping loss recovery on the intake stroke. We've seen gains of almost 75% at only .5 atmospheres in some cases on the dyno.

On the Reno Continental engines, hp gain is almost directly proportional to rpm and manifold pressure assuming good intercooler effectiveness and charge temperature control.

Great info, exactly what I was wanting to learn about. I do have a specific example in mind where my original questions are applicable

A little background, I am referring here to WWI engines and in this case rotary engines.

engine 1 is a 100hp rotary with a fixed pitch prop. However, at 1300rpm it produces 115 hp with 464 lb-ft of torque.

engine 2 is a 110hp rotary with a fixed pitch prop that produces 113 hp at 1200 rpm with 494 lb-ft of torque.

This is a genuine question, no tricks, no stump the dummy: can I say that the engine with the most HP is the engine that will give me the best performance or is it the engine with the most torque?

thanks for the discussion!

At attempt at an answer

Chill31 said:

Great info, exactly what I was wanting to learn about. I do have a specific example in mind where my original questions are applicable

A little background, I am referring here to WWI engines and in this case rotary engines.

engine 1 is a 100hp rotary with a fixed pitch prop. However, at 1300rpm it produces 115 hp with 464 lb-ft of torque.

engine 2 is a 110hp rotary with a fixed pitch prop that produces 113 hp at 1200 rpm with 494 lb-ft of torque.

This is a genuine question, no tricks, no stump the dummy: can I say that the engine with the most HP is the engine that will give me the best performance or is it the engine with the most torque?

thanks for the discussion!

Click to expand...

OK, I checked the HP, torque and RPM numbers and they fit the formula. So that's good.

Performance in this case must mean top speed and it must mean for the same airframe at the same weight. Acceleration and climb with a fixed pitch prop are more complicated. Are the engines, as installed, equal in weight?

The difference in HP is, IMHO, not possible to measure that closely. The HP is what determines top speed, not torque. You could produce 120 HP with 300 ft-pounds of torque and if all other factors were equal that would be the fastest.

The remaining question, not a small one, is the prop. Is it the same prop in both cases? Is it equally efficient at either RPM? The answers could determine which set-up would win a race.

Chill31 said:

Torque RPM and HP. What is important in an engine with a fixed pitch prop?

Would I rather have a little less HP but have greater torque or would I prefer to have more HP and less Torque?

Is torque a valid basis for comparing engines output? Can I use torque as a basis to predict or evaluate aircraft performance?

Looking for some technical explaination from our resident braniacs.

Click to expand...

There are quite a few discussions on HP vs. Torque on this forum. I am sure you will get quite a few replies on this thread. However, spend an hour or two reading this thread and you will have a good understanding of what you are asking about.

Ron Lee said:

Let me add a bit more to clarify it.

More horsepower = more performance.

Constant speed prop improves performance.

Click to expand...

You have just confused me with your clarification.

So, are you saying that an engine with a constant speed prop will have more horsepower than an engine without a constant speed prop? I am having a hard time following that logic.

I would agree that having a constant speed prop will provide a wider performance window for an airplane that has one. However, I am inferring from your statement that you believe because the constant speed prop provides more performance for the airplane you can simply reverse your equation and therefore the engine will have more horsepower.

Increasing horse power does equate to increased performance for an airplane, car, motorcycle, etc. (increased HP = increased Performance) but unless you increase the performance by increasing the HP of an engine the reverse is not true (increased Performance <> increased HP). If you increase the performance of your airplane by reducing drag you have increased the performance of the airplane but this decrease in drag in no way increased the HP of the engine.

I'll take a shot.

Not to put words in his mouth, but I suspect that he's saying that the constant speed prop improves *climb* performance, and, depending on the selection of fixed pitch, may also improve top speed performance. It also has the *potential* to improve cruise efficiency. The reason is that it effectively works like a transmission in a car or other land based vehicle.

If you select a fixed pitch prop purely for initial climb, it can equal the initial climb of a controllable prop. But as soon as the plane begins to accelerate, it will begin to lose out to the constant speed prop, because the constant speed prop will be constantly adjusting its pitch (transmission ratio) to keep the engine turning at full power rpm and coupling that power to the air. Using the car analogy, if you lock one car in 1st gear, it will accelerate equally with an identical car operating in normal mode until the 1st shift point arrives (peak power rpm). When car #2 shifts, it will immediately begin to outrun car #1. Conversely, if you lock car #1 in top gear, both will have the same top speed, assuming you can get car #1 rolling. But by the time it has accelerated to top speed, the race would be over. The reason is that with only top gear available, the engine will be too loaded down at low speed to make full power (turn at full power rpm). The same thing happens in a plane with a cruise prop when it's trying to accelerate on takeoff.
So....
Having a transmission in a car 'improves performance' over one locked in any single gear, and having a constant speed prop 'improves performance' over an airplane with a fixed pitch prop.

Charlie

Oh, and the constant speed may be able to improve efficiency by allowing full throttle setting at lower rpm, which reduces 'pumping losses' in the engine when operating at less than full power. IIRC, aviation lore gives Lindbergh credit for being 1st to use that technique in the Pacific theater during WW2.

steve, that thread is 90% of what I wanted to know. Thank you all for your time

Ok, having read through 10 pages of torque vs hp and rpm and such...

I do have a question for the masses...

If I have two engines that produce the same torque, lets say 500 ftlbs, but engine 1 produces that torque at 2000 rpm and engine 2 produces that torque at 1500 rpm, which engine will make my plane perform better (acceleration, climb, top end speed) assuming equal prop efficiency?

My second question:

If I have the following engines, which one do you chose for max climb, acceleration, top end speed? again assuming equal prop efficiency.

200 hp at 2500 rpm or 200 hp at 1500 rpm?

Thanks!

Same-same with same prop efficiency. But won't the two prop efficiencies deviate from each other with airspeed changes?

another view

I look at this question a little differently. Given a max prop RPM and given a max piston speed, one could either build a big bore direct drive engine (Lycoming) or a gear reduction small bore (Rotex). With the same piston speed and prop speed they will produce roughly the same horsepower. And since horsepower is torque x RPM, the same torque at the prop hub. Now the direct drive is a stroker with a slow speed crank and slow speed cam, etc, so will probably last a lot longer than a high reving gear reduced engine. However, the gear reduced engine could be lighter. This is shown by the Rotax vs Continental o-200 comparison. In the same vain horsepower is just how much gas one burns; it is directly tied to gals per hour. All this fancy porting and stuff just means you are burning more gal/ hour, hence more horsepower. Throttle back to the same gal/hour and you should see equivalent horsepower. JMO

Engines and Props

Chill31 said:

Ok, having read through 10 pages of torque vs hp and rpm and such...

I do have a question for the masses...

If I have two engines that produce the same torque, lets say 500 ftlbs, but engine 1 produces that torque at 2000 rpm and engine 2 produces that torque at 1500 rpm, which engine will make my plane perform better (acceleration, climb, top end speed) assuming equal prop efficiency?

My second question:

If I have the following engines, which one do you chose for max climb, acceleration, top end speed? again assuming equal prop efficiency.

200 hp at 2500 rpm or 200 hp at 1500 rpm?

Thanks!

Click to expand...

I think you need to understand more about the propeller/engine combination in order to be able to answer your question. The power absorbed by a propeller increases proportional to the RPM^3. In other words it is a curve. So even if two propellers have the same efficiency, that is converting HP to thrust, a propeller that absorbs 200 HP @ 1500 rpm will either have a larger diameter or more blades or both compared to one that absorbs 200 HP @ 2000 rpm. So you may not have enough ground to propeller clearance if you try to fit the lower speed engine to your aircraft. In general you want the largest diameter prop turning as slowly as possible to get the greatest thrust/HP.

You also should realize (and if you are talking about actual engines you probably know this) that the lower the rated engine rpm for a given power the larger and heavier the engine will be. This affects the empty weight of the aircraft and will also have an effect on the climb performance.

So if you are thinking of designing/building a WWI replica remember these are draggy airframes (lots of struts and wires), usually with big cowls to house the low speed engines of the period. Using a modern (2500 rpm horizontally opposed air cooled) engine a lot of the propeller is going to be masked by the cowl.

You have asked a question that gets debated by all who use engines. Here is the key, torque is the ability to do work. Horsepower is the speed with wich this work can be done.

What you are interested in is horsepower. However, people talk about engines as if a 200HP engne will always, under all conditions make 200HP. This is not true. Let's take out the complication of density, and assume we are talking about sea level only for a moment. A 200HP engine will only produce that at, say, 2700rpm. At 2500 it will be less. How much less? You need to have the power curves to know, but typically big displacement, high torque engines have broader HP curves, and the reduction of HP with reduced rpm is less than a small displacement low torque engine.

The statement about constant speed vs. fixed is wrapped around the ability of the constant speed to keep an engine at its peak power rpm with differring airspeeds. A fixed will require that an engine be off of its peak power rpm with changing airspeeds. So props do not cause an engine to perform differently, they just enable the engine to operate closer, or farther away, form their full potential.

All that said, this broadness of a power curve is often reffered to as the flexibility of an engine, since a broad power curve means an engine makes high power at a wide (relatively) range of engine speeds. All other things being close to equal, a bigger displacement engine will be moreexibly than a smller one.

Tim