Engine physics question...

[porsche356a]

What causes the sensation and transfer of torque and power?

For example, let's say I have 2 cars:

Car 1 - 3000lbs, manual, 300 hp, 300 ft-lb
Car 2 - 3000lbs, manual, 200 hp, 200 ft-lb

Both are rear wheel, both are geared the same.

I understand how engines make power, how transmissions work, and what the differential does, but I don't really get exactly how the transfer function works.

If the only connection between engine and tranny is the clutch plate to flywheel, how does lateral energy become applied energy?

Suppose both cars are spinning @ 2500rpms, clutch engages fly wheel, all things being the same expect hp/torque, why does Car 1 feel more powerful?

[Jim Sims]

"Google" the terms: force, torque, velocity, angular velocity, acceleration, angular acceleration, linear and angular momentum, friction, traction, and MEP (mean effective pressure). Ponder these definitions and then repost your question.

[ianc]

Suppose both cars are spinning @ 2500rpms, clutch engages fly wheel, all things being the same expect hp/torque, why does Car 1 feel more powerful?

Because the engine has an extra 100 ft-lbs of torque obviously.

Follow Jim's advice and do a little research. Make sure you understand the difference between 'power' (or HP), and torque. The answer will probably become clear.

ianc

[KFJ_SC]

Start here: http://auto.howstuffworks.com/under-the-hood-channel.htm

[RWebb]

torque is pretty easy to visualize as it can be directly sensed - torque is how hard it is to stop a rotating shaft if you grab it. low torque, you can grab and stop it; high torque if you grab it it'll throw you across the room

power is the time derivative of energy, hp is a unit of power measurement - for that you'll need to do some reading & thinking and it still may be vague

your example is even tougher to understand as other factors are involved.


BTW, fpt is not a unit; did you mean ft-lb.? often given as lb-ft. too.

[Steve W]

HP = (torque*rpm)/5252

This is very general, but torque is what you feel as thrust, HP is what is needed to maintain a peak speed, by overcoming resistant forces from wind, tires, transmission and friction. You don't feel speed, but you feel acceleration. Both engines having the same peak hp, can only attain the same peak speed.

True power is really about area under the curve - the hp/torque curve. Compare a rectangle 6000 rpm wide and 300 hp high, vs a triangle also 6000 wide by 300 high. Which one has more area? Peak torque is usually achieved at a lower point in rpm scale, and the one with more peak torque, will make more hp at the same rpm points from idle to peak rpm. For example, using the above formula at 2500 rpm, the 300 ft-lb motor will produce more hp at 2500 than the 200 ft-lb motor, and all rpms increments from idle to peak.

[Hladun]

The answer is throttle opening, which results in manifold vacuum. Engines produce torque (cylinder combustion pressure times crank throw). Horsepower is just torque times rpm. So back to the issue, the less powerful engine, at a given rpm, is producing the same torque at a greater throttle opening. When you “floor it” the more powerful engine has more manifold vacuum to suck in more air and fuel (probably into a larger cylinder as well) so you get more added torque and acceleration.

A simple way to verify this is turbocharging. The turbo increases manifold pressures from a vacuum to a positive pressure and away you go.

[Bullet Bob]

I think it is great when one actually tries to answer the question instead of telling him to go read a book and come back when he has some brains.

[porsche356a]

Thanks all.

Hladun provided the sort of answer for which I was looking. I understand the concepts of physics, but don't have the math to see how the formulas equate to mechanical visualization.

I'm still not certain how the rotational mass of the flywheel (at a fixed weight and relative speed) would make one car put down more power than the other.

I am missing a variable here, but not sure what.

[masraum]

Originally Posted by porsche356a

Thanks all.


I'm still not certain how the rotational mass of the flywheel (at a fixed weight and relative speed) would make one car put down more power than the other.

I am missing a variable here, but not sure what.

Your original question is not really clear to me. "lateral energy become applied energy" Which lateral energy, the only lateral that I can think of is the clutch moving in/out relative to the flywheel, and by applied energy, I assume you mean, the tires turning or the car moving?

To me, the answer to your initial question doesn't really address the difference between the feel due to the extra power.
The pistons/crank, use leverage to create torque, twisting power. The torque is then transmitted from the end of the crank through to where the rubber meets the road.

You're most recent question is slightly less confusing.

The rotational mass of the flywheel/crank, stores energy. When you let the clutch out, that energy is partially transfered to the clutch/pp (what isn't lost to heat, etc). The transfer mechanism is friction the same but opposite to disk brakes.

If you managed to make both the 200hp and 300hp engines have the same rotational/reciprocating masses and created the difference in power through tuning, then they should feel the same if the throttle was fixed on each engine to spin the engine at say 4000rpm. If they both have the same reciprocating/rotating masses, then I suspect that to get both engines to spin 4000rpm should require roughly the same amount of power. If you dumped the clutch (and the engines didn't stall, then I suspect that the resulting acceleration would feel the same. The throttle opening of the 300hp engine would probably be smaller. You'd have more potential to add to the system than you would in the 200hp engine.

If however, you had both engines at 4000rpm, dumped the clutch AND floored the gas on both engines, then you'd feel the difference because by gassing the engines, you were creating more power/adding power into the systems.

I think it's really hard to answer this sort of question because the odds of "all else being equal" part is unlikely to happen in real life. I can only see two places to feel a difference, 1, different rotating masses storing more energy and 2 different potential for pressure to create more force inside the cylinders. The "feel" that you are going to get is going to be due to either or both of these factors.

I'm certainly no expert, this is just my rambling. If I'm wrong, I'd love to have someone more educated chime in and set the record straight.

[Hladun]

Originally Posted by porsche356a

Thanks all.

Hladun provided the sort of answer for which I was looking. I understand the concepts of physics, but don't have the math to see how the formulas equate to mechanical visualization.

I'm still not certain how the rotational mass of the flywheel (at a fixed weight and relative speed) would make one car put down more power than the other.

I am missing a variable here, but not sure what.

I’m not sure you understand. Omitting a list of qualifications, similar cars traveling at the same speed would need the same hp (torque/rpm) regardless of engine size. The difference would be that the car with the small engine might be at full throttle (its top speed…could not accelerate at all) whereas the car with the larger engine might be at, let’s say, half throttle. When you open the throttle (press the accelerator) the second car would accelerate until the hp the car requires at its top speed matches the engine at full throttle. This of course would be faster than the car with the smaller engine.

Engines almost always operate in a “throttled” condition. That is, they produce less power than they are capable of at wide open throttle.

[porsche356a]

The reason for this inquiry is because I accidentally popped the clutch (@ about 5000 rpm) in my 997S on a hill.

The car shot forward with great torque.

I guess my question is, had I been in my '87 or 356 (same basic set-up, but less power) where is the torque difference created?

Does the 997S create more torque (or perhaps forward velocity) because there is more mass to the flywheel? Why does an '87 popped @ 5000rpm shoot forward with less "omph" if the size, weight and speed of the moving parts is basically the same.

If I knew the right terminology, this would be easier to ask.

[47silver]

the torgue from the engine is caused by the force of the piston acting on the lever that is the stroke of the crankshaft.

for example
a single cylinder engine
3 inch stroke = .25 feet
4 inch diameter piston = .087 square feet
1000 psi = force of the fuel.

torque = 1000*.25*.087 = 21.75 foot pounds

now make the stroke (the lever) 1 inch longer and keep everything else the same then
4 inch stroke = .33 feet
t = 1000*.33 * .087 = 28.71 foot pounds of torque
torgue then is the force acting on the lever.
now make the piston bigger say 4.5 inches and keep the length of the stroke the same...and see what happens.

[porsche356a]

Originally Posted by Hladun

I’m not sure you understand. Omitting a list of qualifications, similar cars traveling at the same speed would need the same hp (torque/rpm) regardless of engine size. The difference would be that the car with the small engine might be at full throttle (its top speed…could not accelerate at all) whereas the car with the larger engine might be at, let’s say, half throttle. When you open the throttle (press the accelerator) the second car would accelerate until the hp the car requires at its top speed matches the engine at full throttle. This of course would be faster than the car with the smaller engine.

Engines almost always operate in a “throttled” condition. That is, they produce less power than they are capable of at wide open throttle.

Ah, I think I got it. It is a function of relative speed and potential power (per throttle), not necessarily size or weight.

[Hladun]

Originally Posted by porsche356a

The reason for this inquiry is because I accidentally popped the clutch (@ about 5000 rpm) in my 997S on a hill.

The car shot forward with great torque.

I guess my question is, had I been in my '87 or 356 (same basic set-up, but less power) where is the torque difference created?

Does the 997S create more torque (or perhaps forward velocity) because there is more mass to the flywheel? Why is does an '87 popped @ 5000rpm shoot forward with less "omph" if the size, weight and speed of the moving parts is basically the same.

If I knew the right terminology, this would be easier to ask.

You've now changed the question. One answer per customer.

[Dantilla]

The difference between a 997 and a 356:

Generating power has nothing to do with flywheel weight.

Power/torque is created by harnessing an explosion in the cylinder. The 356 has four little cylinders. The 997 has six much larger cylinders, and on top of that, harnesses the explosions more efficiently.

Six big explosions have more energy than four little ones.

[porsche356a]

Originally Posted by Dantilla

The difference between a 997 and a 356:

Generating power has nothing to do with flywheel weight.

Power/torque is created by harnessing an explosion in the cylinder. The 356 has four little cylinders. The 997 has six much larger cylinders, and on top of that, harnesses the explosions more efficiently.

Six big explosions have more energy than four little ones.

Getting close...

Ok, I understand why larger displacement/more cylinders generate more power... How is it that the power is translated to trust?

Is the flywheel spinning faster because of relative engine power?

Maybe I need to stop by the local university for a physics lesson.

I'm a tech management guy, mechanical physics have always fascinated, but eluded me.

[masraum]

Originally Posted by porsche356a

Getting close...

Ok, I understand why larger displacement/more cylinders generate more power... How is it that the power is translated to trust?

Is the flywheel spinning faster because of relative engine power?

Maybe I need to stop by the local university for a physics lesson.

I'm a tech management guy, mechanical physics have always fascinated, but eluded me.

Actually, you don't need more cylinders or a larger engine to make more power. You can have a 4 cylinder with more power than a 12 cylinder (uncommon, and unlikely, but possible). Part of the reason was above. Imagine that the air/fuel mixture explodes like a firecracker or explosive with each cyle (it doesn't actually explode that would be bad, it just burns really fast). OK, if you can picture each explosion as coming from some sort of fuse lit device, imagine that in my miata, the explosions are like an M-80 firecracker, in my old '76 chevette they were like 'lady fingers' firecrackers, and in a 997 the explosions are like a stick of dynamite. So, which explosion would you rather hold in your hand? Why, because the smaller ones release less energy. More energy means a bigger push on the top of the pistons.

The 'explosion' pushes on the top of the piston, the piston connected to the connecting rod, which is connected to the crank (think bicycle pedals) causes the crank to rotate. The flywheel is bolted to the back of the crank which is rotating. That rotation is the same rotation other than being modified by gears that goes through the tranny, to the driveshaft to the rear to the axles to the tires.

Like I mentioned, the car engine works just like the pedals on a bicycle. The top of each pedal would be like the throw on the crankshaft. You could think of it as your knee is like the top of the piston, the part of your leg below the knee is like the con rod, the pedals are the crankshaft, and your thigh is like the explosion that powers it all. The same way the sprocket goes in circles because you pedal, the flywheel goes in circles because of the explosions in the cylinders. The main difference between the pedals and the crankshaft is that the pedals have their "flywheel" in between the crank throws and an engine crank has it's flywheel at the end.

Your best bet would have been to look at "howstuffworks" like I think someone suggested above.

http://auto.howstuffworks.com/engine1.htm
http://auto.howstuffworks.com/engine2.htm







To address some of the stuff that you ask about the flywheel. A heavier flywheel (and crank, pistons, rods, etc) will make the engine work differently at launch. Generally if that stuff is heavier, the engine will rev up and down slower. Also, if the flywheel is heavier, then a launch will feel a little benefit, but only for a short time. At some point the engine power has to take over. Racecars don't have heavy flywheels, on the contrary, they have ultralight flywheels (Carrera GT too). With a lighter flywheel, it'll be easier to stall a car when starting because you have to feed the power in sooner than in a car with a heavier flywheel.

The flywheel isn't spinning faster in your 997 at 5000 rpm than it was in the 356 at 5000 rpm. 5000rpm is still 5000 revs per minute no matter what engine you're talking about. It's not the flywheel, it's those pesky cylinders burning all of that air and fuel making the stronger explosions.

[911mot]

Think of a push bike. You are the engine. The wheels are the flywheel. How much torque you can produce will decide how steep a hill you can climb and how much power you can produce will define your top speed.

[Robert Buchanan]

After reading all of the "technical replies" I thought that a common sense one should be offered. In the example given...."car #1" simply has 30% more power and torque.
30% is a lot in terms of just about anything....30% more money, 30% more property, 30% more power....it's a common sense thing.
The one I like the best is 30% more 'time'....
If you have ever stepped out of a 3.2 Carrera and jumped into a 3.3 Turbo, you would understand power and torque easily. Going from 205 HP to 282 HP is a vast difference.
911_pilot