Horsepower vs Torque vs Revs

[bajo39]

I have heard that the high speed of F1 engines is what makes them so useful for racing not the high horsepower. Why is it that RPMs are so important? Why is horsepower not the emphasis and what about torque?


I understand that the light-weight and low COG are also very important but I'm interested in the emphasis on revs and why this is. Why can an F1 engine not just have high-horsepower and torque like a sports car?

Thanks a lot for any help!

[bcsolutions]

There is an old saying: "There is no replacement for displacement". Traditionally, the larger the engine displacement (the combined cylinder capacity), the larger the power. While this is a fine rule of thumb for production car manufacturers, increasing the displacement of a F1 engine is not possible (regulations prohibit anything larger than 3.0 litres) and this poses the interesting question of "how do we increase the power of an engine when we can't increase the displacement?".

Take a well developed engine such as the Porsche 911 flat six for example, 3.6 Litre engine making roughly 325hp, the engine probably revs to around 8000rpm. Put this against an F1 engine of 3.0 Litres and with the classic displacement rule you'd imagine the Porsche engine to be more powerful because of its larger size. The fact is however that the F1 engine is nearly three times more powerful.

To put it simply an engines power output is governed by the amount of fuel/air mixture you can get into the cylinder, combust, and get out again. A larger displacement engine can obviously hold more fuel/air mixture at any given time than a smaller one and should (providing the engines are identical barring size) produce more power. As mentioned earlier, F1 engineers have no say in the displacement of F1 engines. What the engineers have to do is increase the amount of air/fuel mixture the engine can get through by some other means. Increasing the rate at which the engine cosumes the fuel/air mixture is an obvious solution. Imagine a small engine consuming half as much mixture as a larger engine but at twice the speed and you have a reasonable mental analogy of why an F1 engine revs so high.

Basically F1 engines have to rev to extremely high speeds in order to produce the high levels of power that F1 demands. Although there are many other considerations to make in designing an engine the basic principles are the same.

Sorry if this is over-simplified!

[Guest]

Visualize a single-cylinder engine, of any displacement. Each time the spark plug fires, it generates power. That power is transferred via the piston. If you have a long stroke, you get terrific leverage 90 degrees past TDC. If you have a shorter stroke, then you get less torque. Hmm, a long stroke engine is poor for RPM, while a short stroke engine sucks at torque. But to get more power, you need to have the power stroke more often. Increase the RPM's. But as the rev's increase, it is preferable to go with a shorter stroke.
So there you go. For more power, you need more RPM. But for an engine to deal with sustained rev's, the designer includes a short stroke. But a short stroke engine has lower values than a long stroke engine of comparable displacement.

[Guest]

But a short stroke engine has lower TORQUE values than a long stroke engine of comparable displacement.

[Guest]

But a short stroke engine has lower TORQUE values than a long stroke engine of comparable displacement.

Okay I understand most of what you said. Could you define TDC though - I'm not sure what that is?

This is my understanding, based on what you and the previous poster have said - please correct me if I am wrong:

The longer the stroke, the further the piston travels (and consequently the connecting rod), and therefore the more torque that is applied to the crank. However, having the piston travel further takes longer, hence the rate of revs is lower.

Long stroke -> more torque but lower rate of revs -> less power

Conversely, a shorter stroke allows a greater rate of revs and therefore more power (because more power cycles per minute) but less torque since the piston is not travelling as far.

Short stroke -> less torque but higher rate of revs -> more power

[Guest]

So I guess the answer is that the F1 teams don't especially care about revs, it is just that they need to have all those revs to generate a lot of power and they are willing to accept lower torque to get it.

[West]

Smaller engines usually go for more revs for power, since the displacement (and most likely the stroke) is smaller than that of a larger engine. Bigger displacements don't need to rev as high because they can get a bigger volume of air at one moment than a smaller displacement engine at the same moment (let's say engine speed).

[bcsolutions]

So I guess the answer is that the F1 teams don't especially care about revs, it is just that they need to have all those revs to generate a lot of power and they are willing to accept lower torque to get it.

Correct! In fact i think they'd rather the engines didn't have to spin to such stratospheric levels.

It is possible to increase the power of an engine by other means than engine speed and that is to force more air and fuel mixture into the combustion chamber. This is how turbo and superchargers work. Forced induction is not allowed within F1, the old turbocharged F1 engines of years gone by prove just how effective forced induction can be. 1500bhp from a 1.5 Litre 4 cylinder engine! Thats 1000bhp per Litre! Current F1 cars are putting out about 320bhp per Litre and thats unheard of for a naturally aspirated engine (outside of F1). If F1 engineers of today could use forced induction then i think they would but without an effective and fair method of restricting engine power it isn't going to happen again soon.

[Guest]

Thank you very much for your response! You have answered my question but alas I have another for you:

"It is possible to increase the power of an engine by other means than engine speed and that is to force more air and fuel mixture into the combustion chamber."

Why do they not just use a higher compression ratio? This would achieve a greater density of air/fuel in the cumbustion chamber, would it not?

[bcsolutions]

Why do they not just use a higher compression ratio? This would achieve a greater density of air/fuel in the cumbustion chamber, would it not?

Hi Guest.

Well, i'm not sure what compression ratio F1 cars are running but it is bound to be high. The problem arises when the compression ratio becomes so high that the fuel/air mixture within the combustion chamber detonates uncontrollably (pinking, knocking etc) before the spark plug has had chance to ignite the mixture (Diesel or Compression ignition engines put this to great use and thusly do without spark plugs). This is an unwanted side effect to running a too high compression ratio.

One way engineers get around this is to run the engine on the borderline of pre-detonation (by advancing the timing or increasing the compression ratio) and use a knock detector, basically a sensor that can tell whether an engine is pre-detonating or not. When the sensor detects knocking it retards the ignition timing so that the spark combusts the mixture before the pressure within the combustion chamber gets chance too (closer to TDC).

Of course fuel plays a massive part in this and you'll find that the race grade unleaded used in F1 is far less susceptible to pre-detonation than regular pump unleaded and is therefore tolerant of a far higher compression ratio! Phew!

[Guest]

Fascinating. And what is TDC?

[bcsolutions]

TDC = top dead centre, the position of the piston at the top of its travel, when the compression (with valves closed) is at it's greatest. Opposite of BDC which is bottom dead centre!

[riff_raff]

bajo39,

The reason F1 engines rev to as high a speed as possible is that they want to make as much horsepower as possible. Horsepower is what determines how fast an F1 car goes around the track, not torque.

All other things being equal, the faster you turn the engine, the more horsepower it will produce. Double the rpm's and you double the horsepower.

If you've seen the data acuisition from an F1 engine, you'll notice that they typically run between about 14,000 and 18,000 rpm. And when they leave the pits from a standstill, they have to rev the engine to above 8,000 rpm and dump the clutch to keep the engine from stalling.[/i]

[Reca]

But a short stroke engine has lower values than a long stroke engine of comparable displacement.

No, the old “for a given displacement short stroke means less leverage hence less torque” is a false myth, for a given displacement the amount of peak torque is pretty much independent by the bore/stroke ratio. To be more precise, there are secondary effects, but they aren’t directly related with the leverage, they are related with the shape of the combustion chamber, over a certain value of the bore/stroke the shape could be very poor and cause a reduction of the combustion efficiency, hence of the mean effective pressure, that is proportional to torque.
Same goes for the myth of “a V8 has more torque than a V12”, simply false, or, as Marmorini once said with notable brevity : “balls”.

Why do they not just use a higher compression ratio? This would achieve a greater density of air/fuel in the cumbustion chamber, would it not?

Compression ratio has nothing to do with the amount of air (hence mixture) entering in the engine, CR is the ratio between the volume with piston at BDC and the volume with piston at TDC. The influence of the CR is on the thermodynamic efficiency of the engine, but efficiency doesn’t growth linearly with CR, the slope of the curve actually decreases with CR, so above a certain value there’s little to gain.

The problem arises when the compression ratio becomes so high that the fuel/air mixture within the combustion chamber detonates uncontrollably

Detonation was certainly a huge issue during the turbo era but at current F1 rpm isn’t, there’s no time enough to have it. In the current F1 engines the CR is in the order of about 13, limited mainly for geometrical reasons. If you consider that for a unitary displacement of 300cc the combustion chamber volume at such CR is little more than 20 cc, with a bore of little less than 10cm you rapidly understand that the combustion chamber volume at TDC is largely formed by the hollows for the valves on the piston upper surface and these also depend by valves size and lift. Further reducing the volume of the hollows would limit the valve design, the reduction of the volumetric efficiency would likely offset the increment of thermodynamic efficiency.

[Odin]

Guest,

I feel it necessary to state that torque, horsepower, and RPM are related to one another. If you know 2, you know the 3rd.

If follows this equation (From the book Mechanical Engineering Design, by Shigley and Mischke)

Horsepower=Torque(in-Lb)*RPM/63000

So, if you can increase the RPM of the engine, for a given torque, the horsepower increases.

Horsepower is roughly constant through a transmission, therefor, if I put 1Hp into a transmission I will get 1 Hp out. Torque and RPM do change though, to keep that horsepower constant. I can put 10 in-Lb of torque in and get 100 in-Lb out relatively easily, but the RPM will drop to keep horsepower constant.

The most important thing is that if I increase the input speed of a transmission (make the engine rev higher), and keep the torque constant, I can get more torque at lower RPMS out of the engine, and therefor accelerate faster.