Torque and Horsepower - A Primer

[PlatinumZealot]

Power is energy! BMEP is just a way of comparing the performance of piston engines. Just the average pressure above the pistons.

NO IT ISNT! Just stop saying wrong misleading things that can be found to be wrong with the most trivial of research. This is a technical forum facutally back up your claims.


If they were the same, then they would have the same units and a similar definition. They have neither.

Look:
http://en.wikipedia.org/wiki/Power_%28physics%29
http://en.wikipedia.org/wiki/Energy


It's like saying speed and acceleration are the same thing.

Also:

Diesels have high BMEP, because diesel fuel has a high calorific value. More energy in the explosion. The drawback is that the explosions can't take place as rapidly as octane.

Also this is clearly wrong, and can be proved so with the most trivial of research.
Calorific value proper name is called HHV (higher heating value).

Diesel - 43-44 MJ/kg
Gasoline - 47-48 MJ/kg

http://www.engineeringtoolbox.com/fuels ... d_169.html



You clearly don't have a clue what you are talking about, and are either deliberately misleading people, or unintentionally doing it becuase you have a pidgeon understanding of the subject matter but enjoy arguing.

Either way, the things you have stated above are factually incorrect.

You see... I get these things from my memory 98% of the time. so there are times my memory will fade. But Big deal 1 MJ/kg difference between deisel and gasoline! (in a science lab!). Lower heating values are very variable. BUT back to the point -the BMEP of deisel is higher!

Looking for every little chink in my argument I see. I already stated that Power is the rate of energy transfer. Of course it's obvious they are not the same.. I already covered that. But somebody asked me what is its physical form. Now.. I am a practical thinker, and putting a "per second" on to the end of a unit doesn't change its physical form for me.[...]

[xxChrisxx]

You are misleading others with very poor science.

[PlatinumZealot]

The only inconsistencies were that I said diesel has a larger, lower heating value than gasoline (just a one megajoule per kilogram mix up n my part) and when I said Power is energy (and I gave a reason why I said that). So to the average man yes it will mislead. But I shall clear it my self of that mistake a third time by saying that Power is the transfer rate of energy. Good?

That is what you want from an engine! Even if you twist it, by using only pure units...lets say you are producing torque at the crankshaft, power allows you to deliver torque at a high rate.

[Raptor22]

n_smikle


WTF has octane got to do with how fast fuel burns? Example F1 Fuel is High MON, not so high RON but burns much faster than pump gasoline.
Diesel has low octane but high cetane. High cetane is the tendancy of the fuel to want to spontaneously combust.
Diesel does have more energy per unit mass but it burns slower due to longer and bigger molecules that makes up Diesel - C8 to C12 HC chain s vs C4 to C8 chains for petrol/gasoline.

[xxChrisxx]

The only inconsistencies were that I said diesel has a larger, lower heating value than gasoline (just a one megajoule per kilogram mix up n my part) and when I said Power is energy (and I gave a reason why I said that). So to the average man yes it will mislead. But I shall clear it my self of that mistake a third time by saying that Power is the transfer rate of energy. Good?

Saying power is generated first, and work is a produced as a consequence is the only thing I have a major issue with. It then allows many incorrect conclusions to be drawn about how an engine functions.


Which is why:

Even if you twist it, by using only pure units...lets say you are producing torque at the crankshaft, power allows you to deliver torque at a high rate.

is a statement I agree with.

You don't have high power if you can't do a lot of work. However, one can equally correctly say that you don't have high amount of work if you don't have a high rate of power. In the context of power generation they go hand in hand. If one rises, so does the other, if one fall so does the other.

As this is the case, it's easier (and more correct) to say that both are simultaneous and of equal importance.

As I agree that power is the most useful metric for an engines ability to do work (by definiton), I would say that the goal of any engine, is to maximise the area under the power curve in it's ideal operating range.

Whether you say you want to tune for increased power at a given rpm or increased torque at a given rpm, is entirely irrelevent, and one inherently alters the other.



To be honest in common conversation I say exactly the same thing you are doing here. Power causes higher work outputs, but it's a bad habit. When discussing something with someone who has a low level of technical understanding, bad habits and careless explainations lead to people who then misunderstand the fundamentals. Which causes a whole host of problems.

[xxChrisxx]

Diesel does have more energy per unit mass but it burns slower due to longer and bigger molecules that makes up Diesel - C8 to C12 HC chain s vs C4 to C8 chains for petrol/gasoline.

Raptor, see above. The energy per unit mass is lower for diesel 44MJ/kg vs 47MJ/kg. The energy per unit volume (energy density) is higher.

[engineer_roy]

In an ideal world one would go for the highest BMEP attainable and sustain that highest EFFECTIVE pressure attributable throughout the entire range of engine speed which is matched to the gearing used.
In practice this means developing the highest REAL/ACTUAL Average Pressure within the cylinder at all speeds. The difference between these two values is the amount of POWER lost to/from/by Mechanical in/efficiency within the engine, remembering that some is also thrown out of the exhaust.
The discussion on calorific values and burning rates are germane to this.

[strad]

Won't you guys please stay in the REAL world.

[machin]

Strad -now I agree with you; who brought BMEP into this?! BMEP is very useful for Engineers who are developing an engine, but is even less useful than flywheel torque when it comes to deciding which engine is going to make your road car/fantasy track car accelerate quicker!!!

BMEP means nothing unless you know even more data about the engine; speed, number of cylinders, Bore & stroke.

[xpensive]

I've made this xample many times before, but with a decent pipe-wrench, I can by hand easily produce more torque than any F1 engine, even a Mercedes one, but without the rpm it would still be completely useless for accellerating a vehicle.

[engineer_roy]

Strad -now I agree with you; who brought BMEP into this?! BMEP is very useful for Engineers who are developing an engine, but is even less useful than flywheel torque when it comes to deciding which engine is going to make your road car/fantasy track car accelerate quicker!!!

BMEP means nothing unless you know even more data about the engine; speed, number of cylinders, Bore & stroke.

OK then strad, explain to me the quantitative difference between Torque and BMEP.
Then explain why it is useless in deciding car performance.
I don't think you have grasped the realities here.

[machin]

I think you aimed that at me, right, not Strad?

The whole point of this topic is to decide on the simplest, easiest way of deciding which engine to put in your car to make it accelerate quickest.

From a single power figure you can work out the maximum possible acceleration of a car at a given road speed without having to know anything else about the engine, assuming gearing can be optimised.

With a single flywheel torque figure you also need to know the engine speed at which that torque is generated in order to work out a maximum possible acceleration of a car at one given road speed, and again assuming gearing can be optimised.

With a BMEP figure you also need to know the Number of cylinders, the Bore, Stroke, and engine speed. If you don't know one of those you can't work out acceleration; its not "useless" (your word, not mine), its just nowhere near as convenient.

That's why Power is best and BMEP is worst.


Another way of looking at it is this:-

You need to do a certain amount of work in the fastest possible time (be that lift some weight, accelerate a car, whatever), which of the following do you choose (you can use any gearbox ratio you want):-

Engine A generates 100bhp

Engine B generates 1000lbft

Engine c generates 10 Bar BMEP.

Which one do you choose? The answer is Engine A because I haven't given you enough info to allow you to assess the output of Engines B and c. e.g. engine B might generate 1000lbft at 0rpm and nothing from 1rpm upwards = useless in lifting a weight. Engine c might have one cylinder which has a tiny bore and stroke and the speed it generates 10Bar BMEP might be really low. You simply don't know; the figure is useless on its own.

[engineer_roy]

OK Machin, I didn't get all the info in one communication and for that I apologise.
However, assuming a constant capacity/displacement we might be a bit closer on this matter. The number of cylinders etc then disappear out of your equations.
The variable is the range at which the engine can rotate, the higher the revs the greater the power output. BMEP sustained at higher revs is what matters and given the same capacity, BMEP equates to torque and when rpm is included BMEP and Power is relevant to BMEP and to torque in equal measure. When developing power from an engine it is BMEP which can be focussed on to change the power, torque merely follows if you get my drift. BMEP is about converting fuel into power, torque is a measure of the result.

[machin]

OK Machin, I didn't get all the info in one communication and for that I apologise.
However, assuming a constant capacity/displacement we might be a bit closer on this matter. The number of cylinders etc then disappear out of your equations.

Yes, assuming that. But what good is that when you want to quickly compare the ability of two completely different engines to do a certain job in as little time as possible?

The chances are you are comparing two engines with different displacements, and power allows you to do that comparison nice and simply. BMEP does not. As I said in my first post on the subject (and you have subsequently confirmed) you need more info to make the BMEP figure useful in determing maximum possible acceleration.

To steal 'Xpensive's way of comparing this:- I can generate much much more pressure than the average pressure on an F1 engine's pistons simply by pushing down on a drawing pin. However I can't make that same pressure over 8 big cylinders, and definitely not at 18,000rpm! So I'd be useless at accelerating an F1 car!!! That's why an average brake piston pressure on its own means nothing.

[engineer_roy]

Para 1, you need the power curves, Para2, different displacements is changing the goalposts.