Torque and Horsepower - A Primer

[engineer_roy]

Raptor22, you are very close now. In BMEP the B or brake relates to the output of the engine at the measuring point where brake is another term for a dynamometer or however else you measure power output.
It refers to the P, pressure in the cylinder AND, being "Brake" it is not the Mean Actual Pressure in the cylinder.
E, effective pressure, is the calculated Mean Pressure after losses from the fuel consumed have been taken into account. It is calculated, as others have reminded us all, from the formula PxLxAxN. (using whatever units you want to, imperial, metric etc) to derive Pressure from Horsepower.

BMEP (torque) usually diminishes at higher rpm because, a) the frictional losses increase at higher rpm faster than the increase in rpm. b) because the engine breathing diminishes and a full charge is not achieved at higher revs.
The former reduces the mechanical efficiency of the engine.
The second reduces the overall efficiency of the engine (it can't burn as much fuel each revolution as it did at lower revs).

[machin]

When engineers set up a car for the race track they work the gearing to keep the engine within its maximum torque band

This is incorrect. They choose the gears to keep the car in its maximum Power band.

Use my calculator:-

Power, Torque and Acceleration (Half way down the page)

Try putting in 190lbft @ 4500rpm (representing a typical 2.5 litre NA petrol engine at mid-range, and then stick in 170lbft @ 6000rpm (representing typical output at the high RPM) -the acceleration is higher in the second case. This shows that it is better to gear for areas of the rev range with higher power regardless of where the highest torque is.

By the way, I'm not a "petrol head". I'm a propulsion systems mechanical engineer.

I hope this is useful for all reading this.

[Raptor22]

sorry that should read, "set up a car for the race track to keep the engine within its peak Torque and peak Power Band".

the other question that willarise is if we are so concerned with power output, why then are Diesel engines so successful in Le Mans Series LMP1?
Bag loads of torque across their RPM range, but lower power output than an equivalant petrol engine.

the answer lies in torque and power at the wheels

[strad]

sorry that should read, "set up a car for the race track to keep the engine within its peak Torque and peak Power Band".

but they aren't in the same place in the rev band.

[Raptor22]

yes I know that.
Sometimes they are fairly close, sometimes they are far apart. all it means is that yhe engine either provides a wide or narrow useable band.

[xxChrisxx]

sorry that should read, "set up a car for the race track to keep the engine within its peak Torque and peak Power Band".

the other question that willarise is if we are so concerned with power output, why then are Diesel engines so successful in Le Mans Series LMP1?
Bag loads of torque across their RPM range, but lower power output than an equivalant petrol engine.the answer lies in torque and power at the wheels

Meaningless conclusion, a win at Le Mans was a function of many, many things.
You could have just as easily said that the R10 consistently beat everyone else because it had red bits on it. And everyone knows red bits make the car go faster.

Remember correlation does not imply causation.

Diesels have been so successful in Le Mans series endurange racing becuase the regs have slightly favoured them. The higher economy allows longer stints, meaning less time lost. The regs allowed for similar power output, and fuel tank sizes that were fairly close.

All the top runners were fairly close on ultimate laptimes. So you have a situation where two equally fast cars, one petrol one diesel, are competing but one has to stop more often.


Also the phrase 'power at the wheels' makes no sense. As it's the same as power at the engine minus losses.

[machin]

yes I know that.
Sometimes they are fairly close, sometimes they are far apart. all it means is that yhe engine either provides a wide or narrow useable band.

to achieve the best possible acceleration you simply ignore flywheel torque, and make sure that the engine is operating in the rev range that makes the highest power.

If anyone can provide calculations that disprove this I'd be happy to point out where these calculations are wrong,

My calculator is on my website for anyone to try.

[machin]

but lower power output than an equivalant petrol engine.

If acceleration was higher this means either:- the mass was lower, the drag was lower, the inertia was lower, or the power was higher (regardless of what the flywheel torque is).

I'm pretty sure that in the first few years of Audi domination at Le Mans it was a given that the Diesels had more power. The rules were written that way to encourage Diesel manufacturers to compete. In recent years the ACO has tried to re-dress that balance, so the outright speed of the diesels and petrols is now more even.

[machin]

Lets do a quick example of how to select gears for maximum acceleration using just torque figures: I've pulled these curves for a diesel engine off the net:-



At peak torque it makes 735lbft at about 1600rpm.
At Peak power it makes about 680lbft at 2800rpm.

You agree that acceleration will be highest when torque at the road wheels is highest, right????

To gear the engine to transmit the peak torque to the road at 60mph, given typical road sized wheels, requires an overall gear ratio in the region of 1.87:1. This means the flywheel torque is multiplied by 1.87. Road wheel torque is therefore 1374lbft.

To gear the same engine to the road to transmit the torque value at peak power to road at 60mph requires a gear ratio of 3.27:1. This means the flywheel torque is multiplied by 3.27. (you can double check this by comparing the peak torque and the peak power speeds:- 2800/1600 x 1.87 = 3.27) Road wheel torque is therefore 2224lbft. A lot more than you get if you gear for peak torque.

This shows that it is better to gear for peak power than peak torque, because that way you will achieve the highest possible torque at the road wheels, and therefore the highest possible acceleration.

This example shows that it is wrong to assume that more flywheel torque means more roadwheel torque; you have to take the relative engine speeds into account, as this will tell you what gearing you can have.

[PlatinumZealot]

Nope. Gearing changes the speed and torque.. but it cannot increase the power, because you can't create or destroy energy only transfer it from one form to the other

Would it not onlychange torque at the axle? I'd imagine the torque figures at the engine side would still be the same.

Nope. Anytime you alter the speed by mating gears the torque changes. The the torque will be changed right on the engaged gear along the output shaft of the gearbox. That is why the lower speed gears are designed with stronger teeth and even broader faces.

[PlatinumZealot]

A battery can be full of energy, but it aint gonna do any work til it's hooked up to a motor with torque to turn the wheels.

The electrical power sent to the motor is the voltage times the current. Torque arises because the motor is set on a spining shaft.

If it were a linear motor you wouldn't have any shaft to spin on. there would be no torque. think of a maglev train. I have been saying a million times torque arises from the geomtery of your machine.

A linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque (rotation) it produces a linear force along its length. The most common mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field .

http://en.wikipedia.org/wiki/Linear_motor

http://www.aerotech.com/products/motors ... otors.html

[PlatinumZealot]

A battery can be full of energy, but it aint gonna do any work til it's hooked up to a motor with torque to turn the wheels.

but a cordless drill motor can make more torque than the motor in a Tesla Roadster.

The speed will be low though, because it's not putting out much power.

Any time the car is being accelerated, power is being converted from electrical (or fuel) to motion.

You can generate torque by taping a weight to the front of the tyre, but it's not going to drive the car more than a quarter of a wheel revolution.

This is the right way. =D>

[PlatinumZealot]

What we are saying is that what is measured is torque. That is converted to a power number to use as comparison.

There is a simple reason why torque is measured. The simplest way to measure the power of an engine was to use a brake, and it is very easy to obtain the torque on the brake. The next step is to calculate the angular speed of the the output shaft. That is just measuring the output. That has no relation to how the power is developed int he engine. In otherwords measuring the height of tree doesn't tell you about how the cells multiply in the tree.

When engineers set up a car for the race track they work the gearing to keep the engine within its maximum torque band whilst producing torque at the wheel produce power.

So why do sports cars rev all the way to 8000rpm when the maximum torque band is at 5000rpm?

I have never argued that power is unimportant. In fact the formula P=2piNT/60 shows the relationship clearly.

This only for a rotating shaft (or disk or whatever rotating object). The pistons don't rotate though! so you see something has changed before torque is created.

The point that is being missed is that power on its own tells us nothing about performance. We need torque and at what RPM that T is being generated to determine where best power is produce to convert in speed (v).

Power is everything.. hehe with power you can make any amount of torque you want. The opposite is a fallacy.

Petrol heads become fixated on the power but that does not exist unless the energy (fuel) is converted into mechanical energy (measured as BMEP) that produces torque and RPM.

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

The only really effective ay to compare engine 1 to engine 2 is to measure its BrakeMEan Effective Pressure (BMEP). Thats translates to T and N which yields P for ease of comparison. But P is always presented at XXXXRPM

One BMEP doesn't translate to overall perfomance though. there are different BMEP's at different rpms. It's just a convenient way to measure engine performance. You have so many other metrics. Specific fuel consumption, indicated power, indcated mean effective pressure, friction mean effective pressure.. brake power..

[PlatinumZealot]

Guys lets face it.. Power is everything. You want more low down torque? Adjust your engine to give you more power at low rpms!
Add a turbo, get more air and fuel in there to get a more powerful explosion! (not a more torquey explosion!)

[xxChrisxx]

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

Diesel is more dense than gasolene so you have more energy per unit volume, but that's not calorific value. It's also not why diesels produce more output. It'smost due to the way they add heat (how combusion occurs) becuase the heat addition is isobaric in diesel cycles and isochoric in otto cycles.


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.