I am writing a car simulation program and want to use data from a real
car in the program.
I have a graph of power and torque against rpm, but I don't know which
to use to calculate the acceleration for the car.
I worked out that if I multiply the engine torque by the final drive
ratio and by the tyre radius, I would get the force acting at the
tyre/road interface, but the torque seems to be almost constant across
the rpm range. The power on the other hand varies almost linearly
with rpm, and I can relate this to my experience when driving, better
acceleration at higher rpm.
I did a little mechanics theory work on a simple disc brake connected
to an engine who's power varied linearly with rpm, and came to the
conclusion that the torque was constant across all rpm. Surely this
would mean constant acceleration whatever the rpm?
If anyone can tell me how they measure the torque and power of an
engine, I would be very grateful, or for any comments on what I have
said.
Thanks
Scott
Quoted text (7 lines)
>I worked out that if I multiply the engine torque by the final drive >ratio and by the tyre radius, I would get the force acting at the >tyre/road interface, but the torque seems to be almost constant across >the rpm range. The power on the other hand varies almost linearly >with rpm, and I can relate this to my experience when driving, better >acceleration at higher rpm. >Since HP is a fictitious number based on Torque, Why not just use the Torque
curve.
--
Kevin Anderson
[email hidden]
"S Boham" <[email hidden]> wrote in message
news:8r8e32$7ke$[email hidden]...
> I am writing a car simulation program and want to use data from a real
> car in the program.
>
> I have a graph of power and torque against rpm, but I don't know which
> to use to calculate the acceleration for the car.
>
> I worked out that if I multiply the engine torque by the final drive
> ratio and by the tyre radius, I would get the force acting at the
> tyre/road interface, but the torque seems to be almost constant across
> the rpm range. The power on the other hand varies almost linearly
> with rpm, and I can relate this to my experience when driving, better
> acceleration at higher rpm.
>
> I did a little mechanics theory work on a simple disc brake connected
> to an engine who's power varied linearly with rpm, and came to the
> conclusion that the torque was constant across all rpm. Surely this
> would mean constant acceleration whatever the rpm?
>
> If anyone can tell me how they measure the torque and power of an
> engine, I would be very grateful, or for any comments on what I have
> said.
>
> Thanks
>
> Scott
>
>
On Sun, 1 Oct 2000 23:33:11 +0100, "S Boham" <[email hidden]> wrote:
>I am writing a car simulation program and want to use data from a real
>car in the program.
>
>I have a graph of power and torque against rpm, but I don't know which
>to use to calculate the acceleration for the car.
Definately torque. As you said, that's the force after going through
main drive, gearbox, and final drive that is transmitted to the
ground. And F = ma.
The top speed should be geared to the max effect rpm, though, where
you maximize the force by lower gearing.
On an electric motor, maybe(disregarding aerodynamic counterforce). My
experience(heh, mostly reading car/bike mags :) is that no petrol or
diesel engine has a totally flat torque curve IRL. And no engine for
sure has a flat torque curve down to 0 rpm :)
Tuned petrol engines seems to have an arched torque curve peaking
about ~10% below max effect rpm. Reallly tuned 2-strokes have a
_really_ sharp torque threshold mid-rpm(something about exhaust
resonances, I think).
Diesels and Harley:s :) have the torque peak early and just goes
downhill thereafter.
Another complication, if were talking general driving, is how the
engine responds to the throttle. Turbo engines, as you may know has
the infamous "turbo lag" because of the time building up intake air
pressure. But all engines responds to throttle variations differently.
The local motorbike shop here has a special braking-bench(dunno if
that's the word in english) to measure just that.
>If anyone can tell me how they measure the torque and power of an
>engine, I would be very grateful, or for any comments on what I have
>said.
Torque = Effect/(rps*2*pi)
It helps if you use SI-units, as everyone should... ;)
In comes Watts and out Nm.
Just my takes on the matter,
/Magnus
GPLRank hcp: -7.50
Quoted text (11 lines)
>I worked out that if I multiply the engine torque by the final drive >ratio and by the tyre radius, I would get the force acting at the >tyre/road interface, but the torque seems to be almost constant across >the rpm range. The power on the other hand varies almost linearly >with rpm, and I can relate this to my experience when driving, better >acceleration at higher rpm. > >I did a little mechanics theory work on a simple disc brake connected >to an engine who's power varied linearly with rpm, and came to the >conclusion that the torque was constant across all rpm. Surely this >would mean constant acceleration whatever the rpm?If I recall correctly, in plain old English units, the formula is:
Power (HP) = Torque (lb-ft) * RPM / 5252
(the 5252 because we ignored Napoleon's metric system--otherwise we'd have
something more elegant involving PI)
Power and torque are deceptively tricky concepts; it took me a while to
fully understand them.
"Invictus" <[email hidden]> wrote in message
news:4v9C5.311545$[email hidden]...
as
Quoted text (20 lines)
> > But surely the force being transmitted to the ground is due to the > > torque of the wheel which is due to the torque of the engine? > > **Absolutely!! Torque is torque, originating at the engines crankshaft and > trasmitted thru the drivetrain/wheels. > The rear wheel torque is a result of engine torque applied. That engine > torque is factured by the multiplication/reduction of the gear box. > > We have to keep in mind that torque is measured as a results of a single > stroke of the engine, whereas power is that single stroke multiplied by > engine RPM. There is a formula that unfortunately slips my mind at the > moment, > > > > Power is the rate of doing work, so I guess this is the rate that the > > car can do work to increase the kinetic energy and do work against > > friction? > > Again, spot on! > > An interesting sidenote is a current F1 engine has aprox the same torqueQuoted text (7 lines)
> a 5.0 Mustang. The F1 engine make considerably more power by creating this > torque at a very high engine speed. > > Mustang aprox 225BHP, torque peak 300LBS/FT @ around 2500RPM > F1 engine 800BHP, torque peak about 325LBS/FT @ around 17,500RPM > >>Also, many 'road' engines have maximum torque at
>about 3000 and maximum power at about 5000 rpm. Which will give
>better acceleration? My experience tells me that keeping the revs
>around 5000 will give the better acceleration.
> It's simple. In a given gear, acceleration will be
maximized at 3000 rpm. At a given speed, acceleration will be maxed
at 5000 rpm, because you are using a lower gear (compared to 3000 rpm)
and getting more wheel torque because of the gearing. It just depends
on what is constant in the experiment, your gear or your speed.
"> Ideally, you want the engine to center around the peak power
(which is
> essentially torque times gear ratio). Simply put, launch in 1stgear at 3000
> rpm, as the most force will be generated at the tires where the peaktorque is.
> Shift above 5000 rpm, keeping the revs as close as possible to peakpower.
> The car you're speaking of will pull hardest "in any one particulargear" at
> 3000 rpm. However, if you downshift and the revs reach 5000 rpm,the torque is
> lower at that point, but the gear ratio is higher. Here, the highergear ratio
> multiplied by the slightly lower torque at peak power results ingreater
> acceleration.
Thank you very much, this last paragraph has explained a lot to me and
will hopefully make my sim a bit more realistic. Cheers.
> Center the revs at the power peak, try your program using torquefor the
> calculations (instead of power), and you'll see what I mean after awhile.
Quoted text (9 lines)
> > Good luck :-) > > Todd Wasson > --- > Performance Simulations > Drag Racing and Top Speed Prediction > Software > http://PerformanceSimulations.Com
S Boham <[email hidden]> wrote in message
news:8r8e32$7ke$[email hidden]...
> I am writing a car simulation program and want to use data from a real
> car in the program.
>
> I have a graph of power and torque against rpm, but I don't know which
> to use to calculate the acceleration for the car.
It doesnt matter!!!!!! The result should be the same b/c of the Power -
Torque dependance. I dont remember the equation but I now that it is. In
this case I would rather use Torque: T=F cross r thats simpler
Jaro
Don't listen to thge rest of the posts, use POWER!!
POWER is the force of the torque factured to take into account the engine
RPM.
The force that is being transmitted to the ground is the total power, not
torque.
"S Boham" <[email hidden]> wrote in message
news:8r8e32$7ke$[email hidden]...
Quoted text (27 lines)
> I am writing a car simulation program and want to use data from a real > car in the program. > > I have a graph of power and torque against rpm, but I don't know which > to use to calculate the acceleration for the car. > > I worked out that if I multiply the engine torque by the final drive > ratio and by the tyre radius, I would get the force acting at the > tyre/road interface, but the torque seems to be almost constant across > the rpm range. The power on the other hand varies almost linearly > with rpm, and I can relate this to my experience when driving, better > acceleration at higher rpm. > > I did a little mechanics theory work on a simple disc brake connected > to an engine who's power varied linearly with rpm, and came to the > conclusion that the torque was constant across all rpm. Surely this > would mean constant acceleration whatever the rpm? > > If anyone can tell me how they measure the torque and power of an > engine, I would be very grateful, or for any comments on what I have > said. > > Thanks > > Scott > >"Kevin Anderson" <[email hidden]> wrote in message
news:x4PB5.53149$[email hidden]...
> Since HP is a fictitious number based on Torque, Why not just usethe Torque
> curve.
The torque curve seems too 'flat' for what I experience when driving.
Most cars I have driven seem to have vastly more acceleration at
higher rpm, and this doesn't seem to be represented in any torque
curves I have seen. Also, many 'road' engines have maximum torque at
about 3000 and maximum power at about 5000 rpm. Which will give
better acceleration? My experience tells me that keeping the revs
around 5000 will give the better acceleration.
"Invictus" <[email hidden]> wrote in message
news:HhQB5.283183$[email hidden]...
> Don't listen to thge rest of the posts, use POWER!!
>
> POWER is the force of the torque factured to take into account theengine
> RPM.
>
> The force that is being transmitted to the ground is the totalpower, not
> torque.
But surely the force being transmitted to the ground is due to the
torque of the wheel which is due to the torque of the engine?
Power is the rate of doing work, so I guess this is the rate that the
car can do work to increase the kinetic energy and do work against
friction?
Quoted text (9 lines)
>The torque curve seems too 'flat' for what I experience when driving. >Most cars I have driven seem to have vastly more acceleration at >higher rpm, and this doesn't seem to be represented in any torque >curves I have seen. Also, many 'road' engines have maximum torque at >about 3000 and maximum power at about 5000 rpm. Which will give >better acceleration? My experience tells me that keeping the revs >around 5000 will give the better acceleration. > >When I was younger, I always had a hard time trying to understand the
differences between torque and HP. I had had it explained to me a
million times and still couldn't visualize how each felt in the seat
of the pants. When I felt a car doing this or that, was it torque or
HP?
Well, it's not near as complicated as I was making it out to be. What
finally made it click for me was to understand how it was measured on
a brake dyno where these numbers come from in the first place.
First of all, only torque is actually measured with a sensor. HP is
then calculated from the torque and RPM.
Torque is typically measured by mounting a load cell so that you can
measure the forces trying to rotate the brake.
The rotor of the brake is mounted to the crankshaft of the motor. In
order to keep the brake from just spinning with the crankshaft, you
must anchor the brake housing, right? Here's where you measure torque.
Take a bar and mount it to the brake housing on one end and anchor it
on the other end to something fixed so that this bar acts as a
rotational stop. To keep the math simple, make this bar so that the
distance from the center of the rotational axis to the point that the
bar anchors is one foot.
At the anchor end of the bar, install a load cell (weight transducer)
so that you can measure the weight felt one foot off of the rotational
axis. Get the weight from the load cell in units of lbs and multiply
by the distance (one foot) to get torque, which would be ft/lbs in
this case.
Every time you take a torque (ft/lbs) reading also take a RPM reading.
At everyone of these data points, apply the formula
HP = (Torque * RPM ) / 5252
And there you have you power curve.
And now you can understand why a single torque value alone, without
considering RPM, is a worthless number. You can always gear something
down to get tremendous torque, yet, if it can't make some RPM at the
same time then it's not going to get much done (very fast) and doesn't
have much "power". That's the beauty of HP; a single value can give
you good idea of how much work you can expect to get done, torque
alone doesn't.
In your case, it all depends what you have your downstream math setup
to accept. The torque against RPM curve doesn't contain any more,
less, or different information than the torque against RPM curve. One
can be calculated from the other. It's just in different units.
Randy
On Sun, 1 Oct 2000 23:33:11 +0100, "S Boham" <[email hidden]> wrote:
Quoted text (27 lines)
>I am writing a car simulation program and want to use data from a real >car in the program. > >I have a graph of power and torque against rpm, but I don't know which >to use to calculate the acceleration for the car. > >I worked out that if I multiply the engine torque by the final drive >ratio and by the tyre radius, I would get the force acting at the >tyre/road interface, but the torque seems to be almost constant across >the rpm range. The power on the other hand varies almost linearly >with rpm, and I can relate this to my experience when driving, better >acceleration at higher rpm. > >I did a little mechanics theory work on a simple disc brake connected >to an engine who's power varied linearly with rpm, and came to the >conclusion that the torque was constant across all rpm. Surely this >would mean constant acceleration whatever the rpm? > >If anyone can tell me how they measure the torque and power of an >engine, I would be very grateful, or for any comments on what I have >said. > >Thanks > >Scott > >> Don't listen to thge rest of the posts, use POWER!!
... and have a non-realistic simulation.
> POWER is the force of the torque factured to take into account the engine
> RPM.
Power = force x speed. The torque at the wheels is directly proportional to
the acceleration of the car (ignoring wheelspin)
> The force that is being transmitted to the ground is the total power, not
> torque.
The force is the torque (torque is a force - it is rotational force). The
power of an engine expresses the ability of an engine to do work. It is not
useful in mathematical calculations.
Andy
--
Visit my site... http://mail.wm-europe.com/~andrew/
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On Mon, 2 Oct 2000 17:28:32 +0100, "Andy Laurence"
<[email hidden]> wrote:
Power is indeed more of a *statistic* than something to base your
calculations on. It's nice though to calculate the resulting power and
check if that matches a bit with real life numbers; aka 100hp for a
person car, and 700hp for a F1 car. And 1 hp=746W=746 J/s etc.
It often helps to look at units, to see whether you're correct with
your calculations.
force=N(ewton), torque=N.m (Newton times meters).
>> The force that is being transmitted to the ground is the total power, not
>> torque.
>
>The force is the torque (torque is a force - it is rotational force). The
>power of an engine expresses the ability of an engine to do work. It is not
>useful in mathematical calculations.
Right. You go from torque to force though by applying at a certain
*distance*. Torque=force*radius.
So if torque=200 Nm, the force at the tire touching the ground at
radius 0.25 = 200/0.25 = 800 N.
But ouch, this isn't all put into accelerating the car. Only roughly.
Ruud van Gaal, GPL Rank +53.25
MarketGraph : http://www.marketgraph.nl
Pencil art : http://www.marketgraph.nl/gallery/
Car simulation: http://www.marketgraph.nl/gallery/racer/
Quoted text (9 lines)
>> Don't listen to thge rest of the posts, use POWER!! > >... and have a non-realistic simulation. > >> POWER is the force of the torque factured to take into account the engine >> RPM. > >Power = force x speed. The torque at the wheels is directly proportional to >the acceleration of the car (ignoring wheelspin)> But surely the force being transmitted to the ground is due to the
> torque of the wheel which is due to the torque of the engine?
**Absolutely!! Torque is torque, originating at the engines crankshaft and
trasmitted thru the drivetrain/wheels.
The rear wheel torque is a result of engine torque applied. That engine
torque is factured by the multiplication/reduction of the gear box.
We have to keep in mind that torque is measured as a results of a single
stroke of the engine, whereas power is that single stroke multiplied by
engine RPM. There is a formula that unfortunately slips my mind at the
moment,
>
> Power is the rate of doing work, so I guess this is the rate that the
> car can do work to increase the kinetic energy and do work against
> friction?
Again, spot on!
An interesting sidenote is a current F1 engine has aprox the same torque as
a 5.0 Mustang. The F1 engine make considerably more power by creating this
torque at a very high engine speed.
Mustang aprox 225BHP, torque peak 300LBS/FT @ around 2500RPM
F1 engine 800BHP, torque peak about 325LBS/FT @ around 17,500RPM
Here's an excellent explanation of torque and horsepower everyone should read.
It really clears up this whole thing.
http://www.datsuns.com/torquehp.htm
Both methods you are speaking of can be used. In fact, you could even make
calculations using energy if you wanted. However, you seem to want to multiply
by gear ratios and use tire radius, etc., to get force, acceleration, and move
your car over little timesteps. For this, use torque. Heck, try it with
horsepower (like I did the first time, many years ago) and see what happens.
Your 6,000 rpm at peak hp engine will need to shift at probably 9,000-10,000
rpm to get the quickest time. Meanwhile, you're launch will be dreadfully slow
and unrealistic. Try both ways if you want, then let us all know which way
worked :-)
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com
