×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

instantaneous hp

instantaneous hp

instantaneous hp

(OP)
I'm looking to get some opinions on a home project idea.  We are wondering if it is possibile to measure instantaneous hp on a car.

Our current thoughts include using the drag force, mass, and friction to determine the total resistive force on the car.  Then using the speed and engine revs calculating hp.

I have done a bit of research online and have not found much so I'm wondering if anyone has done this before.  Any thoughts on our current idea?

We could get a dyno reading, have a computer find the engine speed and look up the hp from a programmed curve (found using a dyno) probably.  Of course, we don't have a dyno so we're looking for our ways.

Cheers

RE: instantaneous hp

instantaneous hp on a driving car?

You could probably get within 10% percent by getting instantaneous fuel rate from the ECM and taking the ratio over peak HP rate (from a quick run down the highway), then multiplying by advertised peak HP.

RE: instantaneous hp

Or by strain gauging the propshaft, or measuring the load in the engine mounts.

However, neither of these is especially easy, your approach of modelling the instantaneous performance of the car is pretty good, but you'll have to factor in gradients and wind, which will ruin the accuracy.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

You can surely come up with some sort of number, but I don't know how meaningful it would be.

Normally when people want engine power, they want just the engine power, not the net output at the wheels.  So your not necessarily measuring the same thing other people are, so can't directly compare numbers.

I don't know how you'd accurately measure drag force or friction, either.  You can measure the rate of deceleration with the car coasting, and get the sum of aerodynamic drag + friction, but you won't get an accurate number necessarily.

We did this for a lab in college (measuring drag by deceleration, that is).  One problem is wind.  You can do the test in two different directions to cancel out wind exactly parallel to path of travel, but can't account for effect of crosswind that way.

RE: instantaneous hp

Oh, you can measure it properly, using wheel force transducers. At a million dollars a set I think they are not the right answer.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

A dynamic on-car HP measurement system was offered commercially, in the late sixties I think.  The price was pretty steep, like two grand when that was a lot of money.

The operating principle was simple enough; it put displacement transducers across the engine mounts.  Because engine mounts are not linear, you'd need to lock the crank and apply a range of known torques, including large ones, to calibrate such a system.  Today's microprocessors could take care of display and any calculations needed, but the transducers would still be a little pricey.

Accelerometer- based systems sold today are potentially as accurate, easier to install, and much cheaper.

  

Mike Halloran
Pembroke Pines, FL, USA

RE: instantaneous hp

49078,

Maybe I am missing something but several companies make accelerometers for cars.  The ones made by G-tech have built in programs that calculate torque and hp based on the weight of the car and the engine rpm.  They sense rpm through the electrical feed even on CI engines.  You do have to add hp due to wind and drag but their web page has some approx. formulae.  Hope this helps.

RE: instantaneous hp

Now work out the error due to small gradients.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

Of course, instantaneous hp and peak hp are rarely the same thing... I thought the OP was looking for a way to measure output HP (engine, tire, or elsewhere) at any given time, including when the vehicle is traveling at a constant velocity (when accn-based techniques won't work).

RE: instantaneous hp

(OP)
Hey guys,

I was looking for a way to measure the HP output.  The friend of mine who i'm 'working' with on this actually has done up a pretty basic accelerometer.  I'm not sure how good it -- or how bad it is -- but as Ivymike pointed out, we were worried it wouldn't work at constant speed.

Actually, what we want to use this the most for is for a long distance commute, on highways...with constant speed.

RE: instantaneous hp

Dick Johnson developed a drag rake for use in flight testing aircraft.  Google will learn you about that.  Interesting challange to adapt to automotive.

Wind could be reasonably estimated by looking at the difference between the vehicle's speedometer and a pitot tube airspeed indicator.

Since I'm pitching aircraft stuff, grade could be estimated using the vehicle's odometer against a variometer.

Bearing drag is a function of speed and load.  You should be able to get an approximation formula from one of the bearing companies.

Is there a real need for this, or is it just to see if you can?


RE: instantaneous hp

(OP)
Just to see if we can.

RE: instantaneous hp

Sounds like an interesting project.  Keep us posted on your progress.

RE: instantaneous hp

could you measure the spring constant of the engine mounts as instaled and then datalog the position of the motor relative to the frame using some basic digital output gagues. (two cheap pair of calipers with holes drilled in the jaws for mounting, a tach, and a datalogger) with one caliper per side you should be able to write some basic code to get good data even with vibration.

RE: instantaneous hp

That's sensible. Even better is to install the callipers as suggested, but measuring the roll of the engine, and then calibrate them for torque by appling a torque to the crank pulley. This will be more sensitive and does not rely on knowing the the possible non linear rates of the engine mounts. Bear in mind that you are measuring driveshaft torque, not engine.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

(OP)
Carnage and Greg,

What sort of calipers are you referring to?  Some sort of vernier caliper or am I overlooking something basic to the autoworld?

RE: instantaneous hp

(OP)
One more method we have considered is essentially all electronic...

using engine speed, fuel consumption, maybe boost, and an efficiency table to figure out how much power the engine is producing

My problem with that is the efficiency table.  Either constructing one or finding one.  Besides that, i don't see how it wouldn't work.

Which reminds me, i should mention the car has a turbo which may make things even more difficult

RE: instantaneous hp

Yes, digital callipers or linear pots. You'd actually be better off with pots in my opinion, some are designed for long lives. I doubt that digital callipers are.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

What EMS system are you using? Any torque based system, such as Bosch ME7, should be able to tell you modelled instantaneous engine brake torque (obviously) and then its a hop-skip-and-jump away from instantaneous engine power.

MS

RE: instantaneous hp

I have heard that it is possible to put a vr pickup at each end of a driveshaft and install a 36-1 tooth ring gear at both ends.  Then use a megasquirt ( http://www.msefi.com ) like blackbox to determine the degrees of twist in a driveshaft of a known strength. I have been out of engineering way to long to remember the math.  This way at least you can get the torque that is making it thru the transmission and is being applied to the differential.

AW

RE: instantaneous hp

That's a good idea. Not very sensitive except in first gear.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

Doesn't the new Veyron display instantaneous HP?
Any ideas how they do it?
(Not cheaply I'd imagine....)

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

RE: instantaneous hp

Probably back calculated from TPS and rpm and MAP

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: instantaneous hp

if I understand correctly, you are wanting instant hp ie the power an engine is puting out at any time such as driving at a constant speed wile adjusting for diffrent grades in the road, wind ect.? the only big problem I can see is that it takes so little energy to keep a vehicle moving at a constant speed. we are talking like 10 hp to keep a full sized car going 65 mph
you would have to have a very sensitive and accurate instrement. there is a engine load percentage that is calculated by the ecu and can be read using a obd2 scan tool (at least on my car which was obd2).

 if you are talking about geting an on-the-road acceleration number than I cant think of a better way than an accelarometer.(or using that math junk)

I am by no means an expert, I could be totally wrong and usually am.

RE: instantaneous hp

Engine mount compression would not give you a good dynamic indication of the driveshaft torque.  The rotational inertia of the engine would have to be added.  Plus you would have to take into account all the stuff hanging off the engine.  Exhaust pipes in particular significantly restrain the engine rotation.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources