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DIY dynamometer

Calculating power output from the engine diagnostics
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(+1, -2)
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Most cars these days have an ECU controlling the engine and recording various bits of data. Access to the diagnostic bits seem to be tightly controlled (but any way).

If you want to know the power output of your engine at various rpms it should be possible to calculate it given the weight of the car, the drive ratio, speed it is travelling at (and other stuff?).

So plug a computer into the diagnostic port and do a few quarter miles. From the data gathered by the computer it should be possible to get a fairly accurate torque and hp curve.

madness, Jul 13 2004

aeolis14umbra's excellent link, for convenience http://www.auterraweb.com
Palm based application does all that is required, and more. [Ling, Oct 05 2004]


       You would need to know the protocol of the data you get out (assuming you can manage to get any). Before that, you'd need to know the plug pin-out, unless you enjoy the prospect of 36 volts up your serial port. It might be simpler to ask the manufacturer for the figures you want, or spend a couple hours on a commercial dyno.
Why do you want to know this anyway?
BTW, "dynamometer".
angel, Jul 13 2004

       I think it would work at higher speeds, but not when you spin the wheels at take off.
I have an accelerometer based system at home which is quite accurate (and also tells you cornering G-force).
If you want a DIY accelerometer system, then use a half full coffee cup, and mark lines up the side ;>)
Ling, Jul 13 2004

       *ponders*, Doesn't everyone want to know the actual power output of their car. (I have a suby, or subaru impreza, with a modified induction and exhaust.) I friend of mine bought a new tvr with an advertised 340 horsepower. He got the car "dynamometered" and found it was missing 40 horse! (I understand that the measurement depends on temperature and the way the wind is blowing.)   

       This device will give the average motorist more accurate information about the expensive piece of machinery they drive... The car I drive already gives some indication (through the check engine light) of problems detected in the engine before something really goes wrong.   

       I have a copy of the service manual for the suby. It gives detailed wiring information for diagnostic port but you can't buy the diagnostic monitor unless you are a dealer or mechanic...
madness, Jul 13 2004

       //Doesn't everyone want to know the actual power output of their car//
Well, no, but then I appreciate that I am in no way representative of anything. My car does all that I want it to; that's all I really need to know.
angel, Jul 13 2004

       The protocol and data stream content and plug pinouts are something that one could fairly readily obtain; particularly easy if it's an OBDII system. There are projects on the internet that describe how to build hardware to convert some ECUs protocols to RS-232.   

       I'm curious as to what pieces of data available from an ECU would be used to calculate what you're looking for. Acceleration and torque are not likely to be in there. I think you'll need one or the other. There's not really any sensor on a car that I can think of to measure torque. In theory, the crank position sensor can be used to determine acceleration, but the time between individual pulses is probably not in the data stream. Maybe there's an airbag sensor that could be coerced in to giving you acceleration, but I doubt that's in the average ECU's data stream.   

       It's been a few years since I looked at this stuff in any significant way so my information is based on some research and a lot of incidentally acquired "facts".   

       I've played with the type of device that Ling describes. For a specified mass, it can calculate horsepower based on acceleration. Adding tachometer, speedometer and maybe throttle position input to something like that would go a long way toward providing the numbers that you want.
half, Jul 13 2004

       If only the output shaft on the engine were equipped with a strain gauge allowing direct measurement of torque.
gabe, Jul 13 2004

       [Ling]'s DIY accelerometer is the basis of how brake-testers used to work on the UK roadworthiness test. It was basically a pendulum with a "maximum" marker. The tester would drive at the designated speed and hit the brake pedal. If the pendulum failed to swing sufficiently high, the car failed.
angel, Jul 13 2004

       The solid state ones built explicitly for horsepower calculation are probably easier to use.
half, Jul 13 2004

       Horsepower will vary by engine load?   

       I agree that various conditions go in to determining final horsepower output, but the formula for determining horsepower won't change based on them. As I recall the formula is rpm x torque / 5252. Now, the torque being produced by the engine to achieve a given RPM under differing loads will definitely be different. I suppose that's technically what you mean and I'm being nit-picky.   

       Regardless, I'm with you on the data available from a typical ECU being insufficient to calculate horsepower.
half, Jul 13 2004

       Assuming you know the wheel diameter, overall drive ratio, coeff. of drag, coeff. of rolling friction and mass of the vehicle, you only need rpm from the ECU to calculate power.
Ling, Jul 13 2004

       Hmmm, I just had my car dyn'od and from what I can gather the maximum engine load is recorded at specific rpms. This data gives the torque and hp curves.   

       I would imagine that acceleration can be obtained by differentiating the speed-time curve. The loads can be calculated from the gearing and vehicle weight.
madness, Jul 13 2004

       [buddha_pest], because you asked......!
On level ground with no wind: Measure how long it takes to coast down from 60 to 55mph, in neutral =T1 seconds.
Measure how long it takes to coast down from 15mph to 10mph, in neutral=T2 seconds.

Coeff. drag = {mass of the car kg x (T2-T1)} divided by {169 x T1 x T2}

Rolling resistance = [{238 x T1}-{11 x T2}] divided by {1000 x T1 x T2}

I copied it out of my instruction manual for the accelerometer test unit.
Ling, Jul 13 2004

torque: lb-ft
rpm: revolutions per minute
horsepower: horsepower
half, Jul 13 2004


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