# Calculate HP base on OBD II data?

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• 03-08-2010, 03:35 AM
cobrajetcarl
Quote:

Originally Posted by WestonAtPPE
I believe that the formula works out like this:

1/2 * 3000 / 32 * (5280 / 3600)^2 * (VSS^2 - VSSi^2) / t / 550

0.5 * VehicleWeight / Gravity * (FeetInMile / WheelRotationPerMile)^2 * (ChangeInSpeed / ChangeInTime) / 550

The 550 I believe converts the force into horsepower, and the 1/2 I'm not sure why that one is in there. Correct me if I'm wrong on any of this.

This is just 1/2 mv^2 delta divided by time. Difference in kinetic energy over time gives power. It will only give a power value when accelerating.

A rough approximation based on fuel flow is HP=2*gallons/hour. This can be found from MAF.
• 03-08-2010, 01:47 PM
jfitzpat
Quote:

Originally Posted by cobrajetcarl
A rough approximation based on fuel flow is HP=2*gallons/hour. This can be found from MAF.

Actually, no. To get fuel flow from MAF you need to know the air fuel ratio. This will generally be 14.7:1 (gve or take) when you are closed loop at light loads. But it will be on a sliding scale much richer if you stomp on the gas to make a HP measurement run.

Also, fuel effiency is a moving target. It will generally be best WOT and at peak torque, but the curve from that point is highly variable.

-jjf
• 03-08-2010, 03:47 PM
nixit
How about
What if you accelerate to Vo (whatever speed you want > 70mph)
Then shift to neutral, time how long it takes to decelerate to Vi (closer to 0 would be best)
Then use the kinematic equations to calculate -acceleration and that will be equal to you vehicles drag.
Finaly you could plug these numbers into the equations to calculate HP

To be accurate you would have to take several recordings of speed and time because drag increases with speed.

I have wanted to do this with a 4axis accelerometer, to compensate for uneven road surface and vehicle pitch, and obtain a true hp reading
• 03-08-2010, 04:01 PM
jfitzpat
Quote:

Originally Posted by nixit
What if you accelerate to Vo (whatever speed you want > 70mph)
Then shift to neutral, time how long it takes to decelerate to Vi (closer to 0 would be best)
Then use the kinematic equations to calculate -acceleration and that will be equal to you vehicles drag.
Finaly you could plug these numbers into the equations to calculate HP

You could get an approximation of drag, but it won't be perfect. Drag changes with atmospheric conditions. Also, there are really a couple of components, like cross section and drag coefficient, which interrelate.

Quote:

Originally Posted by nixit
I have wanted to do this with a 4axis accelerometer, to compensate for uneven road surface and vehicle pitch, and obtain a true hp reading

It's actually hard to do well. It may not seem like it, but the forces involved are relatively modest. Pitch change might be something like 2 degrees per G of forward force or less. But the vibration and noise in readings can be huge. It is actually a pretty interesting challenge in data sampling and filtering - and pretty exciting the first time your calculated speed actually matches your speedo!

-jjf
• 03-08-2010, 04:17 PM
nst6563
Quote:

Originally Posted by nixit
I have wanted to do this with a 4axis accelerometer, to compensate for uneven road surface and vehicle pitch, and obtain a true hp reading

Thought about using a Wii remote? I've seen projects with them being used as accelerometers before. Have a bluetooth connection and some drivers and you're on your way. I don't have a link, but google will.
• 03-08-2010, 05:27 PM
nixit
Quote:

Originally Posted by nst6563
Thought about using a Wii remote? I've seen projects with them being used as accelerometers before. Have a bluetooth connection and some drivers and you're on your way. I don't have a link, but google will.

My skill with microprocessors leave something to be desired. It may be easier to read a wii-mote than the PWM from the accel. but i think its kinda cheating and more expensive.

So what percent of error is there if you calculate without all the drag coefficients.

At work we calculate hp of pumps using displacement, input RPM and Pressure. There is a input for coefficient for efficiency but it make little difference
• 03-08-2010, 07:56 PM
jfitzpat
Quote:

Originally Posted by nixit
My skill with microprocessors leave something to be desired. It may be easier to read a wii-mote than the PWM from the accel. but i think its kinda cheating and more expensive.

FWIW, most low/mid cost 3 axis accell modules put out a voltage, not PWM.

Also, the Wii remote does use the Bluetooth HID profile, so it is relatively easy to interface with if your OS has the HID profile in it's BT stack.

Quote:

Originally Posted by nixit
So what percent of error is there if you calculate without all the drag coefficients.

At what speed? Remember, parasitic drag goes up exponentially to airspeed. There was an SAE paper just a couple years ago that tested very basic aerodynamic tweaks. Basically moving wing boundary layer stuff from small aircraft to surface tweaking on a car.

With basically foil and dimples, they improved average highway mileage on a 2000 Honda Odyssey from about 25 mpg to 31 or 32 mpg. It seems hard to believe until you (carefully) put your hand palm forward out the window at those speeds and see just how much energy is fighting air.

-jjf
• 03-09-2010, 02:28 AM
Vitaliy
Quote:

Originally Posted by jfitzpat
At what speed? Remember, parasitic drag goes up exponentially to airspeed. There was an SAE paper just a couple years ago that tested very basic aerodynamic tweaks. Basically moving wing boundary layer stuff from small aircraft to surface tweaking on a car.
-jjf

Sorry for an off-topic comment.. are you a pilot? I've just resumed ground school after a long break and recently had gone over induced/parasitic drag, maximum lift/drag ratio, etc. The Jeppersen book has a sidebar where they talk about WWII pilots waxing their planes before each flight to reduce parasitic drag and squeeze a few extra knots out of them.

Vitaliy
• 03-09-2010, 02:37 PM
nixit
OK so to accurately calculate hp you would need
wind speed - Pitot static
vehicle speed - OBD
vehicle weight - position sensors to red suspension sag ( In park then store val)
Road incline - 3axis maglemometer
acceleration - 2-axis accel
This would give the hp acting against rolling resistance and weight right?

Why cant you just record deceleration coasting in neutral with no wind? wouldn't this just be the sum of all force acting against the vehicle? I don't understand why it wouldn't work. I know its not linear but you could build a look up table with the data.
• 03-09-2010, 02:47 PM
Vitaliy
Quote:

Originally Posted by nixit
OK so to accurately calculate hp you would need
wind speed - Pitot static
vehicle speed - OBD
vehicle weight - position sensors to red suspension sag ( In park then store val)
Road incline - 3axis maglemometer
acceleration - 2-axis accel
This would give the hp acting against rolling resistance and weight right?

For better accuracy, I would also suggest adding attitude indicator, a stall warning horn, and a windsock.
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