MPGuino release one workspace

[dcb]

weight was optimistically added for things like coefficient of drag and power computations

[rmccomiskie]

With weight and an accelerometer chip, engine power could be calculated. I wonder if, in place of an accelerometer, the change in VSS over time could be used?

[dcb]

My guess is that the vss would be more accurate and consistent than an accelerometer chip (and more convenient if you already have a working mpguino).

[fx57]

I've done some experimenting trying to get an instant acceleration value from the change in VSS over a 1 second interval and the data comes out next to useless. The pulse interval varies too much. Might be just a mechanical issue with my cars VSS. Maybe measuring VSS change over something like a 10 second interval might work.

I was trying to compensate the instant MPG display to account for the kinetic energy stored in the car's motion. Its a really neat feature if the numbers weren't so jumpy.

[dcb]

FX, you might try a 50-100k pulldown on the vss lead to help stabilize it. What are you driving anyway?

[rmccomiskie]

I found this calculation of HP:

1/2 * [veh_weight_lbs] / 32 * (5280 / 3600)^2 * (curr_speed_mph^2 - initial_speed_mph^2) / time / 550

1/2 and 32 adjust for acceleration
5280 adjusts miles to feet
3600 adjusts hours to seconds
550 adjusts to HP

The total weight is the car + fuel (@6lbs per gal) + driver + misc stuff. My car with 1/2 tank of gas is 3837 + 60 + 170 + 30 = 4097lbs.

I set up an array as a ring buffer in loop() that captures mph values over the last few loops. The oldest value is overwritten by the current value each time through loop(). For example, current MPH might be 50mph and 4 loops ago it was 30mph. 4 loops is about 2 seconds. So, currentmph & initialmph, time and weight are known. Plug them into the formula and calc away.

I've tried this in the car once but I' don't think what I'm seeing is correct. All the constants can be factored into a single constant but it will be a fraction so float types will be needed. Working on it.

[Nevyn]

I think you are missing a step of addition - see page below.

Horsepower Calculator

[fx57]

Originally Posted by dcb

FX, you might try a 50-100k pulldown on the vss lead to help stabilize it. What are you driving anyway?

Thanks, I'll try that. My next step was to put a scope on the VSS input to see whats going on but I'm hesitant to run the car on jackstands.

I have a '94 civic vx (49-state). I already hooked up the o2 sensor wire to the MPGuino and implemented a lean-burn monitor which has been very revealing on how it affects the instant MPG.

[rmccomiskie]

Originally Posted by dcb

...P.S. rmccomiskie, it might help if you record both simultaneously, at cd stereo sampling rate but 8 bit depth. the raw wave format is pretty easy to decypher, don't know about compressed, but having both signals captured at the same time could be something of a rosetta stone for that ECU signal if other VW/Audi groups don't have the answers.

Well...maybe not quite a Rosetta Stone but it's an informative comparison. I frankensteined both the injector and ECU fuel flow signals into a stereo audio plug and captured them with my SoundCard Scope. The green signal is from the injector and the red signal is from the ECU.

This is about 30 mph steady on level ground:

You can that there are 6 ECU pulses for every injector pulse as expected.

...and this is about 30 mph uphill accelerating hard.

The ECU pulses appear to be scaled down from the injector pulse width but I expect them to be proportional.

Then I started coasting downhill at about 30 mph...

When coasting, the signals start out looking like the engine is at idle.

...after a couple of seconds, the signal looked like this...

The ECU signal goes completely flat while the injector signal shrinks to a very short pulse. The flat ECU signal may present a problem because the external interrupt routines will not execute.

More analysis needed. I'm working on it.

[rmccomiskie]

I compared the injector pulse width versus the ECU pulse width across a range of speeds and throttle settings to get an average ratio between the two. Here's the graph of the relationship:



The Raw data is for the full injector pulse width and the -500uS data is for the inj pulse minus the settling time. So, the ECU pulse width seems to be about 1/8th the width of the injector pulse and it's consistent across different speeds and acceleration.

If I assume that the ratio is 0.123 and I know that the injector uS/gal is 200500000 for my car, when I switch to the ECU signal input, the uS/gal should be 148000000. Calculated as 0.123 * 200500000 * 6 inj pulses. That gives me a starting point to calibrate the ECU signal.

Of course, it still remains to figure out how to handle the ECU flatline signal when coasting.