greenMeter iPhone/iPod Touch app measures vehicle efficiency

brn, the app has inputs for drag coefficient and frontal area, which can be looked up for most vehicles (or you can use typical numbers depending on the shape and size of the vehicle). From there, the software can compute drag across the speed range.

Awesome! One step away from knowing true vehicle tank-to-wheels efficiency (or maybe you're already there, I can't get to the site from work..). Power / (fuel energy / unit time) or work / fuel energy = unitless efficiency. How does it know how much fuel is used? Do you enter it after travelling a distance recorded by greenMeter? For that matter, how does it know distance? Internal GPS?

I might have to get my daughter an Ipod Touch for Christmas...

The app starts by computing engine power required, which is a function of the speed, acceleration, aero drag, and rolling resistance as it propagates through the driveline (and under the effect of driveline losses).

For a given power level and speed, you can come up with a relation for the energy required. And from there, using information about the engine efficiency and fuel type (which tells you how much of the fuel's chemical energy is converted to mechanical energy) you can back out the amount of fuel used. It's a very strict computation of the physics that basically relates the required engine power to the required fuel energy. At that point, a whole bunch of stuff can be computed, like carbon emissions, barrels of oil used, etc.

Interestingly, distance doesn't enter in at all and doesn't matter -- the speed sets the rate of use. But some of the results are expressed "per 100 mi" and "per 10000mi", or per km to relate back to distance.

One of things I was reminded of when writing the app is just how inefficient gas engines are. Only 18-20% of the chemical energy in the fuel is converted to mechanical energy, meaning we waste about 80% of the energy of the fuel! Diesels are slightly better, at 40% efficiency (and with about 10% more chemical energy per gallon).

GPS is not used in this version, so the app works on all iPhones and iPod Touches. I may add a minor GPS-based feature to the next version of the app, but it won't add a whole lot of functionality.

Very cool.

This should be on every hypermiler's shopping list as a tool to provide even more info for them to optimize their driving.

Kudos, Craig, it even looks neat.

Ah, so you are assuming an efficiency (SI=0.2, CI=0.4) and computing fuel usage. I was thinking of using your app to teach people that there is a difference between the terms 'fuel economy' and 'fuel efficiency', which are used interchangeably in the media and in advertising. People would be interested to know that a Hummer H2 is as fuel efficient as a Honda Fit (more or less). Or a Chevy Duramax is more fuel efficient than a Vespa scooter. To do that, you'd have to enter fuel usage (something people do anyway), and have efficiency be the output. It would probably be easier to do on a long trip where you fill up at least once, so you can enter pointA, PointB, gallons used...

Love this kind of stuff.....although there's no chance of me dreaming this up.

104wb - RE: your H2 vs. Fit or DuraMax vs. Vespa etc statements - does this relate to fuel used / work done? The H2 getting less MPGs but moving a lot more weight at the same time?

Yes, H2 does more work. The degree to which it converts energy in the fuel into work is about the same as the Fit because they both use an SI engine operating at the same BSFC. Craig's app could show that, but a crude way of showing it is multiplying vehicle weight by mpg. Sometimes called TFC/IW. Given a big enough sample size, you'll find distinct populations based on powertrain type: gasoline SI, diesel CI, gasoline hybrid, etc.

It's important to me, because I think I have an obligation to be efficient, but not to minimize the amount of work I do.

Interesting.

Got it - thanks for the info.