A gallon of gas =

[BoyntonStu]

I found this on the net:

A gallon of average gasoline has around 115,000 BTU.

That translates into 45.2 horsepower hours.

This should put the energy stored in a gallon of gas into perspective.

BoyntonStu

[Q-Hack!]

hp of my Saturn Vue 160
Tank size is 15 gal but I always fill up after using about 12 gal.

45.2 * 12 = 542.4 total horse power hours per tank

542.4 / 160 = 3.39 number of hours of driving on a tank of gas.

3.39 * 60mph = 203.4 miles on a tank of gas.

Damn, that is actually pretty close to reality.

[randycat99]

I think that was just luck that your calcs turned out the way it did, but I think this is still useful info.

If you were really putting down a constant 160 hp over 3+ hrs, you would be flying down the hwy (probably with cops in pursuit, to boot). More likely your cruising power was maybe 30-50 hp? That's the number you would plug in. Compare that time to the actual duration you observe on a tank of gas on a hwy trip, and you then have a pretty good idea of the thermal efficiency of your car. ...or if you assume a typical efficiency (30%?), you can back out what your hwy hp is.

If you have one of them ScanGauge devices, the car might be able to indicate directly how much hp you are using at a given hwy speed, and then you can use that in these equations to derive your efficiency.

Anybody else study these sorts of things on their own car? Maybe you have more typical figures to supply (the ones I gave above, I really just pulled out of the air as an example).

[Owen_]

45.2hp *.7455 = 33.71 kilowatt hours... that would be a lot of hho...
I think that must be E10, I think E0 is about 120,000BTU's as pure ethanol is lower (guess: ) 85,000BTU's.

Q-Hack,
there are two major flaws in your calculations, which happened balance each other out quite well. Both having to do with your engine. First; unless you only drive up hill with the throttle wide open your not using 160hp for all your driving. Second your engine is not 100% efficient, most ICE's are about 20% efficient. So for that 45.2hp hrs you get more like 9hp hours.

[myoldyourgold]

Just an added thought here is that an efficient vehicle is only using about 8% of its rated HP at cursing speed.

[Owen_]

Just an added thought here is that an efficient vehicle is only using about 8% of its rated HP at cursing speed.

That sounds about right. So let's go back to Q-Hack's car:

Let's round up to 10% to average in some acceleration and make the math a little easier.
.10 * 160 = 16hp
9hp hr * 12 gal = 108hp hr
108hp hr/16hp= 6.75hr of driving.
16hp sounds really low to me, I would guess you would be using closer to 25hp at 60 which would be:
108\25=4.32hr
which=259.2miles which would be about 21mpg.
Also the 20% efficiency of your ICE greatly varies with engine speed and load. And there is a mechanical loss in the drive train which also varies with speed. So to really calculate this you would need to know a lot of functions, and really be up on your calculus... but remember kids; calculus is fun!

[randycat99]

What's the epa hwy mpg rating on this thing, anyway?