The effect of horsepower on gas mileage

[white90crxhf]

i wouldn't say a force is essentially weight. the drag on the car is a force, same with the wind pushing it forward or against it.

incase anyone is wondering, a slug is the measurement for mass in the engilsh system.

[omgwtfbyobbq]

Originally Posted by The Toecutter

You don't divide the weight of an object in pounds over 32ft/s^2 unless you want to measure mass. What we're concerned with is force, not mass. Pounds are already a measure of force. Kg is mass, and we must multiply by 9.8 N/kg to figure out the force given Earth's gravity.

Then how can you go from the ~3179lb curb weight/force of the vette to the 1,442kg mass if you don't have som operation accounting for acceleration?

[JanGeo]

you measure mass with a balance scale using weight against weight so gravity doesn't matter - weight measured with a spring scale is essentially the same here on earth as mass, just scientifically speaking, mass is used so the units are correct.

[omgwtfbyobbq]

But we can't convert a force to a mass without an acceleration. If the pounds is actually pound-force, then to convert we need to go to another force. In either case, the common use of SI kilogram must actually be an acceleration, meaning kilogram force instead of kilogram, just like pound force instead of pound.

[The Toecutter]

A kilogram of mass is being held by 9.8N of force under the Earth's level gravity. If you were to weigh yourself on Earth, you'd be able to see that you weigh a certain amount of pounds. Divide by 2.2035 and you will find your mass in kilograms. However, if you weigh yourself in space, you will find you weigh zero pounds. But, that doesn't mean you are 0 kilograms mass.

That conversion factor from pounds to kilos and vice versa is only applicable under the Earth's level of gravity. Both are two different parameters measuring two different things, one mass, one force gravity(weight). In metric, force is expressed in Newtons, and that is what you use when calculating forces acting upon a car.

i wouldn't say a force is essentially weight. the drag on the car is a force, same with the wind pushing it forward or against it.

I should have clarified in that post that I meant the force gravity actiing upon an object is essentially weight. That, and I'd probably make more sense if I wouldn't play on the computer while drunk.

[omgwtfbyobbq]

I know this stuff, it hasn't been that long since I bothered with physics. It's just that your comment about pounds being a force but kilograms being mass seemed out of context and was a bit confusing because you converted from lb-f to kg-f, and then to newtons.

[JanGeo]

Pounds are weight(mass) and force
Kilograms is only mass (weight) and Newtons is force

in the english system the same name is used for two different units of measure in the metric system they have unique names. There in lies the confusion.

In space you would still weigh something as gravity is everywhere - if you were in orbit then you would weigh almost nothing.

[GasSavers_Randy]

Originally Posted by omgwtfbyobbq

I think the EPA factors this in because they test the carbon output through the exhaust, so when people buy diesels, they usually end up getting better than the EPA mpg rating.

I thought this was interesting, so I looked it up. Net result is that diesels can put out more carbon for the same mpg. I think they exceed the EPA mpg the same way we do, in addition to diesels being less sensitive mpg-wise to a heavy foot.

They put the actual formulas they use in 40 CFR 600.113. Diesel mileage is equal to 2778 / (.866 * HC + .429 * CO + .273 * CO2), where the values are in grams per mile. The gas one got complicated in 1988, but prior to that it was the same, but with 2421 instead of 2778. It still seems to be within a couple percent of that.

Of course, diesel contains more carbon per gallon. One CA emissions study put the carbon weight fraction at 87% in diesel and 85% in gas, and the density at 840 g/L diesel and 740 g/L gas. So a gallon of diesel should have about 2770 grams carbon, and gas 2380. It's pretty close to the above figures for diesel, and about 1.5% off for gas, but CA implies reformulated gas. So, I'd say the EPA is trying to match the normal idea of mpg for both gas and diesel.

I'm not so sure anymore about the BTU content though. I got 12% from one source that said 124k BTU/gal for gas and 134k for diesel (it probably included water condensation heat). But the EPA says it ranges from 108 to 117 for actual gasoline. A couple different places put the test fuel (TIER 2 unleaded code 61) at 114.3k (0.743 s.g., 18437 btu/lbm). Also, I found one source that said 129.5k for diesel and 118.3k for B100.

More interesting was the EPA pages on testing. They have full details of the test schedules on one page. And another page has full details of each car test. This isn't the normal car list... they're individual test results. These are unadjusted: multiply by .9 city and .78 highway to get the normal ratings. It'd be interesting to figure out what the dyno settings mean, because it would summarize rolling and air resistance factors for each car. I can't find much info on them, though one (non-EPA) source mentions A as rolling, B as 'intrinsic' (whatever that is), and C as air drag, normally expressed as HP at 50 mph. These numbers don't match that though.

[Ted Hart]

Quote: Now, a question I have is. Why don't hybrids just have 30hp engines with battery packs that allow for ~40-50 miles all electric? They'd get something like 80-100mpg in normal driving, with all the kickass acceleration an electric motor can provide. Well, I think this has to do with the manufacturer's unfamiliarity with battery packs and electric motors, they know ICE motors, and they may have heard that electric motors and battery packs can last for so long, but they don't have the long term empirical data to back this up. Preformance concerns are also present, with only 30hp, the car wouldn't be able to cruise at 90-100mph. They also have to deal with contracts that are years in advance, so they can't up and cancel all those ICE engines they've already ordered, which is why GM is in such a tight pickle. Their ~7 year development cycle is still geared for SUV's in a world where gas will probably never go below $2 a gallon

In any event, given Toyota's short ~3 year development cycle, they should be set to roll out a Prius with more power via supercharging, a bigger, plug-in li-ion battery pack, and ~100mpg in 2008.

Why don't hybrids have 30 HP gas motors & more battery capacity? Well, this "rocket scientist" says...Toyota / Honda don't want to make 'em too good (save room for the improved models for the future)! Besides, shouldn't the hybrids use a little gas...to provide the revenue for the oil giants? After all, giants gotta eat, too!

[The Toecutter]

Let the giants go without. We shouldn't be forced to need them, and they could stand to lose a bit of fat.