How many Btu's or calories does it take to move 1 lb. one mile with a conventional gas engine?

Question

For example, a 160 lb. driver of a 3,000 pound car, using a 200 horsepower 4 litre engine wants to carry a 100 pound bag of cement for 1 mile. The car gets 25 mpg with only the driver. How much more gas (or fewer mpgs) will the car use to carry to additional 100 pounds for 1 mile?

Answer 1

To move 1 pound 5280 feet, (= 1 mile) = 5280 foot-pounds.

1 BTU = 778.2 foot-pounds, so to move 1 pound 5280 feet = 5280 foot-ponds /778.2 foot-pounds per BTU = 6.785 BTU.

But a car engine is not 100% efficient, so you have to divide the 6.785 BTU by the engine efficiency.

Answer 2

If you are traveling only one mile, then most of your energy will be used up in accelerating the car up to speed, and in overcoming the rolling resistance of the tires. Both of those require energy roughly proportional to the mass or weight, so your ~3% more weight with the bag will give you about 3% worse mileage.

If you had a hybrid car, then some of the extra energy used to get the car up to speed will be recovered as extra energy when you brake, so things would be better.

Finally, if you are traveling, e.g. 100 miles at freeway speeds, then most of your energy goes into fighting air resistance. Cement in the trunk doesn't cause any extra air drag so the mileage would be pretty much unchanged. On the other hand, if you tied the cement to a roof rack, it could cause a lot more drag and drop your mileage.

A 40-ton semitrailer (18-wheeler) gets about 6-8 mpg highway. This is much better in terms of ton-miles/gallon than a private car because it packs a lot more cargo weight into a somewhat larger frontal area.

Answer 3

Btu To Calories