Engine requirements for 100 mpg

[Ernie Rogers]

Hello, folks,

I thought it might be useful for some to define in a mathematical fashion what the requirement for an IC engine is, in terms of efficiency, to obtain 100 mpg with the engine only.

A calculation like this is built on assumptions. I assume a car running at sea level at a constant speed-- then I will seek to find what IC engine efficiency is required to keep above 100 mpg. Here are the other assumptions I will use:

Frontal area: 24 sq.ft. = 2.23 m^2
Drag coeff: 0.20
Air density: 1.22 kg/m^3
Head wind: 0
Car weight: 3100 pounds, with fuel and full load of passengers
Rolling resistance: Crr = 0.006
Efficiency from shaft to wheels: 0.85
BTU /gallon: 116,090 BTU /gal

With this information, and knowing the rate of fuel consumption is 100 miles per gallon, I can calculate the needed engine efficiency. Here are the results, for a range of speeds:

SPEED....EFFICIENCY
...50......... 0.34
...55......... 0.385
...60......... 0.435
...65......... 0.485
...70......... 0.54
...75......... 0.605

The calculations were made using a spreadsheet calculator I have -- I will send it to anybody that requests it.

Ernie Rogers