TECH ED’S CHOICE

POWER REQUIREMENTS AND FUEL USE

A couple of years ago, CAR published an article explaining the formulas to calculate the power needed to drive a vehicle at specific speeds. I’ve been exploring drive-style improvements and what they do to my Toyota Land Cruiser 4,5’s fuel consumption and the results in town-driving have been an improvement from 20,0 to 13,0 L/100 km.

My problem is during open-road cruising, where I cannot get below 14,0 L/100 km, and this hurts the holiday fuel bill to Zambia. Would an overdrive ratio of 25% help? Secondly, where can I get a brake-specific fuel consumption (BSFC) map of the engine? PIETER DE WAAL Via email

The power requirement of a vehicle on a flat road at constant speed is the power needed to overcome mainly aerodynamic drag and rolling resistance of the tyres to name the main contributors. The equations are: Aerodynamic drag force: ½ ΡAC V2, where: d ρ = air density (1,2 kg/m3 at standard atmospheric conditions); A = frontal area of vehicle (m2); Cd – coefficient of drag; V = vehicle speed (m/s);

Rolling resistance force (simplistic): C mg, where: r Cr = coefficient of rolling resistance (usually around 0,015); m – mass of vehicle (kg); G = gravitational constant (9,81 m/s2).

Therefore, if we plug in your vehicle estimates of m=2 300 kg, Cd = 0,4 and, A= 3,23 into the above, add the two figures together and multiply with vehicle speed (m/s), we get the following power-consumption figures at speed:

It’s clear the power requirement increases quadratically with speed. Without having the map of your engine (it is proprietary to the OEM and difficult to get hold of), changing the gearing would be a shot in the dark. We doubt this will make a significant difference, though, so it’s best just to lower the vehicle’s cruising speed.