The Effect of Travelling Speed (Aerodynamic Drag) on Fuel Consumption

Air is thick stuff. Even a 10km/h headwind can play havoc with fuel consumption. Here’s why:

At 100km/h, aerodynamic drag accounts for about half the total resistance acting against the car’s forward progress. Battling into a 10km/h headwind is like driving into still air at 110km/h. But unfortunately the drag doesn’t go up by just 10 per cent. It’s more like 20 per cent, because drag varies with the square of speed. Twenty per cent of half the total resistance means 10 per cent higher fuel consumption.

(The maths:

tech, fuelJohn Cadogan17 September 2010Comment

Brake Mean Effective Pressure. Huh?

When it comes to engineering sleight of hand, two routines predominate for extracting more power on next season’s model. The first is simple: spin the old engine faster. If it can be forced to produce the same torque at higher revs, more power will result. (Power is a function only of torque produced and the revs at which it’s produced.) Unfortunately, this approach often compromises low-end driveability from greater overlap, etc.

Second trick is to add cubic capacity. Jumping from 2.0 to 2.5 litres means cramming 25 per cent more fuel/air mix into the process. If you’re lucky you’ll get 25 per cent more torque at the same revs for 25 per cent more power. Unfortunately, this option also adds weight.

In other words, a big engine spinning fast can be designed by dummies and yet produce copious power, whereas building a truly efficient, powerful engine (literally more bang for your buck-per-litre) needs a big R&D budget and smarter boffs

techJohn Cadogan17 September 2010Comment