Environmental awareness and economical driving (eco-driving)
Eco-driving questions are quantitative in disguise: they test whether you know that aerodynamic drag rises with the square of speed, that idling burns fuel for nothing, and that anticipation saves more than any gadget. On a 40 t vehicle, driver behaviour typically moves consumption by 10–20 %.
The physics of consumption
Drag force ∝ v², so the power needed rises with the cube of speed. Dropping from 90 to 80 km/h on a motorway costs about 8 minutes per 100 km but saves roughly 10 % fuel. Rolling resistance falls with correct tyre pressure, and every unnecessary tonne of load costs fuel on every gradient. Engine efficiency is best at low revolutions under load, around 1200–1500 rpm in a modern truck.
Technique
- Anticipate: lift off early for a red light, a junction, a descending gradient, and let the vehicle roll rather than braking and re-accelerating.
- Accelerate briskly to cruising speed, then hold it steadily; use cruise control on motorways but not in hills.
- Use the retarder and engine brake rather than the service brake on descents - you save fuel, brake linings and drums.
- Switch off the engine during longer stops; idling for an hour burns roughly 1–2 litres and achieves nothing.
- Plan the route to avoid unnecessary climbs and urban detours, and combine deliveries.
Worked example
A 100 km motorway leg at 90 km/h takes 66.7 minutes; at 80 km/h it takes 75 minutes, a difference of 8.3 minutes. If consumption is about 30 l/100 km at 90 and falls by roughly 10 % at 80, you save 3 litres. Arriving 8 minutes later, the same driver saves fuel, brake wear and tyres - and on a Swiss motorway the truck is limited to 80 km/h anyway, so speeding gains nothing legal.
Traps
Believing a long idle warms the engine better than gentle driving; running part-worn or under-inflated tyres because "the tread is still legal"; coasting in neutral down a pass, which is illegal and dangerous; overfilling AdBlue or ignoring a fault, which derates the engine and wastes fuel.