Fuel, Speed and the Cost of a Hilly Route
Two routes are available for a 300 km trunk run with a 32-tonne articulated lorry. Route A is 300 km of motorway, mostly flat, with a limit of 90 km/h for lorries. Route B is 285 km, includes 40 km of rolling hills with 4–5% gradients and 25 km of urban road with traffic lights, and has a limit of 80 km/h on the rural sections. The lorry's fuel consumption on flat motorway at a steady 85 km/h is 28 L/100 km; on the hilly section it rises by about 35%, and in urban stop-start conditions by about 50%. The driver is paid by the hour, and the company measures fuel per tonne-kilometre.
1. Estimate the fuel used on each route and decide which is better, showing your working.
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Route A. 300 km at 28 L/100 km = 3.00 × 28 = 84 L.
Route B. Break it down:
- Flat/rural remainder: 285 − 40 − 25 = 220 km at 28 L/100 km = 2.20 × 28 = 61.6 L.
- Hilly 40 km at 28 × 1.35 = 37.8 L/100 km = 0.40 × 37.8 = 15.1 L.
- Urban 25 km at 28 × 1.50 = 42 L/100 km = 0.25 × 42 = 10.5 L.
- Total = 61.6 + 15.1 + 10.5 = 87.2 L.
So Route B uses about 3.2 L more despite being 15 km shorter. Per tonne-kilometre the gap widens slightly because Route B also carries the same 32 t over fewer kilometres - the fuel per tonne-km is 87.2/285 = 0.306 L per km versus 84/300 = 0.280 L per km on Route A. On fuel alone, Route A is better.
2. What other factors could reverse that conclusion, and how would you weigh them?
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Several factors can outweigh 3 L of fuel:
- Time. Route A at 85 km/h takes about 3 h 32 min of running; Route B at lower speeds and with urban delays might take 4 h or more. If the driver is paid hourly, the extra time can cost more than the fuel saved.
- Tolls and distance charges. A motorway route may carry tolls that exceed the fuel difference.
- Driving time and rest rules. A longer Route B may push the driver toward a break or a reduced rest, which has operational cost.
- Emissions and urban exposure. The urban section produces more NOx and particulates per kilometre, which matters for environmental scoring even if CO2 is similar.
- Risk. Urban sections have more junctions and vulnerable road users; hilly sections increase brake and driveline wear.
Weighing: if the schedule is tight or the driver is hourly paid, Route A wins clearly. If the motorway is tolled heavily and the schedule has slack, Route B may win on cost despite slightly higher fuel.
3. Give three driving techniques that would reduce fuel use on the hilly section of Route B, and explain the physics behind each.
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- Build speed before the climb and hold a steady throttle up it. Accelerating on the gradient wastes fuel because the engine must overcome both rolling resistance and gravity; momentum gained on the flat is free energy already paid for.
- Use engine braking and gravity on the descent instead of the service brakes. Letting the lorry run down in a suitable gear recovers the potential energy as distance rather than dissipating it as heat in the drums - and it keeps the brakes cool for the next descent.
- Avoid unnecessary stops and harsh acceleration in the urban section. Each stop-start cycle converts fuel into heat in the brakes; anticipating lights and rolling to a stop, and accelerating smoothly, keeps the engine in its efficient band and cuts the urban penalty.
A fourth supporting measure is correct tyre pressure: under-inflation raises rolling resistance and is a common, invisible fuel penalty on long runs.