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It takes a set amount of energy to propel a car at a constant speed, equating to a certain amount of fuel. Therefore, I feel it would be interesting to do comparisons between different vehicles on a level road at 120 km/h in both directions to get an average fuel-consumption figure at that speed. Although these figures will vary owing to the size, weight and shape of the vehicles, it will make for interesting comparisons.
Having worked in the motor trade all my life, I would predict that a lot of small cars would fare rather poorly against much larger and more powerful vehicles in such a test. Some vehicles with larger engines (for example, a V8) are now able to employ cylinder deactivation to improve fuel consumption. On a recent trip to KZN, I managed to get the average consumption of a Mercedes-Benz C250 CDI down to 5,1 L/100 km at an average speed of 119 km/h through the Free State. Yes, it is a diesel, but how many small cars will achieve a similar figure in similar circumstances?
H WEBER via email
A thought-provoking proposal, although we believe most people would rather be interested in a combined-cycle figure (more relevant to real-world use) than a constant-speed, level-road measurement. We do note your request, though. It is all about efficiency, as you stated. Beyond, perhaps, increased rolling resistance at a constant speed on a level road, mass has little to do with the calculation. The resistive force at constant speed (on a level road) is mostly attributed to aerodynamic drag, rolling resistance and mechanical losses due to friction.
The engine then needs to overcome the total resistance force and the most efficient engine in a specific body style will then use the least fuel. Compared to sedans, hatchbacks have a high drag-coefficient and are therefore at a disadvantage. Turbodiesel engines are more efficient than equivalent petrol engines (no pumping losses and higher compression ratio) and diesel has more energy per litre than petrol.
The reason for cylinder deactivation on large petrol engines is to allow the working cylinders (say four in a V8) to run closer to their maximum torque capability, which is more efficient with less pumping losses thanks to a wide throttle opening. Obviously, the general perception that shutting off half the cylinders will result in half the fuel saving is completely incorrect, as the same energy is needed to propel the vehicle at the set speed. Only by upping the efficiency of the engine will you save fuel.