There has been quite a lot of discussion lately regarding the relationship between automobile speed and fuel economy. It's past time that we resolve this dispute once and for all. Faster cars actually conserve fuel! Now, before any of you become too outraged, let's go through the scientific reasoning and sound engineering logic behind this conclusion:
When an engine is running it consumes fuel
The less time that an engine is running, the less fuel it burns
This leads us to the irrefutable conclusion that our goal needs to be one that leaves the engine off for the maximum possible amount of time. If we drive our vehicles slowly, this increases the time that the car's engine is running. This obviously increases the volume of fuel burned and this is a bad thing. If we drive quickly, the engine is on for a shorter period of time and less time equals less fuel burned, and this is a good thing.
Sound engineering practices require us to evaluate our conclusion (to perform a sanity check) to determine whether a study of actual automotive driving speed supports our finding. I considered multiple driving environments; city, highway, and traffic jams, to assess how engine running time affects fuel consumption. I also evaluated the effects of trailer loading on fuel consumption.
The first case studied dealt with city driving. Slow moving cars exhibited the same time duration for engine-on operations as fast moving cars, but engine load differed for these two cases. Slow moving cars had a higher average load with the engine "pulling" the car all the way from one light to the next. These engines were under load for the entire time the car was driving. Fast moving cars had a low average load with the car's engine running at idle each time it stops for a red light. We all know that an idling engine consumes very little fuel so getting to the red light in the shortest amount of time gets the engine back to an idling condition and therefore conserves fuel.
The second case studied was highway driving. Cars moving rapidly down the roadway were seen to arrive at their destination much faster than slower moving traffic. The engines in these vehicles typically ran only 80% as much time as the slower moving vehicle engines. With these engines running 20% less time, fuel savings were the obvious result. Slower moving vehicles required their engines to remain running for much more time than their faster moving counterparts, and all this extra running time required additional fuel to be burned.
The final highway study was overwhelmingly supportive of our conclusion. Fast moving traffic was always headed towards its final destination while slower moving traffic was much more likely to become stopped in traffic jams. These traffic jams were noted to frequently involve cars sitting at a full stop, and with the engine running. This was the worst noted case of fuel consumption-fuel being burned yet the car was not moving or getting any closer to its final destination.
Our final case study involved cars towing trailers. While all vehicles engaged in towing operations were moving slower than vehicles without trailers, cars towing boats were traveling faster than those towing campers. We must conclude that those towing boats drive faster to save fuel that they can later burn in their boats. As campers don't require gasoline, they are free to drive slower and burn all their fuel simply getting to their final destination.
One final discovery of these real world studies was that the faster moving traffic was on the roadways for less time. While this supports the conclusion that these engines were running less time to burn fuel, it also kept the cars off the road for longer periods of time. Less time spent on the highways means less time in a traffic jam, meaning less time to burn or waste valuable fuel. Clearly, the faster moving vehicles had their engines on for significantly less time than their slower moving counterparts and therefore burned less fuel. The debate is closed!
This report is one of a series of occasional columns exploring the not-altogether-serious side of engineering by Ken Foote, a mechanical engineer at GDLS. You can reach Ken at [email protected] or e-mail your comments to us at [email protected].