Hacker News Reader

Comment by nols | Hacker News Reader

Generally the most efficient is highest gear-lowest RPM, and with car makers that's usually around 55-60. You could change the fuel efficiency to different speeds(I'm sure they're adapted to local markets with metric vs Imperial), but it would be worse off for most people to do that. It's not simply a gearing issue to change fuel efficiency, it's also the ECU controlling the fuel/air going into the engine. They aim for optimal fuel efficiency for normal driving, and that seems to be the sweet spot.

Contrary to what I've heard before, changing fuel efficiency to higher speeds wouldn't make things better. It's also not that simple because drag increases exponentially with speed.

View on HN

Supposedly peak engine efficiency is at peak torque, but of course no car is designed to cruise at peak torque.

This is still not any more satisfying than "because this is the way they are built."

Also, I should say that I realize this is a side show. One that I think should get less attention. No matter how much more efficient you make a car, I doubt I would ever be able to do better than local transit. (I, of course, welcome evidence otherwise.)

Right now local transit in the US is often less energy efficient per passenger-mile than a solo-driven Prius or Insight, or even the average-loaded (gasoline) passenger car. The numbers vary of course based on passenger load and route efficiency of course.

http://www.templetons.com/brad/robocars/trans-energy.png

Cars are built for their purpose, designing cars to be super efficient at speeds they're never driven doesn't make any sense. The higher speed efficiency is a trade-off between all the varying speeds the cars are driven and the limitations of physics.

They could also produce extremely fuel efficient cars by ignoring the safety laws (even get around them by building trikes that don't have to abide by the laws) but they'd either not be allowed or never bought because they're impractical.

That is kind of my point, though. Rush hour traffic around here is 60-70, not 55. Why keep targeting 55 in the designs?

The diagrams here discuss the losses:

http://www.fueleconomy.gov/feg/atv.shtml

They are only rough estimates using existing vehicles, but engine losses are pretty constant, and if you take the 'power to wheels' group, wind resistance is always the largest factor (especially in the 'highway' cycle, which is sort the best one for your concern).

Choosing gears to make the engine most efficient at a higher speed will increase that contribution (without really changing any of the others much; I guess there are a bunch of critical points where the different fuel losses cross each other, and they aren't necessarily straight lines).

Reply on news.ycombinator.com