This occurred to me as I was driving along King’s Highway No. 118, which for the most part passes over the rolling hills of the Canadian Shield. Let’s assume road conditions are good, speed limits are okay, no other traffic or moose in the way, etc; just open hilly road. Heck, we can even assume the road profile is a sine wave if needed.
If I want to use the least amount of fuel to go over a series of large hills, what do I do?
[ul][]Coast down into the valleys and partway up the other side, and power up the rest of the way to the peak?[]Accelerate down into the valleys, and coast up the hills?[]Try to maintain a constant speed?[]Something else?[/ul]
Use your brakes as little as possible. Every time you brake you waste gas. Coast down the hill, even if you end up doing 90 mph. Put your transmission in the highest gear you can, or neutral.
Use the throttle only to maintain your desired minimum speed when going up the next hill.
Essentially, use the brakes as little as possible, and use the throttle as little as possible.
When you reach the top of a hill, shut off your engine and coast until down to a speed at which you have determined your vehicle is optimally efficient.
Decades ago there used to be “mileage derbies” or some such, seeing how far a car could go on a given amount of gas. What I remember is that the two consistently best strategies were constant throttle and constant speed, with constant throttle being a bit better.
It seems to me that if you want to get a certain mass to travel along a road that goes up a certain amount and then down a certain amount all while maintaining around a certain speed, there’s no best way to do it, you just do whatever you need to do to stay near that speed. Gravity being what it is, you’ll end up hitting the gas more while going up and less while going down, but there’s no magic formula of when to hit the gas and when to let off, you just do what’s necessary to maintain the speed you want to maintain.
I’ve tried this experimentally a few times. My 'Vette has a pretty decent average fuel consumption readout on the dash.
Braking on any downhill obviously kills mileage, don’t give away hard-earned kinetic energy.
You never want to have to shift down into 5th to make it up the grade - the revs and light throttle will force you into a bad region of the power/engine speed/economy map. So you mustn’t drop below the speed in 6th where your engine stops making decent power.
The faster you go, the less speed you will lose at constant power. A passenger rail line will have steeper grades than a freight line. If you have to climb 10 m by simply rushing the grade and providing no additional power, it will take you 98 J/kg or kinetic energy to climb it. If you are going 20 m/s (45 mph) originally, that’ll drop you to 15 m/s (33 mph). If you are going 40 m/s (90 mph), it’ll only cut you to 37.5 m/s (85 mph).
Cops like to hide so as to give the least possible warning to drivers they are trying to clock. This means that they will hide past crests. You don’t want crest speeds any higher than the speed limit. This goes unless you will drop below your engine’s decent-power speed in high gear but can stay in high gear at a couple over.
Aerodynamic drag and high speed rolling resistance hurts some cars more than others. Opening the throttle on downhills and letting it roll up extra speed will get you ahead on gas mileage in a Corvette. It won’t in a Winnebago.
