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Driving with Engine Efficiency in mind

After a long conversation with recently graduated mechanical engineer and Dartmouth alum on the subject of engines, engine efficiency, and hypermiling, I've decided to attempt to articulate a slightly different theory on what I understand to be the traditional approach to hypermiling. The important points of this assertion are that engines are more efficient per unit of fuel when running further open (this has wrinkle in it as Ben Jones/SVOBoy pointed out, namely that engine computer tend to become contrary above 80% open in certain cases) because an engine must always work to overcome its own load (internal friction, compression, and when in gear, drive train drag, etc) and if it is working to overcome that load and working at 10% throttle, and for the purposes of this argument we say it uses 5% of its power running itself, than it's using 1/3 of its power just to run and only putting 2/3 of its power to use on the road. When running more open, the percentage that is turned into useful driving work increases. A mechanical engineer I spoke to suggested numbers somewhere in the range of <20% efficiency at low throttle position and 30% at full-open. This seems to translate to driving technique by imagining that one would do well to drive hard when the engine is running and leave it off at all other times. (Please don't yell at me about these percentages—I'm sure they're wrong but the concept is sound.)

Contrary to popular thought that driving up hills with load and accepting a certain speed loss is the most efficient way to drive, I now think that perhaps the most efficient way would be to lose no speed at all up hills if possible and perhaps use the hills as an opportunity to accelerate to the desired speed so that one would need to spend no time at the top of the hill regaining lost speed before going into a coast. Intuitively, this doesn't seem right. It seems like one could accellerate more efficiently on a flat or downhill section of road--that less time would be spent accelerating under those conditions--but perhaps by using the engine as efficiently as possible on the uphills, it would save something in the long term. When testing this theory today, I found that I was surprsingly, no, convincingly efficient. My trip meter lured me into thinking this is the way to do it. As a counterpoint to the argument that spending less time speeding up is more efficient, I say again that if one is to ask something more of their engine, they should ask everything they need at once and then let it sleep.

On the highway, where maintaining a higher-than-pulse-and-glide speed is necessary for not getting pulled over on a Saturday evening, I translated this technique to using hills to get to 65mph and then driving down the other side with the throttle at a position where there would be a steady but slow loss of speed. I was able to average about 35% better mpg this way than had I driven with a constant throttle position (according to my non-scientific or mathematic analysis of my Scangauge's output).

Also, a story related to me by Ben/SVOBoy makes me suspect that it would work for seasoned hypermilers. The story goes something like "My MPGuino died/temporarily stopped working and i had to drive without it, so i wasn't thinking about shaving MPG's up hills any more, just maintaining speed, and that tank was my best ever.""

hmmm....