MPG Cruise Control Algorithm
 

After analyzing how I'd try and optimize fuel economy if I were limited to only the information available from the OBDII port, I came up with the following. I can think of a few ways to make it more involved, but it may not be necessary so I figured I'd run it by the more experienced hypermilers out there. A simple system couldn't take advantage of anticipation (speeding up before going uphill or coasting to stop signs), but one with GPS could in theory, so as a separate issue, I would love to see any thoughts you have for that algorithm as well. I hope my pseudo-code is clear. I didn't want to create a flowchart.

Repeat {

if( speed <= MIN_SPEED ) {

// very steep uphill, economy is not the concern
adjust throttle as needed to maintain MIN_SPEED;

} else if( throttle <= MAX_EFFICIENT_THROTTLE ) {

// level travel, downhill, and shallow uphill
adjust throttle to be as low as possible to maintain at least LEVEL_CRUISE_SPEED;

} else {

// moderately steep uphill
keep throttle at MAX_EFFICIENT_THROTTLE, allowing speed to diminish;

}

}

I realize I didn't observe that because this would be used like a traditional cruise control it wouldn't be expected to deal with starting or stopping. Functionality to efficiently accelerate from rest could be special cased, I guess. Might be useful.

Only problem is, it's more efficient to pulse-and-glide up and down hills... :)

Well, that'd be the GPS version. Automated pulse and glide would be very convenient!

I wonder if a modified form of pulse and glide could be done just within the context of OBDII info? It'd be interrupted by hills, but the pulse would just be used to climb it. Maybe I could think up something, but I'll have to research pulse and glide more first since my experience with it is pretty limited. Since pulse and glide is more difficult to do with other cars around a non pulse and glide algorithm would still have value, not to mention being a simpler test case.

Tried driving according to my algorithm and I quickly noticed an area of ambiguity: coasting down shallow hills. As long as the hill is steep enough to keep the speed above the level cruising speed (between 50-55 mph) then it's fine, but what if speed drops below that? Right now the algorithm would add the small amount of power needed to maintain the level cruising speed down the hill. Is the best course of action to coast down all the way to the minimum allowable speed and then go back into powered mode? That approach seems to fit in with pulse and glide methodology better and it's what I'm doing right now when I drive manually: milk the decline for all it's worth. Hill detection would be easy just by examining the speed vs throttle position history data.

Also, thought over a simplified form of pulse and glide that doesn't require GPS to automate. Seems like you'd just carry out a normal pulse and glide routine, interrupting pulses if you encountered an uphill stretch. Does that make sense to those of you familiar with it? If so, I can boil that down to another pseudo code description. Thanks a lot.

It's not Pulse & Glide if it's in gear during the 'glide' portion. I call that Pulse & DFCO, and I've found that it's less efficient than just steady-state driving.

If you're coasting in neutral, then yes it is the best course of action to allow the speed to drop to the minimum on the descent and take the speed up to the maximum on the ascent. Hills amplify P&G's effects by letting you make power most efficiently and then coast the rest of the time.

For your simplified form, if you encountered an uphill stretch, you wouldn't interrupt your pulse, you'd amplify it. The point of P&G is to make use of what we know about BSFC (to produce work in the most efficient way) and reducing throttling losses.

I was told a while ago that some motorcycles have a constant-throttle-position cruise control (instead of constant speed), and the reason was that you get your maximum MPG with constant throttle position. So what you propose is a 1/2-way compromise.

The only problem is that if you are on the highway and someone is passing you, then your speed changes and you wind up leapfrogging them a few times, it is poor road manners and can cause the other drivers annoyances.

I don't have cruise control, but when there is a lot of traffic around I will maintain a dead-constant speed. However given an open road, I'll use the speed-up/slow-down routine like your mini-algorithm.

-BC

In the Insight I find that the highest mileage on the local very small hills is obtained when you allow the vehicle's speed to increase downhill, then use that speed to get as far as you can uphill with minimum throttle.

I do it using the instantaneous FE bar graph and try to stay above 75 MPG on the uphill portion and over 100 on the downhill.

When I follow that procedure my speed will vary between 60 and 40 MPH from the bottom to the top of the hill, which is about 40 feet elevation change.

Not sure if that would work in other vehicles, my CD is very low so the higher speeds don't cost me as much energy.

regards
gary

Sorry, I failed to mention that the hypothetical situation I'm designing for assumes an overunning clutch is in the transmission. I could write versions for both, I guess.

Hey, that makes sense! Avoid big engine running at partial power. Duh. It's so reversed from what I do now that it takes a little adjusting. I'll try and cook something up. I've tried pulse and glide and found it was more effort than I was willing to put in, but it still interests me a lot.

That is extremely interesting to know! The rudeness (and potential safety problems) of this entire idea pretty much relegates it to hypothetical land, I just find it's not only interesting, but that it helps me more systematically explore my hypermiler driving habits.