MPG Cruise Control Algorithm

[R.I.D.E.]

Max, the inefficiency of standard cruise control is the main reason I dont really want one. My car is like a rolling FE video game. The wife's Rogue has cruise so it is available when we travel together.

Not trying to criticize your idea, but my observation is the best mileage is using speeds similar to a roller coaster that uses altitude to get it going then inertai to keep it going. The constant speed maintained by a cruise control costs a significant amount of mileage, and the last speeding ticket I got, the week Princess Diana died, was becasue the cruise on the Altima I was driving did not have enough closed throttle speed reduction to keep my speed at 62 MPH downhill. It went 7 over than speed and got me a ticket.

I needed to shut off the overdrive to create enough engine braking in the 97 Altima, which would have killed my FE anyway.

regards
gary

[theholycow]

I don't think overrunning clutches are common. In your car with a manual transmission, obviously you don't have one. In automatics I don't think they're common at road speeds. I have the complete GM 4L60E manual and read it somewhat thoroughly looking for that and a few other things. The 4L60E does have overrunning clutches on 2nd and 3rd (and maybe 1st, I don't remember for sure), but they don't get used in normal driving because it will generally shift to 4th. They also won't get used if the driver manually selects that gear, so with the gear selector in '3' the 3rd gear overrunning clutch won't overrun (and similar for '2').

[theholycow]

Originally Posted by R.I.D.E.

I needed to shut off the overdrive to create enough engine braking in the 97 Altima, which would have killed my FE anyway.

In that case, since you need to discard the speed anyway, wouldn't DFCO reduce your loss compared to leaving it in overdrive and using the service brakes?

[GasSavers_maximilian]

Originally Posted by R.I.D.E.

The constant speed maintained by a cruise control costs a significant amount of mileage

That's actually the reason I'm interested in thinking about smarter, variable speed cruise controls. A lot people aren't going to adopt fuel saving driving habits that require much effort. I never intend to ever build one of these, although some of the info could wind up in a display that advises you on the best speed to drive. I'll never have time for such a thing, but I'll contribute it to the various DIY car computer projects so they can do something with it if they want. Long term of a lot of the answer is better propulsion systems, I'd imagine.

Originally Posted by theholycow

I don't think overrunning clutches are common. In your car with a manual transmission, obviously you don't have one.

I guess I wasn't clear again. I have to do better. This is all a "what if" design exercise in which cars could have slightly different components. You'd probably need to beef the brakes up on such a vehicle to handle cases with lots of steep hills. I know some older cars had freewheeling ability, does anyone know if their brakes were larger to compensate? I am interested in formalizing one for DFCO behaviour as well, though. I just find my mind turns to this subject whenever I am out hypermiling. Until I formalize it, it won't leave me alone.

On the more practical side, since I am willing to pop my car in neutral I can simulate freewheeling (at least if I'm not too lazy).

[R.I.D.E.]

Suggestions. limit acceleration to a certain point of low vacuum. Increase the systems propensity to downshift to limit downhill speed to a certain point above the set amount, as well as downshifting to maintain a certain lower than set speed uphill, in order to keep vacuum from dropping completely to zero.

On my Insight I have found that it is possible to very gradually increase speed without a significant decrease in fuel mileage. It is also possible to maintain speed on very slight uphill grades without a significant effect on fuel mileage.

It would probably require some kind of grade percentage detection for the cruise to become more anticipatory instead of reactive as they are now.

regards
gaRY

[theholycow]

There were cars with overrunning clutches, but that was made illegal in many places (IIRC, because of the issue with brakes). Modern brakes are much better. Power disc brakes are much less susceptible to fade and (in my experience towing my camper down mountains) still work ok when excessively hot.

Anyway, with tall gears and slippery torque converters on automatics, we effectively have a situation that is equivalent to an overrunning clutch for braking purposes but wastes gas when looking at it from an FE point of view; so brakes are designed for it and IMO there's no need to avoid overrunning clutches anymore.

[GasSavers_maximilian]

A lot of great tips all around. Gives me plenty to chew over. Thanks! I plan to try a few of these out and see how they impact my mpg. Be a fun way to make the drive more interesting.

[R.I.D.E.]

Technically the Toyota Prius which will coast under certain conditions without driver imput, is illegal in Virginia.

The law was passed in the 30s when Plymouth came out with freewheeling which allowed the car to coast in any gear if you let off the throttle. The problem was in mountains the brakes would fail and you died.

regards
gary

[GasSavers_maximilian]

So Plymouth didn't super-size the brakes? Wow, that's kinda scary. Is the law enforced with regard to the Prius? I assume they'll amend it eventually.

[theholycow]

Laws about coasting: http://www.gassavers.org/showthread.php?t=9946