I notice on sedans with automatic transmissions that the deceleration with foot off gas pedal and no brake applied is too gradual for city driving. Frequently, deceleration is desired without having to apply the brake. If I see a possible stop three blocks ahead, I would like to begin to slow without having to apply my brake. It may develop that I do not have to stop after all. Having to brake in order to decelerate moderately is, in my view, wasteful of gasoline and wearing of the brakes? Why is the throttle pattern not set to require keeping the foot on the gas pedal to ride at constant speed?
I don’t think it’s the throttle pattern, I think it’s the torques converter. It’s designed to operate in one direction, easily allowing slippage in the other. Remember that the torque converter is a fluid coupling device.
I’d be interested to hear from Transman on this one, or from someone else more knowledgable in trannys than I am.
Anyone? I’m all ears.
I am aware of your correct observation on the converter. I refer to the throttle, because I think that slowing down the engine RPM would help decelerate even with a fluid link. Fluids do have viscosity and if the crankshaft is slower than the transmission’s drive shaft there would be negative feedback. Maybe the very slow deceleration I do observe is the result, but I think it is too awfully slow. I am guessing that removing the foot from the gas pedal does not slow the engine enough. I don’t think it’s a case of the fluid’s maintaining the engine crankshaft’s RPM against engine compression.
P.S. I guess I should watch the tachometer when I take my foot off the gas. If it does slow down to idle (750RPM), then I shall have to agree with you. But if the fuel injection keeps the engine RPM close to what it was with my foot on the pedal, then there is room for correction. I’ll have look-see and post back later or tomorrow. Thanks!
I probably should have been more clear in suggesting that in a fluid coupling device the design of the impellars is crutial to determining how the transference of force happens between the halves of the coupling. Much as a large spoon moved through the water with the concave side forward encounters much more resistance and creates more turbulance than if it’s moved through the water with the convex side forward, a properly designed impellar system will be much more efficient in tranferring force in one direction than in the other.
The torque converter is designed to efficiently transfer motion from the front half to the back half, at the expense of efficiency when operating in reverse.
I found you clear and appreciate your clarity. I shall simply have to watch the RPM with foot just removed from the gas pedal to see what happens to the engine speed. You are saying, I believe, that with the engine slower than the transmission, the two are almost as if decoupled by the torque converter. Thanks, again!
The more a car coasts without giving it gas the better - you save gas. I would say that your driving habits are based on experience with mostly manual transmissions - you get used to lifting off the gas at a certain distance before the light. I’m the opposite - I’ve driven mostly automatics. Now I have a Jeep Compass with a CVT transmission that acts more like a standard when I lift off the gas and it drives me nuts! I try to coast and it slows down. I find myself having to accelerate again just to reach the red light! I don’t see the fuel efficiency of a CVT if this is how it’s made to work.
That bothers me too.
I like foot-off-the-pedal decelation in my 92 Explorer, 06 Escape hybrid, and 79 chevy pickup.But when I got my 08 Expedition it didn’t drive that way. Down shifting raised the rpm but barely slows the truck.
This is inside the individual brand of transmission and there’s not a whole lot you can do about it.
I think it has a lot to do with the car makers looking for every possible mile per gallon and the coasting factor helps in that endeavor…
Much to the dislike of us who like to slow with the foot off the pedal.
My goodness! Is this problem not solvable by IT! I really should try a CVT; maybe it’s made for me!
The problem of having to accelerate to reach the light does seem an irritant, but from a safety standpoint - if not economy - I’d prefer it (unless the guy behind me rams me in impatience!)
How about this: when I put the auto trans into N, I get more deceleration than when it is in D with my foot off the gas pedal. That means to me that I am still getting some help from the engine in the latter configuration. It is this slight engine assist over the N condition that I would like to avoid without having to put the trans in N. Any thoughts?
So, you think it is possible at least, to have an AT vehicle (not nec a CVT) that will decel with foot off pedal in a moderate way? I just don’t like the continued raped progress toward the light (or whatever) unless I brake.
A Toyota adviser told me most drivers want the continued speed without having to keep foot to pedal. I think, too, it is a matter of design.
Excuse me! I have to leave off now. The exchange has been accelerating, if you will pardon the expression. Thanks!
Yup, that’s exactly my meaning.
Macfisto suggested that it may use less gas that way. I don’t know, but it sounds like a good reason for the design. I’d suspect that designing to get maximum efficiency in the forward mode would save gas during acceleration and the effect of such a “forward thinking” design compromising efficiency in the reverse mode would be inconsequential.
It’s entirely possible, but at the expense of mileage.
Much of driving is taking the foot off of the pedal and then reaccelerating. The more inertia is carried is the less energy will be used reaccelerating. manufacturers are driven by CAFE mandates and the market forces to design cars that will get maximum gas mileage for their class.
I’m actually rethinking what I said earlier. The more I ponder the question is the more I suspect that the coasting is intentional rather than a byproduct of designing the TC for maximum efficiency in the accelerating direction of operation.
Yes, I think your re-think is probably correct, but I don’t blame CAFE mandates. After all, the energy lost in braking in order to slow down counts, too. Market forces, maybe! People do like not having to keep some pressure on the pedal to avoid slowing down. They do like to go fast and not plan their stops ahead - as anyone who drives in traffic and on urban highways knows. Tailgating, passing inappropriately, weaving, are day to day experiences. No law enforcement to speak of in Pinellas County and St Pete, FL. In part, that’s because of insufficient money for patrols. In part, that’s because it’s today’s driving “culture”.
Back to the technicals! I find it hard to believe that driving accelerator, brake, accelerator, brake is fuel efficient. I rather think that requiring a slight amount of foot pressure on the pedal to maintain constant speed is the most reasonable way to design for safety and efficiency. But I am an elder driver; so what do I know?
I have a Ford Freestyle with CVT (continuously variable transmission) and it has negligible engine braking in town. I can see a stop light/sign 2 blocks away and take my foot off the accelerator and still have to touch the brake. I also notice the RPMs stay in the city driving range. As someone later says, this helps gas mileage. I heard that there is an update that will employ more engine braking in the 20 to 45 mph range, but I decided I did not want it. My beef is the lack of engine braking on the downhills in the mountains. On my Volvo 760, 4 speed, I could continuously shift down from OD to High, to 2nd and even to low and almost never need to brake.
So any chance this problem could be caused by one of the following, assuming it’s not a Toyota DeathMobile trying to run amok?
Binding throttle cable.
Sticky throttle plate.
Idle Air Control Valve acting a bit stupid.
Personally, I prefer free-wheeling and coasting. From a 65 MPH throttle release on my Lincoln, that car will easily roll with no transmission drag and at idle over a 1/2 mile on the flat. My old Mercury was the same way.