Hypermiling Techniques: What is ‘forced auto stop’?
You might also see this referred to as 'stop shutdown', 'stop light shutdown', or 'SLSD'. Most of us have seen a version of this technique... when you are at a railroad crossing and you can see that the train will be passing for several minutes many people will turn their car off. Hypermilers do this at traffic signals and moreover they will often turn the car off before reaching the light so as to coast before reaching a halt.
About.com has this to say about it:
The Forced Auto Stop (FAS)
The first technique I learned and tried is Forced Auto Stop (FAS), but executing a FAS in a regular car takes a soul braver than I. This task involves coasting down a hill with the car in neutral and the engine shut off. What? Too difficult to control, and WAY too dangerous for my taste—with the engine off, the power brakes and steering are compromised and the steering wheel could lock. No Way! I've tried a similar trick that involves only putting the car in neutral to coast, but leaves the engine on. This method at least keeps the power steering and brakes fully functional, yet allows a slight increase in fuel mileage since the engine is only idling at 700 RPMs instead of perhaps rolling at 2500 RPMs.
The trick, and in my opinion, the Achilles heel of a FAS, is putting the car back into gear to resume acceleration. I've learned that I need to do a rev match (accelerate the idling engine to match the freewheeling transmission speed) so the tranny gears and clutches don't detonate when the slow rotating engine and fast turning tranny re-engage. Just as important though, maybe more so, a proper rev match keeps the drive wheels from momentarily locking up at the moment of re-engagement; loss of control on a slippery road is a real possibility. I think these concerns are slightly moderated with a manual transmission and clutch, but only slightly. I also wonder how this might affect internal transmission lubrication (at least in automatics), and further, how a continuously variable transmission (CVT) would react to repeated engagement and disengagements. Personally, I don't think it's worth damaging a very expensive component for questionable return. I've heard stories of folks using this technique for years with no problems, and that may be, but at best it's a roll of the dice.
eHow:
The forced-auto stop is when you turn off your car and coast. In hybrids, the internal combustion engine shuts off at stops to conserve fuel; the electric batteries keep the car running. To save even more fuel when decelerating, some hypermilers shift to neutral and turn off the engine while coasting to a stop.
Expert's say that this is highly dangerous.
Stop shutdown
(Transmission mode in B, speed zero)
Often you will pull up at a stop light that has just changed. In some suburban areas this may entail a wait of several minutes, as the lights cycle through various simultaneous or sequential left turns, cross traffic, pedestrians, etc. If the gasoline engine is running when you approached the stop it will take the better part of a minute before the computer recognizes the situation and shuts off the engine. There is a trick to getting the engine to shut off promptly. If you approached the stop in B (engine assist braking), you may then when stopped with the brake on, command D (drive) - the engine will stop immediately - at least in 2004 US versions.
There is a difference of opinion about this technique in the YahooGroup, Prius Technical Stuff. Two senior NHW20 (04-current) Prius owners believe this at most saves 5 seconds of engine run time on the first engine stop event, normally 8 seconds long. The effect does not occur in the NHW11 (01-03) Prius as long as the blue "cold engine" light is on.
The most informative article is at Wikipedia:
Auto-stop, forced stop, and draft-assisted forced stop
In the auto-stop maneuver, the vehicle's transmission is put in neutral, the engine is turned off (a "forced stop"), and the vehicle coasts to a stop. It is possible to coast in neutral with either a manual or automatic transmission. Modern automatic transmissions/transaxles depend on an engine driven fluid pump for lubrication [17]and coasting with the engine off may lead to damage or failure of the transmission[18]. To perform the maneuver, the driver shifts into neutral, and lets the tachometer stabilize, then keys the ignition back to the first position, referred to as "IG-I", to shut off the engine and electronics. The driver then keys forward to IG-II to start the electronics and continue coasting.[5] The key should remain in the ignition in the IG-II position, and not the IG-I position [19], in order to avoid engaging the steering wheel lock.[20] The driver recovers from "stealth mode" by starting the engine in the normal way, by turning the key to IG-III to crank the starter motor, and then releasing the key back to IG-II. Before putting the transmission in gear, if necessary, the driver may "rev" the engine to match the vehicle's gear and speed. The fuel economy from this advanced technique is increased noticeably over any short distance trip, largely because there are no engine idling losses (see figure below).[5] Most modern automatics' computer systems do a very good job at keeping the transmission in the proper gear while coasting in neutral, and the driver should not be conscious of the tachometer when re-engaging, but rather just press half-way down on the accelerator when re-engaging.
Some, but not all, hypermilers use this maneuver, and some may use it more safely than others. The technique is used for general coasting, or as part of the pulse-and-glide maneuver, or when going down hills or in other situations when potential energy or momentum will propel the vehicle without engine power.[1] Some hypermilers may use this maneuver while going downhill, around a corner, and without braking;[19] however, that practice is in all likelihood more dangerous than an auto-stop on a level and straight road, where stopping distance is shorter and visibility is greater. Vehicle control may be somewhat compromised, and this can be more-or-less dangerous or safe depending on the situation. Turning the engine off will cause the power brake assist to be lost after a few applications of the brake pedal. Power steering is quickly lost, although it is not needed at high speed, only at low speed.[5] Steering is still possible at low speed, but can often require considerably more arm strength to turn the wheel.
For safety reasons, the maneuver is not recommended for use in traffic, since the driver will want the car to be in gear if sudden acceleration is needed as an evasive maneuver. The driver should first look for traffic behind the vehicle before attempting the maneuver. It can be considered more courteous to not coast if another vehicle is closely following. [20] The proper etiquette and acceptable driving practices are controversial, and is worsened by a lack of communication between drivers. Both sides of the debate are often argued passionately, yet sometimes neither of the proposed driving methods is in complete accordance with the rules of the road. Both hypermilers and regular drivers may at different times violate the same rule yet blame the other type of driver.
Despite the potential risks, it does in fact save fuel to turn the engine off instead of idling. Traffic lights are in most cases predictable, and it is often possible to anticipate when a light will turn green. Some traffic lights (in Europe) have timers on them, which assists the driver in using this tactic.[20]
Draft-assisted forced stop, a variation of the forced (auto)stop (sometimes abbreviated as D-FAS), involves turning off the engine and gliding in neutral while drafting a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake (This practice is illegal in some areas due to its danger); while tailgating itself is inherently risky, the danger of collision is increased with D-FAS as hydraulic power for power brakes is used up after a few applications of the brake pedal, and there is a loss of hydraulic pressure that provides power steering, [21] however, there is less need for power steering at high speed.[5]
Some hybrids must keep the engine running whenever the vehicle is in motion and the transmission engaged, although they still have an "auto-stop" feature which engages when the vehicle stops, avoiding waste. Maximizing use of auto-stop on these vehicles is critical because idling causes a severe drop in instantaneous fuel-mileage efficiency to zero miles per gallon, and this lowers the average (or accumulated) fuel-mileage efficiency.
