Imagine this situation:

You are coming to the end of a street without a stop sign to take a right turn.
You can see that no one is coming before you need to completely stop.

You have the following two options:
1) Use the brakes and let auto stop take over, shutting off the engine for the last few feet before you come to a complete stop, then hit the gas and make the turn.
2) Coast through the intersection (slowly) without engaging auto-stop.

Now, which is better for fuel economy?

With #1 you get a few feet of no gas use but have to use some extra gas to restart the engine (I assume) and make up for the speed you lost by fully stopping.

With #2, you are coasting but the engine is running and using gas.

I realize the savings here is probably miniscule but I was curious which was better. My bet is #2.

 

#2. And the HCH doesn't use gas at all while coasting

 

Yes, #2 would be better but only when the car is warm. When it is still cold, then using FAS will be the better option.

Generally, the key is to avoid as much loss of forward momentum as possible. By stopping completely we loose all "energy" we had going for us when we were moving... and that means we have to derive it all from the powertrain to get back up to speed again.

Cheers;

MSantos

 

Agreed. #2. But I don't do it (safety - and the law - first)

 

Another option is to reach down and shift into S, thus avoiding autostop, which is what I would probably do. That way I can concentrate on the turn and doing the safe thing (whether that turns out to be pausing or continuing around the corner) while not having to worry about autostop.

Earlier threads here estimated that it takes about 6 seconds in autostop to make up for the additional fuel that it takes to restart the engine. Sorry, I don't have the link - maybe others remember those discussions?

 

I was under the impression that the engine was started by the electric part of the car and did not use gas to start up (unless the batteries are low)?

 

Well, yes, but from where does the battery get its energy? Some from braking, which conserves energy, but in the end it all comes from gasoline. Also, the battery pack will crank the engine to start it, but you've still got to come up to idle speed after it starts. Not much energy is used to restart the car, but energy is used and that energy has to come from gasoline.

 

Bill, I don't think the engine is using fuel when coasting? Of course it does use fuel!, but less than most!!!

 

About Coasting/Deceleration from Honda World http://world.honda.com/news/2005/4050705_b.html : The valves of all four of the engine’s cylinders are closed and combustion is halted. The motor recovers the maximum amount of energy released during deceleration and stores it in the battery.

Whether this means fuel consumption actually stops is hard to tell. The video shows that fuel consumption stops, however the only place on the Honda World's site that actually says "fuel consumption is ZERO" is when the vehicle is stationary (assuming AutoStop).

As far as the options above when stopping at a stop sign, I use option #3: Coast as long as I can to the stop sign (to make up for the FE hit for coming to a full stop). When I'm at 8 or 9 MPH, I shift into S so as not to go into AutoStop at 6 MPH. I then apply the brakes to stop completely (as required by law). The I put the car back in D and lightly press the gas pedal to get moving again, but not accelerate quickly.

90% of stop signs are in residential areas or large shopping/mall lots and speed limits are only at 10 to 30. And where there's one stop sign, there's another just ahead unless you're pulling onto a main road.

 

Just a couple of comments.
Steve's original post said, "You are coming to the end of a street without a stop sign to take a right turn". In an uncontrolled intersection, or where the cross traffic has the stop or yield sign, I think going around the corner without stopping is acceptable. Unless I'm missing something.

So far as coasting is concerned, I'm confused by the comments, but again I'm probably misunderstanding something. Ultimately the forward momentum of the car must be supplied by gasoline. Yes, when coasting the car is not consuming gas, and you can recover energy by braking, so at that moment you are generating energy from the motor while not buring energy in the engine. But there is no energy input into the system other than by burning gasoline. There are internal efficiencies that conserve gas, such as regen, but the only energy incoming to the system is gasoline. Thus, while the battery turns the car over to start it, the energy the battery stores ultimately originates from gasoline, and the 6 seconds was determined by taking all of these factors into consideration.