Real fuel use during coasting

[mecklen]

I've been trying the "pulse and glide" technique to improve gas mileage (see http://en.wikipedia.org/wiki/Pulse_a...ced_techniques). In my commute I find that I can coast about 3.2 miles out of a 19 mile trip. When I do this I leave the engine running and place the automatic transmission in neutral. My speed varies between 64 and 75 mph during these glides.

My question is: does this actually save fuel? Here are the two possible answers:

Yes: When the car is coasting in neutral the engine is idling at about 800 rpm. That will use less gas than revving the engine at 2300 rpm in gear at the same speed. This opinion is supported by a tech at the Honda dealership. This tech said the engine uses more fuel at 2300 rpm than at 800 rpm regardless of the actual load on the engine.

No: I've read several articles recently (lost the citation) that indicated some modern engines will use zero fuel when car is coasting down hill and no load is on the engine.

Is there anyone who knows how the Honda 2007 GX engine works under these conditions? Remember, opinions are welcome, but I'm most interested in facts. Has anyone with a ScanGauge checked this out?

[CNGfamily]

Re: Real fuel use during coasting

... When I do this I leave the engine running and place the automatic transmission in neutral. My speed varies between 64 and 75 mph during these glides.

I have no idea how much you'd save, but you do have less control over the vehicle while coasting in neutral. It would be a bit more risky, but you could just shut off the engine and go w/o any power assist on the steering and brakes during the glide.

[Blueflame]

Re: Real fuel use during coasting

I don't know the answer but I do know it is frustrating to the drivers behind you trying to maintain a steady speed. Probably not saving them any gas when they have to constantly brake and accelerate behind you and their gas costs A LOT more!

[Curtis]

Re: Real fuel use during coasting

The GX engine does not shut down fuel to the engine at any speed. This is a function of the hybrid Civic if you are no load and under 35 miles per hr. No hybrid will shut off fuel to the injectors at those speeds.

As far as asking about gas mileage... what have your results shown. A gas engine is most efficient at its designated cruising speed, not 75+ miles per hr... 65mph in most cases. An engine uses the most fuel under load... getting to cruising speed. That's why a hybrid has a motor assist for load conditions that drops out completely as load is lifted... at least on a Honda.

[wolfrpi]

Re: Real fuel use during coasting

I am surprised at that - I thought almost all engines shut fuel to the engine on decel. This helps save gas when going down a hill and save the brakes also. Its classic engine braking - but our cars don't have that?

[mecklen]

Re: Real fuel use during coasting

... it is frustrating to the drivers behind you trying to maintain a steady speed. Probably not saving them any gas...

I do my best to a) obey the law, b) save the planet, c) provide a good life for my family, d) enjoy life. This is quite a shopping list for me to tackle.

If the driver behind me chooses to drive so close to my bumper that they use the brake that's their choice. I'm just not willing to take on their b) and d) as well.

You're possibly not aware that on my commute the roads are virtually deserted (by LA standards). It is rare that I need to drive closer than 50 yards of the car in front of me and there is no reason for anyone to get closer than that to me.

[mecklen]

Re: Real fuel use during coasting

...what have your results shown. A gas engine is most efficient at its designated cruising speed, not 75+ miles per hr... 65mph in most cases

Averaged over 10 tanks of gas I'm getting combined city/highway of about 38mpg. However, I'm experimenting with several driving techniques and it is difficult to tell how much any one technique contributes.

As to highway speed, all things being equal I drive 65mph using cruise control. My commute is a hilly route, however, so when I come to a downgrade I'm doing 65/66 at the top and place the car in neutral. Depending on winds and the grade I can get going up to 75mph by the time I reach the bottom. This GX rolls better than any car I've ever driven! (These are gentle grades.)

On the way back I've been using the usual trucker technique of accelerating before the grade and allowing the car to slow as it reaches the top. The GX just doesn't have the power of my '91 prelude and cannot tackle these grades in 5th gear without nursing it. So I'll speed up to 73mph when I hit the grade and slow to 64/63 by the time I reach the top. I try hard to keep the car in 5th gear.

I believe these improve my mileage, but (being and engineer) I'm thinking I'll just buy a ScanGauge.

[Curtis]

Re: Real fuel use during coasting

Engine braking is simply the compression of the engine combined with friction of the trans and drive line parts. You'll note that your tach never zeros out, meaning the engine is still running. Try shutting off the key when the engine is running zeros the tach, it would do so while you were going down the freeway, although I wouldn't recommend it as you'd have no control, but if your car ever died on you while you are driving... it coasts to a stop. Without the hybrid electric engine to spin it up again, that's what happens.

[wolfrpi]

Re: Real fuel use during coasting

On a manual trans car, fuel is cut on decel - I think it is the same on autos when the tq converter is locked. If you locked the GX in gear such as in 1st, accelerated to redline, and then let off the gas, it shouldn't coast - it should engine brake. This means that the engine is still revolving, but not actually running because there is no fuel being sent to the injectors in this scenario. This is why its more fuel efficient to leave manual cars in gear when going a slope - because there is not fuel going into the engine whereas if idling there would definitely be fuel still going in. Once you depress the accelerator, fuel is then injected back into the engine to avoid stalling.

I'll have to try it out today on the GX. A air/fuel (or injector) monitor would of course give the definitive answer. I do know however that when going down a hill using cruise my GX will engine brake (and shift down) to keep the correct speed. I would guess that its cutting the fuel to the engine at that point.

[ajgiorgi]

Re: Real fuel use during coasting

You forgot to mention the effect on the tranny. It will wear out faster because your shifting a lot more out of drive and neutral.

[TheBundo]

Re: Real fuel use during coasting

I know the hybrid has fuel cut-off at any speed in the right conditions. Curtis is mistaken, and it is easy to tell when it is cut off. I have even had it pulling on electric only at 60mph on a flat hwy. It takes some throttle trickery, but you end up with the Instant Mileage gauge pegged at 100, and showing up to 5 bars of assist allowing a slower rate of deceleration than otherwise possible. It cannot, however, maintain that speed on electric only, merely slow the deceleration. The battery will also go down fast enough that about a mile is the maximum for this. It probably saves little if any fuel, as charging back up will use extra. There is a noticable "thump" when the engine kicks back in, the Instant Mileage gauge immediately drops, and the assist meter changes to charging.

A lessor but still discernable sensation of the engine kicking back in can be felt, and seen on the instant mileage gauge, when the fuel is cut off but electric not kicked in. It simply goes from pegged to a lower amount, along with the "thump"

This same behavior (the lesser thump in the second example) can be felt on my GX in similar situation. I believe the fuel IS cut off on the GX, and the thump is the result of power once again being made. Likely this involves shutting off and turn on the valves off through some mechanism, turning the engine into an air-pump, or simply shutting the injectors off, or both. I'll have to refresh my knowledge sometime on this at the Civic section of greenhybrid.com, but suffice it to say that the hybrid definitely can coast using no fuel, and I have one of those too, and the behavior is identical on the GX

[TheBundo]

Re: Real fuel use during coasting

Also, since it takes fuel to idle, and none to have the wheels turn the engine (assuming fuel cut-off), then leaving it in gear going downhill is the best way

[TheBundo]

Re: Real fuel use during coasting

One more thing, pulse and glide doesn't work as well on the GX as the hybrid, but easing up a tad can help. The conventional automatic seems to hold back more by nature

[Curtis]

Re: Real fuel use during coasting

Curtis is mistaken,....

You're right! I just spoke to my head mechanic and even the standard Civic will shut off the injectors for very short periods of time under no load conditions, such as deceleration.

Guess I should stick with selling them and let the mechanics handle the tech questions. I'm still trying to get my GX techs to join the group.

[beaker]

Re: Real fuel use during coasting

I dont know about the GX but I can give my experience with my NG powered Dodge Dakota 5 speed manual.
I have a Scan Gauge 2 and when in neutral, going downhill the engine is idling and using very little fuel as my MPG have hit 140 at 70 mph!

Same scenario but in 5th gear coasting. MPG = 60 - 70. Engine is turning approx 2300 rpm but no load and the throttle is closed.

The SG2 is a great tool. It can do so many things. Go to http://www.scangauge.com/ and check it out.

Curtis: You should look into a group buy on this.