Coasting vs. Engine Braking

[skewbe]

Originally Posted by Mighty Mira

What's the difference?

Dunno, was trying to understand jangeos report of horrible fuel consumption on restart.

Originally Posted by Mighty Mira

Also, is there any difference if you are going virtually straight from restart to in gear acceleration?

Not really. The computer won't get a chance to idle high for 30 seconds if you start and go. There won't be a minutes worth of gas wasted in that situation. If you can execute a well-timed bump start then even better.

[Mighty Mira]

Originally Posted by skewbe

Not really. The computer won't get a chance to idle high for 30 seconds if you start and go. There won't be a minutes worth of gas wasted in that situation. If you can execute a well-timed bump start then even better.

This might not be the place to mention it, but I found this useful. Daihatsu does not recommend bump starts, but so far there seems to be little problem with them if done correctly.

The key is to turn ignition on, give it a little throttle, then release the clutch but only momentarily (i.e. depress the pedal again). After that, I rev the engine up to operating range and then depress the clutch properly to change gear. This only drops my speed very slightly, like a kph or something.

I find that if I was to not depress the clutch after bump starting, the engine braking drops 10kph or more off the speed, which is both annoying, dangerous and wastes a lot of kinetic energy.

If I can see that I'm going to only have 4 or 5 seconds before needing to go again, I'll bump start it just before stopping, saving the starter motor, the battery and the energy necessary to use the starter motor and heat the battery up when charging it.

[JanGeo]

Not sure about that if the fuel mixture is affected after a restart but idle between shifts will be affected and should you have to stop for some idiot in the intersection the engine starts off with a high idle and higher fuel burn rate plus battery charging and a cooled off cat. Plus now you are starting off with engine oil that has drained back down from the upper parts of the warmed up engine under load to accelerate. The ECU has gone through another startup sequence, you just put some more wear on the engine rod and crank bushings starting it warm, the rings just made contact with the cylinder walls without the oil film to prevent some wear... except in my Synlube lubricated engine. On a hot day your coolent temperature just spiked from latent engine heat traveling through the engine metal to the coolant paths. This all adds to the wear of the engine for a half a cent of fuel savings MAYBE? Plus you have your hand off the wheel and the shifter when you are restarting with the key and probably leaning forward to reach it too. Then the startup load just clitched all the electronics in the car when the battery voltage dropped or you cycled the ignition off and on and then to start and run working more relays for headlights and accessories. And finally there is that one or two times that you DON'T turn off the engine and reach for the key to start it and grind the starter.

Now maybe for other cars with higher idle fuel burning rates it makes sense but not the Toyota VVTi engine in my xB or the Yaris. Ever wonder why some people DON'T get high mileage in their Prius . . . same engine! The only time they get good mileage is when they know how to take advantage of the regen braking and electric drive with a really steady foot on the gas. All the engine stopping and starting that a "non-hybrid educated" driver goes through by driving it "normally" doesn't save much fuel because the engine doesn't burn a lot of fuel at idle.

[skewbe]

Originally Posted by JanGeo

but idle between shifts will be affected

LOL, I'm suprised you sleep at night with concerns this deep

Edit: Jan, if you have some studies as it regards to real life pulse and glide hypermiling and longevity then please stop speculating and post them. You make far too many rabbit holes for a decent discussion to follow. I know of at least one other person that reports they have been doing pulse and glide since there car was new and hundreds of thousands of miles later they are still doing it.

[JanGeo]

My Geo would start on the first compression when warm and the second or third in the coldest weather by comparison the xB has to crank and crank usually at least 4 compression cycles before it starts and if you don't catch it and have to start over, it takes about 6 to 8 compression cycles to start.

As far as pulse glide since I can't turn off the engine and coast and usually drive in heavy traffic there is not much point in doing it with the xB and at higher speeds the air drag is so high as well as the engine RPM that doing it over 40mph is also not going to work since they have the engine geared at about 40% to 50% load at constant speed, using heavier throttle activates the VVTi and you end up loosing some of the efficiency. I do manage to coast a lot on certain roads and end up getting in the upper 40mpgs but I have also driven in gear constantly on country roads and gotten 55mpg without P&G. As far as driving and testing goes I don't have the time or the gas to waste - my Geo racked up 90k miles in 12 years and a lot of that was when I was commuting to work which I don't do any more. As it is now I go about 400 miles in 20 hours of driving which comes out to about 20mph average speed . . . not going to rack up a lot of miles driving at those speeds either. To make matters even worse I have been driving on the same roads every day for the past 27 years and the only thing that has changed are the speeds limits getting lower and more traffic lights and stop signs have been added. Classic winter drive tonight to go 1.5 miles took about 15 minutes engine cold burning 0.5gph mostly down hill with stopped cars blocking the roads I got 12mpg and by the time I pulled into the driveway the engine was up to 172 degrees. Usually it is a 6 minute trip and I get 36mpg.

end of rant!

[RningOnFumes]

So, is there a feasible way to combine the positive effects of both?


Is there a way to electronically gerrymander the engine to cut off fuel while coasting neutral?

(don't want to turn off the engine, mainly because I like the use of my brakes for emergencies.)

[DarbyWalters]

Doesn't anyone worry about oil pressure drops when moving with engine off? At least with it running in neutral you are getting turnover...fuel pumps turing on and off...for some starters getting used many times more than normal...solenoids getting "clicked" numerous more times...raw feul sitting in cylinders (plug fouling, cylinder washing, no heat) while coasting (pistones, rods, ect still moving)...

Just being the Devils Advocate...but some of these things could be costing more than the fuel saved??? At least idling , things are still operating under a bit of power...

[skewbe]

Generally not, there is still oil on the parts as it hasn't been sitting overnight.

Aslo, I drive a simple machine (not to mention inexpensive) that is easy to work on. I don't lose much sleep over things mechanical in general. I like machines who know who is in charge

[slurp812]

Originally Posted by JanGeo

Pretty sure it does since I have a xB and it uses the same 1.5liter VVTi engine - the way to tell is to get a scangauge to monitor the engine temperature and when the engine is still cold about 120 degrees, go down a long hill and with your foot off the gas, engine braking, you should see the temp stay constant or even drop a little if you are using the heater. This indicates that the fuel is cut off.

I recently went down a parking lot ramp, and it was apparently the right angle, but the can would slow down when the injectors were off, then soon as it hot 1200 rpm, it would speed up till about 1500 rpm where the car would begin to slow again. Oh, and thats a 1994 Honda accord. I use both neutral coasting, and engine braking when I drive, just depends on if I need to slow down that much or not.

[caveatipse]

I rread the article about pulsing and coasting, but I am still unclear about something. Does this mean flooring it then coasting, like pedal to the metal? How does that save gas?