Coasting vs. Engine Braking

[caveatipse]

I know some cars completely cut off the gas flow at a certain RPM when you use engine braking, thus making it more effecient than coasting to a stop. Does anyone know if the Toyota Yaris does this? Thanks.

[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.

[James]

coasting is always more efficient, unless you need to slow down anyway. If you need to slow down anyway, downshifting does use a little bit less fuel. Shift to a gear where the engine is turning between 1500 and 2500 rpm (depending on how quickly you need to slow down).

[caveatipse]

Wait, I am confused. If engine braking shuts off all gas flow, and coasting allows an idle amount of gas flow, and assuming you are covering the same distance and start both at the same time, why would engine braking not use less fuel?

[Jim Dunlop]

Because you can't "coast" as far while engine braking since the vehicle's momentum is slowed down by the resistance of the engine.

[Biffmeistro]

But the engine braking is far better when coming up to a red light than popping it in neutral and hitting the brakes.

[JanGeo]

YEAH guys pay attention . . . craveatipse is COASTING TO A STOP!

[JanGeo]

If it is anything like my xB you should be able to get engine braking down to about 1000 rpm in top gear before the engine suddenly bucks as the injectors start to fire fuel into the engine. In my xB gearing that happens about 18mph in 5th gear.

[caveatipse]

Ok I should rephrase. If I am, in both situations, coming to a stop, and I can choose either putting it in neutral, coasting to a stop, then applying brakes, or engine braking down to a stop, which uses less fuel? Sorry..I did not communicate very well.

[JanGeo]

You said it right - engine braking will use less fuel only the ScanGauge will not show it until you fill it up and it takes less gas to fill than it reports you had used. Usually a few tenths of a gallon per 10 gallon fillup depending upon how many long engine braking stops / hills you did on the tank of fuel.

[caveatipse]

ok thanks.

[skewbe]

Well, it isn't as simple as JanGeo said. If you are engine braking (or any braking) you are bleeding off speed that took gas to make.

I suggest you look at installing a fuel injection kill switch and understand the pros and cons of pulse and glide ( EOC ) if you really want to maximize your mpg in your stickshift Yaris.

[caveatipse]

Originally Posted by skewbe

Well, it isn't as simple as JanGeo said. If you are engine braking (or any braking) you are bleeding off speed that took gas to make.

I suggest you look at installing a fuel injection kill switch and understand the pros and cons of pulse and glide ( EOC ) if you really want to maximize your mpg in your stickshift Yaris.

But if you are slowing down at all, you are bleeding off speed it took gas to make. If all other things are equal, if engine braking actually stops gas from flowing, and coasting keeps it flowing, how can it not be that engine braking saves more gas, all other things being equal??

[skewbe]

The most fuel efficient mode of operation for a gas powered vehicle that I know of:
http://metrompg.com/posts/xfi-pulse-and-glide.htm

The point is to avoid any sort of braking and to accelerate just enough to coast to the next stop. If you are engine braking you will have to accelerate more to cover the same distance, if you are just coasting you will travel farther after you get off the gas. If you can live with killing the injectors while coasting, even better.

We are trying to save gas here, right? Or are we here to debate hypotheticals with little return?

[caveatipse]

Originally Posted by skewbe

The most fuel efficient mode of operation for a gas powered vehicle that I know of:
http://metrompg.com/posts/xfi-pulse-and-glide.htm

The point is to avoid any sort of braking and to accelerate just enough to coast to the next stop. If you are engine braking you will have to accelerate more to cover the same distance, if you are just coasting you will travel farther after you get off the gas. If you can live with killing the injectors while coasting, even better.

We are trying to save gas here, right? Or are we here to debate hypotheticals with little return?

lol no I just do not understand and I want to understand. If I engine brake, why will I have to accelerate more? If I get off the interstate on an off ramp, for example, and I am going to have to come to a complete stop anyway, why not do so in a manner that completely shuts off fuel flow, rather than a manner that leaves some fuel flowing?

[skewbe]

the kill switch takes care of the fuel flow and allows you to cover more ground while using zero fuel. If traffic is light (or you just don't feel like being pushed) you can shut off the engine sooner and coast much farther than you can in gear.

If you are a fan of pool, you may understand that some shots require you to hit the ball just hard enough that it stops exactly where you want it. This is the same concept, you try and kill the engine at just the right speed and distance from the next obstruction that you roll to a stop where you meant to. Takes practice and some brakes are the norm (if I never use the brakes over 20mph I'm doing ok). But in that manner you do not over-accelerate and then throw that energy away in braking.

[caveatipse]

Argh lol but that is not what I am talking about. My Yaris shuts off the fuel flow completely under engine braking. So it is only logical that, if I have to slow down to a stop anyway, doing so with NO fuel is better than with a LITTLE fuel. How am I wrong? (pulls out own hair)

[skewbe]

What is your goal? Does it relate to this hypothetical question in a significant way?

[caveatipse]

Originally Posted by skewbe

What is your goal? Does it relate to this hypothetical question in a significant way?

lol my goal is to use the least amount of fuel possible when coming to a stop where I HAVE to stop.

[skewbe]

Originally Posted by caveatipse

lol my goal is to use the least amount of fuel possible when coming to a stop where I HAVE to stop.

Ok, then still look into pulse and glide and EOC . If you kill the engine with a kill switch it won't be burning any fuel, and you can get off the gas pedal sooner (I assume you can anticipate most of the stops) if you put in the clutch as your speed will not bleed off so fast as it would if you left it in gear. You may find that you can kill the engine way ahead of the off ramp and coast for miles with no fuel.

How much speed you carry into that stop that you HAVE to stop at is going to be the main determinant in how much energy you will lose at that stop by braking.

If it makes you feel better, then you are correct about the hypothetical situation where you could not anticipate the stop (or did not change wasteful driving habits) and managed to trick the engine into turning off the fuel for some portion of the deceleration by engine braking.

[caveatipse]

No, but in my Yaris, during engine braking, the fuel injectors SHUT OFF. that is my point lol.

[skewbe]

Most cars work that way these days.

Do you understand that you cannot coast as far with the engine engaged?

[caveatipse]

Originally Posted by skewbe

Most cars work that way these days.

Do you understand that you cannot coast as far with the engine engaged?

Yes I understand. I am measuring from the point I decide to decelerate, to the point of the complete stop.

[mattW]

Just consider the a situation where EOC is not an option and a traffic light ahead of you turns red. If the car is put in neutral the energy you used to accelerate to that speed is dissipated by the brakes (wasted) and the engine runs at its no load idle. If the car is left in gear then the energy you used to accelerate to that speed is dissipated within the engine friction/pumping losses etc (also wasted) but the engine isn't using any fuel. What the hard core FE guys are saying is that there is a third and better option where you turn off the engine while still moving (car in neutral) and use the brakes to slow down, the advantage being that at the traffic light you continue to use no fuel whereas with the engine braking you would start to use fuel again at <1000rpm.

So if you are going to stop anyway use EOC, if not the next best is to use engine braking, if not that then use a neutral coast.

[skewbe]

Also, if you can adopt EOC, at the sudden red light, or any stop that you mistime, you will not have reason to restart the engine until it is time to move again. A lot of fuel is wasted idiling at stops, and it can mage a big difference in you mpg numbers.

[JanGeo]

Not in a Scion / Toyota VVTi engine - when it starts up again it will be burning more fuel than it would have running at a stop light for 1 minute. And that is if it starts without a lot of cranking. Better off keeping the engine running - remember it only burns .1gph when warmed up which comes out to 0.5 cents per minute - definately not worth the wear and tear on the ignition switch - starter - battery etc in the winter time. Steering is powered by the engine and believe me - you can't turn quickly without it in fact just turning into a parking space is almost impossible while coasting with the engine off - the steering is not designed to be turned without the pump running.

[skewbe]

What is that one minute figure based on? Most of what I've seen is 5 to 10 seconds.

Are you using a kill switch or the key (and resetting the ecu)?

[JanGeo]

Originally Posted by skewbe

What is that one minute figure based on? Most of what I've seen is 5 to 10 seconds.

Cost of gas at $3 a gallon and 0.1gph burn rate. A restart burns 0.4-0.5 gph for more than 30 seconds. Plus in 18k of driving I can count on one hand the number of times the engine fired up after 1 compression cycle of a cylinder and several times I cranked and let go the key and had to crank again a lot more to start. Just better to keep everything running and warmed up.

[skewbe]

Ideally you would not be sitting there for 30 seconds idiling, but you would start the engine and go.

[Mighty Mira]

Originally Posted by skewbe

Are you using a kill switch or the key (and resetting the ecu)?

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