RPMs - Help Settle a Debate

On another site we are discussing what happens to the engine when the car is coasting down hill. EDIT: We are referring to automatic transmission for the sake of this debate.

One side says that in a modern engine, the injectors shut off all fuel and the reported RPMs are due to the engine spinning strictly through the wheels and transmission 'back powering' the engine.

The other side says an engine needs 4 things to run, fuel, air, compression and ignition, and therefore if the injectors cut off all fuel the engine is considered stalled at that point.

Which do you believe to be correct?

 

I would say the engine needs fuel to run, I wouldn't consider the 1GR that 'modern' but since this is general automotive I guess it would pertain to any modern engine.

 

Yes, this debate is for modern engines in general, not Tacoma specific.

 

I would say both are technically correct (not sure about the technical part of how different engines go about shutting down certain cylinders) but engines do need spark, fuel and air to produce power. They do not need all of that just to turn therefore you can cut the fuel to an engine and if there is something else driving it, like the driveline backpowering the engine, it will continue to spin. Coasting down a hill, the engine doesn't need to produce power but the engine can be an effective braking system. If you take an engine like our Tacoma engine, it will still push some fuel to the engine while coasting but I believe some other engines will truely shut off certain cylinders. An engine will stall without fuel but as long as it's got something driving it, like a starter or momentum, when you kick the fuel back on, it will just continue running (think about pop-starting).

 

I remember something about this being brought up in another thread. Toyota, as well as other manufacturers have what is called DFCO or Deceleration Fuel Cut-Off. What this means is that when you take your foot off the gas, the fuel injectors will actually shut off totally.

Let's stick strictly with the general thread of an automatic transmission, for these operate slightly different from a manual.

For starters, unless you are in any other selected gear except D4-5, there will be no engine braking, thus you will not be able to "drive" the engine from coasting. Also, if the engine is not running at all, then you would not be able to have any engine braking either, as there would be no fluid coupling in the torque converter, nor would there be any fluid pressure from the pump, since the pump is driven by the engine.

Here is how the DFCO operates, and it is a VERY short lived operation: When you remove your foot from the accelerator pedal, the ECU will cut off fuel to all of the injectors. when engine RPM has reduced down to an idle speed (i.e. 800 RMP or so) then the ECU will begin firing the injectors to prevent the engine from stalling. While this does not seem like a very long time (Which it isn't) doing this over a long drive does improve fuel economy by a small amount over NOT using DFCO. The DFCO has been around for YEARS. Toyota, as well as GM and others have been using this program since before 2000. My 1997 Tacoma has it, but mine is a manual transmission, and does things slightly different.

There are certain requirements for the DFCO to operate. For one, your engine has to be at normal operating temperature, so it will not engage first thing in the morning or evening after work. On a manual transmission, SOME vehicles will not engage the DFCO unless your engine was above 1800 RPM when you removed your foot from the pedal. This prevents the engine from stalling out, as the engine drops rapidly in RPM in a manual trans.

On all vehicles if you return your foot to the gas pedal at any point, the ECU turns the injectors back on, and in an automatic transmission, if you shift the shifter into neutral, it will not engage.

 

I would imagine if your costing in gear you could say it is stalled, your mpg would read infinite or 99.99999 mpg. thats how it is in a diesel any ways.

 

i pick up speed on a very slight downhill, so i would guess it's still getting fuel. back in my days of driving a manual truck, i'd still slow down unless it's a very steep downgrade if i had the clutch in.

 

Gravity can still help you, even on a slight grade. Again, the OP was being specific about automatic transmission models. The manual trans models work slightly differently, but the end result is pretty much the same.

 

Well duh! That is what I said! I just said it with too much technical detail that's all.

 

I posted this awhile back while discussing DFCO


http://www.autoshop101.com/forms/h43.pdf


cutoffgraph by worthywads, on Flickr


fuelcutofftext by worthywads, on Flickr

No poll necessary, it is fact.

 

My 2012 stick shift Yaris has DFCO. The engine rpms just need to be above idle speed (around 1,400) for it to kick in. And there's about a two second delay before you can feel it going into DFCO mode. It's a great gas saving feature.

 

Strange how people continue to vote no to something that is fact?

 

I use SCT Advantage software to tune on my Lightning and Mustangs. The injectors will shut of until the computer sees drop below a certain RPM, usually no less than 300, and and also uses a few other functions to determin this. It is called Coasting_Fuel_Shut-off in the tune.

 

I voted that fuel is needed...but this was before reading the above posts. I was unaware of all this technical stuff that an ECU can do. In my opinion even though the ECU goes into this DFCO....it only does it for the time it takes for the RPM's to drop to idle. The poll is flawed, in a sense both sides are correct, just to what extent....when coasting downhill...fuel is still used...just after a target rpm is reached. Am I wrong in this reasoning?

 

DFCO (Deceleration Fuel CutOff) is indeed a reality. And in manual transmissions it enables in all gears, so using third is a good idea because it enables DFCO while keeping the engine above idle speed.

 

Is this in the 2012+ tacos or no?

 

Rocket science...

 

If you have a Scan Gauge set it to GPH (gallons per hour) when you let off the gas completely to coast you can see it drop down to around .4 gph. About where it sits at idle. On level road at highway speeds I usually see around 3.8 to 4.5 gph. Put your foot in and you'll hit 10 to 13.

The keep MPHs highs, the GPHs low and you'll get good MPGs.

 

The scangauge does NOT display fuel use correctly when coasting.

If you display the Closed/Open Loop function you will see it go into OPEN LP when you are in deceleration fuel cut off, but it won't show 0.0 gph. Don't know why scangauge can't get this right but it will even show 0.1gph with the engine off?