Something about electronic fuel injection that has been bugging my mind...

[PhilosophStein]

So I just remembered something that has bugged my mind in terms of why a clear cut answer is not easily found on the internet - at least one I could find. I have tried searching at various times and not finding anything.

Here goes: Back in high school, I knew a little about internal combustion engines - certainly not as much as I do now after going to college and getting more hands on with working on small engines. One of my friends knew a lot then (and still does) about all sorts of technology including mechanical, electrical, devices, etc. Like he had a knack for knowing how to explain exactly how something that seemed very complicated worked to his friends and others who were puzzled.

One day we were talking about engines, fuel efficiency, etc. and he mentioned that in vehicles with any type of electronic fuel injection (EFI), if you have your foot totally off the throttle, then no fuel is being injected and no sparks are occurring in the cylinders... Your vehicle continues forward due to its momentum and/or it could be going down a hill etc. Basically, any time with EFI that you don't have your foot on throttle, you aren't using gas.... Is this true?

I swear i searched the internet for whether that was true or not but couldn't find anything... I mean it would make sense because all of that can easily be achieved with electronic controls. This contrasts with carbed engines where even if you were coasting down a steep hill with your foot totally off the gas, the engine would still be consuming gas because the idle airflow would still be entering the engine....

 

[CarPlayLB]

Maybe some of that is plausible, and it is a bold statement to say "all EFI" systems act this way.

If you take your foot off the throttle at a stop light, the engine still consumes fuel...needs it to idle. (as long as we are not talking hybrids). The spark would need to continue just for emission reasons alone. There has to be a device to burn any fuel present. I think there is still a small amount of fuel being introduced as the signal from the o2 sensor would be very lean and tell the computer to add fuel...my .02

 

[PhilosophStein]

Just to clarify in my first post: He meant (and I mean) that if your vehicle is already moving, and you then remove your foot totally off the throttle; that it does not consume any gas.

The first reply in this thread prompted me to add clarification.

 

[CarPlayLB]

As to compression braking, though... the computer WILL NOT permit the engine to run overlean (emissions and temperature issues), and a compression-braking engine draws more air than an idling engine, and a compression-braked vehicle loses speed much more rapidly than a coasting vehicle, so it follows that a compression-braked vehicle will consume more fuel per unit distance than a coasting vehicle will regardless of fuel delivery system

I googled this

 

[jeep2003]

that might be true in the newest of vehicles but certainly not for most of them. Perhaps he was meant the direct injected engines? There is so much wizardry that goes on in those. They can run super lean and cut spark because of the high compression. They have many similarities to diesel engines

 

[Inventorpardue]

If you shift the transmission into neutral while rolling, notice the engine does not die. A stock ECU is not allowed to let the engine die. Its not EPA regulations, its DOT: power steering and brakes must always be powered. Having worked for a US automaker, with responsibility for getting a car through EPA and DOT tests, these are not my opinions, just observations.

Any newer vehicle with a fuel mileage computer will register great, even fantastic mileage during a rolling throttle lift, but the fuel consumption is still there. It may be slight... but its not zero. Powering down the ECU will get you zero fuel flow, and you can feel the difference in compression braking.

Mechanical fuel injection is simpler yet. If the engine is turning, there is always at least minimal fuel from the injectors.

Jon

 

[Mike R.]

what you are referring to is fuel cut, this happens as you describe, on a longer decel when foot is off the throttle most vehicles do this.
added to that, we also have assisted engine braking,"trans down shifts" while under fuel cut. with foot on the brakes.
all this happens under certain perameters. pretty comon in vehicles 10yo or less

 

[Mike R.]

the ecu's will resume idle or any throttle input in milliseconds, even shifting to neutral under fuel cut.
tons of neat stuff in vehicles that happen very fast, that's one reason why everything have an eve, even the master window switch.
they talk back and forth very fast
remember, bring you extended warranty, or check book when you have a problem

 

[wjustice]

Typically in todays engines the tgrottle valve can be completely closed but air still enter the engine. Most have some sort of IAC (idle air control) valves. This allows the ecu to control the idle even though the main valve is completely closed. So with main valve closed enough air gets in to prevent an over lean/rich condition and allow smooth throttle action should power be applied.

 

[Norcalputput]

Yes injectors will always pulse the duration of the signal will change but it will still squirt fuel (I'm not talking about variable displacement)

Now I we are talking about a hybrid or Variable displacement some makes and models will shut both intake and exhaust valves kill the fuel driver and ignition and use the air trapped in the cylinder like a spring


Sent from my iPhone using Mores code

 

[Mike R.]

IAC valves, are history. throttle is fly by wire, ecu does control throttle plate
fuel cut is as named, no fuel enters cylinders. valves will have the timing changed by oil controlled cam gears, some have an extra lobe and rocker, which are activated by a oil control solenoid, and as far as hybrids. a detroit manufacture buys hybrid technology from the Japanese company I have been working for for many years. but the technology is pretty cool, until it goes spoof