Well, no, not really. It’s one PopMech column writer’s opinion, which is then cited on every car-tech forum in existence.
I can think of many good reasons not to let an engine “shut down the fire” while it’s nominally powering the vehicle (again, those vehicles that shut off the engine while at a standstill being excepted). I believe that this guy saw what he saw, but I can’t find another cite from a reliable source that says cars shut off all fuel flow in those conditions. I also believe I would have heard of this function/practice before now.
I can’t believe I have to argue about this. You’re wrong. It is super dooper well known amongst everyone who know about cars that modern fuel injected engines will shut off fuel flow when above a certain engine speed and on zero throttle. NO ONE IS SAYING IDLING CARS HAVE NO FUEL FLOW. THAT WOULD MAKE NO SENSE.
But I have to ask - do you even drive? You can FEEL the fuel cut off, if you have the slightest sympathy with the car. In fact, EVEN WITH AUTOMATICS going through a slushbox, never mind the manuals I usually drive, the transition between fuel on/off on most cars is definitely the jerkiest part of my astonishingly smooth road driving, simply because it’s the only bit I have no control over smoothing.
Simple Linctus is correct here. Any modern fuel injected engine will shut off fuel flow while off-throttle and in-gear, above certain RPM threshold. This has been addressed here many times, a quick search turned up this thread and in particular, this post by Rick, our resident guy in the trenches…
[QUOTE=Rick]
All modern electronic fuel injection systems that I am aware of (and that includes a very large percentage of the market here in the US) shut off fuel on deceleration. The reason for this is NOT fuel savings, it is for emissions. Cars nowadays have to meet some very serious emission requirements. Leaving fuel injectors on during deceleration gives no advantage.* They also have a very serious disadvantage in that if you leave the injectors on you then have to deal with unburnt hydrocarbons, and CO.
This is why Bosch K-Jetronic fuel injection went the way of the dodo bird. The K stood for Konstant, this system just pissed fuel into the engine if there was air coming in. Because it sprayed fuel all the time air was flowing into the engine, it was too dirty to meet newer emission requirements.
As far as the details go, typically the system will look at throttle position (duh!) engine temp, and possibly air temp. If the throttle is closed above say 2,000 RPM the ECU stops activating the injectors until a preset RPM is reached, and then they are turned back on. This cut in RPM might be about 1300 RPM warm, and somewhat higher cold, say 1800 RPM. If the driver steps on the gas, the injectors resume right away, regardless of engine speed. This off and on function of the fuel injectors is imperceptible to the driver, but can be demonstrated in a shop using shop tools. I have shown this to my students hundreds of times.
I am not sure of the first system that used fuel cut, but I know Volvo has used it since mid 1982.
[/QUOTE]
OK. Dumb question. Why wouldn’t the engine stall then? with no fuel shouldn’t it just completely crap out.
In the old days I’ve had carbureted engines stall on me for various reasons at highway speed, and the results were immediately noticeable!
I’m not doubting you. I’m just trying to figure out how an engine with zero fuel intake manages to keep going until the next feed of gas to the injectors.
You’re in gear, rolling along at a good clip, maybe even downhill. The engine has to keep turning because of the connection to the wheels. The difference between the fuel cutoff and your experience is that you had more or less a sudden flame out. The programmed cut off is subtle. Imagine your rolling down a hill, and giving it less and less throttle. At some point, you lift off the gas completely. At that point, the ECU dials it back from idle throttle to zero throttle in a smooth way that I can only describe as, “computers, man.”
Referring to engine braking causing wheels to lock on a slippery surface:
I think “lock” might be the wrong term. On a sufficiently slippery surface engine braking may cause the drive wheels to spin slower than they should given the forward speed of the car. Not so much locking, as skidding, perhaps. I’ve encountered this with a manual transmission when down shifting descending an icy slope and not properly matching the engine speed to the wheel speed. I’ve also encountered it in automatics when forcing a down shift on an icy slope.
Yes that’s a fair point. Getting away from 4x4s, I’ve actually been in a TVR Griffith in a wet road as a passenger where someone who will remain anonymous who didn’t know how to drive changed down a couple of gears without rev matching (i.e. just moving the box from 4th to 2nd or similar without blipping the throttle and just releasing the clutch to bring the engine up to speed - something perfectly fine to do with a a normal crap car but suicidal in those circumstances). Immediate skid she saved through pure luck.
That is why I recommended stopping at the top of the hill!
Which is why the concern expressed about the wheels, even though I now understand when they said locking they meant nothing of the sort, is incorrect. In extreme circumstances like that you really really really have to drive super dooper carefully.
You don’t need an AME or ANSI, just hook your diagnostic tool up to the car and go for a coast in gear, what’s the reading on the fuel injector pulse? It’s fucking zero.
What is the goal here? Even if this practice doesn’t do any damage (probably) what advantage is there to doing it?
I’d say you’re right, and I’d advise her to keep the car in gear for better control. But I don’t live with her, so I can mouth off about it all I want and never suffer the consequences.
You’re wasting fuel while possibly putting yourself in an unsafe situation. I also waste fuel and put myself in an unsafe situation by driving faster than I should so I’m not going to judge you on that basis but what personal pleasure do you derive from coasting down hills in neutral?
Something to do. My last vehicle had a mileage computer and I used to like popping it into neutral and seeing my gas mileage drop to almost zero MPG. I downshift from time to time at traffic lights and upshift pulling away. It’s fun. Driving can get very tedious.
What he said. Think of how when you have to push-start a car, when the wheels and engine are turning, you can bump the ignition and it easily starts right up.
The engine doesn’t stall because it’s already turning and will continue to do so while it’s in gear and the wheels are still turning from the car’s inertia. With the engine turning, resuming fuel/ignition is relatively effortless compared to starting an engine from a dead stop.
Didn’t read the quote from me in post #24 did you?
Why doesn’t the engine stall? The simple and easy answer is that the injectors are turned back on before the engine stalls.
Depending on conditions the injectors might be off for a fraction of a second or for minutes (coasting down a long hill)
BTW I was a technical instructor for a car company for 15 years. I taught this stuff. Good enough cite?
