I didn’t know this about fuel injected engines until reading this thread, but it makes perfect sense. With the crankshaft already being turned by gravity, the engine should be able start at any point in the cycle. I don’t think a carbuerated engine could do that though, you wouldn’t get a good fuel air mix right away.
"Depending on conditions the injectors might be off for a fraction of a second or for minutes (coasting down a long hill)’
Almost had me until this. You are saying the engine stops operating for minutes at a time in a way imperceptible to the driver and without affecting any ancilliary systems driven by the engine?
I’d like to believe you guys, I really would
It is operating. It is being turned by the transmission, which is turned by the wheels, which are turning because gravity is pulling the car downhill. All the accessories are still operating. The only thing missing is the pointless injection of fuel into the cylinders. It doesn’t to provide a spark either, though I don’t know if that is deactivated also.
I think it was a reasonable question, and I’m not the only one who had it; when I was given a third-hand guess by a magazine columnist as a cite, it was reasonable to ask for something more valid. I am convinced; thanks to all for the more verifiable posts.
No, I’ve never heard of this before. I don’t know why, as I have been a well-informed gearhead since I was about six years old, back when dinosaurs roamed the highways. My hands-on expertise ranges from 7+ liter carbureted engines to 18,000 RPM V-12s, including custom chipping and curving of modern engines. Yes, I drive; one of those 7-liter, 500 HP dinosaurs is in my favorite 2200-pound chassis. Good thing I own a Nomex suit for flame jobs such as the responses here.
I would think this is horrifically dangerous. Much like riding a 2 ton sled (with brakes) down an icy hill. If she starts to spin or slide, she will hit the gas which does nothing. If she’s in gear, she may be able to steer her way out of the slide. Plus she has the added benefit of engine braking on the way down.
Sure, if she’s a complete idiot.
Listen, I shift into neutral now and again on down slopes, even in snow! So what. If you’re a confident driver you will have no issues doing this. She’s obviously very familiar with the slope and her car’s abilities.
If things start to get dicey, you slip the transmission back into drive, or low or whatever. It’s not, and has never been, any kind of an issue for me.
::: Thud::: <–sound of Rick beating his head against a brick wall.
Look the car is in gear. The car is going downhill gravity is pulling the car down. The drive wheels turn. The drive wheels drive the drive train, turn the transmission, which inturn drives the engine at a speed above idle. No fuel required. Gravity is doing the work. Gravity is the law and around here we obey the law.
Or think of it this way. Did you ever ride a bike as a kid? You did? Good. Pedaling is fuel being injected into the engine. You go up a hill you have to pedal (inject fuel), right?
But when you crest the hill and coast you stopped pedaling (fuel stops bring injected into the engine)*
Now when you stopped pedaling did you bike come to a sudden stop just over the top of the hill and stall, or did you coast all the way down the hill enjoying the wind in you face until it was time to start pedaling(injecting fuel) again?
Please tell me you understand my brain is starting to hurt.
*Normal bike fixies don’t count.
You.
Sorry to sound like a jerk, but there’s no such thing as “no risk.” Battery failure, mechanical failure, computer failure - any of these could send you screaming to your death off a cliff.
I work with these things (IT, not cars), I’d rather trust my own limited brain and reflexes. At least when I die in a fiery wreck I’ll know I’m the one who screwed up.
You’ll be going faster at the bottom of the hill if you coast down the hill in neutral.
Depending on the situation, coasting downhill in an auto with the engine on and the transmission in neutral CAN save gas.
I’ve verified this with my OBD2 scanner, but I don’t routinely do this nowadays.
What situation is there where an engine burning gas while in neutral uses less gas than one that doesn’t burn any gas while in drive, when both are coasting downhill?
It doesn’t save gas during the coast.
It saves gas because you’re banking momentum that you can leverage to use less gas during the remainder of your trip.
That’s what I thought you meant.
A carb doesn’t use sensors and is not computer-controlled, and wouldn’t shut off the fuel completely while there is airflow.
A better comparison would be CFI (central fuel injection), which was the early inexpensive approach to electronic fuel injection. This design injects fuel in to a “throttle body” which essentially replaces the carb. CFI designs need to compensate for the thickness of the fuel layer in the intake manifold because it acts as a variable reservoir for fuel. Even if the injectors stopped completely, this fuel layer would provide fuel until it evaporated completely. So, while it might be possible to shut off fuel completely, it wouldn’t happen quickly, and when demand comes back, it would take a while to build it back up and it’s just possible that you couldn’t run as rich as you’d like (i.e., reduced power until the layer reached a nominal thickness). I don’t remember whether an engine ever runs on maximally rich air and would suffer from this.
But only if you go faster than if you were engine breaking (i.e., not using your breaks). If you’re sticking to the speed limit in both cases, engine-breaking wins.
:smack:
Brake. It’s spelled B-R-A-K-E. Unless you’re talking about shattering your engine to somehow decelerate the vehicle.
Having a steep , sometimes icy driveway and a Subaru AWD I have had times to play around to see what works best. What I have found is it really doesn’t make much difference N vs D but in certain rare conditions N may have a advantage as it is easier to control the car with brake force alone w/o the engine contributing. motive force (in either direction).
I have also found and sometimes use the ABS method of decent. Slamming on the brakes engages ABS on all 4 wheels which sometimes gives a much more controlled decent, ensures traction, & at a much lower speed.
Peace
When you start up something like that you’ll get some kickback from the engine, it won’t just start matching the coasting speed of the engine as the cylinders start firing again. That’s at a minimum, you could get some heavy vibration, backfire, pre- and post-ignition too. I doubt any non-cylinder injected engine would ever cut fuel flow while coasting. I used to do things like that in cars, cutting the ignition while going downhill, you can definitely feel it when you restore it.
This may be at a tangent, but one of my cars is a Honda with a iVTEC engine. The only way I can tell that it is running on 6 cylinders or 3 is by the ECO light on the sah going on and off - and I have done my best to try and detect any bump, vibration, surge, anything over many thousands of miles of driving. It’s as nearly silent and invisible a switch as I can imagine.
I didn’t think much about this subject until this thread. But yeah, modern engines should be able to just start firing anywhere in the cycle. That one has variable timing on both intake and exhaust valves I think, so it can probably relieve cylinder pressure for idling cylinders also.
Now I’m wondering how the instant start works in the hybrids. Those need to bring the engine up speed to start, though I assume their decoupled from the transmission until they get started. I wonder how much gas we could save if regular gasoline engines could shut off while idling.