Anti-Stall Feature?

[roldan]

I was driving around in traffic the other day and noticed that when coasting in gear, the instantaneous fuel consumption meter stays at 99.9 mpg until you get below ~1,400rpm, when it starts to go up, meaning that the car must be giving the engine some gas.

I figured I'd see how far it goes and let the revs keep dropping -- turns out the car will maintain idle revs without any intervention from the driver, keeping you coasting along at a walking pace in 1st or a bit faster in 2nd. Wild!

Even wilder: try starting in 1st without touching the gas at all. Just let out the clutch really slowly and let the car keep the revs up by itself. It'll move along at a walking pace and you never have to touch the gas even a little bit. Whaaaaaaat?!! (Note: I did all this on a level surface, I bet that doing it on a hill would still make you stall so I wouldn't try it.)

Is this a standard feature on all new(er) cars, or is this a bit of BMW wackiness?

On a side note, there's too much traffic on my commute.. can you tell?

[jeremyc74]

I would guess that this wasn't done intentionally for this purpose. I'm thinking they just have the PID loop that controls idle speed set up so that it happens to be strong enough to keep the car running when it's in gear, or if you pull out slowly. This is probably more of a by-product than a feature.

[imported_mega]

all fuel injected cars with an idle control valve will try to stay at idle rpm, regardless what your doing

there's only so far that valve can open though, I'm not sure how the idle is controlled on the n54, the main butterfly, or another idle valve

[neteagle]

My 97 GTI did the same thing...one of those great side benefits of having a car with self-adjusting idle speed

[lannypoffo]

it also has a similar auto stall feature where it holds the car still for 2-3 seconds if you are taking off on an uphill. so you dont roll back.

[mistermojorizin]

Originally Posted by roldan

Even wilder: try starting in 1st without touching the gas at all. Just let out the clutch really slowly and let the car keep the revs up by itself. It'll move along at a walking pace and you never have to touch the gas even a little bit. Whaaaaaaat?!! (Note: I did all this on a level surface, I bet that doing it on a hill would still make you stall so I wouldn't try it.)

it was the same on my g35.

[Gray135i]

All cars' engines are this way. If they weren't, the car would just stall when you pushed in the clutch. The engine will maintain idle RPM until it's turned off (as a minimum). The main difference is that you're driving a car with a good amount of low end torque that will still pull at idle RPM. In a car with less power, the load on the engine will overcome the engine's ability to maintain idle and will stall.
What is really fun to try is drive a car with 500+ ft/lbs of torque at 1400 RPM. With a good overdrive 6th gear, the car will drive down the highway with the engine just idling.

[neteagle]

Originally Posted by Gray135i

All cars' engines are this way. If they weren't, the car would just stall when you pushed in the clutch. The engine will maintain idle RPM until it's turned off (as a minimum). The main difference is that you're driving a car with a good amount of low end torque that will still pull at idle RPM. In a car with less power, the load on the engine will overcome the engine's ability to maintain idle and will stall.
What is really fun to try is drive a car with 500+ ft/lbs of torque at 1400 RPM. With a good overdrive 6th gear, the car will drive down the highway with the engine just idling.

What you are saying is incorrect...cars with a fixed idle adjustment will not act the same way...they will stall if you put load on them without increasing throttle...

[imported_mega]

maybe he's young, all cars after what, 1985? will have fuel injection and an idle control valve of some type

[amurph182]

I've been driving manual transmission cars for 17 years now, and every single one of them has done the same thing.

[imported_mega]

never drove a car with a carburetor ?

They do NOT adjust air/fuel based on idle load, and have no idle control valve

most old cars, idle is set by a flat head screw on the side of the carb

[roldan]

Originally Posted by Gray135i

All cars' engines are this way. If they weren't, the car would just stall when you pushed in the clutch. The engine will maintain idle RPM until it's turned off (as a minimum). The main difference is that you're driving a car with a good amount of low end torque that will still pull at idle RPM. In a car with less power, the load on the engine will overcome the engine's ability to maintain idle and will stall.
What is really fun to try is drive a car with 500+ ft/lbs of torque at 1400 RPM. With a good overdrive 6th gear, the car will drive down the highway with the engine just idling.

Ah, that's a satisfying explanation - I guess my conceptual model for when idle came into play was forgetting that the idle control system doesn't care whether or not the car is moving or the clutch is in our out.

So moral of the story is torque FTW, I guess. :thumbup:

[Bruck]

my 01 s4 had a lot of torque, I could start from a light with NO gas at all, in fact when I was first learning stick on it, I used that as a "control" .

The release of the clutch has to be VERY gentle to pull that off, once I could engage first with no gas, I just shifted the equation so that the gas I gave it helped. .... pretty soon my starts were extra smooth.

[R32]

Originally Posted by Gray135i

What is really fun to try is drive a car with 500+ ft/lbs of torque at 1400 RPM. With a good overdrive 6th gear, the car will drive down the highway with the engine just idling.

which is why a new corvette will get 28 mpg on the highway-- a uselessly high 6th gear, something like 1300 rpm at 60 mph!

[Sherifftruman]

I love a high 6th gear. IT is a good way to get decent mileage, but have a performance car in all the othe rgears still.

I can also do the no gas takeoff in my MINI which has almost no low end torque.

[akak1997]

this is perfect for really bad traffic. I've do that all the time on flat roads. Just make sure you aren't on a slight incline, or you'll stall, I've tried it :P
Also, I learn how to drive stick using that No Gas start thing. let you get a good feel on how to let go of the clutch.

[esses]

Fuel injection and engine control units are to thank for our variable idle. I bet you could chirp the wheels without throttle if you quick slipped the clutch right after a cold start (but I don't recommend it). I've seen someone leave one foot of rubber by stalling a Viper :biggrin:.

Originally Posted by lannypoffo

it also has a similar auto stall feature where it holds the car still for 2-3 seconds if you are taking off on an uphill. so you dont roll back.

There is enough low end torque in this car to use the clutch alone to stop rollback when stopped on an incline. I don't like the anti-rollback because it requires that I be in gear and braking to engage, which is a pain when there are multiple lights on an incline. This is based on my driving across San Francisco, where virtually every single N/S light is an incline, 2-3 days a week.

[InquisitiveA]

My 123d Coupe will pull up hill in 2nd with no gas. It'll reverse up hill as well with no gas, just a little clutch control.

[Techie]

all cars have an Idle AirControl Actuator known as IAC Valve this valve is controlled by the ECU or car's computer. basically what it does is opening a bypass air intake to maintain idle revs.

order wise when being idle and starting the air conditioner or any torque change would stall the motor.

of course BMW cars have lots of torque enough to startup the car without pressing the gas pedal.

by the way the full torque /Horsepoer is not used at all speeds. so @1400 revs you could be getting 30 % of full torque.


in premium cars that's a lot of HP..