posted
Driving to work today (no beams today, hence my Hatracking...) I noticed a curious behaviour. As I approach a stop, I like to push the clutch in, thus disconnecting my engine. This generally means the RPM will hover around 1000. But then, as I come to a complete stop, the RPM will drop to 750. What causes this? Is there perchance some computer that detects the full stop, and drops the engine RPM to give a better torque for starting up again?
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
posted
But I'm often applying the brake for quite a while, and see a distinct drop the moment I come to an actual stop. It's almost as though the engine is experiencing the same little jolt that I do, from the car leaning forward on its axles due to decelerating, and then swaying back.
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
posted
Years ago, carburetors used to have something called a "dashpot" which prevented the throttle from snapping closed instantly when the driver releases throttle pressure. The reason for this is that if the engine is still spinning fast, closing the throttle too fast will cause a vacuum spike, which will cause a rich mixture and hence higher CO emissions and lowered fuel mileage. The dashpot allowed the throttle to close maybe 85% on throttle release and then it took maybe five seconds to close completely.
With modern fuel injection, I think they've dispensed with the dashpot, but I'm willing to bet that the fuel injection system tries to soften the effect of a sudden throttle closure, which in turn would delay a return to low idle. I doubt that the computer detects the full stop so much as it merely delays the full closure of the off-idle openings in the throttle body. The timing may be merely coincidental. You can test this by disengaging the clutch on long (but not too steep) downhill.
There may actually be a fuel cost associated with using the clutch to disconnect the engine from the drive train during trailing throttle. As I recall, the fuel injection systems (of late 1980's Mazdas, anyway) detected a high vacuum combined with a closed throttle position, and turned of the fuel injectors during this condition. At this point, the engine simply acts as an air pump. By disengaging the clutch and allowing RPM to fall, the computer doesn't see the high vacuum and instead tries to put the engine in an idle state, which requires the fuel injectors to fire.
Posts: 3735 | Registered: Mar 2002
| IP: Logged |
posted
I don't know the answer to that. I've got a couple questions, though, since you brought up coming to a stop in a standard...
Do you still downshift? Meaning - do you get to second and then push the clutch in?
and
Do you leave it in gear with the clutch in until you stop or do you just pop it in to neutral and then put in in first when you start to go?
Posts: 1355 | Registered: Jul 2006
| IP: Logged |
quote:With modern fuel injection, I think they've dispensed with the dashpot, but I'm willing to bet that the fuel injection system tries to soften the effect of a sudden throttle closure, which in turn would delay a return to low idle. I doubt that the computer detects the full stop so much as it merely delays the full closure of the off-idle openings in the throttle body. The timing may be merely coincidental.
I am reasonably convinced this is not the case. Here is how I stop: On seeing a red light ahead, I take my foot off the gas. This produces a slow decline of the speed and RPS, from the roughly 2k of cruising to, say, 1500. When I'm convinced that I will be either stopping or shifting down, I pop the clutch in. The RPS then drop quite rapidly to 1000. Approaching the stop, I then usually need to put the brake in; the RPS remains constant at 1000, often over quite a long distance. Only when I come to a full stop does it drop to 750. There is quite a marked change; it doesn't drop gradually at all.
I believe the car does have ABS, yes.
I don't generally downshift when stopping; put the clutch in, then change to whatever gear is good for beginning to accelerate again.
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
posted
I know it's very common (if not standard) for the computer to drop the engine speed by a couple hundred RPM when the car is stopped, though I don't know what the reason is. As soon as you take your foot off the brake, the engine speed should go up a bit. At least, it does in automatics. I'm not sure exactly how it works in manuals.
Posts: 9945 | Registered: Sep 2002
| IP: Logged |
posted
I'd just like to confirm that this happens in my Honda Accord, too. It's manual transmission, and I always put the clutch in any time I brake--just out of habit (Is there a good reason not to do this?)
I also notice the dropoff in RPMs as soon as I've come to a complete stop. Sometimes it drops so quick that it seems like it would stall. It does seem odd that it would be any different while it's moving or while it's stopped, even though the clutch is in for both.
Posts: 1314 | Registered: Jan 2006
| IP: Logged |
quote:Originally posted by Launchywiggin: I'd just like to confirm that this happens in my Honda Accord, too. It's manual transmission, and I always put the clutch in any time I brake--just out of habit (Is there a good reason not to do this?)
If you push your clutch in you are not using your engine to help you stop...which can put more wear on your breaks...and can also make it harder to stop.
When I first learned to drive a manual, I was taught that it was best to downshift as you slowed (just as you shift up as you speed up).
But I have also heard unless you are very good at downshifting smoothly, this can put a lot more wear on your clutch and can cause problems.
Of course by the time you are all the way stopped you would push the clutch in...though if I was going to sit at a stop light I would switch to neutral.
Posts: 1901 | Registered: May 2004
| IP: Logged |
posted
Your problem is that you spend too much time watching the RPS. I have no idea what my car idles at, and I'm perfectly happy Posts: 3950 | Registered: Mar 2006
| IP: Logged |
The computer may be picking up a signal from the ABS that tells when the wheels actually stop moving. The ABS information is being used increasingly to find out interesting bits of information, such as whether a tire is low on air.
It still seems like an odd input though, since idle speed has nothing to do with whether the car is moving or not.
quote:As soon as you take your foot off the brake, the engine speed should go up a bit.
This makes perfect sense, of course, since the programmers anticipate that pulling your foot off the brake may coincide with starting to release the clutch to begin acceleration. But it's also got an unambiguous input from the brake pedal. Aside from the ABS, there's no signal anywhere else in the car that could detect the moment the car stops, unless they added one just for that purpose, which would be expensive.
Posts: 3735 | Registered: Mar 2002
| IP: Logged |
posted
I'm not sure what ABS or the clutch has to do with anything. I've got an automatic with ABS that drops from 1000 at a moving idle to about 750 at a stop, and an automatic without ABS that goes from 1000 to 800.
Posts: 9945 | Registered: Sep 2002
| IP: Logged |
posted
But with an automatic, it's not clear whether the engine is disengaged at any given time. When you put the clutch in on a stick, you know that the engine is not driving the wheels.
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
quote: When you put the clutch in on a stick, you know that the engine is not driving the wheels
the engine 'drives' everything in the car, not just the wheels. There is a fast idle which is used when the car demands more power for various reasons while moving, and a slow idle for when you are sitting still and all the various pumps and electronics do not need to be fully engaged. The RPMs may change slightly if fans and/or pther pumps need to be turned on to keep the car cool.
Posts: 1918 | Registered: Mar 2005
| IP: Logged |
posted
That's a good point. But why does the car need more power while coasting than while fully stopped?
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
posted
Stronger torque via the-engine-as-a-flywheel effect, and closer matching between engine speed and transmission speed. The assumption being that a moving vehicle might need to be rapidly accelerated, while a stopped vehicle doesn't. There ain't much point in burning extra fuel to sit at a stoplight.
Also take a look at the power band on engines. In any given gear or at any given speed, the rpm producing maximum fuel economy is lower than the rpm producing maximum torque which in turn is lower than the rpm producing maximum horsepower.
quote:Originally posted by Lupus: When I first learned to drive a manual, I was taught that it was best to downshift as you slowed (just as you shift up as you speed up).
But I have also heard unless you are very good at downshifting smoothly, this can put a lot more wear on your clutch and can cause problems.
While I often downshift as I come to a stop, it should be noted that replacing brake pads and components is generally cheaper than replacing a clutch and/or engine parts that may wear faster than normal.
Also, I can't see why always using the clutch while braking would make any difference, just seems to be occasionaly unnecessary.
Posts: 369 | Registered: Apr 2007
| IP: Logged |
posted
I was always taught that you decelerate in gear as a safety precaution, since your vehicle slows (and the engine will eventually turn off) more quickly that way. It was my understanding that this is why, when taking a driving test in a manual car, you're docked points for taking the car out of gear while decelerating.
Posts: 4313 | Registered: Sep 2004
| IP: Logged |
Printer-friendly view of this topic
![[Smile]](smile.gif)
![[Cool]](cool.gif)
)![[Eek!]](eek.gif)
