Coasting up to Lights.

[demarpaint]

Can someone explain how idling at a light the SG reads 9999 LHK, or "infinite". That would mean the car would never run out of gas at a light, ever. I'd be willing to bet it will run out of gas idling at a light. How would the A/C be working if the engine isn't burning fuel [stopped at a light] to turn the belts when the car is not in motion?

Please no flame war, I'm trying to understand this. The engine has to consume fuel to run, coast, idle, or it would shut off wouldn't it, if no fuel was being burnt? How does the AC and the PS work while an engine is coasting and burning fuel, wouldn't you have to turn the engine back on to resume driving if the engine is not burning fuel?

Thanks!

 

[TFB1]

Fuel can be cut completely when coasting in gear because the coupling to the transmission keeps the engine spinning... Engine turning = A/C, alt, P/S, and vacuum operated systems continuing to operate... When the RPM are slowed past a point, the coupling ceases with a automatic so fuel is likely restored at a higher RPM vs with a manual trans... On a manual no doubt fuel is restored when you foot even slightly depresses the clutch, in anticipation of loosing the coupling between the trans and engine...

I have a wide band air fuel ratio gauge on my Marauder... Coasting in gear it's off the scale to lean(no fuel), while idling or coasting out of gear it indicates a 14.7 A/F ratio which is what the PCM strives to maintain for best combustion efficiency...

 

[rationull]

Originally Posted By: demarpaint
Can someone explain how idling at a light the SG reads 9999 LHK, or "infinite". That would mean the car would never run out of gas at a light, ever. I'd be willing to bet it will run out of gas idling at a light. How would the A/C be working if the engine isn't burning fuel [stopped at a light] to turn the belts when the car is not in motion?


Units issue. 9999 ("infinite") liters/100km is burning all the fuel in the world and making no forward progress. It converts to 0 MPG. The fact that the English vs SI units used commonly used for this are inverted with respect to each other does make switching systems in conversation a little confusing

The difference is between a measure of consumption vs a measure of efficiency.

 

[demarpaint]

Ok, so then at idle it has to be burning fuel to run. I can see where coasting it could cut off fuel because the engine being connected to the transmission would keep things spinning. Now if someone decided to coast in N going to a light, then fuel would be needed to stop the engine from stalling. If those statements are correct then I think I've got it. Thanks!

 

[SHOZ]

Coasting Fuel Shut Off happens when the load is very low and the rpms are above 1300 rpm usually while in gear. When in neutral or the clutch is engaged then the fuel idle settings are in place.

 

[cchase]

Originally Posted By: demarpaint
Can someone explain how idling at a light the SG reads 9999 LHK, or "infinite". That would mean the car would never run out of gas at a light, ever.


The key is that the LHK unit is the inverse of an MPG... MPG is distance per fuel, LHK is fuel per distance.

Originally Posted By: Artem

if you are coasting to a definite STOP up ahead, there is no need to coast in neutral while trying to maintain speed (you're going to hit the brakes soon to STOP!) so your statement doesn't make sense to me AT ALL!

Now if you're talking about coasting a bit and then getting back on the gas, coasting 100-200 feet in gear, getting infinite MPGs (as per the Scan Gauge) and then simply reapplying gas (if the light turns green and you can go) is better then;

*Hit the clutch
*take it out of gear
*coast 100-200 feet burning x amount of fuel while the engine is maintaining idle
*reapply the clutch
*put it back into the proper gear
*rev match / ease the clutch to re-engage (however you do it)
*get back on the gas

^seems like more wear and tear on drivetrain components all while burning more fuel.



It was not right of me to quote that part of your post (I did not mean to select that part). My point is that for example, when I'm going down a long, gradual slope my average MPG is higher if I coast in neutral vs coasting in gear and then re-applying gas occasionally.

As long as I rev-match, there's no additional wear and tear.

 

[rationull]

Originally Posted By: demarpaint
Ok, so then at idle it has to be burning fuel to run. I can see where coasting it could cut off fuel because the engine being connected to the transmission would keep things spinning. Now if someone decided to coast in N going to a light, then fuel would be needed to stop the engine from stalling. If those statements are correct then I think I've got it. Thanks!


Exactly! The energy to turn the engine has to be coming from somewhere -- either from fuel or from the car's kinetic energy rolling down the road.

 

[demarpaint]

What happens when a newer car stalls, coasting in gear? Does it lose the power assist of steering, A/C, and vacuum assisted brakes? Older cars do, I'm wondering what newer cars do?

 

[Kestas]

When a car stalls, the brakes have vacuum assist for roughly one application. After that, you better have a strong foot!

 

[yonyon]

That varies. With many dual diaphragm boosters there's usually enough vacuum to provide a decent amount of assist three times. Some small diameter single diaphragm boosters barely give adequate assist with a vacuum source present.

 

[rationull]

Originally Posted By: demarpaint
What happens when a newer car stalls, coasting in gear? Does it lose the power assist of steering, A/C, and vacuum assisted brakes? Older cars do, I'm wondering what newer cars do?


Depends on what you mean. If you're actually coasting in gear (i.e. in gear and moving) then the engine isn't really stalled, it's just not powering itself with gas. You should still have vacuum and your belt driven and electrical accessories would still be getting power (assuming the key is in the right position to let the electrical system stay on). Point is the engine is still "running", it's just bleeding off energy instead of adding more to the system.

Generally the fuel cutoff only lasts until somewhere around 1200 RPM in order to avoid the engine stalling when stopping and disengaging the clutch. If you actually stall (i.e. stop the engine) then you'll lose vacuum and belt driven accessories like you would in any car.

 

[demarpaint]

Originally Posted By: rationull
Originally Posted By: demarpaint
What happens when a newer car stalls, coasting in gear? Does it lose the power assist of steering, A/C, and vacuum assisted brakes? Older cars do, I'm wondering what newer cars do?


Depends on what you mean. If you're actually coasting in gear (i.e. in gear and moving) then the engine isn't really stalled, it's just not powering itself with gas. You should still have vacuum and your belt driven and electrical accessories would still be getting power (assuming the key is in the right position to let the electrical system stay on). Point is the engine is still "running", it's just bleeding off energy instead of adding more to the system.

Generally the fuel cutoff only lasts until somewhere around 1200 RPM in order to avoid the engine stalling when stopping and disengaging the clutch. If you actually stall (i.e. stop the engine) then you'll lose vacuum and belt driven accessories like you would in any car.


I got it now thanks!! The 1200 rpm statement got me to totally understanding it.