Isn't it more efficient coasting in gear/engine on?

[cRiPpLe_rOoStEr]

Originally Posted by Arragonis

maybe I misunderstood but still confused - engine off coasting seems a reasonable idea for carb-fed cars. They won't have DFCO so that would be more efficient in all cases, would it not ?

English is not my native language. So, what's that DFCO?

And engine off-coasting would really increase efficiency in all cases, but the more accurate management from an EFI increases the savings.

[Arragonis]

DFCO in my part of the world is Deceleration Fuel Cut Off - basically the FI system determines that the driver has the throttle closed and the engine is being turned over by the car's travel and it shuts off the fuel injection and ignition.

There may be a lower RPM limit - e.g. my engine won't go into it under 1400 RPM (from my experiments - it may be different) but once in that mode I can go slower and the engine won't add fuel until the engine speed us under the idle speed ~ 870 rpm.

Carbs don't - they just spray in whatever based on the air flow - SU carbs are the most efficient for FE as far as I know due to the rising piston design - not perfect by any means though.

EDIT - to answer your question better - assume you are descending a hill and in top gear you can coast even with engine braking. The engine is being turned by the car so the Fi system shuts off fuel and you get infinity MPG and your car's systems (PAS, ABS, EBD etc.) are still powered.

[radioranger]

How about shutting off the ignition thus shutting off the fuel supply and opening the throttle full to reduce engine braking, , leave in gear so in emergency you just back off throttle to slow down, admittedly sounds complex but just in theory , ?

[bestclimb]

Originally Posted by radioranger

How about shutting off the ignition thus shutting off the fuel supply and opening the throttle full to reduce engine braking, , leave in gear so in emergency you just back off throttle to slow down, admittedly sounds complex but just in theory , ?

I have tried this when I had a kill switch on my Honda, I felt no change in declaration as I opened or closed the throttle. (In a car with a carb you would still be pumping fuel).

5th gear (or what ever the highest gear is) does not provide much braking if you need more down shift or touch the brakes. If you need less coast engine off or engine idling (is still better than any form of braking). If you need some where between top gear engine braking and coasting down just feather the brakes it is not that hard to figure out.

[ecomodded]

The Fuel cut off that may happen from coasting in gear would occur so infrequently in my car that that technique would waste gobs of fuel every time i drove. I live in a mountainous region and yet my car never will coast downhill in 5th gear above 2000 rpm, Most of the time i drive at 1000 rpm meaning the fuel cut off will rarely happen in a unlimited amount of miles of driving, for me in my car.
In gear coasting sucks the distance out of the coast, My estimate is 10-15% less coast distance with the car in gear, meaning the throttle needs to be engaged that much earlier.

I suspect the technique works best in automatic transmission cars, what choice do you have with a auto,in most cases.

I have been unable to detect the RPM at which the fuel cut off occurs on my car.

[Arragonis]

In my Fabia (1.9 130 PD ASZ engine) the engine would go to "infinity mpg" on the overrun at any RPM over 1000. Under 900 or so it would sometimes jerk because the ECU would switch to the idle setting.

As for the answer to the PO - I still say it depends on the cicumstances - road, car, temp, traffic etc. Don't stick to one technique rigidly. On steep declines use DFCO, on lower declines or the flat use neutral to coast further. If its wildly busy traffic maybe neither is a good idea.

[ecomodded]

Originally Posted by Arragonis

In my Fabia (1.9 130 PD ASZ engine) the engine would go to "infinity mpg" on the overrun at any RPM over 1000. Under 900 or so it would sometimes jerk because the ECU would switch to the idle setting.

That's good news! There is a good chance my AGR is similar. I like the sounds of that !

[bestclimb]

The techneque works best when you need to slow down, it's engine braking, that conveniently shuts down fuel while decelerating. If used as braking it helps, unnecessary braking (of any form) is detrimental to good economy.

[cRiPpLe_rOoStEr]

Originally Posted by ecomodded

There is a good chance my AGR is similar. I like the sounds of that !

You might know engine-braking is not so great in Diesels. No wonder trucks are usually fitted with those exhaust-brakes...

[Arragonis]

Depends on the gearing - 6th in the Fabia is about 34 mph / 1000 rpm so slowing in that gear made very little difference and was enough to go to 199 MPG on the on board and 999.99 on the SG2.