Engine braking - Fuel Economy, Hypermiling, EcoModding News and Forum

justjohn · 04-12-2011, 06:17 PM

Saves brakes, but worth the wear and tear on transmission?

Does it increase or decrease fuel economy?

Any other thoughts? (I've heard both answers to both questions. I'm curious to see if anyone can provide a convincing well-reasoned answer to either.) I of course realize answers may depend on the situation somewhat.

Joenavy85 · 04-12-2011, 06:40 PM

the only time i engine brake (by down shifting) is when i have to brake hard for some reason. other than that i sometimes use it to maintain speed going downhill (by simply leaving it in gear and letting off the gas completely), but that is all.

euromodder · 04-13-2011, 08:15 AM

Quote:

Originally Posted by justjohn

Saves brakes, but worth the wear and tear on transmission?

There is no wear and tear on the transmission by using engine braking properly.
It's just lifting your foot off the accelerator.

Many people will downshift frantically early in the process, but that's not the point.
Lift your right foot or cancel the CC.
Shift down when the revs have already dropped down to a point where you're hardly slowing down anymore.
Repeat.

Quote:

Does it increase or decrease fuel economy?

Depends on what you'd do otherwise.

It's more efficient than driving under load longer, then slamming on the brakes.
It's less efficient than coasting (even with the engine on).

justjohn · 04-13-2011, 10:17 AM

Ok that makes sense because the people saying it's hard on the transmission were assuming early downshifting. However, even "gentle" downshifting should put more strain on the transmission than coasting, right? More shifts, each one coming in at a higher rpm than you would want to shift at if you were being gentle with the clutch.

As far as fuel efficiency, the person who told me the opposite of that told me that when engine braking the wheels "drive" the engine, thus using less gas than at idle. Is that just a bunch of baloney then?

Christ · 04-13-2011, 10:23 AM

THE person was referring to dfco, wherein fuel is cut when certain conditions are met. It is not universally applicable.

Its efficiency depends alot on the skill of the operator.

Considering that using gears to decel, or hold speed, case by case, is basically the norm for driving instruction, even in automatics, and these transmissions don't have abnormal wear/failure patterns, Id chance a bet that you're not doing anything harmful, and your friend is probably regurgitating something he heard out of context, in an attempt to sound like he has some knowledge on the subject.

Joenavy85 · 04-13-2011, 10:24 AM

if the engine is below a certain RPM (which varies by vehicle) it will inject fuel regardless of the status of the transmission or throttle. i have yet to find the injector cutoff point on my Jeep, heck, i'm not even sure if it has one.

Christ · 04-13-2011, 10:28 AM

Quote:

Originally Posted by Joenavy85

if the engine is below a certain RPM (which varies by vehicle) it will inject fuel regardless of the status of the transmission or throttle. i have yet to find the injector cutoff point on my Jeep, heck, i'm not even sure if it has one.

Usually 1100-1300rpm.

Joenavy85 · 04-13-2011, 10:30 AM

i've had mine at over 2000, in gear and decelerating, throttle closed, and the injectors were still operating

Christ · 04-13-2011, 10:33 AM

Dfco also depends onfactors like engine temp, on time, etc. It is entirely possible that your Jeep just "no gots".

Joenavy85 · 04-13-2011, 10:39 AM

that's true, like i said "i'm not even sure if it has one" i wired in an indicator LED parallel with one of the injectors (it's fused and i have extensive electrical background to repair if required) and it always stays lit while engine braking , though i haven't done much over 2000 RPM, maybe the ECU only shuts off some of the injectors??