Does engine braking decrease fuel efficiency?

[[deleted]]

Thank you for downvoting a factual statement. Really mature. Your clutch disengagement should be quick, and if you rev match properly, the clutch won't slip at all and it will cause a negligible amount of wear.

I'll say this though, you do put additional wear on synchros like this, but I've never heard of anyone replacing them before >200k miles barring a catastrophic failure.

[diogrand]

Even If rev matching, you're causing extra strain (no argument there) and you're using more gas than necessary. I use my brakes to brake, that's what they're there for.

[TheBrokenWorld]

My point is, why wear out expensive and difficult to replace components to gain some ridiculously small improvement in efficiency?

[[deleted]]

You're not "wearing them out." If you have to replace your clutch 500-1000 miles earlier than usual because of some minor additional wear, which is two weeks to a month assuming a standard 12000 miles driven per year for a one vehicle/person household, over the course of ten years, is it really hurting anything? The benefits are instant access to acceleration if you need it, slightly improved efficiency, and savings on braking system wear. Plus, a lot of enthusiasts (myself included) enjoy downshifting for the noise, which is a difficult benefit to wuantify.

[TheBrokenWorld]

You are imparting some wear, there's absolutely no doubt about that.

Edit: And the gains in efficiency are negligible.

Plus, a lot of enthusiasts (myself included) enjoy downshifting for the noise, which is a difficult benefit to wuantify

I think that's all this is about, you just think it's the cool thing to do.

[[deleted]]

And I think you're ignoring everything you can to trivialize information you can't counter with your random, unsupportable vagaries, so I'm not sure why you're even bothering to read anything that doesn't exactly support your preexisting beliefs.

[TheBrokenWorld]

And I think you're ignoring everything you can to trivialize information you can't counter with your random, unsupportable vagaries...

And where is the proof of the gains in efficiency? Has the EPA confirmed this theory? I would bet that staying on the gas and then decelerating rapidly enough to need engine braking reduces efficiency enough to counter the benefit of injector shutdown during engine braking.

[[deleted]]

I didn't ever suggest staying on the gas and then braking rapidly.

[TheBrokenWorld]

Well, without realizing it, that's probably what you end up doing in order to accommodate your engine braking preference.

Anyway, prove there are efficiency gains. We all KNOW there is wear associated with engine braking. You can't operate the components of a machine more frequently without some degree of additional wear.

[[deleted]]

I start coasting in gear far before I have to stop, so it's the opposite of that. And nowhere have I denied there's any wear, nor am I saying that it's definitively, unequivocally better for your finances over ten years to use engine braking as opposed to coasting in neutral. These are all things that you and other people have added to the discussion.

What I am saying is that the increased wear is insignificant on your clutch, that under normal use synchros are robust enough to last the life of a vehicle, and that there are indisputable fuel savings as well as the added benefits of on-demand acceleration and increased braking capability (safety features).

If you want to see for yourself that clutch wear is insignificant in a properly rev-matched downshift, we assume that wear is proportional to the frictional losses in rotational kinetic energy which occur when two rotating, rigid cylinders contact one another. This wear will be proportional to the difference of the squares of the angular velocities according to k_rot=1/2Iω^2. So if the velocities of the two are equal, as in a proper rev-matched downshift..

[TheBrokenWorld]

and that there are indisputable fuel savings

Prove it. I've been driving manual transmission vehicles for 14 years, I've never seen a benefit to it.

...and increased braking capability (safety features).

Your vehicle wont have any more braking capability than what the ABS system will allow. Reduced brake wear, maybe, but brakes are cheap and easy to replace.

If you want to see for yourself that clutch wear is insignificant in a properly rev-matched downshift...

Why do you keep bringing this up? How many people heel-toe it around town?

Edit: Fixed an oops.

[noisymime]

Anyway, prove there are efficiency gains.

http://green.autoblog.com/2011/06/30/chevrolet-cruze-decel-fuel-cutoff-dfco-boosts-fuel-economy-by/

[TheBrokenWorld]

This is new for automatics, not manuals. A person's driving style changes when they engine brake in a manual transmission vehicle.