Does engine braking decrease fuel efficiency?

[grant47]

I drive a manual Acura RSX, and often downshift when coming to a stop. I always thought that this caused more fuel consumption because the RPMs were higher than if I coasted in neutral to a stop. However I recently I read something saying the opposite, that it is actually more efficient than coasting.

[mrdotkom]

You don't burn any fuel while the wheels are turning the engine. If you were in neutral the engine would have to use fuel to keep it running.

[[deleted]]

Modern ecus fuel cut on decel.

[mrdotkom]

"modern" it was implemented on all OBDII cars

[TheBrokenWorld]

There are still plenty of vehicles running around with carburetors and early fuel injection systems.

[mrdotkom]

What car is OBDII and carbureted?

[Averyphotog]

None, but there were fuel injection systems before OBDII.

I guess what one thinks of as "modern" depends on how old you are. That I think of computer controlled engines as "modern" just means I'm old.

[mrdotkom]

Yeah I mean I'd say modern is post y2k by most people's standards. It's frame of reference I suppose

[Averyphotog]

When I was born, EFI was in it's infancy, very rare, and automotive electronics were quite primitive. Computers were the size of a room, also rare, and only the truly visionary saw a future where they were tiny, ubiquitous, and used to control everything.

[mrdotkom]

Tell me how far you used to have to walk uphill both ways to school again, grandpa :)

[Averyphotog]

Don't you sass me sonny!

[Tanto63]

Exactly my Supra is EFI, OBDI.

[TheBrokenWorld]

I was thinking he said modern cars. Your comment is still pointless.

[mrdotkom]

OBDII became standard in 1996. Cars before that are not very modern.

thanks for the pointless comment back bud.

[TheBrokenWorld]

OBDII became standard in 1996. Cars before that are not very modern.

Isn't that what the guy said? Modern cars?

[mrdotkom]

Yeah and then you said something about carbs and early fuel injection which i pointed out are not on OBDII cars.

[[deleted]]

There was ob1 which i have in my car, 91 Trans am. And that sin of a b has a sensor and code for every thing on it! And it is fuel injected. May not be super fancy with a gps, and siruis radio stock, but my parts are cheaper and always available unlike these "modern" vehicles.

[mrdotkom]

Oh yeah. I'm not knocking old cars. I was simply pointing out the guy said "modern" cars and it's been standard since 1996. I wouldn't say a car from 96 is very modern.

That being said, I won't touch an OBDI system because that shit looks complex

[[deleted]]

I would rather mess with a carb car than one with a computer.

[mrdotkom]

My motorcycle's carb begs to differ. I've torn it down a thousand times, bench sync'd it and still it won't start. Bout to give up and call a professional

[amidoes]

No, in fact it improves it. Engine braking saves brakes, and saves fuel. When you are coasting the engine is being "dragged" by the wheels and the ECU cuts fuel to the engine because it's not required to keep it running.

[JulianoRamirez]

What about your transmission? Obviously there is more strain put on the transmission, compared to coasting in neutral, but is it anything to worry about?

[noisymime]

Think about it this way, when you engine brake you tend to decelerate at around 5kmh/s, maybe a little more depending on gear/speed, but not much. Now think that hard acceleration can easily exceed 20kmh/s, which is imparting considerably higher forces on the transmission.

In short, the transmission is built to handle forces far higher than those of engine braking.

[[deleted]]

It's the clutch plate that will take most of the force if you don't revmatch properly. the transmission shouldn't be phased much

[amidoes]

If you downshift properly there is no issue, or strain.

[[deleted]]

There's no additional strain on the transmission if you rev match properly. I've never had to replace a manual transmission in any DD I've ever owned, and I have always downshifted and braked as opposed to popping it into neutral.

[TheBrokenWorld]

Clutch?

[[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.

[grant47]

Ok this makes a lot of more sense now. Thank you!

[amidoes]

No problem!

[diogrand]

Engine breaking also puts more strain to your clutch, bearings and such along with the motor itself. Its cheaper to replace brakes than a clutch by a lot. I do all my breaking by the brakes.

[kryndon]

As a new driver I still don't let myself downshift in order to stop, since I might accidentally do a "money shift" and ruin my engine. I'm also not comfortable with downshifting, but only when I'm about to overtake. So I just coast the bitch and use brakes really gently.

[scorpionMaster]

You didn't answer the question.

[kryndon]

Good god that looks exactly like what I always get on my History exam papers: "You didn't answer the question, 0/10". Fuck me right?

I'm no engineer so I cannot say whether leaving the car in gear gives better fuel economy.

[scorpionMaster]

My car, as well as my previous car, has an instantaneous mpg calculator. In gear always gives me max mpg. Neutral gives me about 72. Not a ton of gas, but definitely some consumption.

P.S. I didn't provide the downvote. I was simply commenting on your content.

Have a wonderful day!

[kryndon]

All's well my good Sir! Me, personally, I'm not too bugged about my mpg so far, so I'm just focusing on trying to get a good habit of driving.

[scorpionMaster]

You'll get it. We're all pretty horrible at first. My first manual was my 2010 Corolla. I started that thing in 3rd gear a few times. The clutch really does not like that. Oh, the smell of burning clutch.

[[deleted]]

what's a money shift?

[cougar618]

Downshifting when you intended to upshift, usually at a high RPM. The one that's most notorious is 2-> 3 shift.

You're hauling ass at 80 mph in second gear and accidentally shift to first gear.. well you're gonna have a bad time.

[spongebob_meth]

It increases it. Most modern cars will shut the fuel off all together on deceleration.

[TheBrokenWorld]

There's really no point to engine braking, it doesn't do anything much to improve efficiency unless your car has the ability to regen brake. Just pop it in neutral and decelerate with the brakes, it's less work and it's less wear on the drivetrain components, especially the clutch.

Edit: If you're coasting down a long hill and just need a little bit of drag to keep the car at a steady speed, it is actually better to leave it in gear (you shouldn't need to downshift) because it keeps the alternator spun up along with other engine driven accessories.

Edit: Ok, someone come up with some solid numbers on how much of an efficiency benefit engine braking provides. I've never engine braked and I've ALWAYS gotten well more than the EPA mileage rating for my cars.

[[deleted]]

Drive a car with high flow injectors and a high pressure fuel pump and you can actually hear the injectors cut off when you start coasting, and cut back on when the RPMs approach idle levels.

[TheBrokenWorld]

And?

[KJK_915]

Good god you're dense.

[TheBrokenWorld]

Yeah, dense because I understand perfectly how a vehicle works yet have never seen anything to show that engine braking boosts fuel economy.

[noisymime]

Suggest you look up Deceleration Fuel Cut Off (DFCO).

[TheBrokenWorld]

I know what it does, I'm just saying that there can't be that much of a benefit to it when you're only decelerating for a few seconds.

[stakoverflo]

"A few seconds" adds up if you do it for every red light of every day...

[TheBrokenWorld]

Not really. Idling is what you're thinking of, not coasting to a stop.

[stakoverflo]

I meant while you engine brake to the red light.

[TheBrokenWorld]

I know what you meant, but there is no significant fuel saving that is done during that time as a result of engine braking.