MeetJoeAsian

so...if you're driving at high speed in a manual, and you decide to shift down in gear to help the car slow down (aka engine braking)...my question is this...well, since your RPM revs higher, do you burn more gas using engine brake???

shinkuu

iTrader: (4)

yes u will burn more gas. you have to step on the throttle to rev match and any time you step on it, you are using more gas. i've been trying to drive conservatively. whatever mileage i get on the highway gets destroyed on the city streets.

ohw

Actually, it depends ...
If you lift off the gas pedal, the injectors are shut off, no gas is going thru, no matter what RPM you are at, gas consumption at that point is zero. If you have to rev up to match the RPM, that means gas used. The other way to save gas is to use your brake to slow down while leaving it in gear. Only clutch in when the engine is too slow, just before you can feel it start to labor to stay running.

TreeZ

iTrader: (23)

^avatar is priceless. "the world needs ditch diggers too"

sabrefanpc

iTrader: (4)

do the injectors actually switch off? I understand how much fuel is used is a function of rpm and throttle %, but didn't think 0% throttle meant no fuel

roast

iTrader: (2)

.

Kolia

iTrader: (2)

No, ohw is right. When the throttle goes to 0%, the injectors will not open until the rpm goes down to about 1,000. They then reopen for idling.

Read the Fuel injection section in the Nissan Tech manual.

Blipping to rev match will waste a lot more gas though. And clutching and slowing down in neutral too, since the engine is idling.

So, the most economical way will be to brake with the car in gear right until you're about to stall.

ohw

When it's idling, it uses fuel even with no throttle. When you are accelerating, the RPM and throttle determine the amount and rate of fuel intake. When the engine is running in higher RPM and you lift off, the injectors are shut off. The momentum of the car will keep the engine running, regardless of the RPM. This does not apply to older car with carbs and older fuel management systems. That's the theory and a simplified version of things happen under the hood. To know for sure, we need to find out how Nissan actually program the "drive by wire" logic. If someone knows better about this topic, please feel free to chime in.

davidv

iTrader: (2)

I don't think so. You have to press on the accelerator to get fuel into the engine.

ohw

Sorry guys, something needs clarification. When I said "... regardless of the RPM...", I should specify that does not include idling speed.

roast

iTrader: (2)

.

roast

iTrader: (2)

Someone first explain to me why I'm a crackhead. I just went and tested it out lol.... reset my MPG guage right as I started going down a hill and engine braked down the whole down and it only registered MPH but no MPG.... guess I was wrong.

That seems a bit weird to me.

/edit. I just admitted I was wrong!! (and a crackhead) I tested it out myself, and I didn't read the gauges wrong. After engine braking all the way down the hill, at the bottom it registered speed but no MPG. That means no fuel was used. I didn't think the injectors closed 100%, but I was flat out WRONG! Thanks for pointing that out to me, that's nice to know!

jt14894

I'll give it a shot, but I'm no rocket scientist. I just slept at a Holiday Inn Express last night.
If you're coasting down a hill, the engine is basically idiling and isn't doing any work and so your saving fuel as your travelling. The further you go without having to burn fuel will increase your distance travelled per gallon of gas. So, for example, if you're coasting a mile down hill your engine may burn .032 gallons of fuel, but if you're accelerating up hill your engine may burn .100 gallons of fuel. If you calculate that into MPG, your car is getting 31.25 MPG going down the hill and getting 10 MPG while going up.

As for engine braking, it will save gas and increase MPG, but not as much as coasting since you are using the engine to control your speed. I'm sure you've experienced going down a hill and then letting off the accelerator to slighty slowdown, but then apply a little gas to maintain your speed before you got to the downhill slope.
In doing that, your engine may burn .040 of fuel which, if you calculate that into MPG, your car is getting 25 MPG
I did a search and found this write up of Engine Braking on Wikipedia. Check out his link.
http://en.wikipedia.org/wiki/Engine_braking

Again, I just slept at a Holiday Inn Express last night.

Kolia

iTrader: (2)

Cuz you didn't read right !

With no throttle, the engine gets no fuel untill rpm drops near idle speed. It will then give fuel so it doesn't stall.

Going down hill while engine braking, no throttle = no fuel. But the rpm are still higer than 1,000. It just wait.

Going down hill on neutral with no throttle, the engine drops to below 1,000 and switch to idle mode, pumping fuel in the engine.

Better ?

MeetJoeAsian

I guess another way to understand this concept is if you're in gear (especially first gear), and you take your foot off the throttle, your engine will die as it reaches 0 cuz no gas is kicking into the engine....whereas, if you're in Neutral, injector's always open, therefore, there always fuel available to keep the car running....

MeetJoeAsian

so....my next question is this...since using engine braking DOES save gas, what is the wear on the engine? so you save a LITTLE gas and a LITTLE more life on your brakes, how much of a toll is this taking on your engine in the long run, since your car runs in higher RPM so much....it's gotta wear your pistons out faster...and I heard the pistons in the Z are pretty not the strongest..

Kolia

iTrader: (2)

The pistons see no wear ever. They are not in contact with the cylinder wall directly.

Engine wear ? It's been decades now that a car's life span is dictated by the chassis's longevity. Engines will run a long time. In pratical term, you'll probably grow weary of your car's look long before the engines wear out from engine braking.

barthelb

unless its a dodge and the chassis and everything else out live the engine and tranny.

Kolia

iTrader: (2)

LOL!

Then it's Warranty+1k miles !

350Zenophile

iTrader: (20)

Actually, going easy on the gas during acceleration has been proven to use more gas then stomping on it (WOT) and short shifting. The theory being you want to decrease the total amount of time it takes to reach your cruising speed while at the same time keeping revs low. I read about it in last month's Road & Track so it must be true.

You are 100% right about coasting though.