Coasting in Neutral vs engine braking - PeachParts Mercedes-Benz Forum

thayer · #1 08-23-2013, 07:02 AM

So I'm trying some Hyper-Economic driving in my 1995 E320T now that its beginning to cool down.

There is a three mile stretch between my house and work where I can put it in neutral and coast. Because the transmission doesn't like being put into drive while rolling in neutral, I'm trying to work out how much to push the gas pedal to turn off the engine braking without adding more fuel.

I feel about an inch of no tension motion in the fuel pedal before it starts adding fuel. The Question is, how far do I need to push the pedal in to cancel the engine braking.

This morning I'm going to leave it in drive during the 3 mile stretch and see if I coast as far as I do in Neutral with the fuel pedal pushed just to the end of the tension free zone.

Any info regarding manipulation of the engine braking would be extremely helpful

Peter Guenther · #2 08-23-2013, 08:25 AM

I dont know about the E series but somewhere in the shop manual on my 500 S series it stated that "the engine recieves no fuel when coasting or slowing down". The engine control cumputer shuts down the injectors, I would definately not take it out of gear!

oldsinner111 · #3 08-23-2013, 09:44 AM

I tried that with mine,and when I put into drive the trans stayed in neutral.I had to stop pull down to first and back to drive for it to reset.My car has no engine braking as its in a ultra high gear when coasting.Now my SD is different,I can get a tow up to 40mph,and slam into drive she will start

jcyuhn · #4 08-23-2013, 10:22 AM

All modern cars use DFCO - deceleration fuel cutoff. When you close the throttle and the engine is above a set RPM - typically 1200 or so - fuel flow is completely shutoff. As a result, coasting in gear saves more fuel than shifting to neutral because no fuel is consumed to idle the engine. On newer MBs with a trip fuel economy display you can really see the impact of taking advantage of DFCO; the trip economy jumps during long stretches of coasting.

A 1995 model year car almost certainly has DFCO, especially one with engine controls as sophisticated as the M104. I believe this was the first mass market engine with drive by wire throttle.

ILUVMILS · #5 08-23-2013, 12:13 PM

Yes, the HFM-SFI system in '95 has decel-shut-off.

My 210 and 203 both have trip computers and the fuel mileage definitely improves when coasting oppotunities are taken advantage of.

pawoSD · #6 08-23-2013, 03:59 PM

In my TDI it displays "--.-" for the fuel consumption when coasting when it shuts off the fuel. Easy to do with a manual trans...

Even the CIS cars don't really add any fuel when coasting above 1200rpm as I have seen decent MPG gains by coasting as much as possible on a couple tanks.

thayer · #7 08-23-2013, 05:52 PM

PO of my e320T had a hole drilled in the zebrano for a switch to control something. I removed the switch, ran a vacuum line, pulled an old 126 gasser cluster. Works well

.

97 SL320 · #8 08-23-2013, 07:47 PM

By 95 I'm pretty sure the trans didn't have a rear pump so if you hit neutral and shut the engine off you have lost flow through the lube circuit.

More importantly, when a auto trans is driven from the output shaft, planetary gears can reach ludicrous speed ( or maybe even go to plaid ) because they are not no longer out of the power flow.

A better choice would be to shut off power to the injectors and open the throttle then you will coast like a diesel rather than build vacuum. You might force a downshift so plan throttle % accordingly. At the end of the hill, close the throttle and power up injectors.

thayer · #9 08-23-2013, 08:14 PM

how do you kill power to the injectors? I'm assuming close throttle is to let completely off the pedal.

I never shut down the engine, just let it idle.

By the way, the engine is getting additional fuel in the space in the pedal without friction. I ended up going much faster than when I was in neutral.

25.87 mpg so far. No excited...leaning more towards a ticket to rhode island for Shertex's 95 e300d.

"Barf...Not in here mister, this is a mercedes"

97 SL320 · #10 08-23-2013, 09:31 PM

Quote:

Originally Posted by thayer

how do you kill power to the injectors?

The injectors on your car have solenoid coils. I'd have to see a wiring diagram but usually one wire goes to +12V and the other to the engine computer. The computer grounds this wire to fire the injector. If you shut off the +12V they won't spray fuel. The computer opens the ground to shut off the fuel, doing it manually is easier on the + 12 V side because all of the injectors are tied together on that side.

Quote:

Originally Posted by thayer

I'm assuming close throttle is to let completely off the pedal.

Sort of closed, you will want a smooth transition between fuel off and fuel on.

Quote:

Originally Posted by thayer

I never shut down the engine, just let it idle.

This keeps the lube circuit going but the planetaries are still spinning wildly and will fail. Think about driving 65 MPH in first gear.

Quote:

Originally Posted by thayer

By the way, the engine is getting additional fuel in the space in the pedal without friction. I ended up going much faster than when I was in neutral.

pawoSD · #11 08-23-2013, 11:40 PM

Sounds like a ton of work and concentration to save a minuscule amount of fuel. Sell it and get a diesel if you want economy.

I live at both ends of the spectrum, a 15ish mpg 420SEL and and a 40mpg TDI.

Skippy · #12 08-24-2013, 06:03 AM

Info on hypermiling here:

Energy-efficient driving - Wikipedia, the free encyclopedia

Hypermiling :: Maximum Miles Per Gallon

Or just do what I do and select vehicles that can be driven normally to aggressively and still get good mileage. I manage about 25 mpg average in my 300D and 50 on my Ninja 650R, and I typically describe my driving style as "jackass", "raging jackass", or "Ayrton Senna".

thayer · #13 08-24-2013, 07:00 AM

Yeah, going back to diesel may just be the answer. Driving like I have somewhere to go,I'll get 21 in the 320 and 30 in an e300d.

The traffic Waves article is great. It should be required reading for getting a drivers license.

thayer · #14 08-25-2013, 12:43 PM

Quote:

Originally Posted by Peter Guenther

I dont know about the E series but somewhere in the shop manual on my 500 S series it stated that "the engine recieves no fuel when coasting or slowing down". The engine control cumputer shuts down the injectors, I would definately not take it out of gear!

If the engine is getting no fuel, momentum is being transferred from forward motion into turning engine.

That's my logic equation for engine braking.

Hirnbeiss · #15 08-26-2013, 07:40 AM

Thayer hit the nail on the head. Shutting off fuel flow is only one part of the fuel efficiency equation: the engine will be braking the car, thus robbing you of your precious momentum. If the story ends at the bottom of the hill, then fuel cutoff is probably more efficientbthan neutral; if you have to go back up a hill, neutral may win if it saves you having to accelerate back up to speed.
However, if this is how you spend your brain energy, you're probably a good candidtate for an electric vehicle with regenerative braking.