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  #1  
Old 01-02-2002, 10:36 AM
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How is fuel used during downhill with no throttle applied?

Sorry for the long-ish message, but I would like to find out exactly...

Recently a friend started shifting into Neutral when going downhill, claiming that it would save fuel (due to lower RPM). I tried to tell him that it is totally unnecessary (even if fuel is saved the amount would be negligible), if not using more fuel and causing more wear, however he is not convinced.

Here is my understanding of the Fuel Injection, I think I read it somewhere:
For modern EFI models, when going downhill, with trans in Drive the momentum would keep the engine turning, and thus the computer "switches off" injection to save fuel and actually no fuel is used. If instead putting trans in N and fuel (idle amount) would be required to keep the engine turning, thus in actual fact more fuel would be consumed.

Is the above correct?

How about for older cars with carburetor or mechanical injection (W126)? Does higher RPM translates to more fuel? (Referring to coasting downhill with no throttle applied)

My understanding is that it would based in the workings of the W126 Mechanical CIS.

Thanks for reading and any comments would e appreciated.
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  #2  
Old 01-02-2002, 11:39 AM
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From my understanding, for fuel injection engine, going down hill with transmission in "D", fuel is injected into cylinder as long as key is in "on" position, in fact it should always operate at or above the floor.

But it consumes less fuel than normal driving condition, because without the need of extra throttle opening, the computer reduces the duty cycle of the fuel injectors, less fuel is injected into cylinder.

Put into "N" will save negligible amount of gas, but you will lose some maneuverability, sometimes you need to use brake to keep speed down, it may overheat brake system, all these actually increase the possibility of accident.
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  #3  
Old 01-02-2002, 03:31 PM
blackmercedes's Avatar
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I'm sure that on newer injected cars, fuel is shut off completely on no-throttle coasting conditions, and then an idle circuit engages when the engine is truly required to idle.

An MB engineer was explaining to another customer why coasting in gear might use less gasoline than neutral coasting. I was a third party to the conversation (also known as eavesdropping) and might be wrong.
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  #4  
Old 02-21-2002, 10:58 AM
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I have been meaning to add a little comment to this post for awhile.
I think I understand how fuel is used when to throttle is not applied... or while the car is coasting.
About a month ago, I was leaving my house in the morning, terribly late... and I had to run 10 miles south of my house, and then 25 miles north or my house... with no time (SHOULD HAVE!) to stop for gas... I was in a major hurrey, had to drive 35 miles and get to work in about 30 minutes. Well when I pulled out of my garage, the gas needle was in the red, and BARELY moved up.... but moved up nonetheless.
Anyways, to make a long story short, I was about one mile from work (and about 1/2 a mile from the gas station I planned on getting gas at) when I ran out of gas.... I was going about 50 mph when the fuel supply cut off.

I observed that the engine, when coasting (no throttle applied) (RPMs) was running at roughly 1500 (point being, ENGINE WAS RUNNING) with no gas applied. But when I would attempt to push the gas, the engine would sputter down, but then when I let off the gas, the engine ran again... with no fuel.

Just kind of proving that fuel isn't *needed* during coasting.
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  #5  
Old 02-21-2002, 01:42 PM
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Its my understanding that when a car is put in neutral going down a hill the tranny is not getting the proper amount of lubrication to some parts. Obviously it has its own oil so maybe its something to do with the diff in engine vs trans speed. At any rate if this is true then any money saved by putting it in neutral can be used for a new trans later on. But i cant imagine how much hes trying to save unless these are some extremely long hills. And if hes that worried about saving a few pennies maybe he should buy a Dae-woo not a Mercedes-Benz
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  #6  
Old 02-21-2002, 08:43 PM
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Modern cars do shut off the fuel when coasting with no throttle!
The manufactures got this sorted ages ago trying to reduce offical MPG figures!. So shifting to neutral won't help (except at v low speeds).

To test:
I'm not sure if MBs do this, but bAd124 and I both have Audi's with trip computers. If in neutral while coasting, the computer reads '200 mpg' - it's maximum value, but signifying it's still using fuel. Now if you stay in gear and coast it switches to '- - - mpg', and you're now not using any fuel. In an automatic this may only occur when in 'lock-up'.
Also, in a modern(ish) diesel, if coasting in gear, its nearly silent, because no fuel is injected and there's no combustion. Then even the slightest throttle opening introduces the 'clatter' as the injectors inject fuel and combustion occurs.

later,

Russ
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  #7  
Old 02-21-2002, 10:00 PM
Jackd
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There has to be fuel entering the combustion chamber at all time, otherwise, you would have to re-start the engine after coasting.
No fuel=no engine running. The engine can run on the car enertia but the engine would then operate as a brake (Compression)
Proof: Coast down from say 40mph to 0mph, the engine is still running after you stopped. This means that the engine got fuel to keep running, from 40mph to 0mph..
This is true, being in gear or not.
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  #8  
Old 02-22-2002, 05:33 AM
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If you're still in gear and still coasting, the engine is turned over by the wheels turning it. What decides if an engine 'is running' or not, is debatable, but I'd say if its turning round then its running. When you reach lower speeds, fuel is then injected. The engine does not need restarting, because it is being turned round the whole time, exactly the same as the starter motor does.

later,

Russ
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  #9  
Old 02-22-2002, 09:55 AM
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Is there a difference between diesel engines and gasoline in this regard? Im thinking of a phemomena called"engine braking" moreso with trucks where gearing down actually uses engine resistance to reduce speed, implying that the engine is coasting out of sync with tire rotation and in finding equilibrium, one sees black smoke spewing from diesels in these situations, suggesting the engine is active, even when decelerating.

trucks are manual, so perhaps the crankshaft is directly linked to tire rotation, but automatics....im no engineer, but methinks the power flow is one way, therfore fuel injection is governed by the load being placed on the engine, not always speed (i.e., tire rotation) and in automatics the air conditioner, alternator, fuel pump etc. still require an actively running engine.

so the engine, in the case of an automatic, is being fueled at a minimum of idle rpm. my assumption is that an automatic tranny cannot transfer power back to the engine, (i.e. would it need two torque converters at either end? etc.)
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  #10  
Old 02-22-2002, 10:47 AM
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EFI cars enter decel cut-off when the conditions are right: throttle closed, rpm high and in some cases speed above a certain limit. NO FUEL IS delivered. The motor is being turned by the rear wheels and IS acting as a brake. When the conditions change the state changes. Thus when the speed drops (engine or in some cases vehicle) fuel is given back and as pointed out with an engine spinning at 1200rpms what use is a starter (that spins at 300rpms).

When I was young and lived where there were hills (Seattle), I was told that it was either illegal or illogical to coast down hill. Bad's comment about the trans got me thinking and for awhile he had me agreeing that the trans would suffer because the pressure would be reduced at low front pump speeds. I do know that trans system pressure is not stable often till 2000rpm. But then I realized that since the thing would be in neutral no force would be transmitted and idle pressures are definitely adequate for rotational lubrication.

As to the original argument, I agree with Bill, if you were coasting down Vail pass, properly, you could use no fuel for miles. If instead you coasted the engine would use idle fuel consumption for that distance and might actually be significant.

As to the transmission part, I am still wondering about the wear. I can gaurantee that pulling the car in and out of gear at speed is one of the worse things possible and should not regularly be done. Many MB trans will reengage at low gear and can over rev the motor before they shift. Doesn't usually happen but when ever I pull a trans to neutral (for diagnostic purposes) I always slow down before pulling it back to drive.
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  #11  
Old 02-22-2002, 02:41 PM
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There are several strategies used to determine fuel delivery in a vehicle. Typically it is load and air flow, or engine speed and air density along with throttle position. These inputs are used in table lookups and equations with other modifiers such as air charge temperature, coolant temperature and the ego sensor. Remember, we may in closed-loop control and trying to maintain a stochiometric (14.64:1) fuel-air mixture. And of course, there may be a high RPM cut-off and a high speed cut-off. In a no-load or closed throttle situation, the idle-speed control logic will try to maintain rpm within a certain range.

Steve is correct, on hard decels, fuel may be cut off, momentarily. This time can be measured in the milliseconds. When coasting, the engine, if under no load, will try to maintain idle speed. The engine controller does not care if the car is sitting or moving. It will inject fuel at the prescribed amount to maintain idle. To remove fuel would mean having to restart the engine at a later time. For reasons of trying to obtain low emissions, auto manufacturers try to refrain from restarting vehicles. This is a high emission event and also adversly effects the catalytic converter.

Besides, if you were going downhill and wanted to accelerate, you don't want to have to restart the engine, then start the acceleration process which is what the engine strategy would have to do. That isn't efficient nor does it give the performance that cusomers demand and expect.

I remember discussions on this topic during the energy crisis in the '70's. I though also that it was not good for an automatic transmission to be left in neutral while coasting downhill. This was both for lubrication and cooling reasons. In a manual, it is OK.

Now I haven't worked in engine strategy since the 80's, but I haven't been aware of any significant changes in basic fuel control. Electronic throttle control has appeared in the last few years, but that is a whole different topic. (and one I can't talk about). I'll check with others and verify this.
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  #12  
Old 02-22-2002, 03:22 PM
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Like Steve and I are trying to say - you wouldn't have to restart the engine!!! It's being turned over already, Try switching off your engine while coasting (in a manual) - notice anything different? The engine can't just 'stop' - what the hell would it do? Lock up and lock up the rear wheels? Disengage somehow and then stop, so you'd have to restart it? It's just gonna keep turning round.

Starting the car by turning the key does not do any special routine to get the engine to go - as long as it's being turned (by starter motor or the wheels), and fuel is delivered - it will go. (and ignition is on for a spark)


later,

Russ
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  #13  
Old 02-22-2002, 04:47 PM
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I have monitored decel cut-off hundreds of times and I can tell you that if you were in gear, foot off the gas maintaining speed (if not accelerating) down Vail pass there would be NO FUEL FLOW.

If I recollect downhill on Vail pass is ten to twenty miles. NO FUEL .

Since I actually live in flat lands and the state of accelerating while one's foot is off the gas doesn't exist, I can't say for sure that the system won't stay off indefinitely but I have watched it many times from 80 to whatever engine or vehicle speed brings about reset over a minute.
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  #14  
Old 02-22-2002, 06:33 PM
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Whether a starter is used to turn an engine or it is already turning, it is still a "restart".

While in NEUTRAL, the engine does not turn the driveshaft, nor does the driveshaft turn the engine. If fuel is cut off, the engine will stall. Thus if your friend puts his car in NEUTRAL while going down a hill, the engine will maintain idle RPM. He may save a few cc's of fuel per tank.

There is a decel cutoff while in gear, but I have not determined how long it is yet.

Regardless, I do not believe it is good for the transmission.
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  #15  
Old 02-22-2002, 08:12 PM
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Quote:
Originally posted by BAd124
Its my understanding that when a car is put in neutral going down a hill the tranny is not getting the proper amount of lubrication to some parts. Obviously it has its own oil so maybe its something to do with the diff in engine vs trans speed. At any rate if this is true then any money saved by putting it in neutral can be used for a new trans later on. But i cant imagine how much hes trying to save unless these are some extremely long hills. And if hes that worried about saving a few pennies maybe he should buy a Dae-woo not a Mercedes-Benz
With an automatic transmission, the fluid pump is driven by the torque convertor. Shut the engine down, and all fluids stop flowing. With an automatic, yes, he is doing major long term damage.
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