gearing to neutral

[h3ctic]

hey guys,

i was wondering if anyone could shed light onto the whole idea of shifting to neutral when the car is coasting down a hill or stopping at lights. some ppl say that it uses more petrol whilst others claim that it uses less. I used to shift to neutral all the time and now with a little more research ive strayed away from it. also, the constant shifting btw gears, would that potentially damge or make the gear box more susceptible to damage?

thanks

[lateralus]

for a manual, it wouldnt add any more wear and tear to the gearbox than what normal driving would provided you synch the gears properly. for autos, however, being in neutral while in motion is a BIG no-no, being stopped is fine though.

with efi vehicles, there's very very little fuel being injected while youre coasting down hill or at idle so the differences would be relatively insignificant between the two methods. the nrma reported placing the gearbox in neutral when at traffic lights for long periods of time cuts fuel consumption by around 3% which is next to nothing. however, i wouldnt recommended to leave it in neutral when coasting as you have no control over the engine. leaving it in gear gives the advantage of using engine compression to decelerate. if youre in neutral, you would gain momentum because of gravity

i'm sure superdave or hiro protagonist can shed some more expert light on this, they're wizz's when it comes to this stuff.

Edited April 15, 2009 by lateralus

[Hiro]

In a modern EFI car, when you are off the throttle going down a hill in gear (ie engine braking) you are actually using NO petrol. The ECU stops the injectors from firing, and the engine stops being a driving engine and instead turns into a driven engine (this is the principle behind engine braking and regenerative braking). This is also the reason why the engine note changes significantly if you put just a tiny bit of throttle on (when completely off the throttle, it's quite loud and boomy, but squeeze the throttle slightly and it gets a lot quieter).

However, if instead of going down the hill in gear and engine braking, you put the clutch in our shift into neutral, the engine no longer has a load connected to it (or has a driving force pushing it), and will drop to idle. However, fuel must be injected at idle to keep the engine turning over, otherwise it will stall. Thus, coasting down a hill in neutral uses more fuel than going down in gear.

At the lights (once again in a manual car), it is more fuel efficient to push the clutch in as opposed to leaving the transmission in neutral. The reason behind this is if the clutch is disconnected, the only thing being driven by the engine is the engine itself, and the flywheel. If the clutch is engaged and the transmission is in neutral, then the engine is also driving the clutch/pressure plate, and the input shaft of the gearbox. This creates drag, which requires more power from the engine, which means more fuel. It's a marginal difference, but it does exist (of course, sitting at lights with your foot on the clutch gets very tiring, and it can wear out the throwout/release bearing).

Things are changed completely for an auto. The engine-braking situation should remain the same, but sitting at the lights is different, as when the transmission is in Drive but the car is stopped, you are putting a fair bit of resistance against the engine (since the torque converter/fluid coupling cannot 100% disconnect, you need to hold the brakes slightly), once again sapping power and fuel. Can't remember exactly how an auto creates neutral, pretty sure it's a locked torque converter but no clutches engaged, so there's no physical connection to the wheels (unlike Drive, which relies on brake-load and slipping the torque-converter to stay stationary)

EDIT: As an aside, because a lot of people find it hard to believe that the engine uses no petrol when engine braking, try this (only tried it in a manual, so no guarantees for an auto):

1) Get up to around 60km/h or so on a straight road (downhill helps) with no traffic, in gear

2) Take your foot completely off the throttle, listen to the sound the engine makes (this is engine braking)

3) Leaving the car in gear, and without touching the clutch, turn the ignition key to ACC

4) Listen to the car, it will sound exactly the same as when in engine-braking mode and the car turned on

If you pay a bit more attention, you'll notice that it sounds exactly like an electric air compressor, because that is exactly what it is. The sound you are hearing isn't the burning of petrol, it's the compression and release of the air in the piston. At this stage the engine is simply an air pump, powered by the inertia of the vehicle and gravity, which drives the engine through the rotation of the wheels and hence the transmission/clutch/crank. Car engines make pretty good air pumps (that's what they are in essense anyway, just powered by internal combustion), in fact Beetle engines are renowned in the Australian opal-mining industry for being used as air-compressors - one side of the boxer burns fuel like a 2-cylinder internal combustion engine, the other side (which receives no fuel or spark) acts as a 2-cylinder air pump/compressor)

Edited April 15, 2009 by Hiro Protagonist

[ricketyclik]

While I agree with the above post, I use angel gear (these days in my automatic) when I specifically don't want engine breaking, ie, if when coasting downhill the engine would be slowing me more than I wish to be slowed.

[h3ctic]

whats angel gear?

and im still a lil sketchy on the info above. does gearing to neutral wen the car is stationary a good or bad thing? i get the whole driven engine while the car is moving without the gas on. not sure abt the whole if the car isnt moving thing. because there must be some driving force (ie using petrol) because as soon as u lift the brake, the car moves forward.

[mbruce]

Yeah well, basically like lateralus said, the difference is about 3% in fuel consumption. Personally I do shift to N in a long wait (road works ect), but just a personal preference, and it wont harm your trans. So basically it makes little difference over all, and just use what you feel comfortable with.

Edited April 26, 2009 by mbruce

[STYLSH]

whats angel gear?

and im still a lil sketchy on the info above. does gearing to neutral wen the car is stationary a good or bad thing? i get the whole driven engine while the car is moving without the gas on. not sure abt the whole if the car isnt moving thing. because there must be some driving force (ie using petrol) because as soon as u lift the brake, the car moves forward.

Angel gear is the gear that only an angel can save you if things like your brakes fail. --> Neutral. Nothing is slowing the car down for you.

[ricketyclik]

Angel gear is the gear that only an angel can save you if things like your brakes fail. --> Neutral. Nothing is slowing the car down for you.

Gears are funny things... you can change out of them, and back in to them.

I always thought it was called angel gear because it's quiet, and the car accelerates downhill without using fuel, as if by divine intervention.