Hanging In Gears

[LTSBGY]

Me and my mates were having a discussion, dont know how it came up though. We were saying, lets say for example your in second gear auto or manual doesnt matter and you punch it to 5 grand and back off and let it hang in gear. Does it drink more petrol than putting it in neautral and letting it role?

We came to the conclusion that since your foot is off the accelerator then that means the throttle body is closed which means no air goes in, which means that not a lot of petrol is being thrown in. However will it use standard idle petrol or will it squirt a little more because its hanging at high revs??

What are your thoughts?

Nesh

[gogo]

I would expect higher revs will use more fuel. You still have to accelerate and decelerate the pistons, rods valves etc within the motor each cycle, and pumping losses will also be higher as you will move more air/fuel in, exhaust gas out at higher revs per unit time = greater energy loss = need more fuel (energy) to overcome these losses

Cheers,

Ben.

[tab]

But then on deceleration the injectors don't fire until the RPM fall below a specific point. Can't recall exactly what that is, but it's in the workshop manual somewhere

[LTSBGY]

So that would mean that gearing down and hanging the gears will waste more juice than puttin it in neatral and letting it role, even though the throttle body is closed?

I was argueing the point wiht my mates that it should drink more due to the revs being up but then again the throttle body is closed which means no air, less fuel. I dont know though, doesn anyone know for sure?

Nesh

[harvyk]

Try using the real time L/100K setting on your dash, it'd soon tell you if you where using much fuel...

[faygun]

I have noticed that staying in gear to slow down uses less fuel than putting it in neutral and letting it roll down a hill.

check your instant fuel consumption. it will show you.

[Macktheknife]

What ever the answer, running along in neutral is a practice not reccomended by all sorts of people.

[Chooka]

it will use less fuel keeping in gear while decel as tab said and be allot more safe, check your instant fuel and you will see mate

Chooka

Edited 23/03/06 02:21 AM by Chooka

[iTec]

  Macktheknife said:

What ever the answer, running along in neutral is a practice not reccomended by all sorts of people.

not trying to sound dumb/smart but why is it not safe to coast in neutral?

[Lawsy]

  iTec said:

  Macktheknife said:

What ever the answer, running along in neutral is a practice not reccomended by all sorts of people.

not trying to sound dumb/smart but why is it not safe to coast in neutral?

Its like being caught in the wrong gear in an emergency. If you're in 5th and not paying much attention, and have a very real need to go faster 5 seconds ago, you're pretty screwed. If you're in neutral, you're 100% up the creek without a paddle.

Say a big truck loses his brakes and gives you warning with his horns about 50 meters behind you, hes doing 80, you're doing 60 at the time (say its down a hill in a 60 zone and compression braking just doesn't seem to be having much of an effect).... You'd wanna get to 90 or so pretty quickly. In 5th, you might get away with it, just, because you will find it alot easier to pick up 2nd and floor it. If you're in neutral, then your brain is too, it seems, and for some reason picking a lower gear and matching the revs properly is a task many fail.... Thus, acceleration doesn't happen and you find a truck finding its way between your legs, while you're laying down.... With a spread on.

Not pleasant. (has happened, apparently).

that's what the coppers want you to believe.

My theory is that if I'm not retarded, I know what RPM my little oil burning 1.8L astina needs for 2nd gear at 60, by sound rather than the tacho, and I wouldn't have an issue. But I don't make it a practice of rolling around everywhere unless I'm being exceedingly lazy coming up to a set of traffic lights, which is highly uncommon because I heal toe every downshift I do, even at low rpm, and I come to a smooth, quiet, no fuss, no clutch slip or CV joint breaking stop... I found this hard to do in the XR6T though, as my little astina's engine, though it is definately waiting for the right time to explode, turns its own weight rather easily, and jumps up the rev range in an instant between 2500 - 5000rpm, heal toes are like taking a leak in the morning when you just woke up, they happen and you barely remember them.

On either side of that RPM range though, it doesn't do alot other than make noise, and in the later case, burn oil and sound stupid.

But you get that... In the T, it felt like I could explain quantum theory to someone in the time it took to get from idle to, well, 1000rpm. But then when you can chirp every gear from that point on, it all falls into place.

Now, back on topic.

The throttle shut, but a tiny bit of air slips past them obviously otherwise your engine would be sucking the throttle body through the exhaust.... Its tight, but not a perfect seal. The injectors do virtually nothing, they might dribble every now and then on the way down through the revs but that's about it... At idle, the injectors must put just enough fuel to keep a smooth burn happening, which means it needs a specific amount of fuel at all times, this happens to be more than a dribble.

Also, the pumping losses have nothing to do with the fuel. The pistons are already moving at high speeds back and forth because the motor is turning so fast and has the weight of the components, flywheel and indeed the entire car to keep it going. The energy the engine expells is actually heat, its converting the rotational motion of the drivetrain and forward motion of the car into heat and sound..

that's pretty much it. Doesn't need fuel to do that, only enough to keep a bit of a flame happening, which is just a vapour. And sometimes not even that because there's enough exhaust gases hanging around still.

/Long post, bored, procrastinating as usual because I don't want to do an assignment.