Fuel Consumption #2

[23790554]

Thanks to everyone that posted a reply...here's another question

I guess there are several uncontrollable factors in this question, such as weather consitions, turning a corner, etc.

What if you are in an Automatic car, stopped at a traffic light, the ground is flat, and you have to wait about 2 min.

1.Shift to N
2.Stay in D

As I have noticed that whenever I shift into N, I can feel as if the brakes gets more power? The brake pedal sinks deeper by itself.

Also, someone mentioned the computer in the car that measure the amount of fuel used, whenever stopeed, it will always say 99/100km.

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Is #2 saving fuel or using more fuel?

Say you are doing 110km on a highway in a 3.0 V6 Camry and there is a long downhill coming up about 2km. Engine revs at 3000rpm.

1. Let go of the accelerator, engine at 2000rpm for the duration of the downhill.

2. Let go of the accelerator, shift to N, engine drops to 800rpm for the duration of the downhill, shift back to D before the uphill, engine revs very fast back to 3000rpm.

[dingo]

I believe #2 is saving fuel, as alot of people do shift into nuetral on a down-hill road.

[1zippo1]

JC claimed when he did the 1200km on one fuel tank with an Audi A8 4.0TDI that it was more economical in a modern car to let the revs drop and keep the car in gear, then to put it into stationary. I never thought this would be and I still often dump the clutch when I'm rolling down a hill for instance.

[TransAm]

I don't think coasting in N is advised - I read somewhere that it doesnt do your transmission much good. And, as thamar pointed out, it's potentially dangerous...

[AlienDB7]

Yes, #1 is true. I actually had a debate with some guy on this issue and found a website with some data log to confirm the result. On top of the fuel you saved by shifting into gear, the extra engine braking will reduce brake wear.

Normally I downshift to 3rd gear under such condition and let the car cruise at 2000rpm and 80kph. You don't want to use very low gear either, high rpm = more engine wear The little you save can cause you more expensive repairs in the long run.

When you're shifting from N to D before going up the hill, you *can* lose traction on slippery condition. Even on dry road, it may cause a crash when you do that while turning. I'm not sure on this, but when you suddenly shift from N to D at such high speed, you can/will damage the transmission.

[possessed_beaver]

number one, imo

because like clarkson also said, that when the car is in neutrel the engine is pumping gas into it to stop it from stalling.

[sentra_dude]

Here's a question for everyone...

Let's say I am in 3rd gear at about 70mph, pushing ~5000rpm, then I start braking down to a stop, now if I just de-clutch and coast down to a stop with just brakes will I still use more fuel than if I let the engine do a good bit of the braking and bring itself down from 5000rpm to maybe 1500rpm (close to stalling) and then de-clutch and coast to a stop?

My 'feeling' has always been to push the clutch in but leave it in gear and slow down, brakes only, to a stop. It feels like I would be burning a lot of gas & putting useless wear on the engine by keeping it up above 4000rpm while slowing down. I can replace brakes, but engines are a bit more expensive... Is this correct...or will a modern (well 1992 engine ) burn more gas while idling than while engine braking from 5000rpm? I don't see how that could be possible, I'm sure the engine must be burning gobs of fuel at that rpm, even while engine braking...but maybe not?

[sentra_dude]

Originally Posted by brembo @sentra dude. If you let the engine brake its way down to 1500rpm`s instead of leting it idle while braking it down you will burn less fuel. The engine management system will cut the fuel to prevent to much fuel when you blipp that throttle again, it will drown it basiclly when you put down your foot again if it have been pouring fuel into the engine while "enginebraking" it down without the fuelcut... it will not have much more wear by engine braking it as it still will have oilpressure (even more than when idling)... you say that it must burn alot of fuel at that rpm, think about it....if it had burned fuel you would have power instead of braking coming from the engine with fuel being ignited....... your engine do have fuelcut and it will save you fuel to engine brake it down...

Thanks for the explantion, 8) I hadn't thought of it like that, and

you say that it must burn alot of fuel at that rpm, think about it....if it had burned fuel you would have power instead of braking coming from the engine with fuel being ignited.......

makes a lot of sense, I don't know why I didn't think of it that way before...

[sentra_dude]

Originally Posted by thamar + braking distance with both brakes & engine braking is shorter, then when you're only braking with your brakes. Less wear on your tires, brakepads & disks.

Yes, yes of course...but I almost always use the brakes in concert with engine braking, unless I am just lifting off for a sec to bleed away speed, it was a semi-hypothetical question...

I allways downshift, smoothly true the gears. I shift down when it reaches 2k rpm, when I have to brake hard I shift down at 3k rpm. Engine only goes to 4k since it's a diesel. Damn you, you drive a celica and you don't downshift! I hope my buddy never sees you driving, he'll kill you raping that celica like that He really loves those cars, used to have one untill he got a car from the office, and let me say he has a heavy right foot

Well...I wish I could downshift more, but unfortunately the young lad who owned the car before me had something against the gearbox and especially the 1st & 2nd gear synchromesh...so I hardly do any kind of aggressive downshifts to be as easy on the gearbox as possible...

[gottacatchup]

I dont leave the clutch engaged when coming to a stop mainly because drivetrain lash makes it uncomfortable for passangers as i let off the gas and apply the brakes, so i've just gotten in the habit of always putting the clutch in when I brake. However i'm slowling unlearning to leave the clutch out around corners.

[hemi_fan]

^^ lol, i have the opposite feel for passengers... IE; i dont care if its comfy for them, as long as its fun for me. I always thought that u brake harder when ur car is in neutral or the clutch is depressed (at least at low speeds) because your brakes dont have to also slow down the rotation of the engine/tranny as well as the wheels.?

[gottacatchup]

Originally Posted by hemi_fan ^^ lol, i have the opposite feel for passengers... IE; i dont care if its comfy for them, as long as its fun for me. I always thought that u brake harder when ur car is in neutral or the clutch is depressed (at least at low speeds) because your brakes dont have to also slow down the rotation of the engine/tranny as well as the wheels.?

I see what you mean with the braking but if you think about the physics the engine and tranny dont want to keep rotating, other than the flywheel so their loss of momentum should add to the brakes stopping power.

[Vansquish]

Originally Posted by 1zippo1 JC claimed when he did the 1200km on one fuel tank with an Audi A8 4.0TDI that it was more economical in a modern car to let the revs drop and keep the car in gear, then to put it into stationary. I never thought this would be and I still often dump the clutch when I'm rolling down a hill for instance.

The fuel strategy for an automatic isn't the same as it is in a manual transmission vehicle...it's very difficult to predict what a driver is going to do in a car equipped with a stick, therefore the programming is much more primitive. In a car equipped with an automatic transmission under deceleration you should definetely stay in gear, as the ECU "starves" the engine of fuel. When you're sitting at a traffic light, I doubt that it makes much of a whether you're in Neutral or in Drive, the revs should stay roughly at the same spot once the idle-speed controller kicks in and the torque converter sits there straining against the brakes.

[jakaracman]

Originally Posted by thamar 1st gear is not synchronised on most cars, that's why it's harder to shift from 2nd to 1st. One way to do it is rev the engine a little, then it'll go easier ...

Tehre's no modern car in production (or has been for alt least 10-15 years) without syncroed 1st gear ...

[sentra_dude]

Originally Posted by jakaracman Originally Posted by thamar 1st gear is not synchronised on most cars, that's why it's harder to shift from 2nd to 1st. One way to do it is rev the engine a little, then it'll go easier ... Tehre's no modern car in production (or has been for alt least 10-15 years) without syncroed 1st gear ...

Yes, I was wondering that myself, I was pretty sure my car had syncros on 1st gear...but I wasn't 100% sure. Anyone else