So I have a question, which is more fuel efficient, cruising to a stop with the car in gear or out. This is with a manual transmission. My friend is telling me it's better to keep it in gear, but part of me thinks he's wrong because of how many other things he's wrong about.
The difference would be pretty much negligible, I think...
i would say that staying in gear saves an ever so little amount of fuel. in neutral, the car is basically idling and still using gas, but in gear, when you're off the throttle, there is no fuel being burned.
This is an ongoing topic of discussion. My view:
In gear the inertia of the vehicle is keeping the engine running. Many cars are smart enough to sense it and cut fuel altogether. In neutral the engine has to feed fuel and air to itself to keep itself running.
Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).
Quote from: R-inge on January 21, 2009, 04:46:44 PM
This is an ongoing topic of discussion. My view:
In gear the inertia of the vehicle is keeping the engine running. Many cars are smart enough to sense it and cut fuel altogether. In neutral the engine has to feed fuel and air to itself to keep itself running.
Quote from: MX793 on January 21, 2009, 04:48:53 PM
Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).
I thought both of those scenarios applied to auto trans more than manual trans. Might be I'm thinking of the "old days" when we were taught to save gas by avoiding jackrabbit starts and stops.
But, there's something new to learn everyday and computers monitor everything.
Quote from: MX793 on January 21, 2009, 04:48:53 PM
Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).
+1
It depends on the car and the situation.
At home, we have ScanGauge mpg monitors in a 2004 xB and 2005 Focus, both stickshift. In the Scion, the monitor shows fuel supply being cut off when you're in gear and not touching the gas. That's not the case in the Focus, however, which gets better mileage if you take it out of gear.
Another factor to consider is that you don't get engine braking in neutral, so you can coast farther out of gear than in.
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Quote from: MX793 on January 21, 2009, 04:48:53 PM
Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).
Our Mazda 3 shows mileage of 99.9 MPG when I take my foot off of the gas and coast in gear. Tonight I'll try a couple of slow downs coasting in neutral and see what the readout reports.
I think it still uses a tiny bit of fuel even when coasting in gear because as I get really slow the mileage readout changes from 99.9 to 67, then to 34. Then I'm pushing in the clutch and braking.
Quote from: JWC on January 21, 2009, 05:19:24 PM
I thought both of those scenarios applied to auto trans more than manual trans. Might be I'm thinking of the "old days" when we were taught to save gas by avoiding jackrabbit starts and stops.
But, there's something new to learn everyday and computers monitor everything.
According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed. The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.
Quote from: Byteme on January 22, 2009, 08:43:28 AM
According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed. The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.
What if you drive Honda
Quote from: Byteme on January 22, 2009, 08:43:28 AM
According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed. The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.
So you take off at WOT and shift immediately to the highest gear?
A couple of you are saying that it shuts off fuel injection completely. To me, that doesn't sound right at all. That means no combustion. There's no way it's just pumping air at that point. The exhaust note would change drastically. Everything would change. The car would slow considerably more when combustion isn't happening.
I can see it cutting fuel off a lot, along with air flow, but not completely. That doesn't even make sense.
Quote from: MrH on January 22, 2009, 02:58:15 PM
A couple of you are saying that it shuts off fuel injection completely. To me, that doesn't sound right at all. That means no combustion. There's no way it's just pumping air at that point. The exhaust note would change drastically. Everything would change. The car would slow considerably more when combustion isn't happening.
I can see it cutting fuel off a lot, along with air flow, but not completely. That doesn't even make sense.
Let me show you something...
(http://i589.photobucket.com/albums/ss335/47acp/0122091438.jpg)
This is a Buick LeSabre, 2005, 3.8L V6 engine.
As you can clearly see, it cuts off fuel on decel.
As for the drastic changes you imagine, well, have you heard and uncorked car decelerating for a turn? It does in fact sound different. You just can't hear it most of the time with your muffler installed. Pumping air isn't a big deal for a car, anyway. Displacement on demand cars do that in fact.
Well, you put the clutch in as you come to a stop anyway, so what's the difference?
Quote from: Raza on January 22, 2009, 03:14:58 PM
Well, you put the clutch in as you come to a stop anyway, so what's the difference?
I don't put the clutch in till I am nearly to a complete stop.
Quote from: NomisR on January 22, 2009, 01:02:00 PM
So you take off at WOT and shift immediately to the highest gear?
I use a lot of throttle and shift at about 2000-2500 rpm I usually skip 4th and go from third to fifth.
Quote from: R-inge on January 22, 2009, 03:14:46 PM
Let me show you something...
(http://i589.photobucket.com/albums/ss335/47acp/0122091438.jpg)
This is a Buick LeSabre, 2005, 3.8L V6 engine.
As you can clearly see, it cuts off fuel on decel.
As for the drastic changes you imagine, well, have you heard and uncorked car decelerating for a turn? It does in fact sound different. You just can't hear it most of the time with your muffler installed. Pumping air isn't a big deal for a car, anyway. Displacement on demand cars do that in fact.
thought DoD engines shut off the intake/exhaust valves, so that they don't actually pump any air while they're shut off.
Maybe you're right! I'm not entirely sure.
Quote from: Soup DeVille on January 22, 2009, 05:56:36 PM
thought DoD engines shut off the intake/exhaust valves, so that they don't actually pump any air while they're shut off.
Unless they're direct injection, they'd have to. Otherwise, fuel would still be going into the cylinder and if the ignition was shut off, you'd get a bunch of raw gasoline spewing out your tailpipe. Not to mention the added drag of the piston pumping air (vs being an air spring).
Quote from: MX793 on January 22, 2009, 06:26:52 PM
Unless they're direct injection, they'd have to. Otherwise, fuel would still be going into the cylinder and if the ignition was shut off, you'd get a bunch of raw gasoline spewing out your tailpipe. Not to mention the added drag of the piston pumping air (vs being an air spring).
Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.
Quote from: R-inge on January 22, 2009, 03:20:04 PM
I don't put the clutch in till I am nearly to a complete stop.
Interesting. I put the clutch in when I drop below a speed appropriate for that gear, in case emergency acceleration is needed, that way I can get into gear faster.
Quote from: Raza on January 22, 2009, 06:43:55 PM
Interesting. I put the clutch in when I drop below a speed appropriate for that gear, in case emergency acceleration is needed, that way I can get into gear faster.
I practically always keep the car in the most appropriate gear for the speed I'm travelling and keep the clutch engaged as much as possible. That way, I don't even need to shift into gear and drop the clutch, I can just step on the gas if the situation calls for acceleration.
Quote from: R-inge on January 22, 2009, 06:38:40 PM
Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.
You'd still have the added drag of pushing the air around. Better to just shut the valves and let the pistons compress the same air over and over.
For what it's worth, on the Z06 the DIC tells me I get much better mileage when I coast with the gear out and at idle compared to being in gear with the RPM's up. I did this experiment sometime during the summer. With the car in gear and going down a hill the DIC told me I was doing about 60ish MPG. In neutral and just coast the MPG will jump to 99.
I'm not so sure about the fuel being instantly cut off when you lift off the gas in the Z06. I mean coming down from high RPMS that's often when the exhaust is often the loudest.
Quote from: Byteme on January 22, 2009, 08:40:01 AM
Our Mazda 3 shows mileage of 99.9 MPG when I take my foot off of the gas and coast in gear. Tonight I'll try a couple of slow downs coasting in neutral and see what the readout reports.
MY RX-8 reports the same and when looking at the injector pulse width it goes to zilch
Quote from: R-inge on January 22, 2009, 06:38:40 PM
Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.
Y? The intake + exhaust valves are closed.
Quote from: MrH on January 22, 2009, 07:15:48 PM
You'd still have the added drag of pushing the air around. Better to just shut the valves and let the pistons compress the same air over and over.
Gotta balance the fuel savings against the added expense of such a valve train.
Quote from: r0tor on January 23, 2009, 09:30:30 AM
MY RX-8 reports the same and when looking at the injector pulse width it goes to zilch
How do you know the injector pulse width?
he has a datalogger thingy
Aye, Accessport. www.cobbtuning.com
Quote from: sportyaccordy on January 23, 2009, 10:42:11 AM
Y? The intake + exhaust valves are closed.
You're not paying attention. I was suggesting that as an alternative to closing the valves, but then realized it would still be an issue.
At any rate, being an air pump is the whole point of engine braking.
Quote from: R-inge on January 23, 2009, 07:15:20 PM
You're not paying attention. I was suggesting that as an alternative to closing the valves, but then realized it would still be an issue.
At any rate, being an air pump is the whole point of engine braking.
Jake-brake?
Quote from: JWC on January 23, 2009, 07:38:20 PM
Jake-brake?
Not sure how those work. But even with a normal car, the engine is pumping but not creating energy, so it must be a hinderance, right?
Your description sounds a lot like the theory behind jake-brake.
I'm pretty sure that trying to accelerate my VW to get to a higher gear sooner will waste fuel. I can also say that my Volvo's average mpg goes down with continuous hard acceleration.
Quote from: JWC on January 23, 2009, 08:04:44 PM
Your description sounds a lot like the theory behind jake-brake.
I'm pretty sure that trying to accelerate my VW to get to a higher gear sooner will waste fuel. I can also say that my Volvo's average mpg goes down with continuous hard acceleration.
Ditto. In theory is sounds feasible but in real life I do much better by driving gently.
I think you would have to shift very early for it to be effective which is difficult to do, particularly if your engine enjoys higher revs as much as mine does.
Quote from: JWC on January 23, 2009, 08:04:44 PM
Your description sounds a lot like the theory behind jake-brake.
I'm pretty sure that trying to accelerate my VW to get to a higher gear sooner will waste fuel. I can also say that my Volvo's average mpg goes down with continuous hard acceleration.
Each car is going to be a bit different. What Road and track was offering was a general rule.
Large throttle openings while shifting at low RPMs will reduce pumping loses. I don't think they were advocating flooring it and shifting at 3000-3500. Certainly they wern't saying run it up to near the redline and then shift.
BTW I tried coasting doen from 50 in neutral and in 5th gear. No difference, the mileage readout went to 99.9 in both cases.
In the 3 I accelerate just enough to stay up with everybody else and shift at relatively low RPM, skipping 4th entirely, try not to change speeds often, anticipate lights and generally go 70-75 on the freeway half of our commute. I must be doing something right. I generally average 34 MPG in a 50-50 mix of stoplight city driving and crowded freeway driving.
Quote from: Byteme on January 24, 2009, 05:28:02 AM
BTW I tried coasting doen from 50 in neutral and in 5th gear. No difference, the mileage readout went to 99.9 in both cases.
On our fuel economy readouts, we see about 150 mpg coasting in gear in the Focus (depending on speed) and 999 (presumably meaning the fuel supply was cut off) in the Scion, while 250 or so coasting in neutral in either car.
Keep in mind, the VW is carburetor and I can't skip 4th.
Quote from: JWC on January 24, 2009, 07:11:42 AM
Keep in mind, the VW is carburetor and I can't skip 4th.
Is that because you only have four gears?
Quote from: sportyaccordy on January 23, 2009, 10:42:11 AM
Y? The intake + exhaust valves are closed.
He's talking about if they were kept opening and closing...
Hmm, so most people seem to be saying stay in gear. When I keep it in gear, the instating fuel economy gage goes to what I'm going to call infinity, and when I take it out of gear it stay in the 30-50 MPG range. The "infinity" position is also what it stays at when just idling at a stop as well though.
Quote from: cozmik on January 25, 2009, 02:00:15 PM
Hmm, so most people seem to be saying stay in gear. When I keep it in gear, the instating fuel economy gage goes to what I'm going to call infinity, and when I take it out of gear it stay in the 30-50 MPG range. The "infinity" position is also what it stays at when just idling at a stop as well though.
Your economy meter compares vehicle speed to fuel flow. If the vehicle isn't moving, you're not really getting any MPG, so it must just default to the maximum value when it detects that the vehicle is standing still.
It should show 0 mpg in that case; showing infinite would mean the fuel supply is cut off.
Quote from: ifcar on January 25, 2009, 02:12:39 PM
It should show 0 mpg in that case; showing infinite would mean the fuel supply is cut off.
There is no 0 on BMW's fuel economy gage. The lowest it goes to is 12.
Quote from: cozmik on January 25, 2009, 02:13:53 PM
There is no 0 on BMW's fuel economy gage. The lowest it goes to is 12.
Heh, my Passat would show "---" when stopped, and go as high as the 300s when I was coasting downhill. It showed 5 under floored acceleration.
Quote from: cozmik on January 25, 2009, 02:13:53 PM
There is no 0 on BMW's fuel economy gage. The lowest it goes to is 12.
Well, regardless of the range of the gauge, if the engine is running, and the car isn't moving, 0 MPG is the only possible accurate reading.
Quote from: cozmik on January 25, 2009, 02:00:15 PM
Hmm, so most people seem to be saying stay in gear. When I keep it in gear, the instating fuel economy gage goes to what I'm going to call infinity, and when I take it out of gear it stay in the 30-50 MPG range. The "infinity" position is also what it stays at when just idling at a stop as well though.
In my car, it goes to the "infinity" spot when I'm off the gas (idling and coasting). Mine is an auto though, so I don't know what happens when I put it in neutral.
Quote from: Soup DeVille on January 25, 2009, 05:19:14 PM
Well, regardless of the range of the gauge, if the engine is running, and the car isn't moving, 0 MPG is the only possible accurate reading.
Right, and it goes to the same position if I'm stopped idling or coasting in gear. 0 MPG would seem impossible if I'm moving though.
i do seem to get better gas mileage per my accessport when I bog the shit out of the engine