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Old 11-25-2007, 01:28 PM   #41
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Quote:
Originally Posted by lrichard View Post
I was wondering what people think about coasting in neutral.
Generally, I accelerate upto the speed limit, then put the transmisiion in neutral. I do this in neighborhood driving. With tires inflated to maximum recommended, this allows me to coast for quite a distance before the vehicle slows down enough causing me to put it back in drive. This works well if I am nearing a stop sign. My question is. Could this be harmfull for the vehicles transmission? The tranny seems to shift smoothly from driving to coasting and back.
I have not done this enough to record any possible fuel savings. Mainly due to fears I may be hurting the tranny.

It'll help very marginally. If you really want to improve it, turn the engine off. Unless you're coming to a stop, keep in mind you have to regain any speed you lose effectively negating any gains from coasting.

No it won't hurt the transmission. if you're worried about the transmission, give the car a little gas to get it up to the rpm it'll be at when you engage the trans again. Just make sure you get the fluid changed at the recommended interval.


only certain manuafacturers and cars use off-throttle leaning out of the mixture. all are late models.
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Old 11-25-2007, 02:02 PM   #42
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Coasting in neutral works great in a standard-transmission car (or truck). Combined with moderate acceleration to an upper limit speed, coast till you reach a lower limit speed, then accelerate again... It's called Pulse and Glide.

With an auto tranny, you need to check your owner's manual to see if the car can be "flat towed" (section on getting towed). Depending on transmission design this may be safe within certain speed/distance limits. Many auto tranny's can't be flat towed at all.

Confused?? From the auto transmission's viewpoint, coasting in neutral is the same as getting towed with all four wheels on the ground. Some trannys get insufficient lube to internal components in that situation. That's why you need to check your owner's manual.

For standard trannys, don't coast with foot on the clutch. Bad for the throwout bearing. In neutral with foot off the clutch is the way. Some drivers kill the engine, some don't.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 11-25-2007, 02:22 PM   #43
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Bruce,

my car manual says,"if the vehicle must be towed on the front wheels, don't go more than 55mph or farther than 500 miles or your transaxle will be damaged."

when i EOC it's typically under 30mph and only 1/2 mile or less. sorry if this is a dumb question but, does it mean 500 miles lifetime or 500 miles consecutively? so, am i ok to continue?thanks.
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Old 11-25-2007, 02:25 PM   #44
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Quote:
Originally Posted by broadwayline View Post
You will get better fuel economy with your foot off the gas and coasting, the engine goes into a lean burn type mode.

When the engine is idling you are using more fuel.
I'm not so sure about this. Let's try a thought experiment.

Say you are idling at 60kph, at 700rpm. The force of friction on the engine (it will actually be a torque) will be x.

Now say that you are coasting in 5th gear. In a typical small car, that might be 1500rpm. (In my car, it might be 2000rpm. But even here, a 660cc engine is a rarity.) So, the force (actually torque) of friction on the engine is going to be at least 1500/700 = 2.14x. Quite possibly, more. Few things are more efficient comparatively when moving fast.

In the latter case, the car is going to be slowed down by the same forces of rolling and air resistance, + the torque required to rotate an engine at 1500rpm.
In this case, you may be using less fuel instantaneously, but having to accelerate more at a later time. So although the engine may have the injectors turned off when feathered, you still may be losing more energy than the coast in idle.

Feathered engine coasting is only going to get worse fuel economy the higher the speed at which you coast.

Where using feathered or foot completely off, engine on coasting can be valuable is if there are unexpectedly stopped cars ahead. Since you have to slow down, the kinetic energy would be wasted anyway.
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Old 11-25-2007, 05:31 PM   #45
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Mighty Mira gave a good description.

Another way to see the same thing is, energy is needed to keep the engine spinning. Spinning twice as fast needs twice as much energy. That's completely aside from pushing the car! After having provided fuel to spin the engine, additional fuel is used/needed to push the car. But first you have to "pay" to spin the engine.

So in almost all cases, the fewer total revolutions of the engine are used to complete your journey, the higher your fuel economy will be. Engine off at any given speed will give far greater FE than engine turning. Engine idling at any speed will probably/usually give better FE than engine engaged pushing the car. And, engine engaged in a higher gear will give better FE than engaged in a lower gear - as long as you're not lugging the engine, that is, rpm's so low that the engine runs poorly.

My two cents.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 11-25-2007, 09:48 PM   #46
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Thanks brucepick, I wasn't as confident as you about it (wasn't sure if pumping losses might change things), which is why I went the thought experiment route first. But the idea of "paying" to turn the engine was why I asked the question in the first place. Good explanation.

Another bonus of idling as opposed to having the car in gear (especially at high revs, like mine) is that the engine wear is reduced. Most of the wear occurs at the higher RPMs in other engines such as diesel generators, turbines etc. Just a few more RPM results in vastly reduced engine lifetimes.

There must be a sweet spot speed for P&G, assuming engine on, neutral. Since idle fuel burn rate is static wrt speed, the faster you go the better from that perspective. That will work fine until a point, where rolling resistance and air resistance increase to the point where more energy is wasted in moving air etc.

Note that there is also the pulse to consider. For that, we have to look at the engine map. Most of us probably have engines similar to the following engine map, which is a composite of EFI engines in 1995 (2 valve). In order to maximize efficiency during the pulse, we'd want to ensure that the engine was at least at 2000rpm and at about 3/4 of maximum load according to the graph (the line of the "island" represents equal efficiency (or Brake Specific Fuel Consumption, aka energy output versus, the each inner island having more efficiency than the next (read the link!). Hold on, I'll just compare:
2000 72%
3300 74%
3600-4500 70 - 100%



That's a lot higher than I thought. I was giving it 1/3-1/2 throttle, which I read somewhere on this site. I'll try and guesstimate what 75% of load is, since I'm not sure what the throttle required would be.

Anyway, I'll have a think about it some more later.
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Old 11-26-2007, 02:47 PM   #47
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Gassavers was mentioned in msn today!

http://articles.moneycentral.msn.com...ourOwnCar.aspx
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Old 11-30-2007, 02:20 AM   #48
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I've successfully tried ram air induction in the air dam area. With a large enough intake area and tapered correctly, you can increase the volumetric efficiency and get several PSI boost at highway speeds. The drawback, of course, is sucking in dust, rain, rocks, bugs, garbage, etc. Add a grill to filter out the big stuff at least and get a K&N or equivalent filter because you will be cleaning it more often. I might try it on my current car since it is bone-stock right now and compare the results.
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Old 11-30-2007, 10:11 AM   #49
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Quote:
Originally Posted by frmfrk View Post
I've successfully tried ram air induction in the air dam area. With a large enough intake area and tapered correctly, you can increase the volumetric efficiency and get several PSI boost at highway speeds. The drawback, of course, is sucking in dust, rain, rocks, bugs, garbage, etc. Add a grill to filter out the big stuff at least and get a K&N or equivalent filter because you will be cleaning it more often. I might try it on my current car since it is bone-stock right now and compare the results.
Got some data for us?
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Old 12-01-2007, 06:26 AM   #50
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Quote:
Originally Posted by Mighty Mira View Post
Thanks brucepick...

(I copied the link address and opened in a separate window to see graph full screen.)

And thanks to you, Mira!

Can you give me more information so maybe I can understand the chart?
Specifically, what is the meaning of the scale on the right (values shown .70-1.70)?

And where do I/we see these in the graph:
2000 72%
3300 74%
3600-4500 70 - 100%

Percent of what? Where is it shown in the graph?

Any additional explanation would really help. I've seen these charts a few times and I just can't get the concept yet, even though I'm not tooooo bad in math.

Thanks,
Bruce
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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