downside of ZZZZZZ
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From: Scarsdale, NY
Originally Posted by stylett9
What your saying doesn't make any sense.
When your coasting in gear regardless if you apply throttle or not, your engine is still running. Meaning there is still combustion being conducted....So i guess by your reasoning, combustion is happening with oxygen only right? cause oxygen likes to self ignite itself.....
When your coasting in gear regardless if you apply throttle or not, your engine is still running. Meaning there is still combustion being conducted....So i guess by your reasoning, combustion is happening with oxygen only right? cause oxygen likes to self ignite itself.....
It is not so much that the engine is still, "running," i.e. the combustion cycle is still operative--my point was that there is no fuel or air being fed in or burnt.
I do not have concrete proof either way, so I am not 100% sure if I'm right or wrong right now. I need to find a trustworthy source to answer the specific question: do modern fuel injection cars cut off all fuel, air and ignition when coasting above the idle rpm range or not. That's the question I am trying to find the answer for, and I have not found conclusive results stating that they definitely do, or that they definitely do. It may, as someone else mentioned, depend on the car, but I doubt it. I know that in the past I did read, from a reputable source, that modern fuel injection cars deactivate throttle and fuel completely when you completely let off the throttle and the engine is above idle range. This is regardless of the wheel speed and the gear; as soon as RPMs drop to and/or below idle range, or a figure slightly above idle, throttle is reopened to idle throttle. My problem right now is that I am unable to locate my original source. I've never seen and cannot right now find a reputable source that refutes this.
Doesn't mean I'm right, doesn't mean I'm wrong--it means I'm trying to find a concrete answer for everyone.
-Ed
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From: Triangle, NC
EddNog, believe it or not, you are exactly right -- to a point.
Most modern engines, and this includes the VQ series, do not send any gas to the engine when moving in-gear and with the foot completely off the throttle. That's right, zero gas, even at 7000 rpm. If you're going downhill a long stretch, and you leave the car in gear the entire way but don't press the throttle at all, you will use exactly no gas for the entire distance, and it would be infinitely fuel efficient. If you put it in neutral for the same trip, the engine would burn gas keeping idle going.
However, several factors keep this from being useful most of the time in the real world. The engine has its own internal friction and inertia, which increase as RPMs increase. If the engine is in gear, it will act as a brake on the car if it's not burning gas (which it doesn't when you don't hit the accelerator). On a level surface, your speed will drop too quickly for this to be useful. A constant gentle press of the accelerator in real-world situations, as we all know, is more efficient than accelerating, then coasting, then accelerating, then coasting, even if you use no gas when coasting.
However, if you're coasting downhill, and you have to obey the speed limit, you will be better off *not* pressing the accelerator and *not* putting it in neutral. You will burn no gas, while you would burn gas if you put it in neutral. The engine braking effect, which is a problem on a level surface, is countered by the pull of gravity (more or less) going downhill. As the law won't let you accelerate indefinitely going downhill, you might as well burn no gas going a set speed coasting. If you are accelerating too much, change gears to a lower gear (i.e. higher RPM) to get a higher engine braking effect; if you're slowing down too much, put it in a higher gear to lessen the engine braking.
Another situation in which it's more fuel efficient to be in-gear coasting than in neutral is when coming to a stop. Here, again, you will use *no gas* as long as the engine is in gear and you're not pressing the accelerator, whereas you *would* use gas if you put the car in neutral. While you'll get the engine braking effect, this is perfectly fine since you're trying to stop anyway. In fact, you can use the engine braking to help you slow more quickly, all while being more fuel efficient. (Whether this is actually better for your car than using just brakes is a totally separate question.)
I hope this wasn't too long, but I wanted to explain why EddNog was right about when engines burn gas and when they don't, and real-world fuel efficiency results may not always match up with what he said.
Most modern engines, and this includes the VQ series, do not send any gas to the engine when moving in-gear and with the foot completely off the throttle. That's right, zero gas, even at 7000 rpm. If you're going downhill a long stretch, and you leave the car in gear the entire way but don't press the throttle at all, you will use exactly no gas for the entire distance, and it would be infinitely fuel efficient. If you put it in neutral for the same trip, the engine would burn gas keeping idle going.
However, several factors keep this from being useful most of the time in the real world. The engine has its own internal friction and inertia, which increase as RPMs increase. If the engine is in gear, it will act as a brake on the car if it's not burning gas (which it doesn't when you don't hit the accelerator). On a level surface, your speed will drop too quickly for this to be useful. A constant gentle press of the accelerator in real-world situations, as we all know, is more efficient than accelerating, then coasting, then accelerating, then coasting, even if you use no gas when coasting.
However, if you're coasting downhill, and you have to obey the speed limit, you will be better off *not* pressing the accelerator and *not* putting it in neutral. You will burn no gas, while you would burn gas if you put it in neutral. The engine braking effect, which is a problem on a level surface, is countered by the pull of gravity (more or less) going downhill. As the law won't let you accelerate indefinitely going downhill, you might as well burn no gas going a set speed coasting. If you are accelerating too much, change gears to a lower gear (i.e. higher RPM) to get a higher engine braking effect; if you're slowing down too much, put it in a higher gear to lessen the engine braking.
Another situation in which it's more fuel efficient to be in-gear coasting than in neutral is when coming to a stop. Here, again, you will use *no gas* as long as the engine is in gear and you're not pressing the accelerator, whereas you *would* use gas if you put the car in neutral. While you'll get the engine braking effect, this is perfectly fine since you're trying to stop anyway. In fact, you can use the engine braking to help you slow more quickly, all while being more fuel efficient. (Whether this is actually better for your car than using just brakes is a totally separate question.)
I hope this wasn't too long, but I wanted to explain why EddNog was right about when engines burn gas and when they don't, and real-world fuel efficiency results may not always match up with what he said.
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From: Triangle, NC
Originally Posted by stylett9
What your saying doesn't make any sense.
When your coasting in gear regardless if you apply throttle or not, your engine is still running. Meaning there is still combustion being conducted....So i guess by your reasoning, combustion is happening with oxygen only right? cause oxygen likes to self ignite itself.....
When your coasting in gear regardless if you apply throttle or not, your engine is still running. Meaning there is still combustion being conducted....So i guess by your reasoning, combustion is happening with oxygen only right? cause oxygen likes to self ignite itself.....
This is not correct. The engine is still running only in that the pistons are still going up and down. This is from sheer inertia in the drivetrain. No combustion is happening that is forcing the pistons to go up and down until you hit the gas pedal again. This, also, is why you experience engine braking when you let off the gas -- while the rest of the drivetrain has rotational inertia that wants the car to coast, the engine pistons have their own inertia and friction that counters any longitudinal motion. The piston inertia is overcome by the combustion that happens when the gas pedal is pressed, and instead of the pistons slowing down the drivetrain, they instead speed it up.
ok, at this point i'm not really sure what to think.. but, here is what i'm thinking so far.
I see your point that the pistons are moving off of inertia but aren't the pistons still firing? Cause from what your saying is, the pistons are moving without any combustion. In that case, why is exhaust still coming out of the vehicle?
As long as the engine is running, there is always combustion is there not? regardless if your applying throttle or not. When the engine is on, there is always fuel being fed to make small combustion or large. Otherwise, when your engine breaking, your engine shouldn't sound like its on, it should sound like the motor turning over without making a spark. example, when your starting the engine and it hasnt fully ignited yet.
i'm not arguing, just trying to understand and ^^ is what i'm thinking so far.
I see your point that the pistons are moving off of inertia but aren't the pistons still firing? Cause from what your saying is, the pistons are moving without any combustion. In that case, why is exhaust still coming out of the vehicle?
As long as the engine is running, there is always combustion is there not? regardless if your applying throttle or not. When the engine is on, there is always fuel being fed to make small combustion or large. Otherwise, when your engine breaking, your engine shouldn't sound like its on, it should sound like the motor turning over without making a spark. example, when your starting the engine and it hasnt fully ignited yet.
i'm not arguing, just trying to understand and ^^ is what i'm thinking so far.
Originally Posted by ray427
Ed is right. When I had my 86 300zx it would constantly monitor mpg, not average it out like today's z. Anyway whenever I would let off the gas, in gear, for at least 5 seconds the monitor would read 99 mpg, in other words no gas being used at that particular time. Gotta love MT.
I used to have a bmw, and if anyone has a bmw you all know it has that little tach needle that shows your MPG in real time as your driving.
So yes when you let off the gas, the gauge shoots all the way over to the left simulating 99+ MPH, but same thing would happen if you put the car in neutral and are coasting.....
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From: Belmont, CA
Originally Posted by noir06
So you're saying that when you're coasting in gear, no fuel is being burned?
As in, it would be the same if I reached up and shut the key off? I don't think so..... Does your car go silent when you're coasting in gear? LOL
No, engine compression braking keeps the engine running but cuts fuel to the fuel injectors to slow the car, that is why the car stays running at RPM but is burning less gas than coasting. While coasting the PCM is feeding the car fuel, but cutting fuel when you are in gear and using compression to slow the car.
Originally Posted by xenophile
This is not correct. The engine is still running only in that the pistons are still going up and down. This is from sheer inertia in the drivetrain. No combustion is happening that is forcing the pistons to go up and down until you hit the gas pedal again. This, also, is why you experience engine braking when you let off the gas -- while the rest of the drivetrain has rotational inertia that wants the car to coast, the engine pistons have their own inertia and friction that counters any longitudinal motion. The piston inertia is overcome by the combustion that happens when the gas pedal is pressed, and instead of the pistons slowing down the drivetrain, they instead speed it up.
Last edited by Azrael Z06; Jul 10, 2007 at 04:38 PM.
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From: Triangle, NC
Originally Posted by stylett9
As long as the engine is running, there is always combustion is there not? regardless if your applying throttle or not. When the engine is on, there is always fuel being fed to make small combustion or large. Otherwise, when your engine breaking, your engine shouldn't sound like its on, it should sound like the motor turning over without making a spark. example, when your starting the engine and it hasnt fully ignited yet.
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From: Scarsdale, NY
Originally Posted by stylett9
ok, at this point i'm not really sure what to think.. but, here is what i'm thinking so far.
I see your point that the pistons are moving off of inertia but aren't the pistons still firing? Cause from what your saying is, the pistons are moving without any combustion. In that case, why is exhaust still coming out of the vehicle?
As long as the engine is running, there is always combustion is there not? regardless if your applying throttle or not. When the engine is on, there is always fuel being fed to make small combustion or large. Otherwise, when your engine breaking, your engine shouldn't sound like its on, it should sound like the motor turning over without making a spark. example, when your starting the engine and it hasnt fully ignited yet.
i'm not arguing, just trying to understand and ^^ is what i'm thinking so far.
I see your point that the pistons are moving off of inertia but aren't the pistons still firing? Cause from what your saying is, the pistons are moving without any combustion. In that case, why is exhaust still coming out of the vehicle?
As long as the engine is running, there is always combustion is there not? regardless if your applying throttle or not. When the engine is on, there is always fuel being fed to make small combustion or large. Otherwise, when your engine breaking, your engine shouldn't sound like its on, it should sound like the motor turning over without making a spark. example, when your starting the engine and it hasnt fully ignited yet.
i'm not arguing, just trying to understand and ^^ is what i'm thinking so far.
I knew I was right, I'm just not 100% sure of it because I still haven't found a source of concrete proof. the reason why I can be so sure of myself is because I read it from a reputable source in the past! My problem is that it was such a long time ago, I can no longer remember said source, and am now stuck with the task of trying to find that same source or another reputable one.
Or someone could find a reputable source that proves me wrong. That hasn't been done either. Without either, this is all conjecture to me.
-Ed
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From: Belmont, CA
Originally Posted by EddNog
I hope you're being sarcastic. I was completely wrong.
-Ed
-Ed
But back on track. The gas mileage of the Z isn't that bad, but I guess it is also relative to driving styles.
What other things would be considered downsides of the car? Barring the gas mileage issue is a bust based on individual cars, driving styles, and other issues...
To be fair I will start off with a thing I don't like about my "Z" (Z06) and that is the notchy feeling in the shifter. I really need to do the shifter detent modification...
Oh and the Bose(BLose) sound system sucks!
Last edited by Azrael Z06; Jul 10, 2007 at 04:52 PM.
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From: Little Elm, TX
More from Dr Isotope
I said I would try it, and I did. Five tanks worth. Based on Toyco's white paper showing the application of the Deceleration Fuel Cutoff, I left the car in gear at all times above 2k RPM, then shifted into neutral when coming to a full stop.
Tire pressures were cheacked at the beginning, middle, and end of the test period. An oil change was performed about 1 month before the start of testing.
Over the first ~17k miles of driving the tC, my fuel consumption has averaged out to 25.5mpg. while I don't think 5 tanks is really enough to show an irrefutable difference in economy using the DFCI, I think it's an acceptable indicator. On to the numbers.
Tank One: 12.0gal, 300 miles. 25.0 mpg.
This is slightly below my established average, but it included about 85% city driving.
Tank Two: 12.7gal, 322 miles. 25.4 mpg.
Right at my established average. No surprises.
Tank Three: 13.1gal, 361 miles. 27.6 mpg.
Decent mix of highway and city miles (trip to Disneyland).
Tank Four: 12.9gal, 309 miles. 24.0 mpg.
All city, drop in ambient temps could explain slight dip in overall mpg.
Tank Five: 12.2 gal, 314 miles. 25.7 mpg.
Again, right around my established average.
Total: 62.9gal, 1606 miles. 25.5mpg.
And though I know it won't, I hope this dispells some of the claims of DFCI wizardry. Seeing as my 1600 mile test, starting from a baseline of 25.5mpg ended with an average of... 25.5mpg.
I honestly... honestly wanted it to work. Even 1 more MPG would have been a positive result. But the actual result is that there is pretty much no impact. This included a trip to Disneyland and 2 trips to Knotts (about 125 miles roundtrip each), as well as a 40 mile there-and-back trip up the twisty CA Hwy 38. Sure, the ScanGauge said the down trip was sipping along at 37.1mpg... too bad the trip up was 16.5mpg.
Tire pressures were cheacked at the beginning, middle, and end of the test period. An oil change was performed about 1 month before the start of testing.
Over the first ~17k miles of driving the tC, my fuel consumption has averaged out to 25.5mpg. while I don't think 5 tanks is really enough to show an irrefutable difference in economy using the DFCI, I think it's an acceptable indicator. On to the numbers.
Tank One: 12.0gal, 300 miles. 25.0 mpg.
This is slightly below my established average, but it included about 85% city driving.
Tank Two: 12.7gal, 322 miles. 25.4 mpg.
Right at my established average. No surprises.
Tank Three: 13.1gal, 361 miles. 27.6 mpg.
Decent mix of highway and city miles (trip to Disneyland).
Tank Four: 12.9gal, 309 miles. 24.0 mpg.
All city, drop in ambient temps could explain slight dip in overall mpg.
Tank Five: 12.2 gal, 314 miles. 25.7 mpg.
Again, right around my established average.
Total: 62.9gal, 1606 miles. 25.5mpg.
And though I know it won't, I hope this dispells some of the claims of DFCI wizardry. Seeing as my 1600 mile test, starting from a baseline of 25.5mpg ended with an average of... 25.5mpg.
I honestly... honestly wanted it to work. Even 1 more MPG would have been a positive result. But the actual result is that there is pretty much no impact. This included a trip to Disneyland and 2 trips to Knotts (about 125 miles roundtrip each), as well as a 40 mile there-and-back trip up the twisty CA Hwy 38. Sure, the ScanGauge said the down trip was sipping along at 37.1mpg... too bad the trip up was 16.5mpg.
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From: Scarsdale, NY
Doc Iso is a buddy of mine, so believe me when I tell you that I'm not refuting his numbers. Thing you need to realize here is that you're talking about real life driving figures vs. concrete scientific, factual information. I'm not looking for test data--I'm looking for a manufacturer or equally reputable entity confirming whether or not all fuel, throttle and ignition spark are shut off when throttle is fully released and the engine is spinning above idle limit. Seth's figures are definitely concrete, yes, but I think that's more academic--what it proves is that regardless of whether or not throttle is fully shot off, it does not improve fuel efficiency over time for the routes he drives. That's all it proves; however, it most certainly proves it.
-Ed
-Ed
I was posting helpful suggestions (which were 100% accurate). Had I realized how out of control this thread got after my comments I would have set some of you dip****s straight a lot sooner. (no offense to the persons simply asking why).
I will direct any further off topic conversation related to the "idling wastes gas" FACT to the thread that has been created for the topic.
Thank you.
I will direct any further off topic conversation related to the "idling wastes gas" FACT to the thread that has been created for the topic.
Thank you.
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From: Pasadena, Md.
This forum may be more your speed:
www.myprius.com
Every Yin has it's Yang my friend. It's how the universe works. 300hp, well balanced...heavy, drinks gas. It's all about balance.
www.myprius.com
Every Yin has it's Yang my friend. It's how the universe works. 300hp, well balanced...heavy, drinks gas. It's all about balance.
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Joined: Jan 2007
Posts: 37
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From: Ohio
Ed, you can find the proof you are looking by searching through some patents.
Here are a couple of them that have Nissan's name on them:
http://www.freepatentsonline.com/4221191.html
http://www.freepatentsonline.com/4371050.html
Here are a couple of them that have Nissan's name on them:
In electronic fuel injection various engine operating parameters are sensed to give information on engine input and output conditions to an electronic control unit where the sensed variables are processed to optimize the fuel quantity delivered to each cylinder. To decelerate an engine, fuel is conventionally cut off by sensing the throttle being nearly closed while the engine speed is above a predetermined level.
A fuel cut-off control apparatus is disclosed which is responsive to various vehicle running conditions for cutting off the flow of fuel from a fuel supply system to an engine. The fuel cut-off control apparatus is adapted to interrupt the fuel flow when the throttle valve is in its fully closed position, the engine speed is above a first engine speed reference level, and the vehicle speed is above a first vehicle speed reference level. The fuel cut-off control apparatus is adapted to admit the fuel flow to the engine when the throttle valve is in its open positions, the engine speed is below a second engine speed reference level lower than the first engine speed reference level, or the vehicle speed is below a second vehicle speed reference level lower than the first vehicle speed reference level.
