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Old 3rd October 2013, 11:05   #16
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Default Re: Do High RPMs result in unburnt fuel?

The maximum FE is at the peak torque. As you move away from the peak torque (both up and down) the fuel consumption increases impacting FE.

Lower FE means that some fuel is left unburnt, so at very high or very low RPM where the torque is much below the peak torque, fuel will not be fully used up, hence some fuel will remain unburnt.

That said you can design engines with pretty flat torque over a wide RPM range or with torque peaking at higher RPM to produce more power at high speeds. Most of the heavy trucks have pretty narrow torque bands to increase power. The down side is that you need a large number of gears (at times 12 or more) to get a wide range of road speeds. In contrast earth moving machinery engines are designed with flatter torque to accommodate a wide range of engine speeds for powering accessories and implements.
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Old 3rd October 2013, 11:14   #17
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Default Re: Do High RPMs result in unburnt fuel?

Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
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Old 3rd October 2013, 17:08   #18
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Quote:
Originally Posted by Aroy View Post
The maximum FE is at the peak torque. As you move away from the peak torque (both up and down) the fuel consumption increases impacting FE.

Lower FE means that some fuel is left unburnt, so at very high or very low RPM where the torque is much below the peak torque, fuel will not be fully used up, hence some fuel will remain unburnt.

That said you can design engines with pretty flat torque over a wide RPM range or with torque peaking at higher RPM to produce more power at high speeds. Most of the heavy trucks have pretty narrow torque bands to increase power. The down side is that you need a large number of gears (at times 12 or more) to get a wide range of road speeds. In contrast earth moving machinery engines are designed with flatter torque to accommodate a wide range of engine speeds for powering accessories and implements.
Agreed, the engine is at its most efficient rpm at peak torque.
But I tend to think that peak torque theory is good if you are using all that torque, i.e., when acclerating. If you are driving spiritedly, you will be efficient if you stay around the peak torque.
For crusing purpose, I think speeds between 70-80 kmph and 5th gear will generate best results even though peak torque may arrive much later in the rpm range. Air drag being the culprit.

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Originally Posted by devansn View Post
Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
The turbo spools up as far as my experience goes.

Last edited by aah78 : 3rd October 2013 at 17:43. Reason: Posts merged. Please use QUOTE+/MULTI-QUOTE while quoting multiple posts. Thanks!
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Old 3rd October 2013, 21:31   #19
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by DerAlte View Post
No. By design, the mixture is slightly lean under almost all circumstances.
Any range of values?

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* If the injection control is mechanical (in-line or distributor pumps without electronic control), one can consider gap tolerances and fuel pump piston lag (at 'full speed) to figure out whether there is excess fuel being injected
??

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Old 3rd October 2013, 21:37   #20
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by devansn View Post
Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
No, but more to the point why would you want to?

The turbo is directly in the exhaust, so no matter how much or how little exhaust gasses or present it turns. In fact if it would not turn it would actually be an obstruction, so let's keep it spinning!

I'm not sure if there is anything to gain by not having it spinning?

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Old 6th October 2013, 01:13   #21
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Originally Posted by devansn View Post
Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
The Turbo spools based on the exhaust gas pressure and nothing to do with transmission. So the moment the pressure needed to turn the vanes are achieved, the turbo spools.

By the way what is the point in revving an engine in neutral?

Anurag.
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Old 6th October 2013, 10:28   #22
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Quote:
Originally Posted by devansn View Post
Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
Under hard revving (full throttle) the turbo will definitely spool up, but under soft revving (taking rpm to 3-4k with light throttle) the turbo won't spool up completely.
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Old 6th October 2013, 10:46   #23
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by devansn View Post
Don't know if this question would be OT, if we rev a turbo engine hard when in neutral gear, would modern engines have some mechanism not to turn on the turbo?
The ECU could detect the transmission is in neutral and activate the waste-gate on the exhaust side, which will result in the exhaust gases bypassing the turbo, preventing it from spooling up. AFAIK, cars don't have this feature, because the cost of engineering this feature into an engine/EMS will not outweigh the questionable benefit of revving a turbocharged engine in neutral.

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Originally Posted by a4anurag View Post
By the way what is the point in revving an engine in neutral?
+1 to that - I loathe people who rev engines in neutral to 'feel its power' or 'hear its sound'.

Last edited by arunphilip : 6th October 2013 at 10:56.
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Old 7th October 2013, 22:09   #24
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I have a query. I have heard people saying that one should not shift into neutral when on the move, and should use the clutch only for changing gears.

I have a habbit, sometimes I like to accelerate hard and when I know that I will have to take a turn or slow down after say 400 mts I shift into neutral and let the car deaccelerate freely. On the contrary people say I should not shift to neutral. Suppose I take the car to 70 kmph and then shift to neutral as I know I have to slow down eventually, rather than shifting to 5th gear and let the engine run at an odd 2000 rpm. What I feel is that if I shift to neutral, the engine comes down to its idle rpm of 700-800rpm and so the FE will be better to that compared to when the engine is running at 2000rpm. Since we are not pressing the accelerator, the engine is maintaining that rpm with least needed amount of fuel. So naturally, it should be easier to maintain 800 rpm rather than maintaining 2000rpm. And its not the case that fuel is totally cut-off during deacceleration, I have heard some slight quantity is still fed to the engine.


Need experts to throw some light on this. Is free-wheeling good for FE or bad. If supposed speeds are above 70 kmph.
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Old 7th October 2013, 23:48   #25
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Free wheeling is alright as long as you are in control. Many automatic gearbox do it.

Freewheeling is not good for brakes though. That's because entire load of vehicle is handles by the pads alone.
The clutch is stressed when you bring it back to the gear from neutral. The engine speed and wheel speed is difficult to judge. Clutch is a pretty expensive component to repair.

If you feel a sudden pull/slowdown thats not good for the clutch. Similarly stalling and jerks from engine are equally bad.
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Old 8th October 2013, 00:01   #26
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I freewheel only when roads are empty and traffic is low. And I make sure that when I get back to a gear it is the right one and the highest possible one. The brake pads of my Beat got consumed up quite quickly, I think now I now why.
ThankYou for the clarification HillMan.
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Old 8th October 2013, 07:01   #27
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Quote:
Originally Posted by ahmad.007 View Post
I have a habbit, sometimes I like to accelerate hard and when I know that I will have to take a turn or slow down after say 400 mts I shift into neutral and let the car deaccelerate freely.
A very dangerous habit I must say. Please stop this habit. It is best to keep the car in higher gear and keep the foot off the accelerator pedal. This will use the engine braking to slow down the car and you can use the brakes also lights rather than depending on the engine braking.

There would be a situation when your car is at say 50 kmph and in neutral, a emergency arises and you have to swerve! As human tendency in emergency should be to hit the accelerator pedal but when you do this you ain't going anywhere as the car is in neutral so that could lead to an accident but if the car was in gear and you hit the pedal at least you would have got time to steer out to safety faster.

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Originally Posted by ahmad.007 View Post
Need experts to throw some light on this. Is free-wheeling good for FE or bad. If supposed speeds are above 70 kmph.
I don't think free-wheeling improves FE. When the car is say at 70 kmph and in neutral with leg off the accelerator the RPM falls down to say 800 rpm the engine is still running with a small amount of fuel being fed to keep that going on BUT if the car with the same condition above is in fifth gear and leg is off the pedal then fuel supply would be nil and the engine is running on its own inertia. Modem ECU's have a system to stop fueling if the car is in gear + NO accelerator pedal input given. So here your saving fuel and more safer as the car is in gear.

Drive Safe,

Anurag.

Last edited by a4anurag : 8th October 2013 at 07:06.
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Old 8th October 2013, 07:47   #28
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Originally Posted by a4anurag View Post
Modem ECU's have a system to stop fueling if the car is in gear + NO accelerator pedal input given. So here your saving fuel and more safer as the car is in gear.
I'll have to disagree on this point. Modern ECUs do have fuel saving features, but do not stop fuelling. Your brakes and power steering depends on it. The engine needs to maintain a minimum rpm. In this scenario it runs on a lean mix, but will slow down the speed.

Killing the engine at high speed can put enough load to damage the clutch and also cause steering and brake failure.

The micro hybrid tech switches of the engine if hand brake is engaged in neutral, assuming stop at traffic, that too after around a minutes wait.

The key to better FE is to maintain a constant speed in top gear. Every car had a sweet spot, find that rpm. Keep a light foot on the pedals. Once you reach the desired speed, keep it constant.

After freewheeling you will again push the car to reach your cruising speed, there is loss of energy.
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Old 8th October 2013, 08:11   #29
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by HillMan View Post
I'll have to disagree on this point. Modern ECUs do have fuel saving features, but do not stop fuelling. Your brakes and power steering depends on it. The engine needs to maintain a minimum rpm. In this scenario it runs on a lean mix, but will slow down the speed.

Killing the engine at high speed can put enough load to damage the clutch and also cause steering and brake failure.

The micro hybrid tech switches of the engine if hand brake is engaged in neutral, assuming stop at traffic, that too after around a minutes wait.
You are misinformed. The parts in bold as written by you, just do not hold true.
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Old 8th October 2013, 10:37   #30
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by SS-Traveller View Post
You are misinformed. The parts in bold as written by you, just do not hold true.
Sorry, Going

Found these two reference links to make my point. I partly agree with you on the point of power steering and brakes are not affected by engine.

With engine stall the hydraulic and vacuum are directly impacted making the power steering and brakes very hard to use. Not a total failure though.

http://wiki.answers.com/Q/Do_brakes_..._engine_stalls

http://www.howstuffworks.com/steering4.htm

The engine might not have an affect on the new generation electronic steering because they don't work off the hydraulics.

My take is more from what I read around and experience. I have no mechanical engineering background. So, feel free to correct me.
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