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Old 8th October 2013, 10:52   #31
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Default Re: Do High RPMs result in unburnt fuel?

As per my knowledge, the hydraulics, like the power brakes get their 'power' from the vacuum generated from the engine, or negative pressure used by the brake booster. And as long as the engine is spinning, be it by supply of fuel or by the motion of the wheels, it will keep creating the vacuum and hence the hydraulics will function just fine.
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Old 8th October 2013, 11:10   #32
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by HillMan View Post
With engine stall...
The post you replied to is about an engine running in gear without throttle, because the wheels are driving the engine, and not vice versa.
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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.
Even if you turn the ignition off at this point, how does the clutch get damaged?

You need to take a closer look at Mahindra's MicroHybrid system, and you'll realize that (a) use of handbrake is not required to turn off the engine; (b) the engine cuts off 2-3 seconds (not 1 minute) after coming to a rolling stop with clutch pedal disengaged and shift in neutral.

Turning off ignition turns off power supply to electric power steering units too, even if the engine is turning over.

And ECUs are programmed to stop fuelling (zero fuel supply to injectors) when an engine is running in gear without throttle (because the wheels are driving the engine) - if fuelling did happen, and the airflow (throttle) is shut because there's no A-pedal input, unburnt / half-burnt fuel will pass out into the exhaust, get ignited inside the (very hot) catalytic converter, and damage that as a result.
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Old 8th October 2013, 13:17   #33
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Default Re: Do High RPMs result in unburnt fuel?

In carburetor cars, free wheeling did decrease the petrol consumption, but with modern ECU controlled engines the minimum fuel consumption occurs when your foot is off the accelerator pedal. The ECU just gives the enough fuel to turn the engine over.

Another thing to consider is that the AC and breaks are operated by the engine, so free wheeling will impact these too. The best use of engine in gear while coasting is the breaking power that the engine generates.

In general it is better to accelerate and decelerate gently in the city as the moment you have picked up speed you may have to slow down (you save both fuel and brakes). That said aggressive acceleration may be your only recourse when you want to get ahead of the normal traffic or to overtake on the highways.

Free wheeling, coasting with engine off and generally rolling down incline are the tricks used for setting exemplary FE figures - 2 to 5 times the normal, but are not very safe driving methods, especially during normal commuting.
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Old 8th October 2013, 14:38   #34
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by Aroy View Post
... The ECU just gives the enough fuel to turn the engine over. ...
Actually, not even that. If the vehicle is in gear, it is the kinetic energy of the vehicle which moves the crankshaft. The ECU gives a micro burst of fuel every so often - but that is for keeping up the temperature in the exhaust chain for emission control.

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... Free wheeling, coasting with engine off and generally rolling down incline are the tricks used for setting exemplary FE figures - 2 to 5 times the normal, ...
That is how auto drivers think in Bangalore. It is really a myth, since those who do that are not conscious of their driving habits at other times - gear changes, acc pedal, etc.

Doing 50 in 4th can give better mileage than doing 0-70 constantly going up and down gears. Does anyone realize how much petroleum the country will save if a. people do lane driving and follow rules (all towards steady safe driving) and b. if those stupid speed breakers are removed everywhere? What we need in India is education, not just schooling.
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Old 8th October 2013, 15:47   #35
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by DerAlte View Post
Does anyone realize how much petroleum the country will save if a. people do lane driving and follow rules (all towards steady safe driving) and b. if those stupid speed breakers are removed everywhere? What we need in India is education, not just schooling.
+1 to this. Not only saving on fuel, half the traffic jams will disappear by lane discipline. Today we spend most of our time waiting and crawling on roads just because someone cut across an blocked oncoming traffic and some did a similar thing on from other side of the road.


Unless you have a good long stretch of road with a slope, you really wont save any cost by freewheeling. The FE saved will pay for additional wear and tear.
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Old 8th October 2013, 15:59   #36
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by HillMan View Post
... Unless you have a good long stretch of road with a slope, you really wont save any cost by freewheeling. The FE saved will pay for additional wear and tear.
What do you think with be the extra fuel savings - freewheeling over coasting in gear - in one journey in India? Usually that distance is <1% of the average journeys in urban areas, and <5% on highways.
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Old 8th October 2013, 17:34   #37
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I would like to say it again, when a car is in-gear with no accelerator input, some fuel still goes into the cylinders.
Proof- if you can try this, find an empty patch of road, take your beloved car to 60 kmph, slot in the 5th gear, take your foot off the accelerator, notice the deacceleration. Now turn off the ignition, notice the increase in deacceleration.

Fuel is fed to keep the movement of the car smooth and prevent the jerkiness of the engine.

And as I said, I only do this when I know I have to stop or take a turn after 300-400 mts. So eventually I will slow down deaccelerating and probably slot in 3rd or 2nd gear at 20-25 kmph and carry on. For the rest of the journey my speeds remain more or less constant.

Quite a few of you say that we should stay in gear to use engine breaking, what if I am coming down an incline and I do not need the engine breaking.

And there was one incident I remember, I was going at 50-60kmph in 5th gear in my Beat D, rpm would have been around 1500 when suddenly a cycle-wala decides to take a turn. I slam the brakes (without depressing the clutch) the car starts deaccelerating, but after 1-2 seconds when speed drops to 35-40 kmph and rpm drops to 900-1000, the ECU starts kicking in and engine starts pumping and interfering with my breaks and I had to depress the clutch. That moment was particularly unnerving for me, so from there on it became a habit that I always (mostly) depress the clutch while breaking, so that the engine does not interfere and I get confidence and the feel while breaking. At very high speeds and rpms above 2500rpm, I do not depress the clutch while breaking.
I have heard people saying that one can lose the control of the car by doing this, but how? It would help if someone could explain.
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Old 8th October 2013, 18:09   #38
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Default Re: Do High RPMs result in unburnt fuel?

Actually the proper method in such emergencies is to rapidly down shift, while breaking. This is a skill was taught in the driving school.

Coasting down slope is one of the most dangerous things you can do. Before you realise the vehicle will be rolling at quite a pace. Also remember that barring high performance breaks fitted in racing and rally cars, normal brakes will soon over heat and start fading if used for prolonged period (braking for slowing down while coasting in the hills). A lot of accidents happen because the drivers from plains do not realise this. To retain control of the vehicle while going down hill it is advised to use the same gear as used while climbing. On steeper slopes one gear lower will be needed, as the engine breaking may not be sufficient on steep slopes.
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Old 8th October 2013, 18:12   #39
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Quote:
Originally Posted by ahmad.007 View Post
Quite a few of you say that we should stay in gear to use engine breaking, what if I am coming down an incline and I do not need the engine breaking.

And there was one incident I remember, I was going at 50-60kmph in 5th gear in my Beat D, rpm would have been around 1500 when suddenly a cycle-wala decides to take a turn. I slam the brakes (without depressing the clutch) the car starts deaccelerating, but after 1-2 seconds when speed drops to 35-40 kmph and rpm drops to 900-1000, the ECU starts kicking in and engine starts pumping and interfering with my breaks and I had to depress the clutch.
Always stay in gear while driving down an incline. The general rule is to use same gear that you would use to climb up that incline.

The issue you faced while braking is the anti-stall feature kicking in. That's the point you will need to fully depress the clutch. You get used to it after a while.

Brake and engine together will share the load to give a better controlled braking. Hit the brake then the clutch instead of doing the other way.

Last edited by HillMan : 8th October 2013 at 18:14.
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Old 8th October 2013, 18:20   #40
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With incline I was talking about a flyover ending. Hands down agreed that you absolutely have to stay in-gear while descending a hill.
And HillMan, thankyou for the advice for- first brake then clutch.
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Old 8th October 2013, 18:30   #41
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by ahmad.007 View Post
I would like to say it again, when a car is in-gear with no accelerator input, some fuel still goes into the cylinders.
Proof- if you can try this, find an empty patch of road, take your beloved car to 60 kmph, slot in the 5th gear, take your foot off the accelerator, notice the deacceleration. Now turn off the ignition, notice the increase in deacceleration. ...
In 5th gear, wheel rotation will cause higher crankshaft RPM since the ratio is >1. That doesn't mean fuel is being injected. If at 60Kmph you switch the ignition off in 5th gear, the deceleration will be the same as with ignition on.

The behaviour is different in gear with engine on near idling - idling tries to keep the engine on and increase injection to prevent stalling.
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Old 8th October 2013, 18:34   #42
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Quote:
Originally Posted by ahmad.007 View Post
With incline I was talking about a flyover ending.
Here I'll suggest to stay in gear according to your speed. I usually just use engine braking to slow down a bit and give light touch on the brakes so the lights turn on to warn the ones behind. Just to brace for the light impact.
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Old 8th October 2013, 18:48   #43
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Quote:
Originally Posted by DerAlte View Post
In 5th gear, wheel rotation will cause higher crankshaft RPM since the ratio is >1. That doesn't mean fuel is being injected.
I am sorry, I do not understand that >1 ratio thing. What are you trying to signify with it? A little detailed explaination would help.


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Originally Posted by DerAlte View Post
The behaviour is different in gear with engine on near idling - idling tries to keep the engine on and increase injection to prevent stalling.
Near-idling the Ecu pumps in some fuel, agreed.


Deacclerating in any gear, with ignition turned on in one instance and turned off in another instance will have different results (I positively sure). With rpms far north of neutral.

I read somewhere in tbhp only that some guy got his car a diesel tuning box that manipulated the fuel pressure. He explained the working of some modes with reliable citations, he said that in eco mode (economy), the box completely shut off the fuel supply when deaccelerating in-gear, while the stock ECU maintained 1 unit pressure (psu I guess) he said.
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Old 8th October 2013, 19:12   #44
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by ahmad.007 View Post
I am sorry, I do not understand that >1 ratio thing. What are you trying to signify with it? A little detailed explaination would help.
Hope to answer this correctly and answering this on behalf of Der-Alte sir.

Quote:
A 2004 Chevrolet Corvette C5 Z06 with a six-speed manual transmission has the following gear ratios in the transmission:

Gear Ratio
1st gear 2.97:1
2nd gear 2.07:1
3rd gear 1.43:1
4th gear 1.00:1
5th gear 0.84:1
6th gear 0.56:1
reverse 3.38:1

In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmissionís output. In 4th gear, the gear ratio of 1:1 means that the engine and the transmission's output rotate at the same speed. 5th and 6th gears are known as overdrive gears, in which the output of the transmission is revolving faster than the engine's output.
Source: http://en.wikipedia.org/wiki/Gear_ratio

The part in bold will help you understand the part that you were not able to understand.

Anurag.
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Old 8th October 2013, 19:45   #45
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Originally Posted by a4anurag View Post

Hope to answer this correctly and answering this on behalf of Der-Alte sir.
The part in bold will help you understand the part that you were not able to understand.

Anurag.
Well, I understand gear ratios and stuff.
But I am not able to understand in what context is DeAlte sir trying to use that. I mean to say regardless of the ratio, the engine will still put a load on the spinning of the wheels, no matter however small it may become.
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