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Old 26th October 2005, 08:37   #1
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Default Why diesel engines are revved before switching ignition off?

I always have never understood this. Truck drivers, buses, and other big diesel vehicals always revv the engine while turning it off(or should i say just the moment they turn off)? Why is it so? Any technical reasons they are told to do so?
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Old 26th October 2005, 09:02   #2
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Carbon keeps depositing in the vehicles while they are in use and switched on. They if not revved before switching off would tend to have difficulty while restarting the engines.

Also I am sure you know that diesels don't have a spark plug to help them switch on. They use the compression technique that helps them purr to life. So more the snoot, difficult it will be.

You may have noticed that this is on the decline in the new trucks and busses. Also the cars don't need to rev before switching off.

You can go for the Indica Turbo if you are planning for one. It does not need to be revved.

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Old 26th October 2005, 10:13   #3
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Bad practice, bad habit, does more damage then anything, carbon accumulation is burnt off in modern diesels with proper IP and injectors in normal day to day running at operating temps, this stupid practice keeps the mechanics quite happy, if it is a turbo diesel, you would coke the bearings and fry the turbo in no time, sadly old and bad habits die hard, ignorance among our average truck drivers is another cause of this should I say uniquely Indian diesel myth.
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Old 26th October 2005, 10:19   #4
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Quote:
Originally Posted by Gurkha
Bad practice, bad habit, does more damage then anything, carbon accumulation is burnt off in modern diesels with proper IP and injectors in normal day to day running at operating temps, this stupid practice keeps the mechanics quite happy, if it is a turbo diesel, you would coke the bearings and fry the turbo in no time, sadly old and bad habits die hard, ignorance among our average truck drivers is another cause of this should I say uniquely Indian diesel myth.
My thoughts exactly.....
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Old 26th October 2005, 12:12   #5
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Actually the opposite has to be done for modern turbo diesels(even the indica turbo)
After starting keep revs low (below 1200) for first 1 minute
Before switching off keep revs low for one minute.
As for carbon deposit in exaust pipe this problem is solved by once a month water treatment
Fill exaust pipe with water, and then rev engine hard. Washes out all the carbon, but make sure you dont do it in your backyard.
Occaisonal revving up also clears the soot and also takes care of irritating drivers behind you who keep on honking at red lights.
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Old 26th October 2005, 13:15   #6
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fill water in the exhaust pipe??? You sure this can be done once a month??? i always idle for a min before switching off, never tried the idle for a minute before rolling off... would that do any harm to the car??? I don't rev it hard either....
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Old 26th October 2005, 14:32   #7
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Its needed for turbo charged cars. The if you rev the engine the turbo starts revving at high speed. If you switch off the engine immediately the turbo keeps spinning while oil supply is cut off.
While starting 1 min is recommended as by that time the turbo starts getting oil.
As for the starting part, any engine wether petrol or diesel should not be revved up immediately after startup. The reason is that oil circulation takes time to happen. Thats the reason you have these so called magnetic oils(Remeber castrol gtx magnetic) which stick to engine walls reducing startup damage.
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Old 30th January 2006, 17:45   #8
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Default Its got to do with the DYNAMO

Quote:
Originally Posted by dceite
I always have never understood this. Truck drivers, buses, and other big diesel vehicals always revv the engine while turning it off(or should i say just the moment they turn off)? Why is it so? Any technical reasons they are told to do so?
In older trucks 1940s-60 (non-Turbo) the vehicle had a DYNAMO as compared to an Alternator. So the truckers would revup their engines before stopping to
charge up their batteries.

Keepin' Jeepin'
Arka
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Old 30th January 2006, 17:51   #9
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Quote:
Originally Posted by ex670c
In older trucks 1940s-60 (non-Turbo) the vehicle had a DYNAMO as compared to an Alternator. So the truckers would revup their engines before stopping to
charge up their batteries.

Keepin' Jeepin'
Arka
Revving for how long? Because charging batteries should take some time.

RK
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Old 30th January 2006, 18:22   #10
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Quote:
Originally Posted by ananthkamath
My thoughts exactly.....
Mine too...
You can't just change people as fast as the technology changes..
I have seen the neighbours driver, an old man, revving the santro after he puts the car inside the garage at the end of the day..
Try to tell him.... He claims more experience than me and hence more knowledge.. May be true He might have understood which I have not..
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Old 30th January 2006, 18:25   #11
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Wink

One more reason can be when they turn the car off in non MPFI for starting it next time the fuel would be there in the pipe they wont require choke
I have seen drivers rev baleno up like crazy before switching it off. Hard a stone on my heart while hearing it.
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Old 30th January 2006, 23:49   #12
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Quote:
Originally Posted by tsk1979
Its needed for turbo charged cars. The if you rev the engine the turbo starts revving at high speed. If you switch off the engine immediately the turbo keeps spinning while oil supply is cut off.
While starting 1 min is recommended as by that time the turbo starts getting oil.
Also idling before shutting down lets the oil near the turbo cool down.

cya
R
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Old 31st January 2006, 00:18   #13
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Quote:
Originally Posted by Rehaan
Also idling before shutting down lets the oil near the turbo cool down.

cya
R
This was the only bugging thing about the Scorpio when I owned her for a year... I'm fussy and used to idle her for 60-90 seconds everytime before starting to move and before turning off the engine... This can be a real pain when you start and turn off your car more than 10 times a day and even embaressing while on a business meet/ date.

Boom to passenger/s: This your captain, MrBoombastic, please be seated for a minute... We will start to move/ we will turn off the engine in a minute
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Old 31st January 2006, 00:34   #14
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Quote:
Originally Posted by MrBoombastic
This was the only bugging thing about the Scorpio when I owned her for a year... I'm fussy and used to idle her for 60-90 seconds everytime before starting to move and before turning off the engine... This can be a real pain when you start and turn off your car more than 10 times a day and even embaressing while on a business meet/ date.

Boom to passenger/s: This your captain, MrBoombastic, please be seated for a minute... We will start to move/ we will turn off the engine in a minute
You don't have to spent time on cooling down. Since you are already slowed down while entering the parking area for some time, your vehicle engine is cooled down. Also after starting, check the temperature gauge . If it shows warm, the oil will start flowing faster. Just start moving slowly without revving hard.

RK
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Old 31st January 2006, 00:48   #15
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I do all this, but I still here the hum of the turbo winding down AFTER I've switched the Safari off...I've tried different lengths of idling time, but the hum is still there. Sounds nice, though.
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