Why diesel engines are revved before switching ignition off?

[sumitkalindi]

Hi guys, I have an answere to this question.

See most of the trucks and busses have a battery built by the freindly neighbourhood electritian. While building a battery the main concern is how many plates are giong to be in the battery more no of plate better the amper hour rating. As such there is no quality controll over the battery. So after whole day, especially the whole evening with soo many lights on the battery is already under strain.

So the Drivers just rev the engine at a constant high RPM, putting off all the lights trying to add that extra bit of juice in them.

Now the second and more important reason. These vehicles use air pressure to use the brake. The parking brake is just a valve that applies the air pressure or releases it making the brakes to work.

If you can, try to check what meter the driver checks while revving the truck/bus before cutting off the engine. It is the air pressure meter. As soon he has enough air pressuer, he is going to cut off the engine and head home. Battery charged or not charged.
Hope this cleared out some of your doubts.

[sumitkalindi]

By the way it also answeres another myth, why diesel vehicle need to be started atleast every other day. It all started by seeing the truck driders doing it. The reason is the air pressure drops due to leakage from valves and the works, so they have to started atleast every other day to maintain enough air pressure to keep the parking brakes operating.

[santosh.s]

DRC's post #29 is hilarious... felt like I am watching the laughter challenge show :lol
why don't you post "extra dose" of those remaining comments as well?

@ex670c, what is the difference between a dynamo and an alternator ?? I have been believing for loooong that these are just two synonymous words!

@suman, I think all four wheelers use some kind of power assistance for braking, be it air, vaccume or hydraulic pressue. All of them should be prone to leakage problems. You say that for trucks even parking brakes are dependent on this, but doesn't it defeat the purpose of having an alternate machanism, which would still work in case the main one fails?

For light vehicles like cars, parking brake is purely a mechanical arrangement. You apply a reasonable force to the lever, it gets multiplied by the lever action(s) as it reaches actual brake pads... thats it. May be for heavy vehicles this may not be able to generate high enough force at pads to keep them from moving?? I am simply guessing, any confirmation/corrections are welcome...

[jat]

Originally Posted by santosh.s

@ex670c, what is the difference between a dynamo and an alternator ?? I have been believing for loooong that these are just two synonymous words!

Alternating current - Alternator
Direct current - Well they couldn't name it as Director so Dynamo

Originally Posted by santosh.s

@suman, I think all four wheelers use some kind of power assistance for braking, be it air, vaccume or hydraulic pressue. All of them should be prone to leakage problems. You say that for trucks even parking brakes are dependent on this, but doesn't it defeat the purpose of having an alternate machanism, which would still work in case the main one fails?

[sumitkalindi]

Originally Posted by santosh.s

@suman, I think all four wheelers use some kind of power assistance for braking, be it air, vaccume or hydraulic pressue. All of them should be prone to leakage problems. You say that for trucks even parking brakes are dependent on this, but doesn't it defeat the purpose of having an alternate machanism, which would still work in case the main one fails?

For light vehicles like cars, parking brake is purely a mechanical arrangement. You apply a reasonable force to the lever, it gets multiplied by the lever action(s) as it reaches actual brake pads... thats it. May be for heavy vehicles this may not be able to generate high enough force at pads to keep them from moving?? I am simply guessing, any confirmation/corrections are welcome...

Good question. As you said, for heavy vehicles, specially our loaded trucks a mechanical parking brake is of no use, they can not generate high enough force to keep it from rolling in case it is parked in an incline. So it has to be a air pressure assisted one.

About the emergency situation we have one nice little trick on the heavy vehicles, which very few drivers know of in the plains, but every driver worth his salt in the hilly regions use it. It is called an exaust brake
Yes the so much talked after feature of the Volvos. It was present from quite a long time in our home grown TATA and LEYLAND.

It is basically a mechanical valve on the exhaust pipe, which can be activated on the flick of a lever. It basically traps the exhaust gasses from going out of the engine, which in turn resists the turning of the engine, which leads to fantastic engine braking. A definetly novel way of maximising the capability of the rough and tough engines.

Hope fully this will answere your questiones.

[santosh.s]

jat, thanks for the clarification....(especially to an electronics engg, who has done his m.tech. 7 years back )
Whether electricity is generated out of an alternator or dynamo, the battery has to be charged with direct current only. Why should dynamo need revving, whereas alternator dosen't need ?

Moreover "revving for charging" does not make sense, because when somebody is switching the engine off, it means he has travelled for a while and batteries should be already charged fully, isn't it?

Maintaining air pressure seems logical. I have personally observed that truck drivers wait for the air-pressure neddle to raise or the air tank makes that cooker like "vissle" ! But this is only when they are about to begin their journey after a long break or when the vehicle is idle over days. When their travel is finished, the revving just before switching off the engine is very short, lasting for a few seconds only... so, I think the reason is something else. Also, if engine has been running for a while then air pressure too should be full, just like batteries should be charged up!! Am I missing something?

@sumit, that was a good piece of information

[jat]

Originally Posted by santosh.s

jat, thanks for the clarification....(especially to an electronics engg, who has done his m.tech. 7 years back )

I did not know about your qualification. Thanks for reminding me. We would definetly need your help on Automobile electronics.


Any way here it is again
If AC is alternator then why DC can not be called director :-)

As far as explaining the revving I have said the same thing in post #9 and #26.

[santosh.s]

Originally Posted by jat

I did not know about your qualification. Thanks for reminding me. We would definetly need your help on Automobile electronics.

well, I just felt that I have been unaware of something which probably can be considered as very basic knowledge for my profession/qualification, and wanted to admit the same...did not intend to remind anybody of anything

Any way here it is again
If AC is alternator then why DC can not be called director :-)
#26.

If I had an answer to this question, then I would have already known the difference in the first place!

BTW, did some google'ing to know more about the word dynamo, out of curiosity. What it revealed is this- dynamo is a short form of "dynamo-electric machine", which implies "motion" and "electricity". It was used to refer to both AC and DC generators earlier, but later AC generator got the name alternator and dynamo continued to be used mainly for DC generators. (for example, refer to this: http://www.bookrags.com/sciences/sci...rator-woi.html )

[autozealot]

Hello guyz,

I think that the truckers rev the engine before switching off perhaps for the simple reason that the engine does not need multiple number of heaters whilst starting it the next time. (Just my way of thinking)

In smaller vehicles (old jeeps) its nothing but a practice of wasting fuel.

az

[sumitkalindi]

Originally Posted by autozealot

Hello guyz,

I think that the truckers rev the engine before switching off perhaps for the simple reason that the engine does not need multiple number of heaters whilst starting it the next time. (Just my way of thinking)

In smaller vehicles (old jeeps) its nothing but a practice of wasting fuel.

az

It will never work unless they stop for a tea stop, again then it will never be usefull, as the engine never go cold enough. Also If it is parked overnight. Ha Ha Ha no amount of revving is going to keep it warm enough.

[khaadu75]

Guys , I have a swift diesel and I just came to know a few days ago that one should idle the car before switching of the engine . How true is this and how important is it ???

The guys at Maruti didn't tell me this and I hadn't been doing it ... Ive driven almost 10k kms . I hope its not caused any damage and is there any other way of cooling the turbo ??

Cheers

PS - If this has been discussed .. pls post the link for me !

[DieselFan]

Another way to cool your Turbo is to drive at 1500 to 1600 rpm for last 2-3 kms. I do that always and do not wait even for 30 sec before shutting off the car. It was worked for me so far (Done over 20K km on my swift D).

[khaadu75]

Originally Posted by DieselFan

Another way to cool your Turbo is to drive at 1500 to 1600 rpm for last 2-3 kms. I do that always and do not wait even for 30 sec before shutting off the car. It was worked for me so far (Done over 20K km on my swift D).

I only drive at those revs .. so does that mean I dont need to idle before shutting down ?

[GTO]

Originally Posted by khaadu75

Guys , I have a swift diesel and I just came to know a few days ago that one should idle the car before switching of the engine . How true is this and how important is it ???

Idling after start / before shut down is recommended for any turbo-charged car. I'd rather be on the safe side, even if it is not mentioned in the owners manual.

Check this thread out : Linky (Do all Turbos require idling before shut-off?)

[DieselFan]

Originally Posted by khaadu75

I only drive at those revs .. so does that mean I dont need to idle before shutting down ?

Hi Khaadu,

If you have not gone beyond 1500-1600 you have not used the turbo at all so there is no issue.

But why? The fun with Swift D is to rev it beyond 2 K and it see it zip past all those petrol cars.