Why diesel engines are revved before switching ignition off?

[jat]

As long as the engine is running, the turbo will also run and hence you will continue to hear. Idling is to reduce the rpm and stopping the turbo in shortest possible time, so that turbo does not get starved of oil. Once the mininum rpm and sufficiently low exhaust temp is achieved, keeping it running any longer serves no purpose. Revving will rev the turbo and thus it will run without any oil supply for longer time.

[nitrous]

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.

Sounds exactly like a turbo timer. If u have this, u dont need to wait for that 60 secs after switching on/before switching off.

[Sideways]

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?

To answer your question specifically - Most truck and buses have air brakes. This air is stored in a tank and needs to be charged and therefore these guys rev the engines before they stop and just after they start so that they have some kind of braking power. The rest of it is hogwash.

[Zappo]

Bah! So many different answers to the same question? Till now I gathered the following different reasons for why truckers rev before shutting down

• To clean up the accumulated soot.
• To have oil in the pipe. Helps them in restarting, without choke. (!)
• This charges up the battery.
• And the latest one is to recharge the air brake cylinder.

I can't remember but I think there are two more different reasons given there. Come on guys... Can we reach a mutual agreement on the reason for this now?

[Zappo]

Originally Posted by tsk1979

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.

Wow! tanveer bro... that one floored me. You mean I should fill up the exhaust pipe with water? Really? How much water?! :-/ Won't that affect the engine if it gets sucked in?! Yeh koi desi jugad to nahi hai naa?

[Sideways]

I can gaurentee that my answer is 100% right. Also this thread is a classic example of one thing being started and then getting completely diverted to a different topic. I think the mods should intervene and curb this sort of thing.

[Sideways]

I sincerely suggest you dont fill your exhaust pipes with water to clean them out. If the water gets sucked in you are going to get a really huge bill

[SLK]

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?

Nothing to add.. to all that has been said.... but its not just truck/ diesel drivers... this is a common practice for many people doesn't matter if they drive a bike or a truck!.... but its wrong as far as I am aware.

[falcon]

OK, My reasons for doing so...
1. Both my vehicle batteries were in a terrible state... if I did not race the vehicle for at least a minute before turning them off, there was no way they would start later on... so I had to do it! I've changes both batteries now, so its not needed… but... old habits die hard.
2. The Classic's engine is in need of an overhaul, I burn nearly a litre of oil every month... And as a result, my exhaust smokes (thick black smoke) a LOT... A temporary solution (I plan to get the engine rebuilt soon) that I am currently using, every month, fill the entire exhaust with water and then rev the engine till all the water is pumped out... repeat this about 3 times, and then no more smoke!!! To facilitate filling water, I have got a 1/2 inch bend pipe welded at the engine end of the exhaust... Yes, one more thing I have done is make my fuel mixture very lean.... have lost out on power and pickup :-( but at least the PUC is within limits...

[DCEite]

Originally Posted by Zappo

Bah! So many different answers to the same question? Till now I gathered the following different reasons for why truckers rev before shutting down

• To clean up the accumulated soot.
• To have oil in the pipe. Helps them in restarting, without choke. (!)
• This charges up the battery.
• And the latest one is to recharge the air brake cylinder.

I can't remember but I think there are two more different reasons given there. Come on guys... Can we reach a mutual agreement on the reason for this now?

True Zappo. I started this topic last year. Still not 100% clear on the answer though.

[jat]

To answer your question specifically - Most truck and buses have air brakes. This air is stored in a tank and needs to be charged and therefore these guys rev the engines before they stop and just after they start so that they have some kind of braking power. The rest of it is hogwash.

IMO, the air is not going to be charged fully just by revving it, and if it does it can be done even when you start the engine. If you charge the air tank and then keep the vehicle, the air is likely to be drained out in old vehicles and hence it will be always better to charge the tank after starting rather than before stopping.

As far as I know, revving is always bad for the engines whichever way you see it.

RK

[Steeroid]

Originally Posted by nitrous

Sounds exactly like a turbo timer. If u have this, u dont need to wait for that 60 secs after switching on/before switching off.

There is no sticker asking you to wait before switching off/after switching on (as seen in the Scorpios) inside my vehicle. I follow the process, nevertheless - I dont wait 60 seconds, but I wait for the engine to settle down to an even rhythm before I switch off / start moving the vehicle. The few extra seconds dont mean much to me, but it MAY mean something to the engine....

[Sideways]

Originally Posted by jat

IMO, the air is not going to be charged fully just by revving it, and if it does it can be done even when you start the engine. If you charge the air tank and then keep the vehicle, the air is likely to be drained out in old vehicles and hence it will be always better to charge the tank after starting rather than before stopping.

As far as I know, revving is always bad for the engines whichever way you see it.

RK

I completely agree with your last statement. Also agree that air can be charged by just starting and idling the engine. The point is that this way it is going to take longer, thats why they rev it.

I am sure someone on the forum owns a fleet of trucks / buses. Maybe they can have a chat with one of their drivers and clarify.

[DRC]

Anyone has fleet, I am sure he is going to get as many opinions as many drivers he has in his fleet

My personal experience, and no offence intended, all drivers I have talked to so far, have their own justification, which most of the times are too non technical...

Just a few I had ..
A tempo traveller can not have a Powersteering as it is a rear wheel drive..But a tata truck can have one though it is rear wheel drive because it as 4 tyres at the rear. A tempo tevellerr with PS will topple.

Sumos topple for no reason after 60, 80, 100km/h (depending on the driver)

Air breaks dont work in case of a puncture, as the tyre looses air. So they are good for heavy trucks only..

Powersteering is bad for Ghat roads, they take engine power while climbing up and loose control while climbing down

U must use high beam if any one comes in front of you (he says its a rule)

For a truck you should remove radiator cap when climbing uphill to keep the engine cool and keep adding water to the radiator. ( this one I heard during the delivery of a truck in Tata dealer ship, A driver convincing his owner, after the TATA engineer explained in length the importance of radiator cap.

I have many more..

[DRC]

Why so ever, anyone revvs his what so ever engine, follow the owners' manual. Period.