Why diesel engines are revved before switching ignition off?

[anupmathur]

Originally Posted by DieselFan

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

Correction: You have not used turbo BOOST, but the turbochager has been spinning away nonetheless! It's bearings do need their lubrication and cooling.

[DieselFan]

Originally Posted by anupmathur

Correction: You have not used turbo BOOST, but the turbochager has been spinning away nonetheless! It's bearings do need their lubrication and cooling.

Anup - Turbo will spin even when idling. the point I was making is that the bearing will not get hot. Else what is the difference between driving without turbo boost and idling?

[anupmathur]

Originally Posted by DieselFan

the bearing will not get hot.

Agreed DieselFan. I wish you had said it in the first instance itself!

And car makers have to keep the instructions very simple, so that the afficionado and the average joe can both use the car with success!

[greenhorn]

A bit offtopic, but why do buses/lorries idle in a rhythm, instead of constant revs

[sriturl]

Something which I heard long back.
Never followed with my petrol car in India, but followed in US.

When we start the engine, the rpm is slightly higher than the idle rpm(for Swift D idle warm rpm is 800, while at start it touches 900+)
Allow it to come to warm idle rpm (800 in this case)

Is this true? Can that be a bench mark than 30 sec or 1 min idling?
I normally see that it takes more than 1 min for rpm to settle down.

[svsantosh]

Originally Posted by greenhorn

A bit offtopic, but why do buses/lorries idle in a rhythm, instead of constant revs

Thats constant for them.

What I meant is for a human ear, 1000-1200RPM Idling of a car feels like a whine/whisper without a rhytm, But trucks (Indian) Redline at 2000-2500RPM. You can imagine the Idle Speed - 350-600MAX. (Do you know ships Engines Red Line at around 100-150RPM).

What you call a rhytm is a huge 6Cyl - 6-8Litre - DI Diesel Engine slowly clattering away at Idling speed. (Reason - Low Revs + HUGE FLYWHEEL = TORQUE TORQUE and More of It)

Ps - remember hearing the famous Indian Railways Diesel Loco's chugging away at platform's end in idling, what a sound to remember in our childhood. Check out the specs and you will know why that Giant;s idling is sweet music.

Engine: Alco 251-B, 16 cylinder, 2600hp (2430hp site rating) with Alco 710/720/?? turbocharger. 1000rpm max, 400rpm idle; 228mm x 266mm bore/stroke; compression ratio 12.5:1. Direct fuel injection, centrifugal pump cooling system (2457l/min @ 1000rpm), fan driven by eddy current clutch (86hp @ engine rpm 1000).

Source - [IRFCA] Indian Railways FAQ: Locomotives -- Specific classes : AC Electric

Hope that helps.

[Su-47]

Originally Posted by svsantosh

Engine: Alco 251-B, 16 cylinder, 2600hp (2430hp site rating) with Alco 710/720/?? turbocharger. 1000rpm max, 400rpm idle; 228mm x 266mm bore/stroke; compression ratio 12.5:1. Direct fuel injection, centrifugal pump cooling system (2457l/min @ 1000rpm), fan driven by eddy current clutch (86hp @ engine rpm 1000).

Adding to the OT stuff: Do you by any chance know how much Torque it makes?

Originally Posted by Su-47

Adding to the OT stuff: Do you by any chance know how much Torque it makes?

Quick calculation assuming a flat curve shows somewhere around 19K+ N-m. So next time someone says "My car pulls like a loco", we better take notice. Sorry about another post, the 20-min limit had expired :(

[DieselFan]

Originally Posted by sriturl

Something which I heard long back.
Never followed with my petrol car in India, but followed in US.

When we start the engine, the rpm is slightly higher than the idle rpm(for Swift D idle warm rpm is 800, while at start it touches 900+)
Allow it to come to warm idle rpm (800 in this case)

Is this true? Can that be a bench mark than 30 sec or 1 min idling?
I normally see that it takes more than 1 min for rpm to settle down.

Never Idle latest Gen Diesel engines. Engine do not warm up by idling.

Swift D should be driven at a low RPM (Below 2000) initially till the temp needle reaches the halfway mark and stabilizes after that increase the RPM beyond 2000.

[anupmathur]

Actually, the thread title is a bit misleading. Modern diesel engines are NOT revved up before being turned off; they are 'idled' (revved down, if you please) for a while before being switched off. This is done to allow some cooling-down period for the turbo.

[Jomz]

They don't need to do the revvin because of this reason because, at stop when the air is fully discharged, there si some sort of a spring mechanism to lock the brakes.

they need to rev up at start so that to build air pressure against the spring thingy.

Originally Posted by Sideways

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.

[Ignazio]

Originally Posted by Jomz

They don't need to do the revvin because of this reason because, at stop when the air is fully discharged, there si some sort of a spring mechanism to lock the brakes.

they need to rev up at start so that to build air pressure against the spring thingy.

Hi,
simply is a safety issue, big vehicles (bus, trucks) have a brakes system that work in negative way. You need air to unlock the brakes, if your air circuit has a leaking, your truck simply will stop once air is finished and compressor (driven by the engine) is not enough to keep air flow at the right pressure. When I was at National Service, we had a truck with a little leaking, my driver at start, had to revv a lot to get the pressure at the right level, to unlock hand brake and let us go. Many heavy vehicles also have pneumatic suspension and some have pneumatic controlled gear box. Fill in air reservoir (if you have a little leaking) I think is the only reason to revv the truck before stop, just to have top pressure in to reservoir and avoid wait too long at start...

Ciao

Ignazio

[kuttapan]

AH, missed this thread...Ignazio has come up with what I wanted to say...Parking brakes on buses/trucks are always air to deactivate. Remember my Dad telling this long back, when I was a kid and used to quiz him.

The TATA(earlier Benz) buses, though had mechanical parking brakes for a long time, before air brakes became default.

[SirAlec]

they are revved, to fill the air tank, for vaccum assisted brakes. If you don't rev it while on the next start you may have to keep the engine running idle until BAR guage reaches 6 bar (about 87psi). you normally hear warning beep.