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Old 11-19-2005, 10:14 AM   #1 (permalink)
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Pressure blowoff valve for a turbo diesel

I just had a thought this morning as I was getting up.

Ever notice that diesels dont have a 'pop-off' valve like some of the gas-pots running around with turbos?

I looked through some of my diesel tech manuals and none of them talk about releasing pressure in the intake manifold between shifts of the transmission.

I know I'm missing something here and I'm sure it has to do with the diesel engine intake manifold not having a throttle in it.

Also, I do know that all turbos can suffer from compressor stall if they're shut off or pushed beyond their rpm limits.

Thoughts, experiences, information? [img]/ubbthreads/images/graemlins/shrug.gif[/img]
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Old 11-19-2005, 10:54 AM   #2 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

yeah, it's mostly to do with the lack of a throttle. A blow-off valve vents excess manifold pressure that occurs when the throttle is suddenly closed, since the compressor wheel takes time to spin down.
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Old 11-19-2005, 12:28 PM   #3 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

You are correct, since they do not have a throttle plate, there is a larger area for the excess manifold pressure to disperse into, even into the engine itself, although some of the wicked higher HP tractor pull diesels use electronic operated dumps.
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Old 11-19-2005, 03:16 PM   #4 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

Ok that's what I thought. I was looking through my diesel textbooks from when I was learning diesel technologies and saw no reference to a blow off valve on the intake tract of a diesel engine.

And some to think of it, I have heard blow off valves on the tractor pull units from time to time.

Of course some of those diesel are running 4 stage turbo systems with over 200psi of boost! [img]/ubbthreads/images/graemlins/eek.gif[/img] [img]/ubbthreads/images/graemlins/eek.gif[/img]
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Old 11-19-2005, 09:07 PM   #5 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

funny it should come up on the posts here - I use a blow-off ( rather than a wastegate) on my home brewed setup. the Benz spins at higher rpms than most diesels on here, and havign tried it both wastegated, and blowoff, I found I had less turbo lag on shifts with the blow-off. ( ie: the residual rpm in the turbo made up for the momentary off-on / pressure drop in the exhaust flow when I upshift)

here's a pic: [image]http://home.off-road.com/~northern_gypsy/images/engine_top.jpg[/image] (not sure if its going to be too big to show on the BBS if not here's a Link)

the blow-off is the ribbed unit attached to the outside radius of the U bend. outlet vent is on the underside..

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Old 11-19-2005, 09:24 PM   #6 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

One reason you might be seeing better response from a blow-off is it lags behind the boost in its action thus allowing quicker boost; by its nature a wastegate has to be set to start opening slightly early so that it can really start to open when it needs to. A blow-off is of course an energy waster but I guess you have determined the actual amount wasted does not bother you.

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Old 11-19-2005, 10:29 PM   #7 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

Yes, I would figure that you would want as much energy from the exhaust (heat and exhaust pressure) to be recovered by the turbo and put in to compressing the intake air.

I would be curious to see how much energy is absorbed and recovered by the temperature drop across the exhaust side of the turbo. I'll have to dig out all my thermodynamics books and notes.

Should be a good physics project to fill up one of the days of the long weekend coming up here. [img]/ubbthreads/images/graemlins/cool.gif[/img]
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Old 11-19-2005, 11:24 PM   #8 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

Ive heard thru some sources that the blow off is not used in conjution with standard turbos because they tend to over spool the turbo.On the Mack EGR engine there is a spring loaded blo off valve and it works alright??????? in this application because the VGT is helping to control the boost as well!The valve is Aledged to be set to go off at 33 psi even thou the engine will still make 35-8 psi boost Ive seen these valves leaking by at 20 psi.I really don't see the need for this valve on these engines when you have VGT but its there? [img]/ubbthreads/images/graemlins/rolleyes.gif[/img] [img]/ubbthreads/images/graemlins/confused.gif[/img]
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Old 11-20-2005, 09:29 AM   #9 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

Probably because the engineers don't have a very good handle on what exactly makes a VGT tick, yet. That is why they have to measure shaft speed for the computer too, to avoid overspinning the turbo. It is still very nonscientific

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Old 11-20-2005, 12:04 PM   #10 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

all good info. on my frankenmotor, I have more boost available than I can get fuel from the injection pump, so 'losing' air & boost isnt really an issue. The turbo is a KKK K26 which is an oil ocooled and lubed high speed car turbo, that supposedly has titanium alloy compressor, and turbine wheels. (?!?! its from a high performance european spec Audi that revs to ~8500 rpm) ) There is NO wastegate on this setup, just the blow off: its not got a blocked off wastegate, either - the K26 is externally wastegated, and the unit is no longer installed. So far, its been smooth, and reliable with the blow-off is set at 18-20 psi, mostly to ensure that I dont overpressurise the motor: it very rarely goes off.

Given that I have bigger injection pump elements, bigger injectors, and a few other refinements in the works right now, can someone elaborate on what happens when you either over-speed, or stall the turbo? Be nice to know what symptoms / indicators I'm looking for when I finish installing this monster into its new home. (I'm guessing the over-speed part has to do with temperature, lube, and cooking the bushings?)

oh - and birken, as regards wastegate bleedoff: this setup in its original form used an electric over pneumatic solenoid to restrain the wastegate until it 'hit' boost dead on. when it came up to full boost, the solenoid opened, and popped the valve wide. I got rid of it as the electronics kept fouling up when I went in deep water with the 4x4

tia

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Old 11-23-2005, 12:50 AM   #11 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

BD Power (local sponser I believe) has the blow off valves available as an add on, mostly for the bombed Cummins. Helps keep turbo stall (when the higher numbers are being run) from snapping a shaft.
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Old 11-23-2005, 09:16 AM   #12 (permalink)
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Re: Pressure blowoff valve for a turbo diesel

[ QUOTE ]
by its nature a wastegate has to be set to start opening slightly early so that it can really start to open when it needs to.

[/ QUOTE ]

that can be corrected with a pair of check valves or a boost controller.

one spring loaded check at the pressure setting you want max boost to be piped so flow through it goes to the gate, and one check valve [non spring loaded] piped parallel to the spring loaded one in reverse to the spring loaded one. easy to make, and you can leave the spring setting on your wastegate lower than the boost you want to run. once the spring check cracks, the gate will near instantly open limiting the boost, once the boost drops below the spring check settings, it will vent out the regular check valve
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