Turbocharger doubt

Bypass valve may automatically return some charge air to the compressor inlet to improve acceleration
when running upto speed .( refer A.J wharton page:39 )

can anybody plz explain this to me…

Thanks for the question, have you done much research about the theory of the gas exchange processes and scavenging?

yes sir, i have already read about it but this doesn’t make sense to me …
plz sir explain this…

I really would not be the right person to explain this in a more logical way, than what is available already by the people that have written the articles or text books.

no problem sir

If you open a bypass valve so that the turbo doesn’t have to compress air, it can accelerate up to the desired RPM more quickly. Once the bypass closes, the turbo is at operating RPM and lag time to full boost is reduced. If you have ever heard a blow off valve on a turbocharged car; that valve opens when the throttle is slammed shot so that the restriction in the intake system of the car doesn’t slow the turbo down between shifts, same principal. Pumping air in a circle is easy; compressing it takes work.


thank u so much for the answer sir

You have a knack for posting the most useless, empty statements known to humanity.


Bypass valve may automatically return some charge air to the compressor inlet to improve acceleration
when running upto speed .

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Since the header says - Marine Engineering - we are talking about turbochargers used on ships, right ?
There are lots of different kinds of turbochargers but they all have two things in common . They are driven by the exhaust gas of the engine they are attached to and they are used to improve the combustion due to delivering more air into the cylinder as a normal suction cycle could do. This said it narrows down to more or less two types of engines ( wether two-stroke or four-stroke ).

  1. The fast starting and revving up engine with a fixed revolution -like a generator engine.Probably four stroke.
  2. The two or four stroke engine with adjustable revolutions. Mostly used for propulsion purpose.
    For the first option the turbocharger/s are fixed and fitted and deliver due to more or less constant revolutions compressed air in amounts as constructed and adjusted by maker so that performance and environmental results are met.
    The second option has the problem of varying revolutions thus producing various amounts of exhaust gas resulting in different turbocharger speeds. In the engines low revolution range you must have additional electrically driven auxiliary blowers which support the insufficient amount of compressed air by the main chargers in order to meet the air-hunger of a good combustion.
    Once the engine reaches a certain revolution and has “enough” exhaust gas for the T/Cs to produce sufficient compressed air a pressure sensor will decide to cut off the help of the auxiliary blowers.
    Thats all. A " blow off " of surplus air will occur if you are running at full speed or full load with your respective engine and there is a sudden and unexpected considerable drop of load. The surplus air of the T/Cs cannot be consumed due to a crack stop and is blowing off with loud noise or blow off valve or whatever. Hope to gave you a glimpse into the interesting realm of ship engineering problems…
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In order to support the “air-hunger” of a fast running up engine ( mostly used for generator engines) the attached turbocharger gets help of boost air ( compressed air delivered through solenoid valved bypass line ).
Reason is to avoid black smoke from funnel and help the T/C to revv up more quickly. Its only a boost for a short time and once the engine reaches its service speed within appr. 30-60 secs. the attached T/C delivers sufficient air for a clean combustion.

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Is this T/C ‘barking’? Some have a recirculation valve to avoid barking, is that true? I haven’t experienced this yet, but there are some dramatic videos out there.

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@oildrop thanks for joining the discussion your vast experience is appreciated.

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Let’s say a ninety-thousand hp 2 -stroke engine with four turbochargers is driving a container vessel 320 mtrs. long and 46 mtrs. wide with a draft of 14,5 mtrs. in heavy seas.
If the Captain is a cautious man he instructs his mates to adjust accordingly, especially the second mate in dog watch 00-04 am. If not, it might happen that the propeller is emerging and the fuel racks are abruptly retracted by the governor of the main engine. The engine revolution is reduced remarkably and the surplus of air from the turbochargers doesn’t know where to go - it flows back through the suction strainers of the chargers, giving a barking or coughing or whining - however you want to name it - noise. Latest then the C/E is on the bridge and shouting to the poor 2nd. mate to reduce revolutions. I have never seen recirculating valves because cautious seamen avoid situations like that and why invest money to prevent it ?
But you are right, there might be situations on vessels where this is necessary.


As long as this has turned into a stream of conscious dump on turbos, sir you have overlooked one of the most amazing inventions of man, the EMD 645 series, two cycle with a gear driven turbocharger and overriding clutch. Zero to 900 rpm and breaker closed in seconds although I grant you there will be some smoke involved.


Weren’t those the ones with the supercharger and turbocharger?

EMDs turbos are mechanically driven up to about 700 rpms. The drive clutch disengages and goes in to turbocharger mode. Other 2 cycles with roots blowers and turbos are GM 71s, 92s, 149s are some of the high speed diesels. The only medium speed I’ve worked with like that are Wichmann. The direct reversible Bronz were turbocharged but had to operate with a pressurized engine room.


Jeez to many Indians and not enough Chiefs (unlimited) …the correct term for ‘barking’ or ‘coughing’ is turbocharger surging and compressor stall and is an extremely complex interaction between engine speed and load interacting with the gas flows through the turbine and air flows across the compressor. Most engine manuals will have load diagrams detailing the above. For propulsion engines it is exacerbated when you add in CPP and try to meet Tier III.


Feeling frisky today are we?

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