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Chopping the throttles & turbos

Discussion in 'Engines' started by chesapeake46, Dec 3, 2015.

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  1. chesapeake46

    chesapeake46 Senior Member

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    In another thread, PacBlue stated as follows:

    "......Chopping throttles - love to whenever I can, especially coming back to harbor in glassy seas, late evening, it's just a personal thing!

    I always test new boats by chopping throttles at WOT, sea conditions allowing, to see how she glides and at what kind of attitude. Gives me some insight into hull design and efficiency..........."

    So, I am wondering. My hero and mentor once said to me that chopping the throttles is not a good idea because the trubo is spinning at a high RPM and the engine slows and the oil flow to the trubo slows and MAY starve the trubo bearings for oil because the turbo doesn't slow at the same rate as the engine RPM.
    Any truth to that ?

    I used to like to chop them as well, which brought on the conversation about it which made me change my ways.
  2. Oscarvan

    Oscarvan Senior Member

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    I don't like do anything quickly to engines. Gentle.
  3. RER

    RER Senior Member

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    I never do it. When I give operating instructions it's one of the things I tell people never to do. Particularly when entering a slow speed zone. I stress having the boat under the operator's control at all times. When you're surfing your own wake it's not. It bothers me when someone else is driving and they do it. It's actually a big pet peeve of mine. One of many.
  4. Capt J

    Capt J Senior Member

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    Very bad for the turbo's, even if they have a bypass. The turbo's are putting out a lot of boost at cruise RPM's and when you chop the throttles the boost has nowhere to go because the intake is closed and it put's a lot of backpressure on the turbo's, especially the bearings. Can even cause the turbine to hit the housing if the bearings have a little wear in them. Even if they have a bypass it still put's some before the bypass can bleed it off. As well as the turbo's not getting as much oil flow and fuel to cool them down. I never do this.
  5. Marmot

    Marmot Senior Member

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    The occasional case of a turbo "barking" from an overly enthusiastic power reduction is not something to lose much sleep over. The mass of the moving parts is not sufficient to keep the turbo at high speed, particularly when the charge air pressure exceeds the compressor discharge pressure which is what causes barking ... that condition slows the turbine down very rapidly.

    As for the oil pressure, chopping the throttles does not slow the engine down so fast that oil is not delivered to the turbo (which gets more than enough at idle anyway) which is slowing very rapidly in any event and doesn't need a great deal of oil. I have no idea where that myth came from. The only time oil is a problem is if it is not there at all, if it is dirty, if the drain is blocked, or if it is cooking off on a hot engine after shutdown.

    The rapid slowing, the bark, and the strange sound of the compressor rapidly slowing is what probably gets recreational boat operators nervous and adds to the mythology surrounding a phenomenon that they may not understood very well.

    There are probably more turbos damaged from inadequate cool down idling that causes "coking" of the bearings. Dirty inducers and turbines probably cause as many issues with balance and bearing failure. Unless the turbocharger is in such bad shape that it "barks" or surges very frequently, an occasionally chopped throttle shouldn't be cause for concern.
  6. Capt Ralph

    Capt Ralph Senior Member

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    I lost ya on this.

    I can assume your thinking the engine is quickly dropping rpm and the turbo is still delivering some pressure.

    This would be a quick time frame I would think, without expanding exhaust driving one end of the turbo, some restriction would slow the turbo down rapidly.

    With this pressure on the compressor side, maybe some un-natural movement of the shaft?

    Lost here also. Are you thinking waste gate?
    Last edited: Dec 3, 2015
  7. Capt J

    Capt J Senior Member

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    Yes wastegate/bypass valve where it re-routes the boosted air around or exhausts it from the intake track.

    It still puts some pressure on the turbo when you chop the throttles instantly and the boost has nowhere to go.....
  8. Marmot

    Marmot Senior Member

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    ????? It has someplace to go, back where it came from.

    The compressor stalls and the "boost" flows back out the compressor. That is what causes the "bark."

    The bypass you are talking about is used on turbocompressors that operate in a very limited but very high efficiency regime at a constant speed. Any change in inlet conditions or exhaust flow will cause those compressors to stall and surge so they are designed with a bypass to reduce surging. That technique is very rare, if not non existent on marine engines, particularly little ones that are not constant speed constant load devices.

    A wastegate diverts part of the exhaust gas flow, not the charge air. Bleed air is sometimes taken off the compressor discharge to control boost pressure, and in gas turbines is used to relieve compressor discharge on startup or to increase the stall margin. The bypass valve recirculates compressor discharge in order to maintain mass flow through the compressor and reduce the risk of surging.
    Last edited: Dec 3, 2015
  9. Capt J

    Capt J Senior Member

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    Yes, and it's not good for the turbo to come to a sudden halt and the excessive backpressure flowing backwards through a turbine designed to turn in one direction. It puts a lot of stress on both the bearings and driveshaft and superheats the air. It's not a good practice to do and we'll leave it at that. Coking is much worse, but rarely happens on boats because you idle to your slip and that's enough time for the oil to cool off the bearings.

    When I used to race cars, the guys with big turbo's on their cars, if they didn't have a big bypass valve that opened and vented the boost out at the end of the 1/4 mile when the throttle was instantly chopped, a lot of times the turbo's would grenade after chopping the throttle at the end of the 1/4 mile. It wasn't all of the time, or even often, but it was often enough. These were Turbo's with a heck of a lot less hours than your typical marine diesels. Same style turbo's also.

    I ran a supercharger instead of turbo, which was even worse because it was belt driven and if chopped the throttle, the belt was still spinning the supercharger very fast. But I had a big blower bypass that re-route the air around to the other side of the supercharger and worked off of vaccuum. If it saw vaccuum it opened.

    On a boat when you chop the throttles, the engine is still turning RPM's and turbo is spinning producing rated boost for rpm and throttle valve is closed, it's hard on the turbo (and also the blower if it's a D.D.). Detroit Diesel produced some Turbo Aftercooled diesels with a bypass.

    "When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge. Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

    Q. What is compressor surge?
    A. The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side. Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are: "

    https://turbobygarrett.com/turbobygarrett/faq

    Garrett made a lot of the turbo's for Detroit Diesel and other marine diesel engine manufacturers.
    Last edited: Dec 3, 2015
  10. Marmot

    Marmot Senior Member

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    We've already debunked the oil flow myth but I just reread this and noticed the "fuel to cool them down" claim. I would love to read how fuel is used to cool the turbocharger of a diesel engine.
  11. Beau

    Beau Senior Member

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    Just trying learn. Are you validating "chopping" the throttle as an acceptable regular procedure?
  12. Oscarvan

    Oscarvan Senior Member

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    Staying out of the discussion per se (as I said above, I try to be gentle with engines, especially expensive ones) but out of curiosity, why would the turbo DD still have a blower? Isn't that a redundancy?
  13. Capt Ralph

    Capt Ralph Senior Member

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    The DDC 2 stroke design requires the blower immediately to scavenge (blow out) the cylinders on startup and to run.
    While starting and until under a load, the turbo is in the way not producing any air flow at all.

    Later when you need some extra horse power, the throttles open up, hot expanding gasses (exhaust) turn the turbo, compresses fresh air, blows it to/thru the blower and helps develop that extra HP.

    DDC two strokes have to have that blower. They are completely different from the old Torro lawn mower, motor cycle or outboard 2 strokes.
  14. Marmot

    Marmot Senior Member

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    It is a practice that is validated every time a garbage truck works its way down the block.

    I don't recommend it as a way to operate a marine engine (or a boat) but I don't see it as imposing any great risk either.
    Last edited: Dec 4, 2015
  15. Marmot

    Marmot Senior Member

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    Like rcrapps said, the roots blower is just a scavenging blower, it doesn't provide any great degree of supercharging and always adds a parasitic load to the engine, it robs power.

    At high engine loads the bypass blower provides enough supercharging to unload the roots blower so it doesn't rob as much power.

    Some of the larger versions like the EMD 645 series engines do away with the roots blower and drive the turbocharger through an overrunning clutch which makes it an engine driven centrifugal scavenging air compressor. When the engine is loaded highly enough to produce a certain mass of exhaust flow the turbocharger spins faster than the driving shaft and from that point on, no power is "robbed" from the engine to provide supercharging.

    Very large slow-speed 2 strokes that are used to propel ships and large generating plants on land use an electrically powered centrifugal blower to provide scavenging air at startup and low loads and are switched off when power output reaches a certain point.
  16. PacBlue

    PacBlue Senior Member

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    Have done it many times, as I stated in calm sea conditions and no, I would not recommend to do it in a crowded entrance to a harbor or with lots of boats around you.

    On Detroit Diesel 6-71N,TI's, TA's, 8V-71TI's, 8V-92TA's, Series 60, Yanmar's, Cummins, 5.9, 8.3, 11, Volvo Penta 63, 72-74, D 11-12, MAN 8 & 10 Cylinders.

    No harm no foul so far (40 years). But there are plenty of times I ramp up slowly and ramp down slowly, so I would not make too big of a deal about it.

    Good gawd, have some of you ever seen a tournament sportfisher work the throttles on a money fish, now that's some aggressive throttle work there!
  17. Beau

    Beau Senior Member

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    Good gawd, have some of you ever seen a tournament sportfisher work the throttles on a money fish, now that's some aggressive throttle work there!

    Being a fisherman I well know how those boats are run - and that's why you write off those engines and tranny's on a resale/purchase. I am not opposing the notion that you can "chop" the throttle. I'm just opposing the notion that it's the best way to operate a marine engine over the long term.

    Is anybody honestly saying that "chopping' the throttle is the way to regularly run a marine diesel for the long term health of the engine - garbage trucks aside. I'm guessing, but I would say that probably most every owner's manual recommends a gradual acceleration and deceleration.
    Last edited: Dec 4, 2015
  18. Bill106

    Bill106 Senior Member

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    When doing sea trials on the last boat we were at the final full-throttle stage and the factory rep said "OK, now throw the levers full astern". I looked at him like he grew a second head but he said "really, jam them in full rewind, it's part of the test to time how fast she decelerates and goes into reverse" and it was for the QuickShift gears in her! Talk about coming to a screeching halt, as she started slowing the gears reversed and she "brake checked" and black smoked astern. Unnerving but evidently not anything CAT has a problem with.

    And I prefer gently cutting back whenever possible.
  19. olderboater

    olderboater Senior Member

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    The only ones I'll ever do that on are jets. There it's ok to jam them into reverse even, but just make sure it's not at a speed or in a way to dip the bow under. They don't have a transmission and the other components, you're just redirecting water flow. For any other type drive, only in an emergency.
  20. PacBlue

    PacBlue Senior Member

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    Just to clarify, no one is stating that it is the "best way" to operate an engine and no one is advising to do it "regularly". And some boat designs will be quite sensitive to the change in thrust and running attitude, so all on board would need to be warned in advance. And there are some boats that I would not even consider doing it, so a one size fits all approach does not work and was not recommended.

    I have related some of my experience when testing new designs as well, and the goal is to be as hard as you can (including crash stops) during your version of acceptance trials before the product goes to the public.

    But does it even matter with todays modern electronic controls and governors?