Whats a waste gate for

Discussion in 'Diesel Engines' started by Frosty, May 28, 2007.

  1. Frosty

    Frosty Previous Member

    It has already been writan by Stian that a tractor puller with compund turbos boosting up to 250Psi could get 5000Hp from a standard block.

    jees thats some boost and from a standard block????

    Now lets not go so far, say 20PSI from a 7psi boost motor, Thats not so bad is it. I dont think we are lifting heads here.

    However it is the max possible boost the turbo could possibly in its wildest dreams come up with. So what with a waste gate?

    If Im driving along and my boost guages go up a tad over what I THINK not what a spring and a diaphragm thinks I have the throttles in my hand so I can pull back a tad.

    As this is a boat forum I am generally talking about boat engines.

    I wouldnt leave a gen alone 5 minutes with out a waste gate. Especially if shes on full power.

    Some one said they run cooler ??? Are you sure look at a reasonable sized gen puling say 90%-- the manifold will be cherry red and the turbo housing will be more red.

    The thing is they love it like that they run for years.

    Its a wonder sometimes when you see them in the tropics glowing red in the daylight.

    If an engine is wanting to run away i think you should be shutting of some fuel rather than the cold air cooling piston crowns.

    Oh by the way this is me talking not google. I didnt really want a googling competition.
     
  2. StianM
    Joined: May 2006
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    StianM Senior Member

    yes but the option would be to have a engine fitted with a turbo not working at low revs.

    What did you exspect? If you put a turbo on a compresor would it produce bost? You can rev your engine to 200000rpm without geting bost.
    The turbo are running on power lost in the exhaust and if the engine is not producing power it wont have anny power to feed the turbo.
    Put some load on the engine to make it produce bost. Using CP proppeler you can runn 3000rpm and 50% and 50% pitch and you will probartly have no bost. Reduce the RPM to 1500rpm and increase the pitch to 100% and the engine will moust likely deliver more bost.


    If the cycles increase it don't necesary mean your load will increase so the bost won't increase.
     
  3. Frosty

    Frosty Previous Member

    I am totally confused with your replies.

    I am not suggesting you take out the valve from the waste gate just not have it!!!!!

    Why would a waste gateless engine not have boost at low revs --it wouldnt make any difference.

    The engines at 3800 are driving the boat----!! Yes of couse im not developing full power thats my point and I am flat out. And the governor comes in at 4200. Wast gate unused ??

    With the gen story youve got it back to front I am saying too much boost will cause a run away, In this situation I am sayin you need a waste gate.

    cycles are directly relative to RPM

    Ohh jees I throw the towel In.

    If any one wants to continue with this thread-- go ahead its all yours
     
  4. StianM
    Joined: May 2006
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    StianM Senior Member

    A marine diesel can runn 6000hours a year while a tractorpuller probartly have service for every 1hour running time and I gues you don't have custom made crank, cast pistons and custom rods to take the preshure.

    Exhaust from diesels are coolder than gasoline. Reason less Kj combusted/ unit air volume. Turbo deliver more air and therfor make the fuel air ratio leaner. Now if you decide to make power you could inject more fuel, but then the exhaust gas would be hoter. Make sence?


    I use altavista and I only used it for one picture. The rest is from the top off my head or webpages I allready know about.

    Lets say your engine is designed for 1bara bost. Then you would nead to pick a turbo producing 1bara bost on max load and it would probartly produce bost only at the high end off the powercurve.

    Now install a waste gate and you could get a smaler turbo that would start to charge from mid load and all the way to the top of the powercurve, but when you get there the turbo is overspeeding or delivering more bost than your engine is designed for.

    In the later example a wastegate could come in handy would it not?

    So your boat is owerpowered? Do you know what bost the factory had when driving the engine on the teststand?


    But increased speed don't mean increased load. The turbo is driven by exhaust.

    Delta T x kj/kg (1,05kj/kgK exhaust gas if I remember corectly) would give you the power from the turbine.

    The engine is running 100% load and if it overspeed there is no more fuel for the dieselpump to deliver so the power to runn the turbine won't be increased and therfor the compresorweel won't get more power from the turbine to compress air.
     
    Last edited: May 29, 2007
  5. Steve in SoCal
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    Steve in SoCal Junior Member

    Lets back up a bit and look at how the turbo is driven and why a waste gate is fitted.

    The turbo on a diesel is driven by both pressure and heat, an engine under a light load is not developing much heat or exhaust volume, the turbo still produces some boost to optimize combustion. If the waste gate is removed then the turbo could and would overboost the engine under higher load. You could restrict the exhaust side of the turbo in such a way as to effectively limit the boost capabilities without a waste gate but engine performance will suffer greatly. You can increase the boost of the turbo by resetting the waste gate opening pressure. Adding more boost alone is not going to make much if any more power.

    The engine needs more fuel along with higher boost levels to make more power. I don't know what engines you have but just look at Detroit Diesels where the injector size is how the power output is rated, I know that is not the whole story but, to make any significant gains you need more fuel. The amount of increase in boost is how much potential power you can gain, cooling the air charge and getting more fuel to the cylinders is the rest of the equation. If the boost temperature is too hot then the chance of engine damage is very real. Adding cool boost at higher levels with out more fuel can produce a bit more power but not large gains.

    As far as runaway engines; all runaways by definition are caused by an un-metered fuel source and to stop the engine you must stop oxygen flow. Either intake plates, or non-oxidizer like C02.

    If you want increase the power output of your engines and they are in good shape, you should be able to within reason. Remember that the engines will wear out faster and if you stress the engines too much you could break something, a piston most likely or drop a valve. What kind of engines are you talking about?

    Steve
     
  6. Carioca
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    Carioca Junior Member

    To add to Steve´s analysis as to why waste-gates are used:

    Well, I have seen truck diesel´s with and without waste-gates.

    Trucks need torque/power at low rpm´s to overcome inertia and get rolling when heavily loaded.

    So a turbo is fitted that will deliver considerable boost at low rpm´s and the designer meters-in the fuel required to provide the power desired.

    What happens at higher rpm´s ? Well, the boost gets kind of excessive, out of hand, if you like. The adjustment on the fuel-injection pump provides fuel in direct response to boost, so a limiter needs to be built into the pump, to limit power.

    Problem solved ? Well, partially.

    Engines , even diesels, may be damaged when run too lean, which would be the case if excessive boost were provided with limited-fuel injection. So boost needs to be limited. How ? Blow some of the exhaust gases out into the exhaust through a small hole in the casing of the turbine assembly, by-passing the turbine wheel. Less power to the turbine, less boost.

    Can you get by without recourse to a waste-gate ? Yes, but it would be near impossible to generate a flat-torque versus rpm curve.

    Penalty ? Much lower torque/power at low rpms, but acceptable at top rpm range. Truck is sluggish getting off the mark, but OK once it gets going.
     
  7. Frosty

    Frosty Previous Member

    If as you say a waste gate is only operational at high RPM where the boost needs to be regulated then why do you say you would have low torque at low end. To Quote The truck is sluggish getting off the mark?

    At this speed --pulling away, the wast gate is shut tight,---so it wouldnt matter if you didnt have it.
     
  8. Carioca
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    Carioca Junior Member

    At high rpm and, say , no-load, there is hardly any boost developed. Waste-gate is shut tight. Just rev up a turbo-diesel at idle and verify that the boost gauge hardly budges. Ditto for low-rpm and no-load.

    Now at high rpm and as load builds up, fuel is injected progressively into the charge, the exhaust gases are a great deal hotter and exhaust pressure higher. More power to spin the turbo. Boost is developed accordingly.

    If you were able to maintain high rpm but reduce load, as say would be easy to do on a dyno, less fuel would be injected, boost would drop, same for exhaust temperature. If you continue to reduce load, the high-rpm, no-load condition will be reached (negligible boost).

    At low/mid-range rpm and load, there is a much smaller mass-flow of air going through the engine, and consequently turbine. The requisite quantity of fuel is added for combustion. Exhaust pressure and temperature are considerably lower. Less power to spin the turbine. Smaller boost. A reduced engine power output.

    But now if you were to design and fit a turbo that makes the most out of a lower mass-flow of air, lower exhaust pressure and temperature, you could compensate for these unfavourable conditions. Such a turbo will develop more boost than a standard turbo. So adding more fuel, will develop more engine output power than would be possible normally at low/mid-range rpm.

    Then again, such a turbo would over-boost at high-rpm and heavy load. Thus the need to blow some of the exhaust gas energy off with a waste-gate.
     
  9. Frosty

    Frosty Previous Member

     
  10. Steve in SoCal
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    Steve in SoCal Junior Member

    Jack,

    Is this a boat or a car? If this is a car or truck, there are several things to check before you start making any modifications. What is it and what engine is in it, what is the horsepower and torque and, do you know what the max boost is and what it is supposed to be?

    All newer diesels have bigger turbos than they need for better power and drivability. On many diesel trucks just reducing backpressure at the turbo outlet is a big improvement. I have a friend that works on MTV's Pimp My Ride, they just put an 800HP Duramax in an old Impala, it smoked a Lamborghini at the drag strip.

    The idea that just removing the waste gate is make it all better is not true and you will not be able to keep in check with out constantly looking at the EGT and boost gauges. If the engine is rated at 2 bar of boost and you are only seeing half that your power is way down and you have to find out why before you can do any good. Are the injectors good and the valves adjusted? Start by going over all the basics and make sure everything is to spec. Tight valves will lower power quite a bit, is the airfilter too small?

    You sound like an intelligent man, don't do something crazy that can grenade the engine.

    Steve
     
  11. Frosty

    Frosty Previous Member

     
  12. Steve in SoCal
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    Steve in SoCal Junior Member

    Jack,

    I can't agree with what the man from Cat told you and in the last ten years a lot has changed. Engines are far more stressed today because the material science has developed better turbos and lighter engines. I don't know the marine engines as well but on highway engines will do an easy million miles if they are driven and cared for normally. The engines that you say are running 250 PSI boost in a stock block I find hard to believe. I know they don't last long at load and have a lot welding done to begin with. A top fuel dragster makes 8000 horsepower, it is less than 500 CID you would not tune an engine up for a race you change all the pistons, rods, bearings and gaskets. If you don't break anything. There is a huge difference between racing and normal operations. For general use a modest power increase is reasonable, major changes cause too many short comings.

    Steve
     
  13. StianM
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    StianM Senior Member

    A engine don't nead a waste gate if it's runn on constant rpm and constant load because you can match a turbo to that spesific engine. Once you change load and rpm the conditions is changed.

    Using sequensial turbos is one way, but is complicated and exspensive. A wastegate is a good and easy way to control the turbos operation.

    Increasing the turbopershure will increase the dynamic compresion ratio and the cylinder preshure. witch will increase load on all the mecanical components.

    I think now B&W use 4bara on there diesels, but they are two strokes and nead alot off preshure to savange the cylinder. They also burn HFO witch is not as easy to combust as MDO and therfor can benefitt from more air.

    If you have a complete combustion ading more preshure will only make the turbine make more backpreshure for the engine and the increased preshure will only give increased cylinder preshure and dynamic compresion and all you got is more load on the mecanical parts.

    When you think of a turbos operaion just forget the engine.
    The turbo don't nead the engine, The turbo only nead fuel just like a gasturbine. If you take a gasturin and replace the combustion chamber with a piston engine you have a turbo. Add more fuel to the combustion chamber and the power will increase.
     
  14. Carioca
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    Carioca Junior Member

    In this case, you would need to install a boost gauge as well as an EGT gauge and keep your eyes glued to them.

    Plus keep your ears tuned, and teach them to identify the high-boost condition, as a back up for the gauges.

    The engine manufacturer cares about just one thing, viz., the reputation of his product and warranty claims.

    Result ? He throws in a waste-gate, for no extra charge !

    Current state-of-the-art diesels achieve a similar result with VGT (variable geometry turbine).

    I wouldn´t want one on a boat, the added complexity being more a liability than an assett.

    Same holds for electronic diesel fuel-injection. My preference is for the simpler, albeit less efficient, mechanical fuel-injection pumps
     

  15. Frosty

    Frosty Previous Member

    Finally--- some one can accept the concept.

    And for a boat to have electronic injection is just well, plane daft.

    Not on my boat thank you.

    Already have mutiple stage injectors and all sorts of fancy stuff.

    Simple in line ,--them were the days.

    Talking of electronic injection systems I recenty advised a super yacht to carry a full wiring harness and ALL the black boxes ,sensors and relays that go on the engine. TWO of them--- for each engine.

    In thier Catapilar engine manual it says check fuses, re sets, chech this and that, and if it doesnt start call you nearest Catapilar dealer.

    Hello is that Catapilar? yes this is "MV lucky" we are approx 800 miles east of Madagasca, Yes that right lightening strike. Two gens and two main engines wont start.

    Hello hello-- whats that? ---call mayday???
     
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