Boat/Engine performance on varying ambient temperatures

Discussion in 'Boat Design' started by J.N, Sep 21, 2017.

  1. SamSam
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    SamSam Senior Member

    You are right. A turbocharger is a turbine compressor driven by the exhaust gases. A supercharger is a compressor driven by mechanical means. So let me re-phrase what I posted.
    I wonder if the locomotive had a supercharger, rather than a turbocharger, the decrease of power in hot weather could have been rectified if they by running the turbo supercharger faster...?
    I don't know if variable speed is an option on superchargers, but it sounds like it might be a solution to variable engine power output due to atmospheric conditions.
  2. SamSam
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    SamSam Senior Member

    I didn't know water injection is a thing...

    Water injection (engine) - Wikipedia
  3. SamSam
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    SamSam Senior Member

    Actually, if you varied the exhaust gasses running to a turbocharger, you could vary it's speed. They have simple push-pull valves for exhaust, I think they're called muffler cut out valves. Use the valve part way open for normal, all the way open for extra power.

    I'm not talking about retrofitting engines necessarily, but engines built by design so turbo/super chargers could have variable rpm independent of the engine rpm.
  4. kerosene
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    kerosene Senior Member

    They have all kinds of variable vane systems on modern turbos to help them work on broader set of conditions.

    I think that the very nature of turbo evens out the differences. When there is less resistance to compress it allows turbo to spool up - maybe.

    Anyway 80s turbo cars had quite simple controls.
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    They were a turbo, the whistle was quite noticeable, particularly when the throttle was eased. A number of these locos ended their days in Tasmania, some may still be in use there, in a quite cool climate.
  6. J.N
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    J.N Junior Member

    Thanks for your input again all. To stay a bit on-topic this is what I have so far:
    I've established a model based on ISO 15550 (Internal combustion engines - Determination and method for the measurement of engine power) which should give somewhat of a correction based on the circumstances (barometric pressure, relative humidity and temperature). Going to check this model with engine makers. On turbo-charged engines the ambient variances don't have a lot of effect.

    The sea water cooling systems (HT/LT) is still something I am unsure about. Any ideas what cold or hot (sea) water entering the system might do with the performance of the engine??

    I will continue researching on other parameters, like ship propulsion/hull performance vs. density in water.
  7. messabout
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    messabout Senior Member

    Cut to the bolts and nuts of the answer. Let's use a gasoline engine for the sake of explanation. A charge of fuel in the cylinder needs an appropriate charge of oxygen in order for the fuel to burn. Ideally there will be an amount of oxygen to cause the gasoline to burn completely. Seldom happens perfectly, but go along with the concept. The theoretically perfect proportion of the fuel/oxygen content is called stoicheometric ratio. For gasoline engines it is on average 14.7 to1.0 Both fuel and air, which contains oxygen, is measured in pounds or kilograms. So in order to burn one pound of gasoline, we must introduce 14.7 pounds of air into the mixture. That is a lot of air. which varies in weight at different temperatures. At zero degrees F the density of air is 0.0864 pounds per cubic foot. and at 160 degrees F the density is 0.0638 pounds per cubic foot. Quite a difference in oxygen content for a cubic foot of air when temperature variants are involved.. Oxygen is generally considered to be 21% of the total volume of an air mass. We are not measuring oxy but the weight of atmospheric air which ultimately relates to the oxygen content.

    It can be seen that temperature will affect the oxygen content of the air that is breathed by the engine. The engine has a finite capacity to induct air into its cylinders. (here we get into another variable called Volumetric efficiency...which is the efficiency of the engines ability to stuff air into the combustion chamber). Atmospheric pressure is a small but meaningful variable here. If the oxygen content of the air is more or less than optimum then the output of the engine will vary. Output is reasonabally predictable when the variables are accounted for.

    Incidentally when the fuel input to the cylinder is is less than the air induction for perfect combustion the engine is said to be running Lean. Two stroke outboards are famous for seizing when mixtures have been made too stingy of fuel. That is because there is excess oxygen in the combustion process which generates excessive heat and results in melted pistons and all that sort of thing. Just think about how an acetylene cutting torch is able to slice through steel plates....excess oxygen. When the mixture is rich in fuel, the available oxygen cannot burn all the fuel and the result will be wasted fuel and probably a sooty exhaust pipe.

    Turbos and blowers of other sorts do not violate the fuel/air ratio requirements. What they do is improve the ability of the engine to force the engine to breath more air, thus more oxygen, which in turn can burn more fuel, which produces more energy output. Turbos do not have magic properties, they just do what nature can not do. Atmosphere can cause a finite amount of air pressure into an induction system. Atmospheric engines have limitations for output. Air in- power out. Turbos and blowers can increase the amount of air that is inducted The amount of air fuel that can be efficiently burned is the gimmick. More air in the combustion chamber equates to more oxygen which can burn more fuel which can produce more piston pressures which determines the ultimate potential power of the engine.

    In any case, air temperature, which is a determinate of oxygen volume, is a major factor in the production of work that an engine can do.
  8. PAR
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    PAR Yacht Designer/Builder

    Of course ambient temperatures affect the engines performance envelop, sometime quite dramatically. Cooler air is more dense, so there's more oxygen available per cubic volume it swallows. This means the fuel mixture can be richer, causing the air/fuel mixture have more potential. The reverse is also true, with measurable decreases in performance. Water injection on WWII aircraft was used to cool the "charge" when in "war power" mode, which dramatically increased the engine heat. It was also handy at high attitude, where the air was thin. The compressed O2 molecule would release its oxygen, adding to the mixture potential. Additionally, the steam cleaned the combustion chamber, but also was hard on valves.

  9. Barry
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    Barry Senior Member

    To the OP,
    You have mentioned you are trying to find out if there is a relationship between the SEA water temperature and the engine output of the engine.
    Say from Arctic water temp of say 33 degrees F compared to tropic water temperature of maybe 70 degrees F.
    I would expect that your comparison should be between the engine water jacket temperature and engine output as a thermostat with an adequately sized
    cooling system should keep the engine water jacket temperature within a small defined range. (and under various loads at various rpm)

    Re the comments on increasing charge volume/pressure to accommodate less dense induction charges due to higher intake ambient air temperatures on engine mechanically driven superchargers or exhaust driven turbochargers or having them able to vary the volume/pressure of the induction charge,
    I was under the impression that an electronically controlled waste gate can provide this with little effort. But if you need the lower turbo induction charge, ie lots of 800 rpm to say 2000 rpm, power, and size the turbo with this in mind, then as the rpm increases, so does the turbos output and you have to bleed off some of this volume/pressure via the waste gate. This will take some horsepower as you are now moving more air which take energy and throwing it down the exhaust pipe.

    On pulley driven superchargers, a variable diameter pulley can control volume/pressure into the intake. With exhaust driven turbos, the waste gate control is mainly used.
    The advantage of having a variable volume/pressure charging system is that you can attain a higher pressure at lower rpm and/or maintain certain pressure/volume requirements for horsepower needs, say for towing loads at lower rpm etc.

    Volvo D6 diesels, address this issue on some models have a supercharger and a turbo. The supercharger, driven from an electronically controlled pulley, provides a higher intake pressure/ volume at lower rpm until the exhaust driven turbo has enough pressure/volume from the exhaust to drive the turbo. Then the supercharger idles when the pulley kicks open

    Perhaps Fredrosse can comment on this
    Last edited: Mar 4, 2018
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