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  • Mick Broom
    replied
    polini cooling

    HI Wally,

    Thanks for that

    Now that makes some sense

    Mick

    Leave a comment:


  • Wally Hayward
    replied
    polini cooling

    Mick... also got me that we are talking about "pulse supercharged" engine...................

    Partial quote to promote more thoughts, from
    http://www.challengers101.com/ExhaustGasTemp.html
    (Actually a reprint of an article from the April 2003 issue of Ultra-Flight Magazine. )
    .........
    Even though, exhaust systems for aircraft engines are designed to provide power over a wide RPM range, there will be operating ranges where the reflected pressure pulses will not be arriving at the exhaust port at the right time. The high and low pressure waves will shift one way or the other inside the exhaust. This shift will cause a relocation in the hot exhaust gas pulses, as well, and can cause spikes in EGT readings at certain, rather narrow, engine RPM ranges.

    These spikes in EGT, either up or down, generally have nothing to do with fuel-air mixture. The temperature of the burning gasses in the combustion chamber may not have changed at all. But in the area of the exhaust system where the EGT sensing probe is located, the pressure pulses are moving the gasses in such a way as to create radical changes in EGT readings.

    The most common, and misunderstood, condition affecting EGT is engine loading. If the engine is not loaded sufficiently, it will run high EGTs. If the mixture is richened, the EGTs will go up, rather than down. A lightly loaded engine can be enriched to the point that it will barely run, and still have high EGTs. The explanation is found in one of Newton's laws. Simply stated, it says that energy cannot be created, nor destroyed.

    When we burn a fuel-air mixture inside our engine, we are converting chemical energy into heat energy. When applying the pressure created by the expansion of the burning gasses inside the combustion chamber, to a movable piston, we are converting a portion of the heat energy to mechanical energy. The heat energy not used in turning the crankshaft is given off through the exhaust and the engine cooling system. If we add all this up, it would equal the amount of energy present in the fuel before it was introduced into the engine.

    So it would be correct to assume that, for the same amount of fuel-air mixture being burned in the combustion chamber, if less heat energy is being utilized to turn the propeller, then more heat energy will be going out the exhaust. In other words, a lightly loaded engine, will throw considerably more heat out of the exhaust than the same engine, with the same throttle setting, carrying a heavier load. So, not only will a heavier loaded two-stroke engine have lower EGTs, but the engine will produce more power for the same amount of fuel burned. If a lightly loaded engine, with a high EGT, has the mixture enriched, the extra fuel will go out the exhaust, increasing the EGT even more.

    .................................................. ...........................

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  • Mick Broom
    replied
    polini cooling

    Hi Wally
    Thanks for that, made me think!

    Quote"
    EGT can increase to excessive levels in an engine that is correctly jetted, but is artificially leaned out by overrevving in a fast shallow descent. Typically, the throttle is set to less than 1/2, the airspeed is at or above high speed cruise, RPM is at high, and the EGT gets very high. What has happened is that the fuel supply is less than half normal (the needles are down into the jets), and although the slides are partly closed, air volume is artificially very high due to the high RPM, (the prop turns the engine into an overactive air pump) and the engine mixture becomes greatly leaned out. Now you have a situation of low load, high RPM, high carburetor air flow, and low fuel. This equals very high EGT levels, use caution. A change in throttle position will usually solve the problem.

    Me"
    I can see where he is coming from but am not entirely convinced as the effect can happen with a fixed throttle position which I would have thought gives a fixed mixture and the only thing which changes is more air flow through the carb.

    Still thinking

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  • Wally Hayward
    replied
    polini cooling

    Mick.. re Egts and prop loads.....
    Can't say I know any specific direct explanation, and the first relates to Rotax and the second, not is specific to 2 strokes, (I suspect you know this anyway),

    http://www.rotax-owner.com/en/rotax-forum/1-2-stroke-technical-questions/4295-582-high-engine-temp?start=100

    Posts #5210 and 5211 by Dennis Richardson

    also although not specific to 2 strokes

    http://www.askacfi.com/4539/egt-rise-during-leaning-the-mixture.htm

    Leave a comment:


  • Mick Broom
    replied
    polini cooling

    Hi Dominic,

    This is all standard stuff in the development world and has been used since the video camera was small enough to do it. If you look on my Utube channel there is one showing the flow on the Shadow.

    The other breakthrough was the multi channel 12 volt loggers which are very useable for development and not too expensive which allows you to collect both the engine performance , flight performance and dynamic results of both temperature, pressure and speed so you can see what is really going on as against what you think is happening.

    There can be a big difference between your expectations and the real world.

    The radiator is a prime example as most with a problem would try and get air towards the core but if you think about it long enough you will realise its the pressure difference across the core which is encouraging flow and so you can sometimes get better results from improving the negative pressure area behind the rad.

    Air direction does not always do what you think, on a Shadow the engine air behind the bulkhead can flow forward under the body and get to the front footwell.

    I will keep you up to date but you may need to buy a Dragon for it to be any use. On the 462 setup the rads would work ok until you got a big bloke in the back when it would over heat They needed little ears on the outside edge to effectively make them wider but be careful that they don't fracture due to the vibration and dent the prop.

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  • Guest's Avatar
    Guest replied
    polini cooling

    If you can mount a remote camera looking at the wool tufts a video would be a fantastic opportunity to see what is really going on back there. This was one of the reasons i moved the radiators away from the prop on my 462 and mounted them just behind the monopole as i believe that the high pressure area in front of the prop is having a negative effect on the flow through the radiator. (Maybe the radiators dont need to be quite so big now? Further testing to follow). Let us know how this pans out Mick, its all worthy experimentation.

    Leave a comment:


  • Mick Broom
    replied
    polini cooling

    Hi Dominic,
    Nice to see that you are thinking along the same lines as me ;-)
    Interestingly its the low pressure side which tends to have the greater effect.

    Hi Steve,
    I am sure you are correct in that it is a pressure difference or air flow thing but air does really strange things when disturbed which needs to be found out before progress can be expected.
    Just putting a bigger rad in dirty air flow may just increase the weight and the problem is only at full power so if you are not in big rev changes are basically trying to push the same power through the prop in both climb and straight flying.
    Having said that when you unload the prop the mixture changes and the EGT rises which I don't fully understand.
    Great fun this messing about.
    Long live SSDR

    Leave a comment:


  • Guest's Avatar
    Guest replied
    polini cooling

    Mick I have no doubt that you already have a plan but maybe stick tufts of wool on the rear of the radiator much the same as when testing airflow over an aerofoil section.

    Leave a comment:


  • Steve Uzochukwu
    replied
    polini cooling

    Mick Broom wrote: I have found that giving full power during climb out gives 90C at the head but giving full power at fast as you can go speed for any length of time will make the reading climb to over 100C.

    Recovery is easily achieved by either reducing the speed or power and there is no localised boiling in the system or apparent adverse effects.

    I now intend to investigate air flow management around the radiator and water pump flow rate as I like flying in warm air :smhair:
    My gut feeling is that this is a pressure thing. When you are climbing the prop is doing a lot of work, and there is more of a forced element of air being drawn over the radiator which will be more likely overcome any disturbed air flow. In level flight, you'd be doing less work, and less of a pressure difference. In a dive, the airflow would drive the prop at lighter throttle settings.

    Simple option is going to be a larger rad. Other options might involve a scoop to manage the ram air element of things a bit better.

    As a side note, I seem to remember that the Thor 200 is forced air cooled. Are there two versions?

    Leave a comment:


  • Mick Broom
    replied
    polini cooling

    Just an update on this system for anyone interested in changing the standard system after a more extreme sort of flying heat wise in the south of France.

    I have found that giving full power during climb out gives 90C at the head but giving full power at fast as you can go speed for any length of time will make the reading climb to over 100C.

    Recovery is easily achieved by either reducing the speed or power and there is no localised boiling in the system or apparent adverse effects.

    I now intend to investigate air flow management around the radiator and water pump flow rate as I like flying in warm air :smhair:

    I think my rad is slightly smaller than Colins

    Leave a comment:


  • johnkendall
    replied
    polini cooling

    I like that Colin. Toyed with something similar myself. Very neat.

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  • Guest's Avatar
    Guest started a topic polini cooling

    polini cooling

    Hi

    not sure if anybody is interested, but i've had a go at tidying up the water system on the polini. Its basically a copy of what Mick Broom did on his, it seems to work well, saves half a kilo, reduces the height of the trike quite a bit, (so its easier to get into the van), and brings some warm things near to the carb

    The rad is a rotaxmax kart radiator.

    The original polini rad and expansion bottle are up for grabs, for anyone converting a polini 200 etc.

    cheers

    colin

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