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  1. #1
    Click avatar for tech links/info, donation request K447's Avatar
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    Arrow Piston wash on Ficht fuel injected engines

    I have not seen any discussion regarding piston wash patterns on the fuel injected 2-stroke Ficht engines.

    With carburetor engines, the piston wash is used to tune the fuel jetting and carb screws so there is some 'washing' of the carbon from the piston dome, near where the ports are.

    This is often referred to as the extra fuel 'washing' the carbon from the piston, and adjusting the carbs to create the desired wash pattern does seem to result in a well running engine.

    With a Ficht engine, there is no fuel in the crank case or intake tract. Just air, and a little 2-stroke oil from the oil injection system (which is mandatory on Ficht engines).

    The intake air is compressed in the crank case (just like in the carb'ed engines) then the compressed air is transferred up into the combustion chamber through the transfer ports.

    As the piston rises and closes off all the ports (exhaust and intake), the Ficht injector then rams a precise amount of fuel directly into the combustion chamber.

    Since all the ports are covered, none of the injected fuel can escape. The spark plug then ignites the fuel+air, and the normal 2-stroke power pulse occurs, forcing the piston downwards.

    The Ficht engines allow very little unburned fuel to escape, which not only allows them to carry a California Low Emissions rating, but also provide fairly decent fuel economy.

    The interesting thing is that the Ficht pistons still show a wash pattern, yet there is no fuel coming in through the transfer ports, just air.

    This implies that the piston wash patterns are not created directly by the fuel coming through the transfer ports, but that the wash patterns are an indication of piston surface temperatures.

    The intake air flow cools the pistons near the ports on each intake transfer, and the combustion heat reheats the piston on every stroke. When the piston surface is hot enough, it burns and carbonizes the fuel+oil onto the piston surface.

    Where the piston surface is cooler because of the intake charge, there is less carbonizing effect, hence the wash pattern.

    Having said that, why do you think there is a large wash area near the exhaust ports?

    Attached are some borescope photos from my Virage TXi, which I winterized and fogged right after taking these pictures.

    The ProVision borescope I have creates a round image, and the entire piston top doesn't quite fit into the picture at the same time.

    Some of the images are rather fuzzy, and I will need to use a different camera to get them really sharp using the borescope. The borescope lamp sometimes creates a bright area in the center of the image, the piston isn't actually that shiny.

    In each photo, the exhaust port area is at the top of the frame.

    First 3 are PTO, next 4 are CEN, last is the MAG
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  2. #2
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    My off the cuff responds is that even with the FI,you still have unburned fuel washing the area clean...........Or the Rush from the compressed hot air leaving is preventing the carbon.

    Damn good question

  3. #3
    Bernie's Avatar
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    Keith
    The wash on top could be caused by the pressure that the fuel is being sprayed in at .Exhaust wash area could be caused by the velocity of exhaust air with a small amount of fuel in it as the port is still partially uncovered when injection starts .This is also a high heat area which combined with the small amount of fuel and oil present and the velocity of the gases going through when the port is closing will assist in cleaning the residue off. Sound feasable ?There will be info available that could test my theory .Just wondering who might have it??

    Bernie

  4. #4
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    assumption

    you're assuming all the fuel is burnt.the oil is in the crankcase and could cause the wash as it comes out of the ports.the exhaust backpressure could still be sending a fuel charge back across the port openings which better explains why the exhaust ports have more wash

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    We all know that carbureted 2-stroke engines waste a good portion of their fuel because the intake charge doesn't stay entirely inside the combustion chamber. During the transfer phase from crank case into combustion chamber, both the intake and exhaust ports are open.

    The compressed intake charge rushes up through the transfer ports, and a portion of the intake charge loops across the piston and exits into the exhaust manifold.

    In a tuned pipe exhaust (see animation attached), the tuned pressure pulse reflection forces part of that intake charge back into the combustion chamber, just before the rapidly rising piston closes off the exhaust port.

    In a regular (non-tuned pipe) 2-stroke exhaust, that intake charge 'flow through' just exits the cylinder, and keeps going. That is why carburetor 2-stroke engines have so much unburned fuel in the exhaust, and such high hydro-carbon emissions levels (unburned fuel).

    In a Ficht engine, that intake charge cross flow still occurs (just like the carb'ed engine), but it is only air (and a little bit of 2-stroke lubricating oil). As it flows across the combustion chamber, and exits the other side, I expect it cools the piston dome both near the intake ports, and while exiting through the exhaust ports.

    The exhaust port closes later that the intake ports on the upward piston stroke, so the intake charge has more time to exit out the exhaust port, and cool the nearby piston surface.

    What I suspect is happening in a lean engine is that the piston surface temperature overall is higher, and the cooling effect of the intake charge is not sufficient to prevent the carbonizing effect of the hot fuel+oil that is in contact with the piston dome surface. So more of the piston surface is carbon coated (black).

    That implies that piston wash is showing you the temperature pattern of the piston dome, which is affected by the heat of combustion (we know lean burns hotter) balanced against the cooling effects of the intake charge.

    When the piston temperature from lean burn gets way too hot, not only is the vast majority of the piston surface carbonized black, but the center of the dome actually begins to melt. The edges of the piston are still cooler, just not cool enough to avoid charring the fuel and carbonizing the piston.

    Comments?
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  6. #6
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Keith, If anyone could get the definitive answer,It would be one of the guys on this Engineering forum.

    http://www.eng-tips.com/

    Reading this forum sometimes make my head spin and my hair hurt. Just don't get lost over there,we need you here.

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    I was asked about this subject recently, and thought I might post a portion of my reply;

    My own theory (which I have not really dug deeply into) is that piston wash is NOT caused directly by excess fuel in the combustion chamber. From my perspective, saying that piston wash is caused by the cooling effect of excess fuel is a short cut.

    My thinking is that the wash pattern is a result of varying surface temperatures across the piston top. The different temperatures are caused by the cooling effects of AIR flow across the piston during exhaust and intake charge transfer phases. And the cooling effects of piston/ring contact with the cylinder walls, which are in turn water cooled.

    As piston surface temperature rises in some areas, it becomes hot enough to carbonize the fuel/oil that contacts the surface before spark ignition occurs. Carbon burned onto the piston top combusts (burns off) more slowly than it builds up on the next intake stroke, hence the build-up of the wash pattern. I don't know exactly what temperature range the piston surface needs to reach in order to carbonize the fuel, but it is certainly less than the flash point of the mixture.

    Areas of the piston surface that are cooled sufficiently do not carbonize the incoming fuel+oil, so they retain the clean metal look.

    Engines with lean fuel:air ratios burn hotter, which raises piston surface temperatures, and makes the pistons blacker. Rich fuel:air ratios burn less hot, so the pistons don't get as hot, and less surface area gets carbonized.

    An engine that was running 'lean', and then has the fuel:air ratio adjusted to produce cooler piston temperatures, will slowly burn away the accumulated carbon in the now 'cooler' areas, and change the visible piston wash pattern.

    I note that in the posted Ficht piston wash photos (DSCN3279, 3273, 3278 ), the clean metal areas near the intake ports are actually where the sides of the intake ports are. The center region of the intake port has black pattern, which to me indicates that the piston surface is being cooled both by the intake charge AND the proximity to the cool cylinder wall at the sides of the port. Where the piston is facing only the open intake port, the intake charge is not cooling the piston enough to prevent carbonizing.

  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by CrazyA View Post
    What does one do if they have EGT sensors and the readings are saying too lean, but the wash says too rich?

    Piston wash is just a proxy (indirect) indicator for piston surface temperatures - correct?

    The assumption is that if the piston surface is getting hot enough to carbonize fuel in the areas that normally stay cool enough to stay clean, then the hotter areas of the piston (such as the center) must be getting closer to piston damaging/melting temperatures.

    Different piston designs (better piston cooling), and different intake/cylinder/exhaust air flow patterns, can affect the wash pattern, even if the actual temperature during combustion is consistent.

    As long as the pistons are not melting, does a 'lean' or 'rich' wash pattern matter? How much hotter/leaner than a 'safe' wash pattern can an engine be tuned, before the pistons begin to fail?

    EGT is a proxy measurement for internal combustion chamber temperature.

    My thinking is that your riddle has a piston/engine configuration that is able to keep the pistons cool, despite the high EGT readings. That implies that the engine could run even leaner before the pistons would get hot enough to carbonize or melt.

    If the piston is able to cool itself enough to not fail, then the engine will continue to run. Even if the wash pattern is 'wrong'. Would you agree?

    What particular engine does your riddle refer to?

  9. #9
    FLjoyrider's Avatar
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    This is very interesting stuff.. possibly a hint towards the 2-stroke with dry sump oil system and air+fuel injection to make it environmentally friendly.. The resurection of the two stroke! well, the orbital engine in europe has proven that.. why dont we have it in the US?

  10. #10
    Moderator HiPeRcO's Avatar
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    Attached are photos of my pistons from my TXi teardown project. Comments welcome!
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