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Cox engine question

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Old 11-21-2012, 04:03 PM
  #1
Chikokishi
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Default Cox engine question

So i was looking at my little engines, and im curious - How does the fuel get into the ignition chamber??

Chiko
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Old 11-21-2012, 04:15 PM
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Default RE: Cox engine question

Do you want to know how a reed valve engine works..?
Cox made a lot of reed valve engines and also quite a number of rotary valve engines.
Both designs are based on the pumping/sucking action that the piston provides when it goes up and down.
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Old 11-21-2012, 04:56 PM
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Default RE: Cox engine question

How much understanding do you have of engine operation? 2-stroke engine operation to be specific. From your previous queries here I know you have some Cox engines-but Cox engines can be of two types-reed valves, or rotary valves-and you haven't indicated what sort yours are. It is most likely a reed valve type. This concept of 'valves' can cause some confusion to beginners-for a start the 'valves' are not in the head so have nothing to do with it being a 4 stroke as some might initially think from seeing reference to 'valves'
In this case the valve refers to the induction or how the fuel charge is introduced into the engine. Reed valves are a thin membrane-sometimes copper or brass alloy, sometimes stainless steel, sometime mylar sometimes teflon. The one thing common is that they are very thin, and very flexible, and close off the induction tract in the engine [Reed valves are also used in larger commercial engines such as chain saws, weed eaters, lawn mowers-they may have a different shape and structure in these larger engines-but they operate in exactly the same way].
When the piston is at the lowest point in the cylinder, the crankcase volume is also at its lowest volume. Effectively a 2-stroke engine operates as a variable volume pump, pumping fuel and air through the engine, and burning it to produce mechanical energy. As the piston rises, the pressure in the crankcase reduces-ie a suction effect is created, and the reed valve, being flexible is drawn inwards off its seating (it is held captive to a degree-so cannot get completely sucked into the engine!) allowing air to rush past it to equalise pressures and fill the slight vaccuum in the crankcase. As this incoming air has passed down a fairly narrow tube it has sucked fuel with it from the needle valve metering point-called the jet, so what now fills the crankcase is not air, but a mixture of air, fuel vapour and fine fuel droplets. As the piston reaches the top of the cylinder it also compresses the mixture already there from the previous cycle and the glowplug fires it, causing a rapid expansion of the gas and an increase in pressure in the head space, and this pressure drives the piston downwards again. As the expanding gases in the upper cylinder drive the piston downwards, it is also compressing the gas mixture which has flowed into the crankcase when it ascended the cylinder-so the gas mixture in the crankcase is being compressed more and more, raising the pressure there-and this also caused the reed valve to reseat, sealing the inlet track and the crankcase.
Now running up into the cylinder-either on the outside (but still inside the crankcase) or inside the cylinder itself, are the transfer passages (and ports) These are shallow grooves-they may be rectangular, triangular semi circular or crescent shaped depending on the brand of the engine, and the tooling used to manufacture it. The top of these passages will eventually be uncovered by the descending piston-allowing a free route for the compressed gas in the crankcase to travel into the cylinder-in other words the descending piston is pumping the fuel mixture from the lower part of the engine up into the upper cylinder to be burnt.
The engine has one or more exhaust ports-and these are positioned to open before the transfers, the timing is determined simply by how far up or down the cylinder these ports are located, and the duration-how long they stay open-by the depth of the port opening. The transfers are located in relation to the exhaust ports, so that the exhaust will have opened (and so the pressure in the cylinder will have dropped considerably) before the transfer-the exact timing difference varies from engine model to engine model. The opening of the transfer ports (and there might be one or several-up to 8 of them) allows a jet of cold fuel mixture to squirt under pressure into the upper cylinder. This cold incoming mixture also helps sweep away the residual burnt exhaust gas, (some of the fresh charge is lost out of the exhaust as well-which is why 2-strokes are never as efficient as 4-strokes). And finally the piston starts to ascent again, and once it reaches the top of the exhaust port it begins to compress the new fuel charge in the upper cylinder-having, during the ascent caused another fresh fuel charge to be sucked into the crankcase.
The limitation of the reed valve is that it has symmetrical timing, which is not the optimum for power, and permits running in both directions-which again is less efficient, and can be a nuisance in some setups, and it physically obstructs the flow of gases in the induction tract even when fully 'open'.

A rotary valve engine carries out exactly the same functions, but uses a hollow crankshaft (hollow from the rear) with a side hole at a specific location on the periphery. This hole -the crankshaft 'port' lines up with the engine carburettor once every revolution, and the fuel mixture passes through the hole, down the central hollow passage, and into the crankcase through the open inner end of the crankshaft. A rotary valve can be timed much more optimally than a reed valve, can be larger and offer less obstruction to the flow of fuel mixture, so rotary valve engines are usually more powerful than reed valve ones-in model sizes at least. The down side is they are more expensive to manufacture, and the various holes in the crankshaft weaken it-increasing the risk of shaft failure.

This is far more easily seen than explained verbally-so go to www.animatedengines.com where you will find animated demonstrations of every engine you could possibly imagine-and quite a few that you haven't imagined............! Everything I've said here is explained visually there with animated diagrams. Obviously only the 2-stroke example is relevant to this discussion.

ChrisM
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Old 11-21-2012, 10:44 PM
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Default RE: Cox engine question


Quote:
ORIGINAL: Chikokishi
So i was looking at my little engines, and im curious - How does the fuel get into the ignition chamber??

Chiko
Quick answer...when the piston goes up it creates a vacuum inside the crankcase that pulls air and fuel in. When the piston goes down the mixture is compressed and pushed up into the upper cylinder through some "bypass flutes" or grooves in the side of the cylinder (between the exhaust slits).

George
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Old 11-21-2012, 11:40 PM
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Default RE: Cox engine question

That's a fantastic answer, FFKiwi. I was never actually sure how reedies worked. Thank you. I learned something new today. Now I can go to bed.

Tim
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Old 11-22-2012, 05:56 AM
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Default RE: Cox engine question

Gee, being a simpleton, I've always thought of 4 stroke engine cycle as SUCK - SQUEEZE - BANG - WHOOSH and 2 stroke as BANG - WHOOSH - BANG - WHOOSH The Cox's are in the latter category. Model plane fuel keeps the element glowing hot in the glow plug. The rest is magic. Enough theory, it's time to fly. [8D]
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Old 11-22-2012, 01:50 PM
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Default RE: Cox engine question

Wow thanks for the replies! If youve probably noticed, my little engines are now toast. Im lookin for new ones now, poor little guys.

Chiko
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Old 11-22-2012, 08:29 PM
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Default RE: Cox engine question


Quote:
ORIGINAL: Chikokishi

Wow thanks for the replies! If youve probably noticed, my little engines are now toast. Im lookin for new ones now, poor little guys.

Chiko
Toast? These engines can be repaired.

George
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Old 11-23-2012, 10:44 AM
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Chikokishi
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Default RE: Cox engine question

How do you figure gcb?

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Old 11-23-2012, 05:21 PM
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Quote:
ORIGINAL: Chikokishi

How do you figure gcb?

Chiko
Parts...piston/cylinder/conrods, crankcases, crankshafts, gasket sets/glow heads...all available. What is wrong with them?

George
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Old 11-27-2012, 12:28 AM
  #11
Chikokishi
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Default RE: Cox engine question

Its the backplate thats toast haha. Iv been looking for one for a few days now
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