QZ engine questions

Early QZ engines would has the "open" non-slot type exhaust port. They also came with the 1702 Hicomp head as standard, if I remember correctly the RPM difference was very little.

Thanks guys. I have only one thick-wall open-exhaust cylinder, I need to check if it's an SPI or not. If not, I'll check my QRC (post-96, slit exhaust) and try to find a cylinder similar to it, it's probably similar to regular Babe Bee cylinders of that era.

I already have a couple of TeeDee heads, but I will probably try a low-compression one first.

If you can live with a slit cylinder I would suggest you pick this one which is the most powerful non-SPI from what COX Interbnational are selling today:

http://coxengines.ca/cox-.049-cylinder-texaco-medallion-killer-bee-7.html

Yeah, slit-port is fine as long as the performance is adequate for my (not so demanding) application. The muffler covers it so "aesthetically" it'll be OK either way

I am working on a list of parts to order to get my hive of neglected Bees operational, maybe I'll add a couple of those non-SPI assemblies too. Thanks for the tip.

I also prefer open port cylinders not simply because their most authentical "COX"look but also because of the sensibly higher pressure loss of gases leaving through a slit port. This choking effect, just like that of a muffler, will reduce the output of 2-stroke engines.

The double by-pass double booster design of the cylinder I suggested above resembles the most powerful TD cylinder - the only difference is the non-SPI porting which is necessary because of the muffler, and the slit exhaust port that you cannot do anything about.


BTW if you have a TD cylinder with SPI, a few timer shims (also sold by COX Int'l) placed under the cylinder, and another few head gaskets removed, will result in a no_SPI design and more or less unchanged compression.

I am not sure, but I think the open exhaust ports seem almost too big for an engine that size. Not sure how glow engines work in practice, but 2-stroke gas engines suffer severe torque loss if the exhaust system has not enough back pressure as the fresh fuel charge comes out of the bypass ports and a big portion of it goes right out via the open exhaust.

The timing washers sound interesting. Effectively they will raise the ports, which is the tuning trick to get more rpm out of any 2-stroke, as the ports will open earlier and close later than before, making the timing more "aggressive". The later closing of the ports decreases the compression ratio, so lowering the head is a must. I think that for every timing shim you should remove two head shims to keep the compression at a good level. Or maybe compensate with higher nitro content? The more aggressive port timing will increase top rpm but the lower rpm torque will suffer, maybe it is not an issue in control line use, where the engine runs at wide open throttle. More aggressive port timing requires a tuned pipe to work properly on a throttled engine, that might be a cool experiment

The SPI is an interesting concept too. I had never heard of it before I got involved with Cox engines. I have been thinking about it and have an idea how it might work but I am not sure. I'm thinking it may have something to do with the inertia of the fuel charge entering the crankcase, keeping the reed open just a little longer as the piston has already passed the TDC. Or maybe not Anyone have any insight to the theory behind SPI?

Roddie, that muffler design is very innovative

I like the way you can just slip the muffler on and off in a few seconds without having to take the cylinder off. It would be good to keep one in the field kit, just in case somebody complains about the noise, or if the field has noise limits... Just slip the silencer on for quieter running.

KariFS wrote:...............The SPI is an interesting concept too. I had never heard of it before I got involved with Cox engines. I have been thinking about it and have an idea how it might work but I am not sure. I'm thinking it may have something to do with the inertia of the fuel charge entering the crankcase, keeping the reed open just a little longer as the piston has already passed the TDC. Or maybe not Anyone have any insight to the theory behind SPI?

The SPI helps increase the air intake and thus allows a richer needle valve setting and, at the end of the day, more fuel/air mixture to enter the crankcase:

as the piston moves up it creates depression in the crankcase, and when its skirt rises above the bottom edge of the exhaust port, air will be sucked by the depression into the crankcase (hence the abbreviation: Sub-Piston Induction). This is why muffled engines should not have SPI design that would allow the exhaust gases to sucked into the crankcase.

In a non-SPI engine some depression remains in the crankcase even if the piston reaches the top dead center; in a SPI engine the crankcase pressure reaches the ambient pressure when the piston is at the TDC.

As the piston moves downwards and the reed, or the crank porting on the TD-s closes the crankcase space, pressure in the crnakcase increases. When the bypass port opens, the amount of fuel-air mixture pumped into the cylinder by the crankcase overpressure is more with a SPI design than with a non-SPI design, so the heat input into the cycle of a SPI engine is more than in a non-SPI...no wonder that a SPI TD 049 easily exceeds 22krpm on a bench while a non-SPI TD 050 R/C runs at around 19k with the same prop

[/quote]

.....................Interestingly; both cylinders have dual intake porting.. but the #3 (slit exhaust) intake-ports are bigger (wider) than the #1. This seems confusing to me. Why design for a larger volume intake-charge.. and then decrease the exhaust port size?  The #3 is non-SPI.. where the #1 has SPI. I'm sure that figures into the equation somewhere. I'm learning more about it.. and I find it very interesting. [/quote]

The performance loss caused by the slit exhaust is partly compensated by the widened bypass posrts I guess..The slit exhaust was not introduced because of technical considerations.

Some open port Bees have allegeadly caused, when over-primed, occassional burns on the fingers of young modelers, and the slit port was expected to disperse the flames occassionally bursting from an overprimed cylinder.

You nailed it!

"Effectively they will raise the ports, which is the tuning trick to get more rpm out of any 2-stroke, as the ports will open earlier and close later than before, making the timing more "aggressive". The later closing of the ports decreases the compression ratio, so lowering the head is a must."

KariFS wrote:I am not sure, but I think the open exhaust ports seem almost too big for an engine that size. Not sure how glow engines work in practice, but 2-stroke gas engines suffer severe torque loss if the exhaust system has not enough back pressure as the fresh fuel charge comes out of the bypass ports and a big portion of it goes right out via the open exhaust.

The timing washers sound interesting. Effectively they will raise the ports, which is the tuning trick to get more rpm out of any 2-stroke, as the ports will open earlier and close later than before, making the timing more "aggressive". The later closing of the ports decreases the compression ratio, so lowering the head is a must. I think that for every timing shim you should remove two head shims to keep the compression at a good level. Or maybe compensate with higher nitro content? The more aggressive port timing will increase top rpm but the lower rpm torque will suffer, maybe it is not an issue in control line use, where the engine runs at wide open throttle. More aggressive port timing requires a tuned pipe to work properly on a throttled engine, that might be a cool experiment

The SPI is an interesting concept too. I had never heard of it before I got involved with Cox engines. I have been thinking about it and have an idea how it might work but I am not sure. I'm thinking it may have something to do with the inertia of the fuel charge entering the crankcase, keeping the reed open just a little longer as the piston has already passed the TDC. Or maybe not Anyone have any insight to the theory behind SPI?

KariFS wrote:Which cylinder is correct for an early QZ? Non-SPI I think, but were the early ones with or without the slit exhaust ports? Any experience on its performance vs a standard Bee? What would be a good propeller for it?

Peter Chinn's 1967 test report of the QZ is at:
http://www.sceptreflight.net/Model%20Engine%20Tests/Cox%20049%20QZ.html

Engine puts out 0.065 BHP at 15,000 RPM on 15% nitro. Peter stated a 6x3 prop was well suited as it fits well within the power band; 13,400 RPM to 14,100 RPM.

H.R. Warring's 1961 test report of the Babe Bee is at:
http://www.sceptreflight.net/Model%20Engine%20Tests/Cox%20Babe%20Bee%20&%20Golden%20Bee.html

Babe put out 0.055 BHP at 13,000 RPM on 15% nitro. Top Flite nylon 6x3 prop put out 14,400 RPM.

Although the QZ claims higher output, I gather that the power output between both based on static prop RPM that they are a close match.

KariFS wrote:I have a couple of mufflers, and I think I'll build a "QZ replica" for my small sheet-built trainer plane. I have the 6mm balsa sheets, some plywood, wheels and other supplies for the plane, just need to get my rear in gear and start building

I recently noticed that Bernie at Cox Interntional has now a muffler available to fill a nick and at a good price.

http://coxengines.ca/cox-.049-muffler.html


With that, one can turn any non-SPI Cox .049 into a "QZ"

[quote="OVERLORD...........Andras, I think this is not true. Deducting an engine’s power from the size of the exhaust port makes believe that an exhaust port must be fully open first before the exhaust gasses start leaving the cylinder. The way exhaust gasses escape from a cylinder is not at all the same way the fresh mixture enters the cylinder.

The filling grade of a cylinder with air fuel mixture depends on several mechanical factors as piston size, piston speed, diameter of the inlet valve, size of the manifold and carb, the seal between piston and liner. This is why in nowadays petrol engines, the inlet valve is bigger in diameter than the exhaust valve. Also, when tuning engines, a bigger inlet valve will be installed and not necessary a bigger exhaust valve. This is because the evacuation of the exhaust gasses relies for the major part on pressure. As soon as the piston opens the exhaust port, or the exhaust valve opens on a 4 stroke, the exhaust gasses are forced through the opening due to the reigning pressure in the cylinder even after pushing the piston down. What remains is only a volume of exhaust gasses as big as the cylinder volume. This is evacuated by the up-stroke in 4 stroke engines or by the scavenging air in 2 stroke diesels or the scavenging air fuel mixture in glow engines.

At the Cox factory, they found that out as well. This is a simple check you can do at home:

1) Lower the piston until the piston crown reaches the upper edge of the inlet port. The created exhaust opening at this point is used to evacuate the exhaust gasses by the pressure in the cylinder. You can see, when looking sideways, that the opening between the exhaust port ‘s upper edge and the piston crown has approx the same height as the height of the upper slit in a later slit cylinder.

2) When positioning the piston the same way in a slit cylinder engine, the piston crown will be level with the lower edge of the top exhaust slit.

When the piston reaches the inlet port(s), there is not much of exhaust gas left in the cylinder. The opening with the the size of the later exhaust slit is sufficient to get rid of the exhaust gasses.  The power is mainly determined by the size of the inlet ports.

If the size of the exhaust opening(s) would affect power, why would the  Mark II Special with one exhaust port have more power than the conventional .15 TeeDee?[/quote]

Overlord,

I partly agree. My assumptions (sorry for the theoretical derivation):

1. based on the laws of gas-dynamics the flow through an orifice (like the exhaust port) of a given cross section cannot be increased if the ratio of pressures upstream and downstream the orifice is higher than a gas-specific value (for clean air this ratio is 1,78) unless you have a Laval-nozzle orifice cross section variation along the length of the orifice. (Above the said pressure ratio the gas velocity will exceed Mach=1 i.e. the sound velocity).

This is not the case with the COX exhaust port (constant cross section along the path of the exhaust gas, and not the Laval orifice layout),therefore the only way to increase the flow of the gas i.e. to alleviate exhausting is to increase the orifice (i.e. exhaust port) cross section. In this regards the larger cross section of the open port versus the slit port is presumably helping the faster and more efficient exhaust of the gases.

With the above in mind, the choking effect of the slit exhaust port that impairs the exhaust and thus, the entrance of the fresh charge from the crankcase, will negatively impact the performance.

On the other hand the widening of the bypass port (and this happens in TD-s with the addition of 2 booster ports) will rerduce the friction loss of the fresh charge when pumped from the crankcase into the cylinder, thus, helps the more subtle filling of the cylinder.

So I guess the only way for COX to partly compensate the choking effect of the slit exhaust was to help on the charging side by widening the bypass ports.

If the exhaust port cross section (i.e slit or open) is indifferent from the point of view of performance, we may want to ask Bernie at COX International why the aftermarket TD cylinders he sells are made of slit exhaust cylinders with the bar between the slits removed??

http://coxengines.ca/cox-.049-tee-dee-cylinder-and-piston.html

balogh wrote:If the exhaust port cross section (i.e slit or open) is indifferent from the point of view of performance, we may want to ask Bernie at COX International why the aftermarket TD cylinders he sells are made of slit exhaust cylinders with the bar between the slits removed??

http://coxengines.ca/cox-.049-tee-dee-cylinder-and-piston.html

I suppose the intention is to replicate the #4 cylinder. Most SPI cylinders have the open exhaust port, I think there is only one SPI slit-exhaust cylinder model.

Are you sure the pure resemblance to the #4 is worth it to chip off the bar and take the chance of adding a burr to the exhaust area? let alone the added cost?

GallopingGhostler wrote:

I recently noticed that Bernie at Cox Interntional has now a muffler available to fill a nick and at a good price.

http://coxengines.ca/cox-.049-muffler.html


With that, one can turn any non-SPI Cox .049 into a "QZ"

I wonder why there is no "pipe" on it, or if one could be added afterwards? The test results from the old magazines were interesting, showing that open exhaust port Cylinder may actually benefit from the added exhaust back pressure

balogh wrote:Are you sure the pure resemblance to the #4 is worth it to chip off the bar and take the chance of adding a burr to the exhaust area? let alone the added cost?

Well, that depends. For me a replica of a #4 cylinder is an attractive option, even if it costs 5 dollars more. The SPI is the "thing" for me here, not just the open exhaust.

It would be interesting to take an ordinary slit-exhaust cylinder (similar to the ones used as a starting point for Bernie's replicas) and widen the lower ports so that it'll have SPI, and then shape all the ports according to De Laval principle. Or maybe just the outer sides so that they taper outwards, to create a megaphone effect

Indeed there is. Shown here are two genuine COX exhaust mufflers, the one on the right is with the pipe. The one on the left has a strange silicone cuff encapsulating the muffler with no opening left. Exhaust gases are pressed out between the cuff and the rim of the aluminum housing. This really is a choke, I wonder how the engine performance suffers with this type.