meowy84

Ok, wasn't quite sure where to post this question (or 2 questions in one) but since this is my usual stomping ground and there's a lor of 'gurus' here here it goes. I'm dreaming about getting a lathe and machining some of my own motor(s) and parts. Here's one question I have. What are the typical compression ratios for Cox engines? Also what are some typical compression ratios for the larger .09-.25 2-stroke engines? and finally, what are some compression ratios for the 4-stroke motors? as much as I love the little Cox engines, there's just something about those 4 strokes that puts a smile on my face as well, and I think that maybe eventually I'd want to machine a 4 stroke. One of my engines is an old Saito FA30 with exposed rocker arms....beautiful, just beautiful to se those rockers move.

This is an off the wall dreamer's fancy but do any of you think it feasable to have a 4 stroke with a displacement of .049-.061 (Cox and Norvel neighborhood)? I realize momentum and generating enough vacuum might be a problem but what if you use an overly large flywheel and have a gravity-feed tank above the engine. Can such an engine run decently? (For arguments sake let's not worry about power output right now.) The smallest 4 stroke that I've seen was a .19 (might have been a Hinterberger) in addition to the more prolific .20 sized 4 stroke made by O.S.

BMatthews

Compression ratios are not restricted to any particular size of engine. There is a range of ratios that will provide a given level of performance for a given fuel. This applies as much to the 049 Cox's as well as a 61 OS. Too little and it won't deliver much power, too much and it'll preignite, run rough, blow plugs and generally beat itself to death. Some of this is set by the nitro content and some by the type of running you want to get out of it. Just don't ask me at the moment what the range for glow engines is...

Cox used to make a range of plug types for the 049's. There was a dome shaped one with low compression, a straight conical shape with a bit more compression a trumpet bell one for more yet and finally the TeeDee hi comp head that used a trumpet shape with a squish band around the edge for the highest rate.

I flew a sport model with a TeeDee head and 35% fuel with a 6x3. It was way too much for it and the plug blew out within a few runs. Popped the seal actually. I switched to a conical shaped one and ran the same fuel and got a LOT more revs. The TeeDee plug with only one washer was just overcompressed. I've never used a TeeDee plug since. Racers may benifit from fine tuning the TeeDee head by adding head gaskets to tune the engine for the weather conditions of the day and the nitro being used but for general sport flying it's just too much trouble. I think you'll find the same applies to your typical sport flyer. Some of the engines would develop more power with a slightly higher compression but at the espense of good operating manners. You need to play with that to find out what's right for you.

There's been a few very small 4 strokes custom made over the years. I remember seeing a 1 cc engine as well as a much smaller one who's displacement escapes me But it was very tiny with valve stems that could easily be bent by a wayward finger. I turned a small wooden prop about the size of an 010. Some desiels have been made down to as low as .002 cu inches IIRC. No need for a glow plug helps with making the really small ones.

combatpigg

My Feedback: (3)

HI BRUCE,Good point about the diesels being a good choice for tiny home brew projects.

Meowy, a good project for you would be to get a test tube with graduated marks on it and an eye dropper, to figure out the displacement of the combustion chamber. The [dry] measuring of the cylinder volume at BDC and TDC will give you that volume, a machinists' rule can be ground [narrowed] to fit into the cylinder. Don't forget to factor in the amount that the head sits down into the cylinder. A $20 dial caliper is a nice tool for this. I'll bet you come up with about 6 or 7:1.

meowy84

Bruce, I see....so what you're saying is that the compression ratios are more dependant on the type of fuel you plan to run than on anything else. So for the small engines (like Cox/Norvel, etc) running a comparatively high nitro content you'd want a lower compression ratio to prevent preignition? either way though, there must be some rough neighborhood that the comp ratios for these engines crowd around. Good to hear about the tiny 4 strokes being at least able to run. means I can at least dream about making one some day and not have it be an empty dream.

Combatpig, you're talking about cc'ing the engine as they say to get the volume of the combustion chamber? I used to do that (using water) when home (read 'redneck' heh heh) porting and blueprinting full-size car engines (mostly 350 V8 Chevies and a few air cooled VW flat 4s). And it works fine on full size cars...I'm not sure that it would work on something as small as a Cox because of the whole meniscus thing and water's surface tension. I think the measurements would be way off. I might have to find a liquid with a small surface tension (alcohol maybe)? Of course another problem is that with a 2 stroke engines exhaust and intake ports pouring liquid when cylinder at TDC would just drain the liquid out the exhaust ports and down into the crankcase. Would probably have to use a depth guage and a telescoping guage to get the bore and stroke. The depth guage would work fine but the telescoping guage might be a problem as I don't know if they make any small enough that would measure the bore of something as small as a .049.

6 or 7:1? Wow, that's low. I expected that these little motors running as high nitro content as they do would be at least 10:1 and up (like the old muscle cars on high test ). But I suppose maybe the 6 or 7 comp ratios are for the purpose to prevent preignition with the high nitro glow fuels.

meowy84

Combat, DOH! disregard parts of my above post at will. I just re-read it and you did say 'dry' measuring of the cylinder volume. I guess I got so excited (I just get way too excited about engines...) in responding I only read but didn't fully comprehend what you were saying.

meowy84

TDC=BDC.....I really should proof-read my posts a little better...heh heh

BMatthews

Did a google on "glow compression ratio" and turned up [link=http://www.mecoa.com/faq/compression/compression.htm]this page[/link] that has a pretty good description of how it all works together.

Consider that a glow engine is not like a spark engine where the ignition point is controlled by the spark timing. With glow engines we have a catalytic reaction between the alky and platinum on the plug element combined with the heating of the mixture via the compression. Our engines are often described as being glow plug ignition but in reality it would be more accurate to say alcohal diesel ignition with a glow plug assistance.... or glow ignition with a diesel assistance. The compression and glow are inherently linked in the process so it's hard to put one factor above the other.

That Mecoa page says 7.5:1 to 9:1. When I first went looking for "alcohal compression ratio" I found a bunch of drag racer pages that indicate ratios up around 14:1 or 15:1 with the highest being said to be 15.5:1. But this involved conversion of standard spark ignition engines so it probably would work at that high a ratio but only because they are using standard spark. If car engines used a glow plug they would be forced down to compression ratios similar to model engines I suspect.

meowy84

Interesting page. The glow plug thing does bother me because like you mentioned the compression and glow are inherently linked. In spark ignition if you advance or retard the timing inappropriately you'll either pre-ignite way before piston reaches TDC or fire too late when piston is already at the top or going down and lose lots of power. But at least you have some control. I suppose with glow we control that by shims and different nitro content. There was a post on here a little while ago and someone mentioned that they made a miniature coil that could be used with 1/2A stuff.

Conclusion: "More study is needed" as my grad prof used to say. LOL

combatpigg

My Feedback: (3)

You could get a fairly representative figure with just a machinists' rule ground thin enough to reach into a 1/2A cylinder, and the surface tension thing can be overcome by carefully filling the head whilst looking at it closely from the side, you aren't trying to equalize the volumes of a V8 here, just get a ballpark figure. The plus or minus error would only be a couple of tenths . The alky race cars get away with high ratios because pure alky is less likely to detonate, or pre ignite than gasoline. If you throw the least amount of nitro into one of these engines, then be ready to pull engine parts out of the asphalt.

meowy84

So nitro is present in glow fuel to help prevent pre-ignition? So it must burn very slow and not very good right? Also since castor doesn't burn then how much inflammable stuff is there in glow fuel?

Japanman

From what I read it is there to liberate oxygen- and tends to advance the ignition point, hence the need to run lower compression.

I calculate the compression ratio for twostrokes by comparing the volume trapped when the exhaust port closes to the combustioon chamber volume.

The actual running compression ratio gets interesting in two strokes- especially if you are running a pipe which loads the combustion chamber with quite a bit more gas- so you have to run less compression to compensate. I had mega problems before I realised what was happening- because the ignition point is linked to the compression ratio if you have a piped engine with too much compression, it will imediately overheat when it comes on the pipe as the ignition advances suddenly. This is really very obvious if you are running a bit too much load for the engine.

J.M

combatpigg

My Feedback: (3)

Not only does nitro do all the things that JM pointed out, but it also advances the timing[ ignition point]. In the early days of TOP FUEL a group of young chemical engineering students rewrote,"THE BOOK" on how to use nitro.
They lowered the mechanical compression, locked out the distributors' built in timing advance and ran almost 100% nitro . End result was a bunch of shattered records, and a bunch of red faced "experts" who had initially told these boys that they were doing it all wrong.

Yes JM, it is easy to forget that piping IS supercharging and all the rules about running a pressurized engine hold true. It would be interesting to set one of our engines up with a knock sensor, and oscilloscope to see where the ignition point actually is.

meowy84

I bet the ignition point varies depending on the ambient air temperature (of course) and the temperature of the engine itself. It could be that when the engine first starts the ignition point is retarded (because of the cooler metal in the engine the fuel starts to burn later, when the piston is closer to the top) and as the engine heats up the timing advances a little. I bet in an average run of a 1/2A engine for example as installed in a plane the ignition point dances back and forth as the plane is flying and hits pockets of warmer and cooler air. This probably wouldn't be as big a problem in bigger 2 strokes that have more metal and can hold more heat but with 1/2A and their size it is more of a factor.

Also, remember that the mixture doesn't just explode when it is ignited....the flame progresses (albeit very fast) from the point of ignition outwards. That's why there's been so much work done on scavenging in real cars for instance and why side-valve engines are fairly inefficient compared to overhead valves. Fortunately in 2 stroke glow engines we can pretty well control the shape of the combustion chamber much better than in 4 stroke engines.

meowy84

Then again 2 strokes have their own drawbacks like unburnt fuel being spit out the exhaust.

t_kwink

A lot of things that go on in a running engine are easier to understand if they are looked at correctly.
One important topic is "Volumetric Efficiency" which is simply a ratio of the amount of air that goes into the cylinder compared to how much it is capable of holding (it's displacement). The typical VE of a production car engine is in the 80% range. A good NASCAR style engine is up in the 120% range.
You may ask how you can get above 100%. This is possible because the scavenging of the exhaust flow, and the high velocity of the intake charge will actually "ram" more air into the cylinder. The same principles apply in a 2-stroke. Using a pipe was described above as "supercharging." What happens with a pipe is easier for me to understand by thinking of it as improving VE through superior exhaust scavenging.
Another important factor in a glow engine is "cylinder preassure." This is the amount of preasure in the combustion chamber due to 3 factors, compression ratio, VE, and cylinder seal. Cylinder seal is easy to understand; if it leaks, it bleads off preassure. VE effects cylinder preassure because the more air you get in, the more you have to "squeeze." Compression ratio effects cylinder preassure because it determines how small of an area you have to "squeeze" it all into.
A cooler intake charge is more dense and oxygen rich, so this will also raise the cylinder preassure. Nitro does not actually "burn." The nitro enriches the fuel charge with more oxygen, so the higher the nitro content, the more oxygen your putting in, and the higher the cylinder preassure. Another factor in the engines we use is the oil and other fuel ingredients that do not get burnt. All these non-burning ingredients take up space in the combustion chamber, and effectively raise the compression ratio, and thus the cylinder preassure.
The ignition point of a glow (or diesel) engine is determinned a great deal by it's cylinder preassure and the factors that go into it. The higher the cylinder preassure, the sooner it will ignite.
As an engine's RPM is increased, it desires a progressively further advanced ignition point. This is why high nitro and small props go together. The high nitro/high compression causes the cylinder preassure to rise, and the ignition point to advance. With this advanced ignition point, the engine will only run correctly at a higher RPM, and this is achieved by installing a smaller prop (less load).
I have also noticed that if the crankcase volume is decreased, it improves the draw through the carb, and forces more air into the cylinder, which is an increase in VE, and thus an increase in cylinder preassure. When I did this, I was required to lower the compression ratio to make the engine run best with the same prop and fuel.

Wink

meowy84

I found my spec sheet for my Norvel Cmax .061 Revlite and the compression ratio given is 9.5:1 I believe (it's in the 9 range anyways).

T_KWINK, I read your post about installing a carb on a Cox .049 where you explained a lot of these things. Do you have any idea why if the crankcase volume is decreased the fuel draw is better? I know that the air/fuel velocity of a smaller diameter intake pipe is higher than the same length of a larger diameter pipe and that's why say on a street engine (lets use the ubiquitous 350 Chevy) going to a larger carb isn't always the best because you reduce the air velocity and hence less fuel/air goes into the engine making for crappy performance. On an engine that will turn sustained high rpms (like nascar) the bigger carb makes sense. Anyways, I remember from your post that you brought the carb as close to the back of the engine plate as possible and that seemed to improve the running properties. Have you tried going the other way? That is having an excessively long and narrow intake pipe to to give the charge high inertia and increase the air/fuel velocity for a sort of 'ram-air' effect?

Also, what carb did you end up using on that .049? I was thinking of doing a similar experiment and I have access to 2 carbs....one from a pre-revlite AME .049 engine and another from the Norvel .061. any idea as to which one would work better on the cox reedie?

t_kwink

Well, Your analogy of the large vs. small carb on the ol' 350 chevy is close, but not quite. It is really the size and length of the ports in the intake manifold and cylinder head that effect the velocity and other related issues. The reason an overly large carb doesn't work so well on a street car is because a mechanical carburetor (like a holley, quadrajet, carter, etc. etc.) relies on vacuum to meter fuel, much like on a model airplane engine. If the carb is too large, it gets very difficult to achieve a correct idle mixture and speed as well as a WOT high RPM setting that is correct without hindering the mid-range and part throttle transition. Basically, a carb that is too big doesn't have enough vacuum to pull the fuel from the idle, transition and main circuits correctly. That is why modern production vehicles get away with a much larger "venturi" or throttle body. They don't have to rely on vacuum to meter fuel, it is injected with preassure (fuel injection).

The fuel draw was better on my reedie with smaller crankcase volume because as the cylinder rises, it has a smaller volume between it and the carb. It's like compression ratio in reverse. If you start at TDC and move the piston down, it has more resistance on a high compression engine than on one with a low compression ratio. I also think this is why it worked better with a shorter intake port, but there are other forces acting there like "plenum" volume as well as "waves" similer to the ones that make a tuned exhaust pipe work. The cross sectional area or diameter of the port is simply a matter of what size is the best balance between airflow and air velocity.

I am currently using a tee dee .049 carb on my reedie. It doesn't really perform any better than the AP, but the needle is much tighter and secure. It also does not have a throttle, so it is very simple. The AP would have worked great, but the needle was just a loose fit, and I couldn't keep it from vibrating/leaking. I think the G-mark .061 carb I got from Tim would be the best choice for a throttled carb to actually run in an airplane because it's smaller size would give it a better chance of pulling fuel from a non-preassurized fuel tank. Out of the two carbs you have, I would choose whichever one is easiest to mount, or the smaller of the two.

Another note on compression ratio. There are two types of compression ratio.
Theoretical compression ratio is simply the volume above the piston at bottom dead center divided by the volume above the piston at top dead center.
Practical compression ratio is based off the piston location when it closes off all the ports in a 2-cycle, or when the intake valve closes on a 4-cycle.

Wink

meowy84

Wink, you're absolutely right of course about the diameter and length of intake also playing a big part in addition the the carb size (hence the reason tunnel ram intakes and huge carbs make for finnicky street cars with the performance of a dead horse below several thousand rpm and yet so many people run these ridiculous setups). Ah, perhaps they look the part if not act the part.

Wink, as for your Cox reedy how much of a hindrance do you think the reed valve was? I can't seem to locate that thread at the moment but am wondering what reed material you settled upon?

t_kwink

The intake "port" diameter leading up to the reed was .200", and at this diameter, I don't believe the reed was a restriction at all. I tried every kind of reed cox has made, and they made absolutely no difference whatsover. Different shapes or materiels didn't matter. I think the oval shaped ones look like they have a better chance of staying installed correctly, and I think the stainless steel or clear mylar ones would last the longest and not deform or bend.

I actually don't think there is any reel restriction left in the intake tract at all the way I have it set-up. I think the restriction is now in the bypass ports and the exhaust. I haven't touched the thing in a couple of weeks now. I thought I'd let it sit there and let my mind fester about it some more first.

Wink

meowy84

Thanks for the info. I think I'll be following in your footsteps sometime in the not too distant future (I hope). At the very least this will definitely go on the list of possible winter projects.