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Compression

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Compression

Post  Cribbs74 on Fri Jan 18, 2013 2:33 pm

Why is it a cylinder with less compression turns over faster than one with more? I am noticing this with my unlimited entry, I swapped out a modified#1 Cox International cylinder for a Bonifide number#4 that I pulled off of a TD. I lost over 1K RPM with the #4. I swapped back and sure enough gained my 1K back.

My #4 had noticeably more compression not to mention 2 more boost ports. Anyway I just thought it was strange. Anybody else experience this?
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Re: Compression

Post  Admin on Fri Jan 18, 2013 2:38 pm

I remember reading in a old small engines book that increasing the compression raises torque and horsepower while lowering compression allows for more speed but less torque and horsepower.


For your engine, swapping parts can be tricky. The piston may not fit as well with the #4 cylinder as it did with the original cylinder. Over compression or a tight fitting piston would also slow the engine down.

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Re: Compression

Post  Cribbs74 on Fri Jan 18, 2013 2:49 pm

Admin wrote:I remember reading in a old small engines book that increasing the compression raises torque and horsepower while lowering compression allows for more speed but less torque and horsepower.


For your engine, swapping parts can be tricky. The piston may not fit as well with the #4 cylinder as it did with the original cylinder. Over compression or a tight fitting piston would also slow the engine down.

Makes some sense however, I swapped cylinders and pistons not just the cylinders themselves. Who knows, guess I am sticking with Bernies cylinder set.
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Re: Compression

Post  Jason_WI on Fri Jan 18, 2013 2:55 pm

Ron,

My results are the same with the #4 TD set vs the TD set that Bernie sells. I think it may be less drag with the untapered set.

Have you tried messing with TD cylinder shims. These are the ones that adjust port timing. I have a set of the shims but haven't used them yet.
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Re: Compression

Post  Cribbs74 on Fri Jan 18, 2013 3:06 pm

Jason_WI wrote:Ron,

My results are the same with the #4 TD set vs the TD set that Bernie sells. I think it may be less drag with the untapered set.

Have you tried messing with TD cylinder shims. These are the ones that adjust port timing. I have a set of the shims but haven't used them yet.

Yeah I have actually. I ended up removing the cylinder shim as I originally had an interference issue with a Galbreath head. I set it up so it was at TDC and level with the top of the cylinder. Before I went any further I noticed the piston could use a reset, after the reset I was able to remove the shim.
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Re: Compression

Post  Cox International on Fri Jan 18, 2013 3:06 pm

For years I have been advocating that, once a good level has been achived, more compression does not necessarily equate to more RPM.

A Cox production manager once told me that they went to untapered cylinders because it made no difference in the end. Also, Larry Renger (former Cox engineer) has confirmed that the old TD cylinders with 4 ports actually performed less than the 2 port ones (see copy and paste below)

We found the two boost port version ran higher rpm than four. The reason is that it introduces a swirl to the mix that enhances combustion. Four ports are too symmetrical. This trick was used in the Killer Bee, Tee Dee and Venom. I picked up the hint from the “Two Stroke Tuner’s Handbook”, a long out of print 2-stroke bible.
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Re: Compression

Post  Cribbs74 on Fri Jan 18, 2013 3:18 pm

Well, it certainly appears that way Bernie. Quite impressive to actually view it first hand.

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Re: Compression

Post  Mark Boesen on Fri Jan 18, 2013 4:45 pm

Years ago when I was actively racing ½ pylon with Tee Dees, it was a common practice to lap the piston/cylinder fit to the point a light tap would drop the piston out of cylinder. The thought behind this was at high RPM the piston would become ‘tight’ in the cylinder, due to the extra heat/friction causing the piston to expand more than the cylinder, resulting in more stress/load on ball socket a well as reduced RPM.

This wasn’t just a local concept, I had read about this in several model airplane mags during that time and had seen firsthand early ball-socket failure in a couple new unlapped cylinders turning at high RPM.
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Re: Compression

Post  ado1974 on Fri Jan 18, 2013 5:26 pm

ron
just to give you a little more to consider when tuning your entry...

as you change compression you are effectively changing the ignition point(timing) of the air/fuel mix.(same as turning the distributor around on a car engine)
ignition of the a/f mix will occur at a set pressure(compression), with assistance from the glow plug.

so...
if you change compression, you may also need to change the heat range of the plug to get an ideal ignition timing

more comp = more advance
hotter plug = more advance

the joy of engine building, you cant just change one thing, without it changing everything else

cheers
a
p.s.
that reminds me,
must call my brother... i think he still has one of my 2 stroke tuning books DAMMIT!
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Re: Compression

Post  pkrankow on Fri Jan 18, 2013 6:10 pm

Ado, I have a copy of "2 stroke engines for RC aircraft"by C. David Gierke and it states what you say about compression affecting timing.

These glow engines are really compression ignited engines with a little help from the glow plug. Changing the number of gasket shims will change the RPM This is why there is a general rule of thumb that every 10% of nitromethane requires an additional gasket shim.

Phil

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Re: Compression

Post  ado1974 on Fri Jan 18, 2013 6:16 pm

phil

thats the missing book..... lol!

worth reading, for anyone that wants to learn more

a.
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Re: Compression

Post  John Goddard on Fri Jan 18, 2013 6:38 pm

Goodness me we'll all start talking about lambda 1
And be reaching for our afr meters soon...
lol!
I've always been amazed that no one has ever mentioned
Compression ratio on here or (seemingly)documented it in the
World of Cox performance.
Huh...
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Re: Compression

Post  Cox International on Fri Jan 18, 2013 6:43 pm

Big download but good book for tuning 2-stroke engines I have been told.

http://coxengines.ca/files/TS.pdf

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Re: Compression

Post  Mark Boesen on Fri Jan 18, 2013 9:27 pm

pkrankow wrote:Ado, I have a copy of "2 stroke engines for RC aircraft"by C. David Gierke and it states what you say about compression affecting timing.

These glow engines are really compression ignited engines with a little help from the glow plug. Changing the number of gasket shims will change the RPM This is why there is a general rule of thumb that every 10% of nitromethane requires an additional gasket shim.

Phil


yep, this is a big part of a setting up a 2-4-2 break on larger engines for stunt, not so much the 'rule of thumb' but the relationship of nitro, compresion, load, porting, etc. have with each other, something as simple as a hotter plug can make a big differance.
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Re: Compression

Post  RknRusty on Fri Jan 18, 2013 9:51 pm

cribbs74 wrote:Why is it a cylinder with less compression turns over faster than one with more? I am noticing this with my unlimited entry, I swapped out a modified#1 Cox International cylinder for a Bonifide number#4 that I pulled off of a TD. I lost over 1K RPM with the #4. I swapped back and sure enough gained my 1K back.

My #4 had noticeably more compression not to mention 2 more boost ports. Anyway I just thought it was strange. Anybody else experience this?
I had the opposite result. I had a slit SPI set and replaced it with a #4 and gained a thousand. Both passed the hang and tap out test. I also gained a lot with a Babe Bee crankshaft over a Killer Bee crank. I wish I had one of those Tee Dee cylinders that Bernie has modified for SPI with a regular piston. I spent a lot of time trying head and gasket combinations too. I'm out of time for tinkering now though, time to ship it. I wish I'd started earlier. It didn't just hop up and miraculously perform when I laid hands on it. Rolling Eyes

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Compression test.. your thoughts?

Post  roddie on Fri Apr 24, 2015 9:48 am

Blame it on coffee... but this morning I wondered about making something to actually test engine compression. It's accuracy wouldn't necessarily be important.. (although that would be a plus..) but rather; it's ability to show variances between cylinders/heads having the same design.

I have no idea how much pressure is produced within a model engine cylinder's combustion chamber.. but I'm sure that it can be measured somehow. Huh...

Say you're assembling a Cox engine.. and have several P/C sets to choose from. It would be good to know which sets had the best compression.. wouldn't it?

One way of making a tester.. would be to obtain a digital tire inflation gage for the readings.. and make a fitting from a burnt-out glowhead to accept a Schrader-valve housing (w/valve removed..). The question would be whether significant variations could be observed via these gages; which mostly have an accuracy to within .5 lbs.

Testing could be performed on an RTR engine.. or if you're assembling one; by just installing the piston/cylinder into the case/crankshaft.. and rotating it with the test-head tightened in place.. and observe readings on the gage.. which likely retains the reading in it's memory.

For larger engines.. a standard (1/4-32 threaded) burnt-out glowplug could be drilled-out.. and the Schrader-valve brazed to the top.

Is this worth the effort.. or just another one of my crazy ideas????  What?

Here's some photos of components to show how simple it might be to make one for Cox .049 engines.



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Re: Compression

Post  balogh on Fri Apr 24, 2015 10:32 am

Guys,

I am a compression freak and often check it on my COX engines when cold and hot. Initially I thought a perfect compression is indispensable for max power, but in fact a low-drag piston, even if with somewhat reduced compression, may still perform as one with a tighter fit/stronger drag but perfect compression.


Given the very short time while the piston travels down the cylinder, even if the pressure in the compression chamber is some 40 bar when the explosion occurs, the blow-by is limited. Therefore the power lost by blow-by is very low. (According to the laws of gas-dynamics, unless the orifice between two chambers of pressure ratio less than 0,56 is a Laval orifice, the flow between the 2 chambers remains constant, because the gas speed achieves the sound velocity and cannot grow further only in a Laval nozzle.)

So while I am still inclined to prefer an engine with perfect compression (and my TD051 after some 200 hrs runtime on it is having close to perfect compresiion), I dropped my original theory and believe a lose piston fit may still bring you to the same output.
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Re: Compression

Post  balogh on Fri Apr 24, 2015 10:50 am

Admin wrote:I remember reading in a old small engines book that increasing the compression raises torque and horsepower while lowering compression allows for more speed but less torque and horsepower.


For your engine, swapping parts can be tricky. The piston may not fit as well with the #4 cylinder as it did with the original cylinder. Over compression or a tight fitting piston would also slow the engine down.

In principle with the higher compression ratio the burning of the fuel/air mix will occur at higher temperature and thus the average temperature of the heat input into the Carnot cycle is higher. This, based on the law of thermodynamics, will increase the efficiency of the cycle i.e. generate more power from a given amount of fule
burnts.

This is the reason why the high compression head of TD engines (increasing the (swept volume+combustion chamber volume) /combustion chamber volume ratio) will help produce more power in an engine than the normal head.

Of course if the compression in an engine is excessively low due to piston/cylinder wear or fit, then the above theory is applicable with limitations only.
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Re: Compression

Post  pkrankow on Fri Apr 24, 2015 12:04 pm

at 15000 rpm, there is 250 revolutions per second, .004 second per revolution. Out of this revolution roughly 1/6 is with all ports closed.

How much gas can bleed out in .000067 seconds?

Low compression can make for hard starting, a wet cylinder helps here. A head leak is also quite a different problem than bleeding past the skirt. The skirt makes a very long path for gas to travel to escape too, slowing the loss down.

It is possible that low static compression engines perform just fine. I have a few examples of this that I run regularly.

Phil
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Re: Compression

Post  roddie on Fri Apr 24, 2015 12:33 pm

Would a tester be useful? Maybe not for a small Cox glow-engine.. sighting the last few posts that I've read. I would suppose; in addition to providing fuel for the engine to burn.. an exhaust prime also raises the compression available for starting. Maybe this is why some engines start and continue to run off the 1st exhaust-prime.. while others need to be primed several times before they run long enough to draw fuel from a hard-tank?
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Re: Compression

Post  balogh on Fri Apr 24, 2015 12:35 pm

Fully agree. Just check the static compression in a gas (or petrol in British English ) model engine where the fuel contains only around 3% oil and provides for a thin seal around the piston skirt despite the piston rings ...almost none compared to our 20% thick castor lubricated COX engines ...
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Re: Compression

Post  roddie on Fri Apr 24, 2015 1:08 pm

balogh wrote:Fully agree. Just check the static compression in a gas (or petrol in British English ) model engine where the fuel contains only around 3% oil and provides for a thin seal around the piston skirt despite the piston rings ...almost none compared to our 20% thick castor lubricated COX engines ...

It may also be true that modelers who use after-run oil in their engines experience faster cold-starts.. as a result of oil increasing the compression and also creating more of a vacuum in the crankcase for drawing in fuel? Our fuel obviously has oil "in it".. but it doesn't coat the entire piston/cylinder unless flipped through compression a few times.

Most times (if I remember.. Rolling Eyes ) after running an engine.. I'll squirt a few drops of "3-in-one" oil into the exhaust ports.. flip it through a couple of times and call it a day. I bought some air-tool oil last year.. but haven't tried it yet. I'm guessing it's a lot lighter in viscosity than 3-in-one?
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Re: Compression

Post  balogh on Fri Apr 24, 2015 1:13 pm

After run oil also helps prevent corrosion of the bare steel surface caused by the combustion residues of nitromethane that are said to destroy even the stainless steel ball bearings in larger engines if left inside for a winter off season.
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Re: Compression

Post  balogh on Fri Apr 24, 2015 1:21 pm

[quote="roddie........ Our fuel obviously has oil "in it".. but it doesn't coat the entire piston/cylinder unless flipped through compression a few times.

..................?[/quote]

The fuel even with 20% oil in it will not provide as good lubrication when the engine is cold as when hot. This is one reason why cold runs will wear off the engine, and why you want to reach operating temperature the soonest. In a cold engine the methanol and nitro keep the castor diluted and when it precipitates on cold surfaces the diluted oil is still thin...whereas in a hot engine the methanol and nitro evaporate when hitting the hot surface leaving the castor on the cylinder and piston alone behind.
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COX COMPRESSION

Post  coxaddict on Fri Apr 24, 2015 9:01 pm

Hi coxaddict here,
I read that the groove on TD .051 pistons was put there to bleed power from the larger displacement .051 to get the same power as the TD .049s. Can someone explain that concept?
If there was no groove would the larger bore produce more power? Would a shortened piston skirt bleed off power also? Performance tuning is all so new to me.
Thanks in advance
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Re: Compression

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