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David Kerzel,     Pompano Beach, Florida

Horizontally opposed CO2 type 4 cylinder compressed air engine  First posted 2/24/2004

This is my third small bore CO2 engine.  The 1 and 2 cylinder versions were quick and easy so why not a 4.  I was inspired be Dale Dietrich's engines and started moving parts and making a sketch.  Then I decided to reduce the bore from 1/4 to 3/16 inch.

I began working on the engine in October but could not make accurate pistons or connecting rods.  I realized my lathe was not adjusted perfectly so I shimmed and check until everything looked great.  Still the parts were not concentric or not square.  After 3 tries I decided the parts I wanted to make were too small for the Chinese 3 in one lathe/Mill I used.  I need collets and higher spindle speeds for the small parts.  I bought some Sherline equipment on EBay.


The Sherline lathe with collets and higher speeds helped make the parts with good accuracy.  I am not too sure about the machine handling larger parts yet.


The sketch can be seen by clicking here.


The pistons are made from 3/16 drill rod.

      Connecting rods are made from 3/32 brass rod

Rough turned cylinders made from 3/8 drill rod.

    3/28/2004  Heads


The Split Crankcase were made on my larger mill with a DRO.  Worked out perfect, well it did the second time I made the parts.  The first run I drilled the holes for the cylinder screws a size to large.  I was going to just use the next size larger screws (#1). I enlarger the counterbore in the heads.  I went to cut some decorative fins and broke in to the larger counterbores.  So I came back to my senses and started over.  A Center drill broke, restart, a drill broke, restart, a 0-80 tap broke, and finally some luck, it came out.

I think I an loosing my sense of scale, these parts are looking normal sized.


4/5/2004  Milling slots in the fins for the head screws.  It was one of those days, more tools broke than parts got made. 


4/11/2004   Finished milling slots for screws and ran out of bits for exhaust ports.  A few slots for texture.


4/20/2004    Finally enough parts for a fist fitting and a first look at the design.

Pistons and cylinders were lapped with 600 grit compound.

Its getting to big to fit in the mint can.



fitting the crank shaft for the first tests. The ball depth needs to be set and the seat needs to be lapped with a ball glued to a tube.


 First each piston and cylinder will be run as a single set.  When I have 4 sets that have no valve leaks and work freely the engine will be fully assembled.  The first cylinder required 28 PSI to keep running and runs great on 34 PSI.  Design operating pressure is 44.  The pistons and cylinders are lapped, the valve ball makes no leaking air sound with the piston up or down.

The rest of the cylinders and pistons were tested after they were lapped.  All 4 needed 38 PSI to run and ran great on 42.  The consistency was incredible.  I lowered the speed I lapped the part down to about 200 RPM.  If I covered the top of the cylinder and pulled out the piston it popped every time dry.

The crank shaft is LocTited together and is made of 9 separate parts.  The connecting rods are captive to the pins between the disks.  In the center is a bearing and some spacers.

It took several tries to get the 4 pistons centered in the cylinders. The biggest problem was having the parts fitting to snugly and not allowing easy precise.  A little more clearance would have helped.

It is all together, The shaft has more drag than I am used to but that may just be the 4 cylinders (this is my first over 2).  The air in system I plan is ridged so I will first make a manifold and use 4 hoses to the 4  inlets to test the engine.

7/18/2004 The first test with all 4 cylinders powered.

I knew it would take 40 PSI for it to run, I mover the fly wheel about 1/4 turn and it started.  It runs fast.  There are 4 exhaust cycles every revolution so it sounds even faster.  I took. the pressure up and it sounded like a distant chain saw.  After about 30 minutes at lower speed it started to pop and sound like it was missing.  These engines don't miss and the only get better with run time.

It actually stopped.  when I went to restart I noticed the fly wheel wobbled a little.  I had been concerned about the strength of the crank shaft.  It is made of 9 parts held together with LocTite.  The pins that the connecting rods go on are .047 music wire and are not all that ridged. 

I took it apart.  The crank shaft had several joints fail.  The holes in the brass disks are oval now.  The center portion seems OK.

The design had one ball bearing at each end and one in the center.  I suspect each end needs to keep the shafts aligned.  I also suspect the .047 music wire pins is too flexible for a long crank like this ( it is about 1 inch between bearings).

Next step new connecting rods with big ends that can handle .062 crankshaft pins, and dual end bearings.



New  connecting rods made with bigger big ends and hole5s for .062 pins.  Looking at he parts it is obvious it should be 5/64 diameter crank shaft pins.  I have the drill rod on order.


This is the center bearing.  A bal bearing is in the center, spacers on each side.  The Vee slots allow air pressure to be balanced in the crankcase.

10/2/2004 In the past few weeks a lot of things have been done. 
The big ends of the new connecting rods hit the inside of the crank case.  So the crank case was bored out an extra .01 inch between the bearings.
It ran but not as well as expected and stopped after an hour.  The crank LocTite joints failed again.
After a few more attempts with different LocTite products and cleaners I realized it was not strong enough.
I drilled and pined each joint after it was assembled with LocTite.  I used .022 diameter music wire for the pins

While all this was going on I discovered ultrasonic cleaning of parts.  and finally got it back together.

10/16/2004 It was not the problem.  One piston had a loose pin that occasionally moves and jams the engine.  The sudden jolt of the engine getting jammed is what broke the LocTite joints.  Runs good now on any number of cylinders  and runs for hours without stopping.
Now the engine needs some finishing details.  The air inlets will be fed by copper tubes formed to be like a Type 1 VW exhaust system with a resonator and the little glass pack mufflers.  A carburetor and intake manifold for show along with a belt driven generator.

This engine was a real challenge.  I started it nearly 12 months ago, acquired a lot of new tools, but I have learned so much about little parts.

I started a inline 4 cylinder engine during this project but did not work on it when problems started.  It was started with the block made from steel.  I think I will start over using brass.  Go to work in progress.

I will add the drawings and more photos as I proceed.


Copyright 2004,  Florida Association of Model Engineers and engine builder as noted above, All rights reserved.