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ROTARY PRAGMA

What is a One-Rotor-Wankel engine?

A one rotor Wankel engine is the most simple and smooth running reciprocating internal combustion engine existing today.

It only has two basic moving parts. There is no reciprocating masses,  except for the combustion gases, as a result of the cycling combustion  chamber volume.

Such an engine is running with next to no vibration except for torque fluctuations resulting from a reciprocating combustion.

Power is produced in this engine like in a single cylinder two-stroke engine, one combustion for every crankshaft revolution. The basic difference is the fact that each combustion CYCLE takes three crankshaft revolutions to complete. This allowing high crankshaft RPM at a comparatively slow  combustion cycle, resulting in a potential power to weight ratio comparable to the lightest known two stroke engines.

With all this in mind, I decided to put more effort in realizing this engine than one would put in a normal auto engine conversion.

As it turned out, it was way more than I anticipated in the early stages of the project.

What engines are available from in-production hardware?

I found out very quickly, that there is essentially nothing except for MAZDA with the RX7 and later RX8 sports car. As stated earlier, these engines are too big and too powerful for my application. So, take one of these engine and cut it in halve, similar to what has been done to make a 1/2 VW engine some time ago. Doing this with an RX7 engine would  result in about 100hp and an acceptable weight, depending how much  effort I am willing to put into modifications.

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Engine Design

After looking at the parts of my first disassembled 12A engine, it was clear that some major parts needed to be made of aluminum rather than using  the original Mazda cast iron version.Talking in particular about the side housings.

The first issue coming to my mind on aluminum side housings is the wear  surface. Nobody has come up with an after market aluminum side housing answering all the questions about a good wear surface at an acceptable cost.

In order to keep the developmental risk manageable, I decided to retain  the CI(Cast Iron) surface of the side housing. Meaning a compound  housing, made up of an aluminum outer part and a CI insert as the running surface for the rotor.

All aluminum casting are made of AL 356 T6. I made all the patterns and got it cast by a speciality foundry.

Subsequently having to switch to a peripheral intake port, because I did not see a  possibility for integrating side ports with a¬  sound engineering solution. And, off course, as I learned later, the P-Port intake also  has other advantages.

The prove of principle engine made its first run in ....2001. It is running presently with the final version of the compound side housing(end of 2009).

This first configuration had a peripheral port, and in addition, one side  port. The side port idea was to improve low RPM idle. I immediately  observed interference of the two ports at certain RPM ranges. It also turned out that on PP only, Idle around 2000 RPM was satisfactory for AC use, and I decided not to waste any time looking into the port  combination any more.

At the end of the design and development process, the only MAZDA parts left the engine, which are not modified are, the rotor, oil pump, stationary gear and some bearings.

As of today, January 20 20010, the first generation design is frozen and will be installed for flight testing in the PRAGMA air plane.

This should happen in the next couple of month.

Cutting Parts.

The first lesson I learned was: I can not afford getting the necessary machining work done by a machine shop.

So, shelving the project? Not only nooooo, but h.... no

As a result, a gradual building up of machining capability was essential for the success of the project.

Starting out with making an excenter shaft was like jumping in a swimming pool at 35F. But it worked out pretty well for the prove-of-principle engine.

¬  Finally, by the end of 2008, my machine shop was joint by a CNC milling capability, making things possible I was only dreaming off a few years ago.

I was always told, you can do it either quick or good, but not both. Wrong! Once the G-code was written and debugged, everything was quick and came out good. And that every time I made a part.

Things like measuring twenty five times, and cranking a handle the hole day, are a thing of the past.

The picture on the left is just an example what can be done in a matter of hours, including setting up, versus days with manual machine tools.

A first test run with the final configuration engine took place in early  August 2010. Everything was going pretty well, except for¬  excessive oil exhaustion through the crank case breather.

The reason for this was the location of the breather pickup on the engine. I had it on the oil filler, which is on top of the accessory housing.  Apparently, the oil is just too much in motion and foaming in this area  for this solution. Hooking the breather up to a fitting in the oil pan, just below the anti-foam plate solved the problem.

The engine is now running very solid at 6000RPM and WOT. Calculated HP is 92 - 100 , depending on the BSFC No. using.

Demonstration run at Alternative Engine Gathering in Paducah, KY, 2010.

Picture by Douglas Dempsy

In my personal view, it is already more power than I need for flying the PRAGMA.

Configuration Overview.

Engine rotor size                                                  1/2 MAXDA 12A

Excenter shaft                                                         Modified 12A with case                                                                    hardened bearings and 10:1 taper output.

Flywheel                                 Custom design with integrated balance weight.

Oil pump                                                                   MAZDA 12A

Water pump                                                             Subaru

Alternator                                                                   30A PM.

Ignition                                                                       Streetfire CDI , dual coil

Carburetion                                                              AeroV Injector.

Moving into the area of airplane installation presented a bucket full of new challenges. Some of these are still cooking now.

For a motor mount, I choose a 3-point version, with 2 Hard points and one  tension strut. A nice feature on the AVID is, it has six hard points on  the fuselage for mounting the engine. Unfortunately¬  two of those were  in the wrong location for a practical rotary engine mount. With that  corrected, the motor mount turned out pretty decent.

Early fit-check of engine-mount-AC interface.

Engine instrumentation.

Looking into what to use for measuring engine parameters, I ran into several problems.

All engine monitoring systems I found on the market have capabilities enough to monitor 5 engines at the same time.

I also wanted to use a simple data aquisition system I already have. Generally used sensors for temperature and pressure are highly nonliear, and therefore require computer calculation for generating presentable  physical parameters.

The way I am taking out of this is using linear sensors to begin with.

For temperature, I made my own sensor housing with standard dimensions using a LM34 series temperature sensor.

For pressure an industrial sensor from Digikey came in handy.

One disadvantage of these sensors is the fact that they need three and four wire connection. But it worked out for me better then the alternative.

With the linear sensors, I am able to use simple DVMs for indicating the actual value with a minimum of signal conditioning.

 Pressure and temperature sensor

Finally, the engine design has been tested and debugged. A test run simulating a 20 minutes flight around the pattern, including go-around, has been conducted many times. This made me confident that the engine is ready  for flight testing.

While leaving the ground test engine on the test stand for further component testing, the flight test engine is getting assembled.

As shown in  the above picture, I am using studs instead of tension bolts. The reason for this is to avoid wear on the threads in the AL when assembling and  disassembling the engine. From here on it is pretty much like assembling a two rotor, only somewhat quicker.

The side housings are already assembled with the CI insert and the stationary gear in the rear housing.

Next is  bringing the rear housing into a horizontal position and inserting the  rotor housing with MAZDA stock O-Rings. Insert the e-shaft and the rotor with seals, slide on the PTO-side housing,  add washers and nuts, and the block is complete.

As mentioned earlier, the engine was always blowing a considerable amount of oil out through the oil pan breather.

I was hoping that it was caused by excessive blow by on the seals of the ground test engine, because these were not all new when I started testing with this engine. The flight test engine was built with all new seals, so I assumed  that will take care of the problem. That was not so. The flight test engine was no different as far as the oil problem is concerned.

Now it was time to really find out what is causing that.I tested the pressure in the oil pan while running the engine. It turned out to be 1.5 psi pretty much independent of engine RPM. Next test was closing off the oil pan breather. The results where the same, 1.5psi and no change over RPM range. Conclusion was that it must be some kind off dynamic pressure. First suspect:exhaust!.

Digging up an old rotor housing for checking the exhaust port. No openings or ares of possible leaks could be found. Looking at another rotor housing, and there it was, two holes for EGR, which I overlooked when I designed my side hounings were open into the oil pan. This discovery was a reason to dance.

Now, engine tear down and closing those holes. Within a week the ground test engine was running with the fix. It was a full success. I did not know that the rotary had no detectable breathing into the oil pan, it is totally dry, and I had it run up to 7000 RPM.

Now I can use the ground test engine for component and development testing without having to worry about something as annoying as that oil problem.

Future improvements.

The next improvement is adding a magneto ignition for one of the two spark plugs

 I am looking into a full aluminum sandwiched side houning.

For further information about the engine installation  into the Pragma go to the PRAGMA page.

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