The night after I finished the
first post about Clen's Engine I realized there was a fundamental problem with this design. As I hope you can see from the picture, this engine has three cylinders arranged as the three sides of an equilateral triangle. At each apex of the triangle there is a crankshaft. Each crankshaft is connected to two pistons, one in each of the adjacent cylinders. There are no cylinder heads. Combustion is initiated when the two pistons approach each other near the midpoint of the cylinder.
The problem shows up when you try to analyze the timing of this arrangement. In an
opposed piston engine (of which this is one example) the pistons move in opposite directions simultaneously. They move together to compress the air, and then move apart to transfer the force of the explosion to the crankshaft. Starting at any cylinder in this diagram, and moving in a clockwise direction:
- in the first cylinder both pistons are at TDC (Top Dead Center).
- 60 degrees later the pistons in the next cylinder are at TDC.
- 60 degrees after that, the first piston in the third cylinder is at TDC, but
- the second piston in the third cylinder is way off in the boondocks.
You can try changing direction of the cranks, but no matter how you try to arrange it, you have both pistons meeting at TDC in two cylinders, but you will never achieve it in the third one.
Yesterday I did a little studying. I found my first clue in the article titled
I linked to it in the first post. I hadn't read it thoroughly, I was just using it as a source of pictures. Reading it is an experience in and of itself. I don't have words to describe the style of the writing. Kind of amazing. But I digress. Back to the subject at hand. Here's the clue:
... the pistons are not in truly opposite phase, the exhaust piston having a 20 deg. lead so that it opens and closes its ports earlier than the inlet piston.
20 degrees!?! Aha! That would explain it. If you take TDC as zero degrees,
- then add 60 degrees for the angle between cylinders (which is how far the crank would have to turn to bring the next piston to TDC),
- then subtract 20 as you move to the next crank,
- then add 60 for the next piston,
- subtract 20 as you move to the last crank,
- then subtract 60, because the last crank is turning in the opposite direction,
- and then subtract 20 as you move to the original crank,
you end up with 0, which is TDC, which is right where you want to be:
60 -20 + 60 -20 -60 -20
Voila! Okay, I don't know if that verbal explanation helped at all, but it clarified what was going on in my mind.
But is the last crank really going in the opposite direction? I checked my source again, and he agrees. Then I checked the photos, and sure enough, two cranks are connected to the output shaft by a single idler gear, while the third crank has two idlers! Whenever you have two shafts connected by
spur gears (ordinary gears), they will turn in opposite directions. If you put a third gear (the idler) between them, they will both turn in the same direction, while the idler will turn in the opposite direction. If you add a SECOND idler, the two original shafts will be turning in opposite directions once again.
Now all I had to do was figure out the order of the pistons. I thought about it a little bit, and drew some sketches, but I wasn't really getting anywhere. Finally I decided that I could mark down the relative piston positions on
spreadsheet and graph them using the spreadsheet's built-in graphing function. Then by adjusting the relative position of the cranks by changing the numbers in the spreadsheet I should be able to determine
who was on first, so to speak.
For a first attempt, I used an increment of 60 degrees and plotted how close the pistons were to each other. The initial graph was not balanced (all three lines should follow the same pattern), and I was little concerned that given the large increment (60 degrees) I was not following the action close enough to get an accurate representation.
For my second, er, third, er, final solution I used an increment of 5 degrees and plotted each of the piston's positions individually. As you can see, they all now follow the same pattern. Each pair of pistons approaches TDC (3.0 on the vertical axis) together. While they do not follow exactly the same course, all pairs are separated by the same amount of time, i.e. 20 degrees. You may also notice that all three pairs reach TDC within a relatively short time span (100 degrees), so one bank of cylinders by itself is not going to have the smoothest output. Three banks of cylinders would be the minimum required to get an even series of power pulses around the entire circumference.
The original engine had six banks of cylinders, and no flywheel, which is why there is a one way clutch on the drive to the supercharger. When the engine was stopped, with no flywheel it would stop very quickly. The supercharger, spinning at six times engine speed, would be very unhappy and could conceivably damage itself or its' drive train.
I first heard about supercharged two stroke engines along time ago, probably 30 years. Someone had taken two 500 cc Kawasaki two stroke, three cylinder motorcycle engines and combined them into one, three cylinder, opposed piston engine, added a supercharger, and installed it on outboard motor. Now that, I thought, was someone who was using their head! Years later I found out that supercharged, two stroke, opposed piston, diesels where the
primary form of motive power in locomotives. Hmm.
The drawing at the top comes from an
animated GIF image in Wikimedia Commons. Evidently Google only loaded one frame when I inserted it in this post. I wish I knew how to control the speed of the animation. It would make it easier to see what's going on. As it is, the best I can recommend is to watch the center of a single cylinder as the two pistons approach the center. You should be able to see that for a brief period of time they are both moving in the same direction.
Update January 2017 replaced missing pictures.
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