Opposed two cylinder engine question

[Jake2465]

Lohring is an expert but I think I can say 'perfect' is a never ending, perpetually moving target. Today's record breaker will be tomorrows broken record.

Ambient density variation makes its way into several tuning factors throughout intake, power & exhaust. There is interplay & tradeoffs, you don't get something for nothing. A big part of high performance 2-stroke power comes from tuned exhaust - sucking intake charge out past the exhaust flange, but then reverse packing this charge back into the cylinder via the reflective wave, essentially 'boost'. But its all an orchestra of balancing many factors: compression ratio, timing, gas ports, distribution... the more you learn about 2-S the more you realize how complex they actually are.

Sounds like it. Hmm... Perhaps it wouldn't be such a bad idea to get a hold of one of those hobby king twin cylinder engines and dissect it. I would be interested to see what the case volume is with the crankshaft installed compared to the displacement of the cylinders.

I would have to wait till they are back in stock.

 

[lohring]

Two strokes seldom made it in medium power engines. General Motors made a series of two stroke diesels that were used in trucks, boats, and earth moving equipment. Today four strokes dominate these fields. Two strokes are very hard to compete with at the large and small ends of the scale. If you need an inexpensive, light IC engine it will be a two stroke. Most large marine diesels are turbocharged two strokes. The most notable two strokes in aviation were the Junkers Jumo, the Napier Nomad, and the Rolls Royce Crecy. Turbines are a better option by far. Some ultra lights use two strokes, but old (1930s to 1950s) four stroke designs are still the most popular in light planes. They have a proven record and it isn't worth while to get new designs certified.

I think new IC engine designs will start to suffer the same fate as they are increasingly replaced by electric motors. It's seriously started in model engines and is rapidly creeping into lawn equipment. Cars are just beginning to go electric. I still love two strokes as a design challenge. A lot of work has been done to perfect the kart, motorcycle, and similar engines. The 2008 Aprilia 125 still represents the state of the art with 54 hp from 125 cc.

Lohring Miller

 

[MachineTom]

All of what you said makes a lot of sense. For simplicity sake, I thought that I would just build a horizontally opposed twin cylinder engine that has the rod journals 180 out of phase to each other. This way a single carburetor could be used. The only problem is that since the crank is rotating in one direction, I believe the fuel charge will tend to favor one cylinder over the other unless the carb is offset to one side the case. I don't know if that would help.

I can definitely see the benefit of letting each cylinder have it's own carb as they can be individually tuned.

The only way I know where rods for two stoke engines can be added without pressing cranks together or having dedicated oil passages would be if special rods are made where they have needles that would need to be nested in the rod journals and then have the caps bolted over them. I don't know if anyone has done that before. Slots would have to be cut in the rods so the needles could get lubrication just like the standard weed whacker rods have. I feel like that would be good in theory but likely a real pain to get it to operate correctly without the needles brinelling the inner surfaces of the rod journals... Come to think of it, don't outboard two stroke boat motors have this kind of thing? Needle bearings for their rods?

So, perhaps I should think about a two cylinder engine with each cylinder having it's own carb and forgetting about putting the carb on the crank case so I can ditch the reeds and save maintenance costs.

You can't have a 180 degree opposed twin with a common crankcase, it would need to be separated. If one piston went up as the other goes down there would be no volume change in the crankcase to draw in the fuel charge. Unless it had a supercharger to force the charge into the cylinder.

Petertha, yes you are more than likely right about the cost. I think for me it is more for the experience than the immediate practicality. One thing I am noticing here is that the crankcases are pretty tight looking; the castings almost form fit the rotating crankshaft assembly. I would imagine that it would need to be that way so the pistons have more authority in changing pressures within the crankcase.

I have this book from Gordon Blair; "Design and Simulation of Two-Stroke Engines". I have yet to read most of it as I have not yet gone through college level physics. Anyway, when I do get to that point, I am sure that book will be a wealth of information in helping me answer so many of the questions I have.

Yes, I was thinking about buying off the shelf components and I would in turn machine out the crank case to suit whatever I was able to get a hold of. I may be able to machine out a multi-cylinder crankshaft, but if one can be had for a reasonable price, then it may be good to just go with that. Although, the throws on the off the shelf crank would probably limit me on what cylinders I would buy as it would have likely been made for a specific set.

Like you said, I could definitely just shell out a few hundred and be done with it, but I don't think I would be learning very much in doing that. So, I guess I believe that having an applied approach to the theoretical stuff is important. What would be interesting is if everything in the engine was assumed to be done correctly, and in the end it won't fire off and run. Then what? Well, then I see that as a great opportunity to learn and problem solve to develop a fix .

 

[Jake2465]

You can't have a 180 degree opposed twin with a common crankcase, it would need to be separated. If one piston went up as the other goes down there would be no volume change in the crankcase to draw in the fuel charge. Unless it had a supercharger to force the charge into the cylinder.

I don't quite understand why that would be. the crank would have two rod journals 180 deg out of phase to each other. Both would hit TDC at the same time with that arrangement.

 

[lohring]

You can't have a 180 degree opposed twin with a common crankcase, it would need to be separated. If one piston went up as the other goes down there would be no volume change in the crankcase to draw in the fuel charge. Unless it had a supercharger to force the charge into the cylinder.

Look again at the picture I posted for an alternate firing 180 degree twin. Notice the separator between the connecting rods that seals the crankcase into two sections.

Lohring Miller