2-stroke, 2-cylinder engine?

[Jerd]

Hello everyone!

I'm new to this comunity, and allready have a question
I'm currently working on a 2 cycle glow engine (one cylinder), and
I'm more or less done fabricating it.

When doing a 2 cycle engine, the whole concept is to use the down stroke
to compress the airmixture, and then open the intake so the compressed air mixture
can enter the cylinder. How is this archived on a two-cylinder two-cycle engine? Do the
pistons work parallel, or do you have separate crankhouses?

(Ignore my bad English, it's not my native language, and specially technical English is kinda hard for me)

Cheers /Jon

 

[jgarrett]

Jon, Can't answer your questions but welcome to the group. LOT of knowledge here from people willing to share it.
Show and tell us something about your equipment.
Julian G.

 

[AllThumbs]

Generally, seperate crank cases.

Welcome!

Eric

 

[Jerd]

Generally, separate crank cases.

Welcome!

Eric

As i suspected, having them run parallel would put much stress on the crankshaft.

I have access to CNC lathes and mills. I have been working with MasterCam for 4 years, and as an engineer for 6 years. It all begun when i applied my mechanical knowledge creating a small oscillating steam engine for my girlfriends youngest brother. It was a much appreciated pressent. However, since I'm quite involved in active driving (cars) and races with racecars, I thought a small model engine would be a nice start introducing combustion engines to the "small ones"

Cheers /Jon

Edit: It seams to be hard to find good, free, plans for model engines. Is it possible (and appreciated?) to upload PDF-plans of my oscillating steam engine and the 1-cylinder 2-stroke engine to this site?

 

[Steam4ian]

G'day John.

Look up this site "http://modelenginenews.org/" for some info on building small engines, generally of aero type 2 stroke. Google "Jan Ridders" and "engines", Jan has some great work on 2 strokes, stirlings and the like complete with animations and explanations

Two stroke engines are basically 4 stroke engines but with two parts of the cycle happening simultaneously in different places; e.g. as compression takes place in the cylinder, induction is happening in another chamber. If you are using the crank case for induction then you need to separate the crank cases, one per cylinder. BTW Jan Ridders has a two stroke with open crank case but a double acting piston/cylinder like a steam engine. The induction and pre compression takes place at the back of the piston and normal compression and expansion happens at the top of the piston.

Hope this helps

Regards,
Ian

 

[Stan]

A two stroke engine simply designates an engine that has a power stroke with every revolution of the crank. Air can be sourced from a piston or rotary compressor mounted on the engine and the exhaust can be via ports or valves in the head.

Detroit Diesel made two stroke diesels for more than forty years and they used a rotary supercharger (roots blower) and exhaust valves. Many very large engines driving gas pipeline compressors are two stroke natural gas with vertical power cylinder and horizontal air compressor cylinders and exhaust valves.

Lots of different styles for anyone wanting to do a model.

 

[Sprocket]

There was a build article in Model Engine Builder, issue 9 Nov./Dec. 2006. It's called the Hex-2 and is a two cycle, two cylinder engine. www.modelenginebuilder.com . Back
issues should be available. He used a couple of Cox .049 cylinders. It would at least show you how someone else solved that problem.

Doug

 

[PolskiFran]

Hi Jerd, Separate crankcases were the norm in the past. I have pictures of full size marine (boat) engines from the early 1900's with separate crankcases. These engines could be purchased with 1 to 6 cylinders. While they used a common crankshaft and frame, another cylinder and crankcase was added in line to the others. This would probably be the way to go if attempting a model.

Hope this helps,
Frank

 

[wildun]

Often the problem of crankcase compression is solved with opposing (boxer) pistons firing simultaneously and sharing a common crankcase but with opposing crankpins.
These can be low vibration engines and work well except for a slight rocking couple caused by the cranks being offset to each other (looking from the top). One exhaust can be used for both cylinders.

Having said that, there were many very successful two stroke parallel twin motorcycle engines which had acceptable levels of vibration too and they had crankcases seperated by two opposing simple rubber seals on the crankshaft. (They had the two cranks pressed together.)

 

[dman]

there are 3 ways to tackle a multi cylinder 2stroke and all are applied successfully somewhere.

1. boxer/opposed arrangement. the cylinders fire at the same time thus the crank has 2 opposing pins. same pulses per rev but the pistons work to counter balance each other so less counterweight is needed on the crank. I believe this is used in model aviation on occasion.

2. parallel/inline arrangement, divided crankcase. functions as a set of independent engines that share a crankshaft. the sections are sealed from each other. more pulse per rev so it runs smoother. this is used in motorcycles snowmobiles and watercraft. also ultralight aircraft.

3. super charging. a pump feeds the crankcase (or ports directly but then you need lower end oiling). its usually positive displacement type with a displacement a bit more than the combined cylinder volume. vane style is popular for models and roots is used on full size engines though screw or wenkel could work and with the right engineering a centrifugal may work on full scale but on model scale I doubt it (centrifugal chargers don't scale nice and are harder to engineer because they have a pressure per tip velocity relationship rather than a volume per revolution relationship.) super charing has been done on model radial engines though it could work on any engine with a. evenly spread firing pattern. and is probably most notably use on large multi cylinder detroit diesel engines (yes there are full size 2 stroke diesels) sometimes with the addition of a turbo charger for boost. the super charger is the air displacer and he turbo adds charge density. it was an unconventional design. they have pressurized oiling and use cam actuated poppet valves like a 4-stroke but fires every rev.

 

[Niels Abildgaard]

A V 2 90 degree with a single common crancase will have half the pumping volume of a corresponding single cylinder.
This is more than enough for starting and real power from two strokes come from the tuned exhaust pipes anyway.
Vibrations and balance is better than all othe two cylinder configurations and mass is only better for 180 degre two crank engines.
I have a CAD picture somewhere but in meantime enjoy a picture of worlds most best single cylinder motorbike.

Kind regards

Niels

 

[lohring]

The mechanically simple, crankcase pump type two stroke is one of the most complex engines ever developed. Modern versions are all about fluid flow and taking advantage of the pressure waves driven by the pulsating nature of the engine. The crankcase compression is only needed for starting since, as was noted, the tuned pipe provides the varying pressures to draw in mixture and then supercharge the cylinder.

For a better understanding start with the Wikipedia article and it's references, then go to the Pit Lane discussion (requires log in) on the Aprilia 125 by several of the world's most knowledgeable two stroke designers. That discussion is approaching 100 pages and is a course in itself. For a modern, high speed uniflow design take a look at the Eco motors engine. The picture below is a very simple flow visualization that illustrates how engine builders have tried to figure out what happens in a running engine.

Lohring Miller

 

[dman]

2 strokes are interesting because they are so sensitive to flow dynamics.
a 90deg v should run. but i can't say i remember seeing any examples of it in production and on a one off engine it might need extra tuning to get right. most engineers don't try it because there is a level of unevenness to it. evenness doesn't matter as much as many think it would but it's a place designers like to start. i guess the most vibrations come from the crank imbalance with the tall heavy pistons that 2 strokes need to close off the ports. it would be great for a motorcycle but i have to wonder if the same idle quality and off idle torque is achieved. i understand the principles that go into it but i always figure the exhaust tuning would need a fair amount of rpm to take over from the bottom end pumping.

 

[canadianhorsepower]

[/i understand the principles that go into it but i always figure the exhaust tuning would need a fair amount of rpm to take over from the bottom end pumpingQUOTE]

The thing with a "tune pipe" is that it's "tune" for a specific aplication
example: low end torque= longer heather smaller diffuser
you whant to stop your RPM in a specific data =youll change the angle of the baffle the steiper the angle the closer to your target youll be

I've made many crazy thing with chainsaws and snomobile to say that"as long as you remember that a 2 stroke motor
is made for a particular application your fine" otherwise don't get close to it

 

[Rivergypsy]

For a production 90deg V there's always the RGV250 - much fun, and two separate cases ;o)

 

[wildun]

NIELS
I agree, MZ (and Walter Kaaden in particular) is the company which contributed most to the development of the two stroke engine even though it was kept constrained by the Communist regeime in East Germany) and only when the young Ernst Degner, who raced their motorcycles defected to Japan and conveniently took all the designs with him to Suzuki, did his work start to blossom.
The others soon started to copy them and it was all on during the sixties and seventies with phenomenal power outputs being realised, but the environmentalists eventually saw to it that it was stopped in its tracks ( I guess we had to make more room in the atmosphere for for the soot from jet aircraft and factories!).
Now we are back to the four strokes which are complicated bulky and heavy (and expensive) - but don't get me wrong, still great nevertheless.

What I really meant to say before I got carried away, was that in my opinion the MZ in the picture although not fantastic, cool, super duper with the "wow factor" etc. is the most pratical and sensible two stroke ever built!
I will try to upload a drawing of how the V Twin two stroke (in this case BIMOTA )was being developed a few years ago, ie with seperate crankcases, albeit a little more complicated. The crankshafts are geared together on the other side of the picture.
I also might add, this particular one was a finiancial disaster for Bimota, but the design is still interesting nevertheless!

Shared crankcase?? - I don't think so - not without a blower of some sort!

 

[ShedBoy]

Not exactly a model but I am waiting to see this run
http://www.mwv2.com/creating-v16-victa-the-ultimate-useless/Page-3.html

This bloke has built some amazing engines. He has a common, LONG, crankcase which is pressurized by a small car supercharger.

Brock

 

[Paulsv]

I believe there are plans for a two cylinder 2-stroke in the downloads section of this site. Here's a link:

http://www.homemodelenginemachinist.com/downloads/lobo-16-cc-twin-cylinder-2-stroke-diesel-161.html

It appears to use two separate crankcases.

 

[ShedBoy]

Both pistons need to move up and down at the same time to achieve crankcase pumping and the smaller the volume of the crankcase the more efficient it will be.There is a plan available on the web. I have the pdf of it.
Brock

 

[wildun]

Both pistons need to move up and down at the same time to achieve crankcase pumping
Brock

SHEDBOY
Yes I agree with that but as I should have said before, a two stroke V twin could use a common crankcase, but that would require seperate (staggered) crankpins, or two seperate (geared) cranks to allow simultaneously firing pistons.
Using a common crankpin arrangement as in the classical four stroke V twin however - although it might run, won't be of much use unless you use some sort of blower arrangement.

Please forgive me if someone else has said the same thing!

I would like to see the PDF you mention.