First 2 stroke engine build

[grizelli]

I just finished my first engine, something I've wanted to do since god was a boy. Learned a lot, and made lots of mistakes along the way.

Its my own design, a single cylinder 2 stroke glow engine, using a second cylinder to provide the charge, and a off the shelf carburettor from Just Engines. To lubricate the bottom end I included an oil pump with an external gear train to drive it. Nobody was more surprised than me when it started.

Now the bad stuff. First, its a pig to start, I suspect poor compression and port timing the most likely suspects, though the total lack of any help from Just Engines on how to set up the carb didn't make things any easier. Second, all the plumbing between the cylinders had loads of leaks that took a while to find and fix. Third, the fabricated pressed-together crank seems to have de-fabricated internally after being run a few times and the beast vibrates the whole house when it runs. And then there's the noise...

Got the bug now, working on designs for the next one

 

[cfellows]

That's a very interesting engine. Don't suppose you have a video of running????

Chuck

 

[grizelli]

That's a very interesting engine. Don't suppose you have a video of running????

Chuck

O yes This is one of the early runs before its internals started to come unglued.

https://www.youtube.com/watch?v=8eZ3F4NkCq4

I have more, and pictures of most of the bits if anyone is interested

 

[Niels Abildgaard]

Hello Grizelli

Please let us have a lot of pictures.
This place lack two stroke fairytales.
I have tried to make built up cranks a couple of times and has failed miserably Yours ran well ,at least for a while, so let us see and admire.

 

[Fabrickator]

It's a very interesting motor design and I'm sure that you did a good job machining it. Please don't take this wrong, but I guess I don't understand why you chose to make it so complicated? Two-strokes, by their very nature are the most uncomplicated engines there are. A crank, cyl, piston and carb. The real tricks are proper port timing and a tuned exhaust for optimum performance.

 

[grizelli]

Good question about the complexity. I had originally planned a two cylinder, and wanted to see how it might be possible to use a wet sump and no oil in the fuel. I realised early on in the build that a single cylinder would be a better bet, especially since I had no idea if it would run at all or not so I dropped the second pot.

The built up crank was a long shot from the start, relying entirely on heavy duty press fits. For my next build I plan to stick with the built up design but will find a better way to hold everything together.

On the subject of port timing, I would appreciate some guidance especially since I think that this is one of the main reasons why this first engine was such a pig to get started. Firstly for a single cylinder engine, but also for a opposed piston arrangement with inlet at one end and exhaust at the other?

 

[AussieJimG]

Hi there Grizelli,

In his book "Miniature Internal Combustion Engines", Malcolm Stride gives the typical timing for a piston ported two stroke as: Induction 100 degrees about TDC, Transfer 100 degrees about BDC and exhaust 120 degrees about BDC.

I am not too sure how critical the timing is but I can't get my two stroke to run and I suspect that the timing might not be quite correct. It is very difficult to get it right and to get the compression ratio right as well.

I hope this helps.

Jim

 

[Niels Abildgaard]

A lot of hardcore two stroke from people that earned money making lots of power.

http://www.pit-lane.biz/t4072p940-g...-by-mr-jan-thiel-and-mr-frits-overmars-part-3

 

[grizelli]

Hi Aussie Jim,

My engine opens the exhaust at around 90 degrees after TDC and the inlet at around 120. Sounds like I should have made it all happen a bit later.

I am thinking of building a opposed piston 2 stroke next, and the full size engine I have been looking at has the exhaust opening at 112 degrees and the inlet at around 137. This is a 'uniflow' layout with inlet at one end and exhaust at the other, and it also has the exhaust closing a couple of degrees before the inlet to add a bit more squeeze - this is done by offsetting one crank a few degrees from the other. I'm not sure how this would translate into a small model, I guess I'm going to have to build it and see...

 

[Niels Abildgaard]

Hello Grizeli

Old webpages with opposed pistons


http://archive.is/zHnXJ

http://archive.is/s7gTY


The benefit of asymmetric scavenge/exhaust timing is doubtfull.
The worlds most efficient engines have variable exhaust valve timing and are symmetric when going full power.
Hundreds of millions mopeds have symmetrical timing and start and idle and run.

 

[Niels Abildgaard]

The picture shows a Slavia diesel .90mm bore.Made in very great numbers and ran for many hours.
I have had one.
Look at the crank construction.i have tried pressfitting and could not.

 

[grizelli]

Hi Niels,

many thanks, it seems that you are the man to talk to about opposed piston designs!

I think that I will stick with the symmetrical port timing, I only raised it because the original full size engines that I am planning to base my model on used this method. Google Rootes TS3 for more info.

What is your view on port timing for this type of engine?

 

[grizelli]

Hi Niels,

I found that the pressing process created burrs on the pin which prevented the pin from being pressed home properly, or which caused the pin to sit at a slight angle. Either way, not good enough.

I had planned to copy someone elses' idea for using a pinch bolt to secure the pins, similar to the drawing you provided. At small scale this is a bit fiddly, especially with fat fingers!

 

[Niels Abildgaard]

Hello Grizelli

The full size Commer thing used a Roots blower to make scavenge airflow.
As You have to use some kind of blower ,it really does not matter very much what timing You use.
Find a good two stroke of same cubic capacity and copy the timing.120 degree exhaust and 80 degree inlet?
You will still be miles better as the direct air/fuel loss from inlet to exhaust will be much less.
The negative thing with an opposed piston layout is the impossibility of using a tuned exhaust pipe from 6 to 8 ports around cylinder,but if not for racing but for fun then OK.
Why play around with that single crankshaft two piston layout?It is far to many highly loaded bearings and some of them only oscillating giving friction and wear for nothing.
For real two stroke cranks I will try to make one like shown

 

[grizelli]

Hi Niels,

I like the idea of that crank assembly, though I'm not sure of my ability to make it accurately in a small scale. My current thinking is to weld flanges onto the ends of the crankshaft before machining it, and use a pinch bolt arrangement to hold the crankpins into the webs, then I can use a hammered-in pin to secure the crankpin while at the same time plugging the end of the oil hole. I need to try making a rough one first to see how feasible it is.

I am interested in the Commer design because I worked for the company many years ago when these engines were being made. For their relatively small size and weight they were very powerful and fuel efficient. If you only need a single spindle to take the drive from, I'm guessing that there's not much to choose between the friction of the levers and cranks compared with the gear trains that would be needed to take a single drive from two cranks. Not decided whether to build it yet, I'm still doing some drawings!

 

[Niels Abildgaard]

Hi Grizelli

What is the intended bore and strokes and what kind of blower?

 

[grizelli]

Hi Niels,

I'm thinking of making the engine fairly large to make the construction of the complex rods and levers a bit easier for my fat fingers - bore 30mm and total stroke 60mm (30mm per piston). To make it simpler again I am planning only two cylinders rather than 3.

I'm looking at blowers now, and it seems that the Roots type is probably the most efficient but also the most difficult to build because of the tight tolerances, so I will probably go for a simple sliding vane type using Tufnol vanes running in an alloy housing. If I mount the blower after the carburettor, the lubricant in the fuel should keep the blower oiled without introducing external oil into the inlet. I have no idea how much boost this would provide, but I plan to drive it from a toothed belt off the crankshaft and I can speed it up or down by changing pulleys. This engine will probably run relatively slowly so I'm thinking maybe the blower should run at 4 x engine speed as a starting point. It seems to me that the critical issue will be getting enough air into the engine at starting-up speeds, once its running it should be OK.

I'm also thinking that I could make the cylinders air cooled (possibly with an external fan) rather than water cooled, and I will need a gear type oil pump to keep all the parts running smoothly. I've worked a lot with 'real' engines and I plan to use a similar setup for the lubrication in this model.

The more drawings I do, the more keen I am to get started!

 

[Niels Abildgaard]

hello Grizelli

Thats a nice size.
The blower driven electric until You know a little more?
Many years ago there was a fellow describing a V4 blower fed ,twostroke in Model engineer.
If usefull to You I can try to find it again.

 

[grizelli]

Hi Niels,

Running the blower with a motor would be a good idea as that would let me vary the speed and therefore pressure easily - I have seen some model engines where the blower runs at 4 times crank speed, but even with a relatively low engine speed of maybe 4000 revs the blower is going to be running at around 16000 and I have no idea how much boost it will provide until I've made it. Maybe this should be the first thing I make.

If you have any information on blower engines that would be really useful, thanks. I was also originally planning for this engine to be glow plug powered, but I am concerned that the plug may fire the mixture too early and I can't easily control that. So I am thinking of running it on gasoline and spark ignition instead where I have more control over when it fires.