20 cc Horizontal 2 Stroke Diesel

[Roger B]

The studs were finished, I silver soldered the tank sides to the base, made a gasket and fitted it.

[Ye-Ole Steam Dude]

really nice looking engine.

[fumopuc]

Hi Roger, nice to see the progress. It is comming together.

[Roger B]

Thank you both 

I finished putting it together on its base and gave it a trial run. It sort of ran but I think it was four or possibly eight stroking. My cordless drill didn't have the torque required, but my 1kw corded drill got it going 

<a href="https://www.youtube.com/watch?v=4rAfjEW9BsU" target="_blank">http://www.youtube.com/watch?v=4rAfjEW9BsU</a>

I think I may make an adaptor to use a smaller carb 

[fumopuc]

Hi Roger, congratulations for that first run.

[90LX_Notch]

Congratulations Roger.

-Bob

[yogi]

Congratulation on the first run Roger! 

[Admiral_dk]

Nice to see that you has successfully run the engine for the first time 

This is always very gratifying 

[Roger B]

Thank you for your interest   

I made a new adaptor for the smaller carb, the previous one having been converted into a fuel injection manifold for another set of trials. This ran well enough to allow me to adjust the oil pump for just a little blue smoke. Halfway through the trials it suddenly stopped and refused to fire at all  The battery was ok, the points were making contact but no spark at the plug  Solution, the water splashing out of the cooling tank had got into the plug cap  I removed the plug cap and connected the resistor directly to the plug, all was well again.

I can get a fairly stable low speed run, but when I try to accelerate it goes into loud four or eight stroking. This may be due to the porting or scavenging, but I think it runs well enough for some diesel trials 

<a href="https://www.youtube.com/watch?v=fRgmscG2wOY" target="_blank">http://www.youtube.com/watch?v=fRgmscG2wOY</a>

[Ye-Ole Steam Dude]

Looks and sounds great, now you can take your time to do the fine tuning.

[Admiral_dk]

More Progress 

Normally I would associate "four-stroking on a two-stroke" with too much fuel - but in this case I can't help thinking that you got a too restrictive exhaust, that prevents it from scavenging as well as could be wished for .... 

Best wishes

Per

[Roger B]

Thank you 

I will attempt to check the port timing when I take the petrol head off to see if it is anywhere near what I planned. What's the best way to do this, measurement or blowing down the inlet/exhaust and seeing when the flow starts/stops? I guess that late exhaust opening would also cause poor scavenging 

[Admiral_dk]

It's mostly down to a few factors in a simple two-stroke - blow-down and port area are the main ones along with the size and shape of the exhaust pipe.
Blow-down is the time from opening the exhaust to the transfer opens - this needs not be long if you got enough area in the port above the transfer port height compared to the bore diameter.
All this also relates to the rpm range - so low ports are for slow running and high ports are for fast running and high power if I should be very simplistic. Exhaust port with on 50cc mopeds in my youth ranged, from 45-70% of piston diameter. The pipe should have the same area as the port where it starts.

Transfer port duration between 100 and 120 degrees and exhaust port duration some 20 to 25 degrees longer for work engines. If the shape of your exhaust port is round (I can't remember) - I would make it D shaped in the cylinder wall with the round part down and the flat part as the roof of the exhaust port and the port around 50% of piston diameter wide - smoothly morphing to round where it meets the pipe.

If you want it to be slow running start with 100 for transfer and 120 for the exhaust port - you can always remover more metal to make it run faster ....

[Roger B]

Per, thank you for your comprehensive reply 

I took the head off and made a few measurements. The port timing was based on a recommendation from ETW in one of his books.

Transfer 110°
Exhaust 130°

I then used to design programme on the MEN website to come up with port heights.

Transfer 107°   91.5mm from crankshaft
Exhaust 130°    94.5mm from crankshaft

The engine is 25mm bore, 40mm stroke. The conrod is 80mm between centers and the piston crown is 25 mm above the gudgeon pin center.

First I blew down the inlet and exhaust noting the point where the port opened. This gave the following angles.

Exhaust  126°
Transfer  108°
So Blowdown 9°

Next I measured the port positions from the head joint face and calculated the positions based on 125mm from the crankshaft centerline to the head face.

Exhaust 31.5 from head face  = 93.5 from crankshaft
Transfer 33.2 from head face = 91.8 from crankshaft

Putting these numbers back into the MEN program gave

Exhaust 123°
Transfer 110°
So Blowdown 6.5°

I think that the transfer port is ok but I may need to move the top of the exhaust port up by around 1mm   

The ports are in three 25° segments as I am using unpinned piston rings. The width of each segment is about 5.4mm so the total port width is 16.2mm which is around 65% of the piston diameter. Breaking the port into 3 will reduce it's efficiency. The ports are rectangular with radiused corners.

The exhaust port height with the piston at BDC is 8.5mm which gives a total area of 137 mm2. The exhaust pipe bore is 8mm giving an area of about 50mm2. This as you said may also be a problem 

I've got lots to learn about 2 strokes   

[Admiral_dk]

OK - armed with your data I would suggest to start with a bigger pipe as am almost 3:1 reduction in area is a dramatic "choke" - around 14mm diameter gives you 153mm2. and a bit of space if you need to increase the height of the port a bit.

If you by any chance are going for power - consider an expansion chamber exhaust system, since it can easily boost power more than 50% if you get the dimension close enough at the same RPM.