20cc Four Stroke Diesel

[Roger B]

Looks like it blew a lot of oil around there.  Is that the atomized fuel you're referring to?  (Asks a very non-IC person  )

Kim

Hi Kim, to ignite diesel fuel (or almost any liquid fuel) it has to be converted to a vapour. In petrol (gasoline) engines this was carried out by the carburettor (the German term vergasser is much more explanatory). In the case of my diesel I can see from the open air tests of the injector that some of the fuel is converted to a vapour cloud which will ignite under the temperature generated by compression. Some comes out as a fine jet which will not ignite and is just blown unburnt out of the exhaust so what you see everywhere is un atomised fuel.

[Kim]

Thank you for the additional explanation, Roger! 
Kim

[Roger B]

The last set of injector trials came to a stop when the exhaust push rod dropped out    The actual valve clearance was 25 thou, the inlet clearance was 15 thou.

The push rods were made from 3mm piano wire with the ends roughly shaped on the bench grinder and were slightly too short. I assumed that the rough ends bedded in rather quickly increasing the clearance. A new pair were made from 3mm silver steel with hardened ends.

I knew that the clearance between the valves and piston was rather limited at overlap, the valves being 1mm open (full lift 2mm), requiring a 15 thou gap. As there was ample (too much) compression I decided to make a 0.5mm copper shim for the gasket. This was cut out, annealed and punched out using the template I had previously made.

When I took the head off there were obvious signs of contact between the valves and the piston as well as an interesting un sooty strip between the valves and a mark suggesting that the injector had contact with the piston.

I reassembled the engine and set the valve clearance to 10 thou which still gave plenty of clearance between the valves and piston.

[Roger B]

And still plenty of compression 

[Kim]

Fascinating engine forensics work, Roger! 

Kim

[Admiral_dk]

I can't say I'm surpriced that the Valves survived being shut with help from the piston - as long as the Stems are parallel to the cylinder axis and the part of the piston hitting is parallel to the Valve head.
But as you very well know - best avoided ....

2mm max lift isn't much - but you didn't build a race engine ....

Thinking out loud - the combustion is helped enormously if you have an efficient 'Squis-Band' between Piston and Cylinder-Head .... (if memory serves) between 0.6 to 1.2mm - preferbly leading directly to the 'Combustion-Chamber' ....
To me it looks like you should consider having the 'Chamber in the Piston' centered around the Injector ....  Now you kind of 'loose some of the spraypattern' when it hits the piston crown instead ....

Per         

[Roger B]

Hello Per,

The injector is angled into the piston bowl which doesn't really show in the pictures. This should show the combustion chamber and squish band better.

[Admiral_dk]

OK - I was not clear enough ....

I see that you aim for centre of the bottom of the 'Combustion-Chamber' and I meant centre @ the top ....
I feel (no proof at all) that a part of your Fuel hits the side closest to the Nozzle ....
But looking at your drawing - my idea might just end up having some hitting the opposite side 

Idealy - it will all burn in the centre of that void, and not reaching any metal part ....

Now would be a nice time to have one of those big CAD programs that can simulate both the Flow inside the Injector Nozzle and inside the Combustion Chamber .... that way one could play around with it all ....

So the mark close to the Nozzle - I thought that it came from the Jet - and I guess that you feel that it hits the Nozzle itself - or ?

Per       

[Roger B]

Thank you Per, I think that there was mechanical contact between the injector and the piston as there was between the valves and the piston.

I was setting up for the next trials with the new needle injector when the injection suddenly stopped. An inspection showed that the injection pipe had cracked right by the pump union  This was probably due to me letting the injector hang on the pipe when making output trials. I made a new pipe assembly and took the engine off to my work's welding bay (with extraction) for some trials.

It started immediately so the lower compression ratio and the new needle injector were ok. I started adjusting the injection timing to find an optimum when it stopped and would not restart  There was compression so I removed the injector and there was no injection. I thought maybe an air bubble and bled the system with out any success. I tried one of the mushroom injectors, again no success, so I packed it all up and went home.

I tried again to bleed the system but no injection. It would deliver fuel but not under pressure. There were no visible leaks, unlike when the pipe cracked. I tried the injectors with the test pump, all OK. I then started to dismantle the injection pump, nothing obviously broken. There is some wear on the plunger, but I would expect wear to give a gradual degradation in performance, not a sudden stop 

[Brendon M]

Hello Roger,

Bit of a bummer, but I am sure you will figure it out

[Admiral_dk]

Strange - but I'm sure that you will find the reason why 

The usual 'Divide and Concur' springs to mind ....

Per

[Roger B]

Thank you both 

A strange one  I stripped down the pump, checked the clearances, around 5 microns, which should be ok with diesel. I checked the orientation of the helix on the plunger and made a small mark to avoid it being 180° out of position. I checked the movement of the tappet, it's still 3mm dropping around 0.2mm as I move the timing adjuster.

I put it all back together, being very careful with the bleeding, and had injection again with volumes that matched previous measurements  It must have been an airlock somewhere   

[RReid]

I had a boss at one time who would attribute that sort of thing to the "laying on of hands". It is nice when that's all it takes.