Four Stroke Diesel Revisited

[Roger B]

Find Hansen, who is one of the successful model diesel engine makers, recently published two videos reviewing a commercial Chinese model diesel engine based loosely on his design.

I studied the various problems that he found and how he rectified them and then made a similar analysis of my 20cc Four Stroke Diesel. This is the build thread:

https://www.modelenginemaker.com/index.php/topic,10605.0.html

The injection system thread is here:

https://www.modelenginemaker.com/index.php/topic,10603.0.html

Most of the points Find Hansen raised I had already addressed or didn’t apply to my design. The key points I noted were:

- He uses a larger diameter flywheel probably 160mm as opposed to my 80mm.

- His injector design is smaller, I guess based on a 1mm (or smaller??) bore, mine is 1.4mm.

- His injector design uses a floating needle whereas mine is positively guided as a result of some previous trials.

I last tried the engine about 18 months ago when it would start and run but with a lot of smoke and unburnt fuel from the exhaust. I knew that I needed to experiment further with the injector design but decided to move over to the Junkers CLM build for a while.

<a href="https://www.youtube.com/watch?v=nHYDfQaNw98" target="_blank">http://www.youtube.com/watch?v=nHYDfQaNw98</a>

I decided to get the engine off the shelf and look at some possible improvements. I have a spare RCM 125mm Flying Wheel which looks like it will fit with a little adjustment to the fuel injection pump mountings.

Then came the problem     I turned the engine over to see if I had a chance of hand starting it (when warm) as Find Hansen did in this video as well as with one of his own engines. I felt from previous test runs of my engine that either Mr Hansen has a very strong thumb or his engines run with a lower compression ratio. What I found was almost no compression      I squirted some oil into the inlet which helped a little but there was obviously a leak somewhere. Using a plastic tube as a stethoscope I determined it was leaking past the piston into the crankcase. Probably stuck piston rings I thought and proceeded to strip the engine down.

There was a bit of carbon on the cylinder head but the piston bowl was fairly clean. The piston rings were free and looked shiny. The only problem I could see was a strange mottled finish in the cylinder bore. It was not easy to photograph and it is borderline if I can feel anything or not.

I am guessing that it might be some form of corrosion 
   
I put the engine back together and fitted the compression tester. Cranking with the mains powered drill the compression rose to 20 bar and stayed steady but occasionally rose to 30 bar which is the expected level. The compression pressure was originally 40 bar but I added a 0.5 mm shim to the head gasket which reduced it to 30 bar.

What to do? If it is corrosion I can relap the bore and make a new piston and rings or make a new liner and lap it to fit the existing piston. The plug gauge still fits well and will suck the piston in and out.

<a href="https://www.youtube.com/watch?v=JSh7KmDrfGU" target="_blank">http://www.youtube.com/watch?v=JSh7KmDrfGU</a>

[Admiral_dk]

Your pictures of the Cylinderhead shows too much Carbon build up according to Find Hansen => Rough and unsteady running.

Picture 5 looks like you have combustion down the side of the Piston - though there might be other reasons for the discoloration (excessive heat being one) ....

The cylinder walls appears to be very pitted - first thought was corrosion after running for too short time to reach a temperature where Acid residues evaporates away during running ....
If it was a Methanol Engine, I would have thought - result of no 'After Run Oil' => Rust 

No matter what - I feel sorry for your loss .... Put a good Engine in storage and retrive one that is in worse condition 

I will say that seeing Finds Video made me think that some of my understanding on how he achieved his great results are rather different in some aspects from how I thought it should be done (and I've seen previous videos from him some years back).

Best wishes

Per     

[steamer]

Does look like pitting/corrosion in the cylinders Roger....  sorry to say

Probably a good lap and a new piston/ring will sort it out....

and I would not have thought you would need and after run oil with a diesel engine running ....diesel.

But flushing it with something PH neutral and lubricious after a run would seem to be a good idea....

Dave

[Roger B]

Thank you both 

I am now not sure if corrosion is the problem    I looked back through the construction pictures and the seems to be a similar surface finish during lapping. I decided I would make a new liner and lap it to fit the existing piston and rings as I have suitable plug gauge.

Removing the old liner was surprisingly easy as I have had water in the jacket. Using the plug gauge as a mandrel it pushed out with thumb pressure   The surface at the crankshaft end does not look good 
 
I found the original piece of 40 mm cast iron as well as another piece of 35 mm I could use to make the new liner. The old piece looks completely different to the material I normally receive from RCM and seems to show similar surface markings so I will carefully monitor the new liner during machining.

[Roger B]

Another problem I noted was the large cutouts required to clear the connecting rod. I looked at the drawing and made a couple of calculations and I can reduce the diameter from the planned 10 mm (actually 10.5 mm) to 8 mm with an adequate factor of safety against bucking/bending. Most of the centre drilling at the little end had been machined away but I had left enough so I could redrill it.

The modified rod looks much better proportioned and it has also removed the various marks from when an M3 grub screw passed through the engine in it’s two stroke days 

[Roger B]

I went to start on the new liner but found that the M6 cap screw that holds the tailstock lever to the barrel had broken. This is a weak spot as it has a 3.2mm  hole through for the extractor and has failed before. I took the spare lathe down of the shelf and drilled a replacement.

The blank was then turned to size and a centring ring for the hole saw was cut. It was then cut overlong using the band saw and checked for fit in the cylinder block. The flange was then turned down so it protruded aroun 0.1mm from the block to ensure good compression of the head gasket.

[Admiral_dk]

Looks good Roger 

Two questions : How deep is that Hole Saw and does it run nicely in (no chatter etc.) ?

Per         

[AlexS]

Looking good! Great that you then are able to use the cast iron when using the hole saw.

Might the corrosion be caused by the fuel? What kind of fuel do you used?

I try to search:

"After it is burned however, some of the sulphur in the fuel reacts with water in the air during combustion to form limited amounts of sufuric acid. This acid can build up in the lube oil of the engine, and if left there long enough can start or dissolve the metal linings on the bearings. This is why it is recommended to change the oil prior to long term storage of a Diesel engine. This issue has be substantially reduced however by the use of ultra low surfer diesel fuels in recent years."


I see some tips here:

https://adriansmodelaeroengines.com/catalog/main.php?cat_id=72

[Roger B]

Thank you both 

The hole saw will cut to a depth of 40 mm. As long as it has a good starting groove it cuts well in cast iron or aluminium (I haven’t tried anything else). I do the actual cutting with my bench drill as it has a bigger motor than the lathe.

I am not sure if the problem is corrosion or poor grade material or both. I am using automotive diesel fuel which should be very low sulphur.

I set the liner back in the lathe and cut the centring ring at the other end. The core was cut out using the bench drill at 400 rpm. The swarf needs to be emptied out quite frequently. It was then back to the lathe to bore out to just over 25mm so the lap will go in but not the plug gauge.

[Roger B]

The finish looks quite good so I lapped the bore with 40 micron diamond paste until the plug gauge just entered from both ends. It looks rather better than the old liner 

[Roger B]

I centred the liner in the Keats angle plate to cut the chamfers for leading into the water jacket sealing O ring and to allow the pistons rings to enter easily.

This was then moved to the RT on the mill to cut the reliefs for the conrod. I centre drilled the back of the plug gauge on the Proxxon lathe as the Hobbymat was in milling mode.

The reliefs are rather smaller and shouldn’t damage the O ring as it is assembled.

I cleaned the liner in the ultrasonic bath, put it all back together with a new water sealing O ring and gave it a spin. The compression is back up to 30 bar 

<a href="https://www.youtube.com/watch?v=I258v_Pwxak" target="_blank">http://www.youtube.com/watch?v=I258v_Pwxak</a>

It is difficult to crank it slowly with the drill as the rebound from compression pushes the pin out of the dog. A heavier flywheel or lower compression would help that.

[Mike R]

Still following along Roger.

30bar is an encouraging sign.  Would be nice to know what the cause of the pitting on the previous liner was but sometimes you just can't find out. Maybe preventative maintenance is easier than the root cause investigation, and in this case it may very well be the action that would be indicated anyways if you did find out what caused it.
Also the clearances for the conrod look much better. 

[Roger B]

Thank you Mike 

It’s a difficult question    As I had good compression 18 months ago corrosion seems to be the obvious conclusion. There are however a number of points that suggest it was a material problem.

It was run with automotive (low sulphur) diesel fuel and lubricated with automotive multigrade oil, both of which should have avoided corrosion.

The crankcase was always open via the hole in the top plate so moisture shouldn’t have built up in a dry workshop.

There was no sign of corrosion on the piston (cast iron, different stock) or the crankshaft (C45k steel).

The surface finish of the first liner during lapping showed a similar but not so pronounced pattern. As I wasn’t expecting problems I didn’t take much notice at the time and only had one picture.

The cast iron bar stock was different to that which was normally supplied. It appeared to have been machined.

I tend to think it may have been a combination of porous/flaky cast iron and some form of corrosion 

On with the trials.