Author Topic: 25cc Horizontal engine revisited  (Read 2033 times)

Offline Roger B

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25cc Horizontal engine revisited
« on: December 10, 2017, 05:22:18 PM »
I designed and built this engine as a development of my first IC engine. The build thread is here:

http://www.modelenginemaker.com/index.php/topic,2821.0.html

It ran well and I carried out a number of load trials before I started using it as a test bed for my mechanical fuel injection work, here,

http://www.modelenginemaker.com/index.php/topic,4906.0.html

During the last fuel injection trials it did not run well and compression seemed to be lacking. I removed the piston and checked the rings (there had been previous problems with stuck rings) but all seemed ok. There was a break in the weather yesterday so I refitted the carburettor for a trial. I also moved to Alkylate petrol instead of pump petrol (don't change two things at once  ::) )

It would barely run, instead of 250W at 3000rpm it kept stopping and the compression almost completely disappeared  :( (my vertical engine ran fine on the Alkylate petrol so that was not a problem  :) )

I took it back into the workshop and removed the carburettor and exhaust. The was clearly a leak past the exhaust valve. Off with the radiator and the cylinder head. it was all quite sooty/oily but the cylinder bore looked fine. I took the valves out, the inlet valve was ok but the exhaust seating was completely black  :(

After cleaning up with alcohol there did not seem to be significant damage but polishing the exhaust valve face with fine abrasive cloth showed a significant step.
Best regards

Roger

Offline Roger B

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Re: 25cc Horizontal engine revisited
« Reply #1 on: December 10, 2017, 05:37:42 PM »
I decided to recut both valves and seats and lap them in again. This time I used a 20 grade diamond paste instead of the 40 I had previously used. After making a new head gasket and reassembling I got a nice bouncy compression  :) I don't know when I will be able to try it as it is currently -5░C in the R&D area with 10cm of snow  ::)

I then moved on to the water pump. The end of the shaft had been used to drive the fuel injection pump and it showed significant play. The inside of the pump looked ok, I had had previous problems due to the thermal expansion of the Delrin gears causing it to seize. The drive shaft showed around 0.02mm wear and using a tapered mandrel the Oilite bush was about 0.02mm oversize. This is not a large amount of play for the water pump but if I am going to continue the fuel injection work on this engine rather than moving on to the diesel I will have to improve the system in some way. The diesel thread is here:

http://www.modelenginemaker.com/index.php/topic,5545.0.html

I may just replace the bush and make a new shaft  :headscratch:
Best regards

Roger

Offline Admiral_dk

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Re: 25cc Horizontal engine revisited
« Reply #2 on: December 10, 2017, 06:44:12 PM »
Nice to see that you identified the cause of the problem quickly - or is it only the symptom ...?... have you got any explanation to why the exhaust valve was leaking - like problems from the beginning or other factors that later prevented it from closing as it should ?

Best wishes

Per

Offline Roger B

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Re: 25cc Horizontal engine revisited
« Reply #3 on: December 11, 2017, 11:05:45 AM »
Hi Per,

There could be several causes:

During the first runs of the engine it also lost compression due to a speck of swarf on the exhaust valve seat. I just relapped the valve at this time which was probably the cause of the groove in the seating. This groove may have encouraged carbon build up.

During the fuel injection trials the mixture varied quite wildly, this may have caused some carbon build up on the valve until it started to leak. After that it would continue to degrade.

Maybe a lump of something (swarf, carbon, etc) got onto the seat.

I did wonder if the spring was not strong enough, but I don't think that is relavant. Hit and miss engines run with very weak springs on the inlet without apparent problems.
Best regards

Roger

Offline Roger B

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Re: 25cc Horizontal engine revisited
« Reply #4 on: December 23, 2017, 11:23:15 AM »
I turned a new shaft and made a new 2mm square stainless steel key as the swarf gnomes got the old one  :( When I originally made the pump I hoped that the low pressure in the system would avoid leakage along the shaft. This was not the case so I found some 6mm bore, 10mm dia, 2mm thick oil seals that would fit just behind the pump gears. I had to reduce the length of the Oilite bush to give clearance. It all seemed to fit together ok  :) If the weather improves I will give it a try.
Best regards

Roger

Offline Roger B

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Re: 25cc Horizontal engine revisited
« Reply #5 on: December 28, 2017, 10:43:35 AM »
The weather yesterday was slightly above freezing and a lot of the snow had gone so I thought I would give the engine a try. It started ok but wouldn't rev and was very 'touchy'  :headscratch: I tried to think back over anything else that could have changed. Finally I remembered that when I installed the electrical system I had mistakenly installed a 7812 regulator for the ignition instead of a 7806. I spotted this when I checked the voltage on the coil and found it was ~7.5V. The engine ran ok like this for some fuel injection trials but when I had a free moment I installed the correct 7806 but didn't check anything.
The voltage on the coil when the engine was running was around 2.5V  :(  I connected a 6V accumulator directly to the coil and tried again, everything back to normal  :)  :)
I made  few more checks back in the workshop. With the points open 6v on the coil points closed 2.3V on the coil, current limited to 1A. I connected the 6V accumulator again and measured a current of 2.5A points closed and 0.6A with the engine turning at at least 1500rpm. This is the current I had used to decide that a 1A regulator would be ok, but the inrush spikes were obviously activating the current limit. I then connected a 2200 microfarad 25V electrolytic capacitor across the output of the regulator and tried again. The voltage drooped with the points closed but maintained 6V when the engine was turning.
Although the snow has returned I decided to give the engine another try this morning. I spite of sub zero temps it started instantly and will rev to around 4000rpm as before (quite enough for a cast iron spoked flywheel  ::) ). The carb I was using doesn't really match the engine so I have to adjust the needle valve for different speed ranges.
There was also no sign of water leakage from the water pump shaft  :)


This time I have just pasted the link in rather than using the YouTube button above
Best regards

Roger

Offline Admiral_dk

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Re: 25cc Horizontal engine revisited
« Reply #6 on: December 28, 2017, 11:20:32 AM »
I would expect between 2.5 and 7 Ampere DC consumption in a "normal Ignition Coil".
This drops by the dwell factor when the engine is running and often more when you reach the RPM where the inductance of the Primary Coil begins to limit current. Increasing the voltage prevents the inductance limiting - but it increases the current too if not limited  :zap:

Online Dave Otto

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Re: 25cc Horizontal engine revisited
« Reply #7 on: December 28, 2017, 03:50:58 PM »
That sounds great Roger!
Nice work.

Dave

Offline fumopuc

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Re: 25cc Horizontal engine revisited
« Reply #8 on: December 28, 2017, 03:54:26 PM »
Hi Roger, I do like your R&D environment. Great sound.

Kind Regards
Achim

Online Kim

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Re: 25cc Horizontal engine revisited
« Reply #9 on: December 28, 2017, 05:39:43 PM »
Roger, looks like you've got it running well despite the weather!
Kim

Offline Jo

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Re: 25cc Horizontal engine revisited
« Reply #10 on: December 28, 2017, 05:49:18 PM »
That sounds sweet.... rather you than me out in the snow  ;)

Jo
Usus est optimum magister