Author Topic: 20 cc Horizontal 2 Stroke Diesel  (Read 55775 times)

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #300 on: June 03, 2020, 06:03:48 PM »
I made up the fuel tank support which was actually quite fun as I was not working to sub mm tolerances, could happily blast 10mm holes through things  :) and use taps I was not scared of breaking  ::)
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Roger

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #301 on: June 04, 2020, 07:58:07 PM »
Having assembled the tank I tried to prime the system with the new 10░ injector. No success  :( I worked my way back through the system and tried to check the pump for leakage with light machine oil, nothing  :headscratch: I measured the plunger and to the best of my measuring ability it had worn 3-5 microns. Using a new 1.98mm pin gauge In the pump body I couldn't build any pressure so that was also worn. I don't have a 1.99mm pin gauge to check with so I can't judge the wear but I need to get back to a known point with the pump before I can continue experimenting.
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Roger

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #302 on: July 20, 2020, 07:29:14 PM »
I am starting to sort out how to fit the new fuel pump onto this engine. As I dismantled the old system I found that the cam follower bearing had failed again  ::) It seems that at least two of the balls had cracked in half  :headscratch: Maybe a solid follower on a bronze bush or a complete bronze follower will be better  :thinking:
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Roger

Offline Admiral_dk

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #303 on: July 20, 2020, 09:39:59 PM »
I suppose that you have "a shock load" from the way the pump work, on the bearing and axle ....

If this is the case, there are some very different bearing data that you need to peruse ...!

Another reason might be a slight angle that shouldn't be there ...?...

Per

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #304 on: October 11, 2020, 07:18:57 PM »
This thread tends to merge/spilt with the fuel injection thread.

I had some success with a stainless steel injector body and decided to make some more. I had also found that my compression tester tended to leak off far to quickly so I would make a new body/valve seat. The operations required were quite similar so I made them together. The compression tester still is not working as expected. The low range (15 bar gauge) goes off scale easily but the high range (315 bar gauge) barely reads. I think I may need to buy a new ~100 bar gauge. My current gauges are from an old gas bottle regulator.

Whilst I was playing with the compression gauge I noted that the injection pump cam follower bearing broke again  :( This is strange as there was no fuel and no injection pressure  :headscratch: I think I will try a bronze roller.
« Last Edit: October 12, 2020, 07:28:38 AM by Roger B »
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Roger

Offline Brendon M

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #305 on: October 15, 2020, 05:43:05 AM »
Hello Roger

What material is the bearing? Looks to be a ball bearing?

Probably not standing up to impact loading. As you surmised, you will may get better longevity out of a solid roller.
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Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #306 on: October 16, 2020, 05:20:48 PM »
Hello Brendon, Yes it is an RC car ball race which has failed several times already  ::) I have made a bronze version however the end of tappet is looking somewhat abused due to the various failures so I may remake that as well.
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Roger

Offline Admiral_dk

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #307 on: October 16, 2020, 09:57:55 PM »
I think I can explain Roger.
That very tiny ball bearing can probably easily 'carry the load' of the spring in your system, but I'll be very surprised if it can withstand the shock load when the cam hits ...!
I'm almost alway surprised on how fast the load capabilities of a ball bearing rises with size when reading the tables -> it must be very small with a bearing the size you use here ....

I have never seen the inside of a full size injection pump and only a few drawings (curiosity) and I seem to remember that they use a solid roller and not a ball bearing - I can be totally wrong here ....

Great to see that you haven't given up  :ThumbsUp:   :cheers:    :popcorn:

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #308 on: October 22, 2020, 11:22:25 AM »
Thank you Per  :ThumbsUp:

The small ball race was a quick solution at the start but as expected will need to be re-engineered  ::) As the bronze roller will be running on the pivot rather than an inner race I will need to improve that as well.

I put the compression tester back together with the new valve and gauge and had a quick trial.


At low speeds it will read 20 bar and at high speed (~3000rpm, max. for the drill) over 30 bar. There is still unfortunately a leak somewhere which I think is partly responsible for the speed dependancy of the readings. I may be able to lap the valve seating if I glue a ball bearing onto the end of a rod  :thinking:
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Roger

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #309 on: November 14, 2020, 05:26:12 PM »
I tried lapping the seating which produced some improvement but the ball will tend to be abraded more than the seating as it is harder  :headscratch: I also took the cylinder head off to make sure that the bore looked ok. I have been experimenting with the injection nozzles over on the fuel injection thread and it seems that the injection timing is fairly critical. I can adjust it over a 30░ range but only with the engine stopped. I will need to design a system that will work with a running engine but is not affected by the force applied to the pump plunger  :thinking: I also wonder if I should open up the air intake from the 8mm bore used with the carb for the first trials as a petrol engine. It is reed valve induction so the inlet period is quite long but maybe 12mm or 14mm like the exhaust would be better. This was the last trial, I think I am getting real firing but it won't sustain. I like the smoke ring at the end  :)



« Last Edit: November 15, 2020, 07:39:44 AM by Roger B »
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Roger

Offline Admiral_dk

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #310 on: November 14, 2020, 07:31:07 PM »
Shame that it doesn't do sustained running .... though it feels like you're tantalizingly close ...  :thinking:

I'm not surprised about the lapping, as it's always mentioned that the softer part, is where the abrasives bed into -> it grinds the hard part a lot more ....

I'm not sure about the inlet diameter - if it's big enough for the highest RPM's you desire - it should not help increasing it ...!... and I suspect that it might have the opposite effect / rougher running ...

Offline Brendon M

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #311 on: November 15, 2020, 12:10:49 PM »
Keep at it Roger, you will get there :)

I'm not sure about the inlet diameter - if it's big enough for the highest RPM's you desire - it should not help increasing it ...!... and I suspect that it might have the opposite effect / rougher running ...

I think you are saying that greater oxygen content will cause more issues? If that were the case, you would expect supercharged and turbocharged diesels to run rough due to the excess air. If you were not, please ignore this post :)
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Offline dieselpilot

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #312 on: November 15, 2020, 03:24:15 PM »
How much fuel are you injecting per stroke?

The difficult part of a model injected diesel is the truly minuscule injection volume required and atomizing it in a meaningful way in such a small bore.

Diesels always run lean of stoich. Idle can be 60:1.

Offline Roger B

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #313 on: November 15, 2020, 05:43:40 PM »
Thank you all for your interest  :ThumbsUp:  :wine1: 

Two stokes are outside my full size experience. I know that there are a lot of complicated resonance effects due to the compromised port timings which make them run differently to the four strokes I am used to. The design calculations for the injection system are as follows with many assumptions about volumetric efficiencies:

'The density of air is around 1.2kg/m3 so 20 cc of air is around 0.024g. Taking a fuel air ration of 14-1 this would require 0.0017g of diesel fuel. The density of diesel fuel is around 0.8kg /dm3 so at full output around 2mm3 of fuel per stroke  would be required. This could be achieved with a plunger of 2mm diameter and a stroke of 1.5mm (plus a bit to allow for leakage).'

This is backed up with the petrol injection experiments on my 25cc horizontal engine where it runs with a injection pump plunger stroke under 1mm. The actual stroke is difficult to determine as the start of injection will be affected by how square the end of the plunger and inlet port are.
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Roger

Offline dieselpilot

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Re: 20 cc Horizontal 2 Stroke Diesel
« Reply #314 on: November 16, 2020, 02:24:43 AM »
Right, I do think I've asked before. Sorry.