True Solid Injection Diesel

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Bruce W-S
Project of the Month Winner
Jan 30, 2015
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Perth Western Australia
There has been discussion on these pages regarding the building of a true diesel however, the stumbling block is the fuel system and making something that is approximately to scale. I tend to build one-sixth scale (not to big or small). I started this project building a trial engine to see if it was possible to produce a solid injection fuel system. Note, while I have building models for over forty years, I am no diesel specialist. The main problem is while you can scale the model, you can’t scale the physics and so you need a compression pressure of greater than 500 psi with a minimum injection pressure of around 800 psi. The other problem is the amount of fuel to be injected which is in the order of half to three quarters of a water drop so very little.

My test engine has a bore of 40mm and a stroke of 80mm and is based on a Blackstone horizontal engine. The first issue was the engine compression. My original arrangement was based on four 1.5mm rings. I made a temporary head with inline non-return valves to which a pressure gauge was connected to the outlet. The best I could achieve was 300psi so I added three more rings and rather than having butt joins, I used sliding joints and made sure that they were not in alignment. The next series of test produced 600psi and it was quite difficult to turn over with the motor I was using.

Next was installing the head and valves and installing a pressure gauge with a non-return valve where the injector fitted. On testing, I was back down to less than 300psi. So back to lapping valves and fitting stronger springs. The best I could achieve was 500psi and I considered it acceptable.

So far, I have made some fourteen different fuel injectors all with varying degrees of success but non that I considered acceptable. They may have worked with something other than diesel fuel but that is not what I wanted. The problem always came down to the accuracy of grinding the injector valve. I then looked at commercial injectors and in particular the replacement nozzles. I selected a pintal type and made a housing for it (see photo). Finally I had some success however, my pump was not up to the job at about 1200psi.

Note, I had made nine pumps at that stage and I had collected several CAV pump parts. While they are well made, they were too large to be useful. The problem with the pumps is to be able to machine/grind a piston and bore to very close tolerances. I tried and they would work for a while and then would not hold the pressure. I then found on the Internet, a Chinese made injector pump (see photo) that was quite small and I believe could further reduced in size and at a cost of $20.00. Not as good as a CAV unit but it can reach 3000 psi. The worst case was that you now had a piston with barrel and associated outlet valve, which could be fitted, into your own housing. (see photos).

That is my journey so far and if anyone else is interested in or is in the process of building a diesel model I think it would be good to get input from others including constructive input into the subject and hopefully generate some interest in model diesels generally. Very few diesels have been built and even fewer actually run on diesel.

Injector housing. Once I am totally happy that I can't improve the system further, I will look at ways to reduce the size.
Modified Injector 1.png

My injector showing the commercial pintle valve
Modified Injectoe 2.png

CAV Injector pump piston and barrel (left) Two injector
nozzles on the right. Note these can be purchased on Ebay quite cheaply.

Chinese Injector Pump. Note it is fully regulated via the arm with the pointed pin. That is, it is constant stroke.
Injector Pump.png

Thanks for the link. I have studied his system closely however, he dosn't provide any details such as operating pressure, fuel type or timing. Also the size of his spring as used on the injector would suggest that it is low pressure and therefore not suitable for diesel.
He has done some fine work and made a number of excellent models that all appear to work well and deserves the appropiate credt for them.
My focus has been to make an engine that will run on diesel fuel just to be different and also avoid variable pump stroke speed control as it also varies the timing. I am all for sharing knowledge which allows the hobby to grow.

Bruce W-S

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