Boll-Aero 7.5cc Diesel

I have recently completed making this engine.The Boll-Aero 7.5cc Diesel.It is he last of Chris Boll's excellent bar stock designs and has two ball races to the crankshaft. I have now made  all five of them. As drawn,this one is very square looking,with the cooling fins continuing straight up from the crankcase. I did not like the idea of interupted cutting of the fins,or of making a square piston, so changed the profile to a more conventional circular shape. On the test rig it proved to be a reasonable starter,once I had found the correct compressioin point. The photo of it running shows it doing just under 9000 RPM, with a 11" x 4" APC nylon propeller, using Model Technics 1000 fuel. No gaskets, no leaks,no black oil, so all parts must be in the correct place! Multi runs of 4 minutes with cool down between each, it has now had 32 minutes running time. It has that lovely "crackle"to its sound and is quite thirsty, with 100cc of fuel lasting 4 minutes. The recommended method of running in a Diesel engine by a series of high speed, rich runs,with cool down between each, I find really works and I am always amazed how compression increases by doing so.

Subsequently, I have moved the engine to my static thrust rig and changed the fuel to Model Technics 2000 grade. With RPM at 9300 RPM I got 41.8 oz of static thrust. To try and avoid Castor oil spray going everywhere, I rigged an old umbrella behind the set up to try and restrict it, but at these revs it turned inside out straight away! Later use of two 4ftx 2ft x 1/2inch ply boards hinged together as a screen to catch the exhaust flow is working fine.

I have now made 10 I/C engines consecutively, plus restoring as many clapped out Classics back to life. Not quite the output of"Engiineous" Jason, but he is a lot younger than me and more clever.

Edited By KEITH BEAUMONT on 01/04/2023 09:37:40

Edited By JasonB on 01/04/2023 13:06:54

Another fine looking engine Keith and the personal touches work well.

Lovely work as usual Keith.

Rod

01/04/2023 ? Noel

Well spotted Noel !! I When I realised the date I could not resist seeing if anyone would notice an April 1st joke.

Keith

Thank you Keith, on reading the description, it was the square piston that gave the game away, The broken cut for the finning also seemed suspect. Noel.

My original prototype really does have square fins to match the crankcase, but Keith's version does look superb, as usual. I did not find it a problem to cut them in the al alloy cylinder jacket. There are two to be seen running on YouTube, under BollAero 750.

For square pistons, the wind powered drainage mill that was rebuilt at the museum in Stowmarket does have them, and some modern pneumatic cylinders have been made with oval pistons, so the rod cannot rotate.

Honda NR750 had oval pistons. Why, to get 8 valves per cylinder. I suspect in reality it was to get round the maximum 4 cylinders rule for racing

I'm surprised that caster oil is still used for anything. We used it back in the 70's cart racing with the 9E4 class and had regular engine failures even with 16:1 ratios. Then in the 80's a whole season was run without any noticable wear using BelRay synthetic oil at 50:1 in a Suzuki 250 scrambler with 40 hp.

I think over the years, attempts have been made to use oils other than Castor, because of it being a nuisence to clean off a flying model. Its ability to be the best lubricant for model diesel engines is not disputed, plus we are all hooked on the smell of the exhaust !

Thanks all for your comments. It is much appreciated.

Keith.

Hi Keith,

Very nice indeed, well presented, and if I might add, for me inspiring.

I have been considering what project to make this autumn/winter and as it is some time since I made an i/c engine (a whittaker firefly) that will be my choice. After seeing your work I have opted for the Boll aero 18. Just planning and preparing at the moment. When I start I'll open a file as I imagine I will have some questions.

Well done again

Geoff

Hi Geoff,

Thanks for your comments. With my version of the 1.8cc I did change the shape of things from Chris's design. It is the black one in my album. I am tempted to make another, but stick to the drawing this time.

Keith.