BollAero Major 4.4 (oz14657)

About this Plan

BollAero Major 4.4 cc. Diesel engine.

Direct submission to Outerzone.

Quote: "I am the designer of this engine, which has been built by many others, and wondered if you would like to have the plans on Outerzone. It is on 2 sheets and appeared in AMI magazine in 2011. I have pdf scans. Regards, Chris Boll"

Plan cleanup by Circlip. Article scan here is thanks to RFJ.

Article Quote: "Boll-Aero Major 4.4. Two years ago, our construction feature on Chris Boll's original 1.8 cc diesel drew much interest. Now he offers a larger 4.4cc option.

This is my second design for a home made engine, my first, a 1.8 cc diesel appeared in AMI in 2008, and I was very pleased with the positive response which I received from people, including several from outside the UK.

There are very few model aero engine designs available which are really simple to make. In the past, they were designed by expert model engineers such as Edgar Westbury and Laurence Sparey, who used complex shapes which were either made from castings or required a lot of delicate machining. Their designs are still built today, and various casting sets can be obtained, but times have changed since these notable pioneers were active and today there are probably fewer people with the traditional machining skills, nor local foundries willing to make castings in low volumes.

It may be a factor in early designs that many model makers were still using treadle powered lathes, so were anxious not to have to machine away much material due to the effort involved.

So I decided to try another design that is machined from bar stock, but this time with front rotary induction, which I realised is not at all difficult to make, and means that the timing can be optimised more, although it is still very conservative.

This engine is designed above all to be simple to make for the home builder, many of whom, like myself will be primarily aeromodellers, rather than experienced model engineers. It is based on readily available aluminium alloy bar size and no castings are needed. If this is to be your first home made engine, please be assured that it is just as easy to make as my previous 1.8cc, and due to the larger size, is probably even more tolerant of less than ideal fits for the piston and crankshaft.

It runs well, is very easy to start, and has enough power to fly vintage style free flight or R/C models of around 60 inches span. It has a long stroke, which is similar to the early diesels made in the 1940s. This makes it easy to achieve sufficient compression to fire easily when cold, even if the piston fit is less than ideal.

The maximum speed is relatively low, but there is a good torque output, which allows props of 12 inches diameter to be used. With a piston bore of 0.625 inches and stroke of 0.875, the dimensions are identical to a Kemp diesel which was one of the first to be made in England, although I only learned of this similarity from the excellent www.modelenginenews.orq website after I had made my engine.

I arrived at these dimensions because the 0.875 stroke was the biggest crankshaft throw that could be accommodated in the 1.5 inch wide block of alloy that I wanted to use. The ratio of stroke-to-bore of 1.4 : 1 is also the same as the Mills diesels that have remained popular for over 60 years and are still being manufactured in India and China. The Mills range were never the most powerful in their class, but have very easy starting and running characteristics which make them a pleasure to use. I sometimes think that modellers put too much emphasis on sheer power output when choosing an engine because, for the average sports flyer, robustness and ease of operation are more important.

This 4.4cc engine is both physically large and heavy compared to a modern engine of similar displacement, but then it is not intended to be used in a scale Spitfire or a prop hanging Fun Flier, so I make no apologies for this. In the intended application of Vintage R/C or Free Flight, a bit of extra weight is not a problem, my models always end up with lead in the nose anyway. Adding mass to the engine ensures that it is physically very rugged and also helps to damp out vibration, so it is surprisingly smooth running, bearing in mind there is a large cast iron piston, which changes direction 200 times a second at 6,000 rpm..."

Supplementary file notes

Article.

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