Hot air engine
KMYCA club project 2001
Early this year the steam section of our club launched another club project. And this time the choice fell on a hot air engine of the low temperature variety.
As with the previous project a set was offered containing the drawings, to a design of one of the club members, and a parcel with all the raw materials needed to make the engine.
After the presentation of the project we were challenged to make our machines something original, and encouraged not to follow blindly the drawings. This in the hope not to end up with 40 plus engines all looking the same.
I simply could not resist. I always had wanted to build a hot air engine but was put off by the lack of a suitable drawing and the tales of horror about the precision fit the cylinder had to have. In this design a membrane has replaced the cylinder, and that seemed to be the solution to the cylinder problem. However there was still the problem of the originality.
Being a great admirer of the skeleton clocks I taught it would be nice to make something along these lines. The general dimensions for the machine were kept as per original drawing but the outline of the frame and supports were altered to give it the looks of a clock.
As I had no idea of what power to expect from an engine like this I started with this unit first.
The above pictures give an idea of what the power unit looks like. You can clearly see the displacer room with on top the membrane which acts as a cylinder. To get the engine to run more evenly, it's actually a single cylinder, a counterweight is added to balance the weight of the displacer. On test powered by the heat of a cup of hot water it ran first time for about 1/4 of an hour. By then the water was quite cooled down.
So convinced that the power would be ok a start was made on the clock movement. I however realised that it would nearly be impossible to make it run on time. Therefor I decided that the minute hand would directly be coupled to the crank of the engine. Only the hour hand would get an extra reduction of 12/1. As time would obviously pass quickly the name of the 'clock' became 'Time flies by at every heartbeat', the heart being represented by the shape around the membrane.
To give an idea of the size, the flywheel is perplex diameter 160 mm. The displacer is 92 mm dia. and is 11 mm thick, the chamber is made of perplex tube 100 mm outside and 26 mm high. The membrane has a crank with a throw of 3 mm and the displacer 6 mm.
You can have no “real” clock without the polishing. Speaking of a work of patience, well this certainly is one. Working all the way from coarse till fine emery paper and on to polishing it is a rewarding job. But take my advice NEVER EVER use a scriber on a part that has to be polished. It makes a sort of WWI trench which is nearly impossible to remove.
On its first outing it attracted a lot of interest from … women. They all loved the looks and wondered what the power source could be
.
With the idea that taking part in an exhibition like MEX at Sandown would be fun I entered it in the competition.
I must have taken the judges by surprise as they found this one worth a silver medal in the hot air engine class. Must admit this was a total surprise for me but I can sure appreciate it.
As a present for a friend and a test bed for the polishing technique I built another of these hot air engines. This time I stayed close to the original drawings and you could say this is the KMYCA club project. The polishing went quite straight forward and I must say I am pleased with the final result. Presenting it on a mirror on top of a rather slim glass gives it an extra dimension.
