Hot Air Engine   

Technical Stuff (Well the best I can do)

This is a working model of a hot air engine made by my late father more than 30 years ago. He spent many hours in its manufacture but probably even more in trying to explain to me how it worked. In the latter task he never really succeeded but I will recount what I can remember of its workings.

It bears no resemblance to an internal combustion engine but many people mistake it for a steam engine. In fact it is an external combustion engine which will work from any fuel that can provide heat. 

As you will see there are two cylinders of  differing sizes connected by a copper pipe. Whilst the pipe enters at the head of the small cylinder it joins the larger cylinder at the mid-point. I do not know  the relationship of the volumes. Heat is applied to the larger of the cylinders. The piston contained within the larger cylinder has no contact with the cylinder wall whereas the piston within the smaller cylinder is a very precise fit indeed.

Each piston is connected to the flywheel, the con-rods being set at 90 degrees to one another.

The air in the large cylinder is heated by a small mentholated spirits burner (not shown in photo) and the expanded air  passes in part over the piston but at the same time exerts a small pressure against the piston head. Heat is dissipated by the large cooling fins but a pressure differential of one and a half pounds exists between the two cylinders. No air enters or leaves the engine but is continuously exchanged between the two cylinders.

The whole principle of the thing still seems highly implausible to me but, light up the burner, give the flywheel a flick and off it goes at 800 rpm ! The engine can stop at intervals because of water condensing in the small cylinder but once the piston is removed and dried off it will normally run for about 20 minutes before it happens again.

Construction

The engine was built from odd scraps of mainly everyday items. The large piston is a cut down Steradent tube (the aluminium tube that contained tablets for sterilizing false teeth). The cooling block was cast and machined from melted down car pistons ( I think from a Mark 1 Cortina). The con-rods are machined from duralumin that I believe came from some world war II aircraft. The smaller of the the pistons is made from a brass candlestick machined to an accuracy of less than 1000th of an inch and then lapped. The asbestos gasket between the large cylinder and the cooling block came from an old ironing board. All screws were manufactured. The rest of the components are made from off-cuts of steel bars thrown out by a local engineering factory except for the base, which came from a child's toy sewing machine. Total cost of materials £0.00. Total cost of labour -approaching that of "The Dome".

Site of Manufacture

Wash-House at bottom of yard

Tools used

Myford ML7 screw cutting lathe. Cutting tools remanufactured from those thrown out by local factory. Self constructed grinding wheel. Old pans stolen from my mother's kitchen - used for melting down pistons. Gas ring borrowed from kitchen.

Uses

I understand that hot air engines were in use by the army about the time of the first World War They were used to pump water. The main convenience of these engines was their ability to operate anywhere that a source of heat could be found e.g. wood. They were simple engines with little to go wrong - no valves and few working parts. Perhaps they are still in use somewhere but I am not aware of any modern applications.