The Robinson Hot Air Engine

The Robinson hot air engine was common in England. Patented in 1889, large quantites of them were produced as recently as the 1920s. Used in England and Europe until the WW-II era, they were displaced by electric motors and small but powerful internal combusion engines.

The Robinson's displacer moves vertically and the power piston moves horizontally. This 90 arrangement makes for a very compact size. In fact, the power piston connecting rod weaves its way through the displacer linkage.

I've long been intrigued with this style of engine and hoped to buy or build one, some day. Then, in 2004 my friend Carl Carlson made my dream come true. He helped me buy a modest collection of hot air engines, including a quarter-scale Robinson.

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As you can see in Carl's picture, the little rascal looks really cute. But, upon getting it home, it refused to run. This didn't come as any great surprise. That happens.

So, I undertook an overhaul, but, as it turned out, it became a complete re-build. I hope you like how it turned out.


After I stripped off the old paint, I discovered the base had been broken in three pieces and had been brazed, arc welded, and J-B Welded, none of them being very pretty. It took a lot of JB-Welding, filing and sanding to hide the repairs.

You can see the new graphite piston, new crank disk, crank pin and the new cold water inlet. I also sandwiched in as much high temperature gasket material as bolt length would allow to help make a better thermal barrier between the base and mid-section.

Because of the high temperature from the fire, underneath, that makes it run, the base is painted with stove paint. Seeing as how the mechanism requires oil, and flat paint looks ugly when it gets oil on it, the top is glossy.

The cold water inlet requires an explanation. After I finished the re-build, I filled the water reservoir and started it up. Alas, after 20 minutes it slowed to a stop. The cold end had gotten too hot.

I tried everything I could think of, but nothing seemed to work. As soon as the water reached boiling temperature, end of show. It seemed as if external cooling was needed, but I didn't ever recall seeing it on a Robinson; and, for sure, I'd never seen it on a model of one.

So, I started searching through my pictures of full-scale Robinsons and I finally found the answer. One, and only one, picture showed an external coolant tank connected. Time for a re-design!


So, I added the cold water inlet to the bottom. The coolant circulates around the upper end of the displacer chamber. The heated water exits through the original fill hole. The thermo-syphon system provides very good circulation. The external coolant tank is not shown, here.

For show, I replaced the power cylinder's steel end-plate with one of brass. You can also get a glimpse of the results of the filing, sanding and polishing done on the brass linkages.


During the overhaul, the cast-iron piston was replaced with a much lighter one made of graphite. The new aluminum displacer weighed only a third of the old heavy brass unit.

What with the large reduction in reciprocating masses, to achieve balance I removed about 80-percent of the counterweight. What I couldn't turn off in the lathe, I milled out, as shown.

The crankshaft extension isn't necessary, but it may some day sport a small belt pulley.


Here is the nearly finished project. It shows the coolant tank and engine mounted on a cast-iron base. The base now sports a paint job and pin-striping. A picture of the completed project will be added, later.

There is a post-script to this story. Here goes.

Right from the start I was dissatisfied with the huge amount of heat that had to be dissipated to the coolant tank. This is because the Robinson is such a short, squat engine. Too much heat from the hot end moves by conduction to the cold end.

Furthermore, most model Robinsons such as this one do not use a regenerator type of displacer as was used in the original. When I rebuilt this one, I constructed the displacer out of aluminum. This, of course, contributed to the problem. Aluminum is an excellent conductor of heat; therefore, the displacer conducted far too much heat from the hot to the cold end.

Daniel Lyonett's ASAP engine uses a balsa wood displacer. As an experiment, I made a displacer of balsa, capped with brass at the hot end.

This made a huge difference in the operation of my engine. It will now run with no load on it, indefinitely, without needing any coolant. The coolant tank will be used whenever a load is placed on it.

Note: If you attempt to use a balsa wood displacer, dry it out in an oven, first. Otherwise, it will release loads of water to the inside of your engine. Bad!


Here are some of the parts I replaced. The cylinder in the picture is the displacer. This one had suffered some major abuse during construction. Part of the brass had been melted away! The subsequent lead solder repairs made it so heavy it was better to start all over and build another, much lighter, out of aluminum.

Note the broken #2 flathead screws. They were not up to the job of making things air and water tight. Besides, the countersinks were cocked. When the screws were tightened, this bent the heads over, causing them to snap off. It took hours of careful scraping to correct the problem. The screws got replaced with #4s.

The 3/16-inch copper tubing was supposed to be the boundary between the cooling water and the working air (but it leaked). Being glued in place, the only way to get it out was to gently hammer the parts apart and shear it in two. It got replaced with a Teflon-packed seal that withdraws easily whenever disassembly is required.


All in all the Robinson has been a very fun project. It now runs merrily for hours on end, non-stop. Right now the little rascal stands at the top of my "Favorite" list. I hope you liked it, too.


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