Miniature 4-cylinder 4-stroke

Model Airplane News, Sep 2000 by Gudaitis, Frank

Designed and built by George Luhrs of Shoreham, NY, this mechanical masterpiece is a precise miniature of a gas-ignition model airplane engine of the 1930s. It represents 600 hours of design and very fine machine work. The overall length of the crankcase is 1 7/8 inches; in comparison, a pack of gum is 3 inches long.

George has been building engines of all sizes for most of his life, and he has a large collection of restored, WW II-era machine shop tools. He made many of the 230 parts of this particular engine on a fairly large, 13-inch lathe, using homemade fixtures and tools unique to this project. Other parts for the engine were made using a much smaller watchmaker's lathe.

The engine's updraft carburetor has a choke and a needle valve for setting the mixture. The throttle control automatically changes the fuel mix from low to high speed, and ignition is controlled with a spark-and-advance lever. The spark plugs are made of tool steel with Macor insulators. The thread is no. 2-64 (0.086-inch diameter). The distributor is made of Delrin and has tungsten contact points that are swept by a Teflon rotor.

The crankcase, cylinder head, distributor housing, carburetor body and all other aluminum parts are made of high-strength 7075-T7 alloy. The no. 00-90 thread cylinder-head bolts, pushrods, valves and exhaust stacks are all made of stainless steel, and the crankshaft, bearings, valve tappets and guides, and all other parts such as the camshaft, pistons, wristpins, connecting rods and cylinders are all made of hardened tool steel. Think about machining a camshaft and cutting cam lobes on a steel shaft that's smaller than a wooden matchstick! George notes that the most difficult part of building this engine was creating a tiny carburetor that could meter extremely small amounts of fuel. The engine runs on white gas with a 50:1 mix of Marvel Mystery oil for lubrication, and George and a friend are now testing its reliability to see whether they can safely use it to fly an RC model.

This past April, George's subminiature engine was awarded first prize at the national North American Model Engineers Expo in Detroit.

What's next for George? Perhaps an engine that's half the size of this one: he is now working on a small powerplant that has an 1/8-inch-diameter piston!

SPECIFICATIONS

Type: 4-cylinder, 4-stroke gas ignition

Crankcase: 17/8 in.

Piston: 1/4 in.

Stroke: 5/16 in.

Displacement: 0.061 ci

Fuel: white gas with 50:1 Marvel Mystery oil