The Hugon engine was a slightly improved version, but both were quite inefficient (95 and 85 cubic feet of gas per HP hour respectively).

Clerk decided to develop an engine using compression, but with the two-stroke cycle, as he could see benefit to weight and smoothness of operation through having twice as many power strokes.

"Clerk's initial experiment with a Brayton ready motor in 1878 led him to make improvements that would eventually result in the development of the two-stroke cycle.

However it was not until the end of 1880 that he succeeded in producing the Clerk engine operating on the two-stroke cycle, which became the commercial product.

Clerk states "The Clerk engine at present in the market was the first to succeed in introducing compression of this type, combined with ignition at every revolution; many attempts had previously been made by other inventors, including Mr. Otto and the Messrs. Crossley, but all had failed in producing a marketable engine.

Dugald Clerk was the author of three comprehensive books covering the development of the oil and gas engine from its early inception, and including details of his own work in this area.

The pump takes in a charge of gas and air, and the motor piston overruns a port in the side of the cylinder at the out-end of its stroke to discharge the exhaust gases.

Clerk's significant contribution was introducing Otto-styled compression to the two-stroke engine, bringing its efficiency up to date (for the 1880s).

Many years later the two-stroke engine for large capacity diesels using a turbocharger or supercharger has become common, for example in ships and railway locomotives.

[2][3] The crucial simplification of the concept, that made possible small yet powerful two-stroke engines for mass markets, was patented by Joseph Day[3] in 1894.