Scotch crank engines

Gallery opened: 17 May 2016

Updated: 1 Feb 2022

There have been several attempts to make IC engines using a scotch crank rather than the usual crank-and-connecting rod. There are some theoretical advantages, such as fewer moving parts, and an absence of side-forces on the piston, but the idea has not prospered. The concept is sometimes called a scotch yoke.

Left: Action of the scotch crank
As this animation makes clear, the crankpin slides in the slot as well as rotating. This makes effective lubrication much more difficult than for a rotating bearing.

Authorship of this animation has been claimed by Brian Roys.

THE BOURKE ENGINE
One of the early attempts to produce a Scotch Crank engine was the Bourke design, which has a very good Wikipedia page:The Bourke engine, and is generally better documented than most unconventional engines. This two-stroke engine was developed by Russell Bourke from the 1920s, but never reached the market. The Wikipedia article ends with an impressive list of the problems built into the design. While the Wikipedia article shows four cylinders arranged in a cross, most pictures of the Bourke engine show a horizontally-opposed flat four.

Left: The Bourke engine: 1939
The prototype had a cubic capacity of 400 cubic inches, equivalent to 6.5 litres- a big engine. The tower at the right is the petrol tank.

Bourke obtained British and Canadian patents for the engine in 1939; see GB 514,842 and CA 381,959. He also obtained US Patent US 2,172,670 in the same year. Bourke died in 1968, leaving the engine to his friend Neilsen, who promised to continue its development. You can read a most intriguing account of how that worked out here.
There is a Bourke website at bourke-engine.com but none of the links seem to work.

THE PONTIAC ENGINE

Left: The Pontiac scotch crank engine: 1969
The cylinders were of finned aluminum, and air cooling was by fan with ducting around the cylinders. Lubrication was partly by running on a petrol-oil mixture, which sounds very crude for something expected to go into major production. This caused a smoky exhaust, which the article said "causes concern" and that was in 1969. However the engine also apparently had a pump to feed oil to the critical sliding parts, working on a dry-sump basis. The petrol-oil mixture was supposed to lubricate just the cylinder walls.
Two Zenith side-draught caburettors were fitted. The engine had a displacement of about 100 cu in and had a maximum power of 80 HP- not adequate for a large American car. The distributor was a standard part.
Note that this was a 2-stroke engine. Even if it had worked perfectly, it would have been undone by later environmental legislation.
Source: Popular Science April 1969, p64

OTHER SCOTCH-CRANK ENGINES
A scotch crank engine was designed by Brian Collins, of Perth, Australia. Popular Science, Jan 1990. This engine is also a two-stroke and will not do well when faced with emission limits.
A more recent project is the SyTech scotch yoke engine, which can be seen in an Autospeed article, dated 15 May 2001. So far over AUS$35million have been spent. The prototype appears to be a four-stroke. This link is currently not working.