Thursday, 21 April 2011

EMC Split Single engine

This post is detailing an engine which is a bit of a curio from the early 1950’s. The split single engine is a version of a two-stroke engine which uses two cylinders, but share only one combustion chamber.  The EMC split single engine here is based on some Puch components.

Joseph Ehrlich EMC 125cc split single development engine
In the split single intake charge enters the crankcase below the skirt of the front piston. The charge receives primary compression before passing through transport ports into the rear of the two cylinders. The charge travels up the rear cylinder and then transfer to the front cylinder via the bathtub shaped combustion chamber. The exhaust gases then leave the exhaust ports in the front cylinder. The front piston controls both the intake and exhaust ports, whilst the rear piston controls the transfer of charge from the crankcase to the rear cylinder.

Twin exhaust ports

Twin intake ports on side of engine
Asymmetrical port timing is achieved by pivoting the intake ‘slave’ connecting rod on the back of the main exhaust connecting rod. This allows the exhaust port to open before the transfer of fresh charge into the rear cylinder and also allow the transfer port to stay open whilst the exhaust port has closed.

A 'single cylinder' with two cylinders!

Combustion chamber
The benefit of the split single cycle comes with the separation of the intake and exhaust process. This allows much improved low speed and low load engine performance as well as reduced hydrocarbon emissions. Two-stroke technology in the early 1950’s was limited, as rotary valves and expansion chamber exhaust design was yet to be used widely. Instead conventional two-stroke technology often consisted of deflector type pistons to prevent intake charge leaving the exhaust port immediately.
It is clear that the split single has many issues which prevented it becoming widely used. The engine is larger and heavier due to the multiple pistons. High engine speeds are limited by the design and engine friction is higher than with a conventional two-stroke engine. The intake charge also has a tortuous path and combining this with a very large combustion area leads to poor efficiency.


  1. Mr. Ferret: When I was in high school, over 50 years ago, I designed an air cooled V-Single where there were two cylinders in V configuration and one piston which flopped back and forth between the two allowing each cylinder to cool twice as long. It seemed like a good idea at the time. Jim A. Tucson, AZ

  2. Thanks for the comment Jim. Well stranger ideas have made it to fruition! It is always good to think outside the box on occasions (good to question things), but often the old adage about re-inventing the wheel is apt.

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