Sarich orbital engine

The engine promised to be about one third the size and weight of conventional piston engines due to the compact arrangement of the combustion chambers. Another advantage is that there is no high-speed contact area with the engine walls, unlike in the Wankel engine in which edge wear is a problem. However, the combustion chambers are divided by vanes which do have contact with both the walls and the orbiting piston and are more difficult to seal due to the eight corners of the combustion chamber.[1]

In the patent, the engine is described as two stroke internal combustion engine,[2] but the patent claims that with a different valve mechanism it could be used as a four stroke engine.[2] However most of the development work was done on four stroke versions with both poppet and disk valve arrangements. A supercharger is required if operated in two stroke mode since crankcase pumping can't be used to charge the combustion chamber.

Interestingly, in his seminal book [3] researching and documenting all the possible ways to create a rotary piston displacer, Felix Wankel shows the orbiting piston and reciprocating vane mechanism used in the orbital engine.

The Orbital Engine Company, with funding from partner BHP and Federal Government R&D grants,[4] worked on the concept from 1972 until 1983 and had a 3.5L four stroke engine performing as well as the similar petrol car engines of the day at typical road load conditions. A technical paper[5] was presented to the Society of Automotive Engineers in 1982, and is now part of their historic transaction collection.

A major reason for the good performance of this engine was the development of a unique and patented injection system directed into the combustion chamber which created a stratified charge combustion process.

Several auto makers from around the world showed great interest in the engine, however it was realised that there was still at least $100 million of development work required to commercialise the engine and the funding sources decided this was not a sound investment. Instead it was realised the same injection and combustion system could be adapted onto existing two and four stroke petrol engines and this work become the future of the company, being called the Orbital Combustion Process.[6]

The orbital engine has two of fundamental design issues, which also plague the Wankel engine:

1. Large surface to volume ratio combustion chamber which leads to larger combustion chamber heat losses and so loss of power, which can be greatly reduced using stratified combustion;
2. Long sealing paths and multiple corner seals which mean it is harder to contain the chamber gases and so there is some loss of pressure and thus power.

Some conceptual sketches from the engine's patent:

• 
  perspective view with the exhaust end cover and chamber end plates removed
• 
  as before but with the vane and cam member also removed

• Orbital Corporation
• Powerplus supercharger
• William Selwood
• Cecil Hughes