Less Fuel, More Work
Without exception, improving fuel economy has always been an important aspect for all engines. Now in the 21st century, reducing CO2 emissions to preserve the global environment, in addition to using less fuel resources, has also become a vital goal in engine development. The issue becomes: How can fuel efficiency of engines that operate under high loads be drastically improved?
The path to solving this issue began with one engineer's inspiration. In Summer, 2001, the engineer, between meetings in London, visited a museum. His attention was focused on an aircraft radial engine on display. As he imagined how the various components - the crankshaft, connecting rods, and pistons - interacted, he noticed how the structure and movements of the main- and sub-connecting rods differed, and was glued to the spot.
He had intuitively conceived a completely new mechanism using sub-connecting rods - no longer a part of modern engines. By connecting the main-connecting rod to the crankshaft via a sub-connecting rod, with movement by link, ideal piston movement control could be possible.
This was the starting point of the strangely shaped, multi-link extended expansion linkage engine.
The extended expansion linkage engine was invented in the late 19th Century by an English engineer, James Atkinson, and boasted a net thermal efficiency of 18%, revolutionary at the time. This invention had come only a decade or so after Nikolaus Otto, a German engineer, had completed the Otto engine - which modern engines are based on.
If the expansion ratio is larger than the compression ratio, the engine can work more, using less fuel. In theory, this is correct. The problem is how to build such an engine. The efficiency of Atkinson's second extended-expansion cycle engine from 130 years ago was far superior, but as it was too complex to build in a compact size, and was not suited to high-speed operation, and thus, it had no part in mainstream engine development, and faded into obscurity. But, maybe, by using sub-connecting rod links, it could be possible to build a modern day extended expansion linkage engine that is simpler than Atkinson's mechanism…
Pursuing the Ideal Link Mechanism
In 2001, a research project was started to explore the possibilities of the extended expansion linkage engine. Since the engine didn't exist yet, and all they had were conceptual notes based on the radial engine, the team began research practically from scratch.
The team aimed for a simple mechanism. By positioning a new “trigonal link” between the connecting rod and crankpin of a conventional engine, and turning it via a swing rod at half the speed of the engine, the piston's stroke could be changed for each revolution. This multi-link mechanism allowed the stroke to be longer for expansion than that for compression, realizing a extended expansion linkage engine. Would it work, though? The first prototype was tested in December, 2001, and in spite of the air of doubt that surrounded the team, it whirred into action with a lively sound. That, however, was only the beginning of a very long, and hard, path to success.
Although the team had proven the basic structure was right, the first prototype soon broke, as it was a modified conventional engine. The newly designed second prototype was also problematic, with unexpected vibration and noise levels persisting, making performance tests difficult. The team decided to start from scratch to study the link mechanism's specifications, and by the time they had built the third prototype and saw a glimmer of hope, more than a year had passed since the beginning of their research.
Research was then halted for nearly a year due to internal circumstances, and although the team returned with their accumulated knowledge and an even stronger will to realize the ideal multi-link mechanism, the fourth prototype was marred by too much friction.
Since designing the second prototype, the connecting rod was designed to stay straight during expansion, to reduce piston side force. The idea was that if piston friction could be reduced, increased friction due to other link components would be offset, keeping total friction similar to that of a conventional engine. In reality, however, friction was far greater.
By thoroughly researching load and friction conditions of all the links, and changes over time, the team analyzed the causes of the high friction. They then built the fifth prototype, newly designing every component with sufficient rigidity and lubrication while minimizing axis diameter and frictional area to reduce total friction, finally achieving their initial objectives.