Power Products Technology

Performing more work with less fuel—EXlink

(Extended Expansion Linkage Engine)

1. It All Began with the Inspiration of One Engineer

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…


Early 20th century aircraft radical engine
Connecting rod joins main-connecting rod and piston

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.

Early conceptual notes / Commercialized EXlink mechanism

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.

Evolution of the multi-link mechanism

Evolution of the multi-link mechanism

Completing, and Implementing the Cogeneration Unit

Household cogeneration units can benefit the most from highly thermal-efficient internal combustion engines. But, low vibration and noise, and durability, are also prerequisites at an extremely high level. The development team was unperturbed. By running the engine at constant conditions such as engine speed, they were confident their engine would meet even the most stringent criteria, despite being more complex than a conventional engine. The 3 years and 3 months of trial-and-error since the first prototype would undoubtedly bear fruit.

On their first attempt, equipping a cogeneration unit with their engine, they outperformed the conventional engine by 15%, as they had planned.

By carefully selecting materials and scrutinizing component design to ensure the new mechanism's durability, they also passed thousands of hours of durability tests without any problems.

Less fuel, more work. The extended expansion linkage engine that radically improved fuel economy was finally compete. As the world's first mass-produced multi-link engine with the same system, it was named EXlink (Extended Expansion Linkage Engine), and powers Honda's third-generation cogeneration units.

2. EXlink Basic Structure

With EXlink, a trigonal link is positioned between the connecting rod and crankshaft found in a conventional engine. The trigonal link is connected via a swing rods to an eccentric shaft to complete the extended expansion linkage structure. The eccentric shaft turns at half the speed of the crankshaft, allowing the piston's stroke to lengthen and shorten per cycle.

EXlink realizes a high expansion ratio, expanding 110cm3 intake to 163cm3 exhaust, by shortening its stroke for intake/compression and lengthening the stroke for expansion/exhaust. The Atkinson cycle principle of “less fuel, more work” is realized in a simple and compact structure.


EXlink and conventional engine comparison

How Stroke Length is Changed

With EXlink, an aluminum alloy trigonal link (red) is positioned between the connecting rod and crankshaft found in a conventional engine, and by connecting to the eccentric shaft via swing rod (blue), the trigonal link's attitude is controlled. The eccentric shaft rotates at half the speed of the crankshaft, and is connected to the swing rod's lower edge at an offset to the axis center.

The bottom-dead point position of an EXlink piston differs according to the state of the swing rod's lower edge connection. If the swing rod's lower edge connection is at its lowest point (diagram left), the right of the trigonal link is lowered (pivoting on the crankshaft), and the lower edge of the connecting rod is relatively high. The piston is not lowered as much, thus the piston stroke is short.

Stroke Length Comparison

If the swing rod's lower edge connection is at its highest point (diagram right), the right of the trigonal link is pushed up, lowering the connecting rod's lowest position, creating a longer piston stroke. For each crankshaft turn, the piston's stroke is short once, and long once.

By using the short stroke for intake/compression strokes and the long stroke for expansion/exhaust strokes, a mechanism that expands 110cm3 intake to 163cm3 exhaust is realized.

3. Highly efficient EXlink

In principle, the higher the expansion ratio, the higher the engine's efficiency. This is because the further the piston can be pushed towards atmospheric pressure by the high-temperature and -pressure gas created in the engine through combustion, the more work it does.

With a conventional engine, common perception is that engines with a higher compression ratio are more fuel efficient, as the expansion stroke volume is equal to compression stroke volume (with an Otto cycle engine), thus the expansion ratio equals the compression ratio. Simply raising the compression ratio to improve efficiency, however, is known to cause problems such a knocking, and may even damage the engine.

With EXlink, the multi-link mechanism allows the stroke volume for expansion to be larger than that for compression. In other words, the expansion ratio is larger than the compression ratio. The compression ratio is 12.2:1, enough to avoid knocking, while the expansion ratio is raised to 17.6:1. By efficiently compressing less fuel and air, and expanding the combusted gas to a larger volume, the fuel's maximum amount of energy can be used. Since the intake stroke is short, pumping losses during air and fuel intake can be reduced, and thermal efficiency can be improved over conventional engines.

Engine thermal efficiency

Reducing Friction

EXlink is comprised of many linkage components. Why doesn't friction caused by the connections of these components (or energy loss due to friction within the engine) reduce EXlink's fuel efficiency?

With a conventional engine, side forces act on the cylinder wall creating a large amount of friction between the piston and cylinder when the combusting air-fuel mixture puts pressure on the piston during the expansion stroke. Since the amount of side force is stronger for greater connecting rod angles, this can cause more than half of friction loss, depending on the engine.

EXlink has been designed so that the connecting rod is nearly parallel to the cylinder wall during the expansion stroke. As a result, friction loss due to piston side forces is less than half of a conventional engine. Even with its extra linkage components, EXlink has engine friction similar to a conventional engine, realizing the full fuel efficiency benefits of the Atkinson cycle.

Friction Comparison
Connecting rod angles

Knocking: Knocking is the metallic sound and vibration in an engine caused by abnormal combustion in the cylinder, which can lead to engine damage. Common causes of engine knocking are overly high compression ratio, insufficient cooling, and premature combustion.

Pumping loss: Pumping loss is the energy lost from moving air into and out of an engine's cylinder during intake and exhaust.

4. Related Articles

Honda R&D Technical Review

Other References

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Watanabe, S., et al. : An extended expansion stroke using multiple linkage system in general purpose engine, COMODIA 2008 OS-A3

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