🚗 Honda VC Turbo? No, It’s Called EXLink!

Original – Honda Magazine | Japanese Car Enthusiast Journal | 🗓️ 2025-04-20

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In recent years, Nissan has been heavily promoting its VC-Turbo technology,
but actually Honda also has a similar technology. This technology is called EXLink,
and it appeared even earlier than VC-Turbo,
but due to differences in publicity effort and application environment,
this EXLink system is not widely known. 📉

This EXLink technology is still being used in Honda models today. 🧰

Before officially talking about the EXLink technology,
let’s first clarify an issue: how to significantly improve fuel efficiency under high-load engine operation. 💡
This issue has plagued engineers and developers ever since the invention of the internal combustion engine. ⛽

The earliest solution was invented by British engineer James Atkinson in the late 19th century. 🧪
He installed a complex multi-link mechanism on the piston connecting rod.
This mechanism changed the stroke of compression and expansion,
allowing the engine to achieve a higher expansion ratio. 🔁

In theory, if the expansion ratio is greater than the compression ratio,
the engine’s thermal efficiency is higher, consuming less fuel. 🔥

This mechanism was named the Atkinson Cycle.
In the late 1800s, it allowed engines to reach a net thermal efficiency of 18%,
which was quite revolutionary at the time. 💥

However, subsequent engines focused more on power output,
so the fuel-efficiency-focused Atkinson system was not widely adopted by automakers. ❌

Back to the topic—
EXLink stands for Extended Expansion Linkage Engine. 🔗 The inspiration came from a Honda engineer who, during a business trip to London in the summer of 2001,
visited a museum and accidentally saw a radial engine from an aircraft. ✈️

While observing this engine, the engineer kept thinking about
how internal engine components like the crankshaft, connecting rod, and piston interact with each other. 🔩
He noticed the difference between the movements of master and slave connecting rods in the aircraft engine.

Afterward, relying on intuition,
he imagined a brand new mechanical design:
a main and sub connecting rod mechanism, both linked to the crankshaft,
to control the piston movement via linkage to achieve optimal performance. 🛠️

After returning to Honda, he initiated a research project
to explore the potential of this extended linkage.
However, at that time, all they had was the concept from a radial engine,
so the team had to start from scratch. ⚙️

The goal was to construct a simple mechanism—
by adding a triangular link between the traditional connecting rod and crank pin,
and driving it with a swing arm rotating at half the engine’s speed,
to alter the piston’s stroke during each cycle. 🔺

This multi-link setup would shorten the compression stroke and lengthen the power stroke,
thus creating a situation where the expansion ratio is greater than the compression ratio. 📊

The first prototype engine was tested in December 2001.
Even though many in the R&D department were skeptical,
the engine completed the test successfully. ✅

But this was just the first step toward mass production—
many technical challenges still lay ahead. 🚧

Although the team proved the basic structure was feasible,
the first prototype quickly broke down,
as it was only a modified version of a traditional engine. 🔧

The second prototype had noise and vibration issues,
making testing increasingly difficult.
So the team decided to completely redesign the linkage mechanism. 🧱

By the time the third prototype was built and signs of success appeared,
more than a year had passed since the start of the research. ⏳

The fourth prototype still suffered from noise, wear, and vibration issues,
rendering it unusable shortly after. ❗

Then the team conducted in-depth studies on load and friction conditions throughout the entire mechanism,
and discovered the root cause of excessive friction. 🧪

They then built the fifth prototype,
in which every newly designed part had sufficient rigidity and lubrication,
and shaft diameters and friction areas were minimized to reduce resistance. 🛢️

The fifth prototype engine was finally completed in 2005,
and it was the last prototype before entering mass production. 🏁

This engine was installed in a co-generation unit,
and the R&D team carefully selected materials and inspected part designs
to ensure the new system’s durability. 🧱

After thousands of hours of durability testing,
no issues were found. 💯

With less fuel used and better thermal efficiency,
this became the world’s first mass-produced multi-link engine of its kind.
The system was eventually adopted into the Honda i-MMD hybrid system (LFB engine). ⚡

The EXLink uses a triangular link between the connecting rod and crankshaft.
The triangle link rotates on an eccentric shaft to form the extended linkage mechanism.
The eccentric shaft rotates at half the speed of the crankshaft,
allowing the piston to have longer expansion and shorter compression strokes. 🔄

By shortening the intake/compression stroke and lengthening the power/exhaust stroke,
a higher expansion ratio is achieved.
This follows the Atkinson principle of "less fuel, more work",
and all of this is realized in a simple, compact structure. 🧩

EXLink changes the piston stroke dynamically
to achieve asymmetric movement between compression and expansion. 🧬

Using this multi-link EXLink mechanism,
the engine achieves an expansion ratio higher than its compression ratio.
The compression ratio is 12.2:1, which is enough to avoid knocking,
while the expansion ratio is 17.6:1,
extracting maximum energy from fuel-air mixture. 💥

Because the intake stroke is shortened,
air and fuel losses during intake are reduced,
effectively improving thermal efficiency. 🔥

In the image, we not only see the compression and expansion ratios,
but also the engine’s high base RPM—reaching 1950 rpm. 📈

This diagram shows another benefit of EXLink:
during the power stroke, the cylinder wall experiences less stress
compared to conventional engine structures. 🧱

Back to the original question:
How to improve engine efficiency under high load?
Whether it’s Nissan’s VC-Turbo or Honda’s EXLink,
their original intent is the same—
to improve fuel economy under demanding conditions. 🌱