As Takeishi mentioned, Honda's cogeneration unit underwent a major design change in May 2011. Developers managed to shrink the size of the unit by over 30% by volume while also substantially improving its performance, increasing its electric generation efficiency from 22.5% to 26.3% and its combined power and heat efficiency from 85% to 92%.
One driver of this fast-paced evolution was the adoption of EXlink (Extended Expansion Linkage Engine), a linkage engine with an expansion stroke longer than its intake stroke. Thanks to its high expansion ratio, this revolutionary engine extracts more mechanical energy from fuel. It is based on the principle of the Atkinson cycle engine conceived some 130 years ago, and is the first mass-produced engine in the world to commercialize this principle. Referring to the EXlink engine, which took a total of 10 years—5 years in basic research, 5 years in commercialization—to develop, Sei Watanabe, Chief Researcher at the Power Products R&D Center, stated the following.
"It all started when Shimizu, head of the Power Products R&D Center, went on a business trip to Britain 11 years ago and saw a radial engine at a museum. The pistons in the radial engines used on airplanes a long time ago were arranged in a star, all pointing toward one central point. Pistons were attached—or linked—to the crankshaft via rods with flexible joints, so the engines moved very differently from other engines. When Shimizu saw this, he thought maybe using links would lead to the creation of a new type of engine. He brought home a simple sketch of the vision he had."
Shimizu knew that Watanabe was conducting research for a next-generation, high-efficiency power equipment engine, so he gave him the sketch. The idea to use flexible joints was still poorly defined, but it just might be the clue that Watanabe was looking for.
"I drew up some designs based on the sketch and then built a prototype," said Watanabe. "At first I was aiming for an engine with a variable compression ratio that could be changed by changing the compression stroke. That's usually the ideal engine for engine engineers. But as I continued in my research I realized that an engine that uses the same linkage assembly to realize a higher expansion ratio would be more advantageous for use in power products, so I switched course."
Shohei Kawano worked with Watanabe on research for a new linkage engine with a high expansion ratio. For Kawano, who had just joined the company, this project was his first in engine development.
"We built the first engine by modifying an existing engine. We proved that the concept was feasible, but the engine failed to pass later tests. So we built another engine from the ground up, positioning the joints in what seemed, to us, like the best places. That engine turned out to be super loud. It was so loud we had neighbors calling to complain even though we were running it in a sound-insulated room! [laughter] That's how it got the name 'the jackhammer.'"
Just a minor imbalance can transform a linkage engine into a "jackhammer." The various characteristics of the parts that make up such an intricate assembly—angle, length, weight, materials, and so on—influence each other in complex ways.
"Different parameters of the assembly parts, such as angle and length, can drastically change the personality of the engine, even when keeping the expansion ratio the same. With an unlimited number of combinations of these parameters, it was overwhelming trying to find the right combination that would make the engine efficient and run quietly."
After many long hours of building prototypes and making improvements, just when the team finally saw the goal of their research come into view, good news arrived.
"We received an order from upstairs saying they wanted to use the linkage engine in the next cogeneration unit model," said Watanabe.
The cogeneration unit development team sensed that they had reached the upper limit of efficiency with the configuration of engines they were currently using. To aim higher they would need a technological breakthrough, one based on a different principle or much more advanced technology. That was when they heard that basic research on an ultra-efficient engine with a high expansion ratio was reaching completion. "Our linkage engine enabled a dramatic improvement in efficiency—in other words fuel economy—over existing engines," said Watanabe. "The only drawbacks were that it was larger and heavier due to its complex linkage assembly, and that it was ill-suited to running at high speeds. In a cogeneration application, however, we could rule out these drawbacks because the engine would be stationery and run at a constant speed."
"Our cogeneration unit and the linkage engine were a match made in heaven," said Togawa.
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