Performing more work with less fuel… Energy efficiency has been of prime importance in engine design, driving the development of countless new models over the years.
With the goal of taking energy efficiency to a new step, Honda developed its own linkage engine design based on Atkinson’s extended-expansion cycle engine, invented some 130 years ago. The first small engine of its type to be commercially produced, the EXlink has an expansion stroke longer than its compression stroke to realize an expansion ratio higher than its compression ratio.
In contrast to a conventional Otto cycle engine, in which the piston strokes are typically of the same length, the EXlink has expansion and exhaust strokes that are longer than its intake and compression strokes. The result is an expansion ratio that is 1.4 times higher than the compression ratio, allowing EXlink to offer lower pumping losses and substantially higher thermal efficiency than a conventional engine.
The EXlink structure
EXlink features a trigonal link that lies between the connecting rod and crankshaft found in conventional engines. The trigonal link is connected via a swingrod to an eccentric shaft, completing the extended expansion linkage design. The eccentric shaft turns at one-half the speed of the crankshaft, making possible pairs of piston strokes that alternate between short and long.
To realize a high expansion ratio, EXlink uses the short strokes for intake and compression and the long strokes for expansion and exhaust, expanding 110 cc of intake to 163 cc. The key benefit of the Atkinson cycle is taking in a smaller amount of air and fuel and performing more work with it for enhanced fuel efficiency.