Performing more work with less fuel… Energy efficiency has been of prime importance in engine design, driving the development of seemingly countless new models over the years.
With the goal of taking energy efficiency to a new level, 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 revolutionary 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 more than 1.4 times higher than the compression ratio, allowing EXlink to offer lower pumping losses and substantially higher thermal efficiency than a conventional engine.
“EXlink,” short for Extended Expansion Linkage Engine, is a trademark of Honda Motor Co., Ltd.
A hint from the Atkinson cycle engine—invented some 130 years ago.
Serving as the inspiration for the Honda EXlink engine, the Atkinson cycle is a technology first implemented by James Atkinson some 130 years ago.
In the 19th century, Atkinson’s engine was celebrated for its excellent fuel efficiency, but due to its complicated structure, however, this engine was difficult to construct on a compact scale, and it disappeared thereafter from the main stream of engine development.
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 excentric 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 cm3 of intake to 163 cm3. The key benefit of the Atkinson cycle is taking in a smaller amount of air and fuel and performing more work with it for greatly enhanced fuel efficiency, and in EXlink it is implemented in a simple, compact design.
How EXlink enhances thermal efficiency
In an internal combustion engine, the expansion ratio determines the level of thermal efficiency. Since, in a conventional Otto cycle engine, the expansion ratio is the same as the compression ratio, enhancing fuel efficiency by increasing the expansion ratio also ends up increasing the compression ratio. The higher compression ratio results in undesirable engine knocking.*
In EXlink, however, the compression ratio is 12.2:1, which is low enough to avoid the knocking that can occur in a gasoline engine, while the expansion ratio is a high 17.6:1 to enhance thermal efficiency.
*Engine knocking is the metallic sound and vibration that occur due to abnormal combustion in the cylinder of an engine. Knocking can even lead to engine breakdown. Common causes of engine knocking are premature combustion and an overly high compression ratio.
**Pumping losses are the energy lost from moving air into and out of an engine’s cylinder during intake and exhaust.
Because of EXlink’s linkage structure, increased engine friction might seem to be an issue. EXlink, however, features technologies that help reduce engine friction and prevent fuel efficiency losses.
In a conventional engine, when the combusting air-fuel mixture puts the piston under pressure during the expansion stroke, side forces act on the cylinder wall, generating a large amount of friction. Since the amount of side force is in large degree determined by the connecting rod angle, EXlink has been designed so that the connecting rod is nearly parallel to the cylinder walls during the expansion stroke. As a result, the friction loss due to piston side forces is less than half that of a conventional engine. Even with its extra linkage expansion parts, EXlink has the same amount of engine friction as a conventional engine, thereby reaping the full fuel efficiency benefit of the Atkinson cycle.