A newly published patent application details how to apply the efficiency tech to longitudinal engines.

GM engineers spend a lot of time thinking about engines, and the company’s patent-to-production pipeline is hard evidence. Cylinder deactivation and sliding camshafts? GM has brought its own versions of both to production. Making truck V-8s bigger and more powerful without completely sacrificing efficiency? Coming right up. Their next target may be applying existing fuel-saving tech to a wider array of engines.

A few engineers in particular have been tinkering with ways to adopt a variable-compression ratio, and specifically how the necessary hardware can be packaged with a longitudinal engine. Related patents go back about a decade. (Nissan already sells a variable-compression engine of its own, the somewhat disappointing VC-Turbo, but that one is designed for transverse front- and all-wheel-drive applications.)

In a previous patent filing, GM outlined how it would locate the control shaft, necessary for varying compression in its design, up above the crankshaft. The compression variability—approximately 7:1 to 11:1—comes by way of a special connecting-rod linkage. The connecting rod attaches to one end of a center link that’s mounted on the crankshaft, and the other end of the center link connects through a control rod to the control shaft. The design also includes a phaser that adjusts the control shaft’s rotational timing relative to the crankshaft, thus controlling the piston’s movement and varying the compression.

And one bonus compared with Nissan's system, which uses valve timing to achieve the Atkinson-cycle effect, GM’s arrangement would allow for the expansion stroke to be longer than the compression stroke—up to 40 percent longer—which has potential efficiency benefits.

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