What is one of the most critical variables you can set when building an engine? That could be a long debate, but compression is certainly one of the main ones. It dictates not only how much power you make, but how you can make it; with or without boost. In the internal combustion engine as we know it for automobiles, compression ratio is always a fixed setting. Some modern engines can hang a valve open momentarily during a compression stroke to bleed off compression, but physically the engine is still sweeping though the same stroke. While this method burns less fuel, the same energy is being wasted to stroke that piston.
But what if you could slide the depth of Top-Dead-Center (TDC) and Bottom-Dead-Center (BDC) up and down in the cylinder, thus mechanically changing the compression ratio by adjusting the volume of the cylinder? That’s what Nissan is doing with their VC-T (Variable-Compression Turbo) four cylinder in the upcoming 2018 Infiniti QX50. It’s a new variable-compression engine that uses a multi-link rotating assembly to adjust between 8:1 to 14:1, depending on the need.
If you don’t know what a QX50 is, that’s because hot rodders wouldn’t normally take a second look at another four-banger cross-over CUV. But, this breakthrough reinvigorates the argument for the internal combustion engine in the future, as Nissan/Infiniti claims this innovation increases the fuel efficiency of their 256 hp turbo-four by 27 percent. In light use, it can raise the compression ration to gain combustion efficiency. When more horsepower is demanded, the engine can lower the compression ratio to allow more boost pressure, giving the engine incredible control over its power output not just by the throttle, but also by what combination of compression and boost it’s using.
How does it do this? In the video above, you can see that the piston and its connecting rod no longer ride directly on the crankshaft, instead they connect to a lever that’s affixed to a super short-stroke crankshaft (that begins to look like a camshaft in shape) in the center. The fulcrum, or pivot, of the lever is adjustable via an external motor, and the two crank arms you see on the lower-right.
Part 14 is the eccentric shaft that the green connecting rod rode on in the video, which the actuator (basically part 19-28) adjusts change the fulcrum of the lever which the piston rides on, which in effect adjusts TDC and BDC of the piston. Reports say that the 2.0L turbo-four will produce at least 265, which is pretty impressive given the displacement. Additionally, they claim that it only increases the weight of the engine by 22 pounds over a comparable four-banger. This also puts it awfully close to Nissan/Infiniti’s VQ V6, while assuredly offering superior fuel economy.
With the 2025 CAFE 54.5 MPG target, automakers are doing what they can eek out every bit of mileage. Hybrids, EVs, Hydrogen cars, and natural gas are all short steps, but all have relied on a traditional piston-and-crankshaft design, while others seek to remove it from the equation entirely. Other than the Wankel Rotory, this is one of the most radical departures from the norm in automotive history, and stands as an industry-first; and it’s one of the few seeking to squeeze even more efficiency from regular pump gas.
“VC-T technology is a step change for Infiniti,” said Roland Krueger, president of Infiniti Motor Company. “It is a revolutionary next-step in optimizing the efficiency of the internal combustion engine. This technological breakthrough delivers the power of a high-performance 2.0-liter turbo gasoline engine with a high level of efficiency at the same time.”
For more information, you can read Nissan/Infiniti’s Patent application here, which covers their production design.
For background reading on the multi-link, variable-compression engine, you can read Nissan/Infiniti’s earlier patent here.
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