Variable Displacement Engines

Here is number three in our series of posts. The displacement of an engine refers to the total volume covered by the piston stroke inside the cylinders. Note that it does not include the heads. This is because the thermodynamic work done by the engine happens when the piston is forced down under the pressure of the hot combustion products.

Variable displacement technologies use mechanisms that can change this active piston swept volume according to the power demanded of the engine. When the engine needs less power the displacement is reduced and when the engine needs more power it is increased. It is more efficient to run a smaller engine at normal power output than to run a big engine at a bare idle. This is because the big engine has to be throttled way back and it suffers heavy frictional losses trying to suck in air. The energy lost while sucking air into the engine and pushing it back out on the intake and exhaust strokes is known as pumping loss.

The conventional way to reduce the displacement is to shut off some of the cylinders. For example, the 2008 Honda Accord V6 three, four or all six cylinders depending on the load. A management computer directs the switchover between different numbers of cylinders in use.

More advanced non-conventional techniques also exist. The Hefley engine controls the displacement by moving the average position of the pistons up and down the cylinder. To be able to do this required a complete redesign of the engine layout.

According to Wikipedia the first variable displacement engine was built over a hundred years ago (although it was a stationary engine). The first try at commercial use in cars was by Cadillac in the 1980s but failed due to mechanical breakdown being too common. Only as recently as 2004 was there mass commercial deployment of this technology. One cannot help but wonder if this fuel saving tech might have been developed and deployed a decade or two earlier if Detroit had made it a priority.

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