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The Skyactiv D engine is a big departure from conventional diesel technology. While it is a 2.2 liter 4 cylinder turbo diesel much like its predecessor, the MZR-CD. It differs by producing about 20% more torque over a wider range than most diesel engines and revving higher, to an amazing 5200 rpm. The result is that the Skyactiv D feels more like a gasoline engine than a typical diesel.
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| The current diesel crank is beefy and heavy, designed to take the pounding of 16:1 compression. It has big journals, big bearings and a decent amount of journal overlap. All stuff for strength but it is heavy and contributes to increased friction. |
The comer point of the Skyactiv D engine like the Skyactiv G engine is its compression ratio. If the Skyactiv G engine is revolutionary for its high 14:1 compression in a gas engine, the Skyactiv D is also revolutionary for having the lowest compression ratio in a production diesel at 14:1.
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| The Skyactiv D crank is much lighter, the crank counterweights are pared down. The bearings are much smaller in diameter and narrower. All of this reduces reciprocating weight and friction. A lower compression ratio allows a safe reduction in mass. This means more power, better response and better fuel economy. |
For example the compression of our Golf TDI is 16.5:1. Lower compression means a lower peak cylinder pressure of 1860 psi vs around 2430 psi for a TDI. This means that the Skyactiv D engine can use a lightweight alloy block and have lighter internal parts for less reciprocating mass, about 10% less than a typical diesel, in fact the Skyactiv D is 28 kg lighter than the engine it replaces. The lightweight reciprocating parts and the reduction of pressure help result in an overall 20% reduction in friction.
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| It is obvious that the Skyactiv D rod is on top. You can see the much smaller bearing journal diameter and the much smaller piston pin. Extensive use of FEA has pared much of the weight from the rod while still allowing it to be strong enough to sustain a much higher redline. Not having to live with such a high compression ratio helps as well. |
The lower compression ratio is the heart of Skyactiv D technology. With a lower compression ratio the combustion event can be controlled more precisely with no pre-ignition. With a lower compression ratio there is more time to form a homogeneous mixture inside the combustion chamber. With no pre-ignition, ignition is controlled with injection timing. Less compression allows more advance before top dead center injection timing increasing the engine's efficiency by improving the expansion ratio. A conventional high compression diesel injects fuel after top dead center to reduce NOx which results in poor fuel economy.
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| The Skyactiv D piston on the right is obviously much lighter with strut type construction much like a modern racing piston. You can see the larger and deeper dish for lower compression and the anti friction coating on the skirt. The unusual dome configuration is to help promote swirl and better direct injection mixing of the air and fuel. |
