Tuesday, October 11, 2016

Oval Piston Volkswagen Diesel Prototype

You might be familiar with Honda's incredibly exotic oval piston NR750 street bike, itself developed from GP machines that experimented with the same layout. Like me however, you'll probably never have heard of Wolfsburg's take on the concept.


Developed in 1990, the goal was to increase displacement to 2.3 liters without adding to the existing, conventional, 1.6 liter diesel four's external dimensions.

Note that the piston itself is more of an elliptical shape than that of a true oval, as was the case with Honda's bikes. Another key difference is that VW utilized only one conrod per cylinder, whereas the Japanese doubled this component.


Bore was 99.49 mm in the long direction, and 76.5 in the short. Pistons were specially developed by Mahle, but ultimately the project was abandoned. Interestingly, VW was also deep into VR6 prototype development at the time, which likely wasn't a coincidence.

One thing I've always wondered about oval piston designs is how their engineers intended for cylinders to be accurately re-bored, an inevitable need for any engine over a long enough service life.

Though I've yet to discover how it's done, presumably there are existing methods for honing non-round cylinders, as evidenced by the existence of OEM oversized replacement pistons for the NR750.

NR750 piston--note eight valve recesses and twin wrist pin bosses

If was a smoker, and if I was rich, I'd have a full set of four upturned and scattered around to collect my cigar ashes.

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