4-stroke engine Design

From time to time, we’ve reported on the unique Alligator series of feet-first motorcycles designed and built by Dan Gurney’s All American Racers organization in Santa Ana, California. Early examples were powered by modified production engines but inevitably, because of Gurney’s grand past of auto racing and a life spent at the center of high-performance design, he wanted to do an engine of his own.

The work began with S&S-based V-twins with high-tech AAR heads, but we all know that narrow-angle V-twins shake, and we know they are not particularly compact. He and his merry men have therefore designed a compact big-inch (110 cubes) twin that not only cancels its own primary vibration but is designed from the start with every proven element of high-performance design.

Where did vibration go? The patented engine balances itself by having two counter-rotating crankshafts that are geared together; when the pistons are up or down, the crank counterweights are down or up, and when the pistons are at mid-stroke, the crank counterweights are going in opposite directions.

Okay, air-cooling is traditional for big twins, and we love the look, but when Harley has to water-cool the exhaust valve seats of its top twins, the party’s over. Time for water-cooling, to make it possible to make real power, reliably.

Mind you, Gurney wanted his engine to run just fine on pump gas, and he wanted it to last a long time, so killer compression ratios and otherworldly rpm from the race track were out. At a moderate 4, 200 feet-per-minute piston speed, this twin (with a bore and stroke of 5.00 in. x 2.8 in.) will peak at 280 hp at 9, 000 rpm. To make power despite these limits, the engine has to breathe at state-of-the-art level, and to that end Gurney’s long-time associate, ex-dirt-tracker Chuck Palmgren and others have developed highly optimized and rational porting. Each cylinder’s four valves are set at very low included angle to give a flat, open, and fast-burning combustion chamber.

In case you are preparing to scoff, imagining that we’re talking about a giant “GSX-R” twin with a light-switch powerband, forget that. This engine is to have the hydraulic cam phasers that have for years been ho-hum automotive technology (yes, they are now on Ducati’s 1200 Multistrada) for years. At low crank speed, the cam timing shifts to give Harley-like torque, and as the engine revs up, timing shifts to remain optimal, giving GSX-R-like top power. No more wheezing out on top as the price of big torque on the bottom. Let’s call this “having it all.”

Gurney originally wanted to use this engine in a late-version Alligator that was nearly ready. And then he got to thinking of all the other possible applications for a compact, low-vibration powerplant. Why, it could be built with two, four, six, or eight cylinders, for bikes, cars, boats, even helicopters. So, on August 11 of this year, Gurney was granted US patent 9, 103, 277 B1 for a “moment-canceling four-stroke engine.”

What’s “moment-canceling?” One of the things that strongly affects a motorcycle’s resistance to roll maneuvers (“flickability”) is flywheel gyroscopic moment. Just as the gyro moments of Yamaha’s twin-crank YZR500s canceled each other, so do those of Gurney’s new big twin.