Development of the two-cycle high-speed supercharged engine used in the Duesenberg racing cars that competed in the 1926 500-mile Indianapolis race is described. Excessive trouble in starting the engine, which requires considerably higher supercharger pressure than does the four-cycle engine, inclined the designer and builder to abandon the idea of using this type of engine in the race. The supercharger gear-ratio was not laid out for the extreme speeds and, as the engine speed is directly proportional to the supercharger pressure, the pressure was too low to give the desired speed. The speed and efficiency of the engine were increased by increasing the width of the cylinder intake-ports, but the author believes a supercharger pressure of 15 lb. per sq. in. is necessary for good operation. Best results were obtained when the rotary valve opened the intake valve after the exhaust ports had been opened about 3/16 in. As the pressure of the supercharger increased, better results were secured by setting the rotary valve to open the intake ports still later.
Although a number of piston-rings broke and pulled pieces out of the pistons when the car was first run on the Indianapolis Speedway, the engine continued to run, and the author concludes that, even in its imperfect state, the engine is not so delicate as a four-cycle engine and will run better, even though many things may be wrong with it, than will a four-cycle engine with only minor things wrong. The timing was unexpectedly sensitive, resulting in “popping-back” when the intake valve was opened a little too soon and in insufficient mixture charge in the cylinder when the valve was opened a little too late. It is believed this trouble can be controlled by the pressure of the supercharger.
By admitting cooling water around the exhaust port and then circulating it around the spark-plugs, steam pockets were avoided, the engine ran cooler, and the spark-plugs were much cooler than in the four-cycle engine.
Discussion of the paper concerns the amount of engine power required to drive the supercharger, speed of the supercharger, number of piston-rings used on the pistons, the maximum engine-speed, the gear-ratio of the car, the cylinder compression-ratio, the engine idling-speed, the ignition firing order, spark-advance, type of spark-plug used, relative size of supercharger used on the two-cycle and the four-cycle engine, lubrication of the two-cycle engine, location of the rotary valve in the cylinder-head instead of on the side, flexibility or efficient speed-range of the engine, the use of high-speed engines in future private passenger-cars, type of crankshaft used in the Miller racing-car engine, maximum speed of the Miller engine and supercharger, the supercharger drive, probable future of front-wheel drive in passenger-cars, unsprung front-end weight with this type of drive, experiments with worm-drive in racing cars, the use of “doped” fuels, battery versus magneto ignition, steel used in the steering-arms and steering-knuckles, and the like.