Detroit Diesel Series 51

The Detroit Diesel Series 51 was a two-stroke diesel engine built by General Motors, Detroit Diesel Division from 1951 to 1959. Unlike other GM/Detroit Diesel engines, the 51 series designation did not refer to the cylinder displacement, but rather to the year it was introduced. Two versions were produced, an inline two-cylinder (2-51, 108 cu in (1.8 L)) and an inline four-cylinder model (4-51, 216 cu in (3.5 L)). It was a simplified version of the Series 53 that did not have exhaust valves. This was accomplished by using loop scavenging ports for both intake and exhaust along with a mechanical blower (supercharger). This engine used a dry sleeve with 9 intake ports and three exhaust ports in that sleeve. These sleeves were located in such a way that the exhaust ports lined up with exhaust ports in the block that were physically higher than the intake ports in the block. This resulted in pressurized cool air being forced through the intake ports just after the exhaust started exiting (note that the exhaust ports in the sleeves were higher at the top than the intake ports while the bottom of all ports in the sleeve are at the same height. The net compression ratio is 18:1. Unlike a two cycle gasoline engine, the crankcase was not used to draw in the fuel/air mixture. Instead, the supercharger forced air in the intake ports and diesel was injected by a camshaft actuated injector which was pressurized by a gear pump. Since the Diesel fuel is injected just before top dead center virtually no unburnt fuel is expelled in the exhaust unlike a two-cycle gasoline engine. The elimination of exhaust valves reduced the complexity and weight of the engine. The power output and rpm of this engine was controlled by governing the intake air and controlling the opening of the injectors with shutdown accomplished by completely closing off the air intake. The much lower weight of this engine compared to engines with valves made it quite suitable for applications where other motors of the same horsepower were not feasible. Its uses include: Marine propulsion, generators, pumps, air compressors, road graders and other industrial uses. It was never factory installed for automotive purposes but conversion kits and engines were made available for automotive use. This engine design did not see wide use due to not being able to reduce the exhaust noise to an acceptable level due to any exhaust restriction causing a reduction of power and efficiency. It was a very fuel efficient engine with a very high power output to fuel consumption ratio.