ALL IN THEFAMILY

weight of the SD pump; but on the minus side were increased manufacturing costs and a general lack of flexibility in tuning for different applications. It also became a service and warranty nightmare.

Turbos and Pump Changes

In 1954, the NH made a transition in two areas. The first was to a new injection system called PT (for “Pressure-time”), and the second was the introduction of turbocharged engines.

The PT fuel system was a significant upgrade, and all engines were eventually updated to use it. In a strange twist of fate, Cummins’ PT system actually predated the DD, having started development late in 1941, with Knudsen’s DD starting a little later. Internal politics led to the DD moving ahead to production and the PT moving to the back burner. The PT went on the front burner only after the DD system nightmare began.

PT was the industry’s first common rail system— more or less in the way we understand it today. It used a single high-pressure pump, with line pressure controlled by the accelerator pedal. The injectors were the timing and metering part of it. They were actuated via pushrods from special lobes on the camshaft.

In 1954, the turbocharger was just beginning its market dominance, and Cummins engines began having a turbo application at this time, including the NH. With the turbo, the designation became NT or some variation. The HB and HBS-400 continued, although sales were more limited. By this time, the market for the four-cylinder H diesel had diminished. However, they found a stable market into the 1970s in the oilfields, fueled by natural gas.

The Model H progeny continued to 1960, when there was another big evolution and the birth of another Cummins legend. (But that’s for next month. Stay tuned!)

 Clessie Cummins (1888–1968) as an inspired 45-year-old in the early 1930s and as an equally inspired 79-year-old man in 1967—still fleshing out ideas in his home shop. (Source: Cummins Historical Collection)  The PT pump debuted in 1954 as what we would now call a “common rail” system. It was a much simpler system than either the SD or DD and proved superior to both. It also had some teething problems ... but those were worked out.

 This exploded view can give you an idea how the SD pump worked.