Clessie in retirement

In his retirement, Clessie Cummins perfected several innovations that he had tinkered with throughout the years, including the PT fuel pump and the compression brake which came to be affectionately known as the Jake brake.

The PT Fuel System was the first of Clessie’s post-retirement inventions. He had been working on the design for the fuel system for years but hadn’t perfected it. The breakthrough came when Clessie realized fuel could be precisely metered by pressure and timing, rather than by individual injection events. In the PT (Pressure Time) Fuel System, fuel was delivered to all injectors simultaneously, with each injector receiving the exact amount it needed based on pump pressure and how long fuel flowed. This simpler, more reliable approach improved performance and durability and became a defining feature of Cummins diesel engines for decades.

Arguably, the most important of these innovations was the compression release engine brake, more commonly known as the Jake brake. During their 1931 cross-country trip, Clessie and his team found themselves barreling down a hill in California with no hope of braking. At the bottom of the hill a train was zooming past. Clessie and his team missed hitting the train by inches and walked away from near disaster.

Over two decades later, still troubled by his near-death experience, Clessie made it his mission to prevent runaway trucks. With their newly formed Cummins Enterprise Company, Clessie and his son, Lyle, worked tirelessly on the compression brake with little luck until they made a breakthrough in 1957. By 1959, Clessie successfully sold his compression brake design to Jacobs Manufacturing, cementing it as the “Jake brake.” Jacobs first introduced the brake at the 1960 American Trucking Association meeting in New York. The trucking industry was slow to accept this new invention, but soon it grew to be the norm. With the development of the Interstate Highway System in America, the Jake brake was a timely innovation.

In 2022, Cummins Inc. bought Jacobs Manufacturing Company bringing the Jake brake home. 

On August 17, 1968, after a day of working in his shop, Clessie passed peacefully in his sleep just 7 months before the company that bears his name would celebrate its 50th anniversary. His legacy of ingenuity and innovation lives on in Cummins today.