If the fuel in a cylinder could be expanded at constant pressure, it could get closer to Carnot's limit. He patented the concept in 1892, while working at the firm of the refrigeration engineer Carl von Linde in Berlin. Unfortunately, expanding at constant pressure meant that the engines had to be run at a low speed, limiting their power. Diesel insisted that they be run this way, which delayed their development. His family was never willing to accept the idea that it was suicide, but most historians seem to agree that he was despondent and suffering from a form of depression.
Most historians agree that it was likely suicide.
Diesel had had some sort of mental breakdown about 12-14 years earlier, when he was working madly to develop his engine. He had to be hospitalized and he never fully regained control over the project even after he came back to the firm. The more the engine was developed, the farther it seemed to drift from his original conception, and there was the question of whether his patents would stand up if the engine didn't work according to the theories he had advanced to justify its being patented. He was also coming in for criticism, as I mentioned, and this seemed to worsen his depression. His death seems likely to have been self-inflicted.