It was ten o'clock in the evening. Rudolf Diesel had finished dinner and retired to his cabin aboard the SS Dresden, traveling from Belgium across the English Channel.

His night clothes were laid out on his bed, but Diesel didn't change into them. The inventor of the engine that bears his name was thinking about his heavy debts and the interest payments that would soon come due. He couldn't afford it.

In his diary, that date, the 29th of September, 1913, was marked with an ominous X.

Before the trip, Diesel had gathered what cash he could and stuffed it into a bag, together with documents laying bare the financial mess he was in.

He gave the bag to his wife, with instructions not to open it until a week had passed. She seems not to have suspected anything.

Diesel stepped outside his cabin. He removed his coat, folded it, and laid it neatly on the ship's deck.

He looked over the railings, for the black and swirling waters below. And then he jumped. Or did he?

Conspiracy theorists have speculated that Diesel was assisted over board, but who might have had an interest in the impercunious inventors demise?

Two possible candidates have been fingered.

For context, rewind another 20 years. To 1872, and industrial economies where steam supplied the power for trains and factories, but urban transport depended on horses.

At autumn, equine flu brought US cities to a standstill. Grocery store shelves were laid bare, saloons ran out of beer, garbage piled up in the streets.

A city of half a million people might have 100,000 horses, each one liberally coating the streets with 15 kilos of manure and four litres of urine every day.

An affordable, reliable, small-scale engine that could replace the horse would be a godsend.

The steam engine was one candidate. Steam powered cars were coming along nicely.

Another was the internal combustion engine, early versions of which ran on petrol, gas, or even gunpowder.

But when Rudolph Diesel was a student, both types of engine were woefully inefficient. They converted only around 10% of heat into useful work.

The young Diesel's life was changed by a lecture on thermodynamics at the Royal Bavarian Polytechnic of Munich, where he learned that it was theoretically possible to make an internal combustion engine that would convert all heat into work.

Diesel set himself the task of translating theory into practice.

He fell short. His first working engine was only just over 25% efficient. Today, the best diesel engines top 50%. Even so, it was more than twice as good as what had come before.

Unfortunately for Rudolph, in early versions of his engine these efficiency gains were outweighed by reliability issues. He faced a steady stream of refund demands from unhappy customers.

It was this that dug the inventor into the financial hole from which he never managed to escape.

Still, he kept working at his engine and it kept getting better. Other advantages became apparent.

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