In previous charge! articles discussing the history of internal combustion, we’ve looked at everything from alternative power, to steam power, to the early seeds of the Internal Combustion Engine (ICE). So, what happened next?
We’re beginning to understand that the key point in achieving higher efficiencies with an internal combustion engine is pre-compression. So why not compress further? What prevented our inventors and engineers from pushing boundaries even more? There is one main reason, and even today it still proves a limitation for current gasoline and gas engines.
Let’s quickly delve a little deeper into the spark ignited combustion process.
As was already the case with the first Lenoir engine, the spark-ignited 4-stroke ‘Beau de Rochas’ or ‘Otto’ engines are supplied with light fuels, far before the combustion process (during the intake phase). This means that after the compression, just before sparking, the air-gas or air-gasoline mixture has had plenty of time to become perfectly homogeneous, and so ready for burning. In normal operation, once the sparkplug ignites the mixture, the fire progresses step by step, following a spherical flame front: the heat is gradually released, and the peak pressure remains within the limits given by the mechanical parts.
However, by compressing more than a certain ratio – around four times the atmospheric pressure on the first Otto engine (and even more today) – the overly high temperature increase would anarchically self-ignite the mixture and damage or destroy the engine. When this self-ignition phenomenon occurs after sparking, it’s called ‘knocking’. With this combustion mode, it is impossible to compress more than a certain ratio to further improve the efficiency brought by the compression. This is where the story of Diesel comes in.
From passionate child to celebrated inventor
Rudolf Diesel was born in Paris in 1858, and the youngster was particularly interested in technology. He used to visit the museum of the “Conservatoire des Arts et Métiers”, where among others, Sadi Carnot, Alfonse Beau de Rochas and Etienne Lenoir were associated with, and drew the newly displayed machines. Diesel received an intense education, especially from his father, a bookbinder who also created toys with micro mechanisms within the family workshop.
A student at the Königlich Bayerische Technische Hochschule München, (today’s Technische Universität München), Diesel attended lectures by Carl Linde, renowned for his important work and innovations within air liquefaction and refrigeration fields, along with being the founder of the Linde company. In 1878, Carl Linde explained to his students that steam engines can only convert a maximum of 6 to 10% of the heat energy into useful work (5% was typical at the time), but that the Carnot cycle – with two adiabatic and two isothermal transformations – allows the conversion of much more of the heat energy into work.
According to Diesel himself, this inspired the idea of creating a highly efficient engine that could work on the Carnot cycle. From this moment on, Diesel was driven by the desire to build an efficient ‘Carnot engine’. It would be small and economical (with an output of a few kW’s). Such engines would make it possible to distribute mechanical power more widely across countries, and they’d be ideal for family workshops like the one his father had back in Paris.
After his studies, Diesel started working for the Linde company in Paris. In 1889, Diesel went back to Germany and his research took him closer to realizing the ‘rational engine’, able to operate according to the ideal Carnot Cycle. The key point of his invention in its initial form was the isothermal combustion.