During the 17th century, scientists and engineers began exploring the concept of internal combustion engines in earnest. For example, innovators such as Samuel Morland - an Englishman born in 1625 - reportedly described the concept of a ‘gunpowder’ engine to drive a water pump as early as 1661.
Many subsequently began exploring how expanding gases and steam could be used to drive pistons, converting chemical energy into useful mechanical energy. As more time, money and research was sunk into this promising avenue, the understanding of gases and combustion processes advanced throughout the 18th century.
By the early 1800s, the first prototype internal combustion engines - such as the ‘Pyréolophore’ of 1807, built by French engineers Joseph and Claude Niépce - began clattering into life. This early engine relied on a blast-air injection system; powdered fuel would be dispensed in front of the outlet of a bellows, the air from which would blow the fuel-air mixture into the combustion chamber.
A burning fuse would then be mechanically introduced into the fuel-air mixture, which would promptly combust and drive the motor. This engine was tested in a boat and, later, was used to develop the concept of fuel injection.
Before long, a plethora of engines were being experimented with and development continued apace. These early engines were incredibly finickity and inefficient, though; getting the fuel into the combustion chamber was proving to be a fine art - be it using an injection system or carburetion - while triggering the required combustion cycle was even more complicated.
For example, Nikolaus Otto’s early engines - which showcased the new ‘Otto’ four-stroke cycle - relied on an external naked flame for ignition. The variable quality of fuel available similarly meant that these engines were often inefficient or unreliable.
Consequently, most businesses and industries stuck with the tried-and-tested steam engine; the technology was well understood, proven and reliable. It was not, however, particularly efficient. Early reciprocating steam engines had efficiency figures as low as half a per cent and only cutting-edge, complex engines could exceed 20 per cent. They required considerable quantities of fuel as a result, which posed logistical, storage and cost challenges.
Rudolf Diesel, a German engineer studying at Munich’s technical university, was dismayed when he discovered how inefficient these engines were. He had joined the university in 1875 and was being tutored by Carl von Linde, a German scientist who specialised in refrigeration systems; during his time there, and under Linde’s watchful eye, Diesel began investigating ways improve the efficiency of these engines.
Before taking his last exams, though, Diesel fell ill with typhoid. Although he was soon to recover, Diesel missed the exam window and consequently had to find something with which to occupy himself until the next test opportunity. Fortunately, he found a job that matched well with his interests - and spent six months working for the Sulzer Brothers company, which specialised in industrial hardware and steam engines, in Switzerland.
While Diesel was readying himself for his tests, Linde left teaching and established a specialist refrigeration company in 1879. When Diesel graduated with flying colours, he was promptly employed by Linde to design more efficient and effective refrigeration and gas handling systems. In his spare time, though, Diesel continued to work on ways to improve engines.
Others had, by this point, begun focusing more on engines which did not rely on a spark or flame for ignition - instead, they would rely on heat. In 1886, English inventor Herbert Akroyd-Stuart built a prototype engine that ran on paraffin oil, for example, which is similar to kerosene.
The paraffin would auto-ignite when heated sufficiently, meaning a source of ignition in the chamber wasn’t required - greatly simplifying the engine’s design and improving its reliability. Akroyd-Stuart’s engine had a relatively low compression ratio, though, so the act of compressing the fuel-air mixture alone wasn’t sufficient enough to cause it to start combustion of its own accord.
Instead, Akroyd-Stuart integrated a small standalone chamber into the head of each cylinder, which was first heated by an external source. The temperature in this small chamber, called the ‘vaporiser’ or ‘hot bulb’, was high enough to cause the fuel to auto-ignite when injected. These oil-fuelled engines then went into production in 1891.
Similar techniques and designs were demonstrated elsewhere, with the engines relying on the heat of the chamber’s walls - or the vaporizer - to trigger the start of the combustion process when fuel was introduced. These engines were still not hugely efficient, though, due to their low compression ratios.
Diesel, who was aware that a mist of fuel injected into a hot atmosphere would combust, sought to do away with any form of pre-heating system by instead using a higher compression ratio. As the air in the cylinder was compressed, its temperature would increase. If its temperature could be raised sufficiently, it could be hotter than the auto-ignition temperature of a suitable fuel. When fuel was sprayed into the chamber, it would then combust.
This concept would allow Diesel to retain the simplicity of hot bulb engines, and do away with the vaporiser chamber itself, while resulting in significantly improved efficiencies - as the higher compression ratio would extract more energy from the fuel and bolster the engine’s thermal efficiency. A patent for such an engine was subsequently filed in April 1892, then granted in October that year; a more comprehensive patent, and a working prototype engine, followed in 1893.
Many of Diesel’s early prototypes ran on coal dust or peanut oil. However, several of the petroleum by-products from early fuel refineries - which primarily produced kerosene from crude oil - showed potential. Diesel tested many, including kerosene itself, but ultimately settled on a ‘middle distillate’ that had similar properties. This ‘gasoil’, which was removed from the middle of the distillation tower during crude oil distillation, was unwanted and inexpensive - but, when compressed, it would auto-ignite.
Numerous companies subsequently aided Diesel in the development of production engines, including German industrial firm Krupp, as they saw considerable merit in these more efficient powerplants. It would take four years to prepare a production-ready engine but, when released to the market in 1897, they quickly proved successful. Before long, the ‘diesel’ name became recognised as both an efficient compression-ignition engine and a type of fuel.
In the same year, German businessman Adolphus Busch - who in 1857 had emigrated to America at the age of 18 - bought the rights to produce diesel engines in America. He then established the Diesel Motor Company, which later became the American Diesel Engine Company; ultimately, in 1911, a new firm called the Busch-Sulzer Bros. Diesel Engine Company was formed, which was a joint venture with Swiss engineering firm Sulzer.
Other companies then began licensing Diesel’s engines, including Krupp, with their efficiency, durability and comparative simplicity gaining them much support. These inherent traits made them ideal for stationary, marine and military applications - and their appeal was boosted further by the non-flammable nature of the fuel in ambient conditions. The high compression ratios required heavy-duty construction, though, making diesels heavy and impractical for use in cars and trucks at the time.
Diesel seemingly wasn’t the only inventor who used compression ratios to trigger ignition, mind; engineer Thomas Barton had aided in the development of the Akroyd-Stuart engine and subsequently designed his own version. Barton’s engine, like Diesel’s, did away with the vaporizer entirely and relied on a high compression ratio to generate the temperatures required for auto-ignition.
Barton’s engine is reputed to have run successfully for several hours but, according to his obituary, material, financial and equipment restrictions meant that he could take the idea no further. Few records remain regarding Barton’s engine, however, and there is seemingly no indication as to when the engine first ran - so Barton and Diesel likely developed their ideas independently, although it was only the latter who would enjoy success.
Alas, the preceding Akroyd-Stuart engines would later become known as ‘semi-diesel’ - ultimately denying the inventor of the hot-bulb approach of much credit, which was a great shame considering Akroyd-Stuart had produced the first auto-ignition oil engine prior to Diesel.
Unfortunately, Diesel didn’t live to see the widespread adoption of his powerplant. In 1913, he disappeared while sailing aboard the German steamer Dresden; many theories about what happened exist but, following several financially ruinous business decisions, numerous sources state that he committed suicide.
While Diesel’s life had come to an end, his creation was just getting into its stride. Swiss engineer Alfred Büchi had invented turbocharging in 1904, as a means to recover waste energy from the engine. The aim was to significantly improve the engine’s output and efficiency and, before long, Sulzer had a division dedicated solely to developing turbochargers - kick-starting a new era of more powerful, capable diesel engines. Advances in materials and design meant the weight of the engines began to fall, too, enabling the production of the first diesel truck in 1923.
“I am firmly convinced,” wrote Diesel in 1913, the year of his death, “that the automobile engine will come - and then I consider my life’s work complete.”