In an earlier issue, I looked at the fascinating early history of spark-ignition engines, and explained that engines could be classified as either employing internal or external combustion, according to whether the fire was inside or outside the engine. A diesel engine is an internal combustion engine, but it’s not exactly clear what is meant by the term “diesel engine”, as we shall see…
Early Compression-ignition engines
In the beginning, all successful internal combustion engines were fuelled by gas, and ran on a two-stroke cycle that did not have a compression phase. After Otto invented the four-stroke cycle (see CAR Dec 2007) that included a compression cycle, these engines were also produced in four-stroke form. Some inventors tried to run such an engine on heavy oil, that is oil with a higher flash point than petrol, because it was far cheaper than petrol. In 1885, Priestman of Hull started to sell such an engine that utilised a spray vaporiser and electric ignition.
In 1890, Stuart Ackroyd patented a fourstroke oil engine that was able to run without needing a spark. This engine inhaled air into a bulbous projection that communicates with the combustion chamber via a passage, sprayed fuel into it during the intake stroke, and then compressed the mixture. It relied on the heat in the vaporiser, as well as from the compression, to ignite the mixture. The bulb had to be heated from the outside to get the engine started. It was relatively simple to build and cheap to run, and thousands were built by Hornsby, under the Hornsby-Ackroyd trade name. Later, similar engines were produced by a number of companies, and until WW2 such hot-bulb, or semi-diesel, engines were very popular, in two- and four-stroke form, for use in fishing boats and for stationary duties.
Lanz Bulldog tractors were fitted with a horizontal single-cylinder hot-bulb engine of up to 10 litres, and soon attracted a cult following. These engines could run on any combustible fuel, required very little maintenance, but were seldom produced with more than one cylinder, because the crude fuel supply did not lend itself to the fine tuning that a multi-cylinder engine would need.
In addition, most of these were slow-speed engines, with maximum revs limited to about 250 r/min. The fact that they could only be started after heating the vaporiser bulb from the outside by a blowlamp was not a problem, because most of these engines were used in situations where they would run for hours or even days on end.
Life of Rudolf Diesel
Dr Rudolf Diesel was born in Paris from Bavarian parents. They were both from Augsburg, but met and married in the French capital. In 1869, France declared war on Prussia, and the Diesels were ordered to leave the country. They first went to England, but Rudolf was soon shipped to a cousin in Augsburg, where he continued his schooling. Later, he attended what today is called the Technische Universitat Munchen on a scholarship, studied engineering, and passed with distinction. He was fluent in German, French and English, and also had considerable artistic talents.
Carl von Linde, the head and founder of the thermodynamics laboratory at this university, exerted a major influence on the young Diesel. On returning from a trip to Pinang Island in Southeast Asia, Von Linde gave a talk at the University, and casually used a present he was given on the island to light his cigarette. This was a so-called fi re pistol, which had been in use for centuries on some of the islands. It consists of a small rod, with a knob at one end, fitting snugly into a tube containing tinder at the other end. If the rod is pulled outwards, and the knob then given a sharp blow, the air inside is compressed to a temperature high enough to set the tinder alight. This fascinated Rudolf, and he realised that this phenomenon could be used to ignite the mixture inside an engine.
In 1879, Von Linde, who achieved fame by developing a process to liquefy air, retired from the university, and founded what today is known as Linde AG, a large conglomerate. After graduation, Diesel first worked for Sulzer Brothers, a famous Swiss engineering concern, but after two years Linde asked him to be the French representative for his company, so he returned to the city of his birth and set up a workshop in Paris to experiment with engines.
He studied Sadi Carnot’s groundbreaking 1824 booklet on basic thermodynamic principles, read Otto’s technical papers, and realised that Otto had limited his engine’s efficiency by premixing fuel and air, resulting in a limit to the compression and expansion ratios. His answer was to add the fuel only later, just before it was ignited. Linde transferred him to Berlin in 1890, where he continued to study engine design. In 1892, he published a paper on his work and applied for a German patent, which was granted to him early in 1893 provided he could build such an engine. He decided to devote all his time to the engine, resigned from Linde AG, and moved to Augsburg, where his work was funded by Maschinenfabrik Augsburg-Nuremberg (MAN after 1904) and Sulzer. He first tried powdered coal as a fuel, using compressed air to help the injection. When this did not work, he reverted to fuel oil, and finally built a working engine.
On February 17, 1894, his first compression- ignition engine ran for a minute, developing about 13 kW at 154 r/min. It was a single-cylinder four-stroke direct injection unit, running at a thermal efficiency of 26 per cent, more than double of what a steam engine could achieve. He produced a number of improved engines while at MAN, and they were all much more efficient than any other kind of engine. This enabled Diesel to sell patent rights for building his engines in all parts of the world, in spite of the fact that these first engines were slow (less than 200 r/min) and still used compressed air to inject the fuel.
In 1913, tragedy struck. Rudolf went missing during a cross-channel crossing. Conspiracy theories point to a number of possibilities, but most historians agree that he committed suicide, most likely because he spent money faster than he could earn it, and he was bankrupt. In addition, the expiry of his patent rights meant that less money could be expected in the future.
Diesel engine development
Sulzer started to build diesels in 1898, the same year as the first British diesel, built by Mirrlees, saw the light of day. In 1902, the first American diesel was built by Aldolphus Busch, a German immigrant, who founded a company called Busch-Sulzer Bros to build diesel engines. Over 100 of the first design – a three-cylinder 55 kW unit – were sold, and the company went on to produce Busch- Sulzer engines for the American Navy during WW1, as well as afterwards. Busch also started another company that achieved fame as the producer of American Budweiser beer. By 1906, Sulzer was building 12 different engines, with outputs ranging from 11 to 440 kW. One of them was the first two-stroke, reversible, diesel engine. The latter feature is very useful as a means of slowing down a ship.
Until 1909, all diesel engines were of the direct-injection type, meaning that the fuel was injected directly into the combustion chamber on top of the piston, but in this year Prosper l’Orange, a Lebanese engineer, designed a pre-chamber diesel engine. This, the first indirect-injection diesel engine, was smoother and quieter than other diesels, and soon became a popular alternative, despite a decrease in efficiency. With the passage of time, indirect-injection predominated, spurred on by a number of different pre-chamber designs. but direct-injection later made a comeback, as we shall see. Diesel engines also started to get bigger and bigger, especially for marine applications.
In 1912, Burmeister and Wain of Copenhagen built the first ocean-going diesel motor-ship, the MS Selandia. It was fitted with two four-stroke engines each developing 932 kW at 140 r/min. Soon afterwards, the cargo liner MS Monte Penedo became the first ship to be powered by a two-stroke diesel engine. The first diesel locomotive was built in the same year by Krupp, using a Sulzer engine. In the same year, the diesel patents expired in the USA, and new engines were produced by a number of companies.
Another development, dating from the early-’20s, was the Ricardo Comet combustion process, which promoted swirl inside the chamber by utilising inclined ports and a small cylindrical combustion chamber. It rejuvenated direct injection, and since that time both kinds of engines have been designed. In the last ten years, direct-injection has made a serious comeback, no doubt helped by the modern ability to use multiple injection squirts to quieten the diesel combustion noise.
Until 1922, all diesels were heavy slow speed units, because of the need for a compressor to supply the air used to inject the fuel, but the invention of an airless injector by Karl Bosch in this year changed all that. Engines designed after this development were lighter, and revved higher than before, so it was no surprise when, in 1924, three companies exhibited the first diesel trucks at the Berlin Motor Show. The Benz had a prechamber engine, the MAN had a direct injection engine, and the Mercedes was fitted with a compressed-air injected engine.
In the years since then, the diesel concept has been successful in anything from small stationary units to large marine engines, in outputs from a few to 100 000 kilowatts. It has even been fitted into aircraft. Twostroke versions do not employ crankcase compression, but have exhaust valves in the combustion chamber, and inlet ports part way down the bore. These are uncovered by the downward piston movement, and are usually connected to a suitable blower that forces air into the space above the piston, thus scavenging the spent combustion products through the open exhaust valves. This relatively good breathing means that two-stroke diesels tend to develop about 70 per cent more power than the same-sized four-stroke units, making them ideal for large power outputs.
Earlier diesels utilised a number of plungers, driven by the engine, to distribute fuel to the injectors. The long pipes from plunger to injector, as well as the mechanical fuel measuring arrangement, often resulted in unequal fuel delivery to the injectors. This system has now been replaced by the common- rail design, a manifold containing fuel at high pressure, situated close to the injectors, that is able to supply the same pressure to all the injectors. The latter is operated by electronically-controlled solenoids or piezo-electric units, resulting in more precise control of the amount of fuel being delivered. The system is even able to deliver more than one squirt within milliseconds of each other to optimise the combustion process, and reduce engine noise.
Turbochargers are very much part of modern diesel engines, and have only really come to the forefront in the last 30 years, but the idea was patented in 1915 by a Swiss engineer, Dr Alfred J Büchi, while he was chief engineer at the Sulzer Brothers research department. The idea was not well received by the conservative European engineering community, but by the late-’20s a number of marine and locomotive diesels were appearing with turbochargers to improve their outputs. The first turbodiesel truck was produced in 1938 by Saurer of Switzerland, and today very few large diesel engines are produced without one or more turbos. The idea has lately spread to automotive diesels.
Who invented the diesel?
The claim has often been made that Stuart Ackroyd invented the diesel engine. He certainly invented the first successful compression-ignition engine, but Dr Diesel’s engine contains a sufficient number of differences to be considered unique:
- Diesel engines have compression ratios of anything between 12:1 and 22:1, whereas a hot-bulb engine runs at about 3:1.
- Diesel fuel is injected at the end of the compression stroke, instead of during the intake stroke.
- A diesel relies entirely on the heat from the high compression ratio to ignite the fuel, but a semi-diesel also needs the heat from a hot bulb.
- The efficiency that results from the higher compression ratio and more precise mixture control of a diesel, has meant that a well-designed diesel engine has an efficiency of over 40 per cent, against the less than 10 per cent of the hot-bulb engine.
In conclusion, one of the most amazing facts about these early engines is that most of them still exist, either in the Munich Science and Technology museum, or in the museums created by Mercedes-Benz, MAN, Sulzer and other pioneer companies.