Classic stories
…modern diesels don’t run on the combustion cycle proposed by Dr Diesel
RUDOLF Diesel (18581913) was born in Paris from German parents and graduated with distinction as a mechanical engineer in 1880 from what is now Munich University. In February 1893, he was granted a patent for “a new rational heat engine” and was given working facilities at the Machinenfabrik Augsburg-nuremburg, today known as MAN.
Diesel struggled to get the first experimental engine to run and made many modifications. He succeeded on February 1894 to get the powertrain to idle reliably and tested it for the first time on a dynamometer in June 1895.
Early diesel engines ran at low speeds (below 500 r/min) and were used mainly in power stations and ships. They had to run in combination with an air compressor because the fuel was injected by compressed air, making them unsuitable for road transport.
In the early 1920s, the Robert Bosch company started to produce liquid-fuel injectors that were compatible with diesel engines, with the result that Benz produced a 37 kw, four-cylinder diesel and MAN a 30 kw, two-cylinder diesel, both light enough to be fitted to trucks.
THEORY
There are two ways to describe the difference between petrol and diesel engines: the former employs spark ignition and the latter compression ignition; or call the petrol engine a constantvolume combustion engine and the diesel a constant pressure combustion engine.
Petrol-engine combustion happens so fast that the combustion-chamber volume stays nearly constant while the fuel burns, giving rise to the above explanation. In his writings, Dr Diesel described his engine’s cycle as one where the combustion takes place at a constant pressure while the fuel is being injected. In fact, he said that one of the benefits of the diesel cycle is the fact that the pressure caused by the high compression ratio is not increased to any significant degree by the combustion of the fuel.
This ideal can be approximated in a stationary diesel engine running at 500 r/min, but as engine speed rises, fuel injection must commence earlier before top dead-centre. This automatically leads to a pressure rise during combustion that tends to approximate the rise that occurs in the constant-volume cycle. Modern diesels revving to 4 000 r/min are therefore operating on what is known as a mixed cycle, i.e. part constant volume and part constant pressure.