Max Fiedler, Pioneer of HCCI Diesel Combustion
His research work was decades ahead of its time
In 1972, I met Mr. Max Fiedler, a renowned expert on combustion. He had a remarkable background, as he built grand prix racers and manufactured two-stroke marine engines in Germany during the 1920s. Back then, ignition magnetos couldn't keep up with the high RPM demands of two-cycle engines, so he developed a unique solution: an engine without an ignition system.
This innovative engine featured a 12:1 compression ratio and a 2:1 supercharging piston, resulting in an effective 24:1 ratio - quite high for its time. It also had a non-atomizing carburetor and ran well on the gasoline available. This engine was likely the first-ever Homogeneous Charge Compression Ignition (HCCI) engine, apart from the primitive hot bulb kerosene engines.
Max moved to the United States in 1928 and joined the REO Motor Car Company as a research engineer the following year. One of his projects was to design an automotive diesel engine. He based his design on an earlier model he'd made in Germany, but this time, he switched to a state-of-the-art fuel injection system. However, when the engine started, it produced such a terrible knock that the pistons melted.
Max knew there was a problem because he had previously achieved smooth combustion with the carburetor. This sparked a decade-long quest to solve the issue of diesel knock, which he finally did by 1939. His findings were published in the Journal of the Franklin Institute in January and February 1942, in an article titled "A New Theory of Combustion in Diesel Engines." Max discovered that the prevailing theory of diesel engines was flawed. It was impossible to inject fuel at high pressure into the hot air of a cylinder at top dead center (TDC) without damaging the hydrocarbon's chemical bonds. The fuel would enter the cylinder as a cohesive stream and mix poorly. Additionally, the fuel would be heated to a high temperature before forming a combustible mixture with the air, breaking the chemical bonds in the molecule. As a result, free hydrogen and carbon atoms would be released. The hydrogen would burn explosively, causing diesel knock, while the carbon would resist burning, resulting in soot in the exhaust.
During WWII Mr. Fiedler was recruited by Charles Lawrance, inventor of the Wright Whirlwind engine that powered the Spirit of St. Louis, to develop an aircraft diesel. By the time the war ended it was developing a robust 1500 hp. After the war he continued to refine his engine design to what we today would call a high compression HCCI diesel engine, a lightweight ported 2 stroke with direct injection that could turn up to 8000 RPM. Here is what he did differently, in order to avoid knock:
* Injection pressure was kept low to keep the fuel from hitting the cylinder walls and to reduce atomization.
* Injector nozzle sizes were large to pass the fuel quickly and without restriction. Vaporization must be avoided as that leads to knock. Large droplets stay cool as they evaporate.
* Injection was done very early, as soon as the piston covered the exhaust port. Duration was short, about 10 crank degrees.
* Compression ratios can then be high, from 24:1 to as much as 34:1 were successfully used.
Chemical reactions are reversible, given enough heat and pressure, therefore if the fuel is mixed with the air before it is heated, it is possible to prevent pre-ignition by compressing it faster and to a higher temperature than it can burn, known as the equilibrium temperature. Once the piston passes TDC, the temperature comes down to the burning temperature of the fuel, and will proceed at a fairly constant temperature and pressure until it is consumed- the Carnot cycle ideal. Once the fuel is consumed, the temperature drops rapidly due to the high de-compression, and the exhaust temperature will be low, about 400 F (200 C), and the noise will be quieter as well. It is important to note that the combustion temperature of a well mixed hydrocarbon is below the temperature at which NOx is formed.
We are currently upgrading our engines to high compression HCCI combustion.
To learn more, read about Max Fiedler's research into diesel knock and HCCI in his own words below: