Mercedes Benz

On December 16, 1883, Daimler took out a patent on his “gas engine with hot-tube ignition,” and quickly followed it with a second patent on the “regulation of the engine speed by controlling the exhaust valve.” These patents, which were granted in August 1884, were important landmarks in the development of the modern automobile engine.

From 1872, Gottlieb Daimler held the post of technical director at the Deutz gas engine factory, working together with Wilhelm Maybach, mainly on improving Otto’s four-stroke gas engines. At this stage, Daimler had probably already had the idea for a high-speed four-stroke engine using gasoline as fuel. However, only a visionary genius could have implemented the concept of powering a non-rail road vehicle using these machines. The engines of the period based on the principle of Nikolaus August Otto bear no comparison with later high-speed engines. For example, the 10-HP engine from Deutz weighed in at 4.6 tonnes, while the 20-HP engine weighed as much as 6.8 tonnes. Such engines were simply too heavy to employ as drive systems for vehicles on the roads.

In the summer of 1882, Daimler left the Deutz gas engine factory after disagreements with the management and purchased a villa in Cannstatt. Maybach followed him in September 1882. Daimler had the conservatory in the garden extended and equipped as a workshop, and it was here that the two men experimented on test engines. Maybach’s apartment in Ludwigsburger Strasse was turned into a design office.

The fact that the 1883 patents resulting from this work were granted in the name of Gottlieb Daimler, despite Maybach’s crucial innovative contribution, merely reflected common practice at the time. In his celebrated work History of the German Internal Combustion Engine, Friedrich Sass also praised Daimler as the inspired pioneer who recognized the potential of the high-speed four-stroke engine. However, he also singled out Wilhelm Maybach as a highly talented mechanic and designer, who made the vision a mechanical reality.

Daimler and Maybach agreed that they needed to build a small engine that could operate at a higher speed than the Deutz engines, since this could deliver enough power to drive a vehicle, but would be a more practical weight and have more compact dimensions. Maybach believed that reducing the dimensions of the existing Otto engine to a practical vehicle size would be the lesser problem. He compared the new engine they wished to create with the test engines (“smaller model engines”) that he had already designed in Deutz. A much trickier problem would be the search for a new ignition process that the automotive pioneers needed to achieve higher engine speeds, since Otto’s original slide ignition system permitted engine speeds of only up to 250 rpm.

A further problem was the legal situation: the four-stroke system was protected by Otto’s patent (DRP 532). In order to circumvent this obstacle, as many details as possible in Daimler’s engine would be different from the Deutz original. One of Maybach’s most important tasks between October 1882 and the summer of 1884 was to evaluate patents for engine ignition systems. He analyzed several thousand patents, most of which had been taken out by German, English, or American engineers. At an early stage, he and Daimler ruled out an electrical ignition system because of the unreliability of such systems at the time.

Finally, the designer came across a series of designs for controlled hot-tube ignition systems. These featured a hot tube that was connected with the engine combustion chamber with a slide. Wilhelm Maybach realized that these controls (just as with the slide control in the Otto flame ignition system) would limit the speed of the engine. He found his inspiration in a drawing by the British engineer Watson, who had also mainly worked on controlled hot-tube ignition systems. However, Maybach discovered a drawing in Watson’s 4608/1881 patent specifications without an ignition seal, and used this as the basis to develop his unregulated hot-tube system.

The patent specification of December 16, 1883 for the Daimler engine includes this ignition system as the second patent claim: “The ignition cap f, with a permanently open connection to the combustible mixture, is heated so that ignition occurs only at the end of the compression stroke.” A threaded flange installed close to the entry valve provided an airtight connection between the ignition cap and the cylinder head. The hot-tube ignition was used for the first time in the experimental engine of 1883. It also featured later in Daimler’s stationary engine and in the vehicle drive systems developed from it. The Daimler-Motor-Gesellschaft company would also use this ignition system in its automobile engines up until 1898.

Drawings from the foundry show that the engine delivered to Daimler in August 1883 by Kurtz had a cylinder diameter of 42 millimeters and a stroke of 72 millimeters. Maybach and Daimler continued experimenting with the engine into the winter, until they had it working reliably in December of that year. In addition to hot-tube ignition, the prototype also had a new type of valve control featuring a curved groove valve. Just one week after the patent application for the “gas engine with hot-tube ignition,” Gottlieb Daimler submitted another patent for a system to “control the speed of the engine by controlling the exhaust valve,” in order to protect this invention as well.

The patent principle is based on joint control of the intake and exhaust valves in the cylinder head using a single rod. The foot of this rod has a pin in the groove, allowing it to operate a control valve installed at the end of the crankshaft. The groove is cut into the disc and performs two intersecting curves, whose movement is traced by the pin and transferred to the control rod at the same time as the cylinder stroke. The pin moves completely through the dual curve over the course of two revolutions of the crankshaft at speeds of up to 600 rpm, opening the entry and exhaust valves in accordance with the four-stroke principle. However, if the engine speed exceeds this value, flyweights rotate a flap installed at the intersection of the two grooves. This deflects the rotor pin, so that the valves remain closed during the next revolution of the crankshaft.

This was the principle of the intermittent control system that controlled the speed of the early Daimler engines. In the final version, however, Daimler activated only the exhaust valve with a curved groove control, while the intake valve (pressure relief valve, or “snifting valve”) was automatically controlled by a vacuum.

The two patents applied for in 1883 as a result of the work on the horizontal test engine were granted to Daimler on August 4 and 27, 1884, respectively, as DRP Nos. 28022 and 28243. They laid the foundation for further innovations that Daimler und Maybach thought up over the next few years, and that finally led to the development of the “motor carriage” in 1886. Unfortunately, the original test engine did not survive: a major fire in 1903 at the DMG factory in Cannstatt destroyed this embryonic development in engine technology. The engine was later reconstructed on the basis of drawings by its inventors and from records at the foundry.