Fire in the Hole – Tracing the evolution of early ignition systems from open flame to spark plug

Article Richard Simonds • Graeme Morpeth • Gary Anderson

The automobile isn’t so much a single invention as it is a combination of inventive solutions to a variety of different technical problems. One critical problem was how to ignite the compressed fuel-air mixture in the cylinders. The solutions evolved as rapidly in the early years as any other aspect of the invention of the automobile.

The earliest internal-combustion engines used an open flame and a porthole in the top of the cylinder to ignite the fuel mixture. By 1886, Gottlieb Daimler and Carl Benz thought the procedure was both dangerous and difficult to control; they discovered better, but different, approaches.

Daimler’s vertical-cylinder “grandfather clock” engine used a heated platinum tube in the top of the cylinder to cause the fuel mixture to explode when it was sufficiently compressed. In a more advanced approach,  the horizontal-cylinder engine in the Benz Patentwagen used a high-voltage battery to generate a series of electrical sparks from a trembler – a device similar to an electric door buzzer. By 1893, Benz used a switch controlled by the crank, which conserved the battery and for efficiency allowed variable control of the ignition timing via a lever on the steering wheel.

At about the same time, Robert Bosch and chief engineer Gottlob Honold of Bosch’s Stuttgart-based Bosch GmbH electrical-systems company – perhaps the first auto-industry supplier – found a better solution. Bosch used a magneto – a device with a ferrite core rotating within the horns of a permanent magnet – instead of a battery to generate electricity. This low-voltage electricity was transmitted through a make-and-break device inside the cylinder that was opened and closed by a lever outside the cylinder driven by the driveshaft. As the two contacts opened and closed, a spark was created.

By 1898, the device – much lighter and faster-acting than the trembler system – had advanced to the point that a Daimler Phoenix, the first model Emil Jellinek purchased from Daimler-Motoren-Gesellschaft, was fitted with this Bosch system. The car carried out a successful five-day test run through the Austrian Alps. This was the system installed in the Mercedes vehicles that Jellinek promoted with such great success after 1901, including the Mercedes-Simplex 40HP seen here.

But if one looks closely at the photographs of our example to the left, the car in fact uses a spark plug–ignition system very similar to that used on modern vehicles. Patented for Bosch GmbH by Bosch and Honold in 1902, this method uses a distributor to control the high-voltage electric current between the magneto and the spark plug from outside the cylinder, with no moving parts within the cylinder. Maybach first used this ignition system in his 6-cylinder racecars in 1905. When a Renault racecar with the spark plug won the 1906 French Grand Prix, other manufacturers quickly adopted the system.

During the restoration of the Mercedes-Simplex 40HP illustrated here, as the intention was to use it in modern driving events at higher speeds than it was originally designed for, the decision was made to retrofit the Bosch spark plug system now seen on this engine.

1902 magneto & spark plugs: bosch media services

Engine detail showing spark plugs inserted into the top of each cylinder.

Engine detail showing the magneto and distributor