Invention of the boxer engine: The Benz “contra engine” of 1897
Stuttgart
Feb 01, 2007
  • Karl Benz develops a flat two-cylinder engine
  • Horizontally-opposed pistons ensure a good balance of masses
  • Power output increased from 5 hp (1899) to 20 hp (1902)
In 1897 Karl Benz developed the boxer engine. This drive system, in which two horizontally-opposed cylinders turned a single crankshaft, was given the name “contra engine”. The unit was used from 1899 onwards, principally in passenger and racing cars.
In 1897 cars were still a rarity on the roads of Germany – after all, it had been only eleven years since the invention of the automobile. Nevertheless, those who did own cars already had heightened expectations for the performance capabilities of their vehicles – a detail Karl Benz was quick to pick up on. In 1896 he established a dedicated department of motor vehicle engine design at his Mannheim company and set about developing efficient multi-cylinder engines. His first design in 1897 resulted in the prototype for a two-cylinder unit, with cylinders coupled in parallel. This “twin engine” was similar to the in-line engines being developed by other manufacturers of the day.
But it was not long before Benz & Cie. introduced a new engine concept, in which the cylinders were positioned in an arrangement opposing one another. Over the next two years Benz’s engineers developed this so-called “contra engine” to production standard and as early as 1898 were installing a 4.2-liter variant in buses. The fully matured 1.7-liter and 2.7-liter engines, with outputs of five hp (4 kW) and eight hp (5.9 kW) respectively, finally made their debut in the Benz Dos-à-Dos of 1899.
Balanced masses for smoother running
The contra unit was the first boxer-type engine in automotive history – and it found application in passenger cars, commercial vehicles and racing cars. This new drive concept offered several advantages over in-line engines. Above all, with the cylinders set at 180 degrees to one another, dynamic masses were much better balanced, since the timing of the pistons in such an opposed configuration meant that any momentum produced by the power/intake strokes and the exhaust/ compression strokes was effectively counterbalanced by the corresponding piston movement of the opposite side. This piston movement was possible thanks to a double-offset crankshaft, due to which the two cylinders were slightly offset within the open-topped engine frame. This configuration also allowed for a compact and flat design of the boxer engine.
Back-to-back
In addition to the new engine, Benz also came up with a new body concept in 1899, the Dos-à-Dos – a vehicle in which driver and front passenger sat back-to-back with the other passengers. But that same year also saw the appearance alongside the Dos-à-Dos of the Benz Mylord (two rows of seats facing the direction of travel) and the Benz Phaeton (three rows of seats, of which two faced the direction of travel). Benz also presented a twelve-seater Break variant in 1898, fol-lowed in 1899 by an eight-seater Break (a Break’s bodywork could be seen as a precursor of the station wagon).
Following the premiere of the contra engine, power output increased steadily. Whereas the five-hp engine of the Dos-à-Dos initially remained unmodified, the larger variant with a displacement of 2,690 cubic centimeters delivered first eight hp (5.9 kW) in 1899, then nine hp (6.6 kW) in 1900 and finally ten hp (7.4 kW) in 1901. By 1902, with an increased engine speed of 980/min instead of 920/min, the tried-and-tested engine finally achieved twelve hp (8.8 kW). In that same year output of the smaller 1,710-cc engine was increased to nine hp (6.6 kW). And two further new variants were used in the Benz Tonneau and Phaeton models – the 2,940-cc engine with 15 hp (11 kW) and the 3,720-cc unit with 20 hp (14.7 kW).
Moreover, by 1898 a Benz boxer engine with a 4,245-cc displacement was already powering the twelve-seater Break, delivering between 13 hp (9.6 kW) and 15 hp (11 kW). Production of this engine was discontinued in 1900, however, and in 1902 the Benz Ideal was fitted with a contra engine with 2,090-cc displacement and an output of eight hp (5.9 kW) at 1000/min. Predecessor models of the Ideal were still equipped with a single-cylinder engine.
In the field of commercial vehicles, the contra engine also proved itself to be a reliable drive system for the most powerful of the new truck models introduced by Benz in 1900. The five-ton vehicle ran on iron wheels and was powered by a contra engine with 14 hp (10.4 kW). In 1898 Benz sold the twelve-seater Break as a bus in England, but the vehicle never really caught on in Germany.
Racing applications
The boxer engine not only enjoyed success in passenger cars and commercial vehicles. It also proved itself on the race track. The brand’s first ever vehicle built specifically for motor sport competition was the eight-hp Benz racing car introduced in 1899. That same year Fritz Held drove it to a class victory in the Frankfurt – Cologne long-distance race over 193.2 kilometers, completing the distance at an average speed of 22.5 km/h. Held was awarded the principal gold medal, Emil Graf finished runner-up driving an identical vehicle. With engine output increased to twelve hp, Fritz Held also finished second in the 1899 Innsbruck – Munich race. And Fritz Held and Richard Benz also finished with the fastest driving time to take overall victory on the Berlin – Leipzig route.
The next Benz racing car, built in 1900, was based on the successful 1899 vehicle. The contra engine now delivered 16 hp and featured central lubrication, water pump and a gear transmission with four forward speeds and one reverse gear. Weighing in at 1,150 kilograms, the car cost approximately 15,000 Marks. Its first outing resulted in a second place for Fritz Scarisbrick at the 136-kilometer Eisenach – Oberhof – Meiningen – Eisenach mountain rally. Clocking a time of four hours and 23 minutes, Scarisbrick finished with an average speed of 30.1 km/h and recorded a top speed of 65 km/h.
The final evolutionary stage of the Benz racing car equipped with a contra engine was the 20-hp Benz vehicle introduced in 1900. Whereas the two previous models both had a two-cylinder boxer unit, this vehicle – developed by Georg Diehl – featured a four-cylinder boxer with a 5,440-cc displacement. It was the first Benz automobile to feature an inclined steering column. With this car Mathias Bender won the international circuit race in Frankfurt on July 29, 1900, completing a distance of 48 kilometers at an average speed of 47.5 km/h. Runner-up was Fritz Scarisbrick in the 16-hp race car.
Return to the in-line configuration
The era of Benz’s contra engine finally came to an end in 1902. From now on Mannheim vehicles were equipped with a twin-cylinder engine or an in-line four-cylinder version derived from it. But the boxer engine developed by Karl Benz remains a glorious episode in the history of engine development at Benz & Cie.
Model overview: Benz vehicles powered by the contra engine
Passenger cars

Benz Dos-à-Dos, 1899-1900:
1710 cc, 5 hp (4 kW) at 940/min

Benz Dos-à-Dos, 1899:
2690 cc, 8 hp (5.9 kW) at 920/min

Benz Mylord, 1899:
2690 cc, 8 hp (5.9 kW) at 920/min

Benz Phaeton américain, 1899:
2690 cc, 8 hp (5.9 kW) at 920/min

Benz Dos-à-Dos, 1900:
2690 cc, 9 hp (6.6 kW) at 920/min

Benz Mylord, 1900:
2690 cc, 9 hp (6.6 kW) at 920/min

Benz Phaeton américain, 1900:
2690 cc, 9 hp (6.6 kW) at 920/min

Benz Spider, 1900-1901:
2690 cc, 10 hp (7.4 kW) at 920/min

Benz Tonneau, 1900-1901:
2690 cc, 10 hp (7.4 kW) at 920/min

Benz Dos-à-Dos, 1901:
2690 cc, 10 hp (7.4 kW) at 920/min

Benz Mylord coupe, 1901:
2690 cc, 10 hp (7.4 kW) at 920/min

Benz patent motor car Ideal, 1902:
2090 cc, 8 hp (5.9 kW) at 1000/min

Benz Tonneau or Phaeton, 1902:
1710 cc, 9 hp (6.6 kW) at 920/min
Benz Tonneau or Phaeton, 1902:
2690 cc, 12 hp (8.8 kW) at 980/min
Benz Tonneau or Phaeton, 1902:
2940 cc, 15 hp (11 kW) at 1020/min
Benz Tonneau or Phaeton, 1902:
3720 cc, 20 hp (14.7 kW) at 980/min

Racing cars

8-hp Benz, 1899:
2280 cc, 8 hp (5.9 kW) at 750/min

16-hp Benz, 1900:
2280 cc, 16 hp (11.8 kW) at 1000/min

20-hp Benz, 1900:
5440 cc, 20 hp (14.7 kW, 4-cylinder boxer)

Commercial vehicles

Benz Break (eight-seater), 1899-1901:
2000 cc, 8 - 10 hp (5.9 - 7.4 kW) at 920/min

Benz Break (twelve-seater), 1898-1900:
4245 cc, 13 - 15 hp (9.6 - 11 kW) at 820/min

Benz five-ton truck, 1901:
14 hp (10.4 kW)
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