The Boxer engine

Karl Benz

The flat engine, better known as the boxer engine widely used by VW, Porsche and Subaru, is an internal combustion engine with multiple pistons that move horizontally. Generally, the engine has cylinders arranged in two banks on either side of a single crankshaft. The Karl Benz (as in Mercedes-Benz) patented the concept back in 1896.

Boxer engines got their name because each pair of pistons move simultaneously in and out rather than alternately and in doing so, mimic boxers showing they are ready by touching their gloved fists against each other before a fight.

The boxer engine with up to 8 cylinders has proven highly successful in cars, and up to six cylinders in motorcycles (pictured L: 1954 BMW boxer motorcycle engine) and even light airplanes (pictured R: UL260i flat 4 aircraft engine).

The boxer type is one of three cylinder layouts that have a natural dynamic balance; the others being the straight-6 (six pistons moving vertically) and the V-12 (twelve cylinders, six moving on each side of a "V"). The advantage of the boxer's natural dynamic balance is that it runs very smooth and free of unbalanced forces with a four stroke cycle. Also, they need no balance weights on the crankshaft, which is lighter and fast-accelerating. As a consequence, they have a characteristic smoothness throughout the rev range and offer a low center of gravity. On the other hand, they tend to be noisier.

Below there is a short video on the inner workings of a boxer engine. Be sure to watch it to the end because mid way through, it brakes the engine down to the crankshaft and starts adding the pistons, cylinders, spark plugs, intake and exhaust valves.

Below you can see a 30 second animation of the flat 12 cylinder engine that powered the legendary Porsche 917. This is NOT a boxer but a 180 degrees V-engine.

Even though the basic concept dates back to the nineteenth century, the Boxer engine continues to be perfected to this day.
In 2009, Porsche was awarded the prestigious "Best New Engine Award" for its Carrera S 3.8-liter flat-six boxer engine which delivers more power and increased fuel-efficiency. The award is bestowed by the Engine Technology Int'l (journal) for outstanding engine characteristics in applicable technology. It is pictured on the right.

Source: Porsche, Wikipedia.
Animations courtesy of: mekanizmalar.com and mwp917.