Two-Stroke Engines Got Powerful Thanks to Rocket Science

Highlights of the article on the science of exhaust rockets in two-stroke motorcycle engines:

  • Modern two-stroke engines have high horsepower per liter figures, thanks in part to rocket scientist Walter Kaaden
  • Although Kaaden did not invent a specific feature, he developed the modern form of the two-stroke exhaust manifold that effectively uses gas flow to increase horsepower.
  • The latest two-stroke dirt bikes still rely on Kaaden’s refined techniques, although that didn’t help them become street legal.

Cars have more parts than motorcycles, but both have an intense amount of thought, sweat, and math poured into every detail. And seemingly small – and often overlooked – tweaks to these details can often have surprisingly big results. Just look at how things like vortex generators and splitters affect race car behavior. But making such changes sometimes requires outside help. And in the case of the development of the modern two-stroke engine, a real rocket scientist was needed.

WWII rocket scientist helped increase power in two-stroke motorcycle engine

Left to right: Norbert Kaaden, Christian Steiner and Lutz Langer at a DKW and MZ two-stroke motorcycle show in honor of Walter Kaaden | Hendrik Schmidt/picture alliance via Getty Images

Although they have their flaws, two-stroke engines also have several advantages over four-stroke engines. They are simpler, lighter, somewhat easier to maintain and above all, have higher specific yields. In other words, with engines of equal displacement and number of cylinders, two-stroke bikes produce more power.

However, this has not always been the case. Granted, none of the early internal combustion engines were powerful, whether they had a four-stroke or two-stroke cycle. But around World War II, two-stroke motorcycles hit a wall of power, especially after the change in regulations. Due to the way they work, the pistons of two-stroke bikes heat up twice as fast as four-stroke ones, bike world said. Also, while higher rpm helps four-strokes produce more horsepower, two-strokes don’t because of their design.

One of the motorcycle companies most affected by changing two-stroke racing regulations was DKW. Before WWII, the German company used supercharging to get around the scavenging problems of two-stroke engines – more on that later. But with this ban, DKW, now named MZ, needed a workaround. And while several engineers created snippets of the ultimate answer, it was Walter Kaaden who pieced them together.

Kaaden was an honest rocket scientist who wanted to do something more peaceful with his knowledge after the war, Popular mechanics Explain. He discovered that something with MZ’s racing arm was aptly called “MZ Racing”. Here he took the ideas developed by previous DKW engineers like Erich Wolf and refined them with his knowledge of gas flow and resonance. And to say they made a big difference is an understatement.

Prior to Kaaden’s work, MZ two-stroke engines achieved just over 100 bhp/litre. But using just an oscilloscope and metal shaping techniques, Kaaden bumped it up to 200 hp/litre, PM said. Keep in mind that the Bugatti Chiron hypercar produces 187.5 hp/litre.

Making rockets and two-stroke engines more powerful is exhausting work

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It should be noted that Walter Kaaden did not invent anything, bike world said. On the contrary, he combined and refined several disparate ideas into one very successful one. And that involved the exhaust.

Rather than using valves, two-stroke engines cover and uncover their intake and exhaust ports using their pistons. But they still need those ports because, like four-stroke bikes, they work by drawing in fuel and air and venting waste gases. And those waste gases go down the manifolds and down the exhaust pipes, just like they do in four-stroke motorcycles. However, even the best-designed exhaust and intake systems drag waste gases into the combustion chamber. This dilutes the mixture of fuel and fresh air, which means the engine produces less power.

This is where Kaaden’s knowledge of gas flow and resonance comes into play. Gases flowing through the pipes create pressure waves and resonate at specific frequencies – this is how brass instruments work and wind instruments. Wolf found he could use these pressure waves to his advantage, but couldn’t quite optimize his designs, bike world said. But Kaaden could.

Using his oscilloscope, he monitored the exhaust manifold resonance at various rpms. He then reshaped it and checked the frequencies and flow again. Repeat until nauseated, and it ended up with the familiar bulbous exhaust manifold of the two-stroke engine we know today.

This shape allows exhaust gases to exit quickly, creating a wave of low pressure that sucks in more fuel and air. But it also created a back pressure force that prevented too much fuel and cool air from leaking into the exhaust. It’s not quite forced induction to boost or boost, but it’s not too far off, PM Remarks. And Kaaden’s work was so influential that modern two-stroke dirt bikes are still based on the principles he set forth.

They’re strong, but emissions (mostly) keep them from rolling

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However, although MZ Racing had some success with Kaaden’s two-stroke engine upgrades, it would not last. After falling out with Kaaden, driver Ernest Degner took his former partner’s designs to Suzuki, PM reports. Suzuki used them to improve their race bikes, one of which rode Degner to win the 50cc World Championship. Kawasaki then discovered this new two-stroke technology, and after that, the oil-scented cat was out of the bag.

Unfortunately, while Kaaden helped make modern two-strokes powerful, he couldn’t clean them up as much. Do you remember those flaws I mentioned earlier? No matter how well they sweep, two-strokes still squirt unburnt fuel out of their exhaust. Plus, they’re not lubricated like four-strokes—you have to mix two-stroke oil with the fuel. This means that two-stroke engines burn oil, which causes even more pollution.

Certainly, with modern direct injection technology, two-strokes can be as clean as four-strokes, bike world complaints. However, it wasn’t available in the 1980s when the EPA really started taking emissions seriously. So, to play it safe, most companies stopped trying to make street-legal two-strokes in the United States.

And in today’s supplier-driven world, continually refining two-stroke technology to reduce pollution would cost considerably more than buying an off-the-shelf solution. Plus, the rise of electric motorcycles and lawn equipment arguably makes the notion irrelevant.

Still, it’s cool to think that there’s a bit of rocket science in every two-stroke motorcycle engine.

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