Why Five Stroke Engines Are More Efficient But Still a Failure
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As you probably know internal combustion engines are not very efficient. On average modern gasoline engines in passenger cars manage around 35% whereas diesel versions can do a bit above 40% but need complicated and expensive emissions control equipment to be as clean as their gasoline counterparts.
An efficiency of 35% means that of the energy present in the fuel only 35% gets converted into useful work. The rest is lost. Some of it is lost to internal friction but most of it actually escapes as heat and noise out through the exhaust.
So why is so much energy lost? Why can’t we efficiently harness more of it?
The problem we have here is that all four strokes of a four stroke engine are of equal length and duration and of those four strokes it is only the combustion stroke that produces significant energy. The remaining three strokes mostly just consume energy. This means that an engine would be more efficient if it was given more time to actually harness the energy.
One of the ways to increase engine efficiency is to increase the compression ratio of an engine. A compression ratio is simply the ratio between the smallest and largest cylinder volume. But the positive effects of increasing the compression ratio are limited because we are limited in the amount of how much we can increase it. At some point space for combustion becomes so small that combustion occurs so close to the piston that too much energy is transferred too quickly which makes it hard for even the most robust engines to handle these shocks.
So we are limited in what we can do with the compression ratio. That means that we must look for other ways to increase efficiency the ideal thing to do would be to make the strokes unequal. What we actually want to do is have the combustion aka the expansion stroke somehow be longer than the other strokes.
Of course, the conventional rotating assembly does not permit different lengths for different strokes which is why James Atkinson decided to forego the traditional engine anatomy and created a new different engine anatomy that enabled the engine to have a noticeably longer combustion or expansion stroke. Even the inventor of the of the four stroke engine himself Nikolaus Otto saw the limits of his design and wanted to increase the time and space for the expansion and energy harnessing but Otto together with Gottlieb Daimler decided to take a different approach. Instead of creating a novel and unproven rotating assembly they decided to rely on existing engine anatomy. They simply added another cylinder to harness the remaining energy of the exhaust gas.
Instead of letting exhaust gas go out into the atmosphere the high pressure cylinders would send it into the low pressure cylinder where the pressure remaining in the exhaust gas was used to drive the larger middle piston. So the outer cylinders operate like normal four stroke cylinders.
Well all of that sounds great in theory but Otto and Gottlieb’s five stroke engine was a failure. It was commercialized but it suffered from poor performance and production was quickly discontinued. Probably because Gottlieb and Otto were working with technology from late 1800s.
And so the design was abandoned but not forgotten. It laid dormant for 124 years until 2003, when it was awakened by Belgian engineer and inventor Gerhard Schmitz who patented a three-cylinder five-stoke engine which was virtually identical to Otto and Daimler’s design.
Of course getting a patent for a theoretical concept is one thing. Getting that concept materialized into a working prototype is another. But here Gerhard Schmitz managed to convince a very serious company to turn his idea into reality. Ilmor engineering. Maybe you haven’t heard of them but Ilmor is nothing like any the newly sprung-up companies created around novel engine designs. Founded in 1983 by Mario Illien and Paul Morgan Ilmor engineering has been successfully designing and developing engines for Chevrolet in Indycar racing, for Sauber and Mclaren in Formula 1, they even competed in MotoGP.
So when a company like this takes on the development of a novel engine design it definitely gives the design credibility and high hopes of reaching mass production. So Ilmor got busy and just 4 years later in 2007, we got a running prototype.
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