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Old September 22nd, 2022, 02:53 PM   #1
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[cycleworld.com] - The Big Squeeze

Raising an engine’s compression ratio is key to getting the most bang for your buck.

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Ask Kevin Cameron (Cycle World/)We all know that raising compression ratio, provided it doesn’t push our engine into knock/detonation, increases torque at pretty much all rpm. We also know that fuel efficiency rises with higher compression. But perhaps you’d like to know why.

Engineers relate compression ratio to peak combustion pressure, observing that peak pressure is roughly compression ratio multiplied times 100.

When the energy in the fuel is released as heat, combustion raises the temperature of the resulting gas by about 2,600 degrees Kelvin. When you raise compression ratio, what you are actually doing is making the combustion-chamber volume smaller (the volume above the piston at TDC). Early Model T Ford engines had 3:1 compression ratios; the chamber volume was about half of the cylinder volume. But by 1938 the compression ratios of bikes competing in the Isle of Man TT roadraces had risen to about 10:1, and their chamber volume had been reduced to 1/9 of cylinder volume. Big difference! Adding 2,600 degrees of temperature to that smaller compressed volume pushes its pressure to a much higher value than in the Model T’s engine, containing the same energy in a much smaller space. Here’s another rule of thumb: Combustion of a correct fuel-air mixture in a sealed volume raises its pressure about 7.6 times.

Expansion Ratio

If that sounds like getting something for free, let’s turn this around and look at compression ratio as being the engine’s expansion ratio. Clearly, letting the hot combustion gas expand 10 times is going to extract much more of its energy than does expanding it only three times, especially with the higher peak combustion pressure of the high-compression engine. With the low-compression engine, where does the extra energy go, since it doesn’t act on the piston? It goes out past the exhaust valve as a higher exhaust-gas temperature.

The Earliest IC Engines

The very earliest internal combustion engines (other than guns) operated on a cycle that sounds completely crazy today. As the piston descended on its intake stroke, it drew in fuel-air mixture for about the first third of its stroke. Then, with a partial vacuum still present in the cylinder, the piston’s edge slid past a “flame port,” briefly drawing in fire from a little lantern next to the cylinder. This flame ignited the mixture in the cylinder, causing a very limited pressure rise because:
  1. The cylinder was at most only 1/3 filled with mixture and
  2. There was no compression of the mixture before ignition
The result was a weak and inefficient engine, firing every time its piston came to TDC. Yet it found stationary industrial uses because it could be operated anywhere there was city-illuminating gas in the last third of the 19th century.

Nikolaus Otto and the Four Stroke

In Germany, Nikolaus Otto’s education told him this crazy cycle could be greatly improved by compressing the fuel-air charge before igniting it. The scheme he devised in the 1870s had to completely fill the cylinder with fresh charge, compress it, then ignite it to realize a huge increase in peak combustion pressure. To accomplish this would require two full revolutions of the crankshaft. The four piston strokes would accomplish the following:
  1. As the piston descended from TDC to BDC, it would draw in fresh charge through a one-way valve.
  2. As the piston rose from BDC to TDC it would compress that mixture.
  3. Igniting the compressed mixture near TDC would greatly raise the pressure of the gas, forcefully driving the piston on its power stroke from TDC to BDC and accelerating the crankshaft.
  4. Near BDC a mechanically operated exhaust valve would open, releasing any residual combustion pressure in the cylinder. As the piston rose from BDC to TDC it would push out all remaining exhaust gas save for that in the combustion volume above the piston at TDC. The cycle would then repeat.
Otto’s idea drew criticism from engineers more familiar with steam engines, which typically produced two power strokes per revolution (steam being admitted alternately on both sides of the piston). The idea of having only one power stroke every other revolution seemed laughable to them. How could such a device, firing so infrequently, produce useful power?

What they didn’t appreciate was that Otto’s four-stroke concept, employing compression of the charge before igniting it, would produce peak cylinder pressures five to ten times greater than then-existing steam engines. Furthermore, it would soon prove capable of performing its cycle very rapidly.

Surviving photos of pioneers like Otto present them as white-haired granddads, but of course the photos were made long after their ideas had become worldwide big business. When folk like Otto, Gottlieb Daimler, Dugald Clerk, Rudolf Diesel, and Karl Benz were making their discoveries, they were ambitious young hotshots like those presently hailed in the tech business.


Otto’s 1876 four-stroke engine. (Wikimedia Commons/)The Achilles’ heel of Otto’s four-stroke gasoline-burning engine was detonation. Raise the compression ratio enough and eventually combustion becomes abnormal and destructive deto. Understanding of detonation and what causes it began to arrive from the mid-1920s onward, bringing with it much more detonation-resistant fuels.

Next, in the 1960s, science showed us how to speed up combustion through mixture turbulence. Detonation requires both heat and time to mature; if you can make combustion fast enough, it can outrun detonation. This brings us to the present day when so many production motorcycle engines, save for those with the largest cylinders, can operate knock-free at compression ratio as high as 13:1 even on today’s limited octane number pump gasolines.

Even Higher Compression—The Diesel Engine

Rudolf Diesel saw a way to avoid the gasoline engine’s detonation problem by not adding fuel to the cylinder until a very high compression ratio (today around 16- or 17:1) had heated the air in the cylinder to a temperature high enough to ignite injected fuel on contact, without an ignition spark. His experiments in the early 1890s led to a successful engine and patent protection in 1893. The very high compression ratio of the Diesel engine is a major element in its excellent fuel economy, which is roughly 1/3 better than that of comparable gasoline engines.

Alternatives are coming—so many voices say it’s so. But there are roughly two billion internal combustion engines operating in the world today. Replacing them all, not to mention the systems manufacturing them, will take time and investment.
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