The Compression Stroke

Admin · December 21, 2023

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. (Robert Martin/)

The compression stroke begins as the piston reverses direction at Bottom Dead Center (BDC), after filling the cylinder with fresh mixture during the intake stroke. It is commonly supposed that no compression can begin until both valves are closed. Yet because intake velocity can be very high at the end of the intake stroke, it would be a shame to close the intake valve right at BDC. Therefore it is normal to delay intake closing for a number of degrees, and let that velocity convert itself into pressure as it continues to coast into the cylinder.

In fact, useful compression can take place before intake closure—through this process of conversion of velocity into pressure. For example, if intake velocity at BDC is 500 feet per second, 100 percent conversion of that energy into pressure would result in a 14 percent pressure rise. Since sea level atmospheric pressure is 14.7 psi, 14 percent of that is 2 psi. I have digressed in this fashion in order to assure the reader that delaying intake valve closure until many degrees ABDC need not result in delaying the start of compression.

So powerful is this effect—especially when combined with the positive effects of intake and exhaust wave action—that un-supercharged four-stroke IC race engines have been able to fill their cylinders to 125 percent of atmospheric pressure.

Another element in how such high volumetric efficiency can be achieved is the 97 years of intensive airflow optimization studies that have so streamlined the intake ducts and valve ports of engines. Such streamlining has allowed intake velocities to be greatly increased. Each time this has allowed higher intake velocities to be used, making possible the achievement of increased intake flow after bottom dead center.

Related: The Intake Stroke

Compression stroke begins as the piston leaves BDC, even if the intake valve is still open. (Illustration by Robert Martin and Ralph Hermens/)

Now to the task at hand. The purpose of the compression stroke is to raise the peak pressure attained by combustion. The rule of thumb is that complete combustion of a chemically correct hydrocarbon air-fuel mixture multiplies its pressure by a factor of about seven. In the earliest gas-fired IC industrial engines, the intake phase took in only about one-third of a cylinder-full of mixture, and then ignited it without compression. If we multiply seven times one-third, we get a pressure rise factor of 2.3, which is feeble! Those early Lenoir gas engines produced very little power, but their ability to make any power at all, wherever they could be connected to city gas mains, made them useful for a time in the 1860s and first half of the 1870s.

Henry Ford’s Model-T engine’s compression ratio was maybe 4.5 to one, so here we invoke another rule of thumb: The peak pressure of combustion is roughly one hundred times the compression ratio. That is for operation at full throttle and at peak torque rpm. Applying this to the Model-T we get: 100 x 4.50 = 450 psi. That’s not very much in modern terms, so the Model-T’s 2,900cc engine made something like 22 hp at 1,600 rpm (peak torque was down at 900 revs!).

Now consider the engine of a late-model sportbike, with its 13-to-one compression ratio: 100 x 13 = 1,300 psi. Yet in both cases, the air-fuel mixture contains the same potential chemical energy per cubic inch. Why such a difference?

The reason is that the compression ratio of an engine determines how much of that chemical energy will be exerted on the piston crown as combustion gas pressure, and how much will be wasted out the exhaust port as heat when the exhaust valve opens near the end of the power stroke. For this reason, an IC engine’s compression ratio is a prime determinant of its efficiency.

Then why be shy? Why didn’t Ford boost the compression of his Model-T to 13-to-one?

The answer is, if compression was raised any higher than that 4.50 back in 1913, the engine made strange knocking noises, overheated, and soon rattled to a stop with ruined pistons. The name of that “combustion illness” is detonation (and that’s why racers used to refer to the AMA’s big March Florida Speed Week as “Detona”).

Normal combustion in IC engines is a smoothly progressive spread of flame—not anything near being “an explosion.” The flame progresses by heating the unburned mixture ahead of it to its ignition temperature. In this way, combustion spreads from the spark plug outward to the cylinder wall at a moderate speed of 50–150 feet per second. This is given the name “deflagration.”

But under certain conditions, another form of combustion occurs—one that spreads at or above the local speed of sound, and operates by shocking apart the molecules it encounters, allowing them to rapidly recombine with oxygen (which makes up 21 percent of our atmosphere) and release heat. This high-velocity combustion, propagating by shock, bears the name detonation.

Early gasolines were more apt to detonate, and engine design lacked the sophistication that would come later. What engineers did know back then was that they could avoid detonation—and the damage to pistons and bearings caused by its violent shock waves—by sufficiently reducing compression ratio. Other things that help to stave off detonation are:

Limiting the temperature rise of the fresh charge. Temperature drives chemical changes that lead to detonation.
Accelerating combustion by providing mixture turbulence. Because conditions favoring detonation take time to develop, more rapid combustion can often actually outrun detonation.
Lugging an engine, making it pull on large throttle at low rpm, allows conditions favoring deto: intense combustion with plenty of time for chemical change in the unburned mixture.

Detonation damages pistons and bearings with violent shock waves. (Jeff Allen /)

Honda, in research it performed in the early 1960s, discovered that an engine’s octane requirement (Octane Number, or ON, is a measure of a fuel’s resistance to detonation) actually decreased above 12,000 rpm. Up at 20,000 rpm or more, its test engine operated knock-free on dreadful fuel whose ON was less than 40.

Early in the 20th century the value of mixture turbulence in avoiding detonation by speeding up combustion was discovered numerous times and by different people.

One source of turbulence is squish. Areas on the piston are so shaped as to come very close (“close” is in this case 0.7mm or 0.028 inch) to corresponding areas of the cylinder head at TDC. As the mixture between these areas is very rapidly squeezed (or “squished”) out from between, fast-moving jets form which vigorously stir the mixture. Squish area as a percentage of piston area is of the order of 15 percent.

Another path to turbulence is to guide the fast-moving mixture into the cylinder in such a way as to store its kinetic energy as swirl—creating a flywheel-like mass of rotating mixture. That turns into turbulence as the piston nears the end of compression.

Axial swirl is created by making a single intake port approach the cylinder on a tangent. Anyone who has filled a bucket from a hose has seen how easy it is to make the water in the bucket rotate right or left in similar fashion.

Or, in the case of four-valve engines, the entering jet of mixture can be aimed to create a tumble motion as it hits the far cylinder wall, turns down to the piston, flows across its crown to the near cylinder wall, then back up to the head to complete a looping path. Tumble can store much of the kinetic energy of the entering mixture, to be transformed into random turbulence as the piston approaches TDC.

Modern pump gasolines have lower ON than the fuel adopted in 1936 by the US Army Air Corps for its piston-powered aircraft, so it’s not “high-tech fuels” that are preventing today’s sport motorcycles from detonating on their 13-to-one compression.

Then what is? Part of the answer is liquid-cooling, which usefully cools combustion chamber surfaces, thereby slowing the pre-flame chemical reactions that lead to detonation (this is why air-cooled engines typically have lower compression ratios than water-cooled). But most of the credit goes to tumble, generating turbulence that speeds up combustion enough to “outrun”detonation.

Because normal combustion of the mixture takes time, it must be ignited near the end of the compression stroke—many degrees BTDC—in order to reach peak pressure just as the piston begins significant downward motion on its power stroke. For that reason, the last part of the compression stroke is also the first part of the power stroke.

Here’s another thing to think about. The only way to get full effect from an engine’s compression ratio is to make the whole air-fuel mixture burn right at TDC—which is impossible. Mixture still burning at 30 ATDC is burning at a significantly lower compression ratio (like seven-to-one) because the piston has already moved through 8 percent of its power stroke. Or what about the mixture that was pushed into the piston-ring crevice volume during the compression stroke? High-speed photography has shown that the last of that “crevice gas” may still be streaming out as the exhaust stroke begins.

Bottom line here is that, in fact, the four strokes are not separate activities at all, but tend to overlap each other in complex (but fascinating!) ways.

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