Free Supercharging For Four-Stroke Piston Engines

The usual description of the four-stroke piston-engine cycle makes the intake stroke sound like the opening of a popular movie at a local theater. Moviegoers are lined up at the box office, and when the doors open, they file in politely to fill all the seats. Those who don't find seats are asked to wait for the second showing.

In this analogy, the theater lobby is the intake airbox. The moviegoers are the intake air, the theater seats are the cylinder displacement, and in this simplistic model, the “cylinder” is full when all seats are occupied.

For an engine being turned by hand, this analogy works, but at actual operating speed, things with mass—such as the intake air itself and the valves that control its motion—take time to accelerate. The piston reaches its maximum speed in the bore at roughly 76 degrees after top dead center (ATDC), the point at which the crank arm and connecting rod are at right angles to each other. For maximum flow, it’s desirable to have the intake valves almost fully open at this point so they must begin lifting well before top dead center (TDC).

Because air has inertia, it takes time to get the fuel-air mixture in the intake pipe moving. In a high-rpm modern engine, the first half of the intake stroke accomplishes little more than this. If our engine is turning 12,000 rpm, that quarter of a revolution takes 0.00125 second, just long enough for the “message”—that the piston has pulled a deep vacuum into the cylinder—to travel at the speed of sound upstream through the intake duct and into the airbox. In this process, the air accelerates to several hundred feet per second.

As the piston decelerates, nearing the bottom of its intake stroke, the inertia of the now fast-moving air keeps its velocity very high; it is a fortune in kinetic energy. If we closed the intake valves at this point, the cylinder would not yet be filled and all that lovely kinetic energy would be wasted, piling up as pressure against the upstream faces of the closed intake valves.

How much pressure? I turn to a constant companion of many years, a chart entitled, “Pressure Of Air On Coming To Rest From Various Speeds.” It tells me that if the intake air at bottom dead center (BDC) is moving at 500 feet per second, its pressure on being stopped will become 1.14 times greater than atmospheric pressure.

Where did I get that figure of 500 feet per second as intake velocity? The old way to work out what was called “mean intake velocity” was to calculate by geometry the ratio between piston speed and intake velocity. If, for example, an old Kawasaki Z1’s pistons have five times the area of its intake ports, then when the average piston speed is 3,500 feet per minute, average intake velocity will be five times greater than that, or 3,500 x 5 = 17,500 feet per minute, which is 292 feet per second. So where do I now get 500 feet per second as a possible intake velocity?

I get it from the fact that the first half of the piston’s intake stroke moves very little air because, in a high-speed engine, that time is consumed in accelerating the flow. This information comes from instrumented running engines. If most of the flow takes place only in the second half of the intake stroke that suggests the actual peak velocities reached by intake flow will be almost twice the old mean intake velocity calculated from simple geometry. Five hundred feet per second is almost twice the old mean intake velocity of 292 feet per second.

So how do we spend the “fortune in kinetic energy” of intake flow at 500 feet per second? Not by letting it pile up against closed valves, that’s for sure! No, we keep those intake valves open, not just after the piston has stopped at BDC at the end of its intake stroke but as the piston begins to rise on its compression stroke. All that kinetic energy keeps the air rushing into the cylinder.

Only when the fast-moving column of intake flow is rammed to a stop by the rising piston do we close the intake valves. This is, in a sense, free supercharging because it is using intake velocity to overfill the cylinder to a pressure higher than atmospheric. This is what old-timers were talking about when they referred to “intake ram pressure.”

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As it turns out, with excellent intake port shape and size, with just the right valve area and timing, it is possible to fill engine cylinders this way to 125 percent of atmospheric pressure. Working backward on the aforementioned chart, we find that the velocity required to overfill a cylinder to that degree corresponds to an intake velocity of 675 feet per second. (In actual practice, reaching that 125 percent cylinder filling requires a combination of maximum intake ram, intake wave effects, plus some boost from a resonant airbox.)

This 25 percent in natural supercharge is not actually free because with it come compromises. The later we close the intakes in the interest of maximizing cylinder filling at high rpm, the weaker our engine’s bottom end and midrange become. Why? Because at lower rpm, intake velocity is lower, so the piston, rising on its compression stroke, is able to ram it to a stop sooner and then push some mixture backward out of the cylinder into the intake pipe and back toward the airbox. That loss of charge translates into lower torque that gets weaker the slower our engine turns.

Engineers can play games with all these effects, hoping to build something riders will like. If we cam our engine for midrange (earlier intake closing!) and boost intake velocity at all speeds by making the intake ports smaller, we can end up with peak torque at around-town engine rpm, as in Triumph’s 765cc triple. All riders love a torquey engine. But if it’s 2005 and the mission is to win AMA Supersport races no matter what, the compromise has to be pushed the other way, with later intake valve closing that gives peak cylinder filling above 10,000 rpm. The intake ports are enlarged to give peak torque very high up, producing the peak power required for a winning pace at Daytona (1988–2005 AMA Supersport race wins sold a lot of 600s).

If the job is touring or climbing over rock piles, we need strong torque from bottom revs. Cam timings for those jobs look pretty much as they did in 1910: intakes beginning to open just after TDC and closing soon after BDC. Top end? Don’t worry about it. With engines in today’s sizes, you can still get as many speeding tickets as you like with short cam timings and smallish intake ports.