Technologies for Cylinder Filling

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

The percentage by which an engine succeeds in filling its cylinders with air or fuel-air mixture is termed its Volumetric Efficiency, or VE. In unsupercharged engines operating on full throttle over a range of rpm, VE varies widely. A decent peak figure for production engines is 85 percent, while the upper limit for purpose-built competition engines can be as high as 125 percent.

Let’s begin with an imaginary engine that operates so slowly that there are no unsteady-flow effects operating in its intake system. These are effects such as intake and exhaust resonances, airbox resonance, or intake-velocity considerations.

We know that in practice, the intake valve(s) begin to open a short distance before top dead center (BTDC) and close at some time after bottom dead center (ABDC). How long after? In the case of racing two-valve engines, intake closure may be as late as 80 degrees ABDC. For a four-valve engine, the two intakes may close at more like 55 degrees ABDC.

But in our oh-so-slow-turning imaginary engine, we see that such delayed closings just allow the piston, rising on its compression stroke, to push fresh air or charge back out of the cylinder until the intake(s) actually close, thereby losing charge. In the case of 80 degrees ABDC, the loss is 36 percent of the engine’s stroke. At 55 degrees ABDC it is 17 percent.

The Dynamic Answer

Why oh why would anyone use such late intake-valve closing timing? The answer: Precisely because practical engines do not turn this slowly. The faster an engine turns (up to a limit set by intake friction) the higher its intake velocity. In the case of racing or supersport engines, if the piston’s maximum speed down the bore (reached at about 76 degrees ATDC) is as high as 100 feet per second, and if the ratio of piston area to intake area is about 4:1, then from this purely geometric argument we will expect intake velocity to reach four times the peak piston speed, or 400 feet per second. This assumes that average piston speed is 4,500 feet per minute.

Referring to my handy graph, poetically titled “Pressure of Air on Coming to Rest from Various Speeds,” I find that stopping a column of air moving at 400 feet per second ideally produces a pressure of 1.09 times atmospheric pressure.

Now we begin to understand late intake closing timings. Although the piston is rising on compression, that extra 9 percent of pressure, generated as the piston stops inrushing intake air at 400 or more feet per second, begins to look like an improvement in cylinder filling.

Intake Charge Inertia

Yet it’s more complicated than that. The air or fuel-air mixture sitting in the intake pipe cannot move instantly to follow piston motion; it has inertia that must be overcome in order to accelerate it to high speed. Pressure instrumentation in an engine’s intake tracts have resulted in graphs of intake velocity versus crank angle.

They reveal that at higher revs it can take almost half the intake stroke to overcome intake inertia and get that column of gas up to speed. And that, in turn, implies that since most of the flow is occurring in the second half of the stroke, peak intake velocity may be of the order of double what we may call “geometric” intake velocity (figured from average piston speed and the ratio of piston area to intake area).

Honda engineers have authored SAE papers that show us unsupercharged engine cylinders have been filled to pressures as high as 1.25 times atmospheric pressure. That pressure would be reached if an intake velocity of 675 feet per second were converted into pressure by stopping it against a rising piston.

Bear in mind that other elements in attaining that Volumetric Efficiency of 125 percent mentioned above may be provided by intake, exhaust, and airbox-wave effects.

Valve Timing

Why the big difference in intake valve closing timing between two-valve and four-valve engines (as above: 80 degrees ABDC or more for two-valve engines like a 1950s Manx Norton, and 55 degrees ABDC for a modern Superbike-kit intake cam for Kawasaki’s four-valve ZX10-RR)? As a valve begins to open, the flow area its lift creates, called “curtain area,” is the circumference of the valve head (the distance around it) multiplied times valve lift. Circumference for the two intakes of a four-valve engine is much greater than for the single intake of a two-valve design having the same valve-head area, so during valve lift a four-valve engine generates curtain area considerably faster than can a two-valve. Having that advantage, it need not begin to open its valves as early or keep them open as late as does a two-valve. In addition, the greater weight of a single large valve can itself dictate longer valve timing to give the valve springs the time they need to slow, stop, and reverse the motion of a heavier valve.

This thought experiment suggests that the higher we push intake velocity, the more able the intake flow is to continue after BDC. And indeed there has been a trend over many years toward smaller intake ports. Flow resistance, which also increases with intake velocity, sets an upper limit.

“Hogging Out” Ports and Resonance

Conversely, the old practice of “hogging out” intake ports to large diameters just means that the high intake velocity we want only occurs at very high rpm. Engines with either such large-diameter intake ports, or with very long valve timing, produce the infamous “light-switch” power band: As the engine revs up through low and mid there’s little torque, until bang, torque appears up near peak rpm. Not very useful!

There are plenty of other nonsteady flow effects at work in engines. We know we can increase cylinder filling by making use of intake- and exhaust-wave effects, plus the added effect of intake airbox resonance.

The airbox and its intake pipe(s) act as a Helmholtz resonator. We’re all familiar with the handy example: Blowing across the open mouth of a bottle to produce a strong acoustic tone. People who work with these effects say it’s easy to gain 10 or even 15 percent extra torque…over the narrow range of the resonance. An oscillator requires three basics for its operation: a mass that oscillates against some form of spring, and a driving force to set that mass into motion. The mass in our case is the slug of air in the intake pipe leading from the fairing nose into the airbox. The spring is the compressibility of the air in the box (which is sealed to the engine intakes), and the driving force is the series of suction events generated by the engine’s intake strokes. Google “Helmholtz resonator” to learn more.

Use of resonances has a cost: For every resonance (rpm range over which it boosts torque) there is also an anti-resonance (range in which it reduces torque). Years ago, during the 500 two-stroke era in GP roadracing, Honda found the anti-resonance generated by the intake airbox of its NSR500 so troublesome that it placed something resembling the biggest valve from a bass saxophone on the box, programmed to open (killing the anti-resonance) in that rpm range.

Valve Overlap and Euro 5

Always note that everything affects everything else. Consider valve overlap, the time centered on TDC at the end of the exhaust stroke when the intake valve has begun to open but the exhaust has not yet closed. Before Euro 5 discouraged its use, overlap was a way to start the intake process even before the piston had started to move downward on its intake stroke. This occurred when a negative exhaust pipe wave was timed to reflect back to the cylinder just before exhaust-valve closure. That low-pressure wave helped to evacuate inert exhaust residuals from the combustion space; it could then propagate across to the intake valve(s), traveling into the intake duct and  accelerating intake flow into the cylinder.

Why does Euro 5 discourage valve overlap? Any time both valves are open there is a chance that some fresh charge will exit through the exhaust port(s) to become unburned hydrocarbons during emissions testing.

If intake and exhaust valves are at significant overlap lift around TDC, and if we are trying to boost torque by using a high compression ratio, there is danger of valve-to-piston interference unless we machine valve-clearance cuts into the piston crown. A conflict arises, for the deeper we cut into the piston for valve clearance, the lower our compression ratio becomes.

It’s easy to just give up at this point by saying, “Electrics are coming and they make all this antique technology unnecessary.” Don’t be too sure. Consider that there are 260 million cars and light trucks registered in the US, while every year only 6 percent of them are replaced by new vehicles.

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