Accelerating Combustion

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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/)

We can learn some interesting things about the combustion of gasoline and air by looking at the ignition timings commonly used in various engines and circumstances.

For many years, a quite common ignition timing has been 36 degrees before top dead center (BTDC); this results in peak combustion pressure at around 11 degrees ATDC. Various experimental results tell us that combustion lasts after TDC roughly as long as the time from ignition to TDC. This gives us roughly 36 + 36 = 72 crank degrees for the duration of combustion in this case, or two-tenths of a revolution.

In a car engine with a 4-inch bore operating at 2,200 rpm on the highway, that tells us that the flame progresses from a centrally mounted spark plug and across the 2-inch cylinder-bore radius to reach the cylinder wall in 0.0055 second. That is 364 inches per second, or 30 feet per second. Note that this is not, as it is so often wrongly described, an “explosion.”

Now imagine a one-liter Superbike engine at full throttle, 14,000 rpm, and with a 73mm (2.87-inch) bore and the same ignition timing. The time of combustion is 0.00086 second for combustion taking two-tenths of a revolution, and the apparent flame speed is the bore radius, 2.87 2 = 1.44 inches, divided by the time, giving us 1,668 inches per second or 139 feet per second.

That’s 4.6 times greater than for the automotive example above. Why?

Ignition Advance

Long before World War II, practical persons and engineers alike had discovered that cars cruising on the highway on part throttle got much improved gas mileage if their spark timing was advanced about 10 degrees. This was the origin of what came to be called “vacuum advance.” When engine vacuum was high, a small diaphragm connected to intake-manifold vacuum pulled the points plate in the distributor (against a return spring) around to a position of increased spark advance.

In this case, total burn time is increased by 20 degrees, so the 72-degree example above becomes instead 92 degrees—28 percent longer—meaning that at high vacuum, combustion speed is slowed by that same percentage.

This leaves us with interesting questions to consider:

1. Why is flame speed so much faster—by 4.6 times!—in the racing Superbike engine than in the auto engine?
2. Why is flame speed slower in conditions of high intake-manifold vacuum?

One obvious conclusion is that flame speed, at least as based on the time taken from passage of the spark to the end of combustion out at the cylinder wall, is not at all constant. Flame speed varies from engine to engine, and also varies with respect to intake-manifold pressure.

Related: Valve Lift

In the late 1950s I attended a school assembly to see a GM engineer demonstrate some of the science behind the automobile. The memorable part consisted of his squeezing a few drops of gasoline into a 2-foot piece of 3-inch Plexi tubing, plugged at one end. He mixed the gasoline with the air in the tube by dropping in a rubber ball, capping the end, and then rolling the ball end-to-end a couple of times. He then removed the cap and ignited the cylinder with his lighter.

We anticipated a bang, but instead the result was a drawn-out borrrrp! He then asked us, “How long did that take? Maybe a couple of seconds?”

A murmur of assent.

“OK,” he continued. “What this tells us is that a still mixture of gasoline vapor and air burns at about 1 foot per second, because it took maybe two seconds to burn the length of this 2-foot plastic pipe.”

He then laid out the automotive example I gave above, showing that flame speed during actual engine operation was more like 30 times faster than this. Why such a difference?

In the case of the tube’s still-mixture combustion, the flame’s rate of advance was limited by how fast the flame front could heat the unburned mixture before it to its ignition point—pretty slow.

Intake Velocity Can Speed Combustion

In an actual engine, the mixture is not still. The cylinder had just been filled by fast-moving intake mixture, rushing through the cylinder’s intake valve. That motion did not disappear; long ago, old Isaac Newton observed that a body in motion tends to remain in motion. That rapid intake motion persisted to some degree through the compression stroke, so that as the ignition spark lit a flame kernel, that kernel was swept away, shredded into fragments, and those burning fragments were widely distributed in the cylinder. The result of all this was not to accelerate the flame speed, which locally remained the same as in our tube experiment, but to greatly increase the surface area of the flame front, thereby causing much faster burn-up of the mixture in the cylinder.

In the auto engine, loping along at maybe 15 percent throttle, there’s enough energy left from the in-rushing fuel-air mixture to speed up combustion by 30 times. And in the Superbike engine, full-throttle intake velocity is very high—peaking at close to the speed of sound. And, being on full throttle, the intake charge is much denser than in the 15-percent-throttle automotive case, so it contains vastly more energy. And that is what sped up its combustion by another 4.6 times: Energetic motion of the intake charge, persisting through compression and assisting the spread of flame throughout all of combustion.

Now to question No. 2: Why does flame speed slow down in conditions of high intake-manifold vacuum?

Because the mixture under intake vacuum is less dense, its combustion releases less heat per unit of volume, becoming thereby less able to rapidly heat unburned mixture ahead of it. The result: slower combustion.

In an earlier post I wrote about the reductions of combustion time (reflected as ability to reach peak combustion pressure with less ignition lead) that took place as tall piston domes, projecting upward into correspondingly deep “hemi” combustion chambers, were made less tall and their chambers less deep.

What this tells us is that as the combustion chamber is made more open, with fewer friction-producing features such as a dome, valve cutouts, and so on, the turbulence created by the high velocity of the intake process decays less slowly, resulting in faster combustion.

Axial Swirl and Tumble

In the late 1920s and early ‘30s, English engine designers discovered that directing a single intake port into the cylinder more on a tangent than on a diameter, thereby generating axial swirl, improved acceleration and power. This was a means of storing intake velocity until it was needed after ignition to accelerate combustion. Some 40 years later, Keith Duckworth would arrive at another method of accomplishing the same thing: His “barrel motion” (today called “tumble”) in which intake from a pair of valves is directed across the chamber to the far cylinder wall, down that wall to the piston crown, and then back across the piston to the near wall and up to complete the loop. Axial swirl and tumble remain the two successful methods of storing intake flow energy until it is needed to accelerate combustion by transforming into turbulence around TDC.

Squish

We can also give the fuel-air mixture a last-moment stir by employing what is called “squish” between areas on the piston and areas on the cylinder head that are dimensioned to approach one another very closely (as in a vertical clearance of 0.025–0.030 inch or 0.6–0.75mm) at TDC. When the mixture between those surfaces is rapidly squeezed out, it forms fast-moving jets that can provide valuable air-fuel mixing and “refresh” mixture motion that has decayed during compression.

Related: Updraft, Downdraft

In our present era of production bikes with 13:1 compression and racebikes reaching for 15:1, a fresh problem arises: the danger of making the combustion space so thin vertically that charge motion in it decays very fast. The classic case was described to me by a Superbike tuner whose engine gave him a choice between acceleration and top speed, but without a useful compromise: It was either/or. High compression, which increases peak combustion pressure (the rule of thumb being that peak pressure is close to 100 times the compression ratio), also improves acceleration. But the resulting very tight chamber burned so slowly on top end that much heat was lost to the piston crown and combustion chamber, killing the horsepower needed for top- end.

Lowering the compression ratio provided room in which charge motion could persist long enough to achieve fast combustion and more top-end horsepower, but at the cost of a loss of acceleration from reduced peak pressure.

Achieving a compromise better than this has occupied some of the best minds in engine development, but it comes down to some combination of instrumented testing and predictive computer simulation—combustion chambers smooth enough to allow turbulence to continue to TDC, yet tight enough to reach the high compression that boosts acceleration.

Squish and the Era of the Flathead in US Motorcycling

There’s a bit of irony in all this. Early in the 20th century, Charles Franklin, Indian’s engineering force through the 1920s, used his knowledge of squish to allow his company’s flathead PowerPlus engines to frequently defeat made-for-racing OHV eight-valve bikes. The extra turbulence created by squish sped up combustion, allowing Franklin to use higher compression ratios that would have made the eight-valve competition detonate themselves to junk.

At the time, gasolines had poor detonation resistance, so compression ratios were low—in the fours and fives. With such low compression it was impossible to generate squish in an OHV chamber even if the eight-valve builders had known and understood it. But in the PowerPlus flathead, with its two valves in a little “porch” beside the cylinder, their stems pointing downward, it was easy to bring any desired area of the flat piston crown quite close to the head at TDC. And that is how the side-valve or flathead engine got its reputation for low-speed pulling power: Its combustion, sped up by Franklin’s knowledge of squish, could literally outrun the heat-driven chemical processes that led to detonation. Because the eight-valves would knock during acceleration on equal compression, they had to run less compression than Franklin’s flatheads.

OHV eight-valves had their strength on top end; their intake flow path was much more direct, and therefore, higher-flowing. But races seldom take place at constant high revs.

That’s enough typing for today.

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"That’s enough typing for today."

 

That's enough for eternity thanks Kevin.