ASK KEVIN: DO TWO-STROKES WORK BETTER WITH LEADED GASOLINE?

TwoStroke2014-040

Question: Do two-strokes engines work better when they run leaded gas? And if so, why?

TYLER CAREY

CYCLEWORLD.COM

Answer: All spark-ignition gasoline engines are limited in the compression ratio they can use by the onset of engine knock, or detonation. In racing, it has for 80 years been normal to operate at the threshold of detonation because, the higher the compression ratio, the higher engine torque becomes. Detonation is an abnormal form of combustion that produces an audible sound, progressive erosion of metal from pistons and heads, and overheating.

Detonation is not the same as pre-ignition. In pre-ignition, some hot object in the combustion chamber (such as a glowing carbon deposit, or the glowing electrode of a spark plug of too hot a heat range) ignites the fuel-air mixture before the ignition spark occurs. Pre-ignition heats the very center of the piston dome very quickly because it is farthest from the cooler cylinder wall, so in just a couple of engine cycles, the center of the piston softens from the heat and caves in, producing the “hole-in-one” failure characteristic of pre-ignition.

In detonation, the ignition spark ignites the fuel-air charge normally, and a normal flame front expands through the charge at a speed of 50 to 200 feet per second (this is normal burning, not an explosion). As the flame front expands, it compresses the unburned charge ahead of it, heating it rapidly. In normal combustion, the flame front burns the charge all the way to the cylinder wall and all is well.

Eythel-Avgas

But if the unburned charge ahead of that expanding flame front is already unusually hot as a result of abnormally early spark timing, an excessive compression ratio, or an overheated engine, chemical changes in the unburned mixture move faster than normal, generating an increasing population of highly reactive molecular fragments, the most active of which is OH-, the dreaded hydroxyl radical. If this population reaches a certain density, bits of the remaining unburned mixture out near the cylinder wall can auto ignite—go off by themselves—before the flame front reaches them. In their chemically-altered state, these bits mixture burn, not at the usual slow 50 to 200 feet per second, but at the local speed of sound, which may be as high as 3,000 feet per second. This creates shock waves, zones a few molecules thick, across which there is a very steep pressure gradient. When these pressure waves hit internal engine surfaces, we hear engine knock. In a taxi engine, running cheap gas and lugging, it sounds like knocking two stones together under water. In a racing two-stroke, some riders say they hear a squeak.

Working for the Dayton Electrical Lab (Delco) in the early 1920s, Thomas Midgley operated a roomful of knock-test engines, systematically going through one after another possible chemical additive in search of anything that could suppress engine knock. This was important work because fuel efficiency rises with compression ratio, but at the time, car and bike engine compression ratios were stuck in the range of 4 or 5:1 because anything higher produced detonation and internal engine damage (in two-strokes, the first thing you see is a rough, gray, sand-blasted zone right at the edge of the piston, usually on the exhaust-port side).

Midgley discovered that the compound tetraethyl lead, used in a concentration of one gram per gallon of gasoline, could allow compression ratio to be raised at least one whole number without detonation.

Later, the equally systematic work of Dr. Graham Edgar mapped out the knocking behavior of the entire spectrum of hydrocarbon structures found in petroleum. Now it became possible to refine specifically for the most knock-resistant hydrocarbon structures (those which were either ring structures or branched chains of carbon atoms, with hydrogen atoms attached), and later, to synthesize in quantity the very most knock-resistant structures.

Battle_of_britain_air_observer

When these improved gasolines were combined with additions of tetraethyl lead, it became possible by the late 1930s to operate racing engines on compression ratios as high as 10:1. British aircraft, burning US-made 100-octane aviation gas, were able to prevail in the Battle of Britain against their German opponents whose fuel octane number was substantially lower.

After World War 2, it became possible to supply motor gasolines with similar high octane number, making possible those crazy supercars like the 1965 Ford “Thunderbolt,” whose 600-hp owed much to the ability to run 12:1 compression on leaded gasoline.

Wartime avgas employed as much as six grams per gallon of tetraethyl lead.

Two-stroke combustion chambers allow very high compression ratios to be used because, having no overhead valves, they can use as much area of the head as flame-speeding squish as they need. Two-stroke race engines from the 1970s to the end of the two-stroke era in GP racing were able to operate at 15:1 or even 18:1 compression.

When low-lead fuels were required in GP racing in the 1990s, the fuel blenders tried to make up for the loss of octane number by making “chemistry set” gasolines out of pure iso-pentane, octane, triptane, toluene, and MTBE (methyl tertiary-butyl ether). Yamaha worked to reduce combustion chamber surface temperature by making the cylinder caps out of high-conductivity copper instead of the formerly used aluminum.

BoBritain

When FIM rules required all lead to be removed from race fuel, compression ratios had to be reduced and power dropped with them. Even so, the last of the two-stroke 125cc singles in GP racing were making as much as 55 horsepower. Later, fuel rules in racing were changed to ban “chemistry set” fuels in favor of something closer to a normal refinery product.

Simply switching to leaded fuel cannot increase power. The lead in fuel suppresses detonation by acting as a negative rate catalyst for OH-, slowing its rate of production. To increase power by use of leaded fuel, the compression ratio must be increased to the knock threshold of the leaded fuel. It is the torque increase resulting from the increased compression ratio that boosts power.

When the US federal government made the decision to remove tetraethyl lead from motor gasolines, the effective reason was to prevent lead from “poisoning” the new exhaust catalysts being required on automobiles. Also, tetraethyl lead is of itself a highly poisonous substance, requiring special handling techniques. Its presence in motor gasolines in former times was the reason we were told as children never to wash parts in gasoline.

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