Passing gas | General Aviation News

It’s all Tom Midgely’s fault, so quit knocking him for ridding us of the knock.

That was the problem a chemist named Thomas Midgely was tasked to resolve in the early 1900s. His bosses at General Motors were making better engines with higher compression and higher horsepower output. Unfortunately, the gasoline going into those engines was little better than crude coming out of the ground. Pre-detonation, backfiring in plumes of acrid black smoke, and generally acting like a lope-eared jackass in heat plagued the early automotive engine. But knocking was the thing that irritated drivers most. It just didn’t sound right and, what’s more, it seemed like a bad thing to have in a device with hundreds of moving parts.

Midgely’s peers tried ethyl alcohol, but it was expensive and hard to blend. Then came iodine, red dye, and even organic derivatives from plants, but nothing seemed to help. What’s more, the additives stunk so badly that most drivers would rather stand next to an open septic tank than near an exhaust pipe.

Then in 1921 Midgely added a small dose of tetra-ethyl lead. Bingo! No knock and the engine purred like a kitten. It even smelled strangely nice. Lead became known as the “anti-knock” cure.

By this time the ratio of how much gasoline could be compressed before ignition occurred had been defined as “octane.” If compression causes ignition prior to the spark plug firing, you’ve got knock and probably a lower octane rating. But add some lead and voila!, the potential for higher octane with no knock. Adding lead allowed for cheap grade gas to be boosted to a higher octane and — under most circumstances — the higher the octane, the better the engine performs.

While auto engines were advancing in the 1920s, aircraft engines lagged behind with most, such as the Curtiss OX-5, putting out an anemic 90 hp. The compression was so low that anything from tractor gas to dandelion wine would do the trick. About 10 years after Midgely put the knock to rest, aircraft engines caught up quickly thanks to the air racing craze of the 1930s. Pratt & Whitney, Curtiss-Wright, and Lycoming were making 300-500 horsepower engines — some even supercharged with commensurate higher compression ratios.

via Passing gas | General Aviation News.