Piston Engines - Introduction — Page 10, Lesson 16

I want to walk you through the introduction to piston engines, and I'm going to start with a piece of history that most people get wrong. The credit for the design of what we call the 'cold-fuel' compression-ignition engine does not belong to Rudolf Diesel. That's the first thing to fix in your mind. Let me define that term properly. A compression-ignition engine is one where the fuel ignites not from a spark plug, but from the heat generated by compressing the air in the cylinder to a very high pressure. 'Cold-fuel' refers to the fact that the fuel is injected into the engine cold, at ambient temperature, rather than being pre-heated before it enters. So when I say 'cold-fuel compression-ignition', I mean an engine that squirts cold fuel into highly compressed, hot air, and that air's heat ignites it. Now, the history. In 1891, a man named Herbert Akroyd Stuart invented the cold-fuel injection system. His design is similar in operation to modern-day automotive and aero-engine applications, and it actually pre-dates Diesel's design. So Akroyd Stuart came first. Then in 1892, Rudolf Diesel designed and patented a similar engine to Akroyd Stuart's, and this one became known as the 'hot-bulb' system. Here's the key difference: in Diesel's system, the fuel was introduced to the engine using a compressed-air delivery, and that compressed air 'pre-heated' the fuel. That pre-heating allowed an easier start to be achieved. So the distinction is — Akroyd Stuart's cold fuel, Diesel's pre-heated fuel via compressed air. Now here's the twist. Even though it was strictly Akroyd Stuart's design, these compression-ignition engines became known as 'Diesels'. The name stuck, and that's why we call them diesel engines today, even though the original cold-fuel concept was Akroyd Stuart's. Why did cold-fuel compression-ignition engines get developed further? Because they can run faster, they weigh less, and they are simpler to maintain. Those three advantages drove the development. Now, let's bring this to aviation. Diesel engines for use in aircraft are by no means a new idea. Aero-diesels first appeared during the late 1920s. So this isn't some modern novelty — it's been around for nearly a century. Let's look at the mechanical construction. The mechanical parts of a diesel engine are similar to those of a conventional gasoline-driven engine, with one important exception. The diesel's reciprocating parts — that's the pistons, connecting rods, and related moving components that go back and forth — are slightly heavier. Why? To cope with the higher compression-ratios within the engine. Higher compression means higher pressures and stresses, so the parts need to be more robust. Now, here's where recent developments come in. Advances in materials technology, superchargers, and design have brought the diesel to comparable weights with conventional engines. In fact, diesels now have even better power/weight ratios. That means for a given weight, they produce more power than they used to, and they're competitive with gasoline engines. These developments have given way to recent certified retro-fits being trialled in two specific aircraft: the Warrior PA28 and the Cessna 172. A retro-fit means taking an existing airframe and installing a new engine in it, and these are certified — meaning they've been approved by the aviation authorities for use. So you're seeing modern diesels being put into these classic training aircraft. Let me tie this together for you. The key names and dates to hold onto: Akroyd Stuart in 1891 with the cold-fuel injection, Diesel in 1892 with the hot-bulb compressed-air pre-heating system. The engines became known as Diesels regardless. They run faster, weigh less, are simpler to maintain. Aero-diesels appeared in the late 1920s. The mechanical parts mirror a gasoline engine, but the reciprocating parts are heavier to handle higher compression-ratios. And modern materials, superchargers, and design have made diesels weight-competitive with better power/weight ratios, leading to certified retro-fits in the Warrior PA28 and Cessna 172. That's your introduction to piston engines — the diesel side of the story.