Introduction — Page 1, Lesson 7

Let’s begin at the very start of the Piston Engines chapter. I want to walk you through why this engine exists, how it works at its most basic level, and the terminology we’ll need before we ever open the cowling. First, the history and the core problem. Early powered flight failed because there was no suitable engine. The steam engine of the day was heavy and inefficient. Why? Because combustion—the burning of fuel—took place outside the engine, in a separate boiler. That meant much of the heat energy produced was simply wasted to the atmosphere. So you carried a lot of weight and got little useful work. Then, in 1862, a man named Beau de Rochas developed an engine where combustion happened inside the engine. But it was Nikolaus Otto, in 1876, who first succeeded in building a working engine on that principle. That’s the ancestor of what we fly with. Let me give you the operating principle, because everything else builds on it. The engine works by inducing—that is, drawing in—a mixture of air and fuel into a cylinder. That mixture is then compressed by a piston moving up inside the cylinder. The mixture is ignited, and the rapid rise in temperature causes the gas pressure in the cylinder to rise sharply. That high pressure forces the piston down the cylinder. The linear movement of the piston—that straight-line motion—is converted into rotary motion by a connecting rod and a crankshaft. Finally, the burnt gases are exhausted to atmosphere. So, in one sentence: the engine converts heat energy into mechanical energy. Now, the family tree. Internal combustion engines fall into three main categories. First, compression ignition engines—those are the Diesels. Second, two-stroke and four-stroke spark ignition engines. Third, Wankel rotary engines. This course, and this chapter, covers in detail the construction and operation of the four-stroke engine, which is the one commonly used in aviation and is generally referred to as the Piston Engine. Before we dive into operation and construction, I want to flag the groundwork we need. The chapter lists the topics we’ll cover: terminology, dynamics, Bernoulli’s Theorem, the Venturi tube, constant mass flow and the continuity equation, the gas laws, Charles’s Law, the combined gas laws and their application, Diesel engines, and finally terms and formulae. We’ll take those one at a time as we go. Let me pause on one of those because it’s central to carburetion later: Bernoulli’s Theorem and the Venturi tube. The idea is that in a flowing fluid, as the velocity increases, the pressure decreases. A Venturi is a tube that narrows in the middle and then widens again. As the air speeds up through the narrow throat, its pressure drops—and that pressure drop is exactly what we’ll use later to draw fuel into the airstream in a carburettor. I’ll show you that geometry when we get there. So, to anchor what we have: the piston engine is a four-stroke, spark-ignition, internal combustion engine that turns heat into mechanical energy by compressing an air-fuel mixture, igniting it, and converting the piston’s linear motion into rotary motion via the connecting rod and crankshaft. That’s the foundation. Next, we’ll build the terminology and the physics—Bernoulli, the Venturi, and the gas laws—that explain how the air and fuel behave inside that cylinder.