Piston Engines - Introduction — Page 10, Lesson 12

Right, let's get into the very start of the powerplant syllabus. We're opening with the physical laws that make a piston engine work, so I want you to think of this as the foundation layer — the physics that everything else in this book will hang on. We begin with Newton's Laws of Motion, and I want to be precise about the second one. Newton's Second Law is known as the Momentum Law. Now, momentum itself is defined here as Mass × Velocity. It's similar to inertia, but the key distinction is that inertia applies to bodies at rest, whereas momentum only applies to moving bodies. And because it's a product of mass and velocity, it has units of measurement — kilograms and metres per second. So when the piston in an engine is accelerated, its mass is given a velocity, and that product is its momentum. Then we have Newton's Third Law, which is stated as: "For every action there is an equal and opposite reaction." This one is called the Reaction Law. You see it everywhere — the recoil of a gun as the bullet is forced out of the barrel, the snaking of a hose as water is forced from its nozzle, and critically for us, the operation of the jet engine. The action of expelling gas backwards produces the reaction of the engine moving forwards. Now we move from motion to heat. Thermodynamics is the study of Heat/Pressure energy — or, put another way, the behaviour of gases and vapours under variations of temperature and pressure. There are two laws here that matter to us. The First Law states: "Heat and Mechanical energy are mutually convertible and the rate of exchange is constant and can be measured." Let me unpack that. If you rub two moving surfaces together without lubrication, heat is generated, and you can measure it with a temperature gauge. That's mechanical energy being converted into heat energy. Conversely — and this is the heart of the piston engine — when fuel is burned in the engine, the heat energy in the fuel is converted into mechanical energy by the action of the pistons and crankshaft. And that conversion, too, can be measured. So the law is saying energy changes form, but the exchange rate is constant and measurable. The Second Law states: "Heat cannot be transferred from a region at a lower temperature to one at a higher temperature without the expenditure of energy from an external source." Heat naturally flows from a hot radiator to the colder atmosphere around it — that's spontaneous. But to make a refrigerator colder than the surrounding atmosphere, you have to spend energy to force that heat to flow the "wrong" way. That's the expenditure of energy from an external source. Finally, we come to Bernoulli's Theorem, from Daniel Bernoulli, a Swiss scientist who lived from 1700 to 1782. He discovered properties relating to fluids in motion. The mathematical statement is this: the total energy in a moving fluid or gas is made up of three forms of energy — the energy due to height or position, which is the potential energy; the energy due to pressure, which is the pressure energy; and the energy due to movement, which is the kinetic energy. And in the streamline flow of an ideal fluid, the sum of all three is constant. Now, when we're dealing with airflow in aviation, we can make a practical simplification. The potential energy — the height or position term — can be assumed to be constant. So for all practical aerodynamic purposes, the statement becomes: the kinetic energy plus the pressure energy of a smooth flow of air is always constant. The direct consequence is that if the kinetic energy increases, the pressure energy drops proportionately, to keep the total constant. That's the principle that explains how a venturi works — and it's the very foundation of how we measure airspeed and how carburettors meter fuel. So to tie it together: momentum and reaction give us the forces, thermodynamics gives us the energy conversion, and Bernoulli gives us the behaviour of the air and fuel mixture. That's your opening framework.