Piston Engines - Introduction — Page 10, Lesson 9

Right, let's get into the very start of piston engines. This is the introduction, and it's all about the fundamental physics we build everything on. I want to walk you through the terminology first, because these are the exact words we'll use for the rest of your career. First, Force. A force is that which, when acting on a body which is free to move, causes it to move. Conversely, it's that which stops, or changes the direction of, a moving body. So it's a push or a pull that changes motion. Now, force is produced when a mass is accelerated. The equation is Force = Mass × Acceleration, which we write as F = m × a. For example, a force moves the piston down the cylinder. The units are newtons or pounds force. Next, Work. The work done by a force is defined as the product of the force and the distance moved in the direction of the applied force. So it's not just pushing; it's pushing something over a distance. For example, the piston is moved from the top to the bottom of the cylinder by a force. The units are joules or foot pounds. Then we have Energy. Energy is the capacity of a body to do work. It comes in many forms: Heat, Light, Chemical, Kinetic, and Potential. The units are joules. Now, there's a critical rule here called the Law of Conservation of Energy. It states that energy can be neither created nor destroyed; only its form may be changed. Let me give you the exact chain for our engine: the chemical energy of the fuel is converted to heat energy during combustion in the engine. The engine then converts that heat energy into mechanical energy. Finally, Power. Power is the rate of doing work — work done per unit time. The units are joules per second, which is a watt, or foot pounds per minute, which is horsepower. Work is done as the piston moves in the cylinder. It's moved so many times a minute, and so the power can be measured. The horsepower is a measurement of power which is equal to 33,000 foot pounds a minute. So that's a fixed number you should remember. Now, let's move to Dynamics. This is where we bring in Newton's Laws of Motion, which deal with the properties of moving objects, or bodies. It's easy to see a piston or crankshaft move, but here's the key point: air is also a body, and it will obey Newton's Laws. You must remember that air is the working fluid within the engine. That's a crucial concept — we're not just moving metal; we're moving air. First Law: "A body will remain at rest or in uniform motion in a straight line unless acted on by an external force." So to move a stationary object, or to make a moving object change its direction, a force must be applied. Let's apply this to our engine. The mixture of fuel and air for a piston engine does not want to flow into the cylinder; a force must make it flow. And the piston moving down the cylinder does not want to stop. This opposition of a body to change its motion or state of rest is called Inertia. Newton's 1st Law has no units of measurement. It's a property a body possesses, whether stationary or moving. Because of this, Newton's 1st Law is known as the Inertia Law. Second Law: "The acceleration of a body from a state of rest, or uniform motion in a straight line, is proportional to the applied force and inversely proportional to the mass." So the harder you push, the more it accelerates; the heavier it is, the less it accelerates for the same push. Now, how does this apply to us? The energy released by the fuel during combustion increases the pressure energy of the air in the cylinder, and work can be done. The force to move the piston can be controlled by... well, that's exactly where we're heading next.