Understanding the Basics of Internal Combustion Engines

First of all before you can understand how an engine works you must know what it does. An engine is a machine that converts chemical energy to mechanical energy. Converting chemical energy into mechanical energy is achieved by utilizing a fundamental law of physics PV=nRT. We know gas expands with the application of heat and that temperature increases when a gas is compressed. Engines confine this gas expansion process and use it to force the piston downward.

Internal Combustion vs. External Combustion:
There are two main types of engines, internal combustion engines and external combustion engines. In internal combustion engines, the burning of fuel occurs inside the engine within the same cylinder that produces energy to turn the crankshaft. In external combustion engines for instance, steam engines, the burning of fuel takes place outside the engine. For this memo we will be focusing on the internal combustion engines.

Ignition:
Many small engines don’t require a battery, instead they generate power for the spark plug using a magneto. The magneto is an electrical generator on the flywheel. When the starter cord is pulled, magnets imbedded in the flywheel pass by the ignition armature, making a magnetic field in the armature. Once this current reaches maximum capacity, voltage is carrier to the spark plug where the combustion process begins. The spark plug is responsible for providing the spark that ignites the air and fuel mixture.

Design:
Internal combustion engines bring in air and fuel into the carburetor. The choke plate allows air to flow into the carburetor while the throttle plate regulates how much air car flow out the carburetor. The carburetor is specially designed so that it mixes just the right amount of air and fuel so that the engine runs correctly. The carburetor creates a vacuum using a narrowing design called a venturi. Once the air and fuel mixture passes through the carburetor, it is let out into the engine.

The Combustion Process:
When fuel is burned inside the engine it creates combustion, this combustion occurs inside a cylinder causes a metal piece called a piston to move downwards. A piston is a cylindrical piece of metal that shifts up and down in a reciprocating motion. The piston is joined with the connecting rod which is joined with the crankshaft, a part that is responsible for moving the piston up and down. The crankshaft and connecting rod convert the pistons reciprocating motion into rotary motion. Since the crankshaft is connected to the flywheel, the rotary motion of the crankshaft spins the flywheel.

The Four-Stroke Process:
Our 3.5 horsepower combustion engine uses a four-stroke cycle to make chemical energy into mechanical energy. The four stroke cycle was first invented by Nikolaus Otto in 1867. Each single movement of the piston from top to bottom or from bottom to top is called a stroke. Our 3.5 horsepower internal combustion engine has four-strokes, the intake stroke, the compression stroke, the combustion stroke, and the exhaust stroke. For the first stroke in the four-stroke cycle air and fuel mixture is allowed into the cylinder through the intake valve (exhaust valve is closed). For the next stage of the four-stroke process both valves close and the rising piston compresses the air/fuel mixture. Near the end of the compression stroke a spark ignites the mixture. The third stroke of the four-stroke cycle is called the power-stroke. The ignited air and fuel mixture increases in pressure and temperature resulting in expansion forcing the piston downward. Near the end of the stroke the exhaust value opens and begins the forth and final stage of the four-stroke cycle. In the exhaust stage, the exhaust valve is opened allowing the exhaust products to exit. Near the end of the exhaust stage the intake valve opens allowing more air and gas mixture to flow in. It is important to note that both the exhaust and intake valves close during the combustion process. A part called the camshaft is responsible for making sure the valves are timed correctly. The camshaft has lobes, as it spins the lobes open and close the intake and exhaust valves by allowing tappets to move up and down. The camshaft converts rotary motion into reciprocating motion of the tappets.