What Is an Aircraft Internal Combustion Engine?
When piston-powered aircraft are in flight, they are subjected to four fundamental forces that need to be balanced to remain in equilibrium, these being thrust, lift, drag, and weight. While structures like the wings and horizontal stabilizer provide vertical lift to overcome the forces of weight, aircraft will still be faced with drag that combats their forward momentum. To overcome drag, piston-powered aircraft rely on thrust generated by the engine-propeller combination. In particular, light aircraft feature an internal combustion engine that powers the vehicle, and this type of engine works on the principle of converting the reciprocating motion of pistons moving up and down into a rotational motion that drives propellers. The movement of the pistons themselves is driven by force generated by a combusted mixture of fuel and air, wherein the chemical energy of fuel is converted into mechanical energy.
In order to understand how an internal combustion engine of a piston-powered aircraft works, it is important to be familiar with its various parts. To start, these particular engines feature pistons, the reciprocating component of the engine that transfers force from expanding gasses in the combustion chamber to the crankshaft through a connecting rod. Within this system, the piston does not directly touch the cylinder; instead the cylinder wall and piston feature a gas seal that is maintained through piston rings and oil lubrication. Often made of iron, the piston rings are placed on grooves that are located just below the piston crown, and there are three types: compression rings, oil control rings, and oil scraper rings.
As the first type, compression rings are located at the top of each piston, ensuring an airtight seal between the cylinder and the piston during the compression and combustion stroke of a four-stroke cycle. Next, oil control rings are situated underneath the compression rings, and they are designed to permit oil circulation from inside the piston to the cylinder walls. This circulation is made possible by a set of small oil drain holes. Finally, oil scraper rings sit near the bottom of the piston and are shaped in a way that allows them to scrape oil up and down the cylinder while the piston is in motion. Any extra oil is retained during the up-stroke and returned to the crankcase in the downward stroke.
As another core part of the engine, the crankcase is the housing that holds the crankshaft and the connecting rods that attach the piston to the crankshaft. Typically made from a cast or forged aluminum, the crankcase offers the rigidity necessary to keep the crankshaft in place while reaping the benefits of aluminum, rather than steel. Engine lubricating oil is stored at the bottom, where it eventually gets cycled through the engine to lubricate the crankshaft, connecting rod bearings, and other metallic parts. Afterwards, the oil moves within the cylinder walls and passes through the pistons before it empties into the crankcase once more.
The connecting rods create an attachment between a piston and the crankshaft, and both components work together to convert the piston’s reciprocating motion into rotational motion with the help of the rotating crankshaft. As such, when the engine is running, the crankshaft converts the pistons’ up-and-down motion into rotary motion.
Another common internal combustion engine part is a valve, and there are a minimum of two valves per cylinder. One serves as an inlet for fuel and air while the other exhausts gasses following combustion. Working in conjunction with these valves, a device called the camshaft opens and closes the inlet and exhaust valves. Finally, spark plugs help ignite the mixture of fuel and air entering the cylinder inlet by accepting a high voltage from the ignition system. Therein, the voltage enters through a recess that is held in place by a nut and covered with weather-proof sealing. This voltage jumps between a central electrode and the plug exterior, creating a spark.
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