What is unique about an airplane engine? Normally, for smaller aircraft, and airplane engine is an air cooled piston engine. There are two primary manufacturers, Lycoming and Continental. They have similar designations for their engines, a typical indicator will be something similar to:
The fist letter "O" stands for opposed. Some people call this a boxer engine, Subaru, and Porsche are typical manufactures of this layout of an engine. The manufacturers also use an "R" for radial. Radial engines are roundish, and you'll see them on WWII aircraft.O-360A1D
The next number is the displacement of the engine. 360 is for the number of cubic inches of displacement. Common other values are 320, 540 and 720. A Ford 302 is similar, with 302 cubic inches of displacement (or 5 liters).
The values after the the cubic inches define some accessories and other details about the internal parts of the engine.
The engine on my airplane is an O-360A1D engine. It is a 4 cylinder air cooled carbureted engine. It produces about 180HP at 2700RPM. The propeller is attached to the crankshaft through an extension. The slow turning is ideal, and prevents the propeller tip speeds from going supersonic. (I could get into the math, but you can do it if you want. My propeller is 76" diameter, 3ft radius, and arcing at 45 RPSecond you can see pi * d is about 19ft, and 45 times that is 848 ft per second times 3600 and you end up about 578 miles per hour, at the tip).
I want to monitor the cylinder cooling. The baffling on the engine is designed to cool the cylinders, and the air must be efficiently be directed around them. Measuring the individual cylinder head temps (CHT) will help determine the quality of the baffling, and indicate the even flow around them.
The exhaust gas temperature (EGT) can help determine the correct mixture, and even burning of the fuel. As the airplane goes higher, the air is thinner, meaning the mixture must be adjusted to help maintain the proper air fuel mixture. If the temps go down, the extra fuel is coming out the exhaust, and the mixture should be leaned. Cars have mass airflow sensors to maintain good mixture, airplanes have a mixture lever.
The engine RPM will be monitored to be sure the proper speed is maintained, and the engine is running as expected.
Oil is the lifeblood of the engine. Proper temperatures, and pressure will insure the oil is doing it's job. It has two purposes in this engine; provide lubrication and provide some cooling. There is an oil cooler on the engine, and I want to be sure it is operating properly, without too much or too little cooling.
Fuel pressure is something else to watch. There are two fuel pumps on the engine, mechanical and electric. I need to be sure the fuel can get from the tank to the engine as expected. The pumps will help that happen. The fuel pressure will indicate that is working, and if there are any issues.
Those are the main things that I will monitor. Other things will come up. This design is flexible enough that someone with a 6 cylinder engine could use the design.