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The Beating Heart

How Your Engine Works and How to “Treat” It

By James Williams

Source: FAA Safety Briefing Jan/Feb 2020

An airplane’s engine is the closest thing it has to a heart. The engine provides the energy that not only propels the airplane, but also runs all of the other systems. The engine turns the alternator that provides the electricity. It runs the various pumps that power systems like hydraulics, pressurization, etc.

For most of us in general aviation, an engine means an internal combustion engine. Specifically, it means a reciprocating engine, a term that simply denotes the back and forth motion of the pistons. The goal of the engine is to transform potential energy stored in fuel into mechanical energy that powers your airplane, with the help of some air.

reciprocating engine

Basic Anatomy

An engine has several basic components. First is the cylinder, where combustion occurs. Next is the piston, which nests inside the cylinder from the bottom and provides the compression for, and absorption of energy from, combustion. Supporting the piston is the connecting rod, which transmits energy down to the crankshaft, transferring it out of the engine, usually to a propeller.

As its name implies, the cylinder head sits on top of the cylinder and houses critical components like valves and spark plugs. The valves open to allow the air and fuel mixture into the cylinder (the intake valve) and to allow combusted gases out (the exhaust valve). The spark plug ignites the compressed fuel and air, transforming that chemical energy into mechanical energy that spins the crankshaft and turns the propeller.Now that we know the basics, let’s see how these parts work together.

crankshaft

And One, Two, Three, Four, Repeat!

Aircraft engines are, with few exceptions, four-stroke engines with four distinct phases: intake, compression, power, and exhaust. During the intake stroke, the piston lowers from the top of the cylinder while the intake valve opens to let in the fuel/air mixture. The compression stroke begins when the intake valve closes and the piston begins to rise toward the top of the cylinder. The power stroke kicks off when the spark plug ignites the compressed fuel/air mixture, causing combustion that pushes the piston back down forcefully. The exhaust stroke starts when the piston reaches bottom dead center and begins to rise again to push out burned gases through the open exhaust valve. Then we start all over again. Although we break the process down into separate steps, the reality is that it is more of a continuous process.

four stroke cycle

Supporting Cast

Engine cooling is one of the systems that helps your engine work. Internal combustion engines turn most of the energy from combustion into waste heat. While most of that is sent out through the exhaust, a significant amount of heat remains. Our engines are generally air cooled, so logic suggests that more air equals better cooling. Consequently, the nacelle contains ducts and baffles that direct air flow evenly across the engine’s cooling surfaces, thus keeping the engine’s operating temperature balanced. If these baffles are removed or damaged, excessive heat buildup in part of the engine can lead to additional wear and possibly failure.

In addition to cooling, an engine needs air and fuel. An intake manifold guides the mixture into the cylinder and fuel is added via the carburetor or fuel injectors. The carburetor remains the most common solution. Carburetors are the older technology but have the advantage of being a well-tested, less complex, and very reliable solution.

Fuel injection allows greater control and greater efficiency, but is more complex. Carburetors do have one distinct disadvantage: carburetor icing can choke the engine. Carb heat is a simple solution to this specific problem, but you do have to activate it.

Then there’s the exhaust system, which transfers the spent gases and heat out of the cylinder. The exhaust system ushers the hot combustion gases safely out of the engine compartment and into a muffler. Despite its humble description, the exhaust system is absolutely safety critical.

One way to get more power from an engine is to increase the amount of air and fuel in the cylinder during combustion. This can be done through forced induction, more commonly called turbocharging or supercharging. Turbocharging is more common in today’s GA airplanes, but both techniques essentially do the same thing. They compress intake air to force more air and fuel into the engine than normal atmospheric conditions will allow. The difference is that turbocharging uses the engine’s exhaust gases to power the compressor while a supercharger taps the engine’s power output.

cooling the engine

Heart Health

Now that we know how the airplane engine works, let’s look at a few ways this “heart” can run into trouble. During preflight, it’s important to look for any evidence of leaking or damaged fuel or oil lines. Visually check connections to the greatest possible extent; loose wires or lines can chafe and quickly turn a minor issue into a major emergency.

Never forget to check the oil, which is the lifeblood of the engine. It helps transfer heat from the hot parts of the engine to areas where it can be safely dissipated. More importantly, it lubricates the engine so that it can function efficiently. Oil starvation, whether from leakage, burning, or just breakdown is one of the common causes of airplane “cardiac” events. Also bear in mind that oil degrades over time, becoming less effective at its job. Regardless of the cause, insufficient lubrication can result in serious damage. Monitoring not just the amount of oil but also its condition during preflight is critical.

Modern avionics and engine tracking systems have made detecting problems a more proactive process. Data analysis can allow for intervention before an emergency. When coupled with a better understanding of the engine and a thorough preflight, they can be a major positive force. It’s always better to find a problem in the data rather than in the air.

aircraft engine

Learn More

Pilot’s Handbook of Aeronautical Knowledge — Chapter 7 bit.ly/354k5ex

James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.

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