Prerequisite Knowledge:
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Piston engines are found in a wide variety of aircraft, ranging from single engine general aviation planes to four engine commercial aircraft. They come in many different arrangements, but they all share the same basic principles. This article will explore how piston engines work, and some of the different shapes they can take.
The Cylinder
At the heart of all piston engines are the cylinders. The cylinders are where all of the engine's power is generated, and they contain several key components. Inside the cylinder is where the fuel and air is ignited, and the force of the combustion is used to eventually power the propeller. It is important to note that when fuel and air are burned inside of the cylinder, it is not an explosion, but rather a gradual and progressive burning of the fuel. The phenomenon of exploding fuel/air rather than controlled combustion is known as detonation, and it is discussed below.
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Piston engines are found in a wide variety of aircraft, ranging from single engine general aviation planes to four engine commercial aircraft. They come in many different arrangements, but they all share the same basic principles. This article will explore how piston engines work, and some of the different shapes they can take.
The Cylinder
At the heart of all piston engines are the cylinders. The cylinders are where all of the engine's power is generated, and they contain several key components. Inside the cylinder is where the fuel and air is ignited, and the force of the combustion is used to eventually power the propeller. It is important to note that when fuel and air are burned inside of the cylinder, it is not an explosion, but rather a gradual and progressive burning of the fuel. The phenomenon of exploding fuel/air rather than controlled combustion is known as detonation, and it is discussed below.
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- Crankshaft: The shaft driven by the piston, usually what is connected to the propeller and other accessory systems.
- Piston: The part of the assembly that moves up and down. The force of the combustion is what drives the cylinder down, which then rotates the crankshaft. The piston is connected to the crankshaft via the crankpin, crank, connecting rod, and wrist pin.
- Piston rings: Rings that expand to seal the gap between the piston and cylinder wall, in order to keep the combustion gases contained in the combustion chamber.
- Crankcase: The lower part of the entire assembly, which houses the crankshaft, crank, and connecting rod.
- Spark plugs: These generate the spark that is used to ignite the fuel air mixture at a certain time.
There are many things happening simultaneously within an cylinder when an engine is running. Below is a chronological explanation of what exactly happens inside the cylinder:
Modern aircraft engines are classified as four stroke engines, which means that each "cycle" in the cylinder is comprised of four main strokes of the piston (two where the piston is moving down, and two where it is moving up). These four strokes are as follows:
Piston engines are also known as "reciprocating" engines because of all of this (the up and down motion of the pistons).
During each four stroke cycle, the crankshaft makes two revolutions, and the camshaft makes one revolution. This means that when the engine is operating at twelve-hundred rotations per minute (rpm), the crankshaft is rotating twenty times per second. The camshaft would be rotating at ten times per second, and ten of the four stroke cycles would happen in that second (which equates to fourty strokes in that second).
The point is, things are happening very quickly, and timing has to be exact for everything to run smoothly.
In most piston engine aircraft, the crankshaft is connected directly to the propeller, which means the propeller turns at the same speed as the crankshaft.
Engine Arrangements
A single cylinder is not enough to generate sufficient power to turn the propeller and power any accessory devices (e.g. an alternator to generate electrical power). Most general aviation aircraft have engines containing four cylinders. The DC-6 had four engines, each with eighteen cylinders. There are several ways manufacturers can arrange the cylinders in an engine.
- Intake: During the intake stroke, the piston is moving downwards. The intake cam forces the intake valve to the open position, and the air/fuel mixture is drawn into the combustion chamber as the piston continue to move downwards.
- Compression: During the compression stroke, the intake valve closes. The piston starts coming back up, and compresses the air/fuel mixture in the combustion chamber.
- Power: The power stroke is where the spark plug generates a spark, which ignites the air fuel mixture right as the piston begins moving down again. As the gases burn, they expand, forcing the piston down.
- Exhaust: As the piston begins moving up again, the exhaust cam opens the exhaust valve, allowing the waste gases to exit the combustion chamber. As the piston begins moving down again, the exhaust valve closes, and the four stroke cycle begins again.
Piston engines are also known as "reciprocating" engines because of all of this (the up and down motion of the pistons).
During each four stroke cycle, the crankshaft makes two revolutions, and the camshaft makes one revolution. This means that when the engine is operating at twelve-hundred rotations per minute (rpm), the crankshaft is rotating twenty times per second. The camshaft would be rotating at ten times per second, and ten of the four stroke cycles would happen in that second (which equates to fourty strokes in that second).
The point is, things are happening very quickly, and timing has to be exact for everything to run smoothly.
In most piston engine aircraft, the crankshaft is connected directly to the propeller, which means the propeller turns at the same speed as the crankshaft.
Engine Arrangements
A single cylinder is not enough to generate sufficient power to turn the propeller and power any accessory devices (e.g. an alternator to generate electrical power). Most general aviation aircraft have engines containing four cylinders. The DC-6 had four engines, each with eighteen cylinders. There are several ways manufacturers can arrange the cylinders in an engine.
Horizontally opposed: In the horizontally opposed layout, half of the cylinders are placed one side, and the remaining cylinders are located on the other side. All of the cylinders are on the same plane (all horizontal, hence horizontally opposed). To the right is a picture of a Lycoming O-235 four cylinder engine, used to power the Cessna 152. The O-235 utilizes a horizontally opposed engine type, with two cylinders on one side and two on the other.
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Radial: In radial engines, the cylinders are all mounted in a circle. This allows for more cylinders to be mounted than with horizontally opposed arrangements, and the fact that all of the cylinders are up front means they all get better cooling (air cooled horizontally opposed engines can have issues with the cylinders at the back overheating, due to them not getting much airflow). However, radial engines have a much larger profile, and therefore create more profile drag than the slimmer horizontally opposed arrangement.
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Abnormal Piston Engine Operation
Detonation
Detonation occurs when the fuel/air mixture burns explosively rather than progressively in the combustion chambers. Under normal operation, the burning of the fuel/air mixture is a controlled, progressive burn, starting from the top of the combustion chamber at the spark plug and moving down. Detonation is when the fuel/air mixture ignites all at once, creating excessive force that can damage the components of the engine, and eventually lead to catastrophic engine failure.
Detonation can be caused by several factors, such as an engine that is running too hot, or incorrect fuel. Fuel is generally categorized by it's octane rating. Fuels with a higher octane rating can handle higher pressures in the combustion chamber, and are therefore less likely to detonate.
Pre-ignition
Pre-ignition occurs when the fuel/air mixture ignites prematurely, when the pison is still in it's compression stroke. During the compression stroke, the piston is moving upwards, so having the fuel/air mixture at that point would result in the piston being slammed downwards while it is still trying to move up. This too can cause a dramatic failure of the engine.
Pre-ignition can be caused by an overheated engine, a bad fuel rating, or mineral deposits in the engine. Over time, carbon, lead, and other minerals can build up inside the cylinders, which become excessively hot during engine operation. These hot spots can ignite the fuel before the spark plug fires, causing pre-ignition. These mineral buildups are discussed more in the next article.
That's it for piston engine basics! The next article will cover ignition systems in more detail.
Detonation
Detonation occurs when the fuel/air mixture burns explosively rather than progressively in the combustion chambers. Under normal operation, the burning of the fuel/air mixture is a controlled, progressive burn, starting from the top of the combustion chamber at the spark plug and moving down. Detonation is when the fuel/air mixture ignites all at once, creating excessive force that can damage the components of the engine, and eventually lead to catastrophic engine failure.
Detonation can be caused by several factors, such as an engine that is running too hot, or incorrect fuel. Fuel is generally categorized by it's octane rating. Fuels with a higher octane rating can handle higher pressures in the combustion chamber, and are therefore less likely to detonate.
Pre-ignition
Pre-ignition occurs when the fuel/air mixture ignites prematurely, when the pison is still in it's compression stroke. During the compression stroke, the piston is moving upwards, so having the fuel/air mixture at that point would result in the piston being slammed downwards while it is still trying to move up. This too can cause a dramatic failure of the engine.
Pre-ignition can be caused by an overheated engine, a bad fuel rating, or mineral deposits in the engine. Over time, carbon, lead, and other minerals can build up inside the cylinders, which become excessively hot during engine operation. These hot spots can ignite the fuel before the spark plug fires, causing pre-ignition. These mineral buildups are discussed more in the next article.
That's it for piston engine basics! The next article will cover ignition systems in more detail.