top of page

The 4-Stroke Cycle of Reciprocating Aircraft Engines: How Your Powerplant Breathes, Burns, and Flies

Updated: Dec 18, 2025

If you fly an airplane with a piston engine, you’re relying on a marvel of mechanical timing and precision: the 4-stroke cycle. This is the process by which reciprocating aircraft engines turn fuel into thrust-producing power, over and over again, thousands of times per minute.


Although it’s sometimes taught as a simple “suck, squeeze, bang, blow,” the cycle is worth understanding in detail—because it directly affects performance, troubleshooting, and maintenance decisions.



Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >


Overview of a Reciprocating Aircraft Engine

A reciprocating engine uses one or more cylinders, each containing a piston connected to a crankshaft. As the crankshaft rotates, each piston moves up and down in its cylinder. The cylinder’s intake and exhaust valves open and close at precise intervals, allowing air-fuel mixture in and exhaust gases out.


The 4-stroke cycle—intake, compression, power, and exhaust—refers to the four separate piston strokes required to complete one power-producing sequence. Each stroke is a half-rotation of the crankshaft, so a full cycle takes two full crankshaft revolutions.


1. Intake Stroke


What Happens:

  • The piston moves downward from Top Dead Center (TDC) to Bottom Dead Center (BDC).

  • The intake valve opens, while the exhaust valve remains closed.

  • Atmospheric pressure pushes a fuel-air mixture (from the carburetor or fuel injection system) into the cylinder.


Why It Matters in Flight:

Proper fuel-air ratio and unobstructed airflow are critical here. A dirty air filter, intake icing, or improperly set mixture can starve the cylinder and reduce power.


2. Compression Stroke


What Happens:

  • The piston moves upward from BDC back to TDC.

  • Both intake and exhaust valves remain closed.

  • The air-fuel mixture is compressed to a fraction of its original volume (compression ratios in aircraft engines typically range from 6:1 to 9:1).


Why It Matters in Flight:

Higher compression produces more power for a given displacement, but also increases susceptibility to detonation. Pre-ignition, incorrect magneto timing, or low-octane fuel can cause dangerous cylinder pressures here.


3. Power Stroke


What Happens:

  • Just before the piston reaches TDC, the spark plugs fire (magneto-driven).

  • The fuel-air mixture ignites, expanding rapidly.

  • This high-pressure combustion forces the piston downward from TDC to BDC, turning the crankshaft and producing usable engine torque.


Why It Matters in Flight:

This is where all the power comes from. Weak spark, fouled plugs, or incorrect mixture will cause roughness and reduced performance. Engine monitoring systems (EGT/CHT) can help pilots fine-tune combustion efficiency here.


4. Exhaust Stroke


What Happens:

  • The piston moves upward from BDC to TDC again.

  • The exhaust valve opens, allowing burnt gases to escape into the exhaust manifold.

  • The intake valve remains closed until the stroke finishes.


Why It Matters in Flight:

Any restriction here (e.g., cracked muffler baffles, carbon deposits) will increase back pressure, reducing efficiency and potentially causing overheating.


The Cycle in Motion

Here’s the complete sequence:

  1. Intake → Fuel-air charge enters.

  2. Compression → Mixture squeezed for efficient burn.

  3. Power → Combustion drives piston down.

  4. Exhaust → Spent gases leave.


This cycle repeats continuously for each cylinder, but in a multi-cylinder engine, the strokes are staggered so power delivery remains smooth.


Key Points for Pilots and Technicians

  • Valve Timing: Small changes in when valves open and close affect efficiency and power.

  • Ignition Timing: Firing too early or late can cause detonation, pre-ignition, or power loss.

  • Fuel Mixture: Too rich wastes fuel; too lean risks overheating or detonation.

  • Compression Health: Low compression readings in one cylinder indicate leaks (valves, piston rings, or head gasket).


Why Understanding the Cycle Matters

When you understand the four strokes, you can make better in-flight and maintenance decisions. You’ll know why leaning the mixture changes EGT, why a fouled plug makes the engine run rough, and why carb heat reduces power. In short—understanding how your powerplant works makes you a safer, more capable pilot.



Study Full Aviation Courses:

wifiCFI's full suite of aviation courses has everything you need to go from brand new to flight instructor and airline pilot! Check out any of the courses below for free:


Study Courses:


Checkride Lesson Plans:


Teaching Courses:



 
 
bottom of page