On every preflight, you open the small oil door on the cowling and pull the dipstick. For most students, that’s their first look at the Cessna 172 engine: a four-cylinder, air-cooled Lycoming whose basic design has changed very little in decades. All of Pacific Flying Club’s 152s and 172s fly on Lycoming power. In this fourth article in our technical series, we cover how the engine works, what its model name means, and why TBO and unleaded fuel have been in the aviation news this year.
The engine is horizontally opposed. Its four cylinders lie flat, two on each side of a central crankshaft. The opposing pistons balance each other, so the engine runs smoothly, and the flat shape keeps the nose low enough for a good view over the cowling.
There’s no gearbox. The propeller bolts directly to the crankshaft, so the tachometer shows engine and propeller RPM as the same number. That direct connection is why the Cessna 172’s fixed-pitch propeller behaves the way it does in climb and cruise.
The engine is also air-cooled, with no radiator or coolant. Air comes in through the front of the cowling, and baffles guide it over finned cylinders. Oil carries away a good share of the heat as well, which is one reason the oil check on every preflight matters.
In Lycoming’s naming, O means opposed, and an I in front of it means fuel injected. The number is the displacement in cubic inches. An O-320 is a carbureted opposed engine of 320 cubic inches. An IO-360 is a fuel-injected opposed engine of 360 cubic inches. The suffix, such as -L2A, identifies the specific configuration.
The R and SP use the same engine model. The SP gets its extra 20 horsepower from a higher maximum RPM. For a full comparison of the two airframes, see Cessna 152 vs 172.
Each cylinder has two spark plugs, fired by two separate magnetos. A magneto is an engine-driven generator that makes its own electricity for the plugs, so the engine keeps running even if the aircraft’s electrical system fails completely. If one magneto or set of plugs fails, the other keeps every cylinder firing.
Students meet this system during the run-up, when they check each magneto on its own and watch for a small RPM drop. The Pilot’s Operating Handbook (POH) sets the maximum acceptable drop. A drop that’s too large, or no drop at all, means taxiing back and talking to maintenance.
The 152 and 172P are carbureted. As air speeds up through a carburetor, it cools, and ice can form inside even on days well above freezing, especially in humid air like the Lower Mainland’s. That’s why these aircraft have a carburetor heat control, and why students learn to recognize the gradual RPM loss that can signal carb ice.
The 172R and SP are fuel injected. Fuel goes straight to each cylinder’s intake, so there’s no carburetor to ice up and no carb heat control. Hot starts take a slightly different technique, which is covered during the checkout. Both types have a red mixture control for leaning the engine as the aircraft climbs into thinner air.
TBO, or time between overhaul, is the manufacturer’s recommended number of operating hours before an engine is disassembled, inspected, and rebuilt. Lycoming lists the 152’s O-235 at 2,400 hours. The 172’s O-320 and IO-360 have long been listed at 2,000 hours
Caption to come:
In July 2026, Lycoming raised TBO to 2,600 hours for many factory-new and factory-rebuilt engines in those families. Engines must be operated and maintained to Lycoming’s requirements, and the 12-year calendar limit still applies, so not every engine qualifies.
Training aircraft fly a lot of hours, so a busy 172 can use a significant share of its engine life within a few years. At PFC, engine hours and inspections are tracked by our maintenance team as part of each aircraft’s maintenance schedule.
Piston training aircraft burn 100LL, dyed blue so pilots can confirm the grade during the fuel check. The “LL” means low lead. A small amount of tetraethyl lead raises the octane rating and prevents detonation, which matters most in high-compression engines.
Because the 172’s Lycomings are relatively low-compression, they were early candidates for unleaded fuel. GAMI’s G100UL was first approved for Lycoming-powered 172s in 2021, and Swift Fuels’ 100R was approved in 2024 for 172R and 172S models with the IO-360-L2A. In the United States, the FAA and industry are working toward eliminating leaded avgas by the end of 2030.
Canada hasn’t set a deadline. In August 2026, the City of Pitt Meadows asked the federal government for a timeline. Nothing changes in how students fly today, but the list of approved fuels for your aircraft is likely to change during your flying career.
Early 172s used a six-cylinder Continental. Cessna switched to the four-cylinder Lycoming O-320 with the 172I in 1968, and the 172 has been Lycoming-powered ever since. Lycoming, based in Williamsport, Pennsylvania, says its engines power more than half of the world’s general aviation fleet. If you go on to fly Pipers, Diamonds, or Robinson helicopters, you’ll likely meet another one.
You don’t need to be a mechanic to fly a 172. Understanding the engine, though, turns the run-up, carb heat check, and mixture lever into steps that make sense rather than items on a checklist. It’s also good preparation for the more complex aircraft used later in commercial pilot training.
You can see the aircraft on our Cessna 172 fleet page, and an instructor can walk you through the engine on your first preflight. Earlier articles in this series cover the fixed-pitch propeller, the high-wing design, and how the 152 and 172 compare.
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