Every four-stroke aircraft engine depends on a precise sequence of intake, compression, power, and exhaust events — and the exact moments the intake and exhaust valves open and close are no accident. Valve timing refers to the crankshaft angle positions at which each valve begins to open and finally closes, expressed in degrees of crankshaft rotation. Valve overlap is a specific portion of that timing: the brief period when both the intake and exhaust valves are simultaneously open at the end of the exhaust stroke and the very beginning of the intake stroke. Together, these two concepts determine how efficiently the cylinders breathe, how much power the engine develops, and whether the engine remains healthy over time. For the AMT powerplant technician, understanding valve timing is essential for correct engine assembly, troubleshooting poor performance, and interpreting manufacturer specifications.
The Four-Stroke Cycle and Where Valves Fit In
To appreciate valve timing, you must first anchor it in the four-stroke cycle. During the intake stroke, the piston moves from top dead center (TDC) to bottom dead center (BDC) while the intake valve is open, drawing the fuel-air charge into the cylinder. On the compression stroke, both valves are closed and the piston rises to compress the charge. The power stroke follows ignition: both valves remain closed as the burning gases expand and push the piston down. Finally, during the exhaust stroke, the exhaust valve opens and the piston rises again, pushing spent gases out of the cylinder.
In a theoretically perfect engine operating at very low speeds, the intake valve would open exactly at TDC and close exactly at BDC, and the exhaust valve would do the mirror image on the other side. In the real world, however, gases have inertia and the engine operates across a range of RPM. Valve timing is therefore deliberately shifted — measured in degrees of crankshaft rotation before or after TDC and BDC — to take maximum advantage of gas-flow momentum.
How Valve Timing Works in Practice
Intake Valve Opens Early (BTDC)
The intake valve does not wait until TDC to open. It begins to open before top dead center (BTDC) near the end of the exhaust stroke, while the piston is still rising toward TDC. This early opening takes advantage of the remaining exhaust-gas flow, which is still moving outward and creates a slight low-pressure area near the valve. By the time the piston starts the intake stroke, the intake valve is already partially open and ready to admit the fresh charge immediately. The number of degrees BTDC at which the intake valve opens is specified by the engine manufacturer and is set by the camshaft lobe profile and the mechanical relationship between the camshaft and crankshaft (a 2:1 gear ratio in a four-stroke engine).
Intake Valve Closes Late (ABDC)
Equally important, the intake valve does not snap shut at BDC. It remains open for a period after bottom dead center (ABDC) on the compression stroke. Even though the piston has started upward, the inertia of the incoming fuel-air mixture means the gas is still flowing into the cylinder. Closing the valve late captures this additional charge, increasing the volumetric efficiency of the engine. If the valve closed exactly at BDC, that moving column of air-fuel would be wasted. The result is a cylinder that is slightly more completely filled than it would otherwise be — more mixture means more power potential.
Exhaust Valve Opens Early (BBDC)
On the power stroke, the exhaust valve begins to open before bottom dead center (BBDC), while combustion pressure is still above atmospheric. This may seem wasteful — and a small amount of expansion energy is indeed sacrificed — but the benefit is that exhaust gases begin escaping before the piston starts rising. By the time the exhaust stroke begins, a significant portion of the hot gases have already started moving out, reducing the pumping work the piston must do to push out the remaining exhaust. At typical aircraft engine operating speeds, this tradeoff strongly favors early exhaust opening.
Exhaust Valve Closes Late (ATDC)
The exhaust valve does not close at TDC either. It remains open after top dead center (ATDC) of the exhaust stroke, into the beginning of the intake stroke. The momentum of the exiting exhaust gas continues to scavenge the cylinder even as the intake valve has already begun to open. This late closing ensures the maximum amount of burned gas is removed before the fresh charge takes its place.
Valve Overlap Explained
Valve overlap is the angular period, measured in crankshaft degrees, during which both the intake valve and the exhaust valve are simultaneously open. It occurs around TDC between the exhaust and intake strokes — the exhaust valve has not yet closed (it closes ATDC) while the intake valve has already opened (it opens BTDC). This overlap period typically ranges from a few degrees to several tens of degrees, depending on the engine design and its intended operating range.
During overlap, two beneficial effects occur. First, the momentum of the exiting exhaust gas creates a slight venturi-like low pressure in the cylinder and at the exhaust port, which can help draw fresh intake mixture into the cylinder — this is called scavenging. Second, the fresh incoming mixture helps push any remaining exhaust gases out, further purging the cylinder. The net result at the correct operating speed is a cylinder that is more completely filled with fresh charge and more completely emptied of burned gas than would be possible with no overlap.
However, valve overlap is a design compromise. At low RPM or idle, intake velocity is low and the scavenging effect is weak. With both valves open simultaneously, exhaust gases can actually flow back into the intake manifold, or raw mixture can pass straight through the cylinder and out the exhaust — a phenomenon called short-circuiting. This is why engines with large valve overlap (common in high-performance designs) often idle roughly and lose efficiency at low speeds. Aircraft engine designers balance the overlap to provide good scavenging at cruise and takeoff RPM without unacceptable roughness at idle.
The Camshaft and Valve Timing Mechanism
Valve timing is built into the engine through the camshaft, whose lobes are shaped and positioned to open and close each valve at the correct crankshaft angle. Because the four-stroke cycle requires each valve to open once for every two crankshaft revolutions, the camshaft rotates at exactly one-half crankshaft speed. The gear train or timing gears connecting the camshaft to the crankshaft must be assembled with the manufacturer's timing marks aligned; if these marks are off by even one tooth, the valve events will be displaced and engine performance will suffer significantly. On horizontally opposed aircraft engines, the camshaft typically runs along the top or bottom of the crankcase, actuating pushrods that transfer motion through rocker arms to the valve stems.
Valve clearance — the gap between the rocker arm and the valve stem tip (or between the tappet and camshaft lobe on solid-lifter engines) — also affects effective valve timing. Excessive clearance means the valve opens later and closes earlier than designed, reducing the effective opening duration and hurting volumetric efficiency. Insufficient clearance means the valve may not fully close, causing compression loss, burning of the valve face and seat, and potential valve failure. The AMT must always set valve clearance to the manufacturer's specification using the correct feeler gauge technique with the engine in the correct position.
Why Valve Timing Matters for Maintenance and Safety
Incorrect valve timing is a direct cause of poor engine performance and accelerated wear. A stretched or worn timing gear or chain can retard valve timing, causing late intake opening and late exhaust closing — the engine loses power and may run rough. Worn camshaft lobes reduce valve lift and shorten opening duration, again reducing volumetric efficiency. A burnt exhaust valve that cannot fully close allows combustion gases to erode the valve and seat, rapidly destroying both. All of these faults have roots in the precise world of valve timing.
From a powerplant technician's perspective, any time an engine is disassembled and reassembled, verifying correct camshaft-to-crankshaft timing is mandatory. The manufacturer's overhaul manual will specify timing marks, the correct TDC reference, and the method for confirming valve events using a degree wheel if necessary. Aircraft engines certified under FAA type certificates must be overhauled and reassembled to those specifications; deviation is an airworthiness violation.
Key Numbers and Rules
- Camshaft speed: Always one-half of crankshaft speed in a four-stroke engine — one camshaft revolution per two crankshaft revolutions.
- Valve overlap: The simultaneous opening of both intake and exhaust valves, occurring around TDC between the exhaust and intake strokes.
- Intake valve opens: Before TDC (BTDC) on the exhaust stroke.
- Intake valve closes: After BDC (ABDC) on the compression stroke.
- Exhaust valve opens: Before BDC (BBDC) on the power stroke.
- Exhaust valve closes: After TDC (ATDC) on the exhaust stroke.
- Valve clearance: Must be set to manufacturer specifications; too much or too little clearance alters effective timing and causes damage.
- Timing marks: Must be aligned per manufacturer instructions during reassembly; one-tooth error in the camshaft gear causes significant timing displacement.
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