1.2 Valve Timing, Overlap & Duration

1.2 Valve Timing, Overlap & Duration

Quick Answer: Valve timing specifies the precise crankshaft angles at which intake and exhaust valves open and close. Because air-fuel charges and exhaust gases possess physical mass, velocity, and inertia, valves do not open or close at exact dead center. Valve lead (early opening), valve lag (late closing), and valve overlap (both valves open simultaneously at TDC) maximize volumetric efficiency and combustion chamber cooling. Independent FAA AMT Powerplant prep by OpenExamPrep.

---

The Aerodynamics of Cylinder Gas Exchange

In theoretical thermodynamics, valve operation is instantaneous: the intake valve opens at TDC and snaps shut at BDC; the exhaust valve opens at BDC and snaps shut at TDC. In a physical aircraft engine turning at 2,400 to 2,700 RPM, however, this theoretical timing is completely non-viable due to fundamental laws of fluid dynamics and mechanical inertia:

1. Column Inertia of Incoming Mixture: The fresh fuel-air charge inside the induction manifold possesses mass. When the intake valve opens, the stationary column of air-fuel mixture requires time to overcome static inertia and accelerate into the cylinder. Once moving at high velocity, this air column develops immense momentum that resists stopping even after the piston reaches BDC.
2. Exhaust Gas Dynamics: At the end of the power stroke, cylinder gas pressures remain high (40 to 60 psi). If the exhaust valve waited until BDC to open, expanding gases would exert heavy upward backpressure resistance against the ascending piston during the exhaust stroke, robbing brake horsepower.
3. Mechanical Cam Profile Constraints: Poppet valves cannot be jerked open or slammed closed instantaneously without breaking valve stems, collapsing pushrods, or shattering valve seats. Cam lobes must incorporate gradual opening and closing ramps to control valve acceleration, prevent valve bounce (valve float), and maintain valvetrain structural integrity.

To overcome these physical constraints, aircraft engine designers engineer precise amounts of valve lead, valve lag, and valve overlap.

---

Mathematical Definitions & Timing Formulas

Every Aviation Maintenance Technician must master the definitions and mathematical derivations governing valve events, as these form standard calculation items on the FAA Powerplant written examination:

               Top Dead Center (TDC)
                        |
       IVO (Lead)       |       EVC (Lag)
       15° BTDC         |       18° ATDC
              \         |         /
               \        |        /
                \  [OVERLAP: 33°] /
                 \      |       /
       <----------+-----+-----+---------->
                 /      |       \
                /       |        \
               /        |         \
              /         |          \
       EVO (Lead)       |       IVC (Lag)
       55° BBDC         |       60° ABDC
                        |
              Bottom Dead Center (BDC)

1. Valve Lead

Valve lead is the number of crankshaft rotational degrees that an intake or exhaust valve opens before the piston reaches dead center:

• Intake Valve Lead: The intake valve opens before the piston reaches Top Dead Center (BTDC) on the exhaust stroke.
• Exhaust Valve Lead: The exhaust valve opens before the piston reaches Bottom Dead Center (BBDC) on the power stroke.

2. Valve Lag

Valve lag is the number of crankshaft rotational degrees that an intake or exhaust valve remains open after the piston passes dead center:

• Intake Valve Lag: The intake valve remains off its seat past Bottom Dead Center (ABDC) on the compression stroke.
• Exhaust Valve Lag: The exhaust valve remains off its seat past Top Dead Center (ATDC) on the intake stroke.

3. Valve Duration

Valve duration represents the total angular distance in crankshaft degrees that a valve remains lifted off its seat during a complete cycle:

$\text{Intake Valve Duration} = \text{IVO (BTDC)} + 180^\circ + \text{IVC (ABDC)}$ $\text{Exhaust Valve Duration} = \text{EVO (BBDC)} + 180^\circ + \text{EVC (ATDC)}$

Worked Example: Consider an engine where the intake valve opens 15° BTDC and closes 60° ABDC. The total intake duration is: $\text{Intake Duration} = 15^\circ + 180^\circ + 60^\circ = 255^\circ$

4. Valve Overlap

Valve overlap is the angular interval in crankshaft degrees during which both the intake valve and the exhaust valve are off their seats simultaneously at the end of the exhaust stroke and beginning of the intake stroke (at TDC):

$\text{Valve Overlap} = \text{IVO (BTDC)} + \text{EVC (ATDC)}$

Worked Example: If an intake valve opens 15° BTDC and the exhaust valve closes 18° ATDC, the valve overlap is: $\text{Valve Overlap} = 15^\circ + 18^\circ = 33^\circ$

---

The Physics & Operational Benefits of Valve Overlap

Valve overlap is one of the most critical aerodynamic tools available in reciprocating engine design. Why leave both valves open at the same time?

1. Low-Pressure Scavenging Wave

When the exhaust valve opens, high-pressure spent gases rush down the exhaust stack at supersonic to high-subsonic velocities. This rapidly exiting column of gas possesses high mass inertia. As the gas pulse exits, it creates a powerful localized low-pressure depression (rarefaction wave or suction pulse) directly in the combustion chamber behind the exhaust valve.

Because the intake valve is already cracked open during overlap, this exhaust suction depression reaches into the intake runner, overcoming the static inertia of the fresh fuel-air mixture and pulling it into the cylinder before the piston has even begun its downward travel on the intake stroke. This scavenges the clearance volume of stagnant, non-combustible exhaust gases, drastically improving volumetric efficiency.

2. Internal Combustion Chamber Cooling

Aircraft cylinder heads and exhaust valves operate under extreme thermal stress. During valve overlap, the cool, incoming atomized fuel-air charge sweeps across the red-hot exhaust valve head, the piston crown, and the spark plug electrodes. This cooling air wash absorbs localized heat, lowering cylinder head temperatures (CHT) and mitigating hot spots that would otherwise trigger dangerous preignition.

3. Idle Operation Tradeoff

While wide valve overlap yields exceptional power and cooling at high cruise and takeoff RPM, it produces a distinct operational penalty at low engine speeds (600 to 800 RPM). At idle, exhaust gas velocities are low, and the intake manifold depression (vacuum) is high (10 to 15 inHg absolute). As a result, exhaust gases can be pulled backward into the intake runner, diluting the incoming charge and causing the characteristic rough, uneven lope of high-performance reciprocating engines at idle.

---

Poppet Valve Construction & Sodium Cooling

Aircraft reciprocating engines utilize poppet valves (mushroom-shaped valves) that open inward toward the combustion chamber. The design demands placed on intake versus exhaust valves reflect completely different operating environments:

Sodium-Cooled Exhaust Valves

To prevent exhaust valves from burning or melting under 1,600°F exhaust gas streams, aircraft engine manufacturers incorporate sodium-filled exhaust valves:

• Internal Construction: The valve stem and part of the head are hollow, filled approximately 40% to 50% by volume with pure metallic sodium.
• Thermodynamic Action: Metallic sodium melts at a low temperature of 208°F (97.8°C). At normal engine operating temperatures, the sodium is a free-flowing liquid.
• Heat Transfer Path: As the valve reciprocates up and down, the liquid sodium sloshes violently between the hot valve head and the cooler valve stem (a convective pumping action). It absorbs heat from the head and transfers it rapidly to the stem. The heat conducts through the valve stem wall, across the valve guide in the cylinder head, and out through the external aluminum cooling fins.
• Safety Hazard Alert: Metallic sodium reacts violently and exothermically with water or atmospheric moisture, generating caustic sodium hydroxide and explosive hydrogen gas. Discarded sodium-filled valves must never be cut, drilled, or crushed; they must be disposed of in accordance with specialized hazardous material procedures.

---

Valvetrain Kinematics & Valve Clearance (Lash)

In an aircraft valvetrain, the cam lobe actuates a hydraulic or solid lifter (tappet), which pushes a hollow steel or aluminum pushrod, tilting a rocker arm that depresses the valve stem tip against concentric valve springs.

Solid Lifters vs. Cold Valve Clearance

In engines fitted with solid lifters, a specified mechanical clearance (lash) must be set between the rocker arm roller/toe and the valve stem tip when the engine is cold:

• Thermal Expansion Disparity: Aircraft cylinder assemblies consist of forged alloy steel barrels and cast aluminum alloy heads. Aluminum expands at more than twice the rate of the steel pushrods. As the engine warms from ambient to operating temperature, the cylinder head grows outward, increasing the physical distance between the rocker arm and the crankcase.
• Consequences of Excessive Valve Clearance:
  • Lifter contacts the cam opening ramp late and leaves early.
  • Valve opens late and closes early.
  • Valve duration is decreased.
  • Valve overlap is reduced.
  • Accelerated mechanical pounding and wear on valve tips and rocker arms.
• Consequences of Insufficient Valve Clearance:
  • Lifter rides on the cam lobe continuously.
  • Valve opens early and closes late.
  • Valve duration and overlap increase.
  • Critical Failure: When the engine reaches operating temperature, the valve may be prevented from seating fully. Hot combustion gases blow past the unseated valve face, causing localized torching and complete valve failure (burned valve).

Hydraulic Lifters

Modern opposed aircraft engines predominantly employ hydraulic lifters (zero-lash tappets). Pressurized engine oil fills an internal lifter reservoir via an internal check valve, constantly expanding the plunger to take up all mechanical slack. This eliminates manual clearance adjustments and maintains designed valve timing and duration across all engine operating temperatures.

---

Radial Engine Cam Rings

In radial engines, valves are actuated by a circular cam ring (or cam drum) rather than an elongated camshaft. The cam ring is mounted concentrically with the crankshaft and contains one or more tracks of raised cam lobes:

$\text{Cam Ring Speed Ratio} = \frac{1}{2 \times \text{Number of Lobes per Track}}$ $\text{Number of Lobes} = \frac{N \pm 1}{2}$

Where $N$ is the number of cylinders in the row (always an odd number: 5, 7, or 9):

• If $(N - 1) / 2$ is used to determine lobes, the cam ring rotates in the same direction as the crankshaft.
• If $(N + 1) / 2$ is used, the cam ring rotates in the opposite direction to the crankshaft.

Worked Example: In a nine-cylinder radial engine ($N = 9$) utilizing a cam ring that rotates opposite to the crankshaft, the number of lobes is $(9 + 1) / 2 = 5\text{ lobes}$. The rotational speed ratio is $1 / (2 \times 5) = 1/10$ crankshaft speed.