What Do Camshafts Do?

Camshafts open and close an engine’s intake and exhaust valves at precisely timed moments, converting the crankshaft’s rotation into the linear motion that controls airflow through the cylinders. In practice, camshafts set when, how far, and how long valves move—governing power, efficiency, drivability, and emissions across the engine’s operating range.

How a Camshaft Works

At the core of a camshaft are egg-shaped lobes that press on lifters, rockers, or tappets to push valves open against spring pressure; as the lobe rotates away, the spring closes the valve. The camshaft is synchronized to the crankshaft via a timing belt, chain, or gearset so that valve events align with piston position for efficient combustion.

The following points summarize the critical parameters a camshaft controls and why they matter.

• Valve timing: The crank angle when a valve opens and closes, setting the phasing of intake and exhaust events.
• Lift: How far the valve opens, affecting peak airflow and high-rpm breathing.
• Duration: How long the valve stays open, influencing torque curve shape and mixture quality.
• Overlap: The period when intake and exhaust valves are open together, aiding scavenging at high rpm but potentially hurting idle quality and emissions.
• Lobe separation/centerlines: The angular relationship between intake and exhaust lobes, shaping the engine’s character (low-end torque vs. top-end power).

Together, these factors determine how easily the engine breathes at different speeds and loads, balancing smoothness, efficiency, and performance.

Camshaft Layouts and Where They Sit

Engines differ in camshaft placement and valve-actuation geometry, which influences complexity, packaging, and performance potential.

• Cam-in-block (OHV/pushrod): The camshaft sits in the engine block and uses pushrods and rockers to reach the valves. Compact and torquey; common in many American V8s.
• Single overhead cam (SOHC): One cam per cylinder bank operates both intake and exhaust valves, reducing valvetrain mass compared with pushrods.
• Dual overhead cam (DOHC): Two cams per bank separate intake and exhaust control, allowing four valves per cylinder and finer tuning for performance and efficiency.
• Desmodromic (rare): Mechanically opens and closes valves without relying on springs (notably Ducati motorcycles), enabling aggressive high-rpm control.

While all designs aim to time valve motion precisely, DOHC architectures give engineers the most freedom to optimize breathing and integrate modern variable systems.

From Fixed Cams to Variable Control

Modern engines rarely rely on a single, fixed cam profile across all conditions. Variable systems adjust timing, lift, and sometimes duration in real time, improving torque, fuel economy, and emissions without compromising high-rpm power.

• Cam phasing (VVT): Rotates the cam relative to the crank to advance or retard events—brands include Toyota VVT-i, BMW VANOS, Ford Ti-VCT, and many others.
• Profile switching: Swaps between cam lobes with different lift/duration (e.g., Honda VTEC), blending low-speed efficiency with high-speed power.
• Continuously variable lift/duration: Alters valve lift and/or duration without swapping lobes—BMW Valvetronic (lift), Nissan VVEL (lift/events), Fiat/Alfa MultiAir (electro-hydraulic intake lift), and Hyundai-Kia CVVD (duration).
• Camless concepts: Electro-hydraulic/pneumatic actuators control valves individually without a camshaft (e.g., Koenigsegg Freevalve as used on its TFG engine), promising maximum flexibility but still rare in mass-market production.

These technologies let one engine behave like many—strong at low rpm, efficient in cruising, and powerful at high rpm—while meeting increasingly strict emissions standards.

Why Camshafts Matter: Performance, Efficiency, and Emissions

A camshaft’s job is fundamentally about moving air efficiently and cleanly. The trade-offs it manages are central to how an engine feels and performs.

• Low-end drivability: Mild timing and low overlap boost cylinder filling at low rpm, stabilizing idle and improving fuel economy.
• Top-end power: Higher lift and longer duration improve flow at high rpm, trading some low-speed smoothness.
• Emissions control: Optimized timing reduces unburned hydrocarbons and NOx; variable overlap can support internal EGR strategies.
• Turbocharging synergy: Cam timing that limits overlap helps turbos spool and prevents exhaust backflow; advanced phase control tailors response across the rev range.
• Pumping losses: Variable lift systems can reduce throttling losses by controlling air with the valves rather than the throttle plate.

The result is a carefully tuned compromise—or with advanced variability, a broad envelope—that defines an engine’s character and compliance with regulations.

Hardware, Manufacturing, and Reliability

Camshafts are typically cast iron (chilled for hard lobes) or forged/billet steel for strength, with hardened surfaces (nitriding or induction hardening) to resist wear. They ride on bearings and rely on a steady oil film; modern oils and proper service are crucial to longevity.

Owners and technicians should be aware of common maintenance cues and failure symptoms related to camshafts and their drive systems.

• Timing belts and chains: Belts require periodic replacement; chains last longer but can stretch—both can cause timing errors if neglected.
• Noise and rough running: Ticking, misfires, or poor idle can indicate lobe wear, collapsed hydraulic lifters, or phaser faults.
• Check-engine lights: Cam/crank correlation and cam position sensor codes often point to phaser or timing-drive issues.
• Oil quality and pressure: Low oil or infrequent changes accelerate lobe and follower wear, especially with high spring pressures.
• Metal shavings in oil: Can signal accelerated wear of lobes or lifters, requiring prompt inspection to prevent deeper damage.

Proactive service—especially oil changes and adhering to timing-belt intervals—protects the camshaft and the broader valvetrain.

Beyond Cars

Camshafts power valve timing in motorcycles, trucks, small engines (mowers, generators), and many industrial engines. The same principle—turning rotary motion into precise, repeatable linear motion—also appears in pumps and automated machinery, underscoring the cam’s broader mechanical importance.

Summary

Camshafts orchestrate the opening and closing of engine valves with precise timing, lift, and duration, controlling how an engine breathes. Their design—and increasingly, their ability to vary in real time—shapes torque, power, economy, and emissions. From pushrod V8s to DOHC, VVT, and emerging camless systems, cam technology remains central to how modern engines deliver performance and efficiency.