A cam is a mechanical element, usually a hardened profiled disc or block, that rotates and, through the action of a cam follower/ roller and linkage, imparts a precise reciprocating motion to a valve, pump or other component. In large two-stroke marine engines, cams are driven from the camshaft, which in turn is driven from the crankshaft at the same speed (two-stroke) or half speed (four-stroke).

Types of cam profiles used:

1. Tangent (tangential) cam - Flanks are straight lines tangential to the base circle and rise/ fall arcs. Simple to design and machine, gives moderate acceleration, used for fuel pump and exhaust valve drives where a dwell is not essential.
2. Convex / circular arc cam - Flank is an arc of large radius. Gives a smoother acceleration change (no shock at points of inflection), used for exhaust valve cams to reduce noise and wear at high speed.
3. Concave cam - Rarely used; short dwells, higher contact stresses.
4. Zero-lift / parallel and dwell cams - Have base circle only; used for indicator drives where a steady datum is required.
5. Negative cam (see part b) - Used on exhaust valve air spring/hydraulic systems of large two-stroke engines.

Functions and how cam profile affects operation:

• Exhaust valve cam: Opens the exhaust valve against the returning force (air spring or hydraulic). Profile shape controls valve lift, opening/closing timing, periods of dwell when valve stays fully open, and rates of acceleration/deceleration. Long dwell and gentle flanks reduce valve seat impact and stress.
• Fuel pump cam: Rises to displace the plunger for injection. The number of cam lobes and profile determine injection timing, rate of injection (injection pressure build-up), and whether the pump is single- or multi-ram type. The rise must give the required plunger velocity to build fuel pressure quickly.
• Starting air distributor cam: A distribution disc/cam with a series of lobes around its circumference that opens the pilot valves of the air-start valves in correct sequence with cylinder firing order. The number of lobes equals the number of cylinders; profile size sets the duration of air admitting window.

A negative cam is a profile that provides a large movement (lift/dwell) over most of its revolution and only a short period at low (base circle) lift. In practice it is often a cam in which the "active" portion is a depression or a reduced-radius segment.

Purpose: In MAN B&W (and similar) large two-stroke engines with hydraulically operated exhaust valves, the exhaust valve is opened by hydraulic oil pressure from a high-pressure pump driven by the cam; the valve is closed by the valve air spring. A "negative" or long-dwell cam supplies hydraulic oil pressure through most of the rotation so that the valve is held closed under hydraulic pressure, and only at the point where closing is required does the cam lift fall, releasing pressure so the valve air spring slams the valve shut. In this scheme the valve is opened by a fall in the cam lift rather than a rise, hence "negative" cam.

Constructional features: The cam has a large base circle for the majority of the revolution (hydraulic valve held closed) and a single machined depression or reduced-lift sector of short angular extent corresponding to the exhaust open period. It is precision-ground, case-hardened steel, mounted on the camshaft, and the profile is machined to give a fast pressure release for rapid valve closing but with controlled cushioning to avoid hammering.

Typical use: This negative/short-dwell cam principle is used on the exhaust valve hydraulic drive of slow-speed two-stroke engines (e.g. MAN B&W MC/MC-C series using the "fuel oil valve actuator" and hydraulic exhaust drive), where it provides shock-free, precisely timed exhaust valve operation without a conventional high-speed spring cam mechanism. It reduces noise and wear compared to positive cam systems.