Exhaust valves come in for a great deal of abuse as the engine is running, particularly if there are traces of vanadium or sulphur in the fuel. Many exotic, corrosive-resistant materials have been developed and employed on valve seats to combat this problem. Rotation of the valves has also proved to be a successful method of prolonging valve life, by maintaining the valve seating area at a more uniform temperature. A valve which is not rotated tends to heat up in one area more than another due to the directional flow of exhaust gas and air during the gas exchange period.
This non-uniform heating is further accentuated by the heat release from the burning fuel. This localised heating may carry parts of the valve to temperatures beyond the tolerance of the material so that any corrosive products of combustion can more readily attack the metal. The effect is that the valve suffers localised wastage (burning) and holes form through the seat. Another effect of uneven temperature across a valve head is the slight variance in expansion so that effective valve closure may not be achieved. This leads to further heating of the valve, as well as the loss of efficiency, fuel economy arid power associated with blow past. Thus overheating not only weakens the material but also promotes attack from the corrosive products of combustion. Valve cooling should therefore be achieved and maintained as carefully and accurately as possible. Rotation is simply as adjunct to accurate cooling of the seat.
The exhaust valves will be slightly smaller in diameter than the inlet valves, reducing the force required to open them against a given cylinder pressure. The power to open them, coming from the rocker arms and pushrods, is from the engine itself. Although this is a power loss, were larger valves used greater scantlings of running gear would be needed and the power loss would be magnified. Similarly, were the exhaust valve dimensions increased, the loading on the actuating cam face would increase, leading to more problems. Even though the valves are smaller than the inlet valves the gas escape is barely retarded as the cylinder pressure itself plays a great part in expanding the gas from the cylinder. The larger air valves are needed so that they do not offer resistance to flow and unnecessary back pressure on the turbocharger.
Springs for the valves may be in series, one above the other, or in parallel, one inside the other. Springs in series are usually associated with slow speed engines where great lift is required. The fitting of a diaphragm plate half way down the springs prevents them whipping as they operate; this lateral oscillation could lead to premature spring failure or even a form of valve bounce. Springs in parallel allow springs of slightly thinner wire section to be used. This prevents them from becoming coil bound, for a given lift, and if the individual coils are of different section they will have different vibration characteristics so the incidence of resonance is reduced. Also, if one spring fails the other will continue to operate the valve and prevent it dropping into the cylinder.
There are situations where the choice of engine depends only on engine room size; small vessels require medium speed engines. However, on large vessels there may be a choice between a large slow speed engine or a multiple medium speed engine installation. Much of the decision is based on installation costs and expected maintenance costs. In general, slow speed engines can consume lower? And therefore cheaper, grades of fuel. Some reasons for adoption of medium speed engines are outlined below.
- Ship’s reliability increases with more than one engine because failure of one engine does not mean the ship is held up.
- One or more engines can be shut down when the vessel is running in ballast or lightly laden. This allows the running engines to operate at their optimum power and fuel efficiency, which is impossible with a single slow speed engine under the same conditions.
- Maintenance is easier because of the smaller size of component. (In the author’s experience this is often offset by the sheer inaccessibility of some of the engine components.)
- Any engines shut down at sea can be overhauled, within limits. This should save time in port.
- Where vessels of different size are concerned, engines of a common type may be fitted, the differing power requirements being met by varying the number of cylinders or even the number of engines. This provides a fleet of vessels with a common engine type so that replacement is simplified, spare gear costs are lower, and the ship board engineers, becoming familiar with the engines, can move around the fleet without detriment to maintenance.