Crosshead type engines are able to develop much higher power at lower rotational spieeds than trunk-piston type engines, because the space available for the crosshead bearings is greater than the space within the piston for the gudgeon bearing assembly. Trunk-piston engines have the advantage of requiring less head room than crosshead engines. Their working parts are fewer in number and much less costly to produce because their design lends itself to mass production methods. The gudgeon bearing assembly is not particularly suited for highly rated two-stroke engines unless special arrangements are made for its lubrication. Cheaper quality fuels may be used in crosshead engines as it is possible to isolate the cylinder space from the crankcase, thus preventing acidic residues entering the crankcase. The total cost for lubricants is less with crosshead engines than with trunk-piston engines of equivalent power.
Cross-scavenged engines do not require exhaust valves or scavenge valves so some simplicity is obtained Over other engine types.
The cylinder liners of cross-scavenged engines require a complicated pattern
of scavenge and exhaust ports in the lower part of the cylinder. The surfaces left by a core in the casting process of the liner are inadequate in their profile and surface finish. In order to be acceptable the ports must be milled out to give a correct shape and a smooth surface finish. The height of the ports extends relatively high in the cylinder liner and the effective stroke for expansion of the gases is reduced. The cross-sectional area of the ports is relatively large compared with the area of the port bars. This often leads to an excess of liner wear in way of the port bars.
Piston ring breakage is more common in cross-scavenged engines than in uniflow-scavenged engines.
Because they are so complicated the cost of a cylinder liner for a cross- scavenged engine is considerably more than for a uniflow-scavenged engine of similar dimensions.
Uniflow-scavenged engines require an exhaust valve or valves, the number depending on engine speed and cylinder size.
In slow-speed engines only one exhaust valve is required. When one exhaust valve is required two or more fuel injection valves must be fitted whereas in the cross-scavenged engine only one centrally located fuel injection valve is required.
The cylinder liner for a uniflow-scavenged engine has the scavenge ports fitted around the whole of the circumference of the liner. The full circum¬ferential spacoavailable allows the ports to be made circular. This arrangement of ports does not extend as far up the cylinder liner so the effective length of the piston stroke is considerably more in a uniflow-scavenged engine than in a cross-scavenged engine of similar dimensions.
Cylinder liner wear in way of scavenge port bars in uniflow-scavenged engines shows no increase over those parts above and below the ports.
The cylinder liners of uniflow-scavenged engines cost considerably less than those for equivalent cross-scavenged engines.
The arrangements for sealing the bottom of the cooling water space are much simpler in uniflow-scavenged engines.
The two areas requiring maintenance are those associated with (a) combustion, and (b) bearing adjustment, and maintenance of correct alignment in all running parts. There is some overlap between the two areas of activity.
Maintenance work associated with combustion involves scavenge port and valve cleaning, piston ring replacement, air inlet and exhaust valve changes and overhaul, cleaning turboblower blading, compressor air inlet filters, scavenge and charge air cooler, and attending to instrumentation associated with combustion. The items mentioned cover all types of engines.
The other type of maintenance work covers all the moving and static parts of the engine, and includes bearing examination and adjustments, lubrication and cooling services, examination of bedplates, frames, cover, safety devices, etc.