Page 59 - Introduction to Marine Engineering

46 Diesel engines

bottom-end bearings are thin-walled shells lined with white metal. The
forged connecting rod has a 'table top' upper end for the mounting of
the crosshead bearing. A large crosshead, with floating slippers at each
end, is used. The piston rod is bolted directly to the top of the crosshead
pin. The pistons are oil-cooled and somewhat shorter in length than
earlier designs. There is no piston skirt. Five piston rings are fitted
which are designed to rotate within their grooves.
Cylinder liners have a simple, rotationally symmetrical design with the
scavenge ports at the lower end. The deep collar at the upper end is
bore-cooled, as are all components surrounding the combustion
chamber. Cooling water is fed from below through a water guide
arranged around the liner. Cylinder lubrication is provided by eight
quills arranged around the lower edge of the collar on the liner. The
more recently introduced RTA series engines all have oil-cooled pistons
with oil supplied from the crosshead bearing up through the piston
rod.
A piston rod gland separates the crankcase chamber from the under
piston space. Various scraper and sealing rings are Fitted within the
gland.
The cylinder head is a single steel forging arranged for bore cooling
with appropriately drilled holes. Pockets are cut for the air starting valve
and fuel injection valves, the number depending upon the cylinder bore.
The centrally mounted exhaust valve is fitted in a cage with a
bore-cooled valve seat. The valve stem is fitted with a vane-type impeller
to ensure valve rotation. The valve is opened by hydraulic pressure from
pumps driven by the camshaft and closed by compressed air.
The camshaft is located at engine mid-height and is gear driven from
the crankshaft. The initial gear drive is bolted to the rim of the thrust
block and a single intermediate wheel is used. On larger-bore,
high-powered engines the gear drive is in the centre of the engine. The
camshaft extends the length of the engine and each individual segment
carries the exhaust valve actuating and fuel-injection pumps plus the
reversing servo motor for one pair of cylinders.
Constant-pressure turbocharging is used, and electrically driven
blowers cut in automatically when the engine load is at about 40% of the
maximum continuous rating.
Lubricating oil is supplied to a low- and a medium-pressure system.
The low-pressure system supplies the main and other bearings. The
crosshead bearing, reversing servo motors and exhaust valve actuators
are supplied by the medium-pressure system. Cylinder oil is supplied to
lubricators from a high-level service tank.
Double valve controlled, variable injection timing is used to deliver
fuel to multiple, uncooled injectors and an electronic governor is
supplied as standard. Where the engine has oil-cooled pistols they will