Engine Installation, Two Stroke & Four Stroke Engines
Before we examine the two-stroke and four-stroke engines, it is worthwhile to have a look at one method of assembling the engines in the engine room.
Engine Assembly aboard Ship
There are a number of marine diesel engine manufacturers who assemble and test the engines before stripping them down to the main components and lowering them into the ship’s engine room. We used to re-assemble the components in the engine room in the following order. (I imagine the same procedures are used today, except for the main foundation components being of a more modern design.)
The bed plate consists of two longitudinal girders, braced by cast steel traverse cross sections. At selected cross sections, the main bearing pockets are line-bored and two vertical holes drilled through them and the section. The main tie-rods are fitted through these into the bed plate.
The bed plate is lowered into position onto the supports that form part of the ship’s hull, then shimmed level and caulked before being fitted with hold-down bolts that run right around the bottom support frame. The crankshaft main bearing bottom halves are then fitted to the bed plate bearing pockets and the crankshaft lowered into them. The top halves of the main bearing are then fitted and checked for clearance and alignment.
The “A" frames and entablatures follow, being bolted together using fitted bolts, before other components are quickly fitted until the engine is completely rebuilt.
There are two types of marine diesel engines: two-stroke and four-stroke.
Two Stroke Cycle
Starting with the piston at bottom dead center (BDC), the combustion air is supplied to the liner air inlet ports and the piston starts to rise up the liner. Depending on the scavenging system, the piston either ejects the previous cycle exhaust gases out exhaust ports in the liner or out through an exhaust valve in the cylinder head.
The combustion air continues to be compressed to almost top dead center (TDC) when the fuel is injected and combusted by compression ignition, forcing the piston back down the liner.
Four Stroke Cycle
1. Exhaust Stroke - Starting again with the piston at BDC, it begins to rise up the liner, with the exhaust valve in the cylinder head opening and expelling the exhaust gasses during the upward stroke.
2. Inlet Stroke - The inlet valve now opens and combustion air is drawn in as the piston continues downwards.
3. Compression Stroke - After reaching BDC the piston starts to rise again and the inlet valve shuts.
4. Ignition/Power Stroke - As the piston continues to rise with both valves shut, and just before TDC, fuel is injected and is combusted through compression ignition, forcing the piston downwards on its power stroke.