Main Engine Construction and Operation

The main propulsion engine onboard a ship is a type of internal combustion (IC) engine.  IC engine ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. The fuel is injected in the form of a very fine spray, by means of a nozzle, into the combustion chamber. There it is ignited by the heat of compressed air which the chamber has been charged with. 

The low-speed (i.e. 70 to 120 rev/min) two-stroke diesel engine is used for main propulsion units since it can be directly coupled to the propeller and shafting.

Points to remember

• The power produced by an engine is governed by the number of cylinders. The power per cylinder is given by PmLAn/K Kilowatt. Where K=1000. 
• The stroke and bore ratio are between 2.5 and 4:1 or more. Mean piston speed of 8.0 m/sec or slightly more. Slower rotation of the propeller generally increases propulsive efficiency. 
• By allowing the gas to expand further greater fuel efficiency is obtained because more energy is obtained from the expanding gas, however, this requires a longer piston stroke and that in turn results in a lower rotational speed. 
• The compression ratio is given by Total cylinder volume/clearance volume. In a compression ignition engine, fuel starts to burn only when it is raised to a temperature greater than its self-ignition temperature and once ignited, the volatility of the fuel will then dictate the speed of combustion, throughout the fuel. 
• Compression ratio for large bore engine 12:1 and for medium-speed engine 16:1. In small engines more heat is lost to the cylinder surface, thus required a higher compression ratio. 
• As the cylinder dimension increases the volume of the space increases but the ratio of space surface area to the space volume does not increase at the same rate. Engine designers consider cylinder dimensions and ambient starting temperature. 
• Engine Bedplate is supported on a series of steel, C.I or resilient chocks spacing 250mm to 400mm, placed closer to cross girder thus provide good support to prevent any localized distortion. 
• Chocks sit on the foundation plate are fitted in place after the engine has been aligned to shafting. 
•  Solid chocks of CI or steel require an acceptable contact area with the foundation plate and bedplate otherwise fretting damage occur which cause misalignment, fitting them is the time taken, also the forces from engine moving parts are transmitted to the foundation in form of vibration which is damped out within the resilient or non-metallic chocks. Alternatively, accurately machined bedplate and foundation plate faces can be used. 
• Thin sheet aluminium dams are used to form the chock area which is then cleaned and sprayed with a release agent to prevent resin sticking. Resin and hardener are mixed in correct proportions and poured carefully to prevent air entrapment, curing time is 48 hrs after that holding down bolts can be tightened and crankshaft deflection checked for shaft alignment checking.
• The foundation plate forms part of the inner bottom plating of the hull structure. The holding down bolts passes through holes in the bedplates, chokes and foundation plate. 
• Bedplate supports the crankshaft and provides the foundation for the remainder of the engine structure. Made of longitudinal and transverse steel girders. Bed plates are 'Gondola' type, fabricated and only transverse girders are cast. 
• Slow cooling is carried out to eliminates the residual stresses from the fabricated parts otherwise residual stresses with tensile working stress can result in higher stress causing fatigue failure. 
• Frames support the cylinder block, enclose the crankcase and provide mounting points for guide bars, so take side thrust i.e the reaction of angularity of the connecting rod. The Upper & lower faces of the frame and the mounting points are machined. 
• Cylinder blocks made of cast iron contains liner inside, lower part forms scavenge space, containing diaphragm. Also, provide dead space between the scavange space and the upper part of the crankcase. It acts as an insulating space/water-cooled space to prevent explosion in case of scavenge fire. Classification society requirement holding down bolts to be checked by a surveyor every four years. In practice, bolts are to be checked every six months. 
• On older ships, bolts are hammer tested on modern ships are tightened by hydraulic jacks. 
• A-frame, Piston, piston rod, Crosshead bearing are under compressive load while tie rod remains under tensile load. The tensile stress of tie rods tends to lift the cross girders but is resisted by gas force on the piston. These forces are not identical thus setup stress in engine structure. 
• Gas force on the piston and connecting rod varies with time and angular position of the connecting rod, this causes a side thrust to be exerted at the crankshaft and the crosshead. These thrusts are delt by crankshaft bearings and Guide bars respectively. 
• Guide shoes run in a set of guide bars that act to resist the side thrust due to angularity of the connecting rod and by rolling motion of the ship, which prevents the piston from rubbing against the liner wall and keep the piston align with the axis of the cylinder liner. Pitching causes fore and aft thrust also accommodated by rubbing faces on guide shoes and guide bars. 
• Tie rods limit the tensile stresses in the engine component set up during peak pressure by keeping them in compression and thus prevent fatigue cracking. High power to weight ratio is achieved by using tie rods. 
• Tie rod carrying tubes are welded in frames and provided with bracings to prevent transverse oscillations thus prevent fatigue failure. 
• Where engine headroom is limited, tie rods are made in sections with suitably located joints to make removal/fitting easier. 
• The tie rod is equipped with a two-part bush which firmly fasten by two clamp screws located at the bottom of the cylinder block. These guide bushes prevent the tie rod from vibrating. Additional vibration damper at the space around the lower part of tie rod till mid column is filled with oil which enters through a filling bore in way of the crosshead guide plate. In the lower thread, the intermediate ring having a groove that allows oil and possible condensate to drain away. 
• Sealing between cylinder cover and cylinder liner is obtained by means of a sealing ring, made of Mild steel. Oblique or radial Cooling water bores are provided in the cylinder cover, exhaust valve and Liner.    

Hit the button below to read a detailed article about each component of a marine diesel engine.