Running Gear of Diesel Engine

CRANKSHAFT

crankshaft is a component which converts the up and down movement of the pistons of the cylinder into continuous rotary motion which is transmitted further to move the propeller shaft in case of marine diesel engines. Due to its nature of operation, there are several types of forces which come to act upon the crankshaft of engines used in marine propulsion.

The material for construction also depends on the speed on the engine and slow speed marine diesel engines have crankshafts fabricated out of plain carbon steel with a percentage of carbon lying between 0.2 & 0.4%, while the alloy steels are used for engines having a relatively higher speed.

The construction of the crankshaft: it consists of crank-webs, crank-pins and journals along its length. The weight of the crankshaft is supported by the main bearings at the journals.

Crankshaft of Low-speed Marine Diesel Engine

Main Bearing In a piston engine, the main bearings are the bearings on which

the crankshaft rotates, usually plain or journal bearings. The bearings hold the crankshaft in place and prevent the forces created by the piston and transmitted to the crankshaft by the connecting rods from dislodging the crankshaft, instead forcing the crank to convert the reciprocating movement into rotation.

While some small single-cylinder engines have only one main bearing, designed to deal with the bending moment exerted by the crank as a result of the force from the connecting rod, most engines have at least two main bearings, one at each end of the crank shaft and may have as many as one more than the number of crank pins. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also provides stability to the crankshaft, which would otherwise have to endure greater bending moments from having the crank pin further from a bearing. The combination of bearing number and crankshaft design can also have an effect on engine balance; a more stable crankshaft will better avoid unbalance from flexing, but increasing the crankshaft length or girth to accommodate an additional bearing increases the rotational inertia of the crankshaft, which can have the opposite effect.

4-stroke engine main bearing

Thrust block and Bearings

As with any propulsion system the thrust from the propeller must be transmitted into the hull to drive the ship through the water. 

The advantage of using a thrust block is that the propeller thrust load is not taken by the engine-gearbox. This way an optimised flexible mounting system of engine-gearbox can be used, which gives the best results for vibration isolation and reduction of structure borne noise from engine-gearbox to the foundation (hull of the ship). Structure borne noise is transmitted quickly through the hull and causes disturbing noise (resonance) in areas outside of the engine room, this can cause problems or discomfort.

Thrust Block and bearing

Bottom End Bearing (Big end)

Bottom end bearings are bearings on which the crank pin rests. Big end bearings are commonly bushings or plain bearings, but less commonly may be roller bearings.

A forged steel connecting rod complete with bottom end bearing housing has replaced the older style connecting rod with marine palm and separate bearing. The bottom half of the white metal bearing can either be cast into the cap or, like the top half, be a separate steel backed shell. With modern steel backed shell bearings, the clearance is built in and non adjustable; when it becomes excessive the bearing is changed. Older thick walled bearings used shims to adjust the clearance. Because the load on the connecting rod is always downwards on a two stroke engine, it is the top half of the bearing which takes the load. 

Bottom End bearing (2 stroke)

Oil is supplied down from the crosshead, through a bore in the connecting rod and round the outside of the top shell to the ends of the bearing half where it is fed into the bearing through two holes. The bottom half of the bearing may have a groove cut into it to assist in carrying the oil around.

Bottom End Bearing (4 stroke)

Connecting Rod

In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts reciprocating motion into rotating motion.

In four stroke engines, the conrod is connected to the piston through the gudgeon pin at the other end. In two stroke engines, the conrod is connected to the crosshead at the other end.

Connecting rod of a 2 stroke engine

Crosshead and bearing

A crosshead is a mechanism used in long reciprocating engines and reciprocating compressors to eliminate sideways pressure on the piston. Also the crosshead enables the connecting rod to freely move outside the cylinder. The purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the con rod and so bearings are required

Guide and Guide shoes

It is also necessary to provide guides in order to ensure that the side thrust due to the conrod is not transmitted to the piston. This also ensure the piston remains central in the cylinder thus limiting wear in the liner.

To ensure that the crosshead reciprocates in alignment with the piston in the cylinder, guide shoes are attached either side of the crosshead pin. These shoes are lined with white metal, a bearing material and they reciprocate against the crosshead guides, which are bolted to the frame of the engine. The crosshead guides are located inbetween each cylinder.

Guide and Guide Shoes

Lubrication of Main Bearing, Bottom end bearing, and crosshead bearing

1. Crankshaft Oil-hole Drillings

Oil from the main oil gallery reaches each individual main-journal and bearing. Oil is fed through a central circumferential groove in the bearing and it completely surrounds the central region of the journal surface. Diagonal oil hole drills are provided in the crankshaft which pass through the webs between the main and big-end journals for lubrication of the big-end journal. For effective lubrication of the big-end, these oil holes emerge from the crankpin at about 30 degrees on the leading side of the crank’s TDC position. The drilled oil passages should not be close to the side walls of the webs or near the fillet junction between the journal and the webs to avoid high stress concentration, which may cause fatigue failure. Also the oil holes on the journal surfaces must be chamfered to reduce stress concentration, but excessive chamfering can destroy the oil film.

2. Splash Lubrication System

In order to ensure adequate supplies of oil to the engine parts, a reservoir of oil is provided by the sump which is the lower part of the lubrication system and in automobile engines the sump is the oil pan. From the reservoir, oil is distributed throughout the engine either by the splash system or the full pressure system. In case of two-stroke engines, the crankcase cannot be used as an oil reservoir. The lubrication, in this case, is provided by mixing a small proportion of oil with petrol.

In the splash system the oil is maintained in little troughs (Fig. 11.9). There are dippers at the ends of the connecting rods to splash the oil on the various parts like cylinder walls, camshafts, gudgeon pins etc. as they travel through the oil troughs towards the bottom of the stroke of the piston. The oil is supplied to the main bearings under pressure due to an oil pump through drilled passages, in the crankcase, called galleries. The oil pump also replenishes the troughs. The system is now practically obsolete.

3. Full Pressure System

‘forced-feed’ lubrication systems, generally of the wet-sump type in which the sump acts as both an oil-drain return and a storage container. A rotary-type oil-pump provides forced feed. The pump may be driven directly from the crankshaft or indirectly from the camshaft or any auxiliary shaft). Oil from the sump reaches the pump through the submerged gauze strainer and pick-up pipe. The oil is then compressed, which passes through a drilling to the lubrication system. A pressure-relief valve positioned on the output side of the pump controls the oil pressure. If the oil pressure becomes too high, the relief valve opens and bleeds surplus oil back to the sump. The relief valve may be installed on the filter unit, the crankcase, or the pump housing.

The oil-pump forces the oil through drillings in the crankcase to a cylindrical full-flow filter unit. The oil circulates around the filter bowl, passes through the filter towards its center, and flows out to the main oil passage, called main oil gallery which lies parallel to the crankshaft. In most car and commercial vehicle engines, the oil gallery is formed by drilling a hole in the crankcase for full length of the engine and plugging the ends.

The oil is fed to the crankshaft main journal bearings and in some cases to the camshaft bearings through various branch cross-drillings in the crankcase. A few heavy commercial engines use a separate pipe located underneath the main-bearing caps and by pedestal brackets. Drillings in these brackets connect the gallery-pipe oil to the main bearings. By diagonal drillings in the crankshaft a continuous oil is fed to the big-end bearings from the oil grooves around the main-bearings liners. These drillings pass from the main-bearing journal to the big-end crankpins through the crankshaft web.

Main and Big Bearing

Lubrication

Gear wheel transmission

Chain Wheel

Cam Shaft Bearing Arrangement

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.

The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of the air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain called a timing belt or timing chain

This invention relates to a camshaft bearing arrangement for an overhead camshaft engine and more particularly to an improved assembly arrangement for the camshaft and other associated components of the engine.

another feature of the invention is adapted to be embodied in an assembly for an internal combustion engine that comprises a cylinder block, a cylinder head detachably affixed to the cylinder block by a plurality of first threaded fastening means, a plurality of poppet valves supported for reciprocation by the cylinder head and a camshaft for operating the poppet valves. In accordance with this feature of the invention, a cam carrier is detachably affixed by second threaded fastening means to the cylinder head. A bearing cap is detachably affixed to the cam carrier by third threaded fastening means and the cam carrier and bearing cap have bearing surfaces that journal the camshaft.