LABEL DIAGRAM OF 4-stroke-model

Cam Shaft 4 Stroke

4 stroke

4 stroke Crank

The Piston

Pistons for medium speed trunk piston engines which burn residual fuel are composite pistons; i.e the crown and the skirt are made of different materials.

The crown is a heat resisting steel forging which may be alloyed with chromium, molybdenum and nickel to maintain strength at high temperatures and resist corrosion. It is dished to form a combustion chamber with cutouts to allow for the valves opening. The topland (the space between the top ring and the top of the piston) may be tapered to allow for expansion being greater where the piston is hottest.

The skirt can either be a nodular cast iron or forged or cast silicon aluminium alloy. Aluminium has the advantage of being light, with low inertia, reducing bearing loading. However because aluminium has a higher coefficient of expansion than steel, increased clearances must be allowed for during manufacture. This means that the piston skirt clearance in the liner is greater than that for cast iron when running at low loads. The skirt transmits the side thrust, caused by the varying angularity of the con rod, to the liner. Too big a clearance will cause the piston to tilt.

The piston pin for the con rod small end bearing is located in the piston skirt. The piston pin floats in the piston skirt and is located in place by circlips. Depending on the material used for the skirt (esp. cast aluminium), a bushing may be used for the pin.

The piston rings may be located in the crown or in both crown and skirt. Normally, the rings are chrome plated or plasma coated to resist wear. Because the liner is splash lubricated, an oil scraper (oil control) ring is fitted to the piston skirt.

The piston is oil cooled. This is achieved by various means; The simplest is for a jet of oil to be directed upwards from a hole in the top of the con rod onto the underside of the crown. A more efficient method is to use an oil catcher as shown in the picture above. This directs oil into the cooling spaces on the underside of the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is unusual for the oil return temperature to be monitored (unlike the 2 stroke slow speed crosshead engine, where both temperature and quantity are monitored).

Some engines are fitted with one piece pistons manufactured from either cast iron or silicon alloy aluminium . These cannot be used with residual fuel, because the higher temperatures causes burning of the piston crown. Aluminium also suffers from carbon build up above 300º C. Ring grooves in aluminium pistons usually take the form of a chrome plated cast iron insert.

Fuel Pump

Medium speed four stroke engines are equipped with jerk type fuel pumps, one for each cylinder. A plunger operated by a cam reciprocates in a barrel. The plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The barrel is located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor.

As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should therefore be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, then no injection will take place and the engine will stop.

In the example shown above the plunger has a single helix machined into it. More common are pumps with two helices (and thus two no load grooves) diametrically opposite each other. This gives a balanced plunger. (shown left)

On the scroll or helical fuel pump, although the end of injection can be varied, the start of injection (i.e. when the top of the plunger covers the spill ports) is fixed. Fuels of different qualities may require advancing or retarding the start of injection, in addition to which if the injection timing is advanced when the engine is running at loads below the maximum continuous rating, then a saving in fuel can be achieved.

Different engine manufacturers achieve this Variable Injection Timing (VIT) using different methods.

The Wartsila 64 engine uses a fuel pump with two plungers and two barrels with common suction and discharge. The plunger for controlling the start of injection (timing) has a helix in the top of the plunger, while the plunger for controlling the end of injection (metering) is a conventional scroll type fuel pump plunger. Both plungers are operated by the same cam. As they move upwards in their respective barrels injection will not start until the helix on the timing plunger has covered the spill port. This point is controlled by rotating the plunger in the barrel by means of a rack and pinion. End of injection is controlled in the normal way, when the helix on the metering plunger uncovers the spill ports.

The MAN B&W 32/40 engine has a separate camshaft for the fuel pumps that can be advanced or retarded as the engine is running. The final drive gear on the timing gear train has an internal helically toothed sleeve bolted to it. The gear wheel and sleeve can be moved axially by means of a hydraulic piston. The toothed sleeve meshes with a matched helical gear fixed to the camshaft. The camshaft is fixed so that it cannot move axially. Therefore as the sleeve moves up and down controlled by the hydraulic piston, so the fuel pump timing is advanced or retarded. The camshaft for the inlet and exhaust valves utilises two different profiles for economy and full power operation. The camshaft can move axially from one set of cams to the other whilst the engine is running similar to the method used for reversing the engine direction as shown on the camshaft page.

Engine Frame

The engine frame of a modern 4 stroke medium speed diesel can be produced as a single casting or fabricated from cast steel sections and steel plates welded together. With this design, there is no separate bedplate, frame and entablature as with a 2 stroke slow speed engine. The photograph shows the frame of an engine with the liners and crankshaft in place.

An alternative method of construction is shown opposite. A separate bedplate is bolted to an entablature which holds the underslung crankshaft.

Shown here is a partial cross section from a one piece medium speed engine frame. The Crankshaft is underslung, and it can be seen in this example that the load on the bearing caps is transferred back to the frame by the use of tie bolts. Note the use of the side tie bolts which locate the bearing cap, and prevent sideways movement.

Cylinder Liner

The cylinder liner is cast separately from the main cylinder frame for the same reasons as given for the 2 stroke engine which are:

• The liner can be manufactured using a superior material to the cylinder block. While the cylinder block is made from a grey cast iron, the liner is manufactured from a nodular cast iron alloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)
• The cylinder liner will wear with use, and therefore may have to be replaced. The cylinder jacket lasts the life of the engine.
• At working temperature, the liner is a lot hotter than the jacket. The liner will expand more and is free to expand diametrically and lengthwise. If they were cast as one piece, then unacceptable thermal stresses would be set up, causing fracture of the material.
• Less risk of defects. The more complex the casting, the more difficult to produce a homogenous casting with low residual stresses.

Modern liners employ bore cooling at the top of the liner where the pressure stress is high and therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits so that there is not a breakdown in lubrication or excessive thermal stressing. Although the liner is splash lubricated from the revolving crankshaft, cylinder lubricators may be provided on the larger engines. On the example shown opposite, the lubricator drillings are bored from the bottom of the liner circumferentially around the liner wall. Another set of holes are drilled to meet up with these vertically bored holes at the point where the oil is required at the liner surface. Other engines may utilise axial drillings as in a two stroke engine.

Sulzer ZA40 Liner (vee engine; The straight engine is similar)

MAN-B&W L58/64 Liner

Where the cooling water space is formed between the engine frame and the jacket, there is a danger that water could leak down and contaminate the crankcase if the sealing O rings were to fail. As a warning, "tell tale" holes are led from between the O rings to the outside of the engine. modern engines tend not to use this space for cooling water. Instead a separate water jacket is mounted above the cylinder frame. This stops any risk of leakage of water from the cooling space into the crankcase (or oil into the cooling water space), and provides the cooling at the hottest part of the cylinder liner. Note that the liner opposite is fitted with a fireband. This is sometimes known as an antipolishing ring. It is slightly smaller in diameter than the liner, and its purpose is to remove the carbon which builds up on the piston above the top ring. If this carbon is allowed to build up it will eventually rub against the liner wall, polishing it and destroying its oil retention properties.

The liner must be gauged regularly to establish the wear rate and check that it is within manufacturers tolerances. The wear rate for a medium speed liner should be below 0.015mm/1000hrs. Excessive wear is caused by lack of lubrication, impurities in fuel air or Lubricating oil, bad combustion and acid attack.

Cylinder Head

Cylinder heads for 4 stroke engines are of a complex design. They have to house the inlet and exhaust valves, the fuel injector, the air start valve, relief valve and indicator cock. The passages for the inlet air and exhaust gas are incorporated, as are the cooling water passages and spaces.

Normally manufactured from spheroidal graphite or nodular cast iron which is easy to cast. Although not as strong as cast steel, which is difficult to cast into complex shapes due to its poor fluidity, it maintains a reasonable strength under load. Adequate cooling is essential to prevent thermal fatigue due to uneven expansion throughout the casting, and bore cooling has been introduced along with cooling spaces to ensure effective cooling of the "flame plate" (the underside of the cylinder head which forms the top of the combustion chamber).

Cracking of cylinder heads can occur due to poor cooling causing thermal fatigue. Poor cooling can be the result of scale build up within the cooling spaces due to inadequate water treatment. Overloading of the unit causing high peak pressures is also a cause as is incorrect tightening down of the cylinder head. Cracking normally occurs between the valve pockets and/or cooling water spaces. Cracked cylinder heads can be repaired by specialised welding; but this must be done under the guidance and with authorisation from the classification societies.

conrods.doc

The connecting rod in a medium speed 4 stroke engine is subject to an inertia whip loading due to the mass of the con rod swinging about the piston pin. (Because of the lower speed of a 2 stroke engine, the whip loading is not large enough to influence the design of the con rod) Added to this, the inertia loads due to the mass of the reciprocating parts cause a stress reversal from high compressive stress (during power and compression stroke) to a low tensile stress between the exhaust and inlet strokes.

This loading of the rod influences its design, and to withstand the loading described above, connecting rods are often forged from a manganese molybdenum steel in an I or H section which reduces its mass from one made of round section steel (and thus reduces the whip loading) while maintaining strength. This is not always the case, as can be seen from the pictures shown, and often a round section rod is of sufficient strength. .

Because of the large diameter of the crankpin to increase bearing area and decrease bearing load, the width of the bottom end of the con rod is greater than the diameter of the cylinder liner. So that the piston can be withdrawn from the liner, 3 different designs are used: The con rod can be fitted with a marine palm as in the photo above. The con rod can be split in two parts as shown opposite. The bottom end can be split obliquely. Serrations are used to locate the two halves relative to one another.

The advantage of using a vee engine is that the overall length of the engine is reduced for a given power output. If a normal bottom end arrangement is used then the con rods must be placed side by side which means the opposite cylinders are offset. The crankpins must be long enough to accommodate two bottom ends side by side, and of large enough diameter to resist bending. The increased length of the crankshaft means a longer engine. Two alternative arrangements are pictured (right). Both allow the cylinders to be opposite one another. However, both arrangements restrict access to the crankcase, and because the design of the bottom ends are different, more spares have to be carried.

BOTTOM END BOLTS

Because of the stress reversal mentioned above, bottom end bolts have a limited life. This varies from engine to engine, but is generally around 12-15000 hours. If a bottom end bolt was to fail in operation, then the results would be disastrous.

Bottom end bolts should be treated with care when removed from the engine during overhauls. They should be inspected for any damage to the surface from which a crack could start. This damage could be due to corrosion (water in LO) or because of incorrect handling.

The Camshaft

There are several different methods of manufacturing camshafts for medium speed 4 stroke marine diesel engines. On the smaller engines, the camshaft may be a single forging complete with cams.

Alternatively the camshaft can be built up in single cylinder elements, each element made up of the fuel, inlet, and exhaust cam on a section of the camshaft with a flange on each end. So that the element can be used on any unit in the engine, the number of holes for fitted bolts in the flanges must be sufficient to allow the cam to be timed for any unit on the engine. For example, on a six cylinder engine, the flanges must have 6 equi spaced holes or a multiple thereof. The cams must be hard enough to resist the wear and abrasion due to impurities in the lub. oil, yet they must be tough enough to resist shattering due to shock loading. The cams are therefore surface hardened using the nitriding process.

On the larger engines it is usual to manufacture the camshaft and cams separately. The nitrided alloy steel cams are then shrunk on to the steel shaft using heat or hydraulic means. Because the cams are fitted progressively onto the shaft, if the bores in the cams were all the same diameter, it would be very difficult, if not impossible, to fit the first cams all the way along the length of the shaft to the correct position. To overcome this problem the camshaft is stepped, with the largest diameters at the end which has the cams fitted first. The larger bored cams fit easily over the small diameter steps till they reach the correct position on the camshaft.

Keys are not generally used to locate the cams as they would act as stress raisers.

Most medium speed engines are unidirectional (i.e they only rotate one way). This is because they either are driving an alternator, or because if they are used as direct main propulsion they tend to be driving a controllable pitch propeller. In the case where the engine is reversing, then the camshaft has two sets of cams, one for ahead operation, and one for astern.

To reverse the direction of the engine, pressure oil is led to one side of a hydraulic piston which is coupled to the camshaft. The whole camshaft is moved axially and the cam followers slide up or down ramps which connect the ahead and astern cams.

The camshaft is either chain or gear driven from the crankshaft. Because the engine is a four stroke, the camshaft will rotate at half the speed of the crankshaft. (the valves and fuel pump will only operate once for every two revolutions of the crankshaft).

In a case where the cams are shrunk on the camshaft, if a cam becomes damaged and has to be replaced, then it can be cut off using a cutter grinder. Care must be exercised not to damage the camshaft or adjacent cams during the operation. The replacement cam is fitted in two halves which is then bolted on the camshaft in the correct position and the timing rechecked.

This is a damaged fuel cam from a ZA40 engine which has been cut off using a grinding wheel. Note the grooves which are used to distribute the hydraulic oil when expanding the cam on the camshaft. Thanks for the photo Peter

Medium Speed 4 Stroke Trunk Piston Engine

CLICK ON SPECIFIC AREA IN PICTURE TO LEARN MORE ABOUT EACH COMPONENT

The medium speed 4 stroke trunk piston engine can be found on most medium to large merchant vessels even if the main engine is either a steam turbine or a 2 stroke crosshead engine. In these cases it will often be found that the electrical power is supplied by alternators driven by medium speed 4 stroke engines.

They are the favoured method of propulsion on ships where head room is a minimum, for instance, on ferries and passenger vessels, and where, as is the current trend for these ships, diesel electric propulsion is utilised. Diesel electric propulsion allows the engines to be placed wherever is most suitable, as they no longer have to be aligned with reduction gearing and shafting as is the case with conventional installations.

Generally, medium speed engines run at between 250 - 850 RPM. Above this range they are defined as high speed engines. Although not as powerful as their 2 stroke crosshead cousins, the largest 4 stroke engines are delivering just over 2000kW per cylinder. Advances in design and materials have led to an increase in efficiency, together with an increase in turbocharger pressure ratios which allow a greater quantity of fuel to be burnt per cycle. Medium speed engines have a higher power to weight ratio than the slow speed two strokes, but due to the higher speeds tend to have reduced maintenance intervals. The largest of these engines have a bore of 640mm and a stroke of 900mm (Wartsila 64), although engines which are nearly "square" are more the norm: For example, the Sulzer ZA40 has a bore of 400mm and a stroke of 560mm or the MAN-B&W 58/64 which has a bore of 580mm and a stroke of 640mm. A square engine is one where the bore is equal to the stroke.

Wartsila 64

Sulzer ZA40

The name "Trunk Piston" refers to the piston skirt or trunk. The purpose of the skirt or trunk in four-stroke cycle engines is to act in a similar manner to a crosshead. It takes the thrust caused by connecting-rod angularity and transmits it to the side of the cylinder liner, in the same way as the crosshead slipper transmits the thrust to the crosshead guide. With such engines, which are termed trunk-piston engines, the engine height is considerably reduced compared with that of a crosshead engine of similar power and speed. The engine-manufacturing costs are also reduced. It means of course that there is no separation between the crankcase and the liner and piston. This has its disadvantages, especially when considering the choice of lubricating oils when burning high sulphur residual fuels.