MEOclassIIexamnotes

The Miller cycle was developed by Ralph Miller in the 1940s. With the introduction of turbocharging to the 4 stroke diesel engines, the Mean Effective Pressure and thus the power output of 4 stroke engines rose by 50 - 60%. However this was about the limit; If the  inlet air pressure was increased further, then the charge air reached excessive pressures and temperatures on compression causing burning of the LO film and thermal stressing. Miller challenged the thinking of the day by closing the inlet valve before the piston reached bottom dead centre. This had the effect of lowering the cylinder pressure as the piston continued downwards, as well as dropping the temperature of the air (Boyles and Charles' Law). Although the engine is still doing work as the piston is descending on the inlet stroke, there is a saving in work during the compression stroke, and the maximum air temperature and pressure is red...

It is not generally known that the first airless injection system (i.e. not to use compressed air to atomise the fuel) was a common rail system. The invention of this system is often mistakenly credited to Doxford, but it was invented and patented by Vickers of Barrow in Furness. In this early common rail system the engine driven fuel pumps pressurised a fuel rail to about 400 bar from which pipes led to the fuel valves operated by cams and rocking levers. Independently driven pumps were provided to prime the system for starting. Later systems used hydraulically operated injectors, the delivery of fuel being controlled by a cam operated valve. Fuel quantity was controlled by an eccentric on the cam follower. With the integration of industrial electronics into marine engineering systems coupled with the giant strides made in the development of computer technology, it has now become possible to re-introduce...

The pump is basically a jerk type with a plunger moving in a matched barrel, using two helical grooves machined in the plunger to control the end of injection by uncovering spill ports and causing the discharge pressure to drop rapidly, thus causing the needle valve in the injector to close. Oil is supplied to the barrel via the spill ports and a suction valve. The suction valve, situated at the top of the barrel opens when the pressure in the  barrel falls below the supply pump pressure; i.e. during downward stroke of plunger, while spill ports are covered by plunger. Replaceable erosion plugs  are fitted in the pump housing opposite the spill ports. The high pressure oil, spilling back, as the edge of the helix uncovers the s...

The crosshead on a slow speed 2 stroke is a difficult bearing to lubricate effectively. The load is continually downward and because the con rod swings about the pin, changing direction each stroke, true hydrodynamic lubrication cannot take place. Instead the lubrication starts as boundary, and as the rubbing speed increases, a hydrodynamic film is built up. As the rubbing speed decreases the lubrication becomes boundary once again.  As engine powers and thus gas loads have increased, the difficulties with achieving effective lubrication have increased. Larger pin diameters have helped by increasing the linear rubbing speeds, and the continuous lower bearing has reduced the loading/unit area.  The older forked type crosshead as found in earlier engines (up ...

The fuel pump cam on the MAN B&W MC series engine is designed to raise the plunger on the injection stroke and then keep the plunger at the top of its stroke while the follower stays on the peak of the cam until just before the next delivery stroke when the follower returns to the base circle of the cam, and the fuel pump plunger moves down on its suction stroke. The animation on the left shows the cam follower just beginning to move up the slope of the cam with the camshaft rotating in anticlockwise direction. (i.e. start of injection) If the engine direction is reversed at this point, then air will enter the pneumatic cylinder as shown and will move the piston to the right. The cam follower will be moved across and would finish in the position shown which would be at the correct fuel pump timing for running astern. It should be noted that the reversal of the follower only...

If a timing diagram for a two stroke engine is examined, it can be seen that the exhaust valve starts to open at about 110º after TDC (position 4 on the diagram). After the initial blowdown of the exhaust gas from the cylinder, the scavenge ports are opened at about 140º after TDC (position 5), as the piston moves down the cylinder. The position of the scavenge ports is fixed in the cylinder liner, and so it should be obvious that their opening and closing must be symmetrical about BDC, and therefore they close at 140º before TDC as the piston moves up the cylinder on the compression stroke. When the engine is operating in the reverse direction, the timing of the opening and closing of the scavenge ports remains the same.  The exhaust valve can be timed to open and close symmetrically about BDC, and so a...

On a two stroke engine, the fuel pumps must be retimed when the engine is required to reverse direction (i.e. run astern). This is done by moving the fuel pump cams or fuel pump cam follower positions relative to the crankshaft. I f one cylinder of the engine is considered (left), the piston is just before TDC with the engine running ahead and the crankshaft rotating clockwise. The piston is moving up towards TDC. The picture on the right shows the fuel cam at this point; where the cam follower is rising up the lift of the cam as it rotates clockwise. This point can be considered as the start of injection. If, at this point the engine is stopped, and is started in the reverse direction (astern), the crankshaft now moves in an anticlockwise direction. Then the piston in this particular unit is now moving ...