Starting Arrangement of Main Engine

Theory of Engine Starting
Main engine starting and reversing system
Timing adjustment of fuel and air during starting
Important Components of the System

Theory of Engine starting

The compressed air acting on the top of the piston produces the amount of starting torque that is required to rotate the crankshaft at a speed that will produce the desired self-ignition temperature to ignite the fuel in the cylinders.
The best timing of air admission is when the piston is at TDC, but practically the air admits after the TDC to provide a positive torque in the correct direction at the start of working stroke. To achieve this timing the air required to be admitted slightly before TDC as the time lags for pilot valve actuation, air valve opening and full pressure availability are considered. The firing of fuel can be achieved at a speed of 8 to 12% of MCR speed.
Starting air admission period depends upon the valve opening timings. Generally, the scavenge port opens at 110° after TDC and the exhaust valve opens at 140° after the TDC. Starting air can be introduced for a minimum cranking period plus an overlap period to provide sufficient starting torque. Overlapping ensures that at every crank angle position, there is sufficient air turning moment to enable positive starting, it can be reduced with more number of units.
The scavenge port opens at 110° after TDC thus the start angle limit is 110°. For an engine having three units the firing interval is 120° for each unit, thus it is not possible to achieve overlapping for the three-unit engine as the start angle needed to be greater than 120°.

For an engine with four unit firing interval is 90 degrees, and thus an overlapping of 20° can be achieved.

Firing order prevents the crankshaft journal between adjacent cylinders from excessive loads, provide better and regular crankshaft rotation when firing in an equal interval. examples of firing order for two-stroke engine are (1-3-2-4); (1-5-3-4-2-6); (1-7-3-5-4-6-2-8).

Main engine starting and reversing system

System description and working of Main engine starting arrangement:-
(It is recommended to also read the written EKM question.49 on air-stating )

(a) Open the air bottle outlet (30 Bar). Start main engine lube oil pump (4.5 Bar). Start jacket cooling water (4 Bar). Open control air supply (7 Bar). A manually operated stop valve is fitted at the air bottle outlet, that needed to be opened slowly to prevent pressure surge.

(b) See the diagram, after the air bottle stop valve, air can travel to the Automatic valve and Pre-starting valve through the Turning gear block valve. If turning gear is disengaged then air reaches up to the automatic valve and pre-starting valve simultaneously. But if the turning gear is engaged this air will not pass through the turning gear block valve.
Note that the automatic valve is shut because the pilot valve is shut. The 30bar air enters from the top of the automatic valve from where air passes through a drilled passage to keep the automatic valve shut. See the diagram of the automatic valve, once the air that is acting on top is leaked off causes the valve to open because there is a difference in the areas of the piston. This leaked air is called "opening air". Opening air acts underside along with the spring force and pushes the spindle up cause air 30bar air to pass to start air manifold.

Automatic Air Starting Valve - Simple diagram

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(c) Up to this time following interlocks are free:-
1. Main lube oil low pressure
2. Cam-shaft lube oil low pressure
3. Jacket water high temperature
4. Control air low pressure
5. Turning gear interlock

(d) As the bridge gives an order of astern, acknowledge the order by matching the telegraph.
If the telegraph moment does not match the bridge order, the telegraph mechanical interlock will not allow the movement of the air start lever.

(e) Now as the telegraph is matching with the bridge first the reversing will take place. This causes reversing servomotor to move and reverse the camshaft via reversing valve.
Note that in a 2-stroke engine, the same fuel cam is used for AHD and AST operation of the engine, Accompanied by the rotation of the camshaft by means of reversing servomotor. The servomotor is operated by lube oil pressure, this control lube oil is directed to the correct side of the rotary vane, which is connected to the camshaft.

(f) Upon completion of the movement of the camshaft in the desired direction, the interlock on-air start lever is released. As the camshaft reaches the end position, the feedback oil pressure is fed back to the block valve.

(g) Up to this time these following interlocks are free:-
1. Turning gear,
2. Telegraph,
3. Reverse servomotor,
4. M/E lube oil low pressure,
5. Camshaft lube oil low pressure
6. Jacket cooling water high-temperature interlock.
Note that the pressure trips block the starting of the engine if main and camshaft lube oil pressure is low, jacket cooling water temperature is high and control air pressure is low.
Block valve air blocks the moment of air start lever and prevents the starting of the engine.
The relay valve blocks the movement of the fuel lever and prevents the engine from picking up on fuel.

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(h) Now as the air starting lever is moved, which opens the pre-starting valve. This lifts the pilot valve and vents the opening air which was discussed earlier with the automatic valve. The opening air acted along with the spring force to keep the automatic valve shut. The 30bar air now flows through the non-return valve to the distributor and cylinder valve. Thus, the pilot valve allows the passage of opening air to operate the automatic valve and load up distributor slide valves.

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(i) Now the air (30bar) from the automatic valve passes to the individual cylinder valve and distributor via non-return and relief valves. The automatic valve opens and closes automatically to fill the upstart air manifold while starting. It consumes air during the start. The distributor allows air to open the appropriate cylinder valve.
The non-return valve presents the passage of high pressure from the start manifold to the receiver. The distributor control valve regulates the admission of pilot air to operate the cylinder air start valve in the correct sequence. This enables the engine to start in the desired direction.

Cylinder Air Starting Valve

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(j) This air causes the engine to turn on the air. If the running direction matches the telegraph the fuel lever moves to admit the fuel. Running direction interlock checks the direction of main engine rotation with the telegraph. If the direction matches it sends feedback to block air.

Timing adjustment of fuel and air during starting.

Starting air distributor timing:-
The starting air distributor timing is adjusted for the desired direction of rotation. The timing is adjusted by supplying AHD or AST air signal to the pneumatic cylinder that adjusts the distributor cam through a link. this retiming of starting air distributor ensures that air is introduced into the relevant cylinder at the correct time to achieve starting in the desired direction from any position of rest.
Another method involves the distributor cam turning with the fuel pump cam, as it is engaged with the fuel camshaft with bevel gears.

Scavenge and exhaust timing:-
The uniflow scavenging system consists of fixed scavenging ports and no adjustment can take place. Operation of engine with constant pressure turbocharging has almost symmetrical exhaust valve timing and thus no change in timing required for exhaust cams.

Fuel pump timing:-
The fuel pump timing must be readjusted since it will be the opposite flank of the cam, which will now rise the pump plunger to deliver fuel.
    Method-I Shifting the position of rollers:- MAN B&W SMC engines have their fuel pump cams fixed directly to the camshaft but the follower rollers can be displaced to alter the pump timing. The link which displaces each follower is actuated by a pneumatic cylinder and piston, powered by compressed air from the starting system. The link is self-locking in either position and a sensor is fitted to each pump to cut off fuel delivery if the link is not locked in its correct position. The system is simple and safe, reversing can be operated even if one or two-cylinder have not operated to their correct position.
    Method-II Shifting the camshaft axially:-In this method two sets of cams are fitted on the same camshaft each for AHD & AST. A camshaft shifting mechanism called the Reversing gear or camshaft servo motor slides the camshaft axially in its bearing and engages the correct cam with rollers.
The reversing gear consists of a piston that is connected to a camshaft, a cylinder inside which the piston slides, safety gear with a stopper on top of the cylinder and power bottles.
Safety gear is also connected to starting lever and reversing handle through reversing relay. When the reversing handle is shifted, it actuates the reversing relay valve and the control air passes to safety gear. Here the control air pushes the stopper and gets into the power bottles. Hydraulic oil in the power bottle is now pressed up and pushes the piston to slide.
As the piston reaches its end position and shifting of the camshaft is completed, reversing handle gets back to running position, hydraulic oil returns to power bottles and control air releases from reversing relay valve. Now at this point, the stopper will be released and it will hold the piston in the position.
    Method-III Rotating the camshaft or Lost motion:-This method involves the circumferential translation of the camshaft relative to the engine crankshaft by using a servo motor. The servo motor consists of pair of vanes connected to the camshaft and another pair of vanes connected to the gear wheel rim. Oil pressure when applied between opposite pairs of the vanes, the camshaft is moved relative to the gear wheel and the crankshaft.
The oil is taken from the crosshead lube oil line. This oil goes to a reversing control valve which is operated by the telegraph. This reversing control valve directs the lube oil to the servomotor chamber as per the desired direction. This oil then goes to a control slide valve, which gives a hydraulic signal to the hydraulic blocking device which blocks the starting lever in the stop position and cut off the fuel supply to the engine for the time the reversing is taking place. When the telegraph is in stop position the oil from the servomotor is drained into the sump through the reversing control valve.

Important Components of the System

Rotation direction interlock:-
This interlock takes input from camshaft and lube oil. The lube oil is taken from the oil line between the reversing control valve and servomotor. In case of the wrong direction, it sends a signal to cut the fuel oil supply and stops the engine.

Automatic air starting valve:-
The valve keeps the 30bar air holding till it gets a signal from the pilot valve to pass the air. This type of Automatic air starting valve requires a control signal for both opening and closing. This is a simple ball valve that turns through 90 degrees by a Pneumatic actuator fitted on the top. The actuator consists of two pistons. right side piston is fixed and left is moveable, upon which the air pressure acts, its movement causes the rotation of ball valve. Piston transverse movement is translated into rotational movement through Rack and Pinion arrangement.
For starting the air signal acts behind the piston and upon completion of starting sequence, the air acts in the space between the pistons which push them apart and close the ball valve. Once air passed through this valve it waits at distributors and cylinder air starting valve.

Starting Air distributor:-
The air starting sequence of the engine is determined by the Distributor. This admits air to one or more units and is driven by a cam. The new generation of marine engines has a system of controlling starting air to cylinders by individual solenoid valves. This will replace the mechanical Distributor (of conventional engines). Common faults include jamming of the shuttle valve and leakage from pilot air connections.
A series of pilot valves are mounted radially around a cam. They are timed with the engine and allow air to pass through to open the cylinder air starting valve whose piston is in the correct position for admission of starting air. As per the firing order and direction of rotation.

Starting air valve:-
The Starting air valve is used in engines started on compressed air. The signal for opening is given by the Distributor. Air is admitted to the unit by the Distributor, as per the firing order. During operation, the starting-air lines should be felt. In case of leakage, the line adjacent to the valve will become abnormally hot. This can lead to a starting-air line explosion. A leaky valve should be replaced as soon as practicable. This can also be temporarily blanked, however, it may lead to starting problems. The starting valve stem may also jam, with resulting problems for starting. A minimum number of units for a propulsion engine is 3. there will be an overlap provided, during which more than one air starting valve receives air at any instant. Mounted on each cylinder; consists of two pistons mounted on the same valve spindle. The upper piston is called the operating piston having a larger area and having a spring at its bottom which keeps it pushed up; pilot air from the distributor acts on the top of this piston. The lower piston is called the balancing piston; air from the automatic start valve acts below this piston and on top of the valve. This valve remains closed with the effect of spring force and starting air waits here till it the pilot air pushes the top piston. Air now can admit inside the cylinder.

Flame trap and Bursting Disc:-
Air from the automatic starting valve passes through the bursting disc and flame trap before reaching to cylinder air starting valve. A flame trap prevents any flame or spark from the combustion chamber to enter into the starting-air line, as this may cause an explosion in the air starting line.
Bursting disc by sacrificing itself limits any pressure rise and so the damages involved in the stating-air line.


Q. Function of distributor and automatic starting valve.
Q. Theory of engine starting.
Q. Reason for overlapping of air valves. No. of Cylinder for a 4-s engine to start on air.
Q. Air bottle pitting and rectification
Q. Compensating ring in air bottle
Q. Fusible plug function and material.
Q. Relief valve in the air reservoir
Q. Starting-air line safeties
Q. Action and checks upon bursting disc rupturing
Q. M/E turn on air but not firing
Q. Air starting valve stuck, rectification without stopping the engine
Q. Air starting valve leaking during voyage how will you know and action
Q. Overhauling of starting air valve, checks and tests
Q. Pilot signal and function of a pilot valve