Air Starting of Diesel Engines
Classification Society Rules
1. Initial starting arrangements
Equipment for starting the main and
auxiliary engines is to be provided so that
the necessary initial charge of starting air or
initial electric power can be developed on
board ship without external aid. If for this
purpose an emergency air compressor or
electric generator is required, these units
are to be power driven by hand starting oil
engine.
2. Air compressors
• Two or more air compressors are to be fitted having a total
capacity, together with a topping-up compressor where fitted,
capable of charging the air receivers within 1 hour from
atmospheric pressure, to the pressure sufficient for the number
of starts required. The capacity of the main air compressors is
to be approximately equally divided between them. The
compressors are to be so designed that the temperature of the
air discharged to the starting air receivers will not substantially
exceed 93°C in service. A small fusible plug or an alarm device
operating at 1210C is to be provided on each compressor to
give warning of excessive air temperature. Each compressor is
to be fitted with a safety valve so proportioned and adjusted
that the accumulation with the outlet valve closed will not
exceed 10 % of the maximum working pressure. The casings of
the cooling water spaces are to be fitted with a safety valve or
bursting disc so that ample relief will be provided in the event
of the bursting of an air cooler tube.
• 3. Air receiver capacity
• Where the main engine is arranged for
air starting, at least two air receivers of
approximately equal capacity are to be
provided. The total air receiver capacity
is to be sufficient to provide without
replenishment, not less than 12
consecutive starts of the main engine,
alternating between ahead and astern if
of the reversible type. And not less than
six consecutive starts if of the nonreversible type.
4. Draining of Air Bottles
• The air discharge pipe from the compressors is to be
led direct to the starting air receivers. Provision is to
be made for intercepting and draining oil and water
in the air discharge for which purpose a separator or
filter is to be fitted in the discharge pipe between
compressors and receivers. The starting air pipe
system from receivers to main and auxiliary engines
is to be entirely separate from the compressor
discharge pipe system. Stop valves on the receivers
are to permit slow opening to avoid sudden pressure
rises in the piping system. Valve chests and fittings
in the piping system are to be of ductile material.
Drain valves for removing accumulations of oil and
water are to be fitted on compressors, separators,
filters and receivers.
Isolating non-return valve between air bottle
and engine, Bursting disc or Flame arrestor
• The starting air piping system is to be protected
against the effects of explosions by providing an
isolating non-return valve or equivalent at the
starting air supply to each engine. In direct reversing
engines bursting discs or flame arresters are to be
fitted at the starting valves on each cylinder; in nonreversing and auxiliary engines at least one such
device is to be fitted at the supply inlet to the
starting air manifold on each engine. The fitting of
bursting discs or flame arresters may be waived in
engines where the cylinder bore does not exceed
230 mm.
Interlocks on starting
• 1. Turning gear is disengaged : The turning gear acts as a lock to
the engine, preventing it from turning during maintenance etc.
Therefore cranking the engine with the T/G engaged will cause
much damage to the T/G as well as the e n g i n e.
• 2. Distributor position is as ordered and in end position: The starting
air distributor cud position is checked to be in the right end position,
as ordered at the telegraph before a start signal is forwarded.
• 3. Main Lub Oil. pressure is present: A hydraulically -operated valve,
sensing L.O. pressure blocks the start signal in absence of L.O.
pressure.
• 4. Air spring pressure is present: In engines with pneumatic closing
of exhaust valves, the absence of this air spring pressure, causes a
block of starting air signal.
• 5. Aux blower running: In engines with constant pressure
turbocharging, if the auxiliary blower is not started or has failed. the
starting signal is blocked usually.
Checks for leaky start air valves, in port and at sea
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In port
Ideally this has to be done unit by unit, taking each to TDC. Other wise a
minor leak cannot be detected if the unit is at BDC. The steps involved are
as follows:
Open indicator cocks
Engage turning gear
Take unit to TDC
Disengage turning gear
Shut off air to distributor
Pressurize main start air distribution pipe.
Check for air issuing out of indicator cock
Note: In the normal case the engine doesn’t turn on air since the air to
distributor is shut. But in
case of a leaking start air valve the engine would turn. Hence the turning
gear has to be disengaged before the distribution pipe is pressurized.
At sea
Feel for hot start air inlet branch pipes.
Actions in emergencies
• Action in case of engine failing to start due to a
sticking pilot or air start valve, during
• Maneuvering: With a reversible main engine, the
easiest step to follow is to request bridge for an kick in
the other direction and get the engine out of the blind
spot. Note that this is possible since if one unit start air
valve or pilot is sticking and if that is the unit to receive
start air in one direction. (that is when start fails), then on
reversal of the engine. it will be definitely some other unit
which is to receive start air for the other direction.
• Action in case of bursting disc burst.
• The bursting disc hood can be turned to temporarily
regain maneuvering in case of a bursting disc burst.
• Operation with cylinder out of operation
• If the running gear is removed and the piston is
hung , start air to the unit should be blanked as
otherwise the hung piston will be forced down by
start air pressure. Note that in such a case, the
engine may not be fully maneuverable. A similar
situation as mentioned above occurs, the
remedy being the same as given above
Slow turning
• Is an alternative to blow through. In bridge controlled ships, bridge
may take considerable time to start engine even after handing over
engine to bridge. Since F.O., L.O., and F. W. systems are running
after handing over control to bridge. there is a chance of fluid
accumulation in the unit if there is a leak of any of these fluids. To
prevent a hydraulic lock from damaging the engine when starting
with such a fluid accumulation in the unit. slow turning feature is
provided. After a preset time on bridge control, usually 30 mts., the
engine goes into slow turning mode automatically. The engine does
not turn in this mode, only the indication will be on. When the engine
is started from bridge when in this mode, the engine turns over
slowly first for one complete revolution and then starts. Slow turning
is accomplished by controlled admission of starting air. Slow turning
can be initiated manually from E.C.R. and also cancelled from
bridge.
Bridge control
• Three repeated starts are usually carried
out automatically in case of start failure at
the first attempt. However the bridge start
is blocked at low level of start air pressure
which leaves sufficient air for the engineer
attending, to start locally. On bridge
controlled ships the start air from air
bottles is to be always left open during sea
passage also.
Why start air admission before TDC?
• In order to make maximum utilization of the entire start
air period, it is essential that the unit is fully pressurized
with full starting air pressure right at TDC. If the start air
admission starts at TDC, due to mechanical delay, by the
time unit is full pressurized and the air exerts a turning
torque, the unit will be well past TDC, which represents a
loss of starting air range. Hence the start air admission is
slightly before TDC so that at TDC the unit it full
pressurized to turn the engine. Admission of air before
TDC does not turn the engine other way than the
required direction due to the cylinder which is
overlapping which exerts much greater turning torque in
the required direction.
Why inverse cams are used as stating air cam?
• Inverse cam is preferred as starting air
cam since it provides the easiest
arrangement of taking the rollers out of
action once the engine has picked up on
fuel. This will reduce wear and tear of air
distributor valve when engine is running
normally on fuel. Also, with the inverse
cam arrangement, there is no possibility of
mistimed admission of starting air in case
of stuck pilot valves or broken spring.
Air Starting system components
• 1. Main or Automatic air start valve
• 2. Air distributor
• 3 . Start air valves on cylinder head.
Main or Automatic air start valve
• This valve as the name suggests opens automatically
during start and closes after the start. Isolation between
engine and air bottle is the main purpose.
• This valve may incorporate a non return valve to stop the
flame front in case of a starting air line explosion. In
some cases, the non return valve may be a separate
component. The automatic valve is also used for
blocking an engine start or putting into service. Provision
for manual opening can also be provided. Slow turning
device, if fitted, is incorporated in the automatic valve.
Latest B & W design of automatic valve is a simple ball
valve actuated by a piston through a rack.
Air distributor
• The air distributor may be basically of two kinds, the
rotary type, and the pilot valve and inverse cam type.
The rotary distributor designs are common in generators
and also is used in some main engines too. The pilot
valves may be mounted separately and the inverse cams
also separately mounted on the main camshaft next to
each unit instead of grouping them together as a
distributor. This arrangement is found in reversible four
stroke engines. The pilot valve along with the inverse
cam is termed the distributor which controls the timing of
air admission for a particular unit. Pilot valves are also
prone to getting stuck and hence of non corrodible
material
Start air distributor B & W engine
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The pilot valve is a spool type
valve held up by spring pressure
during normal running of the
engine. The other end of the spool
valve bears on an inverse cam
when operating air at 30 bar (or
starting air pressure) is admitted
into the space below piston 1. This
pressure acts on the top of piston
2 and also on bottom of piston 1.
The top of piston 2 area being
larger. the resultant force is to
keep the pilot valve spool pressed
onto the inverse cam against the
spring pressure. If the inverse cam
position allows, the pilot spool
further moves down. This
movement does the following.
• 1. The operating air inlet is cut off by the piston 1 covering the hole
though which operating air was admitted, which traps the 30 bar
operating air pressure in the space between piston 1 & 2. This
means that even if the external operating air line is vented, the
trapped air keeps the pilot valve at the activated position till the
inverse cam mechanically pushes it back. This is the arrangement
that ensures that those units in the starting air range. at the instant
of changing over form air start to fuel run, continues receiving
starting air though external operating air is vented.
• 2. The line to start air valve which is vented is connected to the pilot
air line, This is by the piston 3 covering the vent and at the same
time allowing air into the start air valve line.
• During normal running, the pilot valve is kept off the cam preventing
unnecessary wear.
• There are as many pilot valves as the number of units. Reversal of
start air distributor is by shifting the shaft axially by a pneumatic
cylinder.
Start air valves on cylinder head.
• The starting air valve in
cylinder head of each unit
admits air into the cylinder
during starting.
• Pneumatically operated start
air valves are used and the
start air valve designed such
that if the unit is firing, the
combustion pressure prevents
the start air valve form opening
even if the pneumatic pressure
for opening is present from the
distributor.
• With pneumatic operation of start air valve, the
application of pilot air to start air valve does not
necessarily mean the valve opens, but depends on the
pressure in the cylinder. This has reduced of chance of
starting air line explosion greatly. The valve piston and
liner etc are of non corrodible material, so that moisture
content in the pilot air does not cause corrosion and
eventually jamming of valve. Grease points are provided
outside for lubrication. Flats or screw driver slots on top
of valve spindle enables checking of valve freeness.
SULZER design has both opening and closing pilot air;
whereas B & W uses only opening air, closing is done by
spring force (figure shown here).
Reversing of a Diesel Engine
• Reversed running of a diesel engine can
best be understood by considering
individually the four processes given below
that takes place during engine operation.
• Inlet,
• Exhaust.
• Starting air admission,
• Fuel injection.
• These four processes are timed in relation to power
piston movement, which gives their timings.
• The timings are given in relation to any Dead Center,
TDC or BDC, as “X” degrees before Dead center to “Y”
degrees after Dead Center. Each of the four individual
processes mentioned above are controlled by cams.
Each one may or may not need repositioning for
reversed running of the engine. Whether reversal
arrangement in needed for a particular process depends
on the disposition of the total cam angle about roller
contact point on the cam, when the unit in question is at
dead center. If a certain process commences “X”
degrees before dead centre and terminates “X” degrees
after dead centre (symmetric timing), then there is no
need for a reversal arrangement. Therefore for the first
three processes, reversal arrangement is needed when
the timing is asymmetric.
• Methods of repositioning timings for
engine reversal, can be broadly classed
into following two types.
• 1 . Using same cam for ahead and astern
running.
• 2 . Using a different set of astern cams for
astern running, by shifting the cam shaft
axially.
Using same cam for ahead and astern
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When the same cam is used for ahead and astern
running, it means that the angular relation between the
crank shaft and cam has to change or the point at which
the roller is activated has to be shifted. This angular
relation change can be achieved by any of the following
methods.
1. By turning the cam shaft (along with the cams mounted
on it) through the required angle in the required
direction keeping the crank shaft stationary.
2. By turning the cams alone on the cam shaft.
3. The point of actuation of the roller can be changed by
shifting the roller transverse to the engine.
Using a different set of astern cams
• In this case the cam shaft is shifted axially
to bring into play a different set of astern
cams. In four stroke engines this is the
only method used. Using a different sets of
ahead and astern cams means that the
cam shaft has to be axially shifted for
reversal. In order to achieve this, the cams
have a gradual flow from one to the other
so that the roller cam smoothly move from
one cam to the other.
A Block Diagram of Starting of a Main Diesel Engine