What is the starting air distributor? It is the part of the starting air system responsible for providing pilot air to starting an air valve as per the correct starting order. For an idle six-cylinder engine; the injection timing of all cylinders is timed according to the firing order.
For an inline six-cylinder engine a firing order of 1-5-3-6-2-4 ensures; that more power is produced without putting unwanted stress and opposing forces on rotating shaft.
Normally, a compressed air of 20 to 30 bar is used to start an engine in required direction. This is done by inserting compressed air ( starting air ) in the cylinder when its just past TDC based on their firing order. The starting air is inserted through a valve called starting air valve.
The opening and closing of these valves is controlled using pilot air supplied to them by the starting air distributor. The starting air distributor is the one which ensures; pilot air is supplied to the cylinder as per the proper firing order. Air distributor housing installed in a fixed position; is operated by the movement of camshaft.
The distributor pushrods are in contact with the cam assembly and allow pneumatic air signal ( pilot air ) to pass only when; the poppet valves coincide with the correct timing sequence. A poppet valve is basically located within the distributor housing that avoids distributor push rods operating individual pilot valve; to make contact with cam when the engine is stop, operational or have incorrect timing.
But Why Correct Firing Order Is So Important ?
A single cylinder engine produce enormous gas forces transmitting downward in the form of cylindrical torque. This torque is produced and applied to the shaft using the crank arm assembly. In the single cylinder arrangement almost all the force produced is converted to the rotational motion of the crankshaft.
Now if more than one cylinder is connected to the same shaft; uneven forces such as torsional vibration and cylindrical torque lead to misalignment of shaft and failure of whole engine unit. Thus to balance the system the two cylinders have to be connected; either side by side to the same crank arm or in 180 degree out of phase.
What happens here is that although both piston reach TDC and BDC at the same time; but are always opposite to each other i.e one is at power stroke while other at induction stroke.
Thus a correct firing order ensures proper engine balance with equal distribution of vibration and torque forces. This reduces the risks of damage and improves its durability or life. Furthermore, it balances the output power distributing the load on cylinder over the entire length of the crankshaft.
One can simply calculate the required degree of crank throw by dividing the rotation of crankshaft ( in degree ) between two power stroke; by total number of cylinders for inline engines. For example in single cylinder four stroke engine it is 720o; so the required degree at which crank throw be placed for a six cylinder engine will be 720 / 6 = 120 degree.
Basic Construction Of Starting Air Distributor
Basic starting air distributor consists of a circular casting assembly with distributor push rod and cam assembly. It also contains parts such as; guide screw, disc, liner, lock washer, lock nut, washer and cover. Furthermore many distributor also came with pilot air pressure regulator, timer delay and distributor control cylinder.
A typical starting air distributor assembly is mounted on the control end of the engine frame. The distributor body provide housing to the pilot valve for each cylinder. These valves acts in response to the distributor camshaft connected to the engines upper crankshaft. Under normal condition they are not in contact with the camshaft; thanks to the spring tension holding them.
On the outer periphery of the air distributor an cylindrical air chamber is installed. This holds the pneumatic air signal line which make contact of pilot valve push rod with the cam assembly. These pneumatic air line is sometimes also called as service starting air supply. In reversible engines they are pneumatically connected with speed sensing equipment; to quickly reverse the engine without any damage.
Additional vent arrangement is given in the distributor valve assembly to ensure safety of air distributor. The individual pilot air line from dedicated pilot air valves reach to the starting air valve up-top; to start inserting starting air into cylinder.
How Does Starting Air Distributor work ?
When air starting control valve is operated manually by switching lever to start position; control air is supplied to the automatic valve. This provide starting air to flow to the main starting air valve of each cylinder. Now one branch line from this goes to the pilot air valves housed in the starting air distributor.
Now after checking all the interlocks an pneumatic air signal is given to the air distributor via external air chamber. This acts against the spring tension releasing push rod against cam assembly. The push rod acts against the cam position to determine firing order; and thus operate connected pilot valve.
When the push rod go through the high end of the cam pilot air is sent to main starting air valve of designated cylinder. The pilot air acts against the operating piston within the main starting air valve to inject compressed air. Now in starting air distributor when the push rod reach the low point of the cam profile; the out-end air pressure push the pilot air inward closing the pilot valve.
The correct timing of individual cylinder and correct firing order is obtained by carefully calibrated distributor camshaft; connected to the the upper crankshaft assembly. If you curious and want to understand how the complete starting air system work; check out our old post “Starting Air System in Marine Diesel Engine” clicking link over here.
Reversing Starting Air Distributor
In reversible engines such as marine diesel engine we need to change the timing of scavenging, exhaust valve and fuel injection. But in two stroke engine exhaust and scavenging timing are almost symmetrical in reverse and ahead direction. Thus for two stroke engines such as marine diesel engine; we only need to change the fuel injection timing and starting air distributor position to reverse the firing order.
A starting air distributor is equipped with two sets of two position three way valve which ensures the distributor direction is in match with control direction i.e ahead or reverse. Now if you pay attention to the pilot valve assembly fitted in air distributor; you will notice separate path ways for astern and ahead control air.
A reversible cylinder with double acting piston is used to reverse the distributor by operating the astern valve with the help of moving arm. Moreover when camshaft position is changed the cam profile rotates changing the timing and firing order for the air distributor. In large engine with radial starting air distributor; this is achieved by shifting between the two cam profile fixed on the end of the camshaft.
Note: This article is produced on request from Mariston lerins.
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