It’s not that easy to start marine diesel engine like on a push of a button in our car. Starting an marine engine require a series of checks and procedures to be followed; before the final go ahead. It’s the fuel, lube oil, starting oil, jacket water and interlocks which affects the overall starting procedure.
These checks and balance ensures there is satisfactory jacket water temperature; lube oil pressure, fuel oil circulation, starting air pressure and correct firing order in the diesel engine. Furthermore they ensures components that had been overvalued are truly checked and reinstalled; and no tools or rags still in place.
Marine Diesel Engine basically starts when the compressed air ( Starting Air ) at 12 to 30 bar pressure; impact on the piston to create sufficient torque to rotate the shaft for one complete revolution. This is done by inserting compressed air into the cylinder using starting air valve operation; using starting air distributor.
This is what actually happen when we finally push button to start the engine. But the marine diesel engine requires a series of checks and steps to be done before it; which we will read in a step by step manner in following sub topics.
Preparations And Checks Before Starting Marine Diesel Engine
There are different checks and preparation carried out on four key systems of an engine; i.e lube oil, fuel oil, starting air and jacket water. At first they are taken up to the desired condition for the successful engine running; and then checked for desired requirements. these checks are done on an one to one basis; i.e if any of the condition does not satisfy we do not start the engine.
1 ) Starting Air System
- Ensure all main air compressor’s are on standby and ready to be used.
- Put auxiliary blower to automatic settings.
- Charge up the main air bottles and ensure there is enough pressure to start the engine.
- Check and ensure there is proper supply of control air to the starting air system.
- Drain the main air receiver
- Drain any water present in the air cooler and turbocharger drain’s.
- At last check whether there is sufficient air pressure in the starting air receiver.
2 ) Lube Oil System
- Check the lube oil level in the tanks and engine sump.
- Replenish lube oil if needed.
- Start and keep the lube oil pump running for the piston, camshaft, crankshaft and the bearings.
- Keep an eye on the oil flow from the sight glass.
- In case of four stroke / non reversible engine; start the lube oil pump to the gear box.
- Ensure there is sufficient inlet temperature for the lube oil.
- Make sure there is appropriate lube oil pressure in the system.
- Check the supply of lube oil to the turbocharger
- Make sure there is sufficient lube oil in the cylinder lube oil tank.
3 ) Fuel Oil System
- Ensure there is enough fuel in the fuel oil holding and settling tanks.
- Check whether the purifiers are operational or not ?
- Make sure the fuel oil booster pumps are running.
- Maintain appropriate fuel oil temperature.
- Keep the fuel oil circulation pump running if the engine was stopped on heavy oil; or start it if not.
- Check whether the fuel oil regulators works satisfactory.
- Make sure the fuel circulating is at satisfactory pressure range.
- Check and ensure whether the fuel oil flow meters are functioning properly.
4 ) Cooling Jacket Water
- Start the pumps for the cylinder jacket water.
- Preheat the jacket water to 60-70 degree Celsius.
- Make sure there is a continuous positive flow of the cooling water.
- Check and ensure there is sufficient water level in the expansion tanks.
- Ensure continuous circulation of the jacket water to the preheater.
- Check and ensure there is correct cooling water pressure in the system.
- Make sure there is no leak and rectify if any.
The Term ‘Overlap’ And Its Role In Starting Diesel Engine
Ideally a 30 bar compressed air is used to start the marine engine in required direction. This is done by injecting the starting air within the engine cylinder when the piston is just past the T.D.C. The starting air is injected based on the correct firing order of the engine; so as the engine starts in desired direction.
To enable hassle free startup the engine must have at least two or more starting air valve; injecting starting air at the same time. Now the time period for which there is injection of starting air; in two or more cylinders of the diesel engine is called overlap. Having overlap ensures the engine can be started using compressed air; from any crank position.
While its value depends largely on the total number of cylinders, exhaust and inlet timing; usually engine with less than 4 cylinders does not show any overlap. This is so because for engines with 3 or less cylinders the starting period is less than the total firing interval; i.e 100-110 < 120 on timing diagram.
Having an overall in an engine ensures that when one starting air valve is about to stop injecting; the other one is about to start injecting starting air. This ensures at no times there is less than two starting air valve injecting compressed air into cylinder. If this not be the case an engine can stop in an position from where it just cannot be started using compressed air.
16 Simple Step’s To Start Marine Engine
- Start another generator and take it on load.
- Start all the lube oil, fuel oil and jacket water pumps and maintain their required temperature and pressure.
- Ensure fuel and lube oil purifiers running.
- Check oil level in sump and turbocharger.
- Now engage the turning gear and turn the engine slowly with indicator cock open.
- Check all indicator cocks for any signs of water.
- Prepare the air starting system with enough compressed air in the main air bottle.
- Keep the main air compressor on standby.
- Drain off any water contained from the air start receiver.
- Supply the cylinder lube oil for the time it is turned using turning gear.
- Now disengage the turning gear once you get the go ahead with the bridge and ready to test the engine.
- Start the auxiliary blower on manual for a brief moment and purge the cylinder with fresh air.
- Now turn the engine slowly on starting air with the indicator cock half shut.
- After a minute or two close the indicator cock and drains of air coolers and turbocharger.
- Start and test the engine by giving start signal to air start distributor.
- Check the engine and put it on standby; and inform the bridge.
The Role Of Slowly Turning In The Starting Procedure
On a slow speed engine like that of marine diesel engine it is required to turn the engine in slow speed on air; before finally giving a push start. This is done every time the engine is stopped for more than 20-30 minutes. This is done by allowing a very small part of the total starting air via slow turning valve.
The slow turning valve ensures that the engine rotate at not more than 5 rpm; thus allowing time for any water contained to escape via the partially opened indicator cock. The air used for the purpose could also be other than the small line from starting air line we usually take; just they need to be dry to avoid corrosion.
Turning engine on slow speed basically allows for any oil or water droplet to escape out of the combustion chamber. The liquids in general are in compressible and thus lead to hammering of the piston assembly. Thus to safeguard engine parts from any damage or shock; it is a common procedure to turn large diesel engine on slow speed before starting.
Furthermore it also helps spread the cylinder lube oil evenly within the engine cylinder before the engine start. Many a times engine are also given this slow turn; to protect the bearings and crankshaft from sagging when not used for long. Ideally the engine is turned slowly only for one complete rotation before the final start.
How The Reversing Operation Is Done In The Engine
In general all two stroke diesel engine can be reversed while four stroke can’t. This is so because for normal reversing of the propulsion i.e running engine in reverse direction we need to change fuel injection, scavenging and exhaust valve timings. Thus to make things a more simpler its common practice to use gearbox for the reversing four stroke engine while changing fuel injector timing for the two stroke diesel engines.
1 ) Reversing Two Stroke Diesel Engine
For an diesel engine such as the marine engine we need to change the injection, exhaust and scavenging timing; in order to truly reverse the engine operation. But since for the timing diagram of the two stroke diesel engine we found both scavenging and exhaust timing; almost symmetrical to what in the forward direction.
In large two stroke engine such as ship’s main engine we only adjust the timing for the fuel injection and starting air distributor position to reverse the firing order.
The starting air distributor is a set of valves that sends pilot air to the starting air valve to start injecting starting air to the cylinder; to run engine on air to reach sufficient rpm to self start the engine. These distributors are equipped with a set of three way valve with two positions.
This further ensures that the distributor only initialize main starting air valve in respect to the running direction; i.e ahead or reverse. Now if you invest time going through one of our old post on starting air valve’s and starting air distributor. You will find there is different set of pilot air lines for both ahead and reverse direction.
The double acting piston with the reversible cylinder is used to reverse the operation of starting air distributor. Similarly the position of camshaft is changed with the help of control air. Changing the camshaft position change the rotation angle and thus the fuel injection time w.r.t the firing order.
2 ) Reversing Four Stroke Diesel Engine
Now since its pretty much complex to reverse the complete engine rather than the shaft in four stroke engine. Its no surprise that you guess we use the gearbox. ( Honestly i told you that at the start of this article ). The gearbox consists of definite set of gears which then with helps of idler helps reverse and change shaft speed.
But the main question is how does the reverse gear works ? The different gears on the main and counter shafts in gear box allows for the shaft operation; through a set of different gear ratio. This is done with the help of the synchronizer which engage and disengage two different gear set on main and counter shaft.
But for the reverse gear there is a third gear called idler in between the two gears of the gearbox. The idler helps the output shaft ( Driven Shaft ) to rotate in opposite direction to normal shaft rotation under different gears. Many ships also use reversible motor driven propeller which run by the electric power.
Note: The article is produced on request from RISHIKESH KUMAR.
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