What is an air start system on ships?
An air start system on a marine internal combustion engine is used to start, stop (in an emergency), or reverse the engine. Slow-speed two-stroke marine engines operate at low rotational speeds (<300 rpm), and have a very large mass, this makes it difficult to achieve the pressures and temperatures required for combustion when the engine is initially started. The air start system addresses this problem by introducing compressed air at approx. 30 bar (435 psi) pressure directly into the engine’s cylinders. The compressed air forces the associated piston downwards towards bottom dead centre (BDC) and gives the engine the initial impetus it requires to compress and ignite the air-fuel mixture within the cylinder (combustion chamber). Once the engine reaches a certain speed, regular fuel injection and combustion processes occur, and the engine will continue to operate under its own power.
Note – this article is focused on large slow speed marine diesel engine air start systems.
Why do we need engine air start systems?
Large marine engines need a lot of torque to put the engine’s mass into motion, far more than a typical electric starter could provide. Car engines use electric starter motors to start the engine and correspondingly the combustion process, but this would not work with a large two stroke marine engine because the engine is too big, and the motor amps required would be too large. Compressed air represents stored energy, and it is this energy that is used to put the engine into motion.
What are the main parts of a ship air start system?
A ship air start system has several main components:
1. Air compressor – the source of compressed air. There will almost always be more than one air compressor so that there is some system redundancy. Ships typically use piston air compressors, although other types of compressor are available (screw, centrifugal etc.).
2. Air receiver – stores the compressed air produced by the compressor. An air receiver, or air receivers, are typically sized to allow for several starts of the engine. An engine requires a lot of air to start, thus air must be stored rather than produced on demand.
Good to know - an air receiver is also known as an air vessel or air bottle.
3. Pilot valve – distributors air to the automatic air start valve when open; the automatic air starting valve is closed when air from the pilot valve is supplied. Draining the air from the pilot valve line causes the automatic air start valve to open (drained air is referred to as 'bleed air').
4. Steering gear interlock – allows compressed air to flow to the automatic air start valve only if the steering gear is disengaged. The steering gear consists of an electric motor that can engage with the engine flywheel in order to rotate the engine at slow speed prior to the engine starting.
5. Automatic air start valve – opens once the pilot valve line has been drained and the steering gear disengaged.
6. Air distributor – regulates the flow of compressed air to each air start valve. The distributor ensures that compressed air reaches the designated cylinders at the designated time (air is distributed to the cylinders based on the firing order).
7. Air start valve – the final component of the starting air system prior to air being admitted into the cylinder. The air start valve delivers compressed air into its respective cylinder when it is open. Air start valves are mounted directly onto the cylinder head of each cylinder.
What are the main parts of an air start valve?
The main parts of an air start valve are:
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Valve body – the external casing that houses all of the valve’s internal components.
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Top cover – attaches to the valve body.
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Pilot inlet – compressed air enters through this port.
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Spring – ensures the valve returns to its default closed position after operation.
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Opening piston – compressed air acts upon the piston causing the valve to open.
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Balancing piston – used to guide the valve spindle and ensure it does not misalign.
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Outlet port – when the valve is in the open position, compressed air flows into the cylinder through this port.
Engine Air Start Valve Parts
How do ship air start systems work?
Compressed air is drawn from air receivers, discharged through a pilot valve, and also to the turning gear interlock. The turning gear interlock prevents air from reaching the automatic air start valve until the turning gear has been disengaged. When the pilot valve is open, air is delivered to the automatic air starting valve, which causes the valve to remain in the closed position; removal of the pilot air pressure causes the automatic air start valve to open.
Once the turning gear is disengaged, compressed air flows to the automatic air start valve. Air is held within the automatic air start valve by the valve’s spring pressure, and the compressed air applied from the pilot valve. At this stage, the automatic air start valve is in the closed position. Once a start command is given, compressed air will be drained from the pilot valve line and the automatic air start valve’s spring pressure will be exceeded by the main air start system pressure; this causes the valve to change from the closed position to the open position.
Compressed air now flows to each cylinder air start valve and the air distributor. The distributor will distribute air based upon the firing order of the engine; it uses a valve attached to a cam follower, and a camshaft, to achieve this. As the camshaft rotates, the cam follower moves up or down, which causes the distributor valve to change position (open or close). As the distributor valve changes position, it discharges, or stops discharging, air to each air start valve.
· If air is discharged from the distributor to an air start valve, the air start valve opens, and compressed air is admitted into the respective cylinder.
· If air is not discharged from the distributor to an air start valve, the air start valve remains closed, and no compressed air is admitted into the respective cylinder.
Compressed air is admitted into each cylinder in a specified order, this is known as the firing order (the order of which ignition occurs within each cylinder). As compressed air enters each cylinder, it forces the associated piston downwards towards BDC, but because several pistons are being forced downwards simultaneously, there is enough momentum to force one piston back upwards towards top dead centre (TDC); fuel can then be admitted, and ignition occurs. Once ignition occurs, the engine can continue to operate under its own power.
Good to know - there are safety features built into air start systems to prevent damage to the system and personnel occurring. These include:
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Control valves – to regulate, start, or stop, air flow within the air starting system.
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Relief valves – to relieve any over-pressure that may accumulate within the system.
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Flame traps – installed prior to each air starting valve to arrest any combustion process that may occur in the high pressure air lines. Combustion material within the air lines comes from oil carry-over from the air compressors; this may ignite if an air starting valve is passing/leaking and hot exhaust gases from the combustion space are allowed to escape into the air lines.
How do ship air start valves work?
Compressed air is fed to the lower part of the valve from the automatic air start valve. A large diameter pipe connects the air start system to the valve, which ensures a large volume of air can be admitted into the cylinder quickly. The air distributor will discharge compressed air to the pilot air inlet as it moves through its sequence (firing order), this causes the opening piston to move downwards, and consequently the air start valve opens. At this point, compressed air is admitted into the cylinder. When the air distributor changes position, air will no longer be supplied to the pilot inlet line, it will instead be drained. Draining of the pilot line causes the spring to expand, which causes the valve to close (the air pressure in the main line is no longer sufficient to overcome the spring’s residual pressure, thus the air start valve closes).
The air start valve’s working principle can be seen in the video below.
What are the main designs of compressed air start systems?
There are two main air start system designs for diesel engines.
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Direct start – compressed air is injected directly into each cylinder. This pushes the respective piston downwards, which causes the crankshaft to rotate. This air start design is simple and is used by large marine two stroke diesel engines.
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Air motor start – compressed air is used to drive an air motor, which is connected to the engine flywheel. Compressed air flows through the motor, causing the motor shaft to turn, which in-turn drives the flywheel and consequently the engine crankshaft. This design is used for mid-sized engines.
In both designs, once the engine is operating, the flow of compressed air is stopped and the engine continues to operate using its own power (chemical energy usually from diesel or heavy fuel oil). Once the compressors have refilled the air start cylinders (some compressed air is consumed during start-up), the air start system can be used again for the next start.