The differences between marine and automotive Diesel engines are usually associated with the different usage purpose and the different environments in which they operate. These differences can be summarized by the following points:
Design facilitating onboard maintenance
Among the major differences between marine and automotive Diesel engines, we can mention the particular design of the marine engines.
In the marine field, the installation of the engine in the ship is usually referred as a marriage. This is because once the engine is installed; it should stay there for the whole life of the ship. Hence, a specific design is usually made to facilitate the maintenance onboard and among these facilitating design changes we can list:
- Separate cylinder head for each cylinder
- Gates to access the lower part of the engine (connecting rod, crankshaft…)
- Separate crankcase in some engines
Size, form and lubrication
The size (power) range is one of the chief differences between marine and automotive Diesel engines. The automotive Diesel engines are either small or belong to the medium power range category.
Some marine Diesel engines are huge. These engines are mainly two stroke engines, which mainly produces the double of the power that a four stoke Diesel of an equivalent size can provide. The elements of these engines are designed in a way making their production and their assembly easy. These are some elements affected by this particular design purpose:
- Foundation plate
- Engine mount (frame)
- Crankshaft, camshaft, cylinders, common rail, cylinder head…
Since the generated power is high, the organs of the engines are built using different composite materials. This allows withstanding the high thermal and mechanical constraints.
Additionally, the large marine Diesel engines are crosshead engines. In this category of engines, the piston is not directly connected to the crankshaft via the connecting rod. The crosshead connects the piston rod to the connecting rod and has the role to guide the movement of the piston in the cylinder.
In this type of engine, the lubrication is extended to cover the additional elements in movement like the crosshead, the shoe, the sliding track. It is also not rare in this type of engines, to have some special lubrication circuits, replacing the oil jet nozzle, to lubricate the cylinder liners. These circuits have some lubrication nozzles on the liners, from where the oil flows. Moreover, the lubrication oil can be used as a mean to actuate some hydraulic actuators, like the ones controlling the exhaust valves or the one allowing changing the position of the camshaft to reverse the sense of rotation of the engine.
Air intake and exhaust system
In some marine Diesel engines, the piston rod chamber, or the crankcase, associated with the piston movement, can be used as an air compressor element to increase the charging of the cylinders.
In high power rated marine engines, it is also very often to find configurations with inlet and outlet ports or with inlet ports and exhaust valves instead of inlet and exhaust valves, commonly used in the automotive field. Hence, the resulted type of scavenging can take other forms than the usual loop form met in the automotive field. The engines configured with ports are usually equipped with long piston sleeves. These long sleeves allow closing the ports as the pistons moves away from the BDC (Bottom dead center) and at the same time help maintaining a straight movement of the pistons.
Installation of additional heat exchangers to take advantage of the released heat in the exhaust circuit and also an additional turbine which transfers the recovered energy to the crankshaft is not a rare practice in the marine field.
The cooling in marine Diesel engines, takes advantage of the presence of sea water. Actually, two main variants can be made while speaking about marine Diesel engine cooling:
- engines using sea water as the sole medium of cooling
- engines using sea water, instead of the air commonly used in the automotive field, to cool the coolant with the help of a heat exchanger
This heat exchanger can have a normal form, in which the sea water is pumped into the heat exchanger or a printed form, where the heat exchanger is printed on the ship hull, which is in direct contact with the sea water.
In some high power rated marine Diesel engines, the coolant circuit of the last variant mentioned above (engines using sea water instead of the air), can be divided into two circuits,
- one high temperature circuit which cools the elements in close contact with the combustion chamber such as the liners, the cylinder head…, with high working temperatures
- one low temperature circuit which cools the rest of the components, such as the charge air cooler, oil cooler…, with low working temperatures
In this type of engines, it is also common to cool the piston head. This is made using either the lubrication oil or the coolant.
In the marine field, not only electrical systems but also pneumatic and hydraulic launch systems are used to start up the engine. Some pneumatic systems are directly linked to the cylinders via the air launch valves. These systems distribute high pressured air into the cylinders in respect with the firing order. This allows pushing down the pistons and increase the crankshaft speed until the combustion becomes autonomous.
Use of heavy fuel and low quality fuels is also very common in high rated power marine Diesel engines. This type of fuel is very viscous and fuel heaters are installed to facilitate the usage of this type of fuel. The corresponding injection systems are usually designed to withstand the high temperatures generated in this type of engines.
The marine Diesel engines are in general bigger than the ones used in the automotive field. As a rule, the bigger the engine, the lower its rotation speed will become. This is actually due to inertia of the components in movement and also the quality of the used fuel. The combustion of low quality fuels takes longer than the automotive Diesel fuel.
In some marine Diesel engines, changing the sense of the engine rotation is possible. It means that there is no need to have a gearbox with the rear position to change the direction of rotation. The engine can perform that alone. In general, reversibility implies changes in the camshaft design and in the injection pump (or at least in the injection system control), to fit the new firing order.
Different pollution standards
Until recent years, the usage of electronically controlled engines in the marine field was not welcomed and the engines mechanically controlled were preferred. Actually, the ship owners do not want to deal with complicated system onboard. On the high seas, problems in the propulsion system can be very expensive. It is for this reason that simple systems that mechanics can easily fix were preferred.
With the avenue of the MARPOL convention some changes in the mentality were observed. The requirements are not as stringent as in the automotive field and they take into account the engine speed (size). The slower engine, the more tolerant are the requested values. However, restrictive values are set on the NOx and SOx emissions in certain areas of the world, such as the Baltic Sea and the English Channel.
The use of the heavy fuel, already cited above as one of the major differences between marine and automotive Diesel engines, raises the need to deal with other type of issues. In fact, low quality fuels are rich in sulfur and some requirements are set on their production to reduce their sulfur rate.