In this article, we will take a deep dive into the different types of marine diesel engines and understand the differences between them.
What is a marine diesel engine?
A marine diesel engine is a type of reciprocating internal combustion engine that runs on either diesel or dual fuel. It is bulkier and more complex than a petrol engine but offers higher operational efficiency.
The diesel engine, named after Rudolf Diesel, creates power by adding fuel to high temperature compressed air. The resulting controlled explosion powers the crankshaft through linkages. This power can then be used in many ways such as to rotate motors, propellers and in alternators to create electricity.
Diesel engines have the advantage of being safer and more affordable as diesel is cheaper than other fuels while being less explosive in nature. It also provides greater energy per unit allowing for better mileages or energy extraction per unit.
If you want to learn more about the parts in a marine engine, please click the following link: Parts of a Marine Engine.
Types of marine diesel engines
The industry has been working on improving the diesel engine since its invention in the early 19th century. Today, they are available in many shapes and forms. In this section, we classify the different types of marine diesel engines using their various specifications to understand the difference between them. We will explain the different types of marine diesel engines based on the following aspects:
- Type of use
- Number of strokes
- Single-acting or double-acting
- Type of ignition
- Single or multi-cylinder
- Piston type
- Arrangement pattern of the cylinders
- Charging method
- Unidirectional or reversible
- Operating cycle
- Type of fuel
Type of use
A marine diesel engine is put to different uses onboard. Broadly, they can be used in the following two manners:
Marine propulsion engine
When a marine diesel engine is used to turn the propeller and move the ship, it becomes a marine propulsion engine and is popularly known as the main engine. The engine can be run at various speeds as per need.
A marine diesel engine is also coupled to an alternator and used to generate electric power onboard. There is usually more than one of these onboard and multiple generators may be put into use during critical operations. Such diesel generators are also known as auxiliary engines.
Number of strokes
We define a stroke as the travel of the piston from one end of the cylinder to the other end. Engine cylinder operations such as induction, compression, combustion (power stroke) and exhaust gas removal occur with the help of piston strokes. Based on the number of strokes it takes to complete one power stroke, an engine is classified as:
In a two-stroke engine, a power stroke occurs every two strokes or one revolution of the engine crankshaft. The four operations are completed within one revolution. As a result, two-stroke engines provide a higher power-to-weight ratio, greater stability, simpler construction and are more compact compared to a four-stroke marine diesel engine.
A four-stroke engine needs two complete revolutions of the crankshaft for one power stroke. The four aforementioned cylinder operations occur in four separate strokes in this type of engine.
Due to the fewer power strokes per revolution, these engines are more fuel efficient as the fuel has more time for complete combustion. It also requires less lubrication and is generally more durable than a two-stroke engine. All these qualities make four-stroke engines a more eco-friendly alternative.
Single-acting or double-acting
Reciprocating-type engines are further classified into single-acting and double-acting cylinder engines. In single-acting engines, power stroke occurs on one side of the piston, but it occurs on both sides in a double-acting engine.
There are two combustion spaces and a double piston is used to seal both of them. Power stroke takes place on the upward as well as downward stroke. In the case of a two-stroke double-acting engine, there are two power strokes per crankshaft revolution. Double-acting engines are not as popular as their counterpart as the lower combustion space can be difficult to seal.
A double-acting engine is different from an opposed-piston engine in which there is only one combustion space and two pistons on either side of it.
Type of ignition
Diesel engines ignite fuel by spraying diesel at the end of the compression stroke. The compression stroke increases the temperature of fresh air and such elevated temperatures cause the diesel to self-ignite and execute the combustion or power stroke. As a result, diesel engines do not require an external mechanism to initiate combustion.
The spark ignition engine is a type of internal combustion engine that has spark plugs fitted into the combustion space. Towards the end of the compression stroke, a spark ignites the air-fuel mixture and starts the power stroke. This method is for fuels other than diesel such as petrol, CNG, LPG, ethanol, hydrogen and so on.
Single or multi-cylinder
Marine diesel engines are available as single and multi-cylinder engines. As the name suggests, a single cylinder engine has only one combustion cylinder that produces power. We use them in simpler applications such as boat engines under 50 HP.
Single cylinder engines are easier to cool compared to multi-cylinder engines because of the increased airflow around the cylinder.
Multi-cylinder engines have more than one cylinder. Multiple cylinders reduce the uneven load on the crankshaft while also requiring a smaller flywheel. They can generate higher amounts of power and have a greater number of parts compared to a single-cylinder engine.
Marine diesel engines are available in many different piston configurations but the most popular ones among these are the cross-head and the trunk piston. Let us understand each of them.
In the crosshead engine, an extra part known as the crosshead fits between the piston rod and the connecting rod. The crosshead is especially useful in engines with long strokes and small bores as it eliminates the sideward motion of the piston rod and also the sideward thrust on the cylinder liner.
Crosshead assembly is also preferred for slow-speed engines as it allows the engine to develop greater power at low rotational speeds as the stroke can be increased to increase combustion capacity. A crosshead also prevents the contamination of the crankcase oil reducing the total cost of lubricants compared to a trunk piston engine. Generally, crossheads are a part of main engines onboard ships.
In a trunk piston engine, the piston connects directly to the engine crankshaft through the connecting rod. This type of piston setup is common in diesel generator engines on board.
Arrangement pattern of the cylinders
In a multi-cylinder engine, we can arrange the cylinders in different ways. The in-line pattern is most popular for the main as well as auxiliary engines. However, you will find V, W and X type of marine diesel engines in many ships.
In addition to these, there are patterns such as Radial , Opposed cylinder (aka flat or boxer type engine), opposed piston, delta (aka Napier Deltic), H and U-type in diesel engines.
Another very common way of classifying marine diesel engines is through their running speed. This speed may refer to the shaft rpm or mean piston speed. Broadly, we can divide them into the following three types:
Slow speed engines
Slow speed engines are engines with a shaft speed of up to 150 rpm. Some other classifications have a category known as low speed engines for engines with shaft rpm of up to 350 rpm or mean piston speed between 4.5 m/s and 7 m/s.
Medium speed engines
Medium speed engines are engines with a running speed between 300 – 1000 rpm. Some classifications consider medium speed engines to be between 350 – 750 rpm and mean piston speed between 7 m/s and 10 m/s.
High speed engines
An engine is classified as a high speed engine when the shaft speed is above 1000 rpm. Other classifications categorize an engine as high speed when the engine rpm is between 750 – 2500 rpm and mean piston speed between 10 m/s and 15 m/s.
Each type has their pros and cons for different applications onboard. Slow speed engines have the advantage of being coupled directly to the shaft for propulsion while other engines would require a gearbox to reduce the speed. High speed engines deliver cost and emission benefits.
Charging method refers to the method of introducing fresh air into the combustion chamber for combustion. There are two ways in which we can do this.
Naturally aspirated engine
Naturally aspirated engine is the traditional engine in which the air enters the combustion chamber at atmospheric pressure before compression. The air fills the combustion chamber due to the piston motion creating a vacuum.
We can use the exhaust gases to power a turbine after the exhaust manifold. This turbine can then power a blower which allows us to send compressed air to the combustion chamber.
High pressure air allows us to burn proportionately more fuel in the combustion chamber. This increases the power output of the engine at no additional operating cost. We use this method in most two and four stroke marine diesel engine as we can make the engine smaller for the same power output as a naturally aspirated engine.
However, the compressed air must be cooled before sending it to the combustion chamber for increased efficiency. Hence, you will usually see an air cooler fitted between the turbocharger blower and the scavenge manifold.
Uni-directional or reversible engine
Marine diesel engines may be unidirectional or reversible. In unidirectional engines, we manipulate the gearbox or the controllable pitch propeller to change the vessel movement direction.
Reversible engines, on the other hand, reverse the engine running direction to change the rotation of the crankshaft, and therefore the propeller.
The operating cycle outlines the different processes in the combustion cycle of the engine. There are three main types of cycles in internal combustion engines. These are:
- Otto cycle
- Diesel cycle
- Dual cycle
The otto cycle finds use in spark ignition engines such as petrol-based engines. According to the theoretical concept, heat is added at constant volume in engines based on Otto cycle. This cycle may be used in two stroke as well as four stroke engines. Marine diesel engines do not use this cycle and instead use the other two cycles mentioned in the next two sections.
The diesel cycle is the most common cycle for marine diesel engines. In this cycle, the heat generated in the charge air during the compression phase ignites the fuel. When the fuel enters the combustion chamber, the hot compressed air ignites the fuel and the power stroke begins.
At times, marine diesel engines use the dual cycle. The dual cycle is a combination of the otto and diesel cycles. In the otto cycle, the heat is added at constant volume whereas in the diesel cycle, it is added at constant pressure. However, in dual cycle engines, the heat is added partly at constant pressure and partly at constant volume.
The dual cycle provides more time for a complete combustion compared to the otto and diesel cycles.
Type of fuel
Marine diesel engines are far more robust than petrol engines and are capable of running on different types of fuel. However, we cannot use fuels that require spark ignition in diesel engines without reducing the engine’s compression ratio and introducing spark plugs.
Some of the common types of fuel we can run marine diesel engines on are:
Heavy Fuel Oil
Heavy fuel oil is a residual product left after the distillation of crude oil into refined products. When an engine is capable of using this fuel for propulsion and/or power generation, we call it an HFO engine.
There are about 100,000 sea going vessels out of which almost 65,000 were using heavy fuel oil until the end of 2019. It is the most affordable of all fuels and suitable for marine diesel engines that consume tens of tons each day.
Low Sulphur Marine Fuel Oil (LSFO)
Since 1st January 2020, MARPOL Annex VI has introduced more ECAs. ECA stands for Emission Control Area. The MARPOL convention has imposed strict regulations on the use of heavy fuel oil in these areas that require the use of fuels with lower sulphur content when trading in these areas.
Thus, many marine diesel engines now run on low sulphur fuel oil. There are other sub-groups under LSFO such as VLSFO (Very Low Sulphur Fuel oil) with sulphur content between 0.10% and 0.50% and ULSFO (Ultra Low Sulphur Fuel oil) with sulphur content less than 0.10%.
As the gap between the prices of these alternatives and HFO have increased, many ships are preferring the installation of exhaust gas cleaning systems such as scrubbers.
Marine Diesel Oil or Marine Gas Oil
This is a distillate fuel that does not require heating unlike HFO. MDO can be 50 to 100% more expensive compared to HFO and makes maintenance and operation somewhat easier compared to engines run on HFO. It also addresses the environmental concerns of HFO without the need to use any exhaust gas cleaning systems.
Diesel-electric propulsion is seeing increasing popularity in larger cargo vessels than before. In these engines, the electric motors that derive their power from the ship’s generators power the propeller shaft.
Diesel-electric propulsion provides many advantages of over the conventional HFO-propelled main engine such as lower emissions, higher degree of automation, lower cost, reduced wear and tear, greater payload capacity and low vibration and noise.
To sum it up
Marine diesel engines are the backbone of the shipping industry. And so, it is no surprise that incessant R & D has occurred in developing engine technologies that are faster, cheaper and sustainable, especially in the last few decades.
In this article, we have explained the many categories in which marine engines can be classified and how they operate. We hope that the article was able to enlighten you on the differences between the different types of marine diesel engines.