IMO Tier III and NOx emissions from marine engines

The latest NOx emission regulations will have a significant impact on a lot of marine vessels. The new IMO Tier III regulations mean that almost all marine engines must now be adapted in some way to ensure nitrogen oxide (NOx) emissions are below 2 g/kWh. So, how will this affect boat owners, boatbuilders and shipyards? And how are Volvo Penta’s engines made to meet these new requirements?

What is IMO III, ECA, and what engines are involved?

The IMO Tier III regulation, or IMO III as it is often called, is a new set of emissions standards issued by the International Maritime Organization (IMO), a United Nations special agency. The regulation outlines the permitted levels of NOx in emissions from marine diesel engines with an output power of over 130 kW, other than those used solely for emergency purposes. The previous level, outlined in IMO Tier II, permitted a NOx emission level of up to 7.7 g/kWh. The newer regulation, IMO Tier III, permits only 2 g/kWh.

The IMO III focuses entirely on NOx levels, not carbon dioxide, particles or SOx, for example.

The regulation covers certain defined Emission Controlled Areas, ECAs

Emission Control Areas (ECAs) are marine areas in which stricter control is established to minimize airborne emissions from ships. An area where sulfur is controlled is called a Sulfur Control Areas (SECA). An area where NOx is controlled is called a NOx Emission Control Area (NECA).

The United States has its own regulations for domestic traffic, EPA 3. Accordingly, all international sea traffic to and from the United States, on vessels built after January 1, 2016 must comply with IMO III. In the Baltic Sea and the North Sea, SECA was introduced January 1, 2015. NECA will be introduced there January 1, 2021, requiring engines in boats built after 2021 to comply with IMO III.

  • ECA: Emission Control Area
  • SECA: Sulfur ECA

The solution for transforming NOx into nitrogen and water

Using only in-cylinder technologies, it would be possible to bring the NOx levels down to about 4 g/kWh. In order to come all the way down to 2 g/kWh, you need an exhaust aftertreatment system. One solution is to use a selective catalytic reduction (SCR) unit in which urea is injected into the exhausts. Often referred to as diesel exhaust fluid (DEF), urea is a diluted mixture of ammonia and water. DEF is often sold as AdBlue or with similar brand names.

There are mainly two versions of DEF available, one with 32% and one with 40% concentration. Trucks and cars use 32% DEF, which can handles temperatures down to -11°C. 40% is a marine DEF, big ships do not have to carry as much “water” due to the higher concentration. 40% UREA manages down to 0°C. The Volvo Penta system will manage any concentration automatically and will set an alarm if the concentration level falls below 28%.

The DEF and the hot exhaust gases mix in the SCR unit and react inside the hot catalytic converter causing the nitrogen oxide compounds (NO and NO2) to turn into harmless nitrogen and water. You can read more about Volvo Penta’s various aftertreatment systems in our “Exhaust aftertreatment system – a cleaning process for exhausts” blog article.

How does the IMO III affect boatbuilders and shipyards?

There will be some onboard modifications that will need to be made; this because the exhaust aftertreatment system requires space and might need extra ventilation in the engine room. You need to add at least one more tank, just for the diesel exhaust fluids (DEF). You won’t need an extra silencer because the SCR unit itself acts as a silencer that reduces noise by an average of 35 dBa, and that may save you some space. However, if more noise reduction is required, it has to be added. The SCR unit even acts as a spark arrester, which can be useful if that is a requirement in some applications.

The SCR unit works with temperatures of up to 520°C. Its outer surface must be insulated by the boatbuilder or shipyard just as the rest of the exhaust system has to be insulated to fulfill class rules. The better it is insulated, the less need there will be for ventilation in the engine room. You still have to consider whether extra ventilation may be required to extract heat from the engine room, however.

Depending on the engine configuration, the DEF may require a bulk tank and day tanks for each engine. The bulk DEF tanks should hold about 5-10% of the volume of the diesel tanks. The larger the DEF tanks are, the less likely you are to run out of DEF.

What does this mean for boat owners who operate boats with IMO III engines?

The boat can be operated in exactly the same way as with any other diesel engine. There is thus no difference in driveability, response or fuel consumption.

The only thing that may need to be considered, is whether the engine will be running on low loads for prolonged periods of time. With low engine loads, the SCR unit may not heat up sufficiently and the DEF may cause salt crystals to form. These will burn away once the engine load increases and the temperature inside the SCR unit rises above about 300-400°C.

In addition to filling the tanks with diesel, boat owners or operators will also need to fill the DEF tank. Depending on the size of the boat’s DEF tanks, this may not be required each time you fill up diesel. The IMO III will not affect fuel consumption levels, compared to IMO II engines. If the boat is operating outside of ECA areas for a longer period of time, or in areas where DEF is not available, it is possible to shut down the aftertreatment system.

Our IMO III solution is a complete Volvo Penta System, engine aftertreatment and control system. It has been developed, certified and is serviced by one company. This means that the customers need only one point-of-contact, when installing or servicing the system.

How did Volvo Penta manage to meet the strict requirements of IMO III

Most major engine manufacturers base the development of their marine engines on truck engines. Volvo Penta is no exception. Being part of the Volvo Group, and having access to its technical expertise and latest technology, is a huge advantage. In the group, we have over 10 years of experience of SCR, in different demanding on/offroad applications, and we share the same high quality production facilities.

But we don’t believe in just putting truck parts into boats as is. We have put considerable effort into optimizing the equipment for marine applications and their tough environments. One such specific challenge is the required flexibility when it comes to the installation of the equipment onboard. We have spent a lot of time fitting different concepts in to several challenging installations and engine room configurations. Boatbuilders and shipyards are used to certain specific solutions, such as having bolted flanges with gaskets on all tubes and pipes to the engine.

Volvo Penta carried out 35,000 hours of extensive testing before the 2018 launch, to ensure the reliability of the IMO III solution in marine applications. Read more about one of our field testers in our “Field test – a chance to participate in the development of new marine engines” blog article.


In order to meet the strict requirements of the IMO III regulations, Volvo Penta has chosen to add an exhaust aftertreatment system to our engines. This basically means that the engine is relatively unaffected. Many of the modifications are in the software of the control unit. The engine’s control unit is programmed to communicate a number of parameters to the control unit of the aftertreatment system. During development of the IMO III-approved engines, a number of parameters were measured and mapped. This data was then compiled and programmed into the software to, for example, send a signal about engine load and expected NOx content to the aftertreatment system’s control unit. This unit can then inject just the right amount of DEF into the exhaust gas to keep the NOx levels within the limits.

Volvo Penta IMO III engine with aftertreatment system

The image shows Volvo Penta’s IMO Tier III D8 engine with its exhaust aftertreatment system.

Will the new IMO III engines require extra maintenance?

Marine diesel is regulated to contain a maximum of 1,000 ppm of sulfur. As long as boat owners comply with this, there are no additional service or maintenance requirements. There is a filter for the DEF injectors that is replaced at the normal service intervals of the engine. The aftertreatment system has the same lifespan as the engine.

Are there any restrictions on the fuel?

The system can handle fuel with up to 1000 ppm of sulfur which is the requirement within the SECA areas. It is also possible to operate on HVO (hydrotreated vegetable oil). Depending on the quality of the HVO, it can reduce CO2 with up to 90%.

Which IMO III engines does Volvo Penta offer?

Volvo Penta offers the D13 and D8 as IMO III engines in Volvo Penta IPS, diesel inboard, marine genset and auxiliary applications.

We all need to work hard on reducing emissions

Read more about Volvo Penta’s IMO III engines, exhaust aftertreatment systems and how Volvo Penta works with these technologies and regulations. Visit our website, stay up to date by reading our Professional Power Blog, and feel free to contact me if you have any questions.

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