In this series of twelve articles, we explore the key components necessary for effective emissions monitoring to optimise ship performance, ensure regulatory compliance, and maintain reputation. This article, like the previous one, examines a number of these components including the importance of engine load and conditions, fuel quality and type and the need to consider external factors and the impact they can have on accurate emissions monitoring.
Engine Load and Conditions
In real-world conditions, an engine with a higher load operating at around 80% maximum continuous rating (MCR) will benefit from improved combustion efficiency. At low engine loads, such as when a vessel enters port or is slow steaming, the combustion efficiency decreases, which can lead to incomplete combustion but lower emissions. This means that actual CO2 emissions, on average, tend to be around 3-10% depending on the quality of the fuel-measurement data being used.
Chad Verret, President of Verret Marine Consulting, told Thetius, “Fuel quality affects engine performance significantly – operators need to understand what’s in their fuel, not just assume it’s fine.”
The Challenges Around Fuel Type and Quality
Fuel accounts for around 50-60% of a ship’s total operating cost. While conventional fuels still power 99.4% of the global fleet, the current order book data indicates that 16% of vessels will run on alternative fuels in the near future. According to ClassNK data, LNG-fuelled ships remain the most dominant type of alternative fuel ships in service, but the share of methanol and ammonia-powered vessels is increasing.
With this in mind, it is crucial to use every calorie of fuel in the most efficient way. Different fuels have varying carbon content, energy densities, and combustion efficiencies, which can impact the accuracy of CO2 emissions calculations.
Chad Verret, President of Verret Marine Consulting told Thetius, “Fuel quality affects engine performance significantly – operators need to understand what’s in their fuel, not just assume it’s fine.” While diesel fuel is about 86% carbon, 20 others vary and will therefore produce different amounts of CO2. The IMO’s emissions calculation method does not account for these variations, leading to discrepancies between estimated and actual emissions. For example, the IMO assumes full methane combustion in LNG-fuelled ships. In reality, methane slip significantly impacts GHG calculations and is particularly prevalent during low-power operations, such as when vessels are entering and leaving ports
Continuous Measurements
Emissions data is often captured at specific intervals rather than continuously, potentially missing short-term emission spikes or fluctuations. According to one paper, studies estimating ship or port emissions show that missing ship static data can lead to a significant underestimation of ship emissions, by as much as 49% in some cases. A lack of continuous monitoring could lead to underestimating or misrepresenting actual emission levels.
The Need to Consider External Factors
External factors such as ocean currents and strong winds can influence emissions calculations as they impact the efficiency of engines. An engine working harder because of strong currents will burn more fuel and emit more CO2, but the IMO DCS method does not take this into account.
14 SailPlan (Aug, 2024) How CO2 emissions factors are calculated (and why they are wrong)
15 Liu, L et al., (Jan, 2025) Research on combustion process optimisation of marine diesel engines based on dual-injector system
16 Enshaei, H (2018) In-situ data vs. bottom-up approaches in estimations of marine fuel consumptions and emissions
17 Tzeu-Chen Han and Chih-Min Wang (Apr, 2021) Shipping bunker cost risk assessment and management during the coronavirus oil shock
18 Thetius (Jan, 2024) The maritime alternative fuels barometer
19 ClassNK (Nov, 2024) ClassNK Alternative fuels insight
20 Acomi, N and Acomi, O C (Dec, 2014) The influence of different types of marine fuel over the energy efficiency operational index
21 Comer, B et al., (Jan, 2024) Fugitive and unburned methane emissions from ships
22 Sun, R et al., (Jan, 2025) Enhancing data quality in maritime transportation: A practical method for imputing missing ship static data
For a more detailed understanding and exploration of the topics covered in this article, refer to our report, Transforming Maritime Operations: Unlocking Accurate Data for Sustainability and Compliance, commissioned by SailPlan.
