## 1 Introduction  
Large two-stroke diesel engines are highly efficient and robust internal combustion engines, and as a result they are utilised as main engines in the majority of the world ocean-going fleet, operating under a wide range of conditions [1]. However, main engine breakdown and engine components degradation still present major challenges, leading to significant costs due to vessel down time and engine inefficiencies [2, 3], as well as compromising crew safety [4]. Consequently, early diagnosis of engine abnormalities, degradation and faults is essential in maintaining the vessel's safety, as well as avoiding higher operational expenditure associated with fuel, maintenance, costly downtime and increased repair costs. In order to diagnose the engine's condition, measurement and monitoring of vital performance parameters are required. Amongst the most important parameters indicating the engine health status is the in-cylinder pressure [5, 6], which can effectively be used to analyse combustion quality [7], diagnose cylinder blowby and compression faults [8], and diagnose the fuel injector's condition [9]. However, obtaining the in-cylinder pressure measurement can be intrusive [10] and requires the deployment of large networks of expensive sensors to perform continuous measurement [11], thus increasing costs, creating data synchronisation issues and introducing multiple failure points. To overcome the above shortcomings, other peripheral engine measurements are used to accurately determine the cylinders condition. In specific, a combination engine working media pressures and temperatures are used, however these are not as revealing of the engine cylinders' health status as the in-cylinder pressure [6]. Thus, the Instantaneous Crankshaft Speed (ICS) measurement has been utilised in automotive applications to obtain combustion phasing information [12], identify imbalanced cylinders [13], and cylinders with a decreased mean effective pressure [14]. Regarding larger engines, the ICS signal has been utilised to detect misifiring for medium [15] and slow-speed engines [16], in addition to cylinder imbalances [17]. Thus, the ICS measurement can be utilised due to the low cost of speed sensors, however the raw ICS signal from the angle encoder requires significant processing and high sample rates to provide sufficient detail, which is often not achieved in practice [18]. Hence, by using ICS measurements faults in the engine cylinders are identified, but their specific causes are often not revealed. The Instantaneous Crankshaft Torque (ICT) can be used to acquire detailed information of the cylinders' condition, which can be obtained at sufficiently high sample rates and accuracy [19]. In specific, for automotive applications the peak pressure position [20], and properties such as combustion phasing and the burned mass fraction were estimated for each cylinder [21], by utilising measurements from an ICT sensor. Furthermore, progress was made in reconstructing the in-cylinder pressure diagram, however singularities were encountered during the torque signal inversion [11]. Regarding larger engines the ICT measurement was utilised for a two-stroke diesel engine of a land-based power plant, to determine the indicated power for each cylinder using an Artificial Neural Network (ANN) [18]. The ICT measurement is thus able to provide greater insight into the cylinders condition using just a single sensor, however its utilisation in the marine industry remains a novel concept as more traditional means of monitoring and diagnostics employed [22; 23]. In the shipping industry, the most common form of diagnostics relies on human judgement combined with information on acceptable degradation tolerances of engine parts and performance parameters, which is provided by the manufacturer [24]. However, this is a manual approach which is error-prone due to the involvement of the human factor.