## 1 Introduction  
The diesel engine is the most fuel efficient internal combustion engine, however, the performance of a diesel engine is sub-optimal when the engine is cold [1, 2]. This phenomenon of engine cold start is frequently experienced by motorists in the morning time and during frequent engine stop/start in a typical city driving environment. In fact, a number of studies have suggested that a significant portion of a vehicle's journey is completed before the engine is fully warmed up [3, 4, 5]. While an engine is cold, the high viscosity of the lubricating oil results in higher frictional losses [6]. [PERSON] et al. [7] estimated that the frictional losses in an engine during the early stages of warm up (at approximately 20 \({}^{\circ}\)C ambient) can be up to 2.5 times higher than those observed when the lubricant is fully warmed up. Similarly, the first few minutes of operation, due to low engine block and coolant temperatures, results in higher tailpipe emissions [1, 8]. Since the fuel in a diesel engine burns lean with excess air, higher NOx is produced as a consequence of higher in-cylinder temperatures and pressures [6]. Besides, the fine particles or soot in the form of unburnt HC in a diesel exhaust has also been a concern [6]. Differentstudies have suggested that emissions during cold operation are significantly higher, especially NOx and HC [9; 10; 11]. Numerous volatile hydrocarbons, semi volatile and particle phase organic compounds, carboxyl and fine particles were found in the diesel exhaust from the vehicle driven over a cold start Federal Test Procedure (FTP) cycle [12]. Therefore, it is understandable that the emissions of a diesel engine would be reduced, when the engine coolant, lubricating oil, and all engine components have completely warmed up. There are multiple definitions of engine cold start or engine warm up found in the literature. In general, researchers use the engine cold start definition based on the European Union (EU) Commission's directive 2012/46/EU, where the engine is considered as warmed up, when the engine coolant temperature reaches 70 \({}^{\circ}\)C, for Stage IIIB and Stage IV vehicles (which include heavy duty CI engines) [13]. However, [PERSON] et al. [4] defined engine cold operation being the engine and catalytic converter's temperature below the optimum operating temperature. Similarly, the environment protection agency (EPA) defines hot start as both the engine and catalytic converter being at near operating temperatures, a warm start when the engine is hot but the catalytic converter is relatively cool, while a cold start occurs after a long and uninterrupted soak, such as an overnight soak [14]. According to the EU Commission's directive 2012/46/EU [13], the engine is considered warmed up once the coolant temperature reaches 70 \({}^{\circ}\)C; however, the lubricating oil temperature will still be sub-optimal if it reached this temperature from cold, leading to performance inefficiencies and higher emissions [15]. Engine performance inefficiencies and higher emissions during cold start are highlighted in several studies, such as [15; 16; 17; 18; 19]. [PERSON] et al. [17] experimentally showed elevated fuel consumption throughout the period when the oil temperature was below 90 \({}^{\circ}\)C. It has also been shown that the largest contributors to the total engine friction are the piston and connecting rod assemblies [18], the performance of which deteriorates at suboptimal oil temperatures. Therefore, it is important that engine cold start be investigated more comprehensively. During the last decade, a greater emphasis has been laid on investigating the impact of alternative fuels, such as biodiesel on emissions, in transient diesel engine investigations, as shown in studies such as [20; 21; 22], shifting the focus from diesel fuel to the alternative fuel blends. However, the necessity of diesel fuel as a primary fuel used in industrial applications and the transport sector still remains [23; 24].