# Cycle-to-Cycle Variation of a Diesel Engine Fueled with Fischer-Tropsch Fuel Synthesized from Coal
## 1 Introduction  
With the increasing drive towards energy savings and low-carbon living, greater demand for stringent emissions regulations, and huge fuel consumption, researchers have progressively paid more attention to exploring clean alternative fuels for internal combustion engines in recent years [1; 2]. The Fischer-Tropsch (F-T) catalytic conversion process can be used to synthesize diesel fuels from a variety of feedbacks including coal, natural gas, and biomass. Moreover, liquid fuels produced via the F-T process promise an attractive, clean, carbon-neutral, and sustainable energy source for thetransportation sector. In particular, the F-T diesel synthesized from coal by the F-T method has broad application prospects, as it has matured and entered the industrial stage [3; 4]. Owing to the excellent specifications [5], F-T diesel can be used in unmodified diesel engines; therefore, many diesel engines powering large trucks, buses, and farm, railroad, marine, and construction equipment have been investigated to date [6; 7; 8; 9; 10; 11; 12; 13]. Cycle-to-cycle variations were first analyzed in Spark engines, which are less significant in diesel engines due to the non-premixed combustion. However, this has been a research area of increased interest in the recent past since the two-stage injection strategies on the mixture formation were used in common rail diesel engines. These cyclic variations were considered to cause combustion instability and are detrimental to engine performance [14]. [PERSON] et al. [15] found that variations in in-cylinder pressure were caused by increased ignition delay in cold-start engines. [PERSON] et al. [16] observed cyclic variations in the fuel path of diesel engines. Furthermore, [PERSON] [17] reported that the in-cylinder pressure variations could not be explained in terms of injection variation. Later, a change in the intake pressure and intake temperature resulted in longer ignition delay, which led to an increase in the cycle-to-cycle variations [18; 19]. Moreover, literature reports [18; 20] showed that the variation of in-cylinder pressure decreased in fuels with high cetane number (CN). Furthermore, [PERSON] [21] predicted the influence of cyclic variations on NOx exhaust emissions, and the results showed that the NOx emissions increased by 5% when a point with fluctuating injection timing compared to a stable state. [PERSON] et al. [19] found that cyclic variation in soot was caused by combustion instabilities. Previous studies showed that the cyclic variability, together with in-cylinder pressure fluctuations, had a similar influence, increasing energy release during the combustion phase observed in homogeneous charge compression ignition engines [22; 23; 24]. It is well known that cycle-to-cycle variation in diesel engines can contribute to lower thermal efficiency and output power, as well as higher exhaust emissions [25; 26]. The cycle-to-cycle variation is unexpected, which is harmful to diesel engine performance. Moreover, fuels with high CN and thus short ignition delay exhibited beneficial effects, decreasing cyclic variations of in-cylinder pressure. However, literature reviews showed that the research has been conducted on F-T diesel or F-T diesel blends, and mainly focused on emissions and combustion characteristics. Cycle-to-cycle variation in diesel engine fueled with F-T diesel has rarely been investigated. The main objective of this study was to investigate the variation performance in diesel engines with short ignition delay and long combustion duration conditions. For this purpose, F-T diesel and 0# diesel were used in a common rail engine for comparative analysis. Furthermore, the combustion parameters such as ignition delay and combustion duration were identified by analyzing the in-cylinder pressure and heat release rate (HRR). The cyclic variation coefficient under in-cylinder pressure condition and intensity of fluctuation under different ignition delay and combustion duration conditions were comprehensively analyzed.