## 1 Introduction  
Diesel fuel is a traditional fossil fuel, and it consists of hundreds of hydrocarbons, including 50-60% alkanes, 20-30% cycloalkanes and 7-30% aromatics, according to fuel regulations [1]. The hydrocarbons are required by the engine combustion to some extent, and on the other hand, they are subjected to the petroleum-refining process. Therefore, changing the component hydrocarbons in the fuel on the basis of the China VI standards is of great significance for improving the combustion and emission performance of the engine. The hydrocarbon content in diesel directly affects the combustion and emission characteristics of the engine, as well as the engine power and fuel economy. For instance, as is known to all, the straight-chain alkanes have a significant effect on fuel cetane number (CN), which affects the ignition delay of the combustion process within the cylinder [2]. Poly-aromatics can lead to higher particulate matter (PM) emissions and therefore are limited to below 7% in most current fuel regulations. A higher aromatics fraction has been proven to have negative effects on fuel CN. Different petroleums and their related refining processes output fuel products with different hydrocarbon constitution and cost input, and so on. Thus, the investigation of diesel hydrocarbon constitution has significance for engine combustion and emission control, which means engine operating costs of fuel and urea and control strategy [3]. [PERSON] et al. have studied reducing volatile organic compound emissions from diesel engines using canola oil biodiesel, palm oil biodiesel, and ethanol blends [4; 5; 6; 7]. [PERSON] et al. [8] added 4% and 8%vol. n-pentane into diesel fuel and studied the emission characteristics of the engine. Results showed that CO emissions increased slightly, and NOx emissions were reduced. Meanwhile, the in-cylinder temperature decreased at full load conditions. [PERSON] [9] et al. prepared toluene/diesel, n-butylbenzene/diesel, tetralin/diesel and 1-methylnaphthalene/diesel blended fuels in a ratio of 30% and found that soot emissions decreased and the peak value of heat release rate increased. The importance of decalin and tetralin as bicyclic hydrocarbon components in diesel has been studied for their high thermal stability [10]. According to these studies, n-heptane, n-dodecane, decalin and tetralin were chosen as the additives to diesel in this study. Our pre-tests showed that the blended fuels in the ratio of 10% of the four hydrocarbons have no significant effect on engine performance. Since there is not much difference in lower heating value (LHV), changing the hydrocarbon content has little influence on combustion and engine power but results in a significant improvement in emissions. Furthermore, engine emissions are more restricted in the Heavy Duty Diesel Vehicle Pollutant Emission Limits and Measurement Methods (China VI standard), which is basically equivalent to Euro VI (European exhaust emission regulations) and California Air Resources Board Low Emission Vehicles III (CARB LEV III). Additionally, the technologies and combustion strategies of modern diesel engines are quite different from the conventional ones in order to meet China/EU VI standards, e.g., higher pressure and split injection and higher cylinder temperature and pressure. Currently, limited investigations have been reported on China/EU VI standard engines to study the effects of diesel hydrocarbon constitution on its performance. Hence, four fuel blends that blend the four hydrocarbons with China VI diesel in the fractions of 20% and 50%vol. were studied on a six-cylinder China VI diesel engine. The addition of hydrocarbons affects the fuel properties, including density, lower heat value, and cetane number (CN). Among these, CN is the most important property for combustion. As a macroscopic feature of fuel chemical kinetic behavior, CN can be directly related to ignition quality, and its estimation is of practical significance [11].