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Spray mixing and combustion processes are regarded as the major factors that determine the energy conversion efficiency and emission level of internal combustion engines. However, the related researches mainly rely on 3D flow simulation or optical engine experiments, which are highly expensive and time-consuming. In order to provide a more convenient solution, this paper developed a one-dimensional semi-phenomenological model to achieve accurate and efficient predictions on spray combustion characteristics. The model was constructed by a dimensionless spray theoretical model and limited experimental data of macroscopic spray characteristics. Inside the proposed model, a physical module was used to physical module spray air-fuel mixing conditions like fuel distribution and evaporation. Besides, an improved Arrhenius-typed correlation concerning injection parameters and environment variables was introduced to describe the chemical ignition characteristic of spray combustion. The optical experiment results showed that this method is simple and effective, and the forecasting is satisfactory. Based on the new model, the influences of blending n-pentanol on the spray combustion characteristics of diesel and biodiesel were investigated. On the physical aspect, the critical fuel concentration at liquid length position decreases with n-pentanol is blended into both fuels in the most cases, but witnessed a significant leap in 1200 K condition, especially for diesel blends; the physical ignition delay times of both diesel and biodiesel blends decreases first and go up later. On the chemical aspect, the join of n-pentanol caused the gap between the physical delay time and the chemical delay time much narrower, but the ignition patterns were different in diesel blends and biodiesel blends. Moreover, this paper proposed a series of correlations focused on the liquid length and ignition delay of n-pentanol blends sprays. These correlations covered wide conditions and showed better prediction accuracy than traditional ones.

Recently, diesel/biodiesel/alcohol blend fuels have attracted tremendous attention as a potential alternative fuel in internal combustion engines, because of their beneficial effects on pollution emission and engine performance. However, there are few chemical mechanisms to forecast combustion characteristics of the ternary blends, especially for those blending with long-chain alcohols, such as butanol and pentanol. The objective of this paper is to develop a combined reduced combustion mechanism for the combustion simulation of diesel/biodiesel/alcohol (up to n-pentanol). This new combined mechanism, consisting of 229 species and 902 elementary reactions, was constructed by integrating two separated reduced mechanisms; one is the PRF-alcohols’ mechanism (Liu et al., 2016), and another is the biodiesel mechanism (Luo et al., 2012). The new combined mechanism reproduced well predictions of fuel ignition and flame speed compared with those obtained using the original ones. Extensive validations were performed against various experimental data for 0-D homogenous systems, 1-D freely propagating premixed flames and 3-D spray combustion flames. The proposed combined mechanism was shown to be versatile and robust, successfully integrating the biodiesel reaction mechanism and the long-chain alcohol reaction mechanisms into a single reaction scheme. Coupling with advanced simulation technology, it could be a powerful numerical tool in the design and optimization of new generation engines which are fueled with clean alternative fuels, especially when concerning long-chain alcohols. In addition, the effects of adding biodiesel and different kinds of alcohols on fundamental combustion characteristics of diesel/biodiesel/alcohol blend fuels were investigated using the proposed combined mechanism.

Due to its potential of reducing NOx and soot emissions simultaneously while improving thermal efficiency, water emulsified diesel is considered as one of the most promising fuels for compression ignition engines. In this study, spray and combustion characteristics of neat diesel and water emulsified diesel with various water contents (10%, 20% and 30% by mass) were investigated. The influence of micro-explosion on high pressure spray characteristics of water emulsified diesel was optically observed and discussed. Experiments were conducted in a constant volume combustion chamber with a high-speed schlieren system to capture the spray and combustion processes. The results show that water content plays a significant role in affecting spray and combustion characteristics of water emulsified diesel. Under non-evaporating condition, the spray tip penetration increases with the water content but the corresponding spray angle decreases with the water content. Such an effect was found diminishing under evaporating condition. The spray volume of test fuels increases from non-evaporating to evaporating condition, and the relative volume increase of water emulsified diesel is at least 5 times higher than that of neat diesel. Both the ignition delay and flame lift-off length increase with water content. Consequently, the integrated natural flame luminosity decreases with the increase of water content. In addition, indirect evidences have proven that the occurrence of micro-explosion can enhance the breakup and evaporation processes of water emulsified diesel spray, and the use of water emulsified diesel can effectively reduce soot emission.

Pentanol is considered as one of the most promising alternative biofuels due to its excellent physicochemical properties. The objective of this work was to compare the ignition and combustion characteristics of different n-pentanol/diesel blends in an optical constant volume combustion chamber. The tested fuels included 20% (D80P20) and 40% (D60P40) of n-pentanol blended with diesel in volume, and pure diesel (D100). Broadband chemiluminescence technique was used to measure the timing and location of spray ignition. A high-speed CCD camera with two ND8 dimmer lenses was used to capture the incandescence radiated from the soot particles during combustion. A wide range of experimental conditions was investigated. The ambient temperature ranged from 800 K to 1200 K and the oxygen concentration ranging from 10% to 21%, covering both the conventional and low temperature combustion regimes. The results show that pure diesel has shorter ignition delay and distance comparing to pentanol blends. A larger blending proportion of pentanol D60P40 advances the ignition phase more than the D80P20 in low oxygen concentration conditions. Due to the fuel-borne oxygen and the dilution effect, the natural flame luminosity is reduced significantly with the increase of pentanol ratio in most conditions except under the intermediate temperature region of 1000 K. In that condition, the shorter ignition delay and flame lift-off length of pentanol blends cause a slightly increase in the natural flame luminosity. The natural flame luminosity images showed that the oxygen-contained structure of pentanol could accelerate soot oxidation under all conditions. This indicates that pentanol blends could decrease final soot emissions in internal combustion engines.

Ethanol direct injection has the potentials to increase the engine compression ratio and thermal efficiency by taking advantages of ethanol fuel such as the high octane number and latent heat. In this study, CFD modelling and experiments were carried out to investigate the charge cooling effect and combustion characteristics of ethanol direct injection in a gasoline port injection (EDI + GPI) engine. Experiments were conducted on a single-cylinder spark ignition engine equipped with EDI + GPI over a full range of ethanol ratio from 0% (GPI only) to 100% (EDI only). Multidimensional CFD simulations to the partially premixed dual-fuel spray combustion were performed to understand the experimental results. The simulations were verified by comparing with the experimental results. Simulation results showed that the overall cooling effect of EDI was enhanced with the increase of ethanol ratio from 0% to 58%, but was not enhanced with further increase of ethanol ratio. When the ethanol ratio was greater than 58%, a large number of liquid ethanol droplets were left in the combustion chamber during combustion and fuel impingement on the cylinder wall became significant, leading to local overcooling in the near-wall region and over-lean mixture at the spark plug gap. As a consequence, the CO and HC emissions increased due to incomplete combustion. Compared with GPI only, the faster flame speed of ethanol fuel contributed to the greater peak cylinder pressure of EDI + GPI condition, which resulted in higher power output and thermal efficiency. Meanwhile, the mixture became leaner with the increase of ethanol ratio. As a result, the IMEP was increased, combustion initiation duration and major combustion duration were decreased when ethanol ratio was in 0–58%. The combustion performance was deteriorated when ethanol ratio was greater than 58%. Experimental and numerical results showed that the IMEP, thermal efficiency and emissions of this EDI + GPI engine can be optimised in the range of ethanol ratio of 40–60%.