• Energy Research

Abstract In this study, bench tests of a heavy-duty turbocharged natural gas spark ignition (NGSI) engine were conducted with intake air injection at full load and different engine speeds. The flow characteristic of the compressor was revealed. The flow capacity of the compressor is reduced with air injection, and the reduction range increases gradually with the air injection pressure increasing at a constant speed, which would likely lead the compressor to surge. But the decrease extent of the compressor flow rate is improved as the speed increases, which reduces the tendency of surge. Based on those, prediction models for safe air injection pressure which can avoid compressor surge during various operations were proposed and then validated with experimental data. In addition, the influence of air injection on the pumping loss was also analyzed. The turbocharger efficiency is reduced therefore the pumping loss of the engine is increased during the air injection process. At 1200 rpm, the pumping loss efficiency of the engine without air injection is 0.25%, while it is increased to 1.83% with an air injection pressure of 400kPa at the same load.

Abstract The influences of VVT on engine power performance were well summarized in previous studies from various aspects. However, most of the previous investigations were qualitative analysis and focused on the total effects of VVT without decoupling the influence factors. The primary cause is due to lack of an effective mathematic model to predict the IMEP of VVT engine. In this study, an approach to detect the VVT signals of engine was proposed, and the influence factors of power performance of VVT engines were quantitatively studied. The fundamental relationships among IMEP and its influencing parameters, i.e., intake density, intake VVT, RGF and thermal efficiency, were decoupled and the governing equation of IMEP was derived with the influences of VVT considered, the calculated results through which are in good agreement with tested data. Results indicate that, IMEP is directly proportional to effective cylinder volume at IVC timing and inversely proportional to RGF. The change rate of RGF influence coefficient is almost three times as that of RGF, and the combined effects of these parameters on IMEP equals the product of the coefficients of four influence factors. All these extended engine thermodynamics and provided theory basis for improving VVT engine's power performance.

Abstract In order to evaluate and improve the performance of a vehicle equipped with a gasoline engine, an integrated model of the vehicle coupling with a detailed engine model was established to investigate the performance, combustion and energy distribution under the Worldwide Harmonized Light duty Test Cycle (WLTC) of cold start. The results show that 2.3–3.2 kW of brake power is required for the automatic transmission under idling conditions. Brake thermal efficiency mainly ranges from 17.71% to 31.94% apart from the rapid deceleration conditions and exhibits poor fuel economy under low-speed phase. Combustion phase of 50% mass fraction burnt (CA50) is usually delayed in case of knocking combustion under higher speed conditions. Moreover, it is delayed to 65 °CA at the initial stage of cold start for fast light-off of three-way catalyst (TWC), but 10–90% combustion duration (CA10-90) is not very sensitive to the cold start. The heat cumulated in the engine structure is obvious at the initial stage and up to 37.39% but finally occupies 4.93%. The proportion of heat transfer loss to coolant increases after 300 s and finally occupies 22.71%. Exhaust gas loss mainly ranges from 0.91 kW to 28.42 kW and its proportion first decreases and then increases to 26.46%. After 1160 s, it exhibits higher growth rate. Pumping loss varies from 0.1 kW to 2.67 kW and occupies 2.2% in the total energy. Furthermore, the findings in the paper can be applied to evaluate and optimize the energy distribution by changing different technologies or strategies in the future.

To improve the actual performance of an automotive engine, an approach consisting of dynamic signal measurements coupled with gas dynamics–thermodynamics process simulations is proposed; this is used to detect the working processes of an automotive engine from cycle to cycle in transient conditions. Based on the working principles and the mathematical models of the proposed detection method, corresponding software was developed, and the reliability of this approach was validated on an automotive engine. Automotive road tests were conducted, and various transient parameters of the engine were successfully detected from cycle to cycle by using the developed software. On this basis, the variations in the influencing factors of and the interactions between various engine parameters were analysed. The results showed that the ignition timing has a strong effect on the indicated thermal efficiency. Where there is an unnecessary delay in the ignition timing, there is a decrease in the indicated thermal efficiency. The pumping mean effective pressure is approximately equal to the difference between the exhaust gas pressure and the intake gas pressure for a low to medium load, but it is much higher than the pressure difference and undergoes great fluctuations in the high-speed and high-load operating regions. Both the presented approach and the research results are significant for improving the engine performance in transient conditions.

Abstract Gasoline compression ignition combustion has been regarded as a potential technology for future vehicle power source. Understanding the relationship between gasoline properties and its auto-ignition behaviors is critical to the development of this new combustion technology. Aiming at this goal, a comprehensive parametric study was conducted to investigate the influence mechanism of gasoline octane number, fuel sensitivity and equivalence ratio on the combustion behaviors of an advanced compression engine. A hybrid analysis method was proposed in the study; a reaction kinetics model coupled with a new-developed gasoline surrogate mechanism was used to trace the engine’s combustion characteristics, while a density-based global sensitivity analysis method was applied to decode the composite effect of each factor. Results demonstrate two distinct combustion modes in low temperature conditions and give separation functions in the input space of three factors (R-Squares are above 97%). The reaction kinetics analysis points out the shift of dominant reaction groups is the main reason for incomplete combustion phenomenon. Then the global sensitivity result shows octane number is the dominant factor controlling the rate of the combustion heat release under all conditions (the averaged sensitivity index exceeds 0.7); fuel sensitivity only makes a considerable effect on combustion performances under low temperature regions, with sensitivity indices of 0.2–0.3. Besides, the local sensitivity of each factor on combustion behaviors at specific points is also given to understand the influence mechanism resulted from the variations of fuel properties. This study provides a useful method for getting deeper insights into the combustion dynamics and kinetic reaction in gasoline compression ignition engines.

In this paper, experimental studies were conducted on a single cylinder high speed spark ignition (SI) motorcycle engine under both full load and partial load at 6500 and 8500 rpm with pure gasoline, 30% and 35% volume butanol-gasoline blends. This study is trying to find out the influence on combustion heat release of high speed SI engine by variables including ignition timing, butanol blend ratio and engine load. The results show that butanol-gasoline blend provides higher knocking resistance by allowing advance ignition timing in SI engines, which leads to more efficient combustion. With butanol blend ratio increases, more complete combustion process will achieve with the optimum operating parameters. With engine load increases, the rates of heat release become faster and ascend in peak value for both pure gasoline and butanol-gasoline blends. Furthermore, engine performance parameters such as power, fuel economy and emissions have been compared and analyzed. The results also show that engine power, torque, brake specific energy consumption, HC, CO and O2 emissions are better than those of pure gasoline at full load with 35% volume butanol addition, combined with ignition timing optimization. But NOx and CO2 emissions are higher than those of the original level of pure gasoline.

Abstract In this study, a thermodynamic analysis for the liquefied methane gas (LMG) engine with the variation of compression ratio (CR) was conducted through theoretical and experimental investigations. Firstly, the equations for thermodynamic cycle efficiency were further corrected based on the previous studies, in which the losses due to heat release rate (HRR), exhaust valve opening (EVO) timing, specific heat ratio, incomplete combustion and heat transfer were considered. Then, the sweeping test of CR was conducted on an LMG engine. On this basis, the thermodynamic cycle process was studied and various kinds of energy losses were analyzed. The results show that the improvement of indicated thermal efficiency by increasing CR mainly depends on engine operating conditions, the maximum of which occurs at high load and is close to the theoretical value (4.2 percent points). The actual cycle efficiency of LMG engine is mainly influenced by the specific heat ratio of medium gas, followed by the heat transfer loss and the effective expansion ratio (EER) loss. Compared with combustion duration, the combustion phase plays a much more important role in EER loss. All these have provided theoretical basis and direction for the improvement of actual thermal efficiency of LMG engine.

Abstract 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.

Abstract In this research, a newly proposed method combining Chemkin with CONVERGE was used to study the transient in-cylinder chemical reaction process in NG-diesel dual fuel engine. The selected mechanism was verified by comparing the results of CONVERGE with experimental data, and then the calibrated model of CONVERGE was used to provide boundary conditions for Chemkin. On this basis, the detailed combustion process was simulated at different natural gas substitution ratio (NGSR). The results show that, the chain branching reaction, long-chain to short-chain reaction, and reactions associated with OH radicals have significant impacts on temperature. It can also be found that the combustion of fuel shows a distinct two-stage reaction process. During the low temperature stage, both the CO and NO emissions are little. While at the high temperature stage, the CO emissions first rapidly increase and then decrease due to the consumption reaction, and the NO emissions also have a quick increase. When the NGSR is reduced, a new path for CO generation occurs at low temperature stage, resulting in minor increase (up to 0.0019 mol fraction) of CO concentration. Meanwhile, the ignition delay is reduced significantly (by 88.5%), but the increase of diesel species does not alter the formation mechanism of emissions. All these provide guidance for improving combustion and emission performance of NG-diesel dual fuel engine.