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In general, a leaner mixture condition improves combustion efficiency in compression ignition (CI) combustion using diesel. However, in the case of leaner air–fuel mixture conditions, it disturbs flame propagation in spark ignition combustion using gasoline, i.e., low reactivity fuel, causing a decrease in combustion efficiency. Since dual-fuel combustion in a CI engine typically involves the use of high- and low-reactivity fuels together, the differing reactivity conditions in the cylinder become as important as the local equivalence ratio in the cylinder. Thus, there is a need to verify the effect of a leaner mixture condition on combustion efficiency in dual-fuel CI combustion. For this reason, this study experimentally evaluates the effects of varying equivalence ratios on the combustion efficiency of gasoline/diesel dual-fueled CI combustion in a 0.4-L single-cylinder engine under low-speed (1500 rpm) and low-load (total LHV 570 J/str) conditions. To vary the equivalence ratios, intake pressures and exhaust gas recirculation (EGR) rates were, respectively, changed under the part-load condition. The results emphasize that as the equivalence ratio becomes leaner by increasing the intake pressure, combustion efficiency worsens due to the low reactivity properties and certain flame propagation modes of gasoline combustion. On the contrary, increasing the EGR rate did not significantly influence combustion efficiency, but it effectively helped reduce nitrogen oxide (NOx) emissions. Based on these results, it is concluded that optimizing dual-fuel CI combustion to suppress NOx emissions is better achieved using EGR, rather than creating a leaner mixture condition. [ABSTRACT FROM AUTHOR]

In this study, the effects of varying excess air ratios on a 2.0 L naturally aspirated (NA) hydrogen-fueled spark ignition (SI) engine were evaluated under low-load conditions by using port fuel injection (PFI) and direct injection (DI) systems. The engine speeds chosen were 1,200 rpm and 2,000 rpm. The excess air ratio was varied between 1.0 and 2.7 by controlling the throttle and hydrogen fuel rate under a brake mean effective pressure of 0.4 MPa. The combustion mechanism, net indicated thermal efficiency (ITE), brake thermal efficiency (BTE), gas exchange efficiency, mechanical efficiency, and engine-out nitrogen oxide (NOx) emissions were mainly discussed. The results indicated that the main combustion duration of PFI was shorter than that of DI due to its homogeneity, and the ITE values of PFI were similar or slightly lower than those of DI. However, as the mechanical efficiency of DI was higher than that of PFI, the BTE values of DI were always higher than those of PFI (the maximum BTE was 39.7 %). The NOx reduction potential of DI was superior to that of PFI due to stratified combustion, and its lowest value was 0.03 g/kWh. In addition, as the CO2 concentration in the exhaust gas increased, the brake-specific CO2 reduced (< 5.71 g/kWh). [ABSTRACT FROM AUTHOR]

Hydrogen is advantageous for use in internal combustion engines (ICEs) because of its high laminar flame speed and carbon-neutral characteristics. However, because of its low minimum ignition energy, abnormal combustions such as back-fire, pre-ignition, and knocking hinder its efficient supply as a fuel to ICEs under near stoichiometric conditions. Hence, in this study, the effect of various excessive air ratios (λ) on the performance of a hydrogen-fueled spark ignition engine using port fuel injection (PFI) was evaluated under wide-open throttle conditions. After determining the highest limit for the maximum brake torque, a hydrogen direct injection (DI) system was applied under the same λ and abnormal combustion limits to improve the brake power (BP). Results show that at 1,500 rpm, a maximum BP of 14.7 kW was achieved under a λ value of 1.4 with PFI; however, it can be increased up to 21.1 kW using DI with the same brake thermal efficiency under a compression ratio of 14. [ABSTRACT FROM AUTHOR]

Dual-fueled combustion, using high and low reactivity fuels simultaneously, is a promising combustion method that enables reduction of nitrogen oxides (NOx) emission. However, the total amount of NOx emissions in the exhaust gas is strongly influenced by the overall excessive air ratio (lambda) of air—fuel mixture. Thus, lean mixture condition is important for NOx emissions although RCCI operating condition is applied. In addition, not only reduction of emissions, but improvement in BTE requires the boost pressure to be increased. Thus, variations in boost pressure via a controlling turbocharger system was evaluated under general dual-fueled combustion and RCCI conditions using diesel and natural gas fuels while tracing emissions and BTE. The results indicated that higher boost pressure was more effective on RCCI condition in reducing NOx emissions and enhancing BTE and turbocharging system efficiency with lower carbon dioxides, compared to those of general dual-fueled combustion condition. Especially, turbocharger efficiency of dual-fueled combustion was higher than that of diesel single-fueled combustion related with composition of exhaust gas and exhaust gas temperatures. [ABSTRACT FROM AUTHOR]

In this study, a high-load expansion strategy was investigated by varying the effective compression ratio (CR) using the variable valve duration (VVD) method during combustion in a gasoline/diesel dual-fuel premixed compression ignition engine. The effective CR was varied from 15.0 to 10.6 via the retardation of the intake valve closing timing (IVC) at 1,500 rpm. With respect to optimization, the diesel injection timing, fuel ratio, and exhaust gas recirculation (EGR) rate were adjusted for each optimized condition. The limitations were the maximum pressure rise rate (below 10 bar/deg) and in-cylinder pressure (below 150 bar) with NOx < 40 ppm and soot < 0.2 FSN. The results emphasized that when the effective CR was 12.7, the maximum load increased by 10.5 % (gIMEP from 13.3 to 14.7 bar) compared with that at a CR of 15.0. Although the decrease in CR resulted in a gross thermal efficiency loss, the difference was negligible (from 45.9 to 45.5 %). The main reason for high-load expansion was the increase in intake air volume under the same intake pressure upon lowering the CR and the margin for the maximum in-cylinder pressure. From this result, it can be concluded that the Miller cycle is useful for the expansion of high load ranges in premixed combustion systems. [ABSTRACT FROM AUTHOR]

In-cylinder flow motion is important for enhancing combustion in internal combustion engines. There are two major flow motions: tumble and swirl. Tumble enhances the flame propagation speed to spread throughout the entire cylinder. Swirl affects the behavior of diesel spray; an enhanced swirl ratio has been widely used in conventional diesel engines. Because dual-fueled combustion has the characteristics of premixed combustion from background fuels and mixing-controlled combustion from directly injected fuels such as diesel, understanding the effect of in-cylinder flow motion on combustion characteristics in dual-fuel combustion is critical. In this research, the effect of swirl in gasoline and diesel dual-fuel combustion was evaluated with different diesel injection timing conditions. As the dual-fuel combustion engine used in this research was driven by diesel spray, swirl was selected as the main flow motion rather than tumble. The effect of swirl with different diesel injection timings was investigated under low-speed and low-load conditions. The results demonstrate that increasing swirl intensity provokes fast-burning caused by the enhanced air-fuel mixture, decreasing the diesel fraction and allowing more exhaust gas recirculation (EGR) to be used for the reduction of engine-out nitrogen oxides (NOx) and smoke emissions, without reducing thermal efficiency. [ABSTRACT FROM AUTHOR]

The effects of intake pressures and EGR rates on the natural gas/diesel dual-fuel combustion under a high load condition (brake power of 100 kW) were evaluated using a 6-L CI engine in this study. Natural gas fraction was fixed as 80% of total input energy for all the dual-fuel combustion operations and engine rotation speed was set to 1,800 rpm. Intake pressure was varied from 0.15 to 0.21 MPa by controlling the waste gate type turbocharger system. EGR rate was changed mainly by controlling EGR valve opening from fully closing to mid-opening to fully opening, but it was also affected when intake pressures were varied. In addition to these intake mixture parameters, diesel injection timing was adjusted in two cases—early and late injections and all the experimental results were compared to those of the neat diesel combustion cases. The results emphasized that the dual-fuel combustion was more sensitive to overall equivalence ratio determined by intake pressures and EGR rates than the neat diesel combustion because it had flame propagation process of premixed natural gas and air mixture. Thus, during the dual-fuel operations, in-cylinder temperature and brake thermal efficiency were decreased as intake pressure was increased. [ABSTRACT FROM AUTHOR]

The diesel compression ignition (CI) engine has higher durability and thermal efficiency than the gasoline spark ignition (SI) engine. However, the high levels of nitrogen oxides (NOx) and particulate matter (PM) emissions are major problems of diesel engines due to the auto-ignition of heterogeneous mixtures. The dual-fuel combustion concept could be solution to environmental concerns. Dual-fuel combustion can be implemented by substituting some of the diesel fuel with high-volatility fuels, such as gasoline and natural gas. The premixed mixture condition can be improved to diminish localized rich and stoichiometric regions. Notably, ethanol has greater potential to reduce PM emissions because it is highly volatile and readily oxidized. For these reasons, in this research, the effects of varying the ethanol substitution ratio on engine performance and emissions under the dual-fuel combustion condition were experimentally investigated under various load conditions. The test engine was a heavy-duty single-cylinder diesel engine with two direct injectors. Engine speed was fixed at 1000 rpm and the load condition was varied for an indicated mean effective pressure (IMEP) ranging from 0.2 to 0.8 MPa. The ratio of ethanol to the total input energy was controlled from zero to nearly 50% of the input energy. The NOx and PM emissions decreased with increasing ethanol substitution and the mean size of the PM emissions decreased. For the mid-load condition (IMEP 0.6 MPa), the substitution was increased to 63%, but for low and high loads, higher ethanol fractions could not be used because of insufficient ignition energy at low loads and sharp increment of the in-cylinder pressure under high loads. [ABSTRACT FROM AUTHOR]

In this research, the effects of the propane ratio and diesel direct injection strategy on the emissions and combustion characteristics of a propane-diesel dual fuel PCCI (premixed charge compression ignition) engine were evaluated. Under low speed and low engine load, the propane ratio was varied from 30 to 70% to investigate the effect of the LHV (low heating value) fuel content in the supplied fuel (propane and diesel mixture). The EGR rate was adjusted to suit each propane ratio to ensure combustibility and restricted emissions. Early diesel injection strategies have an excellent effect on the dual fuel combustion in terms of simultaneously reducing NOx (nitrogen oxides) and PM (particulate matter) emissions. This concept is based on PCCI combustion, in which the ignition delay is longer than the diesel injection. Meanwhile, although early single diesel injection has been an effective strategy for reducing NOx and PM emissions simultaneously, it was possible to further reduce NOx emissions using an early split injection strategy with a 30% propane ratio. Additionally, when the propane ratio was 70%, the ignitibility deteriorated due to early single diesel injection, which led to a much leaner air–fuel mixture condition locally prior to auto-ignition, causing unstable combustion. As a result, a multiple injection strategy with earlier main injection and a small diesel post-injection as a triggering source was adopted to stabilize dual fuel PCCI combustion with a high propane ratio. The results emphasized that the diesel injection strategy could be adjusted to suit various propane ratios under dual fuel PCCI combustion to reduce NOx and PM emissions while maintaining thermal efficiency. [ABSTRACT FROM AUTHOR]

As EURO-6 regulations will be enforced in 2014, simultaneous reduction of NO and PM emissions becomes an important issue in recent diesel engine research. New combustion concepts, such as LTDC and pHCCI, have been introduced to overcome the NO and PM trade-off relation. However, these novel combustion concepts are usually implemented with a high EGR rate and by advancing the main injection timing which cause high CO and THC emissions along with poor fuel consumption due to low combustion efficiency. Therefore, the split injection strategy, which was consisted of applying post injection close to the main injection, was carried out in this experiment. Specifically, two different nozzles - a 7-hole conventional nozzle and a 12-hole double-row nozzle - were evaluated to determine the effects of nozzle configurations on engine-out emissions. The result shows that CO emission was reduced by the close post injection strategy regardless of the nozzle configuration. However, THC and PM emissions were reduced only when the 12-hole double-row nozzle was used. Thus, the use of close post injection with the 12-hole double-row nozzle could increase the combustion efficiency in heavy EGR conditions. [ABSTRACT FROM AUTHOR]

• Effects of multi-angle injector on DF combustion was evaluated. • Multi-angle injector helped to reduce NMHC emissions under RCCI. • Multi-angle injector showed better GIE due to lower HT loss. • GWI could be improved more by multi-angle injector under RCCI. In this study, natural gas and diesel dual-fuel combustion in a heavy-duty compression ignition engine is experimentally evaluated by adopting a modified multi-angle nozzle of a diesel injector to reduce the global warming impact (GWI) factor while maintaining high thermal efficiency. The original injector has a single layer with an umbrella angle of 150° and eight holes, whereas the modified injector has an additional layer below with an injection angle of 90° and four holes. The engine speed is 1,200 rpm under low load conditions (intake pressure of 0.11 MPa and overall equivalence ratio of 0.5, such that the torque is varied); meanwhile, the natural gas fraction is varied from 45% to 71% while considering the combustion stability. The diesel injection timing is varied from the top dead center to 70° before top dead center at intervals of 10° for the injector nozzle conditions. Results show that the modified injector improves combustion efficiency under the reactivity-controlled compression ignition (RCCI) regime. In addition, the GWI factor is reduced by 5.6% compared with that of the original injector. Hence, RCCI combustion by early diesel injection using a multi-angle nozzle affords low emissions while considering the GWI factor, as well as maintains a high gross indicated thermal efficiency of 48.6%. [ABSTRACT FROM AUTHOR]

Compressed natural gas (CNG)/diesel reactivity-controlled compression ignition was investigated to understand the combustion and emission characteristics over wide ranges of intake pressure and exhaust gas recirculation (EGR) rate at high load condition. Reactivity-controlled compression ignition is a dual fuel engine combustion strategy that employs in-cylinder fuel blending with two different reactivity fuels, and multiple injections for the in-cylinder fuel reactivity control to optimize the combustion phasing, duration, and magnitude as well as NOx and soot reduction. High CNG substitution was of interest since it could lead to greater CO 2 and particulate matter reductions. The present 80% CNG substitution required substantial intake boosting due to the charge air displaced by CNG. The experimental engine is commissioned for the reactivity-controlled compression ignition regime using natural gas and diesel fuel with a two-stage turbocharging system. The intake boosting system achieved the intake pressure and EGR rate of 210 kPa and 25%, respectively, when the EGR valve was fully open. The intake pressure and EGR levels varied in the ranges of 150 to 210 kPa and 0 to 25%, respectively. The corresponding equivalence ratio ranged from 0.85 to 0.5. The intake system achieved the highest turbocharger efficiency of 56% at the maximum boosting operation. The highest CO 2 reduction was 23.4% at the maximum boosting condition. The improvements under the enhanced intake condition were significant from the standpoints of both efficiency and emissions, including in terms of the particle concentrations. • A two-stage intake boosting system for high-CNG dual-fuel diesel engine is proposed. • Intake system attained the highest turbocharger efficiency under maximum boosting. • Strong correlation between intake conditions and particle concentrations observed. • Significant reduction in CO 2 and NOx emissions and particulate number concentration. [ABSTRACT FROM AUTHOR]

Many previous studies have attempted to substitute diesel fuel with compressed natural gas (CNG) in compression-ignition engines, which can substantially reduce CO 2 and soot emissions. However, the CNG substitution rate cannot be maximized due to several restrictions, most prominently the amount of diesel injection. The minimum injection quantity becomes more significant under conditions of lower torque or higher CNG substitution rate. In this study, engine experiments were performed at 0, 70, and 80% CNG substitution rates under various EGR (exhaust gas recirculation) levels in a six-cylinder, 5899-cm3 diesel engine to determine the combustion and emission characteristics at various CNG substitution rates. The 80% CNG case exhibited the highest thermal efficiency and largest CO 2 reduction with an elevated combustion noise level of 92.3 dB. The thermal efficiency increase and CO 2 reduction with 80% substitution were 2.7% and 18.7%, respectively. A three-dimensional engine combustion simulation was performed to optimize the diesel injector nozzle hole size and rate-of-injection (ROI) for 80% CNG substitution, whereby the mass of diesel injection quantity was reduced to 20% that of the conventional diesel case. The computational result demonstrated that a 25% smaller diameter nozzle (150 μm) with an advanced ROI exhibited a 3.4% increase in the gross indicated mean effective pressure. [ABSTRACT FROM AUTHOR]