High Exhaust Gas Recirculation Rates Research Articles

Impact of Cylinder-to-Cylinder Dispersion of Exhaust Gas Recirculation on the Three-Way Catalyst Performance and Tailpipe Emissions of Spark-Ignition Engines

Abstract New generations of spark-ignition engines include exhaust gas recirculation (EGR) to improve the engine efficiency. Depending on the design of the EGR routing, some differences in the total amount of recirculated gases that reach each cylinder can be induced. This affects the air-to-fuel ratio on each cylinder due to the combination of the different temperature and composition of the gases at the intake valve closure. As a consequence, significant deviations in the combustion process and the subsequent composition upstream the three-way catalyst can be reached. This paper explores these effects on catalyst performance and tailpipe emissions, individualizing the behavior for each regulated species. The study was performed in a four-cylinder naturally aspirated engine with Atkinson cycle and a close-coupled three-way catalyst. The most significant deterioration in conversion efficiency appeared for the nitrogen oxides, directly linked to the EGR dispersion level. In the case of carbon monoxide (CO) emissions, no significant impact was observed except at high average EGR rates, where one or more of the cylinders exceeded the EGR tolerance for that speed and load. Based on these results, a strategy where the fuel injector command is adapted to correct the air-to-fuel ratio deviations induced by the EGR was developed and implemented.

Matching and optimization to minimize fuel consumption and NOx emission for a marine diesel engine with turbo-assisted exhaust gas recirculation

Exhaust gas recirculation (EGR) and two-stage turbocharging technology are widely used in marine diesel engines due to their advantages of reducing nitrogen oxides (NOx) emission and improving fuel economy respectively. The matching of EGR, two-stage turbocharging and diesel engine is significant to realize both advantages simultaneously. Turbo-assisted EGR (TA EGR) can achieve high EGR rate and lower fuel consumption due to a turbocharger driving EGR. In this study, a 1D model of the diesel engine with TA EGR coupling two-stage turbocharging system was established by GT-Power. A theoretical matching method was proposed based on the requirements of excess air coefficient and EGR rate. The matching laws of the coupling system were studied, and the preliminary matching results achieved good effects. A fully connected neural network was used as the optimization agent model of the 1D model, and the NSGA II algorithm was used to carry out the multi-parameter and multi-objective optimizing matching. Considering constrained and unconstrained conditions, independent optimizing matching was carried out under each single load, and the weighted brake specific fuel consumption (BSFC) and weighted NOx under all loads were taken as the target to carry out comprehensive optimizing matching. The results showed that the optimization performance is worse under constrained conditions, while TA EGR coupling adjustable two-stage turbocharging system can achieve comprehensive optimal BSFC and NOx, with weighted BSFC decreasing by 3.57% and weighted NOx decreasing by 75.28% compared with experimental results of the original engine.

Study on the Effect of Coupled Internal and External EGR on Homogeneous Charge Compression Ignition under High Pressure Rise Rate

This paper investigated the effects of exhaust gas recirculation (EGR) on homogeneous charge compression ignition (HCCI) combustion in internal combustion engines. The exhaust valve closing (EVC) timings were scanned to obtain a set of baseline operating points for HCCI, and the coupling control of the internal and external EGR was explored. The results indicate that external EGR delays HCCI ignition timing and slows down the combustion speed. As the internal EGR rate increases, the maximum external EGR ratio that can be tolerated decreases. For HCCI detonation operating points with low internal EGR rates, the addition of up to 10% of external EGR can control the pressure rise rate peak to less than 10 bar/°CA, resulting in reduced fuel consumption and increased indicated mean effective pressure (IMEP). However, for HCCI operating points with high internal EGR rates, the effect of external EGR is mainly observed in the control of the pressure rise rate, with limited increase in IMEP. Additionally, an increasing external EGR rate leads to a significant decrease in nitrogen oxide (NOx) emissions, while carbon monoxide (CO) and hydrocarbon (HC) emissions slightly increase before engine misfire occurs. These findings suggest that the coupling control of internal and external EGR should be explored further, particularly in relation to reducing the negative valve overlap (NVO) angle and improving combustion efficiency.

Reducing Soot Nanoparticles and NOX Emissions in CRDI Diesel Engine by Incorporating TiO2 Nano-Additives into Biodiesel Blends and Using High Rate of EGR

The developments in the field of nano-additives have increased in the recent years due to the desire to reduce the level of exhaust emissions in diesel engines. The soot characteristics of particulate matter (PM) and nitrogen oxides (NOX) were experimentally investigated using two concentrations of titanium dioxide (TiO2) as nano-additives (25 ppm and 40 ppm) blended with C20D (composed of 20% castor oil methyl ester and 80% diesel fuel) and 30% exhaust gas recirculation (EGR). The combustion of C20D + TiO2 increases brake thermal efficiency (BTE) by 2.8% in comparison with neat C20D, while a significant reduction was obtained in BSFC 6.5% and NOX emissions were maintained at a level parallel with diesel. The results indicated that the technique involving a high EGR rate and the addition of 25 ppm and 40 ppm of TiO2 nanoparticles to the C20D exhibits better reductions in NOX emissions by 17.34% and 21.83%, respectively, compared to the technique comprising the use of C20D + TiO2 and C20D. The reduction in the total concentration of PM via the addition of TiO2 nanoparticles to the C20D was 26.74% greater than neat C20D and diesel. In contrast, the incorporation of a high rate of EGR with C20D +TiO2 increased the PM concentrations by 16.85% compared to the technique without EGR. Furthermore, the high concentrations of TiO2 nanoparticles (40 ppm) in the C20D produced 19 nm smaller soot nanoparticles compared to the 23 nm larger soot nanoparticles produced from the low concentrations of TiO2 nanoparticles (25 ppm) added into the C20D. The current investigation reveals that the reduction in NOX emissions and the production of soot nanoparticles notably improved due to the synergic effect of EGR, the TiO2 nanoparticles, and biodiesel.

The multi-parameter optimization of injections on double-layer diesel engines based on genetic algorithm

In this paper, a coupling model of the NSGA-II genetic algorithm and KIVA-3 V program is developed to perform a multi-objective intelligent optimization study of the injection parameters in a diesel engine with double-layer diverging combustion (DLDC) chamber. Four optimization parameters, involving the first injection timing, the first fuel split ratio, the injection timing interval, and the exhaust gas recirculation (EGR) ratio, are optimized after comparing the pollutant emissions and fuel consumption characteristics. The computation results reveal that both soot and fuel consumption function as the optimization objectives, proving a trade-off relationship with the NOx emission. The analysis of the response surface demonstrates that the application of the multiple-injection strategies is beneficial for enhancing the performance of diesel engines. A large first fuel split ratio and a low EGR rate can minimize both ISFC (indicated specific fuel consumption) and soot emission concurrently. A high EGR rate can significantly reduce NOx emission. In addition, the distribution of the final Pareto font shows that 66.7 % percent of the first fuel split ratio is above 0.56, and only two solutions (8.3 %) have values below 0.35. Furthermore, three representative points of the first fuel split ratio are selected for combustion analysis, which are 0.05, 0.35, and 0.88. As the first fuel split ratio rises, the high-temperature areas in the cylinder are gradually enlarged, and the NOx emission is increased while the soot emission and ISFC are further reduced.

Experimental Study and Optimisation of a Non-Conventional Ignition System for Reciprocating Engines Operation with Hydrogen–Methane Blends, Syngas, and Biogas

The paper deals with the experimental study of a medium-load spark ignition engine under operation with different fuel mixtures among those deemed as promising for the transition towards carbon-free energy systems. In particular, the performance of a non-conventional ignition system, which permits the variation of the ignition energy, the spark intensity and duration, was studied fuelling the engine with 60–40% hydrogen–methane blends, three real syngas mixtures and one biogas. The paper is aimed to find the optimal ignition timing for minimum specific fuel consumption and the best setup of the ignition system for each of the fuel mixtures considered. To this end, a series of steady-state tests were performed at the dynamometer by varying the parameters of the ignition system and running the engine with surrogate hydrogen–methane–nitrogen mixtures that permit the simulation of hydrogen–methane blends, real syngas, and biogas. The results quantify the increase of spark advance associated with the decrease of the fuel quality and discuss the risk of knock onset during methane–hydrogen operation. It was demonstrated that the change of the ignition system parameters does not affect the value of optimum spark advance and, except for the ignition duration, all the parameters’ values are generally not very relevant at full load operation. In contrast, at partial load operation with low-quality syngas or high exhaust gas recirculation rate, it was found that an increase of the maximum ignition energy (to 300 mJ) allows for operation down to approximately 66% of the maximum load before combustion becomes incomplete. Further reductions, down to 25% of the maximum load, can be achieved by increasing the gap between the spark plug electrodes (from 0.25 to 0.5 mm).

Experimental investigation on emission characteristics of high reactivity gasoline compression ignition with different EGR and injection pressure strategies

Using a High Reactivity Gasoline (HRG) in diesel engines is a promising approach to reduce NOx and soot emissions simultaneously without modifying the engine hardware. In this study, an experimental investigation on HRG and diesel emission characteristics was conducted in a six-cylinder heavy duty diesel engine with different exhaust gas recirculation (EGR) rate and injection pressure strategies. Four operation points, A25, A75, C25, and C75 in the European Steady Cycle (ESC), were studied. The pilot-main injection strategy was applied, and injection timing was adjusted to maintain a constant Crank Angle of 50 % heat release (CA50). The results show that, compared with diesel, HRG can achieve lower soot mass emission while maintaining an ultra-low NOx emission level (2–3 g/kWh) with a lower injection pressure, indicating lower cost in the fuel injection and aftertreatment systems. The particle size distribution was also analyzed for both fuels. Under high EGR rates, the accumulation mode particles were dominant for both HRG and diesel. However, under low EGR rates, diesel produced much more nucleation mode particles than HRG. The phenomenon can be explained by the difference in the pyrene and acetylene productions between paraffin and aromatic compositions.

Assessment of air management strategies to improve the transient response of advanced gasoline engines operating under high EGR conditions

Advanced gasoline engines may lead the medium-term future of the passenger vehicle market, working in conventional and hybrid powertrains. Downsizing with turbocharging is the most extended way to improve fuel economy in gasoline engines. It is also proven that exhaust gas recirculation (EGR) reduces fuel consumption, but extracting the maximum benefit from EGR requires operating with high EGR rates. This fact can compromise the transient engine operation due to the greater turbocharger dependence. This research evaluates the EGR influence on the transient response of a turbocharged gasoline engine and, mainly, the potential of three air management strategies to accelerate the said response. Tip-in maneuvers at 1500 rpm (6–12 bar BMEP) were tested and simulated to this end. The three strategies are: reducing the EGR dilution by closing the EGR valve simultaneously with the throttle opening, using a pressurized air tank (PAT), and installing an electric supercharger at the compressor outlet in series. Engine tests show that the torque response time with EGR is 2-s slower than without EGR. 1D modeling results reveal that: the PAT connected to the intake manifold provides the fastest response, and the electric supercharger guarantees an excellent tradeoff between fuel consumption and torque response.

Experimental study of gasoline engine with EGR dilution based on reactivity controlled turbulent jet ignition (RCTJI)

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines, but it also deteriorates the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition way for improving ignition stability and burning rate. However, the EGR in the pre-chamber will worsen the reactivity of the fuel-air mixture in the pre-chamber and subsequently lower the intensity of the TJI. To this end, the reactivity controlled turbulent jet ignition (RCTJI) based on a newly air-assisted pre-chamber turbulent jet ignition with a scavenging function has been proposed to improve the reactivity in the pre-chamber for the first time. The combustion characteristics, pressure oscillation, and combustion stability of a single-cylinder spark-ignition engine under four ignition modes of spark ignition (SI), passive TJI, active TJI, and scavenged TJI have been investigated. The results show that three TJI modes can obviously shorten the combustion duration due to the fast jet flame. Compared to the SI mode, the spark timing of TJI is not sensitive to the change in EGR rate. In particular, the scavenged TJI could effectively promote the chemical reactivity in the pre-chamber, and subsequently improves the intensity of the turbulent jet flame and shortens the ignition delay and combustion duration. As a result, using the scavenged TJI can effectively reduce cycle variation and achieve the lowest indicated specific fuel consumption (ISFC) at high EGR rates. However, with the increase in EGR rate, the SI combustion gradually becomes unstable. In contrast, the scavenged TJI can effectively extend the EGR tolerance. In addition, the present work further has confirmed the three main combustion stages of jet ignition in terms of the heat release rate.

Experimental evaluation of performance of heavy-duty SI pure methanol engine with EGR

Because methanol can be synthesized from captured CO2 and green H2, it is a promising alternative fuel for reducing carbon emissions from heavy-duty vehicles. Experiments were conducted on a multi-cylinder port fuel injection spark-ignition (SI) heavy-duty pure methanol engine modified from a natural gas engine. The main objective was to evaluate the performance of a heavy-duty SI pure methanol engine and its improvement over the original natural gas engine. The effects of the exhaust gas recirculation (EGR) rate and spark timing on the brake thermal efficiency (BTE), as well as the NOx, CO and HC emissions, were investigated under a wide range of operating conditions. The experimental results indicated that the pure methanol engine had a BTE of > 40% BTE in a brake mean effective pressure (BMEP) range of 1.27–2.17 MPa at speeds ranging from 1000 to 1700 rpm and that the peak BTE was ∼ 41.4%, which was higher than that of the original natural gas engine by an absolute 3%. The EGR tolerance of methanol engine was high and the coefficient of variation of the indicated mean effective pressure (COVIMEP) could be maintained below 2%, even at 31% EGR. With an increase in the EGR rate, the CO emissions first decreased and then increased. The partial oxidation of unburned methanol contributed to higher CO emissions when the EGR rate reached a critical value. Advancing the spark timing increased the HC emissions at a low EGR because of the lower post-oxidation temperature; however, it reduced the HC emissions at a high EGR rate because of the more stable combustion. At a BMEP above 0.87 MPa, the NOx, HC and CO emissions ranged from 0.055 to 1.185 g/kWh, from 2.4 to 13.05 g/kWh, and from 6.36 to 10.88 g/kWh, respectively. The pure methanol engine had lower NOx and CO emissions but higher HC emissions than the original natural gas engine.

Potential of compression ratio and exhaust gas dilution on improving combustion and nitrogen oxides emission performance on a PFI engine fueled with methanol

Methanol is a promising alternative fuel, and the effects of compression ratio (CR) and exhaust gas recirculation (EGR) of a port fuel injection spark ignition engine modified from a diesel engine fueled with methanol were studied in this paper. The results showed that with the increase of the EGR rate, flame propagation slowed down, heat release rate and the pressure rise rate decreased, and the combustion lag phenomenon was more obvious in the high EGR range. Under light load and high engine speed conditions, the combustion parameters were more sensitive to the variation of EGR rate. Increasing the CR was beneficial to improve combustion stability. Higher CR and introducing exhaust gas into cylinder both can improve break thermal efficiency and fuel economy performance. EGR strategy showed great capacity on reducing NOx emissions. By Taguchi and ANOVA analysis, it was found that higher EGR rate and small CR were benefit for NOx emission, but higher CR was suitable for decreasing BSFC, 17:1 was an optimal CR for this engine. The influence ratio of EGR on NOx emissions reached 88%, and the engine operating condition had the greatest influence on BSFC, followed by EGR rate.

Effects of Intake Components and Stratification on the Particle and Gaseous Emissions of a Diesel Engine.

It is of great significance to improve the performance of diesel engines by adjusting the intake components and their distribution. In this work, various proportions of exhaust gas recirculation (EGR) gas and oxygen (O2) have been introduced to the intake charge of a diesel engine and the effects of different intake components and stratification conditions on pollutant emissions, especially for particles, have been explored. The results show that the introduction of O2 into the intake charge is beneficial to alleviate the deterioration of particles and hydrocarbon (HC) emissions caused by high EGR rates. Compared with the pure air intake condition, the introduction of moderate O2 at high EGR rate conditions can simultaneously reduce nitrogen oxides (NOx) and particles, when the intake oxygen content (IOC) is 0.2 and the EGR rate is 20%, the NOx and particles are reduced by 45.66% and 66.49%, respectively. It is worth noting that different intake components have a significant impact on the particle size distribution (PSD) of diesel engines. In addition, the in-cylinder O2 concentration distribution formed by the stratified intake is advantageous for further improving the combined effect of NOx, particles and HC emissions relative to the homogeneous intake. At a condition of 0.2 IOC and 20% EGR rate, the NOx, particles, and HC emissions are about 8.8%, 14.3%, and 26% lower than that of intake components nonstratification, respectively.

Combustion Performance and Low NOx Emissions of a Dimethyl Ether Compression-Ignition Engine at High Injection Pressure and High Exhaust Gas Recirculation Rate

Dimethyl ether (DME) is a promising alternative to diesel for compression-ignition (CI) engines used in various industrial applications. However, the high nitrogen oxide (NOx) emissions of DME combustion have restricted its use. The primary cause of high NOx emissions is a high combustion temperature. In this study, a high exhaust gas recirculation (EGR) rate was used when testing a common-rail direct injection CI engine suitable (with minor modifications) for a passenger car. A modified fuel supply system created high injection pressure during evaluation of combustion performance. The physical and chemical properties of DME were the principal determinants of the ignition delay, combustion speed, and heat release rate. Although a high injection pressure accelerated formation of the fuel-air mixture and the combustion speed, combustion performance deteriorated with increased NOx emissions. An increased EGR rate affected combustion and the NOx concentration. A high EGR rate effectively reduced NOx formation and emission under low-temperature combustion conditions. Also, the good DME combustion characteristics were maintained when the EGR rate was high, unlike for an ultra-low sulfur diesel engine.

NOx–Smoke Trade-off Characteristics in a Palm Oil-Fueled CRDI Diesel Engine under Various Injection Pressures and EGR Rates

Palm oil is one of the most common and productive vegetable oils, so it is often used as an excellent feedstock for biodiesel production. However, due to the high viscosity and other issues of palm oil, it cannot be directly used as an alternative fuel for diesel engines unless some treatment is carried out. In this study, the effects of palm oil-diesel blend fuel on the nitrogen oxides (NOx)–smoke trade-off characteristics were investigated in a common rail direct injection (CRDI) diesel engine under various injection pressures and exhaust gas recirculation (EGR) rates. It was found that NOx and smoke from the combustion of fuel containing 50% palm oil (P50D50) were simultaneously suppressed by 3% and 3.1% compared with diesel fuel at an injection pressure of 400 bar, respectively. The performance of P50D50 was comparable to that of diesel, but at high injection pressure and high EGR rate, it showed shorter ignition delay (ID) and lower smoke emission.

Experimental analysis of the EGR rate and temperature impact on combustion and emissions characteristics in a heavy-duty NG engine

Cooled exhaust gas recirculation (EGR) is one of the most effective methods in spark ignition (SI) natural gas (NG) engines to improve the fuel economy and reduce the nitrogen oxides (NOx) emissions. The objective of this study is to anatomize the EGR rate and temperature impact on the combustion and emissions characteristics by the experiments in a heavy duty (HD) NG engine under various spark timings. The results showed that the increasing EGR rate reduced the burning rate and extended the combustion duration. Spark timing should be optimized to copy with the retarded combustion phase. Under high EGR rate, the combustion performance could be optimized with the increase of EGR temperature. There exists a proper EGR rate in real application for the fuel consumption of the engine. Higher EGR temperature plays positive effects on fuel economy, owing to the higher combustion efficiency and lower throttle losses, and the decreasing amplitude of BSFC is 1.25% between the EGR temperatures of 333 K and 373 K. With the increase of EGR rate from 5% to 25%, NOx emissions are reduced by up to 76%, the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions are increased by 34.6% and 12.9% respectively. Higher EGR temperature could alleviate the increase of HC and CO emissions, but sacrificing the NOx emissions, and the increase extent of NOx emissions is 35.3%. The results provide an insight into the quantitative comparisons of EGR rate and temperature on the NG engine performance and emissions, which promotes the utilizing EGR strategy in SI NG engines.

Effects of high-pressure and donor-cylinder exhaust gas recirculation on fuel economy and emissions of marine diesel engines

With the increasingly stringent emission legislation for marine diesel engines, a higher exhaust gas recirculation (EGR) rate is used to reduce NOx. However, a high EGR rate deteriorates brake specific fuel consumption (BSFC). Therefore, a suitable EGR pattern is critical to balancing NOx and BSFC. In this study, high-pressure EGR (HP-EGR) and donor-cylinder EGR (DC-EGR) were separately established by GT-Power, and NOx and BSFC comprehensive optimization was conducted by adaptive particle swarm optimization. The effect of the EGR-coupled high-pressure turbine on NOx and BSFC for HP-EGR and the effect of the EGR-coupled throttle valve on the inhomogeneity of different cylinders for DC-EGR were studied. Comparisons of two EGR patterns show that the achievable maximum EGR rate for HP-EGR is more sensitive to engine loads than DC-EGR. DC-EGR can achieve a higher EGR rate and lower NOx at low and medium loads. Under the same NOx, the required EGR rate, λ, boost pressure and exhaust pressure of DC-EGR are lower. However, the BSFC of DC-EGR is slightly higher at a 25% load. As the turbine flow characteristic of HP-EGR is insufficiently adjusted with loads, DC-EGR is more inclined to achieve comprehensive optimization of NOx and BSFC under full loads.

Experimental investigation the effects of spark discharge characteristics on the heavy-duty spark ignition natural gas engine at low load condition

The fuel economy of natural gas (NG) spark ignition (SI) engine could improve significantly when using high exhaust gas recirculation (EGR) dilution. However, the combustion processes become instability and even occur misfire at higher EGR rate. In this paper, the high energy ignition system, which was the technology of variable spark, was utilized to extend the EGR tolerance of a SI engine. Extensive experimental tests were conducted to investigate the effects of spark discharge current and duration on combustion and emissions characteristics at low load condition. The results showed that the fuel economy was improved with the spark discharge energy at the high EGR rate of 30%. The increase amplitude of fuel economy was 1.32% when the spark discharge energy increased from 210 to 320 mJ. The effective spark discharge energy was existed in consideration of the fuel economy. The ignition delay (CA0-5) and the flame propagation period (CA10-90) shortened with an increase in spark discharge energy. Besides, NOx emissions were more sensitive to the spark discharge energy than that of HC and CO emissions. As the spark discharge energy increased, the increase extent of NOx emissions was 26.9%, while HC and CO emissions decrease slightly.

Fuel Stratification Effects on Gasoline Compression Ignition with a Regular-Grade Gasoline on a Single-Cylinder Medium-Duty Diesel Engine at Low Load

<div class="section abstract"><div class="htmlview paragraph">Prior research studies have investigated a wide variety of gasoline compression ignition (GCI) injection strategies and the resulting fuel stratification levels to maintain control over the combustion phasing, duration, and heat release rate. Previous GCI research at the US Department of Energy’s Oak Ridge National Laboratory has shown that for a combustion mode with a low degree of fuel stratification, called “partial fuel stratification” (PFS), gasoline range fuels with anti-knock index values in the range of regular-grade gasoline (~87 anti-knock index or higher) provides very little controllability over the timing of combustion without significant boost pressures. On the contrary, heavy fuel stratification (HFS) provides control over combustion phasing but has challenges achieving low temperature combustion operation, which has the benefits of low NO<sub>X</sub> and soot emissions, because of the air handling burdens associated with the required high exhaust gas recirculation rates. This work investigates HFS and PFS combustion, efficiency, and emissions performance on a single-cylinder, medium-duty engine with a regular-grade gasoline (91 research octane number) at 1,200 rpm, 4.3 bar, and 3.0 nominal gross indicated mean effective pressure operating points with boost levels similar to those in a medium-duty diesel application. Authority of combustion phasing with main injection timing sweeps for HFS and second injection timing sweeps and fuel split sweeps for PFS are shown. In addition, this work is discussed in the context of previous findings with a light-duty diesel platform, and next steps and future direction for this work are presented<span class="xref"><sup>1</sup></span>. </div></div>

Potential of miller timing with synthetic diesel fuels on a single cylinder heavy-duty engine

Upcoming emission limits such as Euro VII will make it necessary to further reduce the NOx emission level of internal combustion engines while stricter CO2 limits demand lower fuel consumption. Early closing of the intake valves (Miller timing) leads to reduced combustion temperatures due to lower effective compression ratio, and therefore lower formation and emission of nitrogen oxides. Miller timing is frequently used in gasoline engines, while in Diesel engines it competes with exhaust gas recirculation (EGR). When both measures are applied simultaneously, this may lead to increased emission of soot using standard Diesel fuel, as combustion temperature and oxygen content of the charge become too low. This work shows the investigation of different intake valve timings on an externally supercharged single-cylinder heavy-duty Diesel engine, stationary operated with hydrogenated vegetable oil (HVO), oxymethylene ether (OME), and standard Diesel fuel (DF). The synthetic fuels have a higher cetane number than DF, which supports ignition at lower temperatures. Moreover, OME has a soot-free combustion, which allows an extension of the operating limits without increased emissions. The results show that especially with Miller timing a high-performance turbocharging system is crucial, since higher boost pressure is required to compensate for the filling losses due to the earlier intake closing. The application of a high EGR rate is limited in this case, leading to a trade-off between Miller timing and EGR. All fuels show a reduction in nitrogen oxides of up to 40% with an improved efficiency of more than 3% at a typical road-load point. Measures to reduce ignition delay were found to be necessary, especially for DF. For OME, increased soot formation does not occur when combining Miller timing with low rail pressure, reduced boost pressure or EGR, which promotes simultaneous application of the measures resulting in minimized emissions of nitrogen oxides.

The combustion and emission characteristics of diesel-ethanol blends with THF as cosolvents in a diesel engine operating with EGR

Tetrahydrofuran (THF) is not only an excellent co-solvent for ethanol but also a promising clean oxygen-containing fuel. The objective of the diesel engine test is to reveal the effects of exhaust gas recirculation (EGR) rates and THF as co-solvents between ethanol and diesel on the combustion and emissions. Results show that the maximum rate of pressure rise increases with the growth of the additive proportion of ethanol or THF under low EGR rates. The indicated thermal efficiency (ITE) of mixedfuels is slightly higher than that of neat diesel fuel when the EGR rate is below 30%. With the increase of THF and ethanol fraction, carbon monoxide (CO) and soot emissions of blends decrease greatly, while total hydrocarbons (THC) emissions gradually increase, especially under medium and high EGR rates. Finally, E30T10 fuel (30% ethanol, 10% THF, and 60% diesel by vol.) presents ultra-low soot emissions under all EGR rates and breaks the conventional trade-off between soot and nitrogen oxides (NOx). Under medium EGR rates, NOx emissions of E30T10 are reduced markedly, but THC and CO emissions are still at low levels. Meanwhile, the ITE of E30T10 still maintains a high level.