High Temperature Heat Release Research Articles

Pilot injection strategy management of gasoline compression ignition (GCI) combustion in a multi-cylinder diesel engine

The present study focuses on the experimental investigation on the optimal pilot injection strategy under GCI combustion mode in a multi-cylinder heavy-duty diesel engine. Three experiments were conducted at a high-speed high-load operating point with different operating parameters and emission targets, namely engine-out NOx target, fuel injection pressure, and main injection timing. The engine-out NOx targets were set to 5.0 g/kWh and 1.5 g/kWh, and the gasoline injection pressures were set to 100 MPa and 140 MPa. These high and low values represent different requirements of SCR efficiency and practical capability of fuel supply system when addressing the hypothetical future tailpipe NOx limit of 0.02 g/hp-hr (0.027 g/kWh). The results show that the use of optimized pilot injection always achieves lower pressure rise rate and soot emissions than the single injection baseline. The pilot gasoline fuel with low injection pressure is more ignitable than that with high injection pressures, hence a distinct heat release spike usually occurs for a pilot injection. The optimal pilot mass should be increased for a higher fuel injection pressure because the pilot fuel stratification level decreases. A relatively late pilot timing is preferable for the early main injection timing. For the late main injection timing, however, a relatively early pilot timing with large pilot mass is preferable and brings about a distinct two stage high temperature heat release, which can reduce the fuel consumption and soot emissions simultaneously. The two stage split combustion process obtained by double injection with retard combustion phasing can be considered to be an important way to alleviate the requirement of GCI fuel system.

Comprehensive analysis of exergy destruction sources in different engine combustion regimes

The exergy destruction sources of different engine combustion regimes are investigated. The results indicate that chemical reaction is the largest sources of exergy destruction. Although the exergy destruction due to chemical reaction in conventional diesel combustion (CDC) engines is the lowest, its total exergy destruction is the largest. The transition from low-temperature heat release (LTHR) to high-temperature heat release (HTHR) accumulates substantial exergy destruction, so adopting one-stage heat release fuels without LTHR is favorable to reduce exergy destruction. Higher combustion temperature benefits to reduce exergy destruction, and the required combustion temperatures to achieve the same exergy destruction fraction are different for various combustion regimes. With the increase of equivalence ratio towards the stoichiometric ratio, the exergy destruction fraction decreases due to higher combustion temperature. When the equivalence ratio exceeds the stoichiometric ratio, equivalence ratio itself plays a more negative effect, leading to larger exergy destruction fraction. Thus, the lowest exergy destruction fraction is at the stoichiometric combustion. Overall, for different combustion regimes, different strategies should be accordingly adopted to reduce exergy destruction, and effective control over the magnitude and stratification of combustion temperature is important to realize low exergy destruction, high fuel efficiency, moderate combustion rate, and low emissions simultaneously.

Experimental Study of Autoignition Characteristics of the Ethanol Effect on Biodiesel/n-Heptane Blend in a Motored Engine and a Constant-Volume Combustion Chamber

To explore the effect of the addition of ethanol (E) on the combustion behavior of biodiesel/n-heptane (BH) blends, autoignition characteristics of the BHE blends were studied in two experimental systems: a modified cooperative fuel research (CFR) engine and a constant-volume combustion chamber (CID 510) used for rating the derived cetane number of fuels. The observations of ignition behavior include the critical compression ratio and heat release profile, which are assessed using the CFR engine. The equivalence ratio is 0.25 and 0.45, respectively, while the physical and chemical ignition delays are measured by the CID 510 under a wide range of air temperatures and oxygen dilution levels. With the addition of the ethanol, the critical compression ratio increases, which indicates that the reactivity decreases. According to the heat profiles, because of the complex composition of the blend, the onset of the high temperature heat release (HTHR) and low temperature heat release (LTHR) did not vary linearly w...

Comparison of late PCCI combustion, performance and emissions of diesel engine for B20 and B100 fuels by KIVA-CHEMKIN coupling

Abstract A numerical study is performed by KIVA-CHEMKIN code to compare the combustion, performance and emission characteristics of neat biodiesel (B100) and biodiesel blend including a mixture of 20% biodiesel and 80% diesel (B20) resulting from the PCCI combustion mode in a light-duty diesel engine. For the biodiesel reaction mechanism, multi-chemistry surrogate mechanism using methyl decanoate (MD) and methyl-9-decenoate (MD9D) is used in this study. The results show that PCCI combustion like high temperature conventional combustion cannot cover the lower ISFC of the biodiesel blend fuel compared to the diesel fuel. A detailed analysis of combustion and emissions, involving the role of formaldehyde (CH2O) and hydroxyl (OH) radicals as well as O and H radicals, was performed in the PCCI combustion stages and emissions formation for the B100 and B20 fuels. The results indicate that higher concentration of formaldehyde as well as lower concentration of hydroxyl radicals for the B20 case advances low temperature heat release (LTHR) and retards high temperature heat release (HTHR) respectively compared to the B100 case. In addition, O and OH radicals as well as cylinder temperature are effective parameters regarding higher NO and CO concentrations for the B20 case compared to the B100 case.

A Detailed Study of the Effects of Biodiesel Addition and Exhaust Gas Recirculation on Diesel Engine PCCI Combustion, Performance and Emission Characteristics by KIVA–CHEMKIN Coupling

In this study, a numerical study is performed by KIVA–CHEMKIN code to investigate the effects of biodiesel addition and exhaust gas recirculation (EGR) on diesel engine premixed charge compression ignition (PCCI) combustion, performance, and emission characteristics. The studies are performed for neat diesel fuel and mixture of 10–40% biodiesel addition at 67%, 50%, and 40% EGR. For this purpose, a multichemistry surrogate mechanism using methyl decanoate (MD) and methyl-9-decenoate (MD9D) is used. The main innovation of this work is analyzing the chemical, thermodynamic, and dilution effects of biodiesel addition as well as different EGR ratios on PCCI combustion behavior. The results show that the main effect of EGR on PCCI combustion of biodiesel blend is related to the high temperature heat release (HTHR), and its effect on low temperature heat release (LTHR) is low. With increasing biodiesel addition, the role of the chemical effect is increased compared to the thermodynamic and dilution effects. Rate of production analysis (ROPA) indicate that for the different biodiesel ratios, the effect of reaction nC7H16 + HO2 = C7H15-2 + H2O2 is more effective on the start of combustion (SOC) compared to the other reactions. For a defined biodiesel addition, with decreasing EGR, total (unburned) hydrocarbon (THC) and CO are decreased, while NOx and indicated specific fuel consumption (ISFC) are increased.

Multiple optical diagnostics on effect of fuel stratification degree on reactivity controlled compression ignition

Reactivity controlled compression ignition (RCCI) was investigated on a light-duty optical engine under different fuel stratification degrees, using multiple laser diagnostic techniques. The engine was run at a speed of 1200rpm and under a load of about 7bar gross IMEP. To form different fuel stratification degrees, the direct-injection timings of n-heptane were changed, while the port-injection timings of iso-octane was kept constant. The fuel/air equivalence ratio and primary reference fuel (PRF) number were quantified by the fuel-tracer planar laser-induced fluorescence (PLIF) under non-combusting condition. The results indicated that with retarding n-heptane injection timing from −90° CA ATDC (RCCI-90 case) to −10°CA ATDC (RCCI-10 case), regions of higher fuel concentration and reactivity moved downstream to the edge of combustion chamber before high-temperature heat release (HTHR) phase. Time-resolved natural combustion luminosity imaging and single-shot OH PLIF imaging indicated that RCCI-10 case presented a staged combustion process that an auto-ignition first happened in the region of high reactivity around the combustion chamber and then another auto-ignition process took place in the region of low reactivity in the central part of the combustion chamber. The staged combustion feature involved in RCCI combustion could result in lower combustion pressure-rise rate. PLIF images of formaldehyde showed that formaldehyde first formed during low-temperature heat release (LTHR) phase in the regions where n-heptane resided. With retarding n-heptane injection timings, both formaldehyde and OH PLIF images presented more stratified distribution, and the consumption of formaldehyde and formation of OH processes got slower. OH PLIF images indicated that HTHR phase of RCCI could extend to the central part of combustion chamber. In the low-load LTC conceptual model proposed by Musculus et al. (2013), no HTHR happened and UHC formed in the central part of combustion chamber. This meant that RCCI could have less UHC emission than LTC in theory.

An investigation on the DME HCCI autoignition under EGR and boosted operation

A numerical study was conducted to investigate the autoignition mechanism for controlling combustion phasing in a homogeneous charge compression ignition (HCCI) engine fueled with DME using zero-dimensional commercial software in a detailed chemical-kinetics model and continued experimentally using single cylinder compression ignition engine. The exhaust gas recirculation (EGR) and boosting method where applied to control the combustion phenomena. The results indicate that EGR addition slows down the decomposition of hydrogen peroxide (H2O2), which contributes to the amount of high temperature heat release by reducing the rate of hydroxyl radical (OH). Since too much EGR reduces the power and raises the carbon monoxide (CO), investigations focus on the autoignition characteristics of DME at boosting with EGR and their effects on variations of autoignition timings, combustion durations in two-stage combustion process in-detail using a contribution matrix to the heat release. It was found that longer duration of cool-flame with boosting due to increased oxygen concentration in the mixture, which finally enhanced the intermediate species reactivity but the duration of combustion dominantly depend on the EGR addition.

Control-oriented premixed charge compression ignition CA50 model for a diesel engine utilizing variable valve actuation

Premixed charge compression ignition (PCCI) is a promising combustion strategy for reducing in-cylinder NOx and particulate matter formation in diesel engines without incurring fuel penalty. However, one of the challenges in PCCI implementation is that the process does not allow direct control of the combustion timing. The crank angle of 50% heat release, known as the CA50, is generally a reasonable proxy for the quality of combustion in terms of maximum pressure rise rate, combustion noise, and fuel conversion efficiency. This paper outlines the development, and validation, of a real-time capable estimation strategy for diesel-fueled PCCI CA50 using production-viable measurements that do not include in-cylinder pressure. The CA50 estimation strategy considers both stages of diesel-fueled PCCI combustion—low-temperature heat release and high-temperature heat release, which contributes most to the cumulative heat released during combustion. The strategy is validated using a PCCI CA50 dataset generated with a wide range of positions of a variable geometry turbocharge, exhaust gas recirculation fractions, and intake valve closing timings. The model estimates CA50 within ±2 CAD for 65 out of 80 data points and exhibits an error standard deviation of 2.55 CAD.

Autoignition studies of C5 isomers in a motored engine

This study explores the autoignition characteristics of three C5 isomers, namely n-pentane, 2-methylbutane (iso-pentane) and 2,2-dimethylpropane (neo-pentane). These measurements are intended to enhance understanding of C5 autoignition chemistry, and provide experimental data to guide improvements to a general hydrocarbon oxidation mechanism. To that end, the autoignition behavior of these three C5 isomers was investigated in a modified CFR engine at an intake temperature of 120°C and a fixed engine speed of 600rpm to determine the critical compression ratio (CCR) at which hot ignition occurs. To find the critical compression ratio, the engine compression ratio (CR) was gradually increased to the point where CO in the engine exhaust rapidly decreased and significant high temperature heat release was observed, while holding equivalence ratio constant. Fundamental ignition behaviors such as the CCR and the calculated percentage of low temperature heat release (%LTHR) demonstrate the impact of chain length and methyl substitutions on ignition reactivity. The %LTHR shows a stronger two stage heat release for n-pentane than for neo-pentane observed at critical ignition conditions. In contrast, single stage heat release is observed for iso-pentane, leading to the weakest overall oxidation reactivity of the three isomers. Key reaction paths forming conjugate alkenes and C5 oxygenated species control the autoignition reactivity of n-pentane and iso-pentane within the low temperature and NTC regimes. However, neo-pentane forms no conjugate alkene due to its unique molecular structure, and instead produces iso-butene to retard its oxidation.

Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: Chemical aspects

Chemical aspects of the ignition of a primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated by analyzing two-dimensional direct numerical simulation (DNS) data with chemical explosive mode (CEM) analysis. CEMA is adopted to provide fundamental insights into the ignition process by identifying controlling species and elementary reactions at different locations and times. It is found that at the first ignition delay, low-temperature chemistry (LTC) represented by the isomerization of alkylperoxy radical, chain branching reactions of keto-hydroperoxide, and H-atom abstraction of n-heptane is predominant for both RCCI and SCCI combustion. In addition, explosion index and participation index analyses together with conditional means on temperature verify that low-temperature heat release (LTHR) from local mixtures with relatively-high n-heptane concentration occurs more intensively in RCCI combustion than in SCCI combustion, which ultimately advances the overall RCCI combustion and distributes its heat release rate over time. It is also found that at the onset of the main combustion, high-temperature heat release (HTHR) occurs primarily in thin deflagrations where temperature, CO, and OH are found to be the most important species for the combustion. The conversion reaction of CO to CO2 and hydrogen chemistry are identified as important reactions for HTHR. The overall RCCI/SCCI combustion can be understood by mapping the variation of 2-D RCCI/SCCI combustion in temperature space onto the temporal evolution of 0-D ignition.

Effects of EGR and boosting on the auto-ignition characteristics of HCCI combustion fueled with natural gas

In this study we computationally investigate the effects of exhaust gas recirculation (EGR) and boosting to clarify the auto-ignition mechanisms of homogeneous charge compression ignition (HCCI) in natural-gas-fueled engines. CHEMKIN-PRO is used to perform the thermodynamic and chemical kinetics analysis. To set the boundaries of the operating range, intake air pressures are varied from naturally aspirated operation to boost pressure (0.3 MPa) and EGR ratios are varied from 0% to 50% at an equivalence ratio of φ = 0.4, intake air temperature of Tin = 430 K, and engine speed of 1200 rpm. Natural gas has a single stage heat release, which results in a high temperature heat release (HTHR). The HTHR is composed of two parts, the thermal ignition preparation and thermal ignition range, in which fuel series reactions, H2O2 loop reactions, and H2-O2 system reactions occur. The reaction paths and contribution ratios that occur within the transient temperature can be described by a contribution matrix. We found that when the EGR ratio is increased, auto-ignition is retarded with a longer duration of combustion. The absolute heat release rate (HRR), forward and reverse reaction rates of reactions, and transient temperature during the thermal ignition stage are decreased, as are misfires. When the intake air pressure is increased, the rates of important reactions also increase rapidly. The auto-ignition points are advanced by a crank angle degree (CAD) of 6°. In the combined case of 50% EGR and 0.2 MPa boosting, all reactions are sped up and the absolute heat release rate (HRR) is higher than in the naturally-aspirated (NA) condition. However, R1 is extremely sensitive to intake air pressure. In the case of NA (no EGR and 0.1 MPa), auto-ignition occurred at a higher temperature (approximately 40 K). The absolute HRR decreased while increasing the boost and EGR ratio, and the forward and reverse reaction rates of most reactions increased.

Homogeneous charge compression ignition (HCCI) combustion of polyoxymethylene dimethyl ethers (PODE)

Polyoxymethylene dimethyl ethers (PODE) are a promising alternative fuel for diesel engines. PODE have high volatility, high ignitability and high oxygen content, and is thus also an ideal additive fuel for blend and dual-fuel combustion due to its low-temperature chemistry. In this work, the characteristics of PODE homogenous charge compression ignition (HCCI) combustion are investigated for the first time. The effects of charge mass equivalence ratio (Φm) and exhaust gas recirculation (EGR) on PODE HCCI are studied. The results indicate that PODE HCCI exhibits two-stage ignition with a strong low temperature heat release (LTHR) before the high temperature heat release (HTHR). HTHR switches from one-stage to two-stage with an increase of Φm due to rapid CO oxidation. At a specific EGR rate, with an increase of Φm, the end-of-compression charge temperature decreases, the ignition timing of LTHR delays. With an increase of Φm, the ignition timing of the HTHR advances at the EGR lower than 42%, but it delays at the EGR of 52% in general. For a specific Φm, with an increase of EGR, the end-of-compression charge temperature decreases, the ignition timing of both LTHR and HTHR delays, and the combustion duration of the HTHR increases. CO emissions decrease with an increase of Φm and a decrease of EGR. Φm and EGR have only a slight effect on HC emissions. This work also provides fundamental data of PODE combustion characteristics for the future development of PODE reaction mechanisms.

Durable flame retardant finish for silk fabric using boron hybrid silica sol

A hybrid silica sol was prepared via sol gel method using tetraethoxysilane (TEOS) as a precursor and boric acid (H3BO3) as flame retardant additive and then applied to silk fabric. In order to endow silk fabric with durable flame retardancy, 1,2,3,4-butanetetracarboxylic acid (BTCA) was used as cross-linking agent for the sake of strong linkage formation between the hybrid silica sol and silk fabric. The FT-IR and XPS analysis demonstrated the Si-O-B formation in the sol system, as well as the linkage between the sol and silk after the treatment. The limiting oxygen index (LOI) and smoke density test indicated good flame retardancy and smoke suppression of the treated silk fabrics. The micro calorimeter combustion (MCC) test and thermo gravimetric (TG) analysis showed that the treated samples had less weight loss in the high temperature and lower heat release rate when burning. The washing durability evaluation results indicated that there was a distinct improvement for the silk samples treated with BTCA even after 30 times washing. In addition, the influence of the processing order of BTCA and silica sol treatment on the limiting oxygen index (LOI) of the finished silk fabric was also investigated. And the results demonstrated that the sample treated with BTCA first and then with the silica sol exhibited better LOI value (32.3%) than that of the sample by the conversed treatment order. Moreover the tensile property of treated samples was nearly unchanged, but the handle of sol treated samples obviously decreased.

A Study on the Autoignition Characteristics of DME–LPG Dual Fuel in the HCCI Engine

This study investigated the potential increase of engine power through the mixture of dimethyl ether (DME) and liquefied petroleum gas (LPG) with homogeneous charge compression ignition combustion. The effects of mixing ratio of DME/LPG at a constant intake temperature were confirmed experimentally in a single-cylinder diesel engine. A numerical analysis was conducted through the detailed chemical kinetics by using CHEMKIN-PRO for the mixing model of DME and n-butane to clarify the underlying mechanism of autoignition, while considering the gas mixture heat loss to the cylinder wall to compare with the experiment. The results show that the increased amount of LPG reduces the low-temperature heat release and activates the high-temperature heat release, which increases in-cylinder pressure. Therefore, it has potential to raise the indicated mean effective pressure when mixing ratio was adjusted appropriately. Also, thermal efficiency was increased to 51.2% at the mixing ratio of 0.6. Finally, engine out emissions including total hydrocarbon increased a small amount and carbon monoxide was decreased to almost zero when the mixing ratio of DME decreased until the combustible zone. Numerical results agreed with the experiment, indicating weakened low-temperature oxidation from the increase of the n-butane amount.

Experimental study on partially premixed compression ignition combustion fueled with a low octane number primary reference fuel using two-stage fuel supplying

An experimental study of partially premixed compression ignition combustion with low octane fuel was conducted on a single-cylinder engine. The effects of the external exhaust gas recirculation and intake boost on this partially premixed compression ignition combustion and emissions were investigated. During the experiments, a trade-off relationship between the NOx and smoke emissions was observed in this partially premixed compression ignition combustion. However, heavy exhaust gas recirculation usage has the potential to decrease NOx and soot emissions simultaneously at the expense of the fuel economy. It is determined that at an increased intake port pressure, the maximum in-cylinder pressure increases, and the ignition timing of the high-temperature combustion is retarded. Also, the peak value of the low-temperature combustion is slightly depressed, the peak value of the high-temperature heat release decreases significantly, and the maximum value of the diffusing burn increases to some extent. Compared to natural aspirated condition, the partially premixed compression ignition combustion with intake boost has the capability of simultaneously reducing NOx and soot emissions to ultra-low levels, that is, the intake boost could be an important strategy for the combustion and emission improvement in this advanced engine combustion mode.

Effect of oxygen concentration on highly diluted charge compression ignition combustion in an optical engine

To implement low temperature diesel combustion, the charge dilution with nitrogen was used in an optical engine to simulate exhaust gas recirculation of a metal engine. In-cylinder visualization is focused on a regime of the simultaneous reduction in soot and nitrogen oxides (NOx) emissions, which is demonstrated by the metal engine experiment. Injection timing was varied from 30 to 20° crank angle (CA) before top dead center (BTDC) by 5° increment, and the oxygen concentration was between approximately 8 and 11%. For an injection timing of a 20° CA BTDC, approximately 10% oxygen concentration showed two peaks of natural luminosity: one corresponds to the OH chemiluminescence from high temperature heat release and the other coincides with soot luminescence in a diffusive flame. Both peaks are reduced as the oxygen concentration at the intake stream decrease; in addition, the luminosity corresponding to the diffusive flame is not apparent below oxygen concentration of approximately 9% in this experiment. For an injection timing of 30° CA BTDC with the oxygen concentration being similar to that of 20° CA BTDC, the luminosity is weak for OH chemiluminescence and not apparent for soot luminescence. The earlier injection timing allows a prolonged mixing period; thus, a relatively well-premixed charge resulted in reduction of soot particles emitting soot incandescence. This result is also supported by the longer ignition delay calculated from the metal engine test.

Observed Differences in Low-Temperature Heat Release and Their Possible Effect on Efficiency between Petroleum Diesel and Soybean Biodiesel Operating in Low-Temperature Combustion Mode

Low-temperature combustion (LTC) in diesel engines has emerged as an enabling technology to simultaneously reduce oxides of nitrogen and smoke emissions. Combustion development continues to face challenges, however, with high emissions of carbon monoxide and unburned hydrocarbons and lower efficiencies than conventional combustion. Study of alternative fuels, such as biodiesel, with LTC shows varied promise of improving both combustion and fuel conversion efficiencies. The results are inconsistent, depending on the type of biodiesel (e.g., palm-based biodiesel relative to soy-based biodiesel). It was originally believed that differences in low-temperature heat release (LTHR) influence the phasing of main heat release (high-temperature heat release, or HTHR), thus creating differences in fuel conversion efficiencies among petroleum diesel and different types of biodiesels. This study attempts to address the issue of seemingly different LTHR behavior for different types of petroleum and biodiesel fuels between a baseline and LTC modes. Further, the study attempts to identify the effect, if any, that LTHR has on main heat release timing and rate. The study suggests, based on experimental analysis and reliance on observed behavior in literature, the roles that liquid and flame lift-off lengths and fuel chemical composition have on the appearance of LTHR in heat release profiles. The study further shows a disconnect between the extent of LTHR and the subsequent timing and rate of HTHR. Consequently, other factors seem to drive the phasing and extent of HTHR (which likely include the aforementioned parameters of liquid and flame lift-off lengths), which has a stronger influence on engine efficiency than the rate and timing of LTHR.

Combined Impact of Branching and Unsaturation on the Autoignition of Binary Blends in a Motored Engine

The impact of a branched and unsaturated compound (diisobutylene) mixed with simple hydrocarbons such as n-heptane and isooctane in binary blends on the autoignition behavior were investigated in a modified cooperative fuel research (CFR) engine at an equivlanece ratio of 0.5 and intake temperature of 120 °C. From this test condition, a homogeneous charge of fuel and intake air can be achieved. The test fuels were prepared by addition of 5–20 vol % diisobutylene into n-heptane and isooctane. The engine compression ratio (CR) was gradually increased from the lowest point to the point where significant high temperature heat release (HTHR) was observed, and this point is also referred to as the critical compression ratio (CCR). Heat release analysis showed that each n-heptane blend had a noticeable low temperature heat release (LTHR), which was not observed in the isooctane blends. The gradual addition of diisobutylene into each primary reference fuel contributed to retarded high temperature heat release in these binary blends, increasing the in-cylinder temperature and decreasing formation of CO. The 15 and 20 vol % blends of diisobutylene in isooctane were not able to reach high temperature heat release in the CFR engine system under these test conditions. The fundamental ignition behavior such as CCR and calculated % LTHR show the impact of the presence of the C–C double bond on ignition reactivity. Species concentration profiles obtained in condensed products from the engine exhaust were measured via gas chromatrography–mass spectrometry and −flame ionization detector. The major intermediate species for each blend were captured at a compression ratio selected just before the high temperature heat release was observed. Most intermediate species were derived from n-heptane and isooctane, while diisobutylene rarely participated in forming any major species, with the exception of the formation of 4,4-dimethyl-2-pentanone. Addition of diisobutylene exhibited opposite trends with regard to the abundance of intermediate species when blended with n-heptane versus with isooctane, due to the different degree of oxidation reactivity of n-heptane in comparison to isooctane.

Experimental Studies of High Efficiency Combustion With Fumigation of Dimethyl Ether and Propane Into Diesel Engine Intake Air

This work explores the role of the ignition quality of a fumigated fuel on combustion phasing and brake thermal efficiency (BTE), which was investigated in a 2.5 l turbocharged common rail light-duty diesel engine. Different combinations of dimethyl ether (DME) and propane were fumigated into the intake air and displaced some of the directly injected ultralow sulfur diesel fuel (ULSD) needed to maintain the engine and a constant speed and load. Fumigation of DME and propane significantly increased BTE and reduced brake specific energy consumption (BSEC) compared to the baseline diesel condition with no fumigation. A mixture of 20% DME with 30% propane provided the maximum BTE, with 24% reduction in BSEC, however, at the expense of increasing peak cylinder pressure by 6 bar, which was even higher at greater DME substitutions. Fumigated DME auto-ignited early, ahead of top dead center (TDC), showing the typical low temperature heat release (LTHR) and high temperature heat release (HTHR) events and propane addition suppressed the early LTHR, shifting more of the DME heat release closer to TDC. Total hydrocarbon (THC) emissions increased, while NOx emissions reduced with increasing fumigation.

Investigation of the combustion and emission characteristics of partially premixed compression ignition in a heavy-duty diesel engine

Partially premixed compression ignition in a diesel engine is a combustion mode between diffusion combustion and homogeneous charge compression ignition combustion, the combustion controllability and the emission performances of which are close to those of a homogeneous charge compression ignition combustion engine; the mixing period plays a key role in realizing partially premixed compression ignition, and a long premixed process before combustion is necessary to make the fuel and air mix well. Partially premixed compression ignition combustion in a boosted six-cylinder heavy-duty diesel engine is realized by adjusting the injection timing and the rate of exhaust gas recirculation based on a single injection. The effects of the injection timing, the exhaust gas recirculation rate and the load rate on the combustion and the heat release pattern are studied. The factors which influence the mixing period and the ignition delay and its regularity are researched. Endoscope technology and the two-colour method are also used to gain an insight into partially premixed compression ignition combustion. The results show that both early injection and late injection have a long mixing period, which helps to form a more homogeneous mixture, and no diffusion combustion is found in the heat release rate curves. Premixed combustion and low-temperature combustion are the key factors in reducing the particulate matter emissions and nitrogen oxide emissions simultaneously. However, the low-temperature combustion and the dilute mixture may lead to incomplete combustion; consequently, for relatively late-injection conditions, the hydrocarbon and carbon monoxide emissions increase dramatically and the fuel consumption becomes worse. In these partially premixed combustion patterns, the effect of the injection pressure on the particulate matter and nitrogen oxide emissions is not clearly observable. At lower load rates, partially premixed compression ignition combustion displays a low-temperature and a high-temperature two-stage heat release. When the engine load rate is increased to 50%, diffusion combustion appears in the early-injection modes, which leads to higher nitrogen oxide and particulate matter emissions.