Early Phase Of Combustion Research Articles

Experimental study on knock suppression by direct water injection and direct ammonia-water injection based on low-flow injector under high load

Under the appropriate injection strategies, both direct water injection and direct ammonia-water injection can effectively suppress the knock, but the effect of the knock suppression is slightly different. In this paper, water and ammonia-water were respectively injected directly into the cylinder of a high performance port gasoline injection engine by modified low-flow direct injectors. The effects of direct injection timing and injection ratio on knock, combustion and emission were investigated. Both water and ammonia-water can effectively suppress knock in SI mode. Compared to water, ammonia-water exhibits a more prominent inhibitory effect on knock at lower DI ratios. However, when the ratio was further increased, the difference between the effects of ammonia-water and water on the inhibition of knockdown became less pronounced. Water and ammonia-water DI affect the knock through different mechanisms due to their unique chemical properties. While water predominantly influences the later combustion phases due to its rapid vaporization and high specific heat capacity. Ammonia-water primarily impacts the early combustion phase because of its complex combustion process and lower laminar flame speed.

Effect of injection timings and injection pressure on knock mitigation with a compression stroke injection of hydrous ethanol in spark ignition

Direct injection fuel systems provide precise control over the amount of fuel injected and can enable higher compression ratio operation and earlier combustion phasing under knock-limited operation, particularly for fuels with a high cooling potential like hydrous ethanol, a blend of 92% ethanol and 8% water. Moving a portion of the total fuel mass from an intake stroke injection to a compression stroke injection can provide a knock suppression benefit, which can enable more efficient operation. In this work, the influence of injection pressure on this split injection spark ignition strategy is examined. The effect of injection pressure on two intake stroke injections were characterized, with an injection pressure of 200 bar improving combustion efficiency by ∼3 percentage points and advancing knock-limited CA50 by 1 crank angle degree over an injection pressure of 30 bar. Then, a compression stroke injection was introduced and swept into the compression stroke while maintaining the two intake stroke injections. Direct injections at an injection pressure of 30 bar enabled a small knock intensity reduction of ∼20%, whereas an injection pressure of 200 bar enabled a larger reduction of ∼90% in knock intensity. The spark timing advance permitted by the reduction in knock intensity with a compression stroke injection timing of −80 degrees after top dead center was 0.3 and 2.0 degrees at an injection pressure of 30 and 200 bar, respectively. Then, the second intake stroke injection was varied at 200 bar to evaluate how the stratification profile prior to the compression stroke injection impacted its ability to reduce knock intensity. It was found that compression stroke injections with an early second intake stroke injection was effective at reducing knock intensity throughout the compression stroke. As the second intake stroke injection was retarded, the early compression stroke injections became less effective at suppressing knock.

Significant catalyst savings and reaction kinetics enhancement for catalytic micro-combustors via non-uniform catalyst segmentation

A recent prioritization of energy-efficient and sustainable technologies has fueled an ongoing demand for improving the heat sources necessary to drive thermally dependent processes in portable and small-scale devices. Previous efforts have explored hydrocarbon-based catalytic combustion (and its kinetics and fuel usage efficiency) for reliable heating in microscale devices (TPVs, TEGs, etc.). However, for mesoscale systems that require a larger length scale combustor, fuel management and the ability to maintain consistent heat flux across the combustor become challenging due to kinetics and mass diffusion limitations, which are extrinsic to microscale combustors. In this work, we present the design and experimental evaluation of a compact parallel plate catalytic micro-combustor with non-uniform, segmented catalytic zones that allow for direct control of heating, spatially. Specifically, we performed a comparative FEM study of various catalyst segmentation patterns to further optimize catalyst bed spacing for decreased fuel consumption and improved combustion reaction kinetics. Due to these enhancements, the proposed patterning scheme can achieve similar temperature profiles to those of a fully-coated surface while using 77.8% less catalyst and reducing extreme thermal gradients during the early transient combustion phase. Finally, we demonstrated the capabilities of the proposed catalytic combustor by testing the combustor in a controlled environment.

Effects of methanol direct injection and high compression ratio on improving the performances of a spark-ignition passenger car engine

The high octane number and high latent heat of vaporization of methanol are beneficial to suppress knock and consequently increase compression ratio (CR) of spark ignition engines, and thus improve the thermal efficiency. Few preious research on methanol engine combuston and performances have been conducted based on spark-ignition passenger car engines with a CR greater than 14. In this study, a 1.5-liter spark ignition passenger car engine with miller cycle was used to explore the potentials of high CR and methanol direct injection (MDI) on improving engine performances. First, the engine was tested under stoichiometric combustion using methanol and gasoline at BMEPs of 1.1 MPa and 1.5 MPa with CR11.5, CR13.8, and CR15.3. Then, the performances of the CR11.5 case with gasoline and the CR15.3 case with methanol were compared under lean burn combustion. The results show that for gasoline, increasing CR from 11.5 to 15.3 results in decreased fuel economy under stoichiometric combustion, whereas the exact opposite tendency is shown for methanol. Integrating a high CR of 15.3 and MDI improves brake thermal efficiency (BTE) to 43.3 % at an excess air/fuel ratio (λ) of 1.0, when compared with 37.4 % of gasoline at CR11.5. Applying lean burn can further improve the BTE of methanol at CR15.3 to 44.9 %. The main causes of the improvement of BTE by high CR and MDI are the reduced exhaust heat loss resulted from earlier combustion phasing. However, high CR and MDI also extend combustion durations at all specified λs, and emit more THC and CO emissions than low CR with gasoline at high λs because of higher surface/volume ratio and flame quenching. The extended combustion duration and the incomplete combustion are the main barriers to greater BTE for MDI sprak ignition engine with high CR.

Effects of turbulence on variations in early development of hydrogen and iso-octane flame kernels under engine conditions

The understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.

Energy efficiency improvements and CO2 emission reduction by CNG use in medium- and heavy-duty spark-ignition engines

This research studies the mechanism of improving energy efficiency and reducing CO2 emissions from medium- and heavy-duty spark-ignition engines by replacing gasoline fuels with natural gas. Detailed experimental analyses are conducted on a multi-cylinder research engine using 91RON, 98RON gasoline, and natural gas fuels. Under 7 key operating conditions, energy efficiency breakdown is performed for each fuel to quantify the individual impact of the fuel's carbon content, pumping work, knock tendency, and fuel enrichment. Complete engine mapping is thereafter carried out with all three fuels, and CO2 emissions are compared over the emission certification test cycle. As demonstrated by the test results, the lower carbon content of natural gas is the major contributor to CO2 emission reduction. At idle and part load conditions, engine operation on natural gas shows reduced pumping loss, improved idle stability, and thus increased fuel efficiency. At high load conditions, the greater resistance to knock allows natural gas to operate at optimal combustion phasing. At the peak power condition, the earlier combustion phasing helps to lower exhaust temperatures, thereby avoiding fuel enrichment. With proper calibration, natural gas outperforms its gasoline counterparts to produce 4% better peak power performance and emits 22.5% lower CO2 emissions.

Exploring the Effects of Piston Bowl Geometry and Injector Included Angle on Dual-Fuel and Single-Fuel RCCI

Abstract Reactivity control compression ignition (RCCI) is a low-temperature combustion technique that has been proposed to meet the current demand for high thermal efficiency and low engine-out emissions. However, its requirement of two separate fuel systems (i.e., a low-reactivity fuel system and a high-reactivity fuel system) has been one of its major challenges in the last decade. This leads to the single-fuel RCCI concept, where the secondary fuel (reformates of diesel) is generated from the primary fuel (diesel) through catalytic partial oxidation reformation. Following the in-depth analysis of the reformate fuel (reformates of diesel) and its benefit as the low-reactivity fuel with diesel, the effects of the start of injection (SOI) timing of diesel and the energy-based blend ratio were also studied in detail. In this study, the effects of piston profile and the injector included angles were experimentally examined using both conventional fuel pairs (gasoline—diesel and natural gas—diesel) and reformate RCCI. A validated computational fluid dynamics (CFD) model was also used for a better understanding of the experimental trends. Comparing a reentrant bowl piston with a shallow bowl piston at a constant compression ratio and SOI, the latter showed better thermal efficiency, regardless of the fuel combination, due to its 10% lower surface area for the heat transfer. Comparing the 150-degree included angle and 60-degree included angle on the shallow bowl piston, the latter showed better combustion efficiency, regardless of the fuel combination, due to its earlier combustion phasing (at constant SOI timing). The effect was particularly prominent on reformate RCCI because of its incredibly high diluent concentration, which retards the combustion further for the 150-deg injector. Later, using convergecfd, seven different injector included angles were studied at a constant SOI. With the change in injector included angle, the region of the cylinder targeted by the fuel spray varies significantly, and it was found to have a significant impact on the combustion efficiency and the engine-out emissions. As the injector included angle changed from 60-deg to 150-deg, the combustion efficiency increased by 15% and the CO, NOx, and HC emissions decreased by 96%, 70%, and 86%, respectively.

Experimental investigation of reactivity controlled compression ignition with n-butanol/n-heptane in a heavy-duty diesel engine

Butanol is a potential alternative fuel to be applied in the internal combustion engine for its sustainability and low-sooting propensity. In this paper, n-butanol is port injected as low reactivity fuel and n-heptane is directly injected into cylinder as high reactivity fuel to achieve high thermal efficiency as well as low soot/NOx emissions. To understand the effects of charge preparation parameters and load range of this reactivity controlled compression ignition combustion, experiments are performed in a single cylinder heavy-duty diesel engine. The results show that single direct injection causes either too early combustion phasing or excessive HC/CO emissions. Increasing the inlet boosting pressure is beneficial to obtain high thermal efficiency but HC/CO emissions deteriorate remarkably. The double direct injection strategy can phase the combustion properly and obtain high gross indicated efficiency without sacrificing CO emissions too much. Additionally, a high exhaust gas recirculation rate is necessary to achieve proper control of combustion phasing as the reactivity of n-butanol is not low enough. It is found that the n-butanol/n-heptane reactivity controlled compression ignition can be operated from low to medium loads. And the sensitivity of combustion phasing to the direct injection timing decreases as the load increases. At medium–high load, combustion couldn't be phased after the top dead center, which leads to high pressure rise rates and high peak pressures. Extremely low particulate matter and NOx emissions are observed throughout this tested load range and a gross indicated efficiency over 50% can be observed from low to medium load.

Exploring the Effects of the Key Multi-Injection Parameters on Combustion and Emissions in Intelligent Charge Compression Ignition (ICCI) Mode

<div class="section abstract"><div class="htmlview paragraph">Developing advanced combustion mode has been the active area for high efficiency and ultra-low emissions of the next-generation internal combustion engines. In this paper, a series of experiments were conducted in a modified single-cylinder compression ignition engine for operating a brand-new combustion mode denoted as intelligent charge compression ignition (ICCI) mode. By using two common-rail systems, commercial gasoline and diesel were alternately directly injected into the cylinder through multi-injection strategies in the injection timing range of 50~320 °CA BTDC. Thus, the in-cylinder stratified condition can be flexibly and accurately adjusted in this unique combustion mode. The key injection parameters, such as gasoline injection timing and diesel split ratio, were investigated to explore their effects on engine combustion, emissions, and fuel consumption. The results showed that the diesel split ratio mainly affected combustion phasing, while the gasoline injection timing had significant effects on the peak value of in-cylinder pressure and pressure rising rate. Higher diesel split ratio in early injection caused earlier combustion phasing and higher in-cylinder temperature, leading to higher NOx emissions, but the accumulation mode particle was decreased. Besides, when more gasoline was injected in compression stroke to form more homogeneous cylinder condition, lower NOx emissions of 0.1 g/kWh were reached, meeting EURO VI standard. Furthermore, lower CO emissions and fuel consumption can be obtained simultaneously, while the accumulation mode particle decreased along with nucleation mode increasing. However, peak pressure and peak pressure rising rate were increased because of more concentrated heat release. HC emissions of all the experimental cases had few vary in the experiment.</div></div>

Experimental Study of the Effect of Start of Injection and Blend Ratio on Single Fuel Reformate RCCI

Abstract A new concept of single fuel reactivity-controlled compression ignition (RCCI) has been proposed through the catalytic partial oxidation (CPOX) reformation of diesel fuel. The reformed fuel mixture is then used as the low reactivity fuel and diesel itself is used as the high reactivity fuel. In this paper, two reformate mixtures from the reformation of diesel were selected for further analysis. Each reformate fuel mixture contained a significant fraction of inert gases (89% and 81%). The effects of the difference in the molar concentrations of the reformate mixtures were studied by experimenting with diesel as the direct injected fuel in RCCI over a varying start of injection timings and different blend ratios (i.e., the fraction of low and high reactivity fuels). The reformate mixture with the lower inert gas concentration had earlier combustion phasing and shorter combustion duration at any given diesel start of injection timing. The higher reactivity separation between reformate mixture and diesel, compared with gasoline and diesel, causes the combustion phasing of reformate-diesel RCCI to be more sensitive to the start of injection timing. The maximum combustion efficiency was found at a CA50 before top dead center (TDC), whereas the maximum thermal efficiency occurs at a CA50 after TDC. The range of energy-based blend ratios in which reformate-diesel RCCI is possible is between 25% and 45%, limited by ringing intensity (RI) at the low limit of blend ratios, and coefficient of variance (COV) of net indicated mean effective pressure (IMEPn) and combustion efficiency at the high limit. Intake boosting becomes necessary due to the oxygen deficiency caused by the low energy density of the reformate mixtures as it displaces intake air.

Reactive computational fluid dynamics modelling of methane–hydrogen admixtures in internal combustion engines: Part I – RANS

The effects of hydrogen addition to internal combustion engines operated by natural gas/methane has been widely demonstrated experimentally in the literature. Already small hydrogen contents in the fuel show promising benefits with respect to increased engine efficiency, lower CO2 emissions, extended lean operating limits and a higher exhaust gas recirculation tolerance while maintaining the knock resistance of methane. In this article, the influence of hydrogen addition to methane on a spark ignited single cylinder engine is investigated. This article proposes a modelling approach to consider hydrogen addition within three-dimensional reactive computational fluid dynamics in order to establish a framework to gain further insights into the involved processes. Experiments have been performed on a single-cylinder spark-ignition engine situated at a test bed and cater as reference data for validating the proposed reactive computational fluid dynamics modelling approach based around the G-Equation combustion model. Within the course of the first part, crucial aspects relevant to the modelling of the mean engine cycle are highlighted. In this article, a simplified early combustion phase model which considers the transition towards a fully developed turbulent flame following ignition is introduced, along with a second submodel considering combined effects of the walls. The sensitivity of the combustion process towards the modelling approach is presented. The submodels were calibrated for a reference operating point, and a sweep in hydrogen content in the fuel as well as stoichiometric and lean operation has been considered. It is shown that the flame speed coefficient A appearing in the used turbulent flame speed closure, weighting the influence of the turbulent fluctuating speed [Formula: see text], has to be adjusted for different hydrogen contents. The introduced submodels allowed for significant improvement of the in-cylinder pressure and heat release rate evolution throughout all considered operating conditions.

Experimental investigation of the influence of ignition system parameters on combustion behavior in large lean burn spark ignited gas engines

To achieve high efficiency and low pollutant emissions with large spark ignited gas engines, operation at high excess air ratios and high turbulence levels in the combustion chamber is necessary. Under these severe boundary conditions, it is a great challenge to reliably ignite the air-fuel mixture and keep cycle-to-cycle variation of the combustion process within reasonable limits. This paper investigates the influence of ignition system parameters on combustion behavior by analyzing the overall ignition and combustion process in detail: the ignition system, the electric arc, the early phase of combustion and finally the whole combustion process.Measurements were carried out on a large bore spark ignited single-cylinder research engine and on a specially designed arc test rig. In the research engine, ion current probes were applied around the spark plug in the spark plug sleeve so that a detailed analysis of the early phase of combustion was possible in order to assess the influence of ignition system parameters on combustion. In addition, crank-angle resolved in-cylinder pressure measurements provided information about the overall combustion process. The arc test rig enables the optical investigation of the influence of ignition system parameters on electric arc behavior under non-combustible atmospheres and at thermodynamic conditions and flow conditions similar to those in the research engine at spark timing. For consistency, the same modulated capacitive discharge ignition system and spark plug were installed on both test rigs. The combination of the two test setups facilitates in-depth understanding of the phenomena observed during ignition and combustion.Near the lean engine operating limit, increasing the nominal spark current duration to a certain level yielded faster combustion progress and lower cycle-to-cycle variation of the combustion process. The investigation of the early phase of combustion by means of the ion current measurements revealed the same trend: Increasing the nominal spark current duration leads to an increase in the flame front velocity during the early phase of combustion. The results from the arc test rig indicated that an increase in nominal spark current duration leads to an increase in the maximum of the cycle-averaged electric arc length. In total, the arc test rig results suggest that besides the increase in overall dissipated secondary energy, the increase in electric arc length is the second important factor in achieving the development of a stable flame kernel and a rapid combustion process in the engine when the nominal spark current duration is varied from 100 to 500 µs at very lean engine operation.

Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy.

Experimental investigation on performance and combustion characteristics of spark-ignition dual-fuel engine fueled with methanol/natural gas

This study aims to evaluate the performance, emissions, and combustion characteristics of a methanol/natural gas dual-fuel spark-ignition engine by conducting several experiments. A heavy-duty natural gas engine is modified by adding a methanol port-injection system to accelerate the burning rate, increase the thermal efficiency, and reduce the hydrocarbon emission. The effects of the methanol energy substitution ratio (MSR) on the brake thermal efficiency, heat release rate, peak cylinder temperature, flame development period, flame propagation period, and emissions are investigated under light, medium, and heavy loads (brake mean effective pressures of 0.387, 0.775, and 1.163 MPa, respectively) at an engine speed of 1600 rpm and a relative air/fuel ratio of 1.3. The results show that the peak cylinder pressure, peak cylinder temperature, and heat release rate increase with an increase in the MSR. A larger MSR leads to a shorter flame development period and flame propagation period as well as earlier combustion phasing. In addition, the brake thermal efficiency increases and the equivalent brake specific fuel consumption decreases significantly with methanol enrichment under the three engine load conditions. For exhaust emissions, the brake specific total hydrocarbon emission decreases while the brake specific nitrogen oxide emission increases slightly with an increase in the MSR. For cycle-by-cycle variation, the coefficient of variation in the indicated mean effective pressure increases slightly with an increase in the MSR. In addition, the peak ringing/knocking intensity is found to be 2.05 MW/m2 that is much less than the critical knocking point (5 MW/m2). Therefore, there was no knocking throughout the entire experiment.

Understanding the effect of operating conditions on thermal stratification and heat release in a homogeneous charge compression ignition engine

Thermal stratification of the unburned charge prior to ignition plays a significant role in governing the heat release rates in a homogeneous charge compression ignition (HCCI) engine. A deep understanding of the conditions affecting thermal stratification is necessary for actively managing HCCI burn rates and expanding its operating range. To that end, a single-cylinder gasoline-fueled HCCI engine was used to characterize the relationship between key operating conditions, such as intake temperature, residual gas fraction, air-to-fuel ratio, and swirl, and thermal stratification. The recently developed Thermal Stratification Analysis was applied to calculate the unburned temperature distribution prior to ignition from heat release.A comparison between re-induction of exhaust gas with an air-to-fuel ratio of 24:1 and air dilution with an air-to-fuel ratio of 43:1 shows that the presence of internal residuals increases the burn duration by 34% and broadens the temperature distribution by as much as 15%. The results from an intake temperature and combustion phasing sweep at an air-to-fuel ratio of 20:1 show that heat release rates increase with advancing CA50 phasing; however, the temperature distributions broaden by 48% when comparing the most advanced to most retarded cases. To add further insight by removing the effect of combustion phasing, an equivalence ratio sweep is compared to an intake temperature sweep. It is shown that a significant part of the broadening of the distributions can be attributed exclusively to the increased intake temperature which elevates the maximum TDC temperature while leaving the wall region unaffected. However, combustion phasing plays a role as well, with earlier combustion phasing being responsible for an additional broadening of the temperature distribution.The addition of swirl elongates the burn duration by broadening the temperature distribution, with this effect being slightly larger at earlier combustion phasings. However, swirl significantly increases heat transfer losses and reduces efficiencies by as much as 1.8 percentage points. Finally, a load sweep with compensation to ensure constant combustion phasing indicates that higher loads result in increased heat release rates and narrower temperature distributions by as much as 20%.

Surface functional groups and sp3/sp2 hybridization ratios of in-cylinder soot from a diesel engine fueled with n-heptane and n-heptane/toluene

This work compared the surface functional group (SFG) types and concentrations and sp3/sp2 hybridization ratios of in-cylinder soot samples generated by a heavy-duty diesel engine when employing n-heptane and a toluene/n-heptane mixture (20% toluene by volume) as the fuels. In-cylinder soot samples were obtained from a total cylinder sampling system, and the SFGs and sp3/sp2 hybridization ratios were analyzed using Fourier transform infrared and X-ray photoelectron spectroscopy. Despite the differences in fuel formulation, both n-heptane and n-heptane/toluene soot exhibited similar trends in terms of changes in the SFGs concentrations and sp3/sp2 hybridization ratios during the combustion process. However, the addition of toluene to the n-heptane was found to increase the concentrations of all SFGs as well as the sp3/sp2 hybridization ratio. The COH and CO group concentrations exhibited a bimodal distribution for both the n-heptane and n-heptane/toluene soot throughout the combustion process, with the concentrations peaking in the premixed and diffusion combustion phases, respectively. In contrast, the relative amounts of aliphatic CH groups decreased in the premixed combustion phase, increased in the early diffusion combustion phase, and then decreased in the subsequent combustion phase. The sp3/sp2 hybridization ratios obtained from both fuel soot were observed to initially decrease, then to increase before a decrease during the combustion process. There was a definite correlation between the sp3/sp2 hybridization ratio and the relative concentration of aliphatic CH groups.

Atom force microscopy analysis of the morphology, attractive force, adhesive force and Young's modulus of diesel in-cylinder soot particles

In this paper, the morphology, attractive force (Fat), adhesive force (Fad) and Young's modulus (EY) of diesel in-cylinder soot particles sampled by a total cylinder sampling system were studied using atom force microscopy. In each combustion phase, the equivalent diameter (ED), Fat, Fad, adhesion energy and EY present broad distributions. The pattern of ED distribution in the late combustion phase is similar to that in the late diffusion combustion phase, especially in the range of ED < 10 nm, suggesting that the particle size distribution changes little after the late diffusion combustion phase. Most isolated soot particles in each combustion phase possess a very low sphericity ratio. Three types of force curves are discovered for in-cylinder particles, and are introduced to identify the particle types. For the test carbonized soot particles, the SR appears to increase with an increase in ED in each combustion phase, and the AED shows a similar trend to that for all test particles during the combustion process. Due to the presence of plastic deformation for the nascent soot particles and the penetration for the liquid materials during the force measurement, the carbonized primary soot particles are employed for force analysis. As the combustion proceeds, the population-averaged values of Fat and Fad show opposite trends; the attractive force, which is mainly driven by the Van der Waals force, gradually decreases. During the combustion process, the population-averaged adhesion energy decreases to a minimum in the late premixed combustion phase after which it then increases. The initial increase in the population-averaged Young's modulus (AEY) is followed by a decrease before gradually increasing again from the early diffusion combustion phase. The AEY shows a negative correlation with the fringe separation distance in the soot structure.

Evolution of in-cylinder polycyclic aromatic hydrocarbons in a diesel engine fueled with n-heptane and n-heptane/toluene

Abstract This work studied the evolution of in-cylinder polycyclic aromatic hydrocarbons (PAHs) in a diesel engine fueled with n -heptane, and explored the effects of adding toluene to n -heptane on in-cylinder PAHs. In-cylinder PAHs were sampled in a direct injection diesel engine using a total cylinder sampling system. PAHs were analyzed by gas chromatography–mass spectrometry with programmed temperature vaporization in the solvent vent mode. For n -heptane, the mass of total in-cylinder PAHs ( Σ M PAHs ) increased in premixed and late diffusion combustion phases, and decreased in early diffusion and late combustion phases. Among the 16 PAHs detected, Naphthalene (Nap) was the most abundant. PAHs with two benzene rings and one five-membered ring, such as Acenaphthylene, Acenaphthene and Fluorene, were also plentiful, implying that PAHs with five-membered ring could play an important role in the growth of in-cylinder PAHs. When adding 20 vol.% toluene to n -heptane, the evolution of ΣM PAHs was varied. The Σ M PAHs for n -heptane/toluene increased in premixed and early diffusion combustion phases, and decreased in late diffusion and late combustion phases. Toluene addition reduced the ΣM PAHs in the premixed combustion phase by up to 48% but increased ΣM PAHs by 30% in the early diffusion combustion phase. In the subsequent combustion phase, the ΣM PAHs was reduced by approximately 67%. For in-cylinder individual PAHs, toluene addition generally reduced the percentages of Nap in ΣM PAHs during the engine combustion process but increased the percentages of larger PAHs, indicating that toluene addition was conducive to the growth of PAHs to form larger PAHs.

Investigation of butanol-fuelled HCCI combustion on a high efficiency diesel engine

Highly-diluted diesel homogeneous charge compression ignition (HCCI) combustion can achieve ultra-low NOx and soot emissions but its implementation is impeded by the lack of control on the ignition timing and excessively early combustion phasing (before TDC) that limit the achievable engine load and result in a reduced energy efficiency. The low volatility and high reactivity of common diesel fuels make it non-conducive for HCCI combustion; hence in this work, n-butanol that has a low reactivity and high volatility is studied for HCCI combustion on a single-cylinder high compression ratio (18.2:1) diesel engine without any modifications to the air-path system. The results indicate that n-butanol-fuelled HCCI combustion offers the benefit of ultra-low NOx and smoke emissions with minimal requirements for intake dilution through exhaust gas recirculation (EGR). The low reactivity helps in realizing an optimal combustion phasing, and thermal efficiency levels comparable to that of conventional diesel combustion are consistently achieved. At low-to-mid engine loads (4–7bar IMEP), the emissions are largely insensitive to the boost pressure, and the boost selection is primarily governed by the trade-off between the combustion instability and the thermal efficiency. At higher engine loads, both boost and EGR are required to limit the high pressure rise rates and to modulate the combustion phasing for high thermal efficiency. The load range is extended up to 10bar IMEP in n-butanol HCCI mode while maintaining ultra-low NOx and soot emissions with improved performance characteristics compared to diesel HCCI.

Combined effects of flow/spray interactions and EGR on combustion variability for a stratified DISI engine

This study investigates combustion variability of a stratified-charge direct-injection spark ignited (DISI) engine, operated with near-TDC injection of E70 fuel and a spark timing that occurs during the early part of the fuel injection. Using EGR, low engine-out NOx can be achieved, but at the expense of increased combustion variability at higher engine speeds. Initial motored tests at different speeds reveal that the in-cylinder gas flow becomes sufficiently strong at 2000rpm to cause significant cycle-to-cycle variations of the spray penetration. Hence, the fired tests focus on operation at 2000rpm with N2 dilution ([O2]=19% and 21%) to simulate EGR. In-cylinder flow, spray, and early-flame measurements are correlated to reveal their effect on the combustion variability.Results reveal two types of flow/spray-interactions that predict the likelihood of a partial burn. (1) Proper flow direction before injection with a more collapsed spray leads to high kinetic energy of the flow during injection, thus generating a rapid early burn, which ensures complete combustion, regardless of the EGR level. (2) Improper flow direction and less collapsed spray generate low flow energy during the early phase of combustion. For this second type of flow/spray-interaction, application of EGR results in a partial-burn frequency of 30%, whereas without EGR, early combustion is shown to be insensitive to flow variations. Flame-probability maps illustrate that the partial-burn cycles for operation with EGR have a weak flame development in that the flame does not develop uniformly and reliably from the spark plug. Without EGR, the flame development is more repeatable regardless of the type of flow/spray-interaction, at the expense of higher NOx emissions.