Low Temperature Combustion Concepts Research Articles

Study of spray/wall interaction in transition zones from HCCI via PPC to CI combustion modes

Low-temperature combustion concepts like homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) offer the benefit of higher engine efficiency while keeping lower exhaust emissions. In this study, a full-transparent optical engine is employed to investigate the effect of fuel spray/wall interactions on combustion characteristics for transition in combustion mode from HCCI, via PPC to CI. Planar laser-induced fluorescence (PLIF) imaging of acetone and high-speed natural flame luminosity imaging techniques are applied to visualize fuel spray/wall interactions, fuel distribution, and combustion development processes. The combustion phasing, IMEP as well as engine-out emissions are analyzed to study the relationships between fuel spray, combustion, and emissions. The experimental results show three distinct zones with different fuel injection timings: the transitional zone between HCCI and PPC, the PPC zone, and the transitional zone between PPC and CI. The fuel/wall impingement position, fuel penetration and distribution, as well as combustion stratification, are investigated in these three zones. Both CA10 (the crank angle when 10% of the overall heat is released) and CA50 followed similar trends at different injection timings and present obvious “bowl” shapes in the PPC zone. Moreover, more fuel distribution near the cylinder wall and crevice is related to the increased UHC emission. The trade-off trend between UHC and NOx emissions, as well as the CO and CO2 emissions during the combustion modes transition from HCCI to CI via PPC, are investigated and analyzed based on the fuel/wall interaction and combustion processes.

Experimental study of the combustion and emission characteristics of oxygenated fuels on a heavy-duty diesel engine

Bio-derived alcohol fuels and oxygenated aromatic hydrocarbons are interesting alternative fuels applicable in low temperature combustion concepts due to their low reactivity. In this paper, three oxygenated fuels are mixed with n-heptane respectively as binary fuel blends to investigate combustion and emission characteristics of the oxygenated fuels with different molecular structures. The n-butanol/n-heptane blend has an identical stoichiometric air/fuel ratio as benzaldehyde/n-heptane blend while anisole is used to assess the importance of the aromatic structure. Standard diesel is also used as a baseline fuel. Experiments are performed on a single-cylinder heavy-duty diesel engine at medium and high loads. It is found that the n-butanol/n-heptane blend presents a higher premixed burn fraction and a higher pressure rise rate at both loads. Increasing injection pressure leads to lower soot emissions and higher NOx emissions are found regardless of fuel type. The n-butanol/n-heptane blend achieves 0.027 g/kWh and 0.2 g/kWh for engine-out soot and NOx emissions with high exhaust gas recirculation rate at high load. For the benzaldehyde/n-heptane blend and diesel, the classical NOx/soot tradeoff is observed under the same operating conditions. The particle size distribution mainly shows accumulation mode particles increase at a high exhaust gas recirculation rate, irrespective of the fuel structure. Only the n-butanol/n-heptane blend emits more nucleation mode particles and much less accumulation mode particles compared to the benzaldehyde/n-heptane and anisole/n-heptane blends. Clearly, the fuel structure influences the soot formation and is therefore not fully determined by the stoichiometric air/fuel ratio of the base fuel.

Two-stage ignition behavior and octane sensitivity of toluene reference fuels as gasoline surrogate

Current approaches to improve the efficiency of Spark-Ignition (SI) gasoline engines have been focusing on turbocharging, increasing the compression ratio, and pursuing advanced low-temperature combustion concepts. In order to maximize these strategies, it is important to optimize the knock resistance of the fuel, and therefore knowledge of the sensitivity of the ignition process under a wide range of engine operating conditions is required. Octane sensitivity (OS), which is defined as the difference between Research Octane Number (RON) and Motored Octane Number (MON), has been introduced to represent how fuel's ignition reactivity changes relative to the primary reference fuels (n-heptane/iso-octane) within RON/MON conditions. Previous works have indicated that OS is intimately related to low temperature reactivity of the fuel, which can be revealed as two-stage heat release characteristics during an ignition event. Prompted by these findings, in this paper, we investigate the relationship between two-stage ignition behavior and OS, using chemical kinetic simulations of 24 Toluene Reference Fuels (TRFs)/ethanol blends. TRFs are ternary mixtures of n-heptane/iso-octane/toluene, which is capable of capturing aromatic content and positive values of OS of real gasoline fuels. Simulation results show that fuels with weak or no two-stage ignition behavior tend to have high OS, due to their lack of Negative Temperature Coefficient (NTC) effect and high sensitivity in ignition delay time. Leveraging such observations, we develop a correlation between two-stage behavior and OS as an OS prediction method. Two metrics that represent the strength of the two-stage ignition behavior are proposed and used as OS predictors, which are Low Temperature Heat Release percentage (LTHR%) and Heat Release Rate at the end of first stage (HRRinf) calculated from a simple kinetic simulation. Regression analysis shows a clear trend between decreases in the proposed two-stage behavior metrics and increases in the value of OS of the fuel. We also test the new metric (LTHR%) using simulation results of 0-D reactors with imposed pressure time histories obtained from engine experiments, as well as using different TRF kinetic mechanisms. The results demonstrate the effectiveness of the metric as a representation of the two-stage ignition behavior in practical combustion systems, highlighting the importance of the proposed relationship, and its potential as a simple and effective OS predictor.

Effects of low-pressure EGR on gaseous emissions and particle size distribution from a dual-mode dual-fuel (DMDF) concept in a medium-duty engine

The application of a low-temperature combustion concept, such as RCCI combustion under real engine operating conditions is extremely complex. However, the implementation of the dual-mode dual-fuel (DMDF) strategy allows operating in low-medium load with the RCCI combustion and in high load with dual-fuel diffusive combustion. This allows taking advantage of the benefits of RCCI combustion as the simultaneous reduction of PM and NOx emissions. However, there are still serious challenges that required to solve, such as the high-pressure rise rate and the excessive CO and HC emissions. In this sense, this work shows how the implementation and an adequate adjustment of the cooled LP-EGR rate significantly minimize these problems and also shows how the LP-EGR has a greater impact on the DMDF than on the CDC concept.This work has been performed in a modern medium-duty diesel engine fueled with standard gasoline and diesel fuels, with which a cooled LP-EGR loop has been coupled. A TSI Scanning Particle Sizer (SMPS 3936L75) was used to measure the particles size distribution and the Horiba MEXA-ONE-D1-EGR gas analyzer system to determine gaseous emissions. A parametric variation of the LP-EGR rate was experimentally performed to analyze the effect over each combustion process that encompasses the DMDF concept (fully premixed RCCI, highly premixed RCCI and dual-fuel diffusion) and its consequent impact on gaseous and particle emissions. In addition, results were compared against the CDC concept to state the benefits of the DMDF concept. Among the different results obtained, it can be highlighted that during the RCCI strategy the increase in LP-EGR rate provided a reduction in NOx emissions. Nonetheless, unlike that fully premixed RCCI in highly premixed RCCI combustion, the PM emissions increased with this increment in the LP-EGR rate, shifting the size distribution of particle toward larger sizes, but decreasing the HC and CO emissions. Finally, with the exception of the high HC and CO emissions in fully premixed RCCI, in all the combustion strategies of the DMDF concept, a reduction of the analyzed pollutants was observed when compared with the CDC mode.

Intake charge temperature effect on performance characteristics of direct injection low-temperature combustion engines

The low-temperature combustion (LTC) concept presents solutions for simultaneous reduction in pollution and fuel consumption of internal combustion engines. However, the most important challenges of this concept are the lack of any direct combustion phasing control method and the limited operating range. Two LTC concepts, namely direct injection (DI) homogeneous charge compression ignition (HCCI) and partially premixed combustion, together with DI diesel can be combined to provide the desired performance for the whole load range. Although this combination provides the solution for the limited operating range, there still remains the problem with the method for combustion phasing control. Inlet temperature modulation may be used to regulate the combustion phasing. Therefore, this study investigates the effect of the intake charge temperature on the DI-HCCI combustion. The investigations are based on a novel two-stage combustion model. The combustion is predicted with the single-zone approach from the intake valve closing to the end of injection. Thereupon, it is predicted by the multi-zone approach. The results indicate that intake charge temperature has profound effects on in-cylinder charge pressure and temperature. It alters heat release rate magnitude and phasing, thus changing combustion timing. Moreover, this parameter affects indicated delivered power of the DI-HCCI combustion engine.

Effects of combustion timing on pressure rise rates in a residual effected HCCI engine

Realization of a low temperature combustion concept in homogeneous charge compression ignition (HCCI) engines is a cutting-edge technology that offers clean combustion in parallel with high thermal efficiency. Low combustion temperature prevents from NOx for-mation whereas homogeneous mixture assures smokeless exhaust. However, achieving the production feasibility by HCCI technology is hampered by high pressure rise rates and the resulting combustion noise at a high load operation. This paper explores combustion tim-ing parameters that are capable of maintaining permissible levels of pressure rise rates under a high load regime. On the basis of exper-imental data collected at a high load HCCI operation, pressure rise level was correlated with combustion duration. Furthermore, com-bustion duration has been found to scale with in-cylinder volume, for which 50% of mass fraction burned appeared. The results showed quantitatively limitations of engine load, pointing out on required combustion timings to achieve acceptable combustion harshness de-pending on engine load.

Experimental investigation of iso-butanol/diesel reactivity controlled compression ignition combustion in a non-road diesel engine

As non-road engine emission control programs are developed largely in number of countries around the world, adopting a high efficiency and low emission combustion techniques has become essential. Flexible fuel operation using in-cylinder blending technique is a primary option for using higher octane fuel in a non-road diesel engine to mitigate emissions and fossil fuel dependency. In the present study, iso-butanol/diesel dual fuel reactivity controlled compression ignition concept is preferred considering the benefits of iso-butanol over other alcohol fuels and better combustion control over other low temperature combustion concepts. An experimental investigation is performed on a suitably modified single cylinder non-road diesel engine for the entire load range (0–100% engine full load) focusing on the effect of operating parameters (iso-butanol mass fraction, diesel injection timing, diesel injection pressure, diesel mass in split injection, and intake air temperature) on the dual fuel combustion, performance and emission characteristics. The reactivity controlled compression ignition combustion is analyzed at each load condition and the best operating points are selected based on brake thermal efficiency and emissions. The results indicated that increase in iso-butanol mass fraction with advanced start of injection of direct injected diesel fuel showed a 99% reduction in oxides of nitrogen and a 91% reduction in smoke emissions with 4.5% increase in brake thermal efficiency. Further the drawback in dual fuel combustion, hydrocarbon emission is reduced by 65% with an increase of intake air temperature from 303 K to 338 K. Also, both hydrocarbon and carbon monoxide emissions are reduced with increase in diesel injection pressure and mass of diesel fuel injected in first injection due to the enhanced mixture formation.

Multi-input–multi-output optimization of reactivity-controlled compression-ignition combustion in a heavy-duty diesel engine running on natural gas/diesel fuel

The aim of this study is to implement the multi-input–multi-output optimization of reactivity-controlled compression-ignition combustion in a heavy-duty diesel engine running on natural gas and diesel fuel. A single-cylinder heavy-duty diesel engine with a modified bathtub piston bowl profile is set on operation at 9.4 bar indicated mean effective pressure and running at a fixed engine speed of 1300 r/min. A certain amount of diesel fuel mass per cycle is fed into the engine at a fixed equivalence ratio without any exhaust gas recirculation. The optimization targets include reduction in engine emissions as much as possible, avoiding diesel knock occurrence, and achieving low temperature combustion concept with the least or no engine power losses. To implement the optimization, the effects of three control factors on the engine performance are assessed by the design of experiment concept—fractional factorial method. These selected control factors are intake temperature and intake pressure (both at intake valve closing) and the diesel fuel start of injection timing. Some randomized treatment combinations of chosen levels from the three selected control factors are employed to simulate reactivity-controlled compression-ignition combustion. Based on the engine’s responses derived from the simulation, reactivity-controlled compression-ignition combustion’s mathematical model is identified directly using an artificial neural network. Next, an optimization process is conducted using two different optimization algorithms, namely, genetic algorithm and particle swarm optimization algorithm. For assessing and validating the obtained optimal results, the obtained data are used to simulate reactivity-controlled compression-ignition combustion as the engine input factors. The results show that the proposed artificial neural network design is effectively capable of identifying reactivity-controlled compression-ignition combustion’s mathematical model. Also, by optimizing reactivity-controlled compression-ignition combustion through different optimization algorithms, the optimal range of the engine operation at 9.4 bar indicated mean effective pressure is well estimated and extended.

Cost Effective BS-VI Solution - A Combined Low Temperature Combustion and Conventional Diesel Combustion Concepts

<div class="section abstract"><div class="htmlview paragraph">Air pollution in India and also global warming are two major concern in the country. To address this situation, India is moving from BS-IV to BS-VI for on-road applications with 90% reduction in NOx and 50% in PM with limit on particulate number. Also moving to Trem-IV and Trem-V for off-road applications subsequently. It needs higher efficiency after-treatment systems like SCR and DPF to achieve such lower emission levels. Addition of these complex after-treatment system, severely increase the cost of diesel power plant with heavy penalty on fuel economy. Hence, it is challenge to auto industry to reduce the complexity and cost, so that it requires an alternate solution to reduce NOx and PM emissions at source to reduce cost and system complexity. Low Temperature Combustion (LTC) is a potential concept to reduce the NOx and PM emissions simultaneously. LTC concept was experimentally demonstrated on engines of passenger car and light commercial segments, but not much work carried out for heavy duty engines.</div><div class="htmlview paragraph">The current research work focused on development of LTC concept to reduce both NOx and PM emissions to a required low levels with fuel efficiency improvement so that aftertreatment system cost and complexity will be reduced to a great extent for BS-VI norms. Extensive simulation work carried out in this research for selecting the combustion system including fuel injection system (nozzle configuration, injection pressures and multiple injection), air handling system (air-fuel ratio, EGR%, and swirl) and piston bowl (combustion bowl geometry, compression ratio) suitable to run the engine on both LTC and conventional modes. Engine was built with all modified features and experiments were performed as per BS-VI cycles (WHSC and WHTC).</div><div class="htmlview paragraph">Experiments carried out over entire engine operating zone with smooth transition between LTC modes in part loads and conventional combustion mode in high load zone. Cycle engine out emissions reduced up to 2.1 g/kWh NOx and ~0.03 g/kWh PM with ~5% overall cycle BSFC improvement in steady state WHSC cycle. Encouraging results have been achieved on transient WHTC cycle also. The lowest BSFC noticed during experiments was close to 191 g/kWh at optimized engine out NOx levels. Utmost important is reduced engine out emissions and improved fuel economy which reduces after-treatment system cost & complexity, operating, maintenance cost and improved field performance with increased regeneration intervals. This result in lower urea and fuel consumption. In summary, LTC is a cost effective solution for BS-VI.</div></div>

Thermally stratified compression ignition enabled by wet ethanol with a split injection strategy: A CFD simulation study

The potential application of partially distilled alcohol biofuels in a new, advanced, low temperature combustion (LTC) concept called Thermally Stratified Compression Ignition (TSCI) was studied in this work. TSCI, which was recently conceived, attempts to intentionally stratify the temperature distribution in the cylinder prior to ignition in a lean, premixed, compression-ignited combustion mode. Preliminary experiments used the latent heat of vaporization from the direct injection of water to stratify the temperatures prior to ignition and therefore provide control over the start and rate of combustion in LTC and expand the operating range. While these preliminary results are very encouraging, the requirement of a separate direct injector for water somewhat limits the commercial applications of this variant of TSCI (i.e. employing direct water injection). Instead, this work investigates using the latent heat of vaporization of wet ethanol to introduce the forced thermal stratification of TSCI to control the combustion process.A 3D CFD model was implemented and validated in CONVERGE with the SAGE detailed chemistry solver. This model was then used to simulate TSCI with wet ethanol employing a split injection strategy. The results indicate that TSCI with split direct injection of wet ethanol has an overall lower heat release rate and longer burn duration compared to HCCI due to the higher level of thermal stratification prior to ignition achieved through the evaporative cooling effect of the wet ethanol and its high latent heat of vaporization. Additionally, the ethanol/water blend ratio was varied to observe its effect on TSCI combustion. The results show that a higher percentage of water reduces the peak heat release rate and elongates combustion. Finally, different split injection mass fractions were simulated and the results indicated that injecting a higher mass fraction of wet ethanol in the second injection reduces the peak heat release rates and can increase the burn duration up to 68.8%. This way, TSCI with wet ethanol provides cycle-cycle control over the combustion process through adjusting the mass fraction of the first and second injection without requiring any engine hardware changes.

GTLine – Gasoline as a potential CN suppressant for GTL

The main driver to investigate low temperature combustion concepts, such as partially premixed combustion (PPC), is the promise of low particulate matter (PM) and nitric oxide (NOx) emissions. A critical prerequisite for PPC is to temporally isolate the fuel injection and combustion events. In practice, exhaust gas recirculation (EGR) is applied in order to sufficiently extend the ignition delay to that effect. Hereby, in general, higher EGR rates are necessary for fuels with higher cetane numbers (CN).Against this background, the objective of this paper is to investigate the efficacy, with respect to PM-NOx emissions and engine efficiency, of gasoline as a potential gas-to-liquid (GTL) CN suppressant in various dosages. The performance of the resulting GTLine blend will be evaluated under PPC operating conditions in a heavy-duty direct-injected diesel engine.Setting aside for a moment any potential practical issues (e.g., flash point, vapor pressure) that fall outside the scope of this study, our data suggest that blending gasoline to otherwise high CN GTL appears to be a promising route to improve not only the efficiency, but also PM and NOx emissions, particularly when operating in PPC mode. Interestingly, this benefit is notwithstanding the high aromaticity of the gasoline compared to GTL.Given the ongoing dieselization trend and associated surplus of gasoline in many regions, notably Europe, along with the fact that the cost price of gasoline is significantly lower than that of GTL, the proposed GTLine approach promises to be a cost effective way to accommodate GTL in a world wherein low temperature combustion concepts, such as PPC, appear to be really taking off.

Triple-injection strategy for model-based control of premixed charge compression ignition diesel engine combustion

Low-temperature combustion concepts are of great interest due to their potential to reduce nitrogen oxides (NOx) and soot simultaneously. However, low-temperature combustion often leads to an increase in total unburnt hydrocarbons and carbon monoxide. Furthermore, combustion sound level becomes a challenge, especially at higher loads. Various studies show that these drawbacks can be compensated by advanced injection strategies, for example, split injections. In this study, a significantly modified triple-injection approach is proposed. First, the corresponding impact on engine performance is evaluated at stationary conditions, and second, its observed advantages are evaluated at transient operation. Stationary results show that NOx, soot, and combustion sound level are simultaneously reduced without losses in fuel efficiency and without any remarkable rise in total unburnt hydrocarbon as well as carbon monoxide emissions, satisfying Euro 6 emission regulations. Under transient conditions, model-based predictive control of the engine, which allows for reliable steady-state measurements and permits validation tests at transient operating points, is successfully demonstrated for single and triple injection. With both injection strategies, control of indicated mean effective pressure, combustion phasing (CA50 (crank angle (CA) when 50% fuel is consumed)), and NOx emissions is achieved. As a result of this work, the identified optimal triple-injection strategy leads to lower total unburnt hydrocarbon emissions and to significantly reduced combustion sound level at the same level for NOx emissions in comparison with the single-injection approach. Thus, the proposed triple-injection strategy combined with sophisticated model-based control is a promising concept for future engine emission control.

Impact of a Yttria-Stabilized Zirconia Thermal Barrier Coating on HCCI Engine Combustion, Emissions, and Efficiency

In-cylinder surface temperature has significant impacts on the thermo-kinetics governing the homogeneous charge compression ignition (HCCI) process. Thermal barrier coatings (TBCs) enable selective manipulation of combustion chamber surface temperature profiles throughout a fired cycle. In this way, TBCs enable a dynamic surface temperature swing, which prevents charge heating during intake while minimizing heat rejection during combustion. This preserves volumetric efficiency while fostering more complete combustion and reducing emissions. This study investigates the effect of a yttria-stabilized zirconia (YSZ) coating on low temperature combustion (LTC), efficiency, and emissions. This is an initial step in a systematic effort to engineer coatings best suited for LTC concepts. A YSZ coating was applied to the top of the aluminum piston using a powder air plasma spray (APS) process; final thickness of the YSZ was approximately 150 μm. The coated piston was subsequently evaluated in the single-cylinder HCCI engine with exhaust re-induction. Engine tests indicated significant advancement of the autoignition point and reduced combustion durations with the YSZ coating. Hydrocarbon and carbon monoxide emissions were reduced, thereby increasing combustion efficiency. The combination of higher combustion efficiency and decreased heat loss during combustion produced tangible improvements in thermal efficiency. When the effects of combustion advance were removed, the overall improvements in emissions and efficiency were lower, but still significant. Overall, the results encourage continued efforts to devise novel coatings for LTC.

On the effects of fuel properties and injection timing in partially premixed compression ignition of low octane fuels

A better understanding on the effects of fuel properties and injection timing is required to improve the performance of advanced engines based on low temperature combustion concepts. In this work, an experimental and computational study was conducted to investigate the effects of physical and chemical kinetic properties of low octane fuels and their surrogates in partially premixed compression ignition (PPCI) engines. The main objective was to identify the relative importance of physical versus chemical kinetic properties in predicting practical fuel combustion behavior across a range of injection timings. Two fuel/surrogate pairs were chosen for comparison: light naphtha (LN) versus the primary reference fuel (PRF) with research octane number of 65 (PRF 65), and FACE (fuels for advanced combustion engines) I gasoline versus PRF 70. Two sets of parametric studies were conducted: the first varied the amount of injected fuel mass at different injection timings to match a fixed combustion phasing, and the second maintained the same injected fuel mass at each injection timing to assess resulting combustion phasing changes. Full-cycle computational fluid dynamic engine simulations were conducted by accounting for differences in the physical properties of the original and surrogate fuels, while employing identical chemical kinetics. The simulations were found to capture trends observed in the experiments, while providing details on spatial mixing and chemical reactivity for different fuels and injection timings. It was found that differences in physical properties become increasingly important as injection timing was progressively delayed from premixed conditions, and this was rationalized by analysis of mixture stratification patterns resulting from injection of fuels with different physical properties. The results suggest that accurate descriptions of both physical and chemical behavior of fuels are critical in predictive simulations of PPCI engines for a wide range of injection timings.

Low temperature combustion strategy in an off-highway diesel engine – Experimental and CFD study

Today, low temperature combustion (LTC) is one of the promising options to meet future emission norms in diesel engines. In this study, the LTC concept is adopted to an existing off-highway diesel engine, by using high quantity of cooled exhaust gas recirculation (EGR), high fuel injection pressures and retarded fuel injection timings, to reduce nitric oxides (NOx) and smoke emissions to meet the off-highway TIER-4 final emission norms. The experiments are performed at 33, 55 and 80% loads to reduce the NOx emissions to the level of 75% of the base engine (target level) and the smoke emissions to the level of the base engine (target level). In addition, a CFD analysis has also been carried out to understand the combustion phenomenon in the LTC mode. From the experimental results, it is found that, the NOx emissions are possible to reduce to the level of 75% of the base engine, at all the three loads. At 33 and 55% loads, the smoke emissions are possible to reduce below the target level; whereas, at 80% load, they are at about 3.7 times more than the target level. Finally, it is proposed that, the existing diesel engine can be operated in the LTC mode up to about 60% load. Beyond this load, a significant increase in the smoke emissions are observed in the LTC mode, hence conventional CI mode is proposed.

Effects of exhaust gas recirculation at various loads on diesel engine performance and exhaust particle size distribution using four blends with a research octane number of 70 and diesel

Partially premixed combustion using gasoline-like fuels on compression ignition engines shows great potentials to break the soot-nitrogen oxides trade off and reduce both emissions simultaneously. By simply adjusting the dilution strategies and injection events, the control of partially premixed combustion is relatively easier compared to other low-temperature combustion concepts. However despite these advantages, recent research shows this concept tends to emit ultra-fine particles. Most previous work on partially premixed combustion only focuses on the soot emissions while the particulate matter in terms of number concentration and size distribution are not well investigated. Ultra-fine particles are dangerous to human health and are getting increasing attentions. Thus the detailed particulate matter emission from partially premixed combustion needs to be further investigated. In this work four gasoline-like ternary fuel blends are designed and experimentally tested under partially premixed combustion. The test blends all share the same two base fuels and blended with different additives. The fuel composition is varied to have the same research octane number. Tests are conducted under different engine loads and dilution strategies since the temperature and oxygen concentration are the key factors in the formation and oxidation of soot. Standard diesel is also tested under the same conditions as a comparison. It is found that these blends are capable of running under partially premixed combustion at low and medium loads and they produce near zero soot emissions when using high exhaust gas recirculation rate. However, these blends do emit smaller particles than diesel under all test loads. Besides, blends with oxygen content yield less soot emissions and smaller particles compared to non-oxygen blends.

Gaseous emissions and particle size distribution of dual-mode dual-fuel diesel-gasoline concept from low to full load

Low temperature combustion concepts are in focus of study nowadays as a method to avoid the NOx-soot trade-off existing with conventional diesel combustion. One of the most promising strategy is known as reactivity controlled compression ignition because of its high thermal efficiency and the ultra-low nitrogen oxides and soot emissions. However, this concept presents several challenges such as the high levels of carbon monoxide and unburned hydrocarbons promoted at low load and unacceptable levels of pressure rise rate at high load. Therefore, to mitigate these shortcomings the dual-mode dual-fuel concept, combining reactivity controlled compression ignition and diffusive dual-fuel diesel-gasoline combustion, has been developed.Total number of particles is also limited by the emission standards. Previous studies focused in particles emissions carried out by the research community present particle size distribution, composition and mass of the particles on reactivity controlled compression ignition combustion mode. Additional studies were carried out in order to identify the components of these particles, being partially formed of volatiles, and reflects that particles are smaller than at conventional diesel combustion, presenting higher number of particles from nucleation mode than from accumulation mode.Dual-Mode Dual-Fuel concept may present a different behavior for particle distribution with respect to the conventional diesel combustion or the traditional low temperature concepts due to the nature of the particles. The objective of the present study is to measure the particle size distribution as well as gaseous emissions of this new Dual-Mode Dual-Fuel concept from low load to full load for a representative engine speed of 1200rpm.Main results of this study suggest that Dual-Mode Dual-Fuel concept promotes higher quantity of particles than conventional diesel combustion despite of providing less smoke. In addition, nucleation mode particles dominate the particle size distribution for the new combustion concept at low load and moves towards accumulation mode domination at full load.

An investigation on the particulate number and size distributions over the whole engine map from an optimized combustion strategy combining RCCI and dual-fuel diesel-gasoline

Literature demonstrates that, for premixed low temperature combustion concepts, particulate matter cannot be directly extrapolated from soot emissions measurements, as typically done for conventional diesel combustion. This is because the particulate matter from low temperature combustion has low fraction of carbonaceous compounds and great amount of soluble organic fraction, which is not captured by the smoke measurement techniques such as the optical reflectometry. By this reason, the study of the particulate matter characteristics from this combustion techniques requires using specific equipment. The aim of the current work is to gain understanding on the particulate matter characteristics from the dual-mode dual-fuel combustion, which is an optimized combustion strategy that combines fully and highly premixed RCCI regimes at low and medium loads, and switches to dual-fuel diffusion combustion at full load. The study was performed over the whole engine map, using a 15.3:1 compression ratio medium-duty EURO VI diesel engine. In particular, the particulate number and size distributions were sampled using a scanning mobility particle sizer and a condensation particle counter, which allow measuring the size distribution and total number of particles from 5 to 250nm. Results demonstrate that the fully premixed RCCI combustion is dominated by small particles (less than 30nm in mobility diameter), the dual-fuel diffusion mode is dominated by larger particles (around 100nm in mobility diameter) showing more diesel-like particle size distributions, and the highly premixed reactivity controlled compression ignition regime shows a transitional particle size distribution with two peaks of mobility diameters around 20 and 80nm.

Partially Homogenous Charge Compression Ignition Engine Development for Low Volatility Fuels

Control of harmful pollutants such as oxides of nitrogen (NOx), particulate matter (PM)/ soot, carbon monoxide (CO), and hydrocarbons (HC) emitted by diesel engines is essential in order to protect the environment and human health. Different versions of low temperature combustion (LTC) concepts have shown their potential in controlling these pollutants, while achieving thermal efficiencies similar to conventional compression ignition (CI) engines. In this study, a diesel fueled partially premixed homogeneous charge compression ignition (PHCCI) engine was investigated to explore the feasibility of LTC combustion. All experiments were performed on a constant speed two cylinder engine prototype, in which one cylinder was modified to operate in PHCCI combustion mode and the other cylinder operated in conventional CI combustion mode. An external mixing device, called a “fuel vaporizer”, was developed for partially homogeneous fuel-air mixing. For a better understanding of the effect of intake charge temperatur...

An investigation on ignition-delay modelling for control

The ignition delay is an important quantity in low temperature combustion concepts where a prolonged ignition delay gives an enhanced fuel-air mixing process, favourable for decreased particulate and NOx emission levels. This article experimentally evaluates three different low-order physics-based correlation models and their ability to predict the ignition delay for the purpose of model-based controller design. The models relate the state of the gas mixture after the point of fuel injection in order to predict the ignition delay. The experiments were all performed on a six-cylinder direct-injection Scania diesel engine with a fuel mixture of 80 vol% gasoline and 20 vol% n-heptane. The results showed that ignition-delay variation was not easily predicted when the injection timing was varied close to top-dead-centre but that the model performance satisfactorily during intake-condition variations. The results also showed that a more complex model did not necessarily give an increased prediction performance.