Particulates Size Distribution of Reactivity Controlled Compression Ignition (RCCI) on a Medium-Duty Engine Fueled with Diesel and Gasoline at Different Engine Speeds.
Increasing environmental problems such as global warming and air pollution from vehicles have promoted more stringent emissions regulations in different countries around the world. Vehicle manufacturers have been worked hardly for updating the engines to face these new emissions regulations without penalizing the engine performance.
Compression ignition (CI) engines are extensively used in vehicle applications due to its high thermal efficiency. However, the nitrogen oxides (NOx) and smoke emissions still represent a challenge for the research community. To reduce NOx and smoke emissions under the limits imposed by the emissions regulations, CI engines require the addition of aftertreatment systems that increase the cost and complexity of the marketed vehicles. For the NOx reduction, the injection of urea fluid upwards the selective catalytic reduction (SCR) system is required to enhance the conversion reactions, which further increases the costs for the vehicle users.
During last decades, several strategies have been explored trying to maintain the benefits of the conventional diesel combustion (CDC) engines, but reducing the costs related to engine production, urea consumption and fuel consumption increase due to the passive diesel particulate filter (DPF) regeneration . Among them, the low temperature combustion (LTC) strategies have been found able to provide a reduction of the smoke and NOx emissions and improve the efficiency simultaneously . From these, it is worthy to highlight the homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), reactivity controlled compression ignition (RCCI) or dual-fuel combustion [4,5,6].
HCCI combustion technology was widely investigated by the research community. This strategy characterizes of using fully premixed lean air-fuel mixtures to achieve high thermal efficiency, low smoke and low NOx emissions simultaneously. However, HCCI combustion process is dominated by chemical kinetics, which difficult the concept in terms of ignition control, cold start and produces excessive levels of carbon monoxide (CO) and unburned hydrocarbons (HC) . Hence, HCCI mode is limited to the partial load range . To solve this shortcoming, Bessonette et al.  suggested that low loads would require the use of high reactivity fuels and low reactivity fuels should be used at high engine loads.
PCI strategy has been also deeply studied by the research community [10-11]. PCI emerges as a solution for the weaknesses of the HCCI mode by using low reactivity fuels. In this sense, the high ignitability of diesel fuel makes it suitable to be used for low load operation, requiring excessively high exhaust gas recirculation (EGR) rates as load increases . Thus, the use of gasoline improves the heat release rate control and reduces the NOx and smoke emissions [13-14]. Different octane number (ON) gasolines were tested showing that the use of excessively high research octane number (RON) leads to unburning problems, being critical for gasolines with RON higher than 91. In this sense, gasoline presents a high resistance to ignite, which makes difficult to manage the combustion process at low load [15-16].
Park et al.  studied the effects of the fuel blends formed with diesel and gasolines. Inagaki et al.  proposed the dual-fuel PCI combustion concept feeding the engine with two fuels of different reactivity by means of separated injection systems. The different reactivity of the fuels was found to be a key factor for triggering the combustion phasing, also showing very promising results in terms of performance and emissions. Kokjohn et al.  continued this investigation and referred the combustion concept as reactivity controlled compression ignition (RCCI). This concept is the most promising LTC, achieving simultaneously low NOx and smoke emissions and high thermal efficiency under different fueling strategies  and hardware conditions . RCCI presents excellent capabilities in order to overcome the challenges found with traditional LTC strategies. RCCI provides simultaneously low NOx and smoke emissions and high thermal efficiency. This strategy allows to reach high engine load while the combustion timing is under control .
From [25,26,27] is known that RCCI is capable enough to meet EU VI in terms of NOx at steady state operation points with low levels of smoke for heavy-duty engines. However, current emission regulations also limit the number of particles as well as the particle matter (PM). CI diesel engines usually form carbonaceous particles and their measurement is often mass-based. CI particle characterization studies have been widely carried out [28-29] in order to obtain the particle size distribution (PSD). These studies suggest that CI PSD present a bimodal shape, presenting both nucleation and accumulation modes, depending on the mobility diameter. For CDC operation mode, the cutoff diameter is typically located at 50 nm. Therefore, nucleation mode particles would have mobility diameters lower than 50 nm and accumulation mode particles would contain particles with diameter larger than 50 nm. For the traditional CI engines, Kittelson et al.  stated that nucleation mode particles might contain up to the 90% of the number of the particles but less than 20% of the PM mass emissions.
Storey et al.  carried out a speciation about the particles emitted under RCCI operation. The most important results indicate that the high boiling range of diesel hydrocarbons was surely responsible for the PM mass captured on the filter media. Additional studies showed that RCCI produces lower quantity of particles compared to other LTC strategies [31-32]. In this sense, Prikhodko et al.  carried out a PM study in a light-duty engine and stated that RCCI was highly dominated by nucleation mode particles and compared the smoke results in terms of filter smoke number (FSN) and PM filter mass measurements. The main conclusions were that RCCI PM is mainly organic carbon with almost no elemental carbon. Thus, implies that it is not possible to convert FSN in PM because of the low values of FSN measured at RCCI mode  due to the organic fraction.
The present study shows the results of the gaseous and PM emissions of a medium-duty engine operating under RCCI mode at 25% engine load. These results are also compared to the results of the same engine operated under CDC mode. The novelty of the study is to study the impact of varying the engine speed from 950 rpm to 2200 rpm in a medium-duty engine. The study will focus on particles emission with a scanning mobility diameter between 5-250 nm by carrying out a PSD analysis.
Test Cell and Engine Description
RCCI operation mode has been implemented in a single-cylinder engine (SCE) adapted from a multi-cylinder engine (MCE). The MCE is a four-stroke, four in-line cylinders, medium-duty diesel engine that meets the EURO VI regulation. Main engine properties are shown in the table below.
The adaptation to operate the engine as a SCE was carried out at CMT facilities and consists of removing the after-treatment elements as well as modifying the intake and exhaust manifolds to isolate the air management of one of the cylinders. Thus, one of the cylinders can be operated under RCCI regime while the other three cylinders run under CDC mode to balance the cylinder-to-cylinder pressure peaks. To allow this, an in-house control system was developed to manage the injection settings for the RCCI cylinder, while the ECU was in charge of the other three cylinders.
As can be observed in Figure 1, the test cell is fully instrumented and the engine is monitored in real time. The air management for the SCE has been redesigned and new elements such as an intake and exhaust air conditioners have been incorporated to the test bench. Intake air conditioner is formed by a screw compressor and an air dryer stage, controlling the humidity and a heater in order to adjust the temperature. Exhaust air management is in charge of the exhaust gases, controlling the back pressure, and the EGR supply. The low pressure (LP) EGR loop is formed by a screw compressor, a diesel particulate filter (DPF), a dryer to control the humidity and a heater to adequate the temperature. Additional equipment such as the instrumentation and the injection system is also needed for the SCE. As can be observed in Figure 1, CDC side engine has a cooler for the boosted air and the corresponding instrumentation required for the correct operating and safety of the engine.
Engine-raw gas emissions were sampled with a five gas Horiba MEXA-ONE-D1-EGR analyzer. Measurements were carried out by averaging 40 s after attaining steady state operation. An AVL 415 smoke meter was used for the smoke emissions measurements. Measurements were performed by averaging three samples of 1 liter volume each with paper-saving mode off. Results of these measurements are presented in filter smoke number (FSN) units.
In-cylinder pressure was measured with a Kistler 6125C pressure transducer coupled with a Kistler 5011B10 charge amplifier. A shaft encoder with 1800 pulses per revolution is used to obtain a crank angle degree (CAD) with 0.2 CAD resolution.
From the injection system standpoint, the stock diesel injector is used to supply diesel fuel into the combustion chamber and a port fuel injector (PFI) is installed at the intake manifold to supply the gasoline fuel. Main characteristics of the injectors are presented in table 2. Both injection systems are completed with two AVL 733S in order to measure the fuel mass flow. Both injectors are driven using a National Instruments (NI) DRIVVEN control system. The in-house developed control allows to operate the engine between different engine points automatically, being able to carry out transitions between engine speeds for instance.
The low reactivity fuel selected has been gasoline with 95 RON (EN 228) and the high reactivity fuel used is the regular diesel (EN 590). The main characteristics are presented in table 3.
As shown in Figure 2, particle size distribution measurements has been carried out by using a diluter, a scanning mobility particle size (SMPS) and a condensation particle counter (CPC). The measurement equipment is located after the exhaust backpressure, which is located immediately before the exhaust settling chamber. The diluter chosen is a TSI Rotating Disk thermodiluter, which dilutes the sample by means of a rotating disk method. Every measurement unit is provided by two disks in order to obtain different number of cavities. The next step consist of a raw portion of the exhaust flow captured by the cavities driven into the mixer with particle-free air. This step ensures good stability and accuracy. Due to the volatiles could cause condensates, air sample must be heated and, hence the equipment will not confuse nanodroplets with particles. This step adapt the inlet concentration of the particles, which is required by the SMPS equipment.
DMA measurement principle consists of the ability of a particle to cross an electric field is directly related to the size of the particle. Following to this step, a (CPC) counts particles between 5-250 nm. Finally, an Aerosol Instrument Manager (AIM, TSI) software is in charge of the data acquisition and PSD processing.
RESULTS AND DISCUSSION
Results have been divided in two subsections. The first one analyzes the RCCI combustion development comparing it versus the CDC operation. The second subsection is completely focused on the particle matter analysis.
RCCI Combustion Analysis
Figure 3 presents the steps followed to reach RCCI operating conditions. The first step proposed consists of achieving the desired engine load, while the mechanical constraints in terms of maximum pressure rise rate (PRR) and the maximum in-cylinder pressure are below 15 bar/CAD and 190 bar, respectively. The second step is required to reduce smoke emissions below 0.1 FSN and NOx emissions below the limit imposed by the EURO VI regulation, 0.4 g/kWh. The third step consists of a fine tuning of the injection timing and gasoline fraction (GF) in order to reduce the fuel consumption to similar or lower values than with CDC reference. A detailed explanation of this methodology can be found at .
Tables 4 and 5 summarize the indicated specific emissions of NOx, CO, smoke and HC for RCCI and CDC modes at different engine speeds. Moreover, the indicated mean effective pressure (IMEP) and EGR rate are depicted in both tables.
The strategy followed to perform the CDC operating points is based on replicating the stock combustion settings. Small differences are observed at the intake air conditions due to the low pressure (LP) EGR loop respect from the high pressure EGR loop.
To facilitate the comparison between results from both combustion modes and foresee the emissions trends with respect to the engine speed, the emissions measurements of tables 4 and 5 have been represented in Figure 4.
Figure 5 and 6 show instantaneous profiles for the most important parameters related to the combustion development. In particular, the in-cylinder pressure, rate of heat release (RoHR) and bulk gas temperature are depicted for both combustion strategies at all the engine speeds.
From the smoke emissions standpoint, both concepts present low values. As it can be observed in Figure 4, RCCI presents a bigger difference from 1800 rpm to 2200 rpm. As depicted in Figure 3, the smoke target is set at 0.1 FSN for the RCCI concept. However, Figure 4 shows that two points (1500 rpm and 1800 rpm) exceed the maximum value despite of having a mixing time of around 4 ms. As the premixing conditions are not explaining this values of smoke obtained, the reason might be related with the nature of the particles.
Thus, this particular case would be explained in detail in the PSD section. Figure 7 shows the mixing time (CA10-EoI), the CAD where the 90% of the fuel mass is burnt (CA90), the CAD where the 10% of the fuel mass is burnt (CA10), the combustion duration (CA90-CA10) and the bulk gas temperature for both operating modes. Additionally, gasoline fraction (GF) is also shown for RCCI mode. RCCI has a clearly premixing behavior due to the mixing time, which is almost 4 times greater than the CDC mode. The combustion duration increases in angle for the RCCI mode, but the duration in time remains quite constant (1500 rpm: 1.53 ms and 2200 rpm: 1.36 ms). On the other hand, CDC cases shows an increasing combustion duration in angle, but the duration of the combustion in time decreases with the engine speed. (1500 rpm: 2.73 ms and 2200 rpm: 2.32 ms).
Figure 8 presents the maximum pressure rise rate (PRR) and the maximum in-cylinder pressure. The CDC pressure curves depicted in Figure 6 show that the in-cylinder pressure curves reach a higher peak values as the engine speed increases. Therefore, the values of IMEP presented in table 5 are maximum at the highest engine speed. RCCI cases have higher maximum pressure peaks than CDC at almost all engine speeds, only showing lower values at the two highest engine speeds. The pressure rise rate presents a similar trend, reaching the maximum value at 1500 rpm. As it can be figured out, the engine torque curve would present higher torque points for the mid-engine speed range, decreasing at lower and at higher engine speeds. However, CDC cases increase the IMEP at higher engine speeds.
Regarding NOx emissions presented in Figure 4, the values measured for RCCI mode are below EU VI limitation, while CDC mode presents emissions which exceed the standard emission regulation. Those ultra-low levels of NOx for RCCI mode suggest that are a consequence of thermal mechanisms. Despite of having higher maximum bulk gas temperatures than CDC, as it is observed in Figure 7, CDC mode promotes higher values of NOx. RCCI often presents higher values of bulk gas temperature than CDC at the same load, however CDC mode achieves higher peak local gas temperatures temperature in the rich reaction zone of the diesel spray. Hence, this difference between the peak bulk and the peak local temperatures might explain the differences of NOx emissions obtained between both combustion modes.
In Figure 4, CO and unburned HC emissions are presented. RCCI promotes higher levels of both emissions than CDC. Main differences between both operating modes are due to the gasoline injection at RCCI mode. In this sense, the gasoline fraction depicted in Figure 7 shows that more than 80% of the fuel mass injected is gasoline. The homogeneous mixture of the port fuel injected gasoline and air may produce high levels of gasoline trapped at the crevice volumes. Thus, during the combustion process, this gasoline might be not completely burnt producing high levels of CO and unburned HC.
The evolution of the CO and unburned HC might be explained with the evolution of the bulk gas temperature. The higher the bulk peak gas temperature, the lower the emissions. The inverted U-shaped curve described by the bulk peak gas temperature produce a U-shaped curve for CO and uHC emissions as it is observed in Figure 7.
Particle Size and Number Emissions
The following section summarizes the particle matter characterization for the engine points tested. The PSD measurements are depicted in figures 10, 11 and 12.
Figure 9 shows the total number of particles emitted from 5-250 nm. As it is seen, the particles number increases with the engine speed for both combustion modes, which was also observed in the smoke emissions trend up to 1800 rpm (Figure 4). The figure also shows that the particles number is very similar for CDC and RCCI. This result differs from previous works [31,32,33], in which it is seen that CDC operation mode produces higher number of particles than RCCI. The different behavior found in the current investigation can be explained considering the stock calibration and hardware of the diesel engines used for comparison. In this sense, the majority of engines used in literature are calibrated to accomplish the EU IV type approval, while in the case of the present research the engine used is a currently under production EU VI heavy-duty engine. Thus, the stock diesel calibration relies on producing high levels of NOx, reduced later by means of a twin parallel SCR system, and low levels of smoke.
Figures 10, 11 and 12 present the PSD at each engine speed for both operating modes. Tests have been performed at the same engine load (25%), while the engine speed has been divided in six steps from 950 to 2200 rpm. Thus, six graphs compare the PSD of the RCCI mode versus the CDC mode. At first sight, RCCI curves at 950 and 2000 rpm seem to be noisy for larger particles, probably due to the low particle concentrations in the diluted exhaust. Something similar occurs at the CDC curve operated at 1500 rpm, where some noise affects the smoothness of the depicted curve.
Figure 10 shows the PSD obtained at 950 and 1200 rpm. CDC shows lower particle concentrations at low mobility diameters (<30 nm), being the difference higher at 950 rpm. On the other hand, CDC shows higher particle concentrations at high mobility diameter (>30 nm). In any case, the differences in particles number are lower, suggesting that the total number of particles would be at the same order of magnitude. At 1200 rpm, the PSD curve present a bimodal shape. There is one main peak at 20 nm of diameter and a second peak at 80 nm. This second peak presents lower number of particles that the particles measured at CDC mode. Moreover, CDC case presents a peak at 60 nm of mobility diameter, lower than for RCCI mode.
Figure 11 presents PSD curves for 1500 and 1800 rpm. The trend followed is very similar to the shapes presented in Figure 9. Main differences are found at RCCI mode, with a bimodal shaped curve at 1500 rpm. Operating under RCCI, the first peak observed at the PSD curve stands at 18 nm, while the second peak is located at 90 nm of mobility diameter. These two peaks observed at the curve could be explained by the coagulation phenomenon. Particle coagulation is defined as a combination of growth by coalescence and agglomeration . The ultra-low sized particles of soot added and the hydrocarbons particles might form the first peak, while the second peak might be formed by larger carbonaceous particles. In this sense, when high levels of small particles appear, new larger particles are formed by this phenomenon, increasing the number of large particles at the PSD curve. These particles usually come from volatile organics particles adhered to a carbonaceous nucleus, existing the possibility of being detected by the smokemeter (Figure 4). Hence, the higher value of FSN respect the other points. In addition, this phenomenon might be replicated at 1200 rpm and 1800 rpm with lower magnitude of the second peak due to the smoke measurements presented in Figure 4 and the PSD curves showed in Figures 10 and 11.
PSD curves for CDC operation shows the same curve than that observed at 1200 rpm. In this sense, the total number of particles seems to be very similar, with a peak located at the same mobility diameter of 60 nm.
Figure 12 presents the PSD curves for 2000 and 2200 rpm. Both RCCI PSD curves show big differences with the previous RCCI PSD curves for 1500 rpm and 1800 rpm, and also differ from those of CDC. The most relevant difference is that the second peak has disappeared. In this sense, only one peak is observed in both curves at 20 nm of mobility diameter in both engine speeds presented in the figure. Therefore, it is worthy to note that the phenomenon of coagulation observed at 1500 and 1800 rpm cases in not observed at these high engine speeds. Thus, low sized particles dominate the PSD at high engine speeds under RCCI.
PSD curves for CDC operation shows again the same trend that it has been observed at the other engine speeds. In this sense, the total number of particles for the PSD peak seems to be slightly increased and the diameter where the PSD peak appears is kept constant around 60 nm.
From the nature of the particles standpoint, PSD analysis provides information about the mobility diameter of the particles and the number of particles corresponding with each diameter measured. Additionally, a nucleation or accumulation mode classification has been performed in order to identify the nature of the particles and the portion of representation for each mode type respect the total number of particles.
Figure 13 presents the particle classification in nucleation or accumulation mode, the total number of particles and the smoke emissions in FSN. Smoke emissions has been added to the graph in order to improve the understanding of the classification and observe the correlation existing between the smoke emissions and the accumulation mode particles emission
CDC PSD curves presented in Figures 10, 11 and 12, show the PSD peaks at a mobility diameter around 60 nm. Therefore, large sized particle domination of the PSD indicates that the majority of the particles have a carbonaceous origin. In this sense, the cutoff diameter used to limit as the nucleation mode as the accumulation mode has been 30 nm. This cutoff diameter ensures that the nature of the particles is kept in the correct particle counter. Thus, CDC classification is presented on the right graph. It is observed that the particle emission measured is dominated by large sized particles, which are provided by the soot produced during the combustion. Smoke emissions are also depicted in the figure and is directly related with the accumulation mode particles. As the smoke emissions increase with the engine speed, the number of accumulation particle increases as well, whereas the number of nucleation mode particles remains almost constant along the engine speeds tested.
RCCI graph shows the classification of the total number of particles and the corresponding nucleation mode and accumulation mode particles. Considering the explanation given at the previous paragraph, the cutoff diameter selected has been 30 nm. As it is depicted in Figure 13, the smoke emissions measurement and the total number of particles do not follow the same trend. Indeed, the smoke measurement presents the maximum value at 1500 rpm, and the total number of particles presents a minimum point. Therefore, it is possible to assume that RCCI is dominated by lower sized particles for all engine speeds. Nucleation mode particles are usually formed by volatile organic and sulfur compounds that are produced during the exhaust dilution. So that, smoke measurement is not able to detect this small particles where they are captured in the paper sample.
Comparing the total number of particles emitted by both modes, except at high engine speed that offers similar results, the number of particles is higher for RCCI than for CDC mode.
Figure 14 shows the percentage of nucleation and accumulation mode particles respect the total concentration of particles obtained. Big differences are found between the RCCI mode and the CDC mode, confirming that RCCI is dominated by the nucleation mode particles and CDC is dominated by accumulation mode particles.
Both combustion modes present a trend between the smokemeter results and the accumulation percentage depicted in the Figures 13 and 14. As the smoke measurements increase, the percentage of accumulation mode particles also increase. This trend might be explained by the diameter of the particles. At CDC mode, it is well-known the different correlations to convert the smoke measurements in soot mass as well as the nature and morphology of the particle. However, the nature of the particles and the correlations for RCCI are not accurately defined even more these big particles come from a coagulation phenomenon process.
The present investigation has been focused on the particle size distribution (PSD) comparison between RCCI and CDC mode. This investigation has been carried out in a medium-duty EURO VI diesel engine adapted to run with two different fuels at the same time. The points tested were performed at 25% engine load and six steps of engine speed from 950 to 2200 rpm.
From the gaseous emissions standpoint, the most relevant conclusions from the study are:
* Ultra-low NOx emissions obtained for RCCI. In spite of having the RCCI concept higher bulk gas temperatures than CDC, local peak gas temperatures are lower. Hence RCCI promotes lower NOx values.
* Unacceptable levels of CO and uHC have been obtained at all engine speeds. These levels should be caused by the crevices volume, which difficult the gasoline burning during the combustion process.
From the PSD analysis standpoint, the most relevant conclusions from the study are:
* Small-sized particles dominate the RCCI particle emissions. PSD curves present the peaks at a mobility diameter of around 20 nm in all the engine speeds tested.
* PSD analysis carried out for a medium-duty engine shows a dependency between the total number of particles and the engine speed for both combustion modes. Despite of having lower smoke values, RCCI mode promotes slightly higher number of particles at high engine speeds. These small particles might be caused by the gasoline (CO and uHC emissions) at RCCI mode and the smoke at CDC mode.
* FSN measurements have not a direct correlation with the total number of particles for RCCI concept. As literature describes, RCCI particles present a carbonaceous nucleus nature in which semi-volatile organics are adhered and thereby, the smallest particles cannot be detected by the smokemeter.
* FSN measurements for 1200 rpm, 1500 rpm and 1800 rpm and their respective PSD curves suggest that the particles derived from a coagulation phenomenon might be detected by the smokemeter. This finding is coherent with the literature, which shows that RCCI particles present a nucleus of carbon and brown colored particles that might be detected by the smokemeter.
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Dr. Antonio Garcia
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This investigation has been funded by VOLVO Group Trucks Technology. The authors also acknowledge the Spanish economy and competitiveness ministry for partially supporting this research (HiReCo TRA2014-58870-R). The predoctoral contract of the author V. Boronat (FPI-S2-2017-2882) is granted by the Programa de Apoyo para la Investigacion y Desarrollo (PAID) of the Universitat Politecnica de Valencia. The author J. Monsalve-Serrano acknowledges the financial support from the Universitat Politecnica de Valencia under the grant "Ayudas Para la Contratacion de Doctores para el Acceso al Sistema Espanol de Ciencia, Tecnologia e Innovacion".
AIM - Aerosol Instrument Manager
aTDC - After Top Dead Center
CAD - Crank Angle Degree
CA10 - Crank angle at 10% mass fraction burned
CA50 - Crank angle at 50% mass fraction burned
CA90 - Crank angle at 90% mass fraction burned
CDC - Conventional Diesel Combustion
CI - Compression Ignition
CO - Carbon Monoxide
CPC - Condensation Particle Counter
CR - Compression Ratio
DI - Direct Injection
DM - Dual Mode
DMA - Differential Mobility Analyzer
DMDF - Dual Mode Dual Fuel
ECU - Engine Control Unit
EGR - Exhaust Gas Recirculation
EOI - End of Injection
EU - European Union
EVO - Exhaust Valve Open
GF - Gasoline Fraction
FSN - Filter Smoke Number
uHC - Unburned Hydro Carbons
HCCI - Homogeneous Charge Compression Ignition
IMEP - Indicated Mean Effective Pressure
ISFC - Indicated Specific Fuel Consumption
IVC - Intake Valve Close
IVO - Intake Valve Open
LHV - Lower Heating Value
LTC - Low Temperature Combustion
MCE - Multi Cylinder Engine
MON - Motor Octane Number
MPRR - Maximum Pressure Rise Rate
OEM - Original Equipment Manufacturer
ON - Octane Number
PFI - Port Fuel Injection
PPC - Partially Premixed Charge
PRR - Pressure Rise Rate
PSD - Particle Size Distribution
RCCI - Reactivity Controlled Compression Ignition
RoHR - Rate of Heat Release
RON - Research Octane Number
SC - Screw Compressor
SCE - Single Cylinder Engine
SMPS - Scanning Mobility Particle Sizer
SOC - Start of Combustion
SOI - Start of Injection
DC - Top Dead Center
Jesus Benajes, Antonio Garcia, Javier Monsalve-Serrano, and Vicente Boronat
Universitat Politecnica de Valencia
Table 1. Main characteristics of the engine used. Style 4 Stroke, DI diesel engine Manufacturer / model VOLVO/D5K240 OEM ECU calibration EURO VI Piston bowl geometry Re-entrant Maximum power 177 kW @ 2200 rpm Maximum brake torque 900Nm @ 1200-1600 rpm Maximum in-cylinder pressure 190 bar Bore x Stroke 110 mm x 135 mm Connecting rod length 212.5 mm Crank length 67.5 mm Total displaced volume 5100 cm3 Number of cylinders 4 Compression ratio 15.3: 1 Table 2. Main properties of the diesel and gasoline injector. Diesel injector Actuation Type Solenoid Steady flow rate @ 100 bar 1300 [[cm.sup.3]/min] Number of Holes 7 Hole diameter [[micro]m] 177 Included Spray Angle [[degrees]] 150 Maximum injection pressure (bar) 1200 Gasoline injector Injector Driver Saturated Steady flow rate @ 3 bar 980 [[cm.sup.3]/min] Included Spray Angle [[degrees]] 30 Injection Pressure [bar] 5.5 Injection Strategy Single Start of Injection Timing 340 CAD ATDC Table 3. Physical and chemical properties of the fuels used. Diesel EN590 Gasoline Density [kg/[m.sup.3]] (T= 15 [degrees]C) 820 720 Viscosity [m[m.sup.2]/s] (T= 40 [degrees]C) 2.8 - RON [-] - 95.0 MON [-] - 85.0 Cetane number [-] >51 - Lower heating value [MJ/kg] 42.97 42.4 Table 4. Engine-out emissions and EGR rate for RCCI mode. Speed IMEP EGR ISNOx ISCO ISHC Smoke rpm bar % g/kWh g/kWh g/kWh FSN 950 4.9 0.069 54.4 9.8 0 1200 6.3 0.051 20.7 7.2 0.08 1500 7.3 20 0.157 9.7 7.3 0.12 1800 7.4 0.203 12.0 7.6 0.11 2000 7.1 0.203 16.9 8.2 0.03 2200 6.8 0.114 35.7 9.2 0.02 Table 5. Engine-out emissions and EGR rate for CDC mode. Speed IMEP EGR ISNOx ISCO ISHC Smoke rpm bar % g/kWh g/kWh g/kWh FSN 950 4.5 20.0 3.5 0.8 0.3 0.07 1200 6.3 18.2 3.8 0.6 0.2 0.1 1500 6.9 24.1 3.0 0.4 0.2 0.1 1800 7.0 23.8 3.1 0.6 0.2 0.14 2000 7.0 25.2 2.6 0.7 0.2 0.17 2200 7.1 13.3 3.4 0.7 0.2 0.2
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|Author:||Benajes, Jesus; Garcia, Antonio; Monsalve-Serrano, Javier; Boronat, Vicente|
|Publication:||SAE International Journal of Engines|
|Date:||Dec 1, 2017|
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