Implementation of the Closed-Loop Combustion Control Methodology

Jan 10, 2012 - ABSTRACT: The interaction among biodiesel, engine calibration, and turbocharger system was investigated in a modern automotive diesel ...
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Implementation of the Closed-Loop Combustion Control Methodology in Modern Automotive Diesel Engines for Low-End Torque Increment Burning Biodiesel Carlo Beatrice,*,‡ Chiara Guido,‡ and Pierpaolo Napolitano‡,† ‡

Istituto Motori, Consiglio Nazionale delle Ricerche, Viale Marconi, 4, 80125 Naples, Italy Università degli Studi di Napoli "Federico II", Naples, Italy

†

ABSTRACT: The interaction among biodiesel, engine calibration, and turbocharger system was investigated in a modern automotive diesel engine. In consideration of the poorer maximum torque curve derived from the use of fatty acid methyl ester (FAME) biodiesels, pure or in blends, with respect to the conventional diesel one, an experimental activity was dedicated to the investigation of the capability of the closed-loop combustion control (CLCC) technology to mitigate or improve the impact of FAME characteristics on the engine full torque curve. With respect to the well-known penalty effect of biodiesel on engine performance, the results highlighted the capability of the CLCC technology to reset the torque loss burning biodiesel. Furthermore, the study has provided evidence of the possibility to exploit the very low soot emissions burning FAME fuels, increasing the low-end maximum torque curve with respect to a conventional diesel fuel. The described engine re-optimization could be activated automatically in the engine electronic control unit, once a fuel-blending detection procedure based on the CLCC system (currently in development) has been implemented. In this respect, the study represents a further step toward the future actual full flex-fuel engine.

1. INTRODUCTION It is well-known that, from an environmental point of view, biofuels can contribute to a significant well-to-wheel reduction of greenhouse gas emissions.1 In particular, the use of the firstgeneration biodiesel, the fatty acid methyl ester (FAME), in a light-duty diesel engine shows interesting advantages, not only in terms of emission reduction but also as the future European legislation will foresee their usage.2 Notwithstanding the biodiesels have inherent characteristics that allow their feeding in modern diesel engines without introducing significant modifications to their design, several studies have evidenced that the impact of biodiesels on the modern diesel engine is significant.3−5 Therefore, it is increasingly felt by the automobile industries and the research centers the exploration of new technologies aimed to mitigate the impact of biofuel usage in engine developed to be primarily fed with petroleumbased fuels and to minimize the negative aspects linked to biodiesel supply. At the same time, an efficient use of alternative diesel fuels, allowing full exploitation of all their high potentials, can be successfully achieved through an “ad hoc” calibration of engine parameters and its control strategy [injection set and exhaust gas recirculation (EGR) rate on all].6 One of the main negative aspects for the end-user derived from biodiesels fueling is the reduction of the maximum torque curve,3,4 because the engine normally operates at a maximum settled fueling rate. This drawback is a consequence of the lesser lower heating value (LHV) of FAME with respect to a petroleum-based diesel fuel. The torque and power gap can be recovered if the fueling rate could be automatically incremented. The oxygen content of the FAME (about 10% of the total mass) is responsible for its LHV reduction of about the same magnitude, which, in turn, determines the torque © 2012 American Chemical Society

performance reduction (approximately of a similar amount of the oxygen content). In turbocharged engines, the problem is intensified at low speed for the so-called low-end maximum torque curve. In these conditions, the exhaust gas temperature fall, directly related to the reduction of energy released during the combustion (burning biodiesel), reduces the enthalpy gap in the turbine and the energy available for engine boosting. Moreover, for an electronic-controlled variable geometry turbocharger (VGT), the exhaust enthalpy reduction brings the turbine to a more pronounced closure of the variable rack. At medium and high speeds, this does not represent a problem, but at low engine speed, where normally the VGT already works with the rack closure very near the limit position, the turbocharging system is no longer able to ensure the boosting level provided by the engine calibration. A similar situation occurs for waste-gate turbochargers. Therefore, the decrease of boost pressure at low speed with the use of biodiesel is responsible for further engine maximum torque reduction. The combined effect of the two described mechanisms can account for a torque reduction up to 25% with evident consequences for the engine handling. The declared torque decrement, burning biodiesel, can be due not only to the LHV reduction but also to the turbocharging operating features. In this sense, recent experiences7 have shown that the lowend torque curve reduction mainly affects small displacement waste-gate turbocharger engines (smaller than the engine employed in this study). In the low-end torque range, the waste-gate typically works near its closest position without Received: September 5, 2011 Revised: January 3, 2012 Published: January 10, 2012 1305

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The CCLC technology, available on the chosen engine, enables individual and real-time control of the indicated mean effective pressure (IMEP), cycle-by-cycle and cylinder-by-cylinder by means of piezoresistive pressure sensors integrated in the glow plugs.10 In particular, on the basis of in-cylinder pressure trace acquisition, the electronic control unit (ECU) calculates the actual value for IMEP and then compares it to the target value derived from the accelerator pedal request. As a consequence, the deviations are continuously resettled by adjusting the main injection quantity for the following combustion cycle. Figure 2 shows the sketch of the CLCC working. In the figure, the ETmain is the solenoid energizing time of the injector that controls the injected fuel quantity. The engine was installed on a dyno engine test bench to perform the tests in steady-state condition at full load for different engine speeds. During the tests, ambient air pressure and temperature were fixed at 1000 mbar and 25 °C, respectively; also, the fuel and the engine-cooling temperatures were kept constant. The engine was instrumented for temperatures and pressures monitoring in the intake and exhaust lines; the analogue signals of the pressure transducers and thermocouples were conditioned and acquired with a modular data logger (National Instruments CompactDAQ). The engine was also instrumented for the indicated analysis (cylinder pressure, injection pressure, and energizing injector current). More in detail, the cylinder pressure signals were measured by the pressure sensor glow plugs in cylinders 2−4, while the cylinder 1 signal was measured by means of a piezoelectric sensor (AVLGH13P). The fuel pressure inside the common rail and the energizing injector current were measured by means of a piezoresistive pressure sensor (Kistler A4067) and a halleffect current transducer. All high-frequency signals were acquired with the AVL Indimicro device. At the engine exhaust, gaseous emissions (FID, total unburned hydrocarbon; CLD, NOx, CO, CO2, and O2) were measured up- and downstream of the diesel aftertreatment device by means of a raw emission analysis test bench (AVL-CEB-2). To have a direct measure of the emitted soot, a high-resolution soot meter (MicroSoot AVL483)11 was employed. The maximum torque curve was reproduced testing the engine at 12 engine speed steps, from 1250 to 4500 rpm. In all test points, the torque request to engine control unit was set at 100% (corresponding to a maximum accelerator pedal position requested from the ECU), fueling the engine with a reference petroleum-based diesel fuel and with pure biodiesel. To evaluate the effect of CLCC varying the fuel quality, two engine operation modes were considered, according to the following scheme and abbreviations: (1) with closed-loop control on IMEP disabled [indicated as open-loop control (OLC)] and (2) with

margins to compensate for the boost pressure penalty derived from the exhaust gas temperature reduction. The purpose of the present research was to evaluate the capability of the so-called closed-loop combustion control (CLCC) technology to annul the torque loss burning biodiesel, in particular, at low engine speeds. The operating characteristics of the CLCC system and its potentialities on combustion and emission control burning alternative fuels have been described by the authors in some previous papers.6−9 In the present work, the adoption of the CLCC technology has led to good results in terms of low-end torque performance relatively to biodiesel use. Furthermore, the characteristic of FAME to produce low soot emissions (also at maximum torque conditions) has suggested the possibility to increase the maximum attainable torque, whereas burning a standard diesel fuel, the exhaust soot emission is normally a limiting parameter. In this sense, the best exploitation of the biodiesel potentiality can be obtained by a calibration adaptation of the engine parameters when the actual biodiesel content in the fuel is known, for example, thanks to a fuel blending detection methodology implementation.9

2. EXPERIMENTAL APPARATUS, TEST PLAN, AND FUELS The experimental activity was carried out on the four-cylinder in-line 2.0 L Euro 5 diesel engine, whose main characteristics are reported in Table 1, while in Figure 1, the engine layout is displayed.

Table 1. Main Features of the Multi-cylinder Engine engine type certification bore × stroke (mm) displacement (cm3) compression ratio valves per cylinder rated power and torque injection system turbocharger catalyst system

four cylinders in-line Euro 5 83.0 × 90.4 1956 16.5 4 118 kW at 4000 rpm, 380 N m at 2250 rpm common rail Garret single-stage VGT integrated closed-coupled DOC and DPF

Figure 1. Engine layout. 1306

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Figure 2. Scheme of the CLCC procedure. closed-loop control on IMEP enabled [indicated as closed-loop control (CLC)]. Furthermore, to exploit the very low soot emissions burning biodiesel and to increment the engine low-end torque curve with respect to the conventional diesel fuel, a dedicated test series was performed with FAME at 1250, 1500, 1750, and 2000 rpm, with a proper engine recalibration. The adopted pure fuels were an EU certification diesel fuel (CEC, RF-03-A-84) compliant with EN590 European regulation, as reference fuel (REF), and an EU widely available rapeseed methyl ester

For each test point between 1250 and 3500 rpm, the injection strategy was composed by a pilot injection and a main injection. In Table 3, the most important injection parameter values relative to all test points with reference fuel are reported.

Table 3. Most Significant Injection Parameters

Table 2. Main Fuel Parameters feature A/Fst LHV (MJ/kg) LHV/A/Fst ratio carbon (%, m/m) hydrogen (%, m/m) oxygen (%, m/m) cetane number density at 15 °C (kg/m3) viscosity at 40 °C (mm2/s) distillation (°C) IBP 10 vol % 50 vol % 90 vol % 95 vol % FBP oxidation stability (mg/100 mL) oxidation thermal stability at 110 °C (h) cold filter plugging point (CFPP) (°C) lubricity at 60 °C (μm) peroxide value (POV) (mequiv of O2/kg) total acid number (TAN) (mg of KOH/g)

method

REF

FAME

14.54 45.965 3.16 85.220 13.030 1.450 51.8 833.1 3.141

12.44 37.570 3.02 77.110 11.600 11.250 52.6 883.1 4.431

158.9 194.3 267.6 333.4 350.0 360.9 EN ISO 12205 EN 14112

318 331.0 335.0 344.0 353.0 355.0 0.6 6.5

EN 116

−14

EN ISO 1215601 NGD Fa 4

179 16.60

UNI EN 14104

0.13

ASTM D4868 ASTM D5291 ASTM D5291 ASTM D5291 EN ISO 5165 EN ISO 12185 EN ISO 3104 EN ISO 3405

engine test point (speed per torque) (rpm × Nm)

fuel pressure in the common rail (bar)

main injection timing (crank angle before top dead center)

main injection duration (μs)

1250 × 228 1500 × 318 1750 × 353 2000 × 362 2250 × 371 2500 × 371 2750 × 360 3000 × 342 3250 × 327 3500 × 312 4000 × 275 4500 × 226

920 1050 1130 1230 1340 1450 1540 1600 1600 1600 1600 1600

4 5 5 7 9 10 12 14 16 18 24 28

986 1153 1113 1066 1034 999 929 892 875 858 827 750

pilot injection timing (crank angle before top dead center)

pilot injection duration (μs)

12 14 16 19 23 25 29 32 36 40

227 222 218 215 215 214 213 213 212 211

According to the experimental procedures typically used by the authors, the accuracy and reproducibility of the measures were checked repeating all tests 3 times in different days and considering as results the mean values. For a better result analysis of the diagrams, the error bars are reported only on the curves relative to the reference fuel case. Their weights are estimated considering both the accuracy of the single acquisition apparatus and the variations of the results of the present test campaign. In the figure captions, the relative measurement accuracy of the displayed data are reported. The effects induced by these properties on combustion noise, engine performances and pollutant emissions at partial loads are largely described in the literature,3−7 and then they were out of the main scope of the present research.

3. RESULTS AND DISCUSSION 3.1. Biodiesel Impact on Engine Full-Load Torque. In this section, the impact of biodiesel on the engine performance at different engine speeds is analyzed, comparing the behavior shown in the two engine operative modes (OLC and CLC) with respect to the performance offered by the reference fuel only in CLC mode. The data relative to the reference fuel in OLC are not reported, because the main effect of the CLC mode is a more uniform and balanced combustion among the cylinders with respect to the OLC conditions when the engine operates at low loads and high EGR rates.6,8 The CLC

compliant with EN14112 European regulation, as FAME. Table 2 reports, for the two fuels, some of the most important parameters affecting the engine performance. The most relevant differences of FAME (in the following also named indistinguishably biodiesel) with respect to conventional diesel, as expected, lie in its lower LHV and in the stoichiometric air/fuel ratio (A/Fst), both due to the oxygen content of biodiesel (about 11% by mass). 1307

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Figure 3. Engine maximum torque curve comparison at different speeds (on the left; note that lines “Diesel_CLC” and “BD_CLC” are completely overlapped), and relative torque deviations between biodiesel and reference diesel fuel (on the right). The accuracy is about 1% of the measured torque values.

advantage becomes negligible at full-load conditions, fueling conventional diesel fuels. The diagrams in Figure 3 report, on the left, the maximum torque curves for the REF fuel, BD in OLC, and BD in CLC operating mode, with all the values scaled with respect to the peak value, and, on the right, the relative torque deviations for biodiesel with respect to REF fuel. In OLC mode, a torque reduction burning FAME is observable; the gap is present at every engine speed step, and it increases with the speed. More in detail, the torque reduction is about 3−4% in the range of 1250−1750 rpm, is progressively increased at medium engine speed, and is about 7−8% above 3000 rpm. Furthermore, in the diagram, a relative minimum in torque deviation for FAME in OLC mode is notable at 2500 rpm. This effect is due to the engine calibration transition from the maximum soot limited range to the maximum peak firing pressure limited range. This aspect will be better clarified in the next section. As explained later, the different A/Fst values of the two fuels and the high fuel conversion efficiency of FAME are the reasons of the reduced torque penalty with FAME at the lowest engine speeds. As expected, when the CLC mode is switched on and the fueling becomes IMEP target controlled, the differences in the maximum torque curve between the two fuels are resettled. This confirms the capability of CLCC technology to compensate for the effect of the fuel LHV differences on the engine maximum torque curve. Indeed, the deviations between diesel and biodiesel in CLC mode are less than 1% on the whole full-load torque curve; therefore, they can easily be considered comparable to the test to test differences. In Figure 4, the absolute boost pressure values relative to all three performed torque curves are shown. No boost pressure differences between REF and FAME (in both operation modes) were detected, suggesting that the turbocharger is able to guarantee the same boosting level (and the same incylinder trapped air mass); therefore, the LHV penalty is the only parameter responsible for full-load torque reduction observed in OLC mode. Differently from other similar experiences relative to engines smaller than the present one,7 in the described case, it is evident

Figure 4. Boost pressure comparison at different speeds. The accuracy is about 1.5% of the full-scale value of 3000 mbar.

that, in the low-end torque curve range, the turbine works far from its own limit. The operating turbine conditions for all full-load curves are shown in Figure 5, where the exhaust gas temperatures at the turbo inlet (on the left) and the percentage of the turbine rack closure (VGT closure rack, on the right) are reported, in all of the tested cases. The diagrams provide evidence of the inlet turbo temperature reduction burning FAME in OLC mode and, consequently, of the VGT duty cycle increment (turbine rack closing) to keep the absolute intake boost pressure constant. It has to be mentioned that, at full-load conditions, the pumping losses are about 2 orders of magnitude lower than the IMEP, and therefore, no appreciable differences between FAME and REF were detected. For this reason, pressure values upstream of the turbine are not shown. The low inlet turbine temperature in OLC is directly linked to the reduction of chemical energy introduced with biodiesel (lower LHV). The turbine rack closure is the ECU response to the exhaust gas enthalpy reduction. In CLC operation mode, the ECU automatically increments the fuel injection quantity to compensate for the LHV reduction of FAME, leading to a higher exhaust gas temperature than in the OLC mode. 1308

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Figure 5. Exhaust gas temperature (at the turbocharger inlet) and VGT closure rack percentage comparison at different speeds. The temperature accuracy is about 1% of the full-scale value of 1000 °C.

soot limit is set in the REF fuel engine calibration in OLC mode while is bypassed in CLC mode, this effect justifies the lower torque penalty burning FAME in OLC mode at 2500 rpm (see Figure 3). This result suggests that the CLCC system could be a partial solution to boosting problems fueling biodiesel in small displacement engines, where reductions up to 25% of lowend torque values were measured.7 Diagrams of Figure 7 show the brake-specific fuel consumption (BSFC) and specific fuel energy (SFE) introduced in the engine cylinders versus engine speed. In particular, SFE is calculated with the following formula:

However, a small exhaust gas temperature difference between REF and FAME remains and depends upon the different A/Fst values of the fuels. Indeed, the exhaust temperature is directly linked to the operating λ value. As the same air is trapped in the cylinder burning the two fuels, when the engine is fueled with FAME, it works with a higher λ and then a lower exhaust temperature. To confirm such analysis, the following Figure 6

SFE =

Q fuel LHV 3.6

(kW)

(1)

where Qfuel is the total fuel quantity injected in the cylinders and is expressed in kilograms per a whole engine cycle. The curves in the diagrams reflect the fuel characteristics and the effect of the engine control mode. Due to the low LHV and high density of FAME, as expected, the results show higher fuel consumption for FAME with respect to the REF fuel in OLC mode (about 3.5% more) on the whole full-load torque. The switch to CLC mode permits the further fueling increment to match the SFE target value of the REF fuel case, as observed in the right diagram of Figure 7. The RME torque decrement in the OLC condition (observed in Figure 3) is less than the LHV gap between RME and REF and, in particular, is very low (mean value of about 5%) in the engine speed range between 1250 and 2000 rpm. This behavior is explained considering the diagrams of fuel conversion efficiency (FCE) reported in the left diagram of Figure 8, where the FCE is

Figure 6. λ curve versus engine speed for all fuels and two engine control modes. The accuracy is about 3% of measured values.

shows the λ curves for all fuels calculated from fuel and air mass flow measurements. It is easy to note that, in both operating modes, FAME has higher λ values than REF and the differences increase in the low speed range, where stoichiometric conditions are approached for FAME. In Figure 6, it is also possible to note a relative minimum of λ at 2500 rpm for all fuels. This minimum value results from the increment of the fueling rate that is possible because, when the speed value is raised, the engine is no longer in the soot limited range. As the

FCE =

1 BSFC LHV 3.6

(2)

In the left diagram of Figure 8, the FCE increment burning FAME in OLC mode is evident and is more significant at low engine speed. 1309

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Figure 7. BSFC (on the left) and SFE (on the right) versus engine speed for all fuels and engine control modes. The accuracy is about 1.5% of measured values.

Figure 8. FCE (on the left) and mechanical efficiency (on the right) versus engine speed for all fuels and engine control modes. The accuracy is about 1.5% of measured values.

The FCE increment is related to the combined effect of a better engine thermodynamic and combustion efficiency of FAME with respect to the REF fuel in OLC mode. Burning FAME, its low A/Fst value and then its high operating λ (see Figure 6) produce a shorter combustion and a higher thermodynamic efficiency with respect to the REF fuel. The plots of the indicated cycles and heat release (HR) curves reported in Figure 9, for the test case at 2250 rpm corresponding to the maximum torque engine speed, confirm the analysis. The diagrams in the figure show that the HR is faster in the case of biodiesel in OLC mode, even if the start of injection is the same for RF and RME. The correspondent ROHR and HR profiles reveal 50% of the total mass burned fraction (indicated as MBF50) closer to the top dead center (TDC) and a shorter combustion duration burning FAME in OCL mode. Such behaviors justify the higher FCE value of FAME, especially in OLC mode. These effects burning FAME become

more evident in the low engine speed range, where stoichiometric conditions are approached and the limiting factor is the exhaust soot level. In these conditions, the efficiency gap between FAME and REF increases. The effects of CLCC on torque compensation burning FAME are evident, as seen from the cylinder pressure plot of Figure 9. Moreover, in CLC mode with FAME, the diagram clearly shows the increment of the main injection duration, expressed as the solenoid energizing time of the injector, because of the automatic compensation of the CLCC system. Moreover, in CLC mode with FAME, the pressure trace almost matches that of the REF fuel, and this is also valid for ROHR and HR traces (see right plots). However, even if the pressure cycle of the REF is matched in the CLC mode, the low A/Fst value of FAME permits operation with higher λ values and then higher FCE with respect to the REF case. Looking at the cylinder pressure plot, it is possible to appreciate that the torque gap in OLC mode is mainly 1310

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Figure 9. Cylinder pressure and injector energizing current (on the left), rate of heat release (ROHR) and HR (on the right) at 2250 rpm and full load for all fuels and both engine control modes.

Figure 10. Soot (on the left) and NOx (on the right) emissions versus engine speed for all of the tested fuels and engine control modes. The soot accuracy is about 3% of the maximum value of the measuring range (0−150 mg/m3). The NOx accuracy is about 4% of measured values.

dependent on the lower cylinder pressure history in the late expansion stroke. A small reduction of the frictional losses was measured for RME in OLC mode, and it is dependent on the lower cylinder pressure, as observable from the right diagram of Figure 8, reporting the mechanical efficiency versus engine speed and fuels. However, looking at the error bars in the diagram, the contribution of the friction loss reduction on the FCE increment for RME in OLC mode can be considered negligible. Therefore, the FCE increment is mainly due to the thermodynamic efficiency rise. As expected and in line with other results available in the literature,3 the differences in A/Fst between the two fuels and the presence of oxygen are the causes of the soot drop and the NOx increase burning FAME with respect to the REF case. In Figure 10, soot and NOx emission values versus engine speed for both operation modes are shown. The differences in soot emissions in OLC mode are significant in the whole speed range. However, it appears important to underline that, also, in CLC mode, in the soot

limited speed range, the gap in soot emission between REF and FAME remains considerable. The present results highlight the benefits offered from the CLCC technology to reset the performance gap of biodiesel with respect to the conventional diesel fuels, simultaneously reducing the soot emission. 3.2. Low-End Torque Increment Fueling FAME. Taking into account that, in the low-end torque range (1000−2000 rpm), the soot emission represents the limiting factor in the raising of the torque output, the soot reduction burning FAME in CLC mode can be exploited to obtain a torque increment over the engine performance with diesel fuel. This result can be easily obtained with the CLCC technology by means of a torque target modification. For a better readability, only the results in the engine speed range between 1250 and 2250 rpm will be discussed in the following, comparing the behavior of diesel, FAME in CLC mode, and FAME in “optimized” CLC mode. The left plots of Figure 11 report the torque increment obtained burning FAME, in the low-end torque curve area, by 1311

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Figure 11. Torque increment by CCLC set point value adaptation with FAME in the low-end torque curve range. Comparison among reference fuel, FAME in CLC mode, and FAME in CLC mode with torque set increment (on the left) and corresponding λ values (on the right).

Figure 12. Soot emissions (on the left) and NOx emissions (on the right) for the low-end torque curve range.

as already demonstrated by Millo et al.12 in recent experiences. The effect is the λ reduction, as observed in the right plot of Figure 11. It is very interesting to note that, when the injection quantity is increased, λ values are pushed near the limit values of REF fuel values and the difference between FAME in CLC conditions with and without torque increment follows the same trend of the engine torque gap. At 2250 rpm, the maximum reached PFP value becomes the limiting factor and no further increment on the torque setting point is possible. The corresponding soot and NOx emissions for torque increment conditions with respect to FAME in CLC mode are displayed on the left and right diagrams of Figure 12, respectively. The left diagram of Figure 12 shows the respect of the imposed soot limit for torque set increment and also confirmed that, at the full-load condition, the main driver of soot emissions is the effective λ value. Also, NOx emissions are mainly dependent upon the λ value because they blindly follow the λ trace and the soot trace versus engine speed.

means of the IMEP set point value adaptation of the CLCC technology, using as limiting value the same exhaust soot emission of the REF fuel but without overcoming the maximum admissible peak firing cylinder pressure (PFP). In the right diagram of Figure 11, the corresponding λ values for all test conditions are displayed. The torque increment was possible in the whole low-end range from 1250 to 2250 rpm and is about 4%, which, at low engine speed, represents a good improvement in the engine performance. Such an increment is roughly equal to the percentage difference of the LHV/A/Fst ratio between the diesel and biodiesel (about 4.3%). As known, the maximum brake torque attainable at a fixed λ is proportional to the LHV/ A/Fst ratio of the fuel. As mentioned before, this increment is obtained by means of the CLC operation that oversees the fuel quantity augment without changing the engine calibration. It is worth noting that, with a specific calibration, changing the injection setting and boost conditions, a higher torque increment with respect to the present results could be reached, 1312

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A higher torque increment could be possible by means of specific whole engine recalibration. (5) It has been demonstrated also that, at full torque operating conditions, soot and NOx emissions are influenced by from the A/Fst value of the fuel, while the other characteristics have a secondary importance. These performance improvements burning biodiesel in a CLCC diesel engine could be easily attained if the automatic fuel blending methodologies, today under development, will be accurately validated with reliability tests in the near future. In this respect, the study represents a further step toward the future actual full flex-fuel engine.

The two last observations require additional comments. As reported in the valuable recent review work by Lapuerta and co-workers,3 several reasons can explain the NOx increment and soot reduction burning biodiesel in a diesel engine. It was reported that NOx increment is mainly based on the interaction between biodiesel and injection parameters and soot reduction is linked to the oxygen content of biodiesel. Even though all claimed reasons can be considered valid in principle, they are strictly dependent upon the engine operating mode (low, partial, or full load) and the injection system technology (e.g., injection pump, unit injectors, and common rail). In the present case, where the biodiesel has no significant impact on the injection characteristics of the common-rail system,5 differently from the experiments reported in the literature, the increment of engine torque over the calibration target was possible. Moreover, even though past experiences have provided evidence of different influences on combustion if the oxygen is available from the air side or fuel side,13 it has been demonstrated that at full-load/power engine conditions, the effective λ is the driver parameter controlling NOx and soot emissions and there is no difference if the oxygen is available from the air or from the fuel. Therefore, the higher NOx and lower soot emissions burning biodiesel are mainly dependent upon a lower A/Fst with respect to conventional diesel fuel. At full-load/power conditions, the A/Fst value affects the oxygen available for combustion and also for NOx formation and soot formation/oxidation, and if the same λ values of diesel are reached, there is no significant difference between biodiesel and diesel in terms of NOx and soot emission performance, while other factors, such as, for example, the fuel oxygen content or the other fuel characteristics, have a second-order influence. In the engine equipped with CLCC technology, the maximum torque recovery in the low speed range burning biodiesel could be simply exploited using minimum λ as the limiting factor and if a fuel blending detection methodology (BDM) is implemented in the ECU system. A simple BDM based on the use of the CLCC system was defined and implemented by the same authors on the same engine and described in a previous work.9 Such BDM could allow for the estimation of the biodiesel−diesel blending level in the tank and then a proper engine recalibration for the best exploitation of the biodiesel characteristics.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +39-081-7177186. Fax: +39-081-2396097. E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Giovanni Alovisi, Giuseppe Corcione, Augusto Piccolo, Roberto Maniscalco, and Alessio Schiavone for their technical assistance in the engine testing.

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REFERENCES

(1) European Commission Joint Research Centre, Institute for Energy (JRC-IE). EU Renewable Energy Targets in 2020: Analysis of Scenarios for Transport; JRC-IE: Petten, The Netherlands, 2011; Report EUR24770 EN; http://ies.jrc.ec.europa.eu/uploads/jec/ JECBiofuels%20Report_2011_PRINT.pdf. (2) European Parliament and the Council of the European Union. Directive 2009/28/EC of the European Parliament and of the Council. Off. J. Eur. Union L 140, 16, April 23, 2009; www.energy.eu/directives/ pro-re.pdf. (3) Lapuerta, M.; Armas, O.; Rodríguez-Fernández, J. Effect of biodiesel fuels on diesel engine emissions. Prog. Energy Combust. Sci. 2008, 34, 198−223. (4) Kawano, D.; Hajime, I.; Yuichi, G.; Akira, N.; Yuzo, A. Application of biodiesel fuel to modern diesel engine. SAE [Tech. Pap.] 2006, DOI: 10.4271/2006-01-0233. (5) Beatrice, C.; Di Iorio, S.; Guido, C.; Mancaruso, E.; Vaglieco, B. M.; Vassallo, A. Alternative diesel fuels effects on combustion and emissions of an Euro 4 automotive diesel engine. SAE [Tech. Pap.] 2009, DOI: 10.4271/2009-24-0088. (6) Beatrice, C.; Guido, C.; Di Iorio, S. Experimental analysis of alternative fuel impact on a new ‘torque-controlled’ light-duty diesel engine for passenger cars. Fuel 2010, 89, 3278−3286. (7) Millo, A.; Bianco, A.; Grange, T.; Voicu, I. The effects of neat biodiesel usage on performance and exhaust emissions from a small displacement passenger car diesel engine. SAE [Tech. Pap.] 2010, DOI: 10.4271/2010-01-1515. (8) Guido, C.; Beatrice, C.; Di Iorio, S.; Fraioli, V.; Di Blasio, G.; Vassallo, A.; Ciaravino, C. Alternative diesel fuels effects on combustion and emissions of an Euro5 automotive diesel engine. SAE [Tech. Pap.] 2010, DOI: 10.4271/2010-01-0472. (9) Beatrice, C.; Guido, C.; Napolitano, P.; Di Iorio, S.; Del Giacomo, N. Assessment of biodiesel blending detection capability of the on-board diagnostic of the last generation automotive diesel engines. Fuel 2011, 90, 2039−2044. (10) Houben, H.; Marto, A.; Pechhold, F.; Haußner, M.; Borgers, M. Pressure sensor glow plug (PSG) for diesel engines. MTZ Worldwide 2004, 60, No. 11/2004. (11) Schindler, W.; Haisch, C.; Beck, H. A.; Niessner, R.; Jacob, E.; Rothe, D. A photoacoustic sensor system for time resolved quantification of diesel soot emissions. SAE [Tech. Pap.] 2011, DOI: 10.4271/2004-01-0968.

4. CONCLUSIONS In the present paper, the experimental results of a research activity devoted to the investigation of the potentiality of the CLCC technology to reset and improve the full torque engine performance gap burning biodiesel have been described. The following main conclusions can be highlighted: (1) When the engine is switched in IMEP target controlled mode by means of the CLCC system, the lower biodiesel performance with respect to diesel fuel in terms of the full torque curve is resettled. (2) Notwithstanding the reset of the full torque curve gap on the whole engine speed range, thanks to the low A/Fst value, biodiesel shows higher FCE values and lower soot emission with respect to the diesel fuel. (3) It has been demonstrated that the low soot emission at the same torque performance of biodiesel enables the possibility to increase the low-end torque curve by means of the recalibration of the IMEP target. (4) At the same exhaust soot emission of diesel and with the same engine calibration (boost pressure, etc.), the low-end torque curve increment was about 4%, like the difference in the LHV/A/Fst ratio between diesel and biodiesel. 1313

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(12) Millo, F.; Ferraro, C.; Vezza, D.; Vlachos, T. Analysis of performance and emissions of an automotive Euro 5 diesel engine fuelled with B30 from RME and JME. SAE [Tech. Pap.] 2011, DOI: 10.4271/2011-01-0328. (13) Zannis, T. C.; Pariotis, E. G.; Hountalas, D. T.; Rakopoulos, D. C.; Levendis, Y. A. Theoretical study of DI diesel engine performance and pollutant emissions using comparable air-side and fuel-side oxygen addition. Energy Convers. Manage. 2007, 48, 2962−2970.

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