ARTICLE pubs.acs.org/EF
Evaluation of Diesel Engine Cold-Start Performance: Definition of a Grading System To Assess the Impact of Fuel L. Starck,* H. Perrin, B. Walter, and N. Jeuland IFP Energies nouvelles, 1 & 4 avenue de Bois-Preau, 92852 Rueil-Malmaison, France ABSTRACT: The objective of this study is first to develop and validate a scientific methodology for the evaluation of diesel engine cold-start performance and then to use this methodology to assess the impact of fuels with respect to two selected fuel properties: variation in the cetane number (from 55 to 70) and use of biodiesel [rapeseed methyl ester (RME) and soybean methyl ester (SME)]. Engine tests were performed at 25 °C on a common rail, four-cylinder diesel engine (Euro 4). The methodology developed shows that the cetane number has a significant impact on the cold-start performance and that the use of cetane improvers (“procetane additives”) does not lead to an acceptable performance for a low cetane fuel. The addition of ester has a negative impact on cold operation by increasing the starting delay and opacity. Nevertheless, when the injection settings were simply optimized, the performance obtained in cold conditions with the biodiesel fuel tested here can equal those obtained with a conventional fuel.
1. INTRODUCTION Diesel engines now play a vital role in the transport sector, as a result, in particular, of their high thermal efficiency and low CO2 emissions. Diesel engines are particularly interesting because further progress can still be expected via downsizing.1 Fuel consumption could be reduced by around 9% with a reduction in displacement from 2 to 1.6 L.2 The most common way to maintain the same performance (specific torque and power) on downsized engines is to use high supercharging. Reduction of the volumetric compression ratio (CR) can prevent higher mechanical and thermal loads,3 and this means that significant downsizing is possible with a reasonable level of constraints. Moreover, the use of new combustion modes is improved with the reduction of CR on a broader load and speed range, which leads to a reduction of raw nitrogen oxide (NOx) emissions,4 something that is another huge challenge for diesel engines.5,6 Therefore, reducing the CR clearly emerges as a key point in the development of diesel engines. However, this reduction in CR to values between 14:1 and 16:1 leads to another limitation:7,8 cold-operation performance (start delay, idle stability just after start, and smoke opacity) at extremely low temperatures ( 20 °C and below). In previous papers,9 11 the consequences of reducing CR from 17:1 to 14:1 and then optimizing the injection settings to obtain good start capabilities were investigated. The problem of the cold-start performance is often related to spark-ignition engines.12 15 Indeed, it is well-known that ethanol has a higher heat of vaporization than gasoline; adding more ethanol to gasoline can make the cold-start performance worse because of the lower in-cylinder temperature.16 It is also important to note that a test exists at 7 °C for spark-ignition engines, and the European Union (EU) wants to extend this 7 °C test to diesel vehicles. At the same time, it is important to take into account fuel developments and their impact on the cold-start performance.17 20 Fuel properties are a key factor in cold operation. The chemical composition, density, volatility, viscosity, and autoignition properties of fuels can have a substantial impact on cold operation. Moreover, the increase in the use of alternative fuels around the r 2011 American Chemical Society
world, as in the EU, for example,21 can have an impact on the properties of fuels. The biodiesel ratio of fuels is increasing worldwide, especially in Europe.22 Fatty acid methyl ester (FAME) is commonly referred to as “biodiesel” and is used as a blend component for diesel fuel. Because FAME can differ significantly from conventional fuels in terms of cold-flow properties [cloud point (CP) and viscosity],23 26 it seems important to study the impact of biodiesel on the cold-start performance. The objective of this study is, first, to develop a methodology using a grading system to assess the performance of a diesel engine taking into account a variety of parameters: start delay, engine stability, opacity. The definition and validation of this grading system represent the main originality of this work. The second objective is to use this methodology to assess the impact of two selected diesel fuel properties [cetane number (CN) and biodiesel content] on the cold-operation performance. The engine tests are performed at 25 °C, on a common rail, 2 L displacement, four-cylinder diesel engine (Euro 4), with a compression ratio of 16:1, first of all using the original settings for all of the fuels (to understand the phenomena involved) and then optimizing injection settings for each fuel using a very simple methodology. Various biodiesels are tested: soybean methyl ester (SME) and rapeseed methyl ester (RME). SME is more widely used in the U.S.A., and RME is more widely used in Europe. The type of ester used in a country is closely correlated to farming practices, with RME and SME being representative to the FAME markets in Europe and the U.S.A., respectively. In the EU, the specification permits a maximum FAME content of 7 vol % in diesel fuel and B30, i.e., 30 vol%, of FAME in diesel fuel for captive fleet applications. The target of the EU for renewable energy is 20% by 2020, with 10% for the transport sector. It is for this reason that it has been decided to blend SME or RME at a ratio of 10% with conventional diesel fuel in the fuels tested. The value of testing different biodiesels is that this makes it possible to study Received: June 8, 2011 Revised: September 19, 2011 Published: September 22, 2011 4906
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Table 1. Characteristics of the Fuels reference diesel fuel
RME
SME
initial boiling point (IBP)
171.8
302.4
324.2
T50
268.4
DIESEL REF + RME
DIESEL REF + SME
DIESEL-60
DIESEL-70
distillation (°C) 173.4
174.8
171.8
172.6
287.8
278.6
268.4
268.2
final boiling point (FBP)
359.2
367.6
367.6
360.6
357.2
359.2
359.2
density (kg/m3)
830.6
884.0
886.1
839.4
836.8
830.8
831.1
energy content (MJ/kg)
42.72
42.43
42.58
42.72
42.81
viscosity at 40 °C (mm2/s)
2.486
4.544
4.209
2.804
2.601
2.486
2.477
cetane number (CN) cloud point (CP) (°C)
55.1 6.2
5.9
1.3
54.4 6.1
54.4 7.3
61.1 6.2
68.6 6.2
20
12
6
23
24
20
20
11.4
7.6
cold filter plugging point (CFPP) (°C) oxydation stability at 110 °C
the impact of both the length of the hydrocarbon chain and the double carbon bond. The CN is increased from 55 to 70 by the addition of procetane. The purpose of using an additive is to obtain a variation in ignition properties only without modifying the other properties of the fuel.
2. EXPERIMENTAL SECTION 2.1. Fuel Matrix. The reference fuel, called “DIESEL REF”, is a conventional diesel fuel that does not contain FAME. The properties of the fuels tested are presented in Table 1. One of the objectives of this paper is to study the impact of biodiesel on the cold-start performance. The name biodiesel is given to transesterified fatty acids. The oils can be converted to their methyl esters via a transesterification process in the presence of a catalyst. Ideally, transesterification is potentially a less expensive way of transforming the large, branched molecular structure of bio-oils into smaller, straight-chain molecules of the type required in regular diesel combustion engines. In general, the term biodiesel covers a variety of materials made from vegetable oils or animal fats. Various crops are used in different parts of the world to make biodiesel (esters). In North America, soybean oil is the largest source for biodiesel, whereas it is rapeseed oil in Europe and palm and coconut oils in Asia. Biodiesel can also be made (marginally) from other feedstocks, including animal fats or used cooking oils. The esters have physical properties close to those of conventional fossil fuels, but these properties depend upon the raw material. Esters can have different numbers of carbon atoms and varying degrees of double carbon bonds. To evaluate the impact of this variation in chemical composition, different biodiesels are tested in this study: SME and RME. Blends of the diesel reference fuel and 10% of ester are prepared and called “DIESEL REF + RME” and “DIESEL REF + SME”. The ester ratio in the reference fuel was determined to avoid freezing problems during the test. FAME has the chemical formula CH3COOR. The conventional method of notation for the R group is Cx:y, with x for the number of carbon atoms in the chain and y for the number of double carbon bonds. The theoretical composition of RME and SME is given in Figure 1. RME mainly has one carbon carbon double bond, whereas SME mainly has two. There is a difference in cold-flow properties and oxidation stability for FAME (Table 1). SME has poor cold-flow properties compared to RME, and this point can be explained by the presence of around 10% C16:0 for SME. The oxidation stability is better for RME compared to SME because of the presence of more double carbon bonds in SME. Although FAME does not have good cold-flow properties, the blends present an acceptable cold filter plugging point (CFPP) (Table 1). The tests are performed at 25 °C. The CNs are similar, at around 55 for all of the blends (Table 1).
Figure 1. Theoretical composition of ester (just the main components). Another objective of this study is to assess the impact of the ignition delay on the cold-start performance. The method used to boost the CN is to add ignition-improver additives. The benefit of this is that it modifies the CN without affecting the other characteristics of the fuels. The procetane additive used is ethylhexyl nitrate (EHN). The chemical formula is C8H17NO3. Three levels of CN were targeted 55, 60, and 70. The first level is obtained with the reference fuel, and the second and the third levels are obtained with the addition of 1800 and 4400 ppm procetane, respectively. The last two fuels are called DIESEL-60 and DIESEL-70. Table 1 presents the CN and also the distillation properties of fuels containing procetane additives. It appears that the addition of 1800 ppm EHN enables a CN of 61.1 to be achieved, and with the addition of 4400 ppm, the CN is 68.6. 2.2. Test Cell and Measuring Equipment. The engine is installed in a cold and dehumidified room, with the inside temperature controlled at 25 °C. During the test, electronic control unit (ECU) data are acquired by an ETAS Inca System. In addition, conventional engine data, such as air, lubricant, water, and exhaust gas temperatures, air/fuel ratio in the exhaust line, or voltage and intensity of the starter and glow plugs, are recorded. Exhaust gas opacity is measured using an AVL 439 system. Each cylinder is fitted with an AVL GU12P pressure transducer to measure the gas pressure inside the combustion chamber. In addition, an optical shaft encoder determines the crank angle inside the engine cycle. It is well-known that heat-transfer losses are very high during cold starts. However, the objective of the combustion analysis conducted here is not to evaluate these losses but to compare the effect of different fuels on combustion when the engine is in idle mode just after starting. For this reason, we adopt the hypothesis that heat-transfer losses are not impacted by fuels, and we therefore consider the apparent “net heatrelease” rate, calculated as proposed by Heywood27 and already applied 4907
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Figure 3. Methodology for finding optimized settings.
Table 3. Definition of the Grade
speed increase delay
Figure 2. Illustration of the main criteria used to qualify a cold start.
value of the
value of the criteria
criteria for grade 0
for grade 10
2
0.5
exhaust gas opacity during start
50
10
stable idle delay
30
1
exhaust gas opacity during idle
11
1
engine speed stability after 20 s
40
10
Table 2. Engine Specifications engine reference
Renault M9R 740
engine type
four cylinder, turbocharged, direct injection
cylinder head
four valve/cylinder
compression ratio displacement (cm3)
nominal, 16:1 1995
bore (mm)
84
stroke (mm)
90
fuel injection system
common rail Bosch (Piezo)
glow plug
ceramic
before.28 This allows us to obtain information about the combustion process (nCA10, nCA50, and nCA90). 2.3. Soaking and Starting Procedure. Before a test, the engine, the battery, and the fuel tank are soaked in the cold test cell for at least 8 h. At the end of this pre-glowing period, the electric starter of the engine is used to crank the engine, which is declutched for the test. After starting, the engine is run in idle mode to characterize idle behavior in these conditions and the test is stopped after 2 min (see Figure 2). The engine temperature and load are then increased gradually to clean the engine before stopping it and starting a new soaking procedure. 2.4. Engine Test. The engine tests are performed on a RENAULT M9R four-cylinder diesel engine with a CR of 16:1. The specifications and dimensions of the test engine are indicated in Table 2. The tests are performed in two stages: (1) The first stage is with the original settings for all of the fuels to compare the impact of the fuel properties on the engine performance. (2) A simple methodology is then used to look for the optimal injection settings with each fuel. This methodology is described in Figure 3 and is divided into two stages. The first stage consists of varying the start of injection (SOI). This variation is achieved by a step of 3° crank angle (CA) around the standard settings and is applied to the whole injection train. At the end, the optimized SOI can be selected. The second stage consists of varying the quantity of fuel injected during the starting phase and is performed with the optimized SOI. Finally, we determine the optimized engine settings (SOI and injected quantity). A test is repeated with these settings to confirm the trend. 2.5. Methodology for Evaluating a Cold Start. 2.5.1. Main Assessment Criteria. As illustrated in Figure 2, several criteria are used to assess the engine behavior in cold operation. The criteria used during the starting phase are listed as follows: (i) rail pressure increase delay, defined as the duration between the moment when the starter is switched on and the moment when the rail pressure reaches 20 MPa, authorizing fuel injection; (ii) first combustion delay, defined as the
Table 4. Definition of the Grade for the Starting Phase and the Idle Phase and the Total Grade weighting (%) grade for the starting phase speed increase delay exhaust gas opacity during start stable idle delay grade for the idle phase
global grade
70 30 50
exhaust gas opacity during idle
20
engine speed stability after 20 s
30
grade for the starting phase
60
grade for the idle phase
40
duration between the first fuel injection and the first combustion during cold starts on one cylinder; (iii) speed increase delay, defined as the duration between the first combustion and the moment when the idle mode is reached (at the end of the starting phase); and (iv) exhaust gas opacity during start, assessed on the basis of the average gas opacity over a period of 5 s from the moment when the opacity signal increases. The criteria used during the idle phase are listed as follows: (i) stable idle delay, defined as the time required to reach a “stable idle” speed, corresponding to a standard deviation in engine speed (calculated on the basis of a moving average for 1 s) of under 25 rpm; (ii) engine speed stability after 20 s, assessed on the basis of the standard deviation in engine speed over a period of 1 s, 20 s after the moment when the “idle mode” is reached; (iii) engine speed standard deviation in idle, assessed on the basis of the standard deviation from 5 s after the moment when the “idle mode” is reached and until the end of the test; and (iv) exhaust gas opacity in idle, assessed on the basis of the average gas opacity from 20 s after the moment when the idle mode is reached and for a period of 60 s. 2.5.2. Grading System. On the basis of previous experience in the field of cold starts,29 it appears that it is difficult to make a global comparison of different tests because a lot of criteria are used to define cold-condition performance and do not vary in the same way. It is for this reason that a rigorous methodology designed to evaluate and compare performance in these conditions has been developed. The idea is to define a grade for each test that is representative of the overall performance of the engine. The higher the grade, the better the cold-start performance of the engine. It is thus possible to choose the optimum setting and make a more rigorous comparison of the performance obtained with different fuels. First of all, the criteria used to define the grade are those that demonstrate robustness, i.e., speed increase delay and exhaust gas 4908
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Figure 4. Impact on the grades with a variation of the SOI for fuel DIESEL REF + SME.
opacity for the starting phase and stable idle delay, exhaust gas opacity in idle, and engine speed stability after 20 s for the idle phase. Second, a method for calculation of the grade has been defined. For each criterion, a worst value and a best value are determined, corresponding to a worst grade of 0 and a best grade of 10. Between these two values, there is linear interpolation of the grade (Table 3). A grade for the starting phase and a grade for the idle phase are then defined by weighting these criteria (Table 4). From these two grades, a total grade is determined. With this grading system, it is easy to compare cold-start performances of different tests and also to assess behavior during the starting phase and the idle phase. Repeatability tests have been performed on this methodology (a total of 13 tests) to adjust the weightings and demonstrate the robustness of the method. The repeatability of the grade is (0.5. It is also important to note that the grading system has to be adapted on the basis of the objective and the engine. This paper presents an example of how the grading system can be applied.
3. RESULTS AND DISCUSSION 3.1. Impact of Biodiesel. The optimal settings were sought first by varying the SOI and then varying the quantity of fuel introduced during the start phase, as explained above. The two biodiesel blends (RME and SME) have the same behavior, which is why only the results for DIESEL REF + SME are presented in Figures 4 and 5. It appears that the cold-start performance is better with early injection timing. The trend observed for the starting phase and idle phase grades is the same, although the impact of SOI appears to be slightly greater for the starting phase. The optimum is obtained with an offset of +6 crank angle degrees (CADs) and +3 CADs relative to the initial settings. It should be noted that the difference between the CN of the fuels is not so important to explain the choice relative to optimized SOI. There is no difference in grade when the fuel quantity injected during the starting phase is varied, and this does not influence the grades in idle. This can be explained by the fact that there is little difference (no more than 1%) between the energy contents and
Figure 5. Impact on the grade for the starting phase with a variation of the fuel quantity injected during the starting phase for fuel DIESEL REF + SME.
densities of the fuels. Finally, the optimal setting for DIESEL REF + SME and DIESEL REF + RME is the initial SOI with an offset of +3 CADs. Figure 6 presents the evolution in grades when biodiesel is added to conventional diesel without any modification of engine settings. It can be observed that there is the same trend for the starting and idle phases. The total grade falls from 8.2 to 7.4, and the deterioration in the cold-start performance is the same for blends with RME and SME. The reduction in the total grade when biodiesel is introduced (with the initial engine settings) is primarily due to a slight increase in the speed increase delay and the increase in opacity during the idle phase (Table 5). After optimization, the speed increase delay is improved in comparison to the reference fuel and the opacity decreases, although it still remains slightly higher for biodiesel than for the reference fuel. The higher opacity for blends with FAME compared to conventional diesel fuel can be explained by their distillation properties. The T50 is 10 20 °C higher for blends with FAME. This can lead to poor vaporization of the fuel within the combustion chamber and to an increase of 4909
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Figure 6. Biodiesel impact on the grades representing cold-start performance with the original engine settings.
opacity. Thus, with optimized settings (Figure 7), it is possible to obtain the same performance for conventional diesels and blends with biodiesel. To gain a clear understanding of the phenomena involved, it is important to examine the combustion process. The addition of FAME to the reference fuel leads to an increase in combustion duration (Figure 8). The start of combustion is similar for blends with FAME and for the reference fuel. This means that the addition of biodiesel addition slows the end of the combustion process. However, optimizing the settings to give more time for the preparation of the fuel and air mixture can compensate for this slowdown at the end of the combustion process when FAME is added (Figure 9). 3.2. Impact of the CN. Optimizing the injection settings involves modifying the SOI for blends using procetane additives. The results are presented in Figure 10. The two blends, DIESEL60 and DIESEL-70, have the same behavior. It can be seen that the impact of SOI variation is not the same for the starting phase or the idle phase. Early injection timing is favorable to the starting phase. The results are more disparate for the idle phase. However, for the overall grade, it appears that the cold-start performance is better with early injection timing. The optimum is with an offset of +3 CADs relative to the initial settings. It is surprising that the optimal settings for these fuels with a high CN (short delay) consist of early injection timing. Figure 11 presents the evolution of the grade when the CN of the fuel is increased. For the same engine settings, it appears that an increase of the CN of around 15 points implies a decrease of the total grade of 20%. It is essentially due to a degradation of the idle phase. Generally, it is observed that the increase of the CN allows for a better cold-start performance. After optimization of the settings (Figure 12), the performance between the fuels is similar, despite the fact that the opacity during the idle phase remains important with DIESEL-60 and DIESEL-70. It is surprising to see that the addition of EHN does not change the ignition delay and the beginning of combustion (Figure 13)
Table 5. Biodiesel Impact on the Speed Increase Delay and the Opacity original engine settings speed
exhaust gas
optimized engine settings speed
exhaust gas
increase opacity during increase opacity during delay (s)
idle (%)
delay (s)
idle (%)
DIESEL REF
0.92
4.2
0.92
4.2
DIESEL REF + RME
1.10
7.4
0.84
5.0
DIESEL REF + SME
1.05
7.6
0.86
5.1
with the same settings as other blends, whereas the CN of the fuels is different. The addition of large quantities of cetane improver, which works by generating radicals when it decomposes,30,31 is not favorable to stable combustion. This can be illustrated by the loss in indicated mean effective pressure (IMEP) stability (Figure 14). IMEP stability represents the standard deviation of IMEP for all cycles. However, with optimized settings, the IMEP stability of the fuels is similar. This may be due to the fact that optimization gives more time for the preparation of the air and fuel mixture. The action of EHN is not usual, and it can be explained by the large quantity of additives added because of the CN targeted. Moreover, the evaporation properties are different for the reference fuel and EHN, the behavior of which is not known in cold conditions. Bickerton et al. observed that an increase in the CN obtained with the help of an additive is not as effective at 25 °C as an increase in the natural CN.32
4. SUMMARY, CONCLUSION, AND FUTURE STUDIES The objective of this study was to develop a methodology based on a grading system, for the assessment of the cold-start performance on diesel engines. The grade is interesting because 4910
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Figure 7. Biodiesel impact on the grades representing the cold-start performance with optimized engine settings.
Figure 8. Biodiesel impact on combustion duration (on the left), the start of combustion, and the ignition delay (on the right) with the original engine settings.
Figure 9. Biodiesel impact on combustion duration (on the left), the start of combustion, and the ignition delay (on the right) with the optimized engine settings.
it enables rapid assessment of the engine performance in cold conditions either overall or focusing specifically on the different phases (starting and idle). It also makes it easier to make comparisons between several tests. This grade takes into account several criteria related to the cold-start performance (starting time, combustion stability, opacity, etc.) and enables the criteria to be weighted as a function of the objective of the study. The second objective of this study was to use this methodology to assess the impact of fuel on the cold-start performance.
Different biodiesels were tested: SME and RME. The value of testing different biodiesels is that this makes it possible to study the impact of both the length of the hydrocarbon chain and the double carbon bond. The results demonstrate that there is no significant difference between SME and RME, showing that the structure of the ester is secondary to the cold-start behavior. The addition of ester to diesel fuel has an impact on the cold-start performance by raising the speed increase delay and smoke opacity, as a result of the slower combustion of FAME at the late 4911
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Figure 10. Impact on the grades with a variation of the SOI for DIESEL-60 and DIESEL-70.
Figure 11. CN impact on the grades representing the cold-start performance with the original engine settings.
angles. However, when the settings are simply optimized, it is possible to recover a performance close to that of a conventional fuel with a fuel containing FAME. However, it has been reported that the extensive usage of esters may have an impact on diesel fuel filter plugging capacity because of a depletion in saturated monoglycerides, saturated esters, and sterol glucosides, which could affect the cold-start performance. The CN is modified by adding a cetane improver (EHN) to increase it from 55 to 70. The advantage of using an additive is that this modifies the ignition properties without affecting the other properties of the fuel. The main conclusions show that modifying the CN by adding a procetane compound in large quantities is not a good solution. Indeed, it has been observed
that adding EHN does not have any impact on the ignition delay when tests are performed at 25 °C, probably because of the specific evaporation properties of this additive. Another important point that has not been studied in this paper is the effect of the fuel on engine emissions because the only pollutant considered here is smoke. However, HC, CO, and NOx emissions are probably modified as a result of the fuel. To investigate these phenomena in more depth, IFP Energies nouvelles (IFPEN) has launched a specific consortium: CODE I (Cold Operation Diesel Emissions Improvement). The goal is to develop measurement methods to assess emissions and the potential for reducing them using specific technologies/ strategies. 4912
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Figure 12. CN impact on the grades representing the cold-start performance with optimized engine settings.
PEUGEOT-CITROEN, RENAULT, and TOTAL) and ANR (Program CALIFOMEDO ANR-07-PDIT-007-01).
Figure 13. CN impact on the start of combustion and the ignition delay with the original engine settings.
’ NOMENCLATURE nCAxx = crank angle corresponding to xx% of the final net burned mass fraction CN = cetane number EHN = ethylhexyl nitrate EN590 = fuel within European specification limits FAME = fatty acid methyl ester FBP = final boiling point IBP = initial boiling point IMEP = indicated mean effective pressure NOx = nitrogen oxide SOI = start of injection TX% = temperature of which X% of the product has been distillated ’ REFERENCES
Figure 14. CN impact on the IMEP stability during idle phase.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +33-(0)1-47-52-71-95. Fax: +33-(0)1-47-52-66-85. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was performed in the context of a research program funded by the “Groupement Scientifique Moteurs” (IFP, PSA
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