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Biodiesel blend effects on common-rail diesel combustion and emissions
Fuel 89 (2010) 3442–3449
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Biodiesel blend effects on common-rail diesel combustion and emissions Marina Kousoulidou, Georgios Fontaras, Leonidas Ntziachristos, Zissis Samaras * Laboratory of Applied Thermodynamics, Aristotle University, P.O. Box 458, GR 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 20 June 2010 Accepted 22 June 2010 Available online 6 July 2010 Keywords: Biodiesel Emissions Fuel consumption Heat-release rates Particles
a b s t r a c t Biodiesel (fatty acid methylesters) blends with fossil diesel at a mixing ratio between 0.5 and 5 vol.% are widely offered as automotive fuels in Europe. The target for the future is to bring this ratio to at least 10%, in order to increase the share of renewable energy in transport. There is however limited evidence on the effects of such blends on the combustion and emissions of diesel engines not originally designed to operate on biodiesel blends. In this study, a number of experiments with 10 vol.% (B10) biodiesel fuel of palmoil origin were performed on a light-duty common-rail Euro 3 engine. The measurements included in-cylinder pressure, pollutants emissions, and fuel consumption. Combustion effects were limited but changes in the start of ignition and heat-release rate could be identified. Emission effects included both higher and lower smoke and NOx, depending on the operation point. The results on the engine bench were compared against a Euro 3 common-rail light-duty vehicle driven on the chassis dynamometer, in order to include the effects of emission control systems (EGR and oxidation catalyst). In addition to the palm biodiesel, an RME-diesel blend was also tested to examine the effects of a fuel with different characteristics. Both biodiesel blends reduced PM emissions and only marginal effects on NOx over the certification test could be identified. The results of this study show that up to 10% biodiesels could be used on current diesel vehicles, without significantly affecting vehicle emission performance. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biofuels have attracted considerable attention during the past decade as renewable, biodegradable, and non-toxic fuels. In 2009 the new European regulation (directive 2009/28/EC) introduced new targets for the European Union (EU) member states stating that each state shall ensure that the share of energy from renewable sources in all forms of transport in 2020 is at least 10% of the final energy consumption in transport. Today, biodiesel is the biofuel most commonly used in Europe. Biodiesel differs than fossil diesel in chemical character, as it primarily consists of esters, compared to the paraffinic and aromatic character of fossil diesel [1]. As a result of the different chemical composition, biodiesel also exhibits different physical properties than fossil diesel, such as higher cetane number, lower heating value, higher viscosity and flash-point. The different properties may in turn affect the combustion and emissions in a diesel engine. Studies of the biodiesel effects on emissions have indicated substantial reductions of particulate matter (PM) [1], carbon monoxide (CO), and unburned hydrocarbons (HC) [16]. However, fuel consumption
* Corresponding author. Tel.: +30 23 10 996014; fax: +30 23 10 996019. E-mail address: [email protected] (Z. Samaras). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.06.034
and nitrogen oxides (NOx) emissions usually increase [2,3]. The rise in fuel consumption is caused by differences in fuel energy content. For the case of the increased NOx emissions from biodiesel, studies suggest that the NOx emission levels increase is not determined by a change in a single fuel property, but rather is the result of a number of coupled mechanisms whose effects may tend to reinforce or cancel one another under different conditions, depending on specific combustion and fuel characteristics [25]. A number of fuel properties (viscosity, heating value, density and cetane number), as well as engine operating conditions and technology have all been shown to play a role on biodiesel emission effects [4–8]. In this framework, aim of the current study is to evaluate the effect of biodiesel on a current technology common-rail light-duty diesel engine. First, the effects on combustion were studied by measuring the in-cylinder pressure during combustion, with use of fossil diesel and a 10% blend of biodiesel of palmoil origin. In addition, regulated pollutants and fuel consumption were measured over steady state tests. The results collected on the engine bench were then compared against a common-rail light-duty vehicle driven on the chassis dynamometer, to also include the effect of typical diesel after treatment system (oxidation catalyst). In addition to the palmoil biodiesel, a rapeseed oil derived biodiesel blend was also tested on the vehicle to study the effect of biodiesel properties on emissions.
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Table 1 Fuel properties. Property
Base diesel
PME 10
RME 10
Test method
Density (g/cm3, 15 °C) Viscosity(mm2/s, 40 °C) Cetane number CFPP (°C) Higher heating value (MJ/kg)
0.834 3.61 55.5 12 45.78
0.837 3.72 55 5 45.09
0.835 3.73 55 9 45.00
EN ISO 3104 EN ISO 3675 EN ISO 5165 EN 116 CEN/TS 14918:2005
Table 2 Engine characteristics. Capacity Max power Max torque Injection system Aspiration Number of cylinders Compression ratio
2179 cm3 96 kW at 4000 rpm 310 Nm at 2100 rpm Common rail Turbocharged 4 18:1
2. Methodology 2.1. Fuels Two biodiesel blends were used in this study, one of palmoil (PME) and the second of rapeseed oil origin (RME). The two oils were blended in fossil diesel (base fuel), which was a typical market diesel fuel already containing an unknown biodiesel fraction, ranging from 1 to 2.5 vol.%. The base fuel was blended with the two biodiesels at a ratio of 10 vol.%, thus bringing the total ratio of biodiesel in the final fuels within the range of 11–12.5 vol.%. Only the PME blend was used for the engine tests while both the PME and RME blends were used for the vehicle testing. The fuels were prepared by splash blending just before the initiation of the measurement campaign. The properties of the base fuel and the two biodiesel blends are shown in Table 1. The cetane number was found well above the specification limit (51 min) for the base diesel and the two biodiesel blends tested. All fuels tested were found to comply with the EN 590 specifications. The higher heating value (HHR) is reported on Table 1, because this was experimentally determined. The ratio of the lower heating value (LHR) over HHR is practically the same between the fuels as the carbon to hydrogen ratio is little differentiated between the fuels (1.85 for the reference diesel and 1.86 for the biodiesel). Therefore, the relative reduction of LHV of biodiesel over the reference diesel is the same as for HHV.
Fig. 1. Engine map. Dots correspond to the 15 operating points measured. Shaded areas correspond to the engine map areas covered by the NEDC for a typical lightduty diesel engine.
All tests were performed on an AVL-Dynoperform (eddy current, max power 130 kW) engine dynamometer. Overall, 15 operating points were measured (Fig. 1) The areas where a typical light-duty vehicle engine operates over the certification test in Europe (NEDC) and a real-world motorway cycle (ARTEMIS) are also superimposed as the shaded areas in Fig. 1. The 15 points selected cover most of the NEDC and ARTEMIS driving cycles. A number of parameters were measured during testing. Fuel consumption was determined by real-time weighting of an external tank which was connected to the fuel supply system of the engine. In-cylinder pressure was measured using a Kistler hightemperature pressure sensor (type 6056A), mounted on the glow plug bore of the first cylinder. The intake air airflow rate was recorded using the engine’s hot-wire anemometer. Engine smoke was measured by an AVL 415S Variable Sampling Smoke meter, while CO2, NOx and concentrations in the raw exhaust were determined with non-dispersive infrared and chemiluminscence laboratory analyzers. All pollutants were measured directly in the raw exhaust. Using measured data regarding air flow and exhaust gas temperature the concentrations recorded were converted to pollutant mass flow. The pollutant mass flow were then expressed as grams of pollutant/kW h. Fuel consumption was measured directly via weight balance. The balance is certified and calibrated with an accuracy of 0.1 g. The balance is certified and calibrated with an accuracy of 0.1 g. Fuel consumption in all cases was recorded after the engine operation was stabilized at each operating point and the balance data show a steady fuel consumption over time.
2.2. Engine testing
2.3. Vehicle and protocol
The engine used for testing was a 2.2 l PSA DW12A TED diesel one (specs in Table 2). This was a common rail, direct injection, turbocharged and intercooled engine with exhaust gas recirculation (EGR). In this engine, three fuel injections per combustion cycle are possible (pilot, main, and post). The operating conditions required no post-injection while pilot and main injections followed stock engine calibration over all operating points. The EGR was disabled during the test bench experiments, in order to ensure that the intake airflow would remain constant during the measurements, for both fuels. This will have as a result an increase in flame temperature and the NOx levels at low load conditions, where EGR is mostly implemented. However, this was deemed necessary to establish the same engine operation conditions. The engine was operated for 4 h at a speed of 2500 rpm and a load of 100 Nm for conditioning between fuel changes.
In addition to the engine tests, a Renault Laguna 1.9 l dCi common-rail turbocharged passenger car was tested with the PME and RME blends, and the base fuel for reference, covering a total distance of 2000 km for testing. The vehicle was equipped with an oxidation catalyst to meet the Euro 3 emission limits. Both the vehicle engine and the test cell engines have the main characteristics of a Euro 4 engine such as high-fuel injection pressure (both over 1250 bar), direct injection, multiple injections per cycle, and EGR. Therefore the basic observations and conclusions drawn from his study can be extended to current technology engines. The measurement protocol consisted of one cold-start New European Driving Cycle – NEDC, one hot Urban Driving Cycle – UDC (urban sub-cycle of NEDC) and these were followed by three real-world driving cycles [10]. The so-called Artemis cycles are highly transient and cover a range of driving conditions, including
M. Kousoulidou et al. / Fuel 89 (2010) 3442–3449
(a)
350
2 250 1.5 200 1
0.2
1.8 100
3 2.6 8 1. 1
50 0
1000
1500
-0.9
0.5 0
4 -0. -0.5
2000
2500
3000
3500
4000
RPM
By using the in-cylinder pressure measurement one may calculate the heat-release rate using Eq. (1) [9]:
(b)
350
10
300
8
ð1Þ
6
200
4
150
where Tcyl is the in-cylinder temperature, mcyl is the mass of compressible mixture, calculated as the sum of masses of intake air and fuel, and R is the gas constant, taken constant for all fuels
50 0
-2 .9
6.5
100
-3.6
3.6 0.7
0
3.6
6. 9.4 5 1000
2
-1.5 0.7
1500
2000
7 0.
ð2Þ
6 3.
Load (Nm)
250
where dQ is the heat-release rate, p is the in-cylinder pressure, V is dt the cylinder volume, and c is the specific heat ratio. The average in-cylinder temperature can be calculated using Eq. (2):
p V ¼ mcyl R T cyl
1
150
2.4. Calculations
dQ c dV 1 dp ¼ þ p V c 1 dt c 1 dt dt
2.5
300
0.2
urban, rural (road) and motorway driving. Two repetitions of each cycle were conducted per fuel. The measurement series started with the base fuel, the PME 10% followed next, then the base fuel again, to conclude with the RME 10% measurement. At least 1000 km (up to 3000 km) intervened between each fuel change. Emission measurements were conducted following the regulations. The exhaust was sampled in a dilution tunnel following the constant volume sampling (CVS) principle and proportional diluted exhaust samples were collected in bags for gaseous pollutants measurement. PM was collected on Teflon-coated borosilicate filters (Pallflex TX40 £47 mm) which were conditioned before and after the measurement in a constant humidity/constant temperature room.
Load (Nm)
3444
-2 2500
3000
3500
4000
RPM
3. Results and discussion 3.1. Engine combustion
(c)
350
4
300
2
250 0
1.1
Load (Nm)
Fig. 2a shows the difference (in percentage units) of the max incylinder pressure (pmax) developed with use of the biodiesel blend, compared to the base fuel. Limited differences in pmax provide an indication that the combustion evolves in much the same way between the two fuels. Also, for comparison, the fuel consumption is shown in Fig. 2b, again as a percentage difference of the biodiesel blend over the base fuel. The differences observed between the two fuels are in general limited over the entire engine map, especially for pmax. The maximum fuel consumption difference measured was in the order of 11% but at very low load (1500 rpm and 30 Nm), which is not representative of a typical real-world transient engine operation. At higher loads and speeds, the difference in fuel consumption was much smaller, in the range of 3% to +4%. The corresponding variation in pmax was even less, from 1% to +3%. The corresponding differences in heat release were from 5% to +7%. There is no specific pattern of the effect of biodiesel on fuel consumption. Over the modes covered by the NEDC driving cycle (see Fig. 1), use of PME would increase fuel consumption. For higher loads at moderate speed the consumption is reduced with use of biodiesel, albeit not significantly. Increased fuel consumption generally also leads to higher pmax, except of a few modes of moderate load and speed where fuel consumption and pmax exhibit opposite trends. On average, overall operating modes, the calculated brake specific fuel consumption (bsfc) seems to be slightly increased by 1.3% with the use of the biodiesel fuel compared to the conventional diesel. The reasons for increased consumption with use of biodiesel maybe twofold. First, the different physical character of the blend,
-1.7
200 -2.7
2.5
-2
1.2
150 2.3
4.8
-4
-1
100 1.8
-1.5
2.5
-6
-2.7
50 -3.4
-8.4
0 1000
1500
2000
2500
3000
3500
4000
-8
RPM Fig. 2. Difference of (a) in-cylinder pressure maxima, (b) fuel consumption and (c) thermal efficiency with use of biodiesel blend over the bas fuel, over the engine map. All values are in percentage units.
in terms of viscosity and surface tension may influence the spray development and hence the combustion characteristics. This might lead to non-optimized ignition timing and hence fuel consumption increase. The second reason is fuel chemistry. FAME contains oxygen which reduces the specific energy content of the fuel; therefore a higher fuel mass is required to counterbalance the energy loss. On the other hand, biodiesels contain unsaturated bonds which may modify the combustion chain mechanism and may increase the combustion temperature. Obviously, a lower pmax with increased consumption denotes a reduced engine efficiency and vice versa.
M. Kousoulidou et al. / Fuel 89 (2010) 3442–3449
Fig. 2c shows the deviation (in percentage units) of the engine thermal efficiency with use of the biodiesel blend, compared to the base fuel. Over the modes covered by the NEDC driving cycle, use of PME would lead to lower values of engine thermal efficiency. However, for higher loads at moderate speed the thermal efficiency is slightly increased with the use of biodiesel. In order to provide more insight into the in-cylinder combustion process, Fig. 3 shows the cylinder pressure build-up and the heat-release profile for four engine operation modes which range from a low-load/low-speed condition (1500 rpm and 30 Nm) to a high-load, high-speed one (3500 rpm and 170 Nm). The horizontal axis is in crank-angle degrees (CAD) after top dead center (ATDC). At 1500 rpm, 30 Nm, use of the biodiesel blend leads to higher exothermy during the premixed phase which follows the pilot injection. However, the main combustion develops in much the same way with the base fuel. The fuel consumption is 5% higher with the biodiesel blend while the heat released was identical between the two fuels. At higher load (1500 rpm and 170 Nm) the fuel injection duration is much longer while premixed combustion starts much earlier (20 CAD) than at low load (10 CAD). In this case, it is the base fuel which leads to higher exothermy in the premixed phase than the biodiesel blend. Despite the lower temperature at the end of the premixed phase, it is the biodiesel blend that starts the main combustion phase 0.5 CAD earlier than the base fuel, which denotes a different chemical mechanism of low temperature combustion. The biodiesel combustion proceeds faster and the total heat release almost matches the diesel combustion at 12 CAD. Due to the earlier and faster combustion, use of the biodiesel actually leads to reduced fuel consumption (1%) and heat release (3%) over the base fuel for the same power output. At higher speed (3500 rpm), the premixed combustion phase is negligible and fuel combustion occurs at one stage, as no pilot injection takes place. At low load (75Nm) the ignition point – defined as the extension of the max heat-release gradient on the x-axis – is identical for both fuels at 3 CAD. However, B10 combustion evolves faster until about 19 CAD. Then, possibly because of
3445
the higher enthalpy, conventional diesel takes over and results in higher heat release by 3% over the biodiesel blend. Similar to the 1500 rpm, 170 Nm case, the fast combustion leads to better efficiency and the fuel consumption with biodiesel drops overall by 1%. At 170 Nm, it is diesel combustion that starts 0.5 CAD faster while combustion evolves at the same rate. Due to this reason, 4% higher fuel combustion is required for the same power output with use of the biodiesel and the heat release increases by 2%. The analysis of the four engine modes shows that the effect of biodiesel on the diesel combustion process is a complicated one and it also depends on the local conditions. The ignition point may appear both earlier or later (by as much as ±0.5 CAD) compared to the base fuel. This is rather expected as the base fuel and the biodiesel blend have almost identical cetane number (55). However, in most cases biodiesel combustion evolves faster than fossil diesel during the initial phases of expansion, possibly owned to the different combustion chemical chain mechanism. The faster combustion rate counterbalances – to a certain extent – the lower by 1.5% (1% on a volumetric basis) heating value of the biodiesel blend. As a result, while one would expect increased fuel consumption over the entire engine map both because of the lower heating value and due to the lack of engine calibration for the particular fuel, there are engine modes where fuel consumption actually decreases with use of the biodiesel. Fig. 2 provides a good summary of these areas where either positive or negative effects by the use of biodiesel appear. The discussion shows that different conclusions may be reached depending on the actual tests performed and the engine points selected. This will obviously have an effect on engine emissions and, by extrapolation, to vehicle emissions over the type-approval test. The following sections will assist to link combustion characteristics with emission performance for the particular engine and a vehicle. 3.2. Engine emissions The difference in combustion due to biodiesel use has repercussions to emissions. NOx emissions (Fig. 4a) vary between 6% and
Fig. 3. Pressure and heat-release profiles of fossil diesel (Diesel) and a PME biofuel blend (B10) over four engine operating points.
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+4% over the engine map. The corresponding range for smoke – in principle soot measured by the smokemeter – is much higher and may exceed 100% for some of the points measured (b). Better insight may be gained by looking at the four individual engine modes, as in the case of the ‘engine combustion’ section. Under conditions of low load and low RPM (1500 rpm and 30 Nm), NOx emissions increase by 1% and smoke decreases by 14% with the biodiesel blend. As it was shown in Fig. 3, combustion evolved in much the same way in this mode. Therefore, the increase of NOx and decrease of PM should probably be attributed to the difference in the chemistry of pollutants formation, mainly because of the oxygen content in the fuel molecule of biodiesel [11,12]. The effect of biodiesel chemistry on NOx emissions has been also shown in previous studies [8,11–14]. At the same speed but higher load (1500 rpm and 170 Nm), NOx decreases by 1% and smoke seems to increase quite substantially, by 118% with use of the biodiesel. Using the in-cylinder pressure profile and Eq. (2), one may calculate that the average max cylinder temperature in the case of biodiesel reaches 935 °C, compared to 1068 °C with the base fuel. This significant drop in temperature and heat release, especially in the premixed phase, seems to be responsible for the much higher smoke emissions. They can also be considered responsible for the marginal drop in NOx emissions. Comparison of the two modes at the same speed (1500 rpm) shows that the actual effect of biodiesel is much dependant on the engine mode of operation. Different engine loads may lead to
opposite conclusions regarding the effect on NOx and smoke. This is something that should be considered when comparing heavyduty and light-duty engine effects of biodiesel. The former are usually tested including higher load points, compared to the light-duty ones. At higher engine speeds the pilot injection is eliminated while combustion has less time to evolve, and this may lead to a variation of the emission performance. At low load (75 Nm) NOx emissions decrease by 6% while smoke increases by 13%. There is again reduced heat release and temperature in this mode with use of the biodiesel, which is consistent with the observed impact on emissions. On the contrary, at high-load, high-speed, both NOx and smoke increase with use of the biodiesel blend, by 2% and 60%, respectively, following the increase in both fuel consumption and heat release. The increase in all variables with use of biodiesel is an indication of non-optimized combustion. Since the effect of the physical properties of biodiesel on the injection advance has not been proved in engines with common-rail injection system, a possible explanation for increased emissions in this case is longer injection times due to the higher biodiesel viscosity [6,15]. This means that part of the fuel is injected late in the combustion stroke at this high-speed operation point. Although this does not significantly affect the total heat released, it may lead to significant smoke formation due to partial oxidation of this late injected fuel. Finally one could suggest the possibility that the lower energy content of the biodiesel blend also requires a longer injection time to inject the energy required to meet the load demand. However in the 10% blends the lack in energy content is compensated by an increase in fuel density. Since the engine operates on a volumetric flow basis the actual energy shortfall is very small (less than 1%) and hence such increases are less likely. The variable impact of biodiesel use appears over the entire engine map, as can be seen in Fig. 4, and general trends are not easy to extract. This trend also comes to agreement with [26] where at higher speed and loads the change in engine control settings due to lower energy content of the blended fuel led to a NOx increase. In our case, three general profiles can be identified: (a) increased NOx and reduced smoke emissions over the zone of 2000– 3000 rpm and up to 50% load (b) reduced NOx and increased smoke emissions over either low speed (<2000 rpm) and high-load (>50%) or over high-speed (>3000 rpm) and low load (<50%), and (c) both increased NOx and smoke emissions (high-speed, highload). According to [26], because of the difference in NOx effect at low and high loads and the sensitivity to ignition properties of the fuel, the magnitude and the direction of the NOx effect associated with burning biodiesel blends over a duty cycle depends on the duty cycle average power and the cetane number of the blend. 3.3. Vehicle emissions
Fig. 4. Difference in (a) NOx emissions, and (b) smoke emissions over the engine map with the use of biodiesel blend over the base fuel. All values are in percentage units.
The vehicle tests were conducted in order to check the biodiesel effects over transient testing and to include the effect of emission control, such as EGR and oxidation catalyst. In addition, a second biodiesel blend consisting of 10% RME was also tested in order to examine the effect of a different FAME composition on consumption and emissions. Generally, cetane number, heat of combustion, melting point, and viscosity of neat fatty compounds increase with increasing chain length and decrease with increasing unsaturation [8,17] of the FAME molecule. Generally, rapeseed FAME contains a higher proportion of larger chain unsaturated components (oleic and linoleic) than PME palmitic, oleic. Although the effect of fuel composition was not observed in the final fuel physical properties (Table 1) due to the relatively low mixing ratio, it is still interesting to observe potential effects on consumption and emissions. The fuel consumption was calculated on the basis of CO2 emissions, using the carbon balance between engine intake and
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exhaust. Eq. (3) was used to derive fuel consumption from the CO2 and CO measurements in the exhaust. For some of the measurements, fuel weighing took place and confirmed the accuracy of this calculation
FC ¼
ECO2 ECO ð12:011 þ 1:008rH:C þ 16:000r O:C Þ þ 44:011 28:011
ð3Þ
where rH:C is the hydrogen to carbon ratio in the fuel molecule. This was 1.85 for the base fuel and 1.865 for the biodiesel blends, and rO:C is the oxygen to carbon ratio which was zero for the base fuel and 0.011 for the biodiesel blend. Fig. 5 shows the CO2 emissions and fuel consumption as a function of driving cycle, for the three different fuels tested. There are only limited differences observed for tailpipe CO2 emissions between the three fuels. In practice, biodiesel emissions fall within the measurement scatter of the base fuel measurements. The only exception is the RME blend over the urban driving cycle, which exhibits a 4% reduction over the base fuel. The fuel consumption pattern rather follows the CO2 emission pattern, with some deviation due to the different fuel composition. The small differences in fuel consumption are consistent to the limited differences in consumption observed in engine tests (Fig. 2b). The increase in the engine fuel consumption at low loads and moderate speeds was either masked by the EGR operation or the averaging effect of the neighboring points in vehicle tests. Therefore, these measurements demonstrate that one should not expect any large impacts on vehicle tailpipe CO2 emissions from the use of biodiesel, at least up to 10% blends. This was confirmed on the vehicle with the use of two different biodiesels. However, small, driving-cycle and biodiesel-specific differences may be observed, which are the result of the biodiesel composition and its impact on combustion and engine efficiency.
a
Base Fuel
250
a
PME 200
RME
100
50
0.6
PME RME
Euro 3 CO Limit
0.4 0.3
0.1
NEDC
Urban
Road
Motorway
0.0
NEDC
Urban
Road
Motorway
9
b
8 7
0.035 0.030
6
0.025
5
HC [g/km]
FC [l/100 km]
Base Fuel
0.2
0
b
0.7
0.5
150
CO [g/km]
CO2 [g/km]
Fig. 6 presents measurement results for CO and HC emissions. With regard to CO, the vehicle emission level is much below the type-approval limit even over the cold-start NEDC, while it is at very low absolute levels over the hot-start Artemis cycles. Therefore no safe conclusions may be drawn with regard to biodiesel effects on CO emissions. In general, biodiesel blends do not exhibit statistically significant differences over the base fuel over none of the cycles tested. On the contrary, use of the 10% PME blend results to significantly higher HC emissions (Fig. 6b), starting from 41% higher over the Urban and 37% higher over the NEDC, down to no increase over the motorway cycle. The 10% RME blend also shows some increase (15%) over the NEDC but this is within measurement uncertainty. It is difficult to explain the biodiesel effects on HC emissions as this largely depends on the catalyst efficiency, further to the engine out emissions. The poor volatilization and combustibility of specific fuel components may be responsible for HC increases. Injection timing differences may also have a negative effect. Also, the catalyst selectivity in specific species may also be responsible. Another source of higher HC emissions may be lubricant oil, as biodiesel is known for its higher solvent effects compared to fossil diesel. Lube oil degradation can lead to the development of lighter components which evaporate and not burn on cylinder walls [27,28]. From a diesel emissions perspective, it is more interesting to study the effects of biodiesel on NOx and PM emissions. These are shown in Fig. 7 for all cycles tested. NOx emissions are not directly comparable to the engine emissions, especially over the engine operation range for the certification test, because the EGR was enabled in the vehicle tests while this was disabled for engine testing. Over the type-approval NEDC, NOx effects are marginal and within the measurement uncertainty of the base fuel. PME biodiesel exactly matches the mean base fuel measurement while the
4 3 2
0.020 0.015 0.010
1 0.005
0
NEDC
Urban
Road
Motorway
0.000
NEDC Fig. 5. (a) Emissions of tailpipe CO2, and (b) calculated fuel consumption over different driving cycles.
Urban
Road
Motorway
Fig. 6. Vehicle emissions of (a) CO, and (b) HC over different driving cycles.
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a
1.4
Base Fuel 1.2
NOx [g/km]
1.0
PME Euro 3
RME
NOx Limit
0.8 0.6
the oxygen in the biodiesel molecules on soot formation, PM emission increases are more difficult to assess. These may either be due to soot or semi-volatile species increase, as a result of incomplete combustion. The current study shows that although PM decreases with use of the biodiesel blends, their effect is less pronounced or even slightly negative over a high-speed and high-load driving cycle. In future studies, full evaluation of biodiesel effects on vehicle emissions need to consider non-type-approval (off-cycle) emissions as well.
0.4
4. Conclusions
0.2 0.0
NEDC
b 0.08
Urban
Road
Motorway
Euro 3 PM Limit
0.07
PM [g/km]
0.06 0.05 0.04 0.03 0.02 0.01 0.00
NEDC
Urban
Road
Motorway
Fig. 7. Vehicle emissions of (a) NOx, and (b) PM over different driving cycles.
RME appears some 5% higher, but within uncertainty. On the other hand, RME NOx emissions are almost identical to the base fuel over all other driving cycles while it is mostly PME that shows differences up to 20% over the Motorway cycle. The higher NOx emissions as load (and speed) increases and as we move into the non-EGR area, is consistent both to the NOx increase in the engine measurements but also to literature studies which show an increase in NOx as load increases [6,18]. For this light-duty vehicle, use of biodiesel up to 10% does not seem to raise issues regarding its conformity with the emission standard. However, as at least one of the two blends led to statistically significant increases over real-world driving cycles, this may be an issue to consider in areas suffering from photochemical pollution. With regard to PM, this study confirms earlier findings that biodiesel blends reduce PM emissions [19–24], mainly attributed to the oxygen content of the biodiesel molecule, which enables more complete combustion even in regions of the combustion chamber with fuel-rich diffusion flames [22]. The PME and RME led to PM NEDC reductions in the order of 17% and 24%, respectively. There is still some variability with the base fuel emissions, but in this case the overlap with the biodiesel emission levels is rather small to allow doubts about the emission reducing effects. The same effects are also observed over the Artemis driving cycles. Only over the Motorway driving cycle seems that the RME blend leads to higher emissions, but still within the uncertainty of the base fuel emissions. PM is much more complicated than smoke, which was measured in engine tests (Fig. 4b), as it consists of both soot particles, and semi-volatile or volatile organic, and ionic species. Although reductions in PM may be attributed to the suppressing effect of
Based on the tests on the engine and vehicle conducted in this study, it is concluded that blending of biodiesel in petroleum-derived diesel at 10 vol.% changes fuel properties to a degree that can affect combustion characteristics, such as start of ignition and heat-release rate. The effects on the engine depend on the operation point considered and range from 0.5 CAD to +0.5 CAD for the start of injection and 5% to +7% (one point at +13%) for the total heat release, over 15 engine operation points measured. As a result of the impact to combustion, NOx and smoke emissions differ over the engine map with use of biodiesel. NOx effects ranges from 6% to +4%, while smoke emissions appeared much more to biodiesel use and ranged from 50% to +70% (one point at +118%), compared to fossil diesel. Further to areas where the NOx-PM trade-off is observed and biodiesel use leads to opposite effects in these two pollutants, there is also an area of high-load and speed where concurrent increases in both pollutants are observed. With regard to vehicle emissions, use of the two biodiesel blends led to tailpipe CO2 differences which are within experimental uncertainty for most driving conditions. Only the RME blend exhibited a reduction of 4% over a real-world urban cycle. This demonstrates that potential CO2 benefits from biodiesel fuels should only be expected from their production and not use on a vehicle. The CO emission level of the car was found much below the emission standard over the certification test and at very low absolute level over the real-world hot-start cycles. Therefore, use of the biodiesel did not have an effect on its CO performance. HC emissions increased both over the certification tests and over real-world cycles, in particular for the PME derived biodiesel, and the maximum difference reached 40%. NOx emissions did not significantly change with either the PME or the RME biodiesel blends. The PME blend led to up to 20% higher emissions over the base fuel over the more transient real-world cycles. Finally, PM emissions were reduced when the vehicle was driven over low load conditions, including the certification test. The reductions were 17% for the PME and 24% for the RME blend. Driving over the highspeed and high-load motorway cycle led to much smaller reductions over the PME and a marginal increase with the RME blend. Acknowledgements Part of this work was funded by Prefecture of Western Macedonia and the Greek Scholarships Foundation (IKY). The authors would like to thank Dr. Panayotis Pistikopoulos and Mr. Georgios Vagiatas for assisting in the experimental part of this work and Mr. Elias Saltas for his help in the data analysis. References [1] Graboski MS, McCormick RL. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog Energy Combust Sci 1998;24:125–64. [2] McCormick RL, Williams A, Ireland J, Brimhall M, Hayes RR. Effects of biodiesel blends on vehicle emissions. Int J Eng Res 2006;2:249–61. [3] Fontaras G, Karavalakis G, Kousoulidou M, Tzamkiozis T, Ntziachristos L, Bakeas E, et al. Effects of biodiesel on passenger car fuel consumption,
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regulated and non-regulated pollutant emissions over legislated and realworld driving cycles. Fuel 2009;88:1608–17. Marshall W, Schumacher L, Howell S. Engine exhaust emissions evaluation of a cummins L10E when fueled with a biodiesel blend, society of automotive engineers. SAE technical paper 952363; 1995. Sze C. Impact of test cycle and biodiesel concentration on emissions. SAE 200701-4040; 2007. Tat M. Investigation of oxides of nitrogen emissions from biodiesel-fueled engines. In: Mechanical engineering department. Iowa: Iowa State University; 2003. EPA. A comprehensive analysis of biodiesel impacts on exhaust emissions. Draft technical report, USEP agency, editor; 2002. Knothe G, Steidley K. Kinematic viscosity of biodiesel fuel components and related compounds, influence of compound structure and comparison to petrodiesel fuel components. Fuel Process Technol 2005;84:1059–65. Heywood JB. Internal combustion engine fundamentals. McGraw-Hill; 1998. Andre M. The ARTEMIS European driving cycles for measuring car pollutant emissions. Sci Total Environ 2004;334:73–84. Ban-Weiss GA, Chen JY, Buchholz BA, Dibble RW. A numerical investigation into the anomalous slight Nox increase when burning biodiesel; a new (old) theory. Fuel Process Technol 2007;88:659–67. Fernando S, Hall C, Jha S. NOx reduction from biodiesel fuels. Energy Fuels 2006;20:376–82. McCormick RL, Graboski MS, Alleman TL, Herring AM, Tyson KS. Impact of biodiesel source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. Environ Sci Technol 2001;35:1742–7. Cardone M, Prati MV, Seggiani M, Senatore A, Vitolo S. Brassica carinata as an alternative oil crop for the production of biodiesel in Italy: engine performance and regulated and unregulated exhaust emissions. Environ Sci Technol 2002;36:4656–62. Szybist JP, Song J, Alam M, Boehman AL. Biodiesel combustion, emissions and emission control. Fuel Process Technol 2007;88:679–91. Sharp C, Howell S, Jobe J. The effect of biodiesel fuels on transient emissions from modern diesel engines, part II. SAE paper no. 2000-01-1967; 2000. Yamane K, Ueta A, Shimamoto Y. Influence of physical and chemical properties of biodiesel fuels on injection, combustion and exhaust emission
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characteristics in a direct injection compression ignition engine. Int J Eng Res 2001;2:249–61. Peterson C, Taberski J, Thompson J, Chase C. The effect of biodiesel feedstock on regulated emissions in chassis dynamometer tests of a pickup truck. Trans ASAE 2000;43:1371–81. Durbin TD, Cocker III DR, Sawant AA, Johnson K, Miller JW, Holden BB, et al. Regulated emissions from biodiesel fuels from on/offroad applications. Atmos Environ 2007;41:5647–58. Hass MJ, Scott KM, Alleman TL, McCormick RL. Engine performance of biodiesel fuel prepared from soybean soap stock: a high quality renewable fuel produced from a waste feedstock. Energy Fuels 2001;82: 1207–12. Baldassarri LT, Battistelli CL, Conti L, Crebelli R, Berardis B, Iamicelli AL, et al. Emission comparison of urban bus engine fueled with diesel oil and biodiesel blend. Sci Total Environ 2004;327:147–62. Lapuerta M, Armas O, Fernandez JR. Effect of biodiesel fuels on diesel engine emissions. Prog Energy Combust Sci 2008;34:198–223. Lapuerta M, Armas O, Ballesteros R, Fernandez. Diesel emissions from biofuels derived from Spanish potential vegetable oils. J Fuel 2005;84:773–80. Dwivedi D, Agarwal AK, Sharma M. Particulate emission characterization of a biodiesel vs. diesel-fuelled compression ignition transport engine: a comparative study. Atmos Environ 2006;40:5586–95. Mueller CJ, Boehman AL, Martin GC. An experimental investigation of the origin of increased NOx emissions when fuelling a heavy-duty compression–ignition engine with soy biodiesel. SAE technical paper no. 2009-01-1792. Eckerle WA, Lyford-Pike EJ, Stanton DW, LaPointe LA, Whitacre SD, Wall JC. Effects of methylester biodiesel blends on NOx emissions. SAE Int J Fuels Lubricants 2008;1:102–18. Sappok A, Wong V. Impact of biodiesel on ash emissions and lubricant properties affecting fuel economy and engine wear: comparison with conventional diesel fuel. SAE technical paper no. 2008-01-1395. Thornton MJ, Alleman TL, Luecke J, McCormick RL. Impacts of biodiesel fuel blends oil dilution on light-duty diesel engine operation, conference paper NREL/CP-540-44833 August 2009. Available at: http://www.nrel.gov/ vehiclesandfuels/npbf/pdfs/44833.pdf.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Biodiesel blend effects on common-rail diesel combustion and emissions Marina Kousoulidou, Georgios Fontaras, Leonidas Ntziachristos, Zissis Samaras * Laboratory of Applied Thermodynamics, Aristotle University, P.O. Box 458, GR 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 20 June 2010 Accepted 22 June 2010 Available online 6 July 2010 Keywords: Biodiesel Emissions Fuel consumption Heat-release rates Particles
a b s t r a c t Biodiesel (fatty acid methylesters) blends with fossil diesel at a mixing ratio between 0.5 and 5 vol.% are widely offered as automotive fuels in Europe. The target for the future is to bring this ratio to at least 10%, in order to increase the share of renewable energy in transport. There is however limited evidence on the effects of such blends on the combustion and emissions of diesel engines not originally designed to operate on biodiesel blends. In this study, a number of experiments with 10 vol.% (B10) biodiesel fuel of palmoil origin were performed on a light-duty common-rail Euro 3 engine. The measurements included in-cylinder pressure, pollutants emissions, and fuel consumption. Combustion effects were limited but changes in the start of ignition and heat-release rate could be identified. Emission effects included both higher and lower smoke and NOx, depending on the operation point. The results on the engine bench were compared against a Euro 3 common-rail light-duty vehicle driven on the chassis dynamometer, in order to include the effects of emission control systems (EGR and oxidation catalyst). In addition to the palm biodiesel, an RME-diesel blend was also tested to examine the effects of a fuel with different characteristics. Both biodiesel blends reduced PM emissions and only marginal effects on NOx over the certification test could be identified. The results of this study show that up to 10% biodiesels could be used on current diesel vehicles, without significantly affecting vehicle emission performance. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biofuels have attracted considerable attention during the past decade as renewable, biodegradable, and non-toxic fuels. In 2009 the new European regulation (directive 2009/28/EC) introduced new targets for the European Union (EU) member states stating that each state shall ensure that the share of energy from renewable sources in all forms of transport in 2020 is at least 10% of the final energy consumption in transport. Today, biodiesel is the biofuel most commonly used in Europe. Biodiesel differs than fossil diesel in chemical character, as it primarily consists of esters, compared to the paraffinic and aromatic character of fossil diesel [1]. As a result of the different chemical composition, biodiesel also exhibits different physical properties than fossil diesel, such as higher cetane number, lower heating value, higher viscosity and flash-point. The different properties may in turn affect the combustion and emissions in a diesel engine. Studies of the biodiesel effects on emissions have indicated substantial reductions of particulate matter (PM) [1], carbon monoxide (CO), and unburned hydrocarbons (HC) [16]. However, fuel consumption
* Corresponding author. Tel.: +30 23 10 996014; fax: +30 23 10 996019. E-mail address: [email protected] (Z. Samaras). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.06.034
and nitrogen oxides (NOx) emissions usually increase [2,3]. The rise in fuel consumption is caused by differences in fuel energy content. For the case of the increased NOx emissions from biodiesel, studies suggest that the NOx emission levels increase is not determined by a change in a single fuel property, but rather is the result of a number of coupled mechanisms whose effects may tend to reinforce or cancel one another under different conditions, depending on specific combustion and fuel characteristics [25]. A number of fuel properties (viscosity, heating value, density and cetane number), as well as engine operating conditions and technology have all been shown to play a role on biodiesel emission effects [4–8]. In this framework, aim of the current study is to evaluate the effect of biodiesel on a current technology common-rail light-duty diesel engine. First, the effects on combustion were studied by measuring the in-cylinder pressure during combustion, with use of fossil diesel and a 10% blend of biodiesel of palmoil origin. In addition, regulated pollutants and fuel consumption were measured over steady state tests. The results collected on the engine bench were then compared against a common-rail light-duty vehicle driven on the chassis dynamometer, to also include the effect of typical diesel after treatment system (oxidation catalyst). In addition to the palmoil biodiesel, a rapeseed oil derived biodiesel blend was also tested on the vehicle to study the effect of biodiesel properties on emissions.
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Table 1 Fuel properties. Property
Base diesel
PME 10
RME 10
Test method
Density (g/cm3, 15 °C) Viscosity(mm2/s, 40 °C) Cetane number CFPP (°C) Higher heating value (MJ/kg)
0.834 3.61 55.5 12 45.78
0.837 3.72 55 5 45.09
0.835 3.73 55 9 45.00
EN ISO 3104 EN ISO 3675 EN ISO 5165 EN 116 CEN/TS 14918:2005
Table 2 Engine characteristics. Capacity Max power Max torque Injection system Aspiration Number of cylinders Compression ratio
2179 cm3 96 kW at 4000 rpm 310 Nm at 2100 rpm Common rail Turbocharged 4 18:1
2. Methodology 2.1. Fuels Two biodiesel blends were used in this study, one of palmoil (PME) and the second of rapeseed oil origin (RME). The two oils were blended in fossil diesel (base fuel), which was a typical market diesel fuel already containing an unknown biodiesel fraction, ranging from 1 to 2.5 vol.%. The base fuel was blended with the two biodiesels at a ratio of 10 vol.%, thus bringing the total ratio of biodiesel in the final fuels within the range of 11–12.5 vol.%. Only the PME blend was used for the engine tests while both the PME and RME blends were used for the vehicle testing. The fuels were prepared by splash blending just before the initiation of the measurement campaign. The properties of the base fuel and the two biodiesel blends are shown in Table 1. The cetane number was found well above the specification limit (51 min) for the base diesel and the two biodiesel blends tested. All fuels tested were found to comply with the EN 590 specifications. The higher heating value (HHR) is reported on Table 1, because this was experimentally determined. The ratio of the lower heating value (LHR) over HHR is practically the same between the fuels as the carbon to hydrogen ratio is little differentiated between the fuels (1.85 for the reference diesel and 1.86 for the biodiesel). Therefore, the relative reduction of LHV of biodiesel over the reference diesel is the same as for HHV.
Fig. 1. Engine map. Dots correspond to the 15 operating points measured. Shaded areas correspond to the engine map areas covered by the NEDC for a typical lightduty diesel engine.
All tests were performed on an AVL-Dynoperform (eddy current, max power 130 kW) engine dynamometer. Overall, 15 operating points were measured (Fig. 1) The areas where a typical light-duty vehicle engine operates over the certification test in Europe (NEDC) and a real-world motorway cycle (ARTEMIS) are also superimposed as the shaded areas in Fig. 1. The 15 points selected cover most of the NEDC and ARTEMIS driving cycles. A number of parameters were measured during testing. Fuel consumption was determined by real-time weighting of an external tank which was connected to the fuel supply system of the engine. In-cylinder pressure was measured using a Kistler hightemperature pressure sensor (type 6056A), mounted on the glow plug bore of the first cylinder. The intake air airflow rate was recorded using the engine’s hot-wire anemometer. Engine smoke was measured by an AVL 415S Variable Sampling Smoke meter, while CO2, NOx and concentrations in the raw exhaust were determined with non-dispersive infrared and chemiluminscence laboratory analyzers. All pollutants were measured directly in the raw exhaust. Using measured data regarding air flow and exhaust gas temperature the concentrations recorded were converted to pollutant mass flow. The pollutant mass flow were then expressed as grams of pollutant/kW h. Fuel consumption was measured directly via weight balance. The balance is certified and calibrated with an accuracy of 0.1 g. The balance is certified and calibrated with an accuracy of 0.1 g. Fuel consumption in all cases was recorded after the engine operation was stabilized at each operating point and the balance data show a steady fuel consumption over time.
2.2. Engine testing
2.3. Vehicle and protocol
The engine used for testing was a 2.2 l PSA DW12A TED diesel one (specs in Table 2). This was a common rail, direct injection, turbocharged and intercooled engine with exhaust gas recirculation (EGR). In this engine, three fuel injections per combustion cycle are possible (pilot, main, and post). The operating conditions required no post-injection while pilot and main injections followed stock engine calibration over all operating points. The EGR was disabled during the test bench experiments, in order to ensure that the intake airflow would remain constant during the measurements, for both fuels. This will have as a result an increase in flame temperature and the NOx levels at low load conditions, where EGR is mostly implemented. However, this was deemed necessary to establish the same engine operation conditions. The engine was operated for 4 h at a speed of 2500 rpm and a load of 100 Nm for conditioning between fuel changes.
In addition to the engine tests, a Renault Laguna 1.9 l dCi common-rail turbocharged passenger car was tested with the PME and RME blends, and the base fuel for reference, covering a total distance of 2000 km for testing. The vehicle was equipped with an oxidation catalyst to meet the Euro 3 emission limits. Both the vehicle engine and the test cell engines have the main characteristics of a Euro 4 engine such as high-fuel injection pressure (both over 1250 bar), direct injection, multiple injections per cycle, and EGR. Therefore the basic observations and conclusions drawn from his study can be extended to current technology engines. The measurement protocol consisted of one cold-start New European Driving Cycle – NEDC, one hot Urban Driving Cycle – UDC (urban sub-cycle of NEDC) and these were followed by three real-world driving cycles [10]. The so-called Artemis cycles are highly transient and cover a range of driving conditions, including
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(a)
350
2 250 1.5 200 1
0.2
1.8 100
3 2.6 8 1. 1
50 0
1000
1500
-0.9
0.5 0
4 -0. -0.5
2000
2500
3000
3500
4000
RPM
By using the in-cylinder pressure measurement one may calculate the heat-release rate using Eq. (1) [9]:
(b)
350
10
300
8
ð1Þ
6
200
4
150
where Tcyl is the in-cylinder temperature, mcyl is the mass of compressible mixture, calculated as the sum of masses of intake air and fuel, and R is the gas constant, taken constant for all fuels
50 0
-2 .9
6.5
100
-3.6
3.6 0.7
0
3.6
6. 9.4 5 1000
2
-1.5 0.7
1500
2000
7 0.
ð2Þ
6 3.
Load (Nm)
250
where dQ is the heat-release rate, p is the in-cylinder pressure, V is dt the cylinder volume, and c is the specific heat ratio. The average in-cylinder temperature can be calculated using Eq. (2):
p V ¼ mcyl R T cyl
1
150
2.4. Calculations
dQ c dV 1 dp ¼ þ p V c 1 dt c 1 dt dt
2.5
300
0.2
urban, rural (road) and motorway driving. Two repetitions of each cycle were conducted per fuel. The measurement series started with the base fuel, the PME 10% followed next, then the base fuel again, to conclude with the RME 10% measurement. At least 1000 km (up to 3000 km) intervened between each fuel change. Emission measurements were conducted following the regulations. The exhaust was sampled in a dilution tunnel following the constant volume sampling (CVS) principle and proportional diluted exhaust samples were collected in bags for gaseous pollutants measurement. PM was collected on Teflon-coated borosilicate filters (Pallflex TX40 £47 mm) which were conditioned before and after the measurement in a constant humidity/constant temperature room.
Load (Nm)
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-2 2500
3000
3500
4000
RPM
3. Results and discussion 3.1. Engine combustion
(c)
350
4
300
2
250 0
1.1
Load (Nm)
Fig. 2a shows the difference (in percentage units) of the max incylinder pressure (pmax) developed with use of the biodiesel blend, compared to the base fuel. Limited differences in pmax provide an indication that the combustion evolves in much the same way between the two fuels. Also, for comparison, the fuel consumption is shown in Fig. 2b, again as a percentage difference of the biodiesel blend over the base fuel. The differences observed between the two fuels are in general limited over the entire engine map, especially for pmax. The maximum fuel consumption difference measured was in the order of 11% but at very low load (1500 rpm and 30 Nm), which is not representative of a typical real-world transient engine operation. At higher loads and speeds, the difference in fuel consumption was much smaller, in the range of 3% to +4%. The corresponding variation in pmax was even less, from 1% to +3%. The corresponding differences in heat release were from 5% to +7%. There is no specific pattern of the effect of biodiesel on fuel consumption. Over the modes covered by the NEDC driving cycle (see Fig. 1), use of PME would increase fuel consumption. For higher loads at moderate speed the consumption is reduced with use of biodiesel, albeit not significantly. Increased fuel consumption generally also leads to higher pmax, except of a few modes of moderate load and speed where fuel consumption and pmax exhibit opposite trends. On average, overall operating modes, the calculated brake specific fuel consumption (bsfc) seems to be slightly increased by 1.3% with the use of the biodiesel fuel compared to the conventional diesel. The reasons for increased consumption with use of biodiesel maybe twofold. First, the different physical character of the blend,
-1.7
200 -2.7
2.5
-2
1.2
150 2.3
4.8
-4
-1
100 1.8
-1.5
2.5
-6
-2.7
50 -3.4
-8.4
0 1000
1500
2000
2500
3000
3500
4000
-8
RPM Fig. 2. Difference of (a) in-cylinder pressure maxima, (b) fuel consumption and (c) thermal efficiency with use of biodiesel blend over the bas fuel, over the engine map. All values are in percentage units.
in terms of viscosity and surface tension may influence the spray development and hence the combustion characteristics. This might lead to non-optimized ignition timing and hence fuel consumption increase. The second reason is fuel chemistry. FAME contains oxygen which reduces the specific energy content of the fuel; therefore a higher fuel mass is required to counterbalance the energy loss. On the other hand, biodiesels contain unsaturated bonds which may modify the combustion chain mechanism and may increase the combustion temperature. Obviously, a lower pmax with increased consumption denotes a reduced engine efficiency and vice versa.
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Fig. 2c shows the deviation (in percentage units) of the engine thermal efficiency with use of the biodiesel blend, compared to the base fuel. Over the modes covered by the NEDC driving cycle, use of PME would lead to lower values of engine thermal efficiency. However, for higher loads at moderate speed the thermal efficiency is slightly increased with the use of biodiesel. In order to provide more insight into the in-cylinder combustion process, Fig. 3 shows the cylinder pressure build-up and the heat-release profile for four engine operation modes which range from a low-load/low-speed condition (1500 rpm and 30 Nm) to a high-load, high-speed one (3500 rpm and 170 Nm). The horizontal axis is in crank-angle degrees (CAD) after top dead center (ATDC). At 1500 rpm, 30 Nm, use of the biodiesel blend leads to higher exothermy during the premixed phase which follows the pilot injection. However, the main combustion develops in much the same way with the base fuel. The fuel consumption is 5% higher with the biodiesel blend while the heat released was identical between the two fuels. At higher load (1500 rpm and 170 Nm) the fuel injection duration is much longer while premixed combustion starts much earlier (20 CAD) than at low load (10 CAD). In this case, it is the base fuel which leads to higher exothermy in the premixed phase than the biodiesel blend. Despite the lower temperature at the end of the premixed phase, it is the biodiesel blend that starts the main combustion phase 0.5 CAD earlier than the base fuel, which denotes a different chemical mechanism of low temperature combustion. The biodiesel combustion proceeds faster and the total heat release almost matches the diesel combustion at 12 CAD. Due to the earlier and faster combustion, use of the biodiesel actually leads to reduced fuel consumption (1%) and heat release (3%) over the base fuel for the same power output. At higher speed (3500 rpm), the premixed combustion phase is negligible and fuel combustion occurs at one stage, as no pilot injection takes place. At low load (75Nm) the ignition point – defined as the extension of the max heat-release gradient on the x-axis – is identical for both fuels at 3 CAD. However, B10 combustion evolves faster until about 19 CAD. Then, possibly because of
3445
the higher enthalpy, conventional diesel takes over and results in higher heat release by 3% over the biodiesel blend. Similar to the 1500 rpm, 170 Nm case, the fast combustion leads to better efficiency and the fuel consumption with biodiesel drops overall by 1%. At 170 Nm, it is diesel combustion that starts 0.5 CAD faster while combustion evolves at the same rate. Due to this reason, 4% higher fuel combustion is required for the same power output with use of the biodiesel and the heat release increases by 2%. The analysis of the four engine modes shows that the effect of biodiesel on the diesel combustion process is a complicated one and it also depends on the local conditions. The ignition point may appear both earlier or later (by as much as ±0.5 CAD) compared to the base fuel. This is rather expected as the base fuel and the biodiesel blend have almost identical cetane number (55). However, in most cases biodiesel combustion evolves faster than fossil diesel during the initial phases of expansion, possibly owned to the different combustion chemical chain mechanism. The faster combustion rate counterbalances – to a certain extent – the lower by 1.5% (1% on a volumetric basis) heating value of the biodiesel blend. As a result, while one would expect increased fuel consumption over the entire engine map both because of the lower heating value and due to the lack of engine calibration for the particular fuel, there are engine modes where fuel consumption actually decreases with use of the biodiesel. Fig. 2 provides a good summary of these areas where either positive or negative effects by the use of biodiesel appear. The discussion shows that different conclusions may be reached depending on the actual tests performed and the engine points selected. This will obviously have an effect on engine emissions and, by extrapolation, to vehicle emissions over the type-approval test. The following sections will assist to link combustion characteristics with emission performance for the particular engine and a vehicle. 3.2. Engine emissions The difference in combustion due to biodiesel use has repercussions to emissions. NOx emissions (Fig. 4a) vary between 6% and
Fig. 3. Pressure and heat-release profiles of fossil diesel (Diesel) and a PME biofuel blend (B10) over four engine operating points.
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+4% over the engine map. The corresponding range for smoke – in principle soot measured by the smokemeter – is much higher and may exceed 100% for some of the points measured (b). Better insight may be gained by looking at the four individual engine modes, as in the case of the ‘engine combustion’ section. Under conditions of low load and low RPM (1500 rpm and 30 Nm), NOx emissions increase by 1% and smoke decreases by 14% with the biodiesel blend. As it was shown in Fig. 3, combustion evolved in much the same way in this mode. Therefore, the increase of NOx and decrease of PM should probably be attributed to the difference in the chemistry of pollutants formation, mainly because of the oxygen content in the fuel molecule of biodiesel [11,12]. The effect of biodiesel chemistry on NOx emissions has been also shown in previous studies [8,11–14]. At the same speed but higher load (1500 rpm and 170 Nm), NOx decreases by 1% and smoke seems to increase quite substantially, by 118% with use of the biodiesel. Using the in-cylinder pressure profile and Eq. (2), one may calculate that the average max cylinder temperature in the case of biodiesel reaches 935 °C, compared to 1068 °C with the base fuel. This significant drop in temperature and heat release, especially in the premixed phase, seems to be responsible for the much higher smoke emissions. They can also be considered responsible for the marginal drop in NOx emissions. Comparison of the two modes at the same speed (1500 rpm) shows that the actual effect of biodiesel is much dependant on the engine mode of operation. Different engine loads may lead to
opposite conclusions regarding the effect on NOx and smoke. This is something that should be considered when comparing heavyduty and light-duty engine effects of biodiesel. The former are usually tested including higher load points, compared to the light-duty ones. At higher engine speeds the pilot injection is eliminated while combustion has less time to evolve, and this may lead to a variation of the emission performance. At low load (75 Nm) NOx emissions decrease by 6% while smoke increases by 13%. There is again reduced heat release and temperature in this mode with use of the biodiesel, which is consistent with the observed impact on emissions. On the contrary, at high-load, high-speed, both NOx and smoke increase with use of the biodiesel blend, by 2% and 60%, respectively, following the increase in both fuel consumption and heat release. The increase in all variables with use of biodiesel is an indication of non-optimized combustion. Since the effect of the physical properties of biodiesel on the injection advance has not been proved in engines with common-rail injection system, a possible explanation for increased emissions in this case is longer injection times due to the higher biodiesel viscosity [6,15]. This means that part of the fuel is injected late in the combustion stroke at this high-speed operation point. Although this does not significantly affect the total heat released, it may lead to significant smoke formation due to partial oxidation of this late injected fuel. Finally one could suggest the possibility that the lower energy content of the biodiesel blend also requires a longer injection time to inject the energy required to meet the load demand. However in the 10% blends the lack in energy content is compensated by an increase in fuel density. Since the engine operates on a volumetric flow basis the actual energy shortfall is very small (less than 1%) and hence such increases are less likely. The variable impact of biodiesel use appears over the entire engine map, as can be seen in Fig. 4, and general trends are not easy to extract. This trend also comes to agreement with [26] where at higher speed and loads the change in engine control settings due to lower energy content of the blended fuel led to a NOx increase. In our case, three general profiles can be identified: (a) increased NOx and reduced smoke emissions over the zone of 2000– 3000 rpm and up to 50% load (b) reduced NOx and increased smoke emissions over either low speed (<2000 rpm) and high-load (>50%) or over high-speed (>3000 rpm) and low load (<50%), and (c) both increased NOx and smoke emissions (high-speed, highload). According to [26], because of the difference in NOx effect at low and high loads and the sensitivity to ignition properties of the fuel, the magnitude and the direction of the NOx effect associated with burning biodiesel blends over a duty cycle depends on the duty cycle average power and the cetane number of the blend. 3.3. Vehicle emissions
Fig. 4. Difference in (a) NOx emissions, and (b) smoke emissions over the engine map with the use of biodiesel blend over the base fuel. All values are in percentage units.
The vehicle tests were conducted in order to check the biodiesel effects over transient testing and to include the effect of emission control, such as EGR and oxidation catalyst. In addition, a second biodiesel blend consisting of 10% RME was also tested in order to examine the effect of a different FAME composition on consumption and emissions. Generally, cetane number, heat of combustion, melting point, and viscosity of neat fatty compounds increase with increasing chain length and decrease with increasing unsaturation [8,17] of the FAME molecule. Generally, rapeseed FAME contains a higher proportion of larger chain unsaturated components (oleic and linoleic) than PME palmitic, oleic. Although the effect of fuel composition was not observed in the final fuel physical properties (Table 1) due to the relatively low mixing ratio, it is still interesting to observe potential effects on consumption and emissions. The fuel consumption was calculated on the basis of CO2 emissions, using the carbon balance between engine intake and
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exhaust. Eq. (3) was used to derive fuel consumption from the CO2 and CO measurements in the exhaust. For some of the measurements, fuel weighing took place and confirmed the accuracy of this calculation
FC ¼
ECO2 ECO ð12:011 þ 1:008rH:C þ 16:000r O:C Þ þ 44:011 28:011
ð3Þ
where rH:C is the hydrogen to carbon ratio in the fuel molecule. This was 1.85 for the base fuel and 1.865 for the biodiesel blends, and rO:C is the oxygen to carbon ratio which was zero for the base fuel and 0.011 for the biodiesel blend. Fig. 5 shows the CO2 emissions and fuel consumption as a function of driving cycle, for the three different fuels tested. There are only limited differences observed for tailpipe CO2 emissions between the three fuels. In practice, biodiesel emissions fall within the measurement scatter of the base fuel measurements. The only exception is the RME blend over the urban driving cycle, which exhibits a 4% reduction over the base fuel. The fuel consumption pattern rather follows the CO2 emission pattern, with some deviation due to the different fuel composition. The small differences in fuel consumption are consistent to the limited differences in consumption observed in engine tests (Fig. 2b). The increase in the engine fuel consumption at low loads and moderate speeds was either masked by the EGR operation or the averaging effect of the neighboring points in vehicle tests. Therefore, these measurements demonstrate that one should not expect any large impacts on vehicle tailpipe CO2 emissions from the use of biodiesel, at least up to 10% blends. This was confirmed on the vehicle with the use of two different biodiesels. However, small, driving-cycle and biodiesel-specific differences may be observed, which are the result of the biodiesel composition and its impact on combustion and engine efficiency.
a
Base Fuel
250
a
PME 200
RME
100
50
0.6
PME RME
Euro 3 CO Limit
0.4 0.3
0.1
NEDC
Urban
Road
Motorway
0.0
NEDC
Urban
Road
Motorway
9
b
8 7
0.035 0.030
6
0.025
5
HC [g/km]
FC [l/100 km]
Base Fuel
0.2
0
b
0.7
0.5
150
CO [g/km]
CO2 [g/km]
Fig. 6 presents measurement results for CO and HC emissions. With regard to CO, the vehicle emission level is much below the type-approval limit even over the cold-start NEDC, while it is at very low absolute levels over the hot-start Artemis cycles. Therefore no safe conclusions may be drawn with regard to biodiesel effects on CO emissions. In general, biodiesel blends do not exhibit statistically significant differences over the base fuel over none of the cycles tested. On the contrary, use of the 10% PME blend results to significantly higher HC emissions (Fig. 6b), starting from 41% higher over the Urban and 37% higher over the NEDC, down to no increase over the motorway cycle. The 10% RME blend also shows some increase (15%) over the NEDC but this is within measurement uncertainty. It is difficult to explain the biodiesel effects on HC emissions as this largely depends on the catalyst efficiency, further to the engine out emissions. The poor volatilization and combustibility of specific fuel components may be responsible for HC increases. Injection timing differences may also have a negative effect. Also, the catalyst selectivity in specific species may also be responsible. Another source of higher HC emissions may be lubricant oil, as biodiesel is known for its higher solvent effects compared to fossil diesel. Lube oil degradation can lead to the development of lighter components which evaporate and not burn on cylinder walls [27,28]. From a diesel emissions perspective, it is more interesting to study the effects of biodiesel on NOx and PM emissions. These are shown in Fig. 7 for all cycles tested. NOx emissions are not directly comparable to the engine emissions, especially over the engine operation range for the certification test, because the EGR was enabled in the vehicle tests while this was disabled for engine testing. Over the type-approval NEDC, NOx effects are marginal and within the measurement uncertainty of the base fuel. PME biodiesel exactly matches the mean base fuel measurement while the
4 3 2
0.020 0.015 0.010
1 0.005
0
NEDC
Urban
Road
Motorway
0.000
NEDC Fig. 5. (a) Emissions of tailpipe CO2, and (b) calculated fuel consumption over different driving cycles.
Urban
Road
Motorway
Fig. 6. Vehicle emissions of (a) CO, and (b) HC over different driving cycles.
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a
1.4
Base Fuel 1.2
NOx [g/km]
1.0
PME Euro 3
RME
NOx Limit
0.8 0.6
the oxygen in the biodiesel molecules on soot formation, PM emission increases are more difficult to assess. These may either be due to soot or semi-volatile species increase, as a result of incomplete combustion. The current study shows that although PM decreases with use of the biodiesel blends, their effect is less pronounced or even slightly negative over a high-speed and high-load driving cycle. In future studies, full evaluation of biodiesel effects on vehicle emissions need to consider non-type-approval (off-cycle) emissions as well.
0.4
4. Conclusions
0.2 0.0
NEDC
b 0.08
Urban
Road
Motorway
Euro 3 PM Limit
0.07
PM [g/km]
0.06 0.05 0.04 0.03 0.02 0.01 0.00
NEDC
Urban
Road
Motorway
Fig. 7. Vehicle emissions of (a) NOx, and (b) PM over different driving cycles.
RME appears some 5% higher, but within uncertainty. On the other hand, RME NOx emissions are almost identical to the base fuel over all other driving cycles while it is mostly PME that shows differences up to 20% over the Motorway cycle. The higher NOx emissions as load (and speed) increases and as we move into the non-EGR area, is consistent both to the NOx increase in the engine measurements but also to literature studies which show an increase in NOx as load increases [6,18]. For this light-duty vehicle, use of biodiesel up to 10% does not seem to raise issues regarding its conformity with the emission standard. However, as at least one of the two blends led to statistically significant increases over real-world driving cycles, this may be an issue to consider in areas suffering from photochemical pollution. With regard to PM, this study confirms earlier findings that biodiesel blends reduce PM emissions [19–24], mainly attributed to the oxygen content of the biodiesel molecule, which enables more complete combustion even in regions of the combustion chamber with fuel-rich diffusion flames [22]. The PME and RME led to PM NEDC reductions in the order of 17% and 24%, respectively. There is still some variability with the base fuel emissions, but in this case the overlap with the biodiesel emission levels is rather small to allow doubts about the emission reducing effects. The same effects are also observed over the Artemis driving cycles. Only over the Motorway driving cycle seems that the RME blend leads to higher emissions, but still within the uncertainty of the base fuel emissions. PM is much more complicated than smoke, which was measured in engine tests (Fig. 4b), as it consists of both soot particles, and semi-volatile or volatile organic, and ionic species. Although reductions in PM may be attributed to the suppressing effect of
Based on the tests on the engine and vehicle conducted in this study, it is concluded that blending of biodiesel in petroleum-derived diesel at 10 vol.% changes fuel properties to a degree that can affect combustion characteristics, such as start of ignition and heat-release rate. The effects on the engine depend on the operation point considered and range from 0.5 CAD to +0.5 CAD for the start of injection and 5% to +7% (one point at +13%) for the total heat release, over 15 engine operation points measured. As a result of the impact to combustion, NOx and smoke emissions differ over the engine map with use of biodiesel. NOx effects ranges from 6% to +4%, while smoke emissions appeared much more to biodiesel use and ranged from 50% to +70% (one point at +118%), compared to fossil diesel. Further to areas where the NOx-PM trade-off is observed and biodiesel use leads to opposite effects in these two pollutants, there is also an area of high-load and speed where concurrent increases in both pollutants are observed. With regard to vehicle emissions, use of the two biodiesel blends led to tailpipe CO2 differences which are within experimental uncertainty for most driving conditions. Only the RME blend exhibited a reduction of 4% over a real-world urban cycle. This demonstrates that potential CO2 benefits from biodiesel fuels should only be expected from their production and not use on a vehicle. The CO emission level of the car was found much below the emission standard over the certification test and at very low absolute level over the real-world hot-start cycles. Therefore, use of the biodiesel did not have an effect on its CO performance. HC emissions increased both over the certification tests and over real-world cycles, in particular for the PME derived biodiesel, and the maximum difference reached 40%. NOx emissions did not significantly change with either the PME or the RME biodiesel blends. The PME blend led to up to 20% higher emissions over the base fuel over the more transient real-world cycles. Finally, PM emissions were reduced when the vehicle was driven over low load conditions, including the certification test. The reductions were 17% for the PME and 24% for the RME blend. Driving over the highspeed and high-load motorway cycle led to much smaller reductions over the PME and a marginal increase with the RME blend. Acknowledgements Part of this work was funded by Prefecture of Western Macedonia and the Greek Scholarships Foundation (IKY). The authors would like to thank Dr. Panayotis Pistikopoulos and Mr. Georgios Vagiatas for assisting in the experimental part of this work and Mr. Elias Saltas for his help in the data analysis. References [1] Graboski MS, McCormick RL. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog Energy Combust Sci 1998;24:125–64. [2] McCormick RL, Williams A, Ireland J, Brimhall M, Hayes RR. Effects of biodiesel blends on vehicle emissions. Int J Eng Res 2006;2:249–61. [3] Fontaras G, Karavalakis G, Kousoulidou M, Tzamkiozis T, Ntziachristos L, Bakeas E, et al. Effects of biodiesel on passenger car fuel consumption,
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