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Direct heat recovery from the ICE exhaust gas
Direct heat recovery from the ICE exhaust gas R Cipollone, D Di Battista, A Gualtieri Department of Mechanical Engineering, Energy and Management, University of L’Aquila, Italy
ABSTRACT In the last years “thinking green” has become the keyword for a more sustainable future; the prevention of the worsening of the environmental conditions produced a strong reduction on ICE emissions that has been, in the recent past, the driving force of the engine evolution. Today, a new interest appeared on the scene and it is CO2 reduction: this goal is focusing the attention of the actual ICE technological evolution and a huge interest is characterizing the market. In this paper the Authors studied an energy recovery system from the exhaust gasses, integrated with the turbocharger, considering that the enthalpy drop across the turbine is usually higher than that requested by the compressor. 1.
INTRODUCTION
Emissions of CO2 from light and heavy duty vehicles (L&HDV) fleet currently account for a significant proportion of the total emissions due to transportation sector. Measures to reduce emissions are, therefore, particularly appreciated and requested by Governments. The two sectors have different impacts: the freight transport is especially challenging due to close links with economic development while the passenger and light duty sector is more oriented to social expectations, related to concept of the quality of life. In the last years, the CO2 emissions of the transportation sector have been rising substantially. In the meantime, legislations concerning CO2 for LDV have been introduced to reduce energy consumption and GHG emissions (Figure 1).
Figure 1: CO2 emissions for LDV in different regions: solid lines represent historical data, dashed lines enacted and proposed targets [1]
____________________________________________ © The author(s) and/or their employer(s), 2012
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Although LDV produce the largest energy consumption in the transportation sector, the projections of CO2 emissions are rising [2] as well as the transportation volumes: this situation suggests regulations in the HDV sector, too. The actual HDV approval cycle for regulated emission (NOx, CO, HC, PN and PM) is based on engine measurements done on test beds and they are expressed in g/kWh, being the energy produced by the engine (instead of the distance as for LDV) a more significant factor. Waiting for new standards, it is expected that in the near future the same test procedure will be adopted for fuel consumption and CO2 as well. In HDV the interest appears to be focused also towards individual engine components and sectors [3] (powertrain, energy recovery, weight reduction, etc…). Figure 2 offers a contribution on the expected CO2 reduction among such sectors. The major reduction is achievable with hybridization technology, but it has a nonnegligible cost increase. Technologies which improve thermal efficiency, or those oriented to recover the great amount of waste heat produced by the engine seem to have a more immediate application [4], due to lower cost increase per unit of CO2 saved.
Figure 2: Expected percentage of CO2 saving for different technological options and HDV segments Exhaust gas leaving cylinders has two mains energy contributions (so, energy losses): the first is related to its pressure, the second to its temperature. Both values are greater than ambient values. Work, therefore, could be recovered by means of a direct expansion (to the atmospheric pressure) or by means of a heat recovery [5-6]. This acts as a high temperature energy source for a working fluid and, thanks to it, the fluid vaporizes at high pressure and produces work inside a proper expander. Its condensation is followed by a pressurization in liquid phase and continuity is done on the energy recovery in mechanical form. Thermoelectric recovery has been studied as well [7] but they still require some material science advancements in order to reduce cost and increase performances. First recovery can be done directly on the turbocharging system [8-10] and it is labelled as “direct heat recovery”, DHR. The interest to this recovery is huge, considering that almost all diesel engines are turbocharged and that the downsizing of gasoline engines requires the need of intake air boosting [11]. The paper focuses the attention on DHR potential.
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Actual turbocharging technology requires that the expansion work produced by the turbine must be equal to the compression work requested by the compressor, for a given boost pressure whose values are stored in a look-up table as a function of engine speed and load. This matching is done wasting the excess of energy: (a) by passing part of the expanding gas inside the waste gate valve; (b) throttling the exhaust gas inside a variable geometry turbine (VGT). In many engine working points the energy unbalance between turbine and compressor is great and, so, the energy dissipated. The Authors already presented two different concepts of direct heat recovery (DHR) from the waste gasses [12] when a VGT technology is used. The two solutions depend mainly on the level of modification that is accepted with respect to the existing turbocharging system. The first does not change the existing turbine (rack controlled); the second asks for a more invasive intervention, but the energy recovery degree is more promising. In a previous work [12], the authors showed that when considering LDV, the energy recovery fits in the range 3-7 % of the overall propulsion energy when NEDC is considered. This is due to the low engine load and speed working points characterizing NEDC. A greater recovery is expected when a more realistic approval cycle will be adopted. In this paper the Authors focus the attention on HDV where energy efficiency means fuel cost reduction, particularly welcome in the sector. In order to evaluate the potential of this recovery, a 1D model of the engine and the turbocharging group has been developed and calibrated on a F1C IVECO 3L Euro 4 engine equipped as HDV. Engine performances, as well as relevant data concerning the behaviour of the turbo-charging system (flow rates, pressure drops, temperatures, VGT rack positions, etc…) have been measured and predicted with a high degree of accuracy. Once the model was deeply calibrated, it has been used as virtual platform to calculate the energy recovery according to ESC-13: each engine working point has been reproduced and all the engine properties calculated in order to predict energy recovery. The model has been enhanced considering different possibility to use the recovered energy in mechanical or in electrical form. Fuel consumption reduction and CO2 emission saving have been estimated. 2.
MODEL DESCRIPTION AND VALIDATION
The methodology used to evaluate the potential of the DHR required the development of a procedure organized as follows. Once the engine has been defined: a) a virtual model of the engine was developed; it is able to represent engine performances, including the implementation of control strategies of major significance (boost pressure control, fuel injection, cooling, EGR, etc...); b) the model was tested and calibrated with experimental data; c) the model was used as a virtual platform for the evaluation of the potential of the energy recovery technologies. The engine considered in this paper is an EURO 4 IVECO F1C 3.0 L with a variable geometry turbine. This engine has the possibility to set up ECU even for a HD application and it has been tested on a bench, checking the model with the experimental data. Figure 3 shows a comparison between calculated data and experimental data for pressure ratio both for compressor and turbine, which are important variables in this work. They are represented as a function of engine speed, at full load at every speed.
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Figure 3: Comparison between measured and calculated engine pressure ratio for compressor and turbine The agreement is quite satisfactory and produces a 10% average error on the engine torque (in the full operating engine conditions), value that is reduced to 2% within the characteristic region of the ESC-13 approval cycle. The model is very comprehensive and it is able to predict many engine characteristic variables. 3.
DIRECT HEAT RECOVERY TECHNOLOGIES
As mentioned above, the enthalpy of the exhaust gasses leaving cylinders is greater than what is required to compress intake air. It follows, therefore, a potential energy recovery through a turbine. The turbocharger on the engine tested has a variable geometry turbo (VGT) system that matches the turbocharger on higher efficiencies, reduces the response time of the group, allows a robust control of the rack position according to the boost pressure, allows for a dynamic overboost on specific engine points. Unfortunately, it produces a pressure loss at the impeller inlet with an energy waste. This loss allows to balance the mechanical work produced by the turbine with that required by the compressor. A look-up table stored inside the ECU gives the desired boost pressure as a function of engine speed and load. Two technologies of direct heat recovery were conceived: • DHR-1: semi-conventional solution, done using a variable geometry turbine (as currently installed on the engine) with an additional turbine operating in parallel with the previous one. This turbine is crossed by a mass flow rate that frequently exceeds what is strictly necessary to ensure the boost pressure given by the compressor. The turbine is partially throttled (VGT) so insuring the mechanical equilibrium with the compressor. DHR-1 considers a fully opened VGT and bypasses part of the exhaust gas in an additional turbine (Figure 4a).This solution requires the development of a map for the bypass valve opening. • DHR-2: the solution considers a new turbocharging group equipped with a fixed geometry turbine, which drives the compressor, and an additional parallel fixed geometry turbine for energy recovery (Figure 4b). This solution enhances the energy recovery, being the pressure loss at the main turbine inlet nil. In fact, the presence of an adjustable stator stage (VGT) produces a pressure loss even when it is fully opened. The boost pressure is achieved through the control of the mass flow rate passing through the turbine that drives the compressor, while the remaining part of the flow goes toward the power recovery turbine. Also in this case, a map should be implemented on the engine, which defines the main flow rate, leaving the residual flow rate on the other turbine. As for the other case, this map has been determined in a virtual way.
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The potential of the two recovery systems have been evaluated according to the approval cycle for HD engines.
Figure 4: Scheme of DHR-1 (left) and DHR-2 (right) technologies 4.
SIMULATION OVER ESC-13
Considering the interest toward HD engines, the ESC-13 cycle has been adopted as sequence of engine operating conditions. Exhaust gas temperature and pressure at the turbine were calculated using the engine virtual platform. Similarly, all the other engine variables needed to evaluate the performances of the DHR technologies were calculated. From mode duration, the mechanical energy produced by the engine was calculated. The advantages which can be obtained using DHR-1 were estimated only in terms of mechanical energy and on a mean engine working point of ESC-13, obtaining a recovery of about 8.7% of the energy used to propel the engine in that point; DHR-2 has a higher efficiency and so, it has been considered more interesting than the previous one [12]. To determine the possible recovery from the DHR-2 technology in an ESC-13 cycle, it was necessary to implement the energy balance of the turbocharger according to the following steps: • Calculation of the specific work of compression (Wc), knowing the boost pressure required by the engine (from its specific map). The values of compressor efficiency are derived from the actual maps (Figure 5): k
Wc
k 1
k 1
RT1 c k 1
(1)
c
Figure 5: Pressure ratio map for compressor. Dots represent ESC-13 working points
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Calculation of the air flow rate entering the compressor ( m air ), knowing the
fuel ), the speed of the engine in RPM, the air/fuel ratio α, the fuel injected ( m crankshaft throw ε and, therefore, the power (Pc) needed by it, according to:
air m
fuel RPM m 60
(2)
air Wc Pc m
(3)
Calculation of the specific work (Wt) required by the new turbine and the gas mass flow rate (mmin) strictly required to balance the turbo:
Wt
k k 1
min m
1 k k
RTin,turb 1 t
Pc
t
(4)
(5)
Wt m
The difference between the mass flow rate of exhaust gasses of the engine (as shown by the simulation of the engine point) and eq. 5 is the mass flow rate that expands in the auxiliary turbine and recovers energy. Results are shown in Figure 6 and in Table 1. In particular, Figure 6 shows the power required by the vehicle and the power recovered from the auxiliary turbine for the operating points of the ESC13, while the dashed line represents the cumulative energy obtained from the same turbine once mode duration is known. Data show that turbine must be sized close to 30-35 kW to allow energy recovery available on mode n.10. This sensibly increase energy recovered. During mode 8 and 12 a power close to 20 kW could be recovered and it decreases to 15 kW during mode n. 2 and 4. This result demonstrates a huge interest toward this technology: considering the heavy duty service of this engine, a similar saving is expected on fuel consumption and CO2 saving. This in spite of the inherent simplicity and a reduced cost required to implement an additional turbine on board.
Figure 6: Energy required for an ESC-13 cycle and energy recovered from the auxiliary turbine with DHR-2 technology The sum of the correspondent recovered energies, compared to the energy needed to propel the whole ESC13 cycle, offers an energy recovery of about 16% (Table 1).
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Table 1: Summary of the performances of DHR-2 technology on ESC13 cycle PARAMETER ESC13 Cycle Energy [kWh] Mean Propulsive Power [kW] Mech. Recovered Energy [kWh] Recovered/Required [%]
RESULTS 27.792 59.554 4.414 15.88%
An energy saving close to 16 % on the ESC 13 cycle represents a result very interesting considering that: a) The sequence of the engine operating conditions fixed by the 13 mode procedure represents a duty cycle close to the reality. So, the results are really representative of real driving conditions (more than for the NEDC as representative of a typical duty cycle for passenger car or LDV); b) The cost reduction due to fuel saving must have the same order of magnitude: this issue represents a very interesting prospective when moving freights; c) CO2 reduction must be lower in percentage terms, but still very promising considering the potential of others technologies which today are considered needed and market ready; d) The cost increase due an additional turbine referred to the unit of energy or CO2 saved is very low and among the “most cheap” green technologies. All these aspects invited the Authors toward a further analysis which considered a possible utilization of this energy recovered. Concerning these aspects two possibilities were considered: a) Excess energy converted to additional mechanical energy; b) Excess energy converted to electrical energy. The intention was to evaluate with a deeper degree of precision the real net energy and CO2 saved. The first evaluation required an iterative method, due to the fact that the ICE and the auxiliary turbine transfer power on the same shaft. In particular, the auxiliary turbine transfers energy (through a gear with a 97% efficiency, reported as ηgear) to the shaft whose speed ω is fixed, and this means that a certain torque is given to the power train. The propulsion torque contribution Taux produced by the auxiliary turbine giving Paux is:
Taux
Paux gear
(6)
Thanks to this contribution, the ICE reaches a new working point but this new state would produce a new energy recovery from the auxiliary turbine: from this consideration an iterative procedure must be started. For each mode, the following condition has to be reached:
T i 1ESC 13 T i ESC 13 Taux i+1
(7)
i
where TESC-13 and TESC-13 are the engine torque values referred to two subsequent iterations: equation (6) is iteratively used to recalculate the contribution due to the auxiliary turbine until the two torque values in equation (7) respect a given maximum error (<1%). This is because at every iteration, as engine torque is changed the energy recovered by the turbine changes too. Applying this iterative
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procedure to the engine working points, the results in Table 2 are obtained: they report an average power saving close to 10%: maximum saving is close to 18 % for modes n. 10 and 11 and 15 % for modes n. 8 and 12. A still interesting 10 % saving is reached for modes 2,3,4,9,13. The difference in torque can be seen as difference in energy (ΔW), and this difference drives the CO2 saving:
W T t
(8)
Table 2: “Engine torque saved” for each ESC-13 mode when a mechanical recovery is applied Mode
RPM
LOAD [%]
Original engine torque [Nm]
1 2 3 4 5 6 7 8 9 10 11 12 13
850 2175 2750 2750 2175 2175 2175 2750 2750 3325 3325 3325 3325
0 100 50 75 50 75 25 100 25 100 25 75 50
0 400.2 200 300 200.1 300.15 100.05 400 100 365.2 91.3 273.9 182.6
Engine torque at convergence [Nm]
0 354.2 179.9 263.0 193.2 281.2 99.4 342.1 89.7 301.2 75.1 233.9 157.6
ΔTorque
0.0% -11.5% -10.0% -12.3% -3.4% -6.3% -0.6% -14.5% -10.3% -17.5% -17.8% -14.6% -13.7%
Figure 7: Engine efficiency map. Dots represent ESC-13 working points The CO2 emissions reduction needs to evaluate the fuel consumption for each working point, both after and before the energy recovery. The calculation of the fuel consumption requires the knowledge of the engine efficiency (Figure 7). Table 3 summarizes the fuel consumption for each engine point: introducing mode duration, the fuel saving can be predicted. In this table also the weight factor suggested by the ESC-13 normative are presented, which suggest how to consider the emissions concerning each point for a overall calculation:
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Table 3: Fuel consumption for each mode of the ESC-13 when a mechanical recover is applied Mode 1 2 3 4 5 6 7 8 9 10 11 12 13
Duration Weight [s] factor 240 15 120 8 120 10 120 10 120 5 120 5 120 5 120 9 120 10 120 8 120 5 120 5 120 5 TOTAL [kg]
Original fuel consumption [g/s] 0.12 5.04 3.46 4.99 2.86 4.02 1.55 6.65 1.94 7.91 2.36 5.74 3.96 6.09
Real fuel consumption [g/s] 0.12 4.50 3.16 4.41 2.78 3.80 1.54 5.63 1.81 6.35 2.12 4.92 3.51 5.37
CO2 emissions were calculated adopting the methodology suggested by the ESC-13 normative for pollutant emissions, waiting for a specific normative.
CO2 C
(fuel weight) (power weight)
(9)
In the equation 9 the C factor is a conversion factor, which is different as the fuel changes. In this case a C13.5H23.6 is considered, that means a C factor of 3.2 ⁄ . The initial value of CO2 emissions is 696.5 g/kWh, while the “new” emissions are 611.6 g/kWh. The important reduction of 12.2% is reached when using this technology “directly” linked (via gears) to the crankshaft. The second way to re-use energy recovered considers its transformation in electrical energy, its storage inside the batteries and its re-use in an electrical form. This availability would push toward a massive electrification of the auxiliaries (cooling fluid, oil, brakes, compressed air service, etc…). Electricity on board is provided by the engine, and so it costs in terms of fuel: the energy recovered in electrical form means, therefore, a fuel and CO2 saving. Starting from the mechanical energy recovered from the auxiliary turbine, considering a new technology for generating electricity on board (like Permanent Magnets Generators or Switched Reluctance) a “new” alternator efficiency equal to 85% has been fixed, and so the saving estimated equal to 15.88% (Table 2) becomes equal to 13.5% in electrical form. The CO2 and fuel saving advantages when electricity is produced can be evaluated with respect to the same electrical energy produced in a traditional way by the engine itself. This calls for the knowledge of engine efficiency (ηeng) on each ESC-13 working point. Table 4 reports for each mode the energy efficiency, the mechanical energy required by the engine and the electrical energy to be produced. This corresponds to the energy recovered by the auxiliary turbine and transformed in electrical form (ηgen = 0.85). Table 4 shows also the fuel required by the engine to produce this energy (ηalt = 0.75, Hi = 43.25 MJ/kg): this fuel is equivalent to a fuel saved thanks to the auxiliary turbine.
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Table 4: Fuel request scenario for electrical generation in ESC-13 cycle Mode 1 2 3 4 5 6 7 8 9 10 11 12 13
ηeng [%] 0% 42% 38% 40% 37% 39% 34% 40% 34% 37% 31% 38% 37%
Erec-el,i, [kWh] 0 0.44 0.22 0.45 0.06 0.17 0.01 0.72 0.10 1.11 0.19 0.62 0.34
Erec-mec,i, [kWh] 0 0.58 0.30 0.60 0.08 0.22 0.01 0.96 0.14 1.48 0.26 0.82 0.43
Fuel required [g] 0 98 54 106 14 41 1 169 29 282 59 152 85
The engine without energy recovery requires 6.1 kg of fuel to cover ESC-13 and 1.1 kg of fuel to produce an equivalent electrical energy (recovered by the DHR-2). This means that a DHR-2 is responsible of a saving up to 15%. A similar saving can be obtained referring to CO2. 5.
CONCLUSIONS
The enthalpy of the gas leaving cylinders in ICE represents a great source of loss which influences significantly overall engine efficiency. Temperature and pressure of this exhaust are high enough to justify a further transformation in mechanical energy directly or by means of a thermodynamic cycle. The paper focused the attention on the first recovery, really more easier and characterized by a lower engine cost increase. Considering that the mean exhaust gas pressure leaving the cylinders is higher than the atmospheric pressure and that, usually, the work required by the compressor to boost intake air is lower than the exhaust gas enthalpy, excess energy can be directly recovered on an auxiliary turbine and transformed into work. This turbine operates in parallel to the one which drives the compressor. It is a proven technology, so component validation could be considered as insured. The control variable is represented by gas flow rate which crosses one of the two turbines. The paper makes reference to an existing engine (IVECO F1C, 3L) equipped in HD mode, extensively tested in order to validate a comprehensive mathematical model. The use of this virtual platform allowed the knowledge of all the variables which were needed in order to estimate the potential of the recovery. F1C engine has a VGT whose closing is used to match turbine and compressor work. The paper discusses two recovery configurations having different potential but characterized by greater engine modifications. The first keeps the same turbine driving the compressor with its VGT which introduces, even when it is completely opened, some pressure losses. The second replaces the actual turbine with a new one without any inlet stage (and therefore without any residual pressure loss). Being the engine set up as HD, the ESC-13 has been considered as sequence of engine working points on which the saving has been calculated.
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The following results were found: a) The turbine of the auxiliary power on which energy is recovered should be sized at 30-35 kW; b) Mean energy saving on the overall ESC-13 test procedure is equal to 15.9 % of the overall energy propulsive power; According to a different use of the mechanical energy recovered, it followed: a) if the energy is used for propulsion, a saving of 11.8% is achievable on fuel consumption, the CO2 saving is about 12.2% of the original emissions (without any recovery); b) if the energy is transformed in an electrical form a fuel saving of 15.3% is reached with respect to the fuel consumption required for propulsion and electrical energy (in the traditional engine). Same result applies to CO2. Considering the simplicity of the turbine control and also the technology which is proven and characterized by low cost (increase) and high reliability, the DHR recovery has all the characteristic to be ready to market. REFERENCE LIST [1]
International Council on Clean Transportation, Global Passenger Vehicles Program - Global Comparison of Light-Duty Vehicle Fuel Economy/GHG Emissions Standards – August 2011 [2] N. Hill, S. Finnegan, J. Norris, C. Brannigan, D. Wynn, H. Baker, I. Skinner Reduction and Testing of Greenhouse Gas (GHG) Emissions from Heavy Duty Vehicles Lot 1: Strategy - Didcot : AEA, DG ENV. 070307/2009/548572/SER/ C3, 22 Feb 2011. [3] University of Technology Graz, Institute for Internal Combustion Engines and Thermodynamics - Reduction and Testing of Greenhouse Gas Emissions from Heavy Duty Vehicles (LOT 2) - Development and testing of a certification procedure for CO2 emissions and fuel consumption of HDV - Contract N° 070307/2009/548300/SER/C3, Final Report, 9 January 2012 [4] K. Law, M. D. Jackson, M. Chan - European Union Greenhouse Gas Reduction Potential for Heavy-Duty Vehicles – IPCC Report, TIAX Reference No. D5625, December 23, 2011 [5] F. Jianqin, L. Jingping, Y. Yanping, Y. Hanqian - A Study on the Prospect of Engine Exhaust Gas Energy Recovery - 978-1-4244-8039-5/11, 2011 IEEE [6] R. Toom – Waste heat regeneration system for internal combustion engines – Engine Expo, May 8th, 2007, Messe Stuttgart [7] M.A. Korzhuev, I.V. Katin - On the Placement of Thermoelectric Generators in Automobiles - Journal of electronic materials, Vol. 39, No. 9, 2010 DOI: 10.1007/s11664-010-1332-z [8] A.T.C. Patterson, R. J. Tett, J. McGuire - Exhaust heat recovery using electroturbogenerators - SAE Paper 2009-01-1604, 2009 [9] D.T. Hountalas, C.O. Katsanos, V.T. Lamaris - Recovering energy from the diesel engine exhaust using mechanical and electrical turbocompounding SAE Paper 2007-01-1563, 2007 [10] M. Michon, S.D. Calverley, R.E. Clark, D. Howe, J.D.A. Chambers, P.A. Sykes, P.G. Dickinson, M. McClelland, G. Johnstone, R. Quinn, G. Morris - Modelling and Testing of a Turbo-generator System for Exhaust Gas Energy Recovery 0-7803-9761-4/07, 2007 IEEE [11] W. Knecht - Diesel engine development in view of reduced emission standards - Energy 33 (2008) 264–271 [12] R. Cipollone, D. Di Battista, A. Gualtieri - Energy recovery from the turbocharging system of internal combustion engines - 11th Biennal Conference on Engineering Systems Design and Analysis ESDA12, July 2-4, 2012, Nantes, France
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ABSTRACT In the last years “thinking green” has become the keyword for a more sustainable future; the prevention of the worsening of the environmental conditions produced a strong reduction on ICE emissions that has been, in the recent past, the driving force of the engine evolution. Today, a new interest appeared on the scene and it is CO2 reduction: this goal is focusing the attention of the actual ICE technological evolution and a huge interest is characterizing the market. In this paper the Authors studied an energy recovery system from the exhaust gasses, integrated with the turbocharger, considering that the enthalpy drop across the turbine is usually higher than that requested by the compressor. 1.
INTRODUCTION
Emissions of CO2 from light and heavy duty vehicles (L&HDV) fleet currently account for a significant proportion of the total emissions due to transportation sector. Measures to reduce emissions are, therefore, particularly appreciated and requested by Governments. The two sectors have different impacts: the freight transport is especially challenging due to close links with economic development while the passenger and light duty sector is more oriented to social expectations, related to concept of the quality of life. In the last years, the CO2 emissions of the transportation sector have been rising substantially. In the meantime, legislations concerning CO2 for LDV have been introduced to reduce energy consumption and GHG emissions (Figure 1).
Figure 1: CO2 emissions for LDV in different regions: solid lines represent historical data, dashed lines enacted and proposed targets [1]
____________________________________________ © The author(s) and/or their employer(s), 2012
177
Although LDV produce the largest energy consumption in the transportation sector, the projections of CO2 emissions are rising [2] as well as the transportation volumes: this situation suggests regulations in the HDV sector, too. The actual HDV approval cycle for regulated emission (NOx, CO, HC, PN and PM) is based on engine measurements done on test beds and they are expressed in g/kWh, being the energy produced by the engine (instead of the distance as for LDV) a more significant factor. Waiting for new standards, it is expected that in the near future the same test procedure will be adopted for fuel consumption and CO2 as well. In HDV the interest appears to be focused also towards individual engine components and sectors [3] (powertrain, energy recovery, weight reduction, etc…). Figure 2 offers a contribution on the expected CO2 reduction among such sectors. The major reduction is achievable with hybridization technology, but it has a nonnegligible cost increase. Technologies which improve thermal efficiency, or those oriented to recover the great amount of waste heat produced by the engine seem to have a more immediate application [4], due to lower cost increase per unit of CO2 saved.
Figure 2: Expected percentage of CO2 saving for different technological options and HDV segments Exhaust gas leaving cylinders has two mains energy contributions (so, energy losses): the first is related to its pressure, the second to its temperature. Both values are greater than ambient values. Work, therefore, could be recovered by means of a direct expansion (to the atmospheric pressure) or by means of a heat recovery [5-6]. This acts as a high temperature energy source for a working fluid and, thanks to it, the fluid vaporizes at high pressure and produces work inside a proper expander. Its condensation is followed by a pressurization in liquid phase and continuity is done on the energy recovery in mechanical form. Thermoelectric recovery has been studied as well [7] but they still require some material science advancements in order to reduce cost and increase performances. First recovery can be done directly on the turbocharging system [8-10] and it is labelled as “direct heat recovery”, DHR. The interest to this recovery is huge, considering that almost all diesel engines are turbocharged and that the downsizing of gasoline engines requires the need of intake air boosting [11]. The paper focuses the attention on DHR potential.
178
Actual turbocharging technology requires that the expansion work produced by the turbine must be equal to the compression work requested by the compressor, for a given boost pressure whose values are stored in a look-up table as a function of engine speed and load. This matching is done wasting the excess of energy: (a) by passing part of the expanding gas inside the waste gate valve; (b) throttling the exhaust gas inside a variable geometry turbine (VGT). In many engine working points the energy unbalance between turbine and compressor is great and, so, the energy dissipated. The Authors already presented two different concepts of direct heat recovery (DHR) from the waste gasses [12] when a VGT technology is used. The two solutions depend mainly on the level of modification that is accepted with respect to the existing turbocharging system. The first does not change the existing turbine (rack controlled); the second asks for a more invasive intervention, but the energy recovery degree is more promising. In a previous work [12], the authors showed that when considering LDV, the energy recovery fits in the range 3-7 % of the overall propulsion energy when NEDC is considered. This is due to the low engine load and speed working points characterizing NEDC. A greater recovery is expected when a more realistic approval cycle will be adopted. In this paper the Authors focus the attention on HDV where energy efficiency means fuel cost reduction, particularly welcome in the sector. In order to evaluate the potential of this recovery, a 1D model of the engine and the turbocharging group has been developed and calibrated on a F1C IVECO 3L Euro 4 engine equipped as HDV. Engine performances, as well as relevant data concerning the behaviour of the turbo-charging system (flow rates, pressure drops, temperatures, VGT rack positions, etc…) have been measured and predicted with a high degree of accuracy. Once the model was deeply calibrated, it has been used as virtual platform to calculate the energy recovery according to ESC-13: each engine working point has been reproduced and all the engine properties calculated in order to predict energy recovery. The model has been enhanced considering different possibility to use the recovered energy in mechanical or in electrical form. Fuel consumption reduction and CO2 emission saving have been estimated. 2.
MODEL DESCRIPTION AND VALIDATION
The methodology used to evaluate the potential of the DHR required the development of a procedure organized as follows. Once the engine has been defined: a) a virtual model of the engine was developed; it is able to represent engine performances, including the implementation of control strategies of major significance (boost pressure control, fuel injection, cooling, EGR, etc...); b) the model was tested and calibrated with experimental data; c) the model was used as a virtual platform for the evaluation of the potential of the energy recovery technologies. The engine considered in this paper is an EURO 4 IVECO F1C 3.0 L with a variable geometry turbine. This engine has the possibility to set up ECU even for a HD application and it has been tested on a bench, checking the model with the experimental data. Figure 3 shows a comparison between calculated data and experimental data for pressure ratio both for compressor and turbine, which are important variables in this work. They are represented as a function of engine speed, at full load at every speed.
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Figure 3: Comparison between measured and calculated engine pressure ratio for compressor and turbine The agreement is quite satisfactory and produces a 10% average error on the engine torque (in the full operating engine conditions), value that is reduced to 2% within the characteristic region of the ESC-13 approval cycle. The model is very comprehensive and it is able to predict many engine characteristic variables. 3.
DIRECT HEAT RECOVERY TECHNOLOGIES
As mentioned above, the enthalpy of the exhaust gasses leaving cylinders is greater than what is required to compress intake air. It follows, therefore, a potential energy recovery through a turbine. The turbocharger on the engine tested has a variable geometry turbo (VGT) system that matches the turbocharger on higher efficiencies, reduces the response time of the group, allows a robust control of the rack position according to the boost pressure, allows for a dynamic overboost on specific engine points. Unfortunately, it produces a pressure loss at the impeller inlet with an energy waste. This loss allows to balance the mechanical work produced by the turbine with that required by the compressor. A look-up table stored inside the ECU gives the desired boost pressure as a function of engine speed and load. Two technologies of direct heat recovery were conceived: • DHR-1: semi-conventional solution, done using a variable geometry turbine (as currently installed on the engine) with an additional turbine operating in parallel with the previous one. This turbine is crossed by a mass flow rate that frequently exceeds what is strictly necessary to ensure the boost pressure given by the compressor. The turbine is partially throttled (VGT) so insuring the mechanical equilibrium with the compressor. DHR-1 considers a fully opened VGT and bypasses part of the exhaust gas in an additional turbine (Figure 4a).This solution requires the development of a map for the bypass valve opening. • DHR-2: the solution considers a new turbocharging group equipped with a fixed geometry turbine, which drives the compressor, and an additional parallel fixed geometry turbine for energy recovery (Figure 4b). This solution enhances the energy recovery, being the pressure loss at the main turbine inlet nil. In fact, the presence of an adjustable stator stage (VGT) produces a pressure loss even when it is fully opened. The boost pressure is achieved through the control of the mass flow rate passing through the turbine that drives the compressor, while the remaining part of the flow goes toward the power recovery turbine. Also in this case, a map should be implemented on the engine, which defines the main flow rate, leaving the residual flow rate on the other turbine. As for the other case, this map has been determined in a virtual way.
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The potential of the two recovery systems have been evaluated according to the approval cycle for HD engines.
Figure 4: Scheme of DHR-1 (left) and DHR-2 (right) technologies 4.
SIMULATION OVER ESC-13
Considering the interest toward HD engines, the ESC-13 cycle has been adopted as sequence of engine operating conditions. Exhaust gas temperature and pressure at the turbine were calculated using the engine virtual platform. Similarly, all the other engine variables needed to evaluate the performances of the DHR technologies were calculated. From mode duration, the mechanical energy produced by the engine was calculated. The advantages which can be obtained using DHR-1 were estimated only in terms of mechanical energy and on a mean engine working point of ESC-13, obtaining a recovery of about 8.7% of the energy used to propel the engine in that point; DHR-2 has a higher efficiency and so, it has been considered more interesting than the previous one [12]. To determine the possible recovery from the DHR-2 technology in an ESC-13 cycle, it was necessary to implement the energy balance of the turbocharger according to the following steps: • Calculation of the specific work of compression (Wc), knowing the boost pressure required by the engine (from its specific map). The values of compressor efficiency are derived from the actual maps (Figure 5): k
Wc
k 1
k 1
RT1 c k 1
(1)
c
Figure 5: Pressure ratio map for compressor. Dots represent ESC-13 working points
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Calculation of the air flow rate entering the compressor ( m air ), knowing the
fuel ), the speed of the engine in RPM, the air/fuel ratio α, the fuel injected ( m crankshaft throw ε and, therefore, the power (Pc) needed by it, according to:
air m
fuel RPM m 60
(2)
air Wc Pc m
(3)
Calculation of the specific work (Wt) required by the new turbine and the gas mass flow rate (mmin) strictly required to balance the turbo:
Wt
k k 1
min m
1 k k
RTin,turb 1 t
Pc
t
(4)
(5)
Wt m
The difference between the mass flow rate of exhaust gasses of the engine (as shown by the simulation of the engine point) and eq. 5 is the mass flow rate that expands in the auxiliary turbine and recovers energy. Results are shown in Figure 6 and in Table 1. In particular, Figure 6 shows the power required by the vehicle and the power recovered from the auxiliary turbine for the operating points of the ESC13, while the dashed line represents the cumulative energy obtained from the same turbine once mode duration is known. Data show that turbine must be sized close to 30-35 kW to allow energy recovery available on mode n.10. This sensibly increase energy recovered. During mode 8 and 12 a power close to 20 kW could be recovered and it decreases to 15 kW during mode n. 2 and 4. This result demonstrates a huge interest toward this technology: considering the heavy duty service of this engine, a similar saving is expected on fuel consumption and CO2 saving. This in spite of the inherent simplicity and a reduced cost required to implement an additional turbine on board.
Figure 6: Energy required for an ESC-13 cycle and energy recovered from the auxiliary turbine with DHR-2 technology The sum of the correspondent recovered energies, compared to the energy needed to propel the whole ESC13 cycle, offers an energy recovery of about 16% (Table 1).
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Table 1: Summary of the performances of DHR-2 technology on ESC13 cycle PARAMETER ESC13 Cycle Energy [kWh] Mean Propulsive Power [kW] Mech. Recovered Energy [kWh] Recovered/Required [%]
RESULTS 27.792 59.554 4.414 15.88%
An energy saving close to 16 % on the ESC 13 cycle represents a result very interesting considering that: a) The sequence of the engine operating conditions fixed by the 13 mode procedure represents a duty cycle close to the reality. So, the results are really representative of real driving conditions (more than for the NEDC as representative of a typical duty cycle for passenger car or LDV); b) The cost reduction due to fuel saving must have the same order of magnitude: this issue represents a very interesting prospective when moving freights; c) CO2 reduction must be lower in percentage terms, but still very promising considering the potential of others technologies which today are considered needed and market ready; d) The cost increase due an additional turbine referred to the unit of energy or CO2 saved is very low and among the “most cheap” green technologies. All these aspects invited the Authors toward a further analysis which considered a possible utilization of this energy recovered. Concerning these aspects two possibilities were considered: a) Excess energy converted to additional mechanical energy; b) Excess energy converted to electrical energy. The intention was to evaluate with a deeper degree of precision the real net energy and CO2 saved. The first evaluation required an iterative method, due to the fact that the ICE and the auxiliary turbine transfer power on the same shaft. In particular, the auxiliary turbine transfers energy (through a gear with a 97% efficiency, reported as ηgear) to the shaft whose speed ω is fixed, and this means that a certain torque is given to the power train. The propulsion torque contribution Taux produced by the auxiliary turbine giving Paux is:
Taux
Paux gear
(6)
Thanks to this contribution, the ICE reaches a new working point but this new state would produce a new energy recovery from the auxiliary turbine: from this consideration an iterative procedure must be started. For each mode, the following condition has to be reached:
T i 1ESC 13 T i ESC 13 Taux i+1
(7)
i
where TESC-13 and TESC-13 are the engine torque values referred to two subsequent iterations: equation (6) is iteratively used to recalculate the contribution due to the auxiliary turbine until the two torque values in equation (7) respect a given maximum error (<1%). This is because at every iteration, as engine torque is changed the energy recovered by the turbine changes too. Applying this iterative
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procedure to the engine working points, the results in Table 2 are obtained: they report an average power saving close to 10%: maximum saving is close to 18 % for modes n. 10 and 11 and 15 % for modes n. 8 and 12. A still interesting 10 % saving is reached for modes 2,3,4,9,13. The difference in torque can be seen as difference in energy (ΔW), and this difference drives the CO2 saving:
W T t
(8)
Table 2: “Engine torque saved” for each ESC-13 mode when a mechanical recovery is applied Mode
RPM
LOAD [%]
Original engine torque [Nm]
1 2 3 4 5 6 7 8 9 10 11 12 13
850 2175 2750 2750 2175 2175 2175 2750 2750 3325 3325 3325 3325
0 100 50 75 50 75 25 100 25 100 25 75 50
0 400.2 200 300 200.1 300.15 100.05 400 100 365.2 91.3 273.9 182.6
Engine torque at convergence [Nm]
0 354.2 179.9 263.0 193.2 281.2 99.4 342.1 89.7 301.2 75.1 233.9 157.6
ΔTorque
0.0% -11.5% -10.0% -12.3% -3.4% -6.3% -0.6% -14.5% -10.3% -17.5% -17.8% -14.6% -13.7%
Figure 7: Engine efficiency map. Dots represent ESC-13 working points The CO2 emissions reduction needs to evaluate the fuel consumption for each working point, both after and before the energy recovery. The calculation of the fuel consumption requires the knowledge of the engine efficiency (Figure 7). Table 3 summarizes the fuel consumption for each engine point: introducing mode duration, the fuel saving can be predicted. In this table also the weight factor suggested by the ESC-13 normative are presented, which suggest how to consider the emissions concerning each point for a overall calculation:
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Table 3: Fuel consumption for each mode of the ESC-13 when a mechanical recover is applied Mode 1 2 3 4 5 6 7 8 9 10 11 12 13
Duration Weight [s] factor 240 15 120 8 120 10 120 10 120 5 120 5 120 5 120 9 120 10 120 8 120 5 120 5 120 5 TOTAL [kg]
Original fuel consumption [g/s] 0.12 5.04 3.46 4.99 2.86 4.02 1.55 6.65 1.94 7.91 2.36 5.74 3.96 6.09
Real fuel consumption [g/s] 0.12 4.50 3.16 4.41 2.78 3.80 1.54 5.63 1.81 6.35 2.12 4.92 3.51 5.37
CO2 emissions were calculated adopting the methodology suggested by the ESC-13 normative for pollutant emissions, waiting for a specific normative.
CO2 C
(fuel weight) (power weight)
(9)
In the equation 9 the C factor is a conversion factor, which is different as the fuel changes. In this case a C13.5H23.6 is considered, that means a C factor of 3.2 ⁄ . The initial value of CO2 emissions is 696.5 g/kWh, while the “new” emissions are 611.6 g/kWh. The important reduction of 12.2% is reached when using this technology “directly” linked (via gears) to the crankshaft. The second way to re-use energy recovered considers its transformation in electrical energy, its storage inside the batteries and its re-use in an electrical form. This availability would push toward a massive electrification of the auxiliaries (cooling fluid, oil, brakes, compressed air service, etc…). Electricity on board is provided by the engine, and so it costs in terms of fuel: the energy recovered in electrical form means, therefore, a fuel and CO2 saving. Starting from the mechanical energy recovered from the auxiliary turbine, considering a new technology for generating electricity on board (like Permanent Magnets Generators or Switched Reluctance) a “new” alternator efficiency equal to 85% has been fixed, and so the saving estimated equal to 15.88% (Table 2) becomes equal to 13.5% in electrical form. The CO2 and fuel saving advantages when electricity is produced can be evaluated with respect to the same electrical energy produced in a traditional way by the engine itself. This calls for the knowledge of engine efficiency (ηeng) on each ESC-13 working point. Table 4 reports for each mode the energy efficiency, the mechanical energy required by the engine and the electrical energy to be produced. This corresponds to the energy recovered by the auxiliary turbine and transformed in electrical form (ηgen = 0.85). Table 4 shows also the fuel required by the engine to produce this energy (ηalt = 0.75, Hi = 43.25 MJ/kg): this fuel is equivalent to a fuel saved thanks to the auxiliary turbine.
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Table 4: Fuel request scenario for electrical generation in ESC-13 cycle Mode 1 2 3 4 5 6 7 8 9 10 11 12 13
ηeng [%] 0% 42% 38% 40% 37% 39% 34% 40% 34% 37% 31% 38% 37%
Erec-el,i, [kWh] 0 0.44 0.22 0.45 0.06 0.17 0.01 0.72 0.10 1.11 0.19 0.62 0.34
Erec-mec,i, [kWh] 0 0.58 0.30 0.60 0.08 0.22 0.01 0.96 0.14 1.48 0.26 0.82 0.43
Fuel required [g] 0 98 54 106 14 41 1 169 29 282 59 152 85
The engine without energy recovery requires 6.1 kg of fuel to cover ESC-13 and 1.1 kg of fuel to produce an equivalent electrical energy (recovered by the DHR-2). This means that a DHR-2 is responsible of a saving up to 15%. A similar saving can be obtained referring to CO2. 5.
CONCLUSIONS
The enthalpy of the gas leaving cylinders in ICE represents a great source of loss which influences significantly overall engine efficiency. Temperature and pressure of this exhaust are high enough to justify a further transformation in mechanical energy directly or by means of a thermodynamic cycle. The paper focused the attention on the first recovery, really more easier and characterized by a lower engine cost increase. Considering that the mean exhaust gas pressure leaving the cylinders is higher than the atmospheric pressure and that, usually, the work required by the compressor to boost intake air is lower than the exhaust gas enthalpy, excess energy can be directly recovered on an auxiliary turbine and transformed into work. This turbine operates in parallel to the one which drives the compressor. It is a proven technology, so component validation could be considered as insured. The control variable is represented by gas flow rate which crosses one of the two turbines. The paper makes reference to an existing engine (IVECO F1C, 3L) equipped in HD mode, extensively tested in order to validate a comprehensive mathematical model. The use of this virtual platform allowed the knowledge of all the variables which were needed in order to estimate the potential of the recovery. F1C engine has a VGT whose closing is used to match turbine and compressor work. The paper discusses two recovery configurations having different potential but characterized by greater engine modifications. The first keeps the same turbine driving the compressor with its VGT which introduces, even when it is completely opened, some pressure losses. The second replaces the actual turbine with a new one without any inlet stage (and therefore without any residual pressure loss). Being the engine set up as HD, the ESC-13 has been considered as sequence of engine working points on which the saving has been calculated.
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The following results were found: a) The turbine of the auxiliary power on which energy is recovered should be sized at 30-35 kW; b) Mean energy saving on the overall ESC-13 test procedure is equal to 15.9 % of the overall energy propulsive power; According to a different use of the mechanical energy recovered, it followed: a) if the energy is used for propulsion, a saving of 11.8% is achievable on fuel consumption, the CO2 saving is about 12.2% of the original emissions (without any recovery); b) if the energy is transformed in an electrical form a fuel saving of 15.3% is reached with respect to the fuel consumption required for propulsion and electrical energy (in the traditional engine). Same result applies to CO2. Considering the simplicity of the turbine control and also the technology which is proven and characterized by low cost (increase) and high reliability, the DHR recovery has all the characteristic to be ready to market. REFERENCE LIST [1]
International Council on Clean Transportation, Global Passenger Vehicles Program - Global Comparison of Light-Duty Vehicle Fuel Economy/GHG Emissions Standards – August 2011 [2] N. Hill, S. Finnegan, J. Norris, C. Brannigan, D. Wynn, H. Baker, I. Skinner Reduction and Testing of Greenhouse Gas (GHG) Emissions from Heavy Duty Vehicles Lot 1: Strategy - Didcot : AEA, DG ENV. 070307/2009/548572/SER/ C3, 22 Feb 2011. [3] University of Technology Graz, Institute for Internal Combustion Engines and Thermodynamics - Reduction and Testing of Greenhouse Gas Emissions from Heavy Duty Vehicles (LOT 2) - Development and testing of a certification procedure for CO2 emissions and fuel consumption of HDV - Contract N° 070307/2009/548300/SER/C3, Final Report, 9 January 2012 [4] K. Law, M. D. Jackson, M. Chan - European Union Greenhouse Gas Reduction Potential for Heavy-Duty Vehicles – IPCC Report, TIAX Reference No. D5625, December 23, 2011 [5] F. Jianqin, L. Jingping, Y. Yanping, Y. Hanqian - A Study on the Prospect of Engine Exhaust Gas Energy Recovery - 978-1-4244-8039-5/11, 2011 IEEE [6] R. Toom – Waste heat regeneration system for internal combustion engines – Engine Expo, May 8th, 2007, Messe Stuttgart [7] M.A. Korzhuev, I.V. Katin - On the Placement of Thermoelectric Generators in Automobiles - Journal of electronic materials, Vol. 39, No. 9, 2010 DOI: 10.1007/s11664-010-1332-z [8] A.T.C. Patterson, R. J. Tett, J. McGuire - Exhaust heat recovery using electroturbogenerators - SAE Paper 2009-01-1604, 2009 [9] D.T. Hountalas, C.O. Katsanos, V.T. Lamaris - Recovering energy from the diesel engine exhaust using mechanical and electrical turbocompounding SAE Paper 2007-01-1563, 2007 [10] M. Michon, S.D. Calverley, R.E. Clark, D. Howe, J.D.A. Chambers, P.A. Sykes, P.G. Dickinson, M. McClelland, G. Johnstone, R. Quinn, G. Morris - Modelling and Testing of a Turbo-generator System for Exhaust Gas Energy Recovery 0-7803-9761-4/07, 2007 IEEE [11] W. Knecht - Diesel engine development in view of reduced emission standards - Energy 33 (2008) 264–271 [12] R. Cipollone, D. Di Battista, A. Gualtieri - Energy recovery from the turbocharging system of internal combustion engines - 11th Biennal Conference on Engineering Systems Design and Analysis ESDA12, July 2-4, 2012, Nantes, France
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