Waste heat recovery from heavy duty truck diesel engines T. Henriques Mechanical Engineering Department Instituto Superior Técnico Av. Rovisco Pais, 1049-001 Lisboa Portugal tiago.r.henriques@ist.utl.pt Abstract- This study is focused on the evaluation of the potential energy recovery in internal combustion engines (ICE) for heavy vehicles. Four sources of heat dissipation of an ICE representative were studied. The EGR and the exhaust gas were identified as the best sources of heat to recover. The two hot sources selected are in parallel and the Rankine cycle (RC) also uses as heat sink the cooling system of the ICE. The chosen RC expander is piston type. The RC maintains the pressure and temperature stationary using evaporators of variable geometry and a condenser with variable bypass coolant. It is concluded that ethanol is the working fluid to be used, the RC working at a evaporation pressure of 49 bar, condensation pressure of 2.3 bar and an overheating temperature of 240 C. After optimization, the EGR cooling temperature is 120 C and the exhaust gases cooling temperature varies from 110 C to 230 C, depending on the regime of the ICE. It was selected the heat exchanger with finned tubes for evaporators. The heat exchanger selected for the condenser has concentric tubes with axial flow. The proposed system achieves a maximum reduction of fuel consumption and carbon dioxide (CO 2 ) emissions of 6.6% at high speed. Mechanical power achieves a maximum increase of 7%. The original cooling system is not undersized. Keywords: waste heat recovery in internal combustion engines; Rankine cycle; heavy duty diesel trucks; performance maps; variable geometry heat exchanger. 1. Introduction In the European continent about 75% of the transport of materials is assured by road through trucks. Nowadays the internal combustion engines (ICE)present maximum thermal efficiency of about 40% [3], wasting the remaining energy fraction in form of heat in the various systems of the thermal management of the ICE. It is thus essential to find solutions to improve the performance of these equipments. The cycle of Rankine (RC) presents one the highest potential in the recovery of thermal energy from such systems. In this context, the main objective of the present work is to evaluate the potential for energy recovery from ICE of heavy vehicles. 2. Engine in study Figure 1 presents a schematic of the system of the heavy duty truck diesel engine used in this study and the main characteristics of the ICE are presented in Table 1 and Figure 2. 1
Cooling System (Radiator) Exhaust Gases DPF e SCR Turbo Intercooler Figure1 Schematic of the system of the heavy duty truck diesel engine used in study. Table 1 Characteristics of the ICE used in this present study[4,6]. Item Parameter Unit Cycle 4 strokes - Displacement 15999 cm 3 Number of cylinders 6 - Valves per cylinder 4 - Fuel Diesel - Fuelinjectionsystem Common rail - Engine speed range [800 ; 2000] r/min Maximumpower 384 (515) kw (cv) Maximum torque 2292 at 1200 N.mat r/min Emissionsstandarts EUROVI - Air Cooling Air Figure 2 Brake specific fuel consumption and efective power, air-fuel ratio and EGR fraction. 3. System design and waste heat potential evaluation Exergy is a useful concept for evaluating the performance of various energy systems. Figure 3 presents the potential evaluation of heat recovery for each of the four sources of heat dissipation of the ICE examined in this study. The EGR and the exhaust gas were identified as the best sources for heat recovery. 2
Figure3 Exergy of each waste heat source of ICE in study. Figure 4 presents the exhaust gas temperature and mass flow rate at the outlet of the exhaust gas treatment system, Figure 5 presents the EGR temperature and mass flow rate at the inlet of the EGR system, and Figure 6 presents the coolant temperature and mass flow rate at the outlet of the radiator. Figure 4 Exhaustgas temperature and mass flow rate at the outlet of the exhaust gas treatment system. Figure 5 EGR temperature and mass flow rate at the inlet of EGR system. 3
Coolant temperature out. radiator (ºC) Coolant mass flow rate (kg/s) re and mass flow rate at outlet of radiator. Figure 6 Coolant temperature and mass flow rate at outlet of radiator. Figure 7 presents the RC proposed to recover heat from the ICE studied. Figure7 Schematic of a RC system combined with a heavy duty truck diesel engine. 4. Thermodynamicanalysis Table 2 presents the assumptions considered in the thermodynamics analysis of the RC. Table2 Assumptions considered in thermodynamics analysis of the RC combined with theice. T cond = 100 (ensures operation in any weather; allows placing the RC condenser at any place; allows the cold source is the cooling system;) p cond 101, 325kPa(prevents the inclusion of air in the RC components;) Isentropic expander efficiency, η e = 0, 70 [2, 5]; Isentropic pump efficiency, η p = 0, 80; Cooling of the EGR at 120ºC and optimum temperature for cooling of the exhaust gases variable from 110 C to 240 C, depending on the ICE regime) Negligible pressure losses in the heat exchangers and pipes; Dryexpansion; Minimum temperature difference between heat source and the working fluid in the evaporation of 20 C 4
ARC control strategy was developed with constant parameters of pressure and temperature. It was concluded that ethanol was the most suitable working fluid.figure 8 presents the optimum parameters for the RC for all ICE regimes. η e = 70% η p = 80% η RC = 13,75% State Pressure [bar] Temperature [ºC] 1 2,3 100 2 49 102 3 49 227 4 49 227 5 49 240 6 2,3 100 Figure8 T-s diagram of base RC with ethanol and thermodynamic states of cycle. Figure 9 presents the optimum cooling temperature of the exhaust gases. 5. Evaporators Figure 9 Optimum cooling temperature of the exhaust gases. The evaporator is an essential component in any vehicle waste heat recovery application. The following characteristicsare important in such applications: i) Maximization of heat transfer area for hot gases; ii) Minimization of weigth; iii) Compactness with EGR HEX volume limit of ~0.025m 3 and ESC HEX volume limit of ~1m 3 ; iv) Low pressure drop trough the heat exchanger, principally in hot gases flow, with the limit of 0.2bar; v) Easy to make and easy to clean; To implement the RC control strategy, evaporators of variable geometry had to be developed on the side of the hot sources.figure 10 shows the evaporator concept developed with variable geometry. 5
Maximum load of heat recovery. All area for hot gases is in utilization. The gates are inactive. Minimum load of heat recovery. Minimum area for hot gases is in utilization. The gates are in lower position. Intermedium load of heat recovery. Some area for hot gases is in utilization. The gates are in intermedium position. Figure10 -Evaporator concept developed with variable geometry. Table 3 presents the main characteristics of the evaporators. The last column of the table represents the standard deviation of the length required for the heat exchanger in the various regimes of the ICE.This parameter is subject to minimization by seeking the best cross-section of the heat exchanger. In this way the effect on pressure drop caused by the gates can be minimized. Evaporator Heigth [m] Table3 Main characteristics of evaporators design. Width [m] Length [m] Volume [m 3 ] Mass [kg] Pipes in series Pipes in parallel Number of plates EGR 0.18 0.15 0.75 0.02 ~41 30 6 90 3.343 Escape 0.3 0.3 1.17 0.11 ~213 47 12 150 8.273 Std The maximum pressure drop of hot gases on evaporators takes place on the exhaust of the evaporator and hasa value of 0.043bar.Figure 11 shows the pressure drop of the hot gases in each HEX. Figure11 Pressure drop of thermal gases in each evaporator: EGR (left), exhaust (rigth). 6
Figure 12presents the details of the designed EGR heat exchanger. Working Fluid Inlet Working Fluid Outlet Hot Gases Outlet Hot Gases Inlet Lockers of the gates Figure12 Geometric details of the EGR heat exchanger designed: bank of tubes and conduct with the lockers of the gates. 6. Condenser To implement the RC control strategy,a condenser with variable bypass coolant was developed. The heat exchanger selected for the condenser has concentric tubes with axial flow [1]. Figure 13 shows the bypass fraction of coolant and the temperature of coolant at the outlet of the condenser. f coolant,condenser T coolant,out,condenser (ºC) Figure 13 Bypass fraction of coolant and temperature of coolant at outlet of condenser. The condenser has a diameter of 71mm and a length of 7.1m and it is divided into seven equal lengths. Figure 14 shows the geometric details of the designed condenser. 7
Cool outl Working fluid inlet Coolant outlet Bypass 1014mm Coolant inlet (dimensions in milimeters) Figure13 Geometric details of condenser designed: cross section and global dimensions. 7. Combined system performance analysis Working fluid outlet Figure 14 shows the improvement of the engine effective power and the improvement of the brake specific fuel consumption (bsfc). Figure14- Improvement of engine effective power and the improvement of bsfc. The proposed system achieves a reduction of fuel consumption and CO2 emissions of 5.5% in most of the ICE regimes and 6.6% at high speed. Mechanical power increases by about 6% in most of the ICE regimes and 7% at high speed. Figure 15 shows the net heat added to the refrigeration system and the coolant temperature at engine block outlet. Figure15 Net heat added to refrigeration system and coolant temperature at engine block outlet. 8
Figure 16 shows the original system and the modified system in a European heavy duty vehicle. The evaporator designed to recover heat in the EGR is placed on the original location of the EGR cooler and the evaporator designed to recover heat from the exhaust replaces the original muffler of the vehicle. Radiator Original System EGR cooler Original muffler Sistema ICE+RC MCI System + RC Pump, Expander and Generator Exhaust evaporator (horizontal position) Condenser ICE Gearbox Fuel tank EGR Evaporador evaporator do EGR Figure16 Original truck and a truck with RC designed implemented. The proposed system requires an investment of about 10,000 and has a payback of 5 years. 8. Conclusions The main conclusions from this study are as follows: The strategy of control of the RC involved the development of evaporators of changeable geometry for the hot sources and a condenser with variablebypass of the refrigeration fluid. The study showed that ethanol is the more suitable fluid, working the RC with a pressure of evaporation of 49 bar, a pressure of condensation of 2.3 bar, a temperature of superheating of 240 ºC and a thermal efficiency of 13.75%. The restrictions of volume of the evaporators and, therefore, the efficiency of exchange, and the remaining design restrictions lead to an optimized EGR cooling temperature of 120 ºC and to an optimized cooling temperature of the exhaust gases between 110 ºC and 230 ºC, in accordance with the ICE regime. A heat exchanger with finned tubes and continuous fins for evaporators was chosen. A concept of variable geometry was developed for blocking the flow of the hot gases upstream and downstream of the bank of tubes. The variable locking is made with two gates which allow a different number of channels of the bank of finned tubes. This way it is possible to artificially vary the swap area and the convection coefficient of the gases, thereby controlling the effectiveness of the evaporators. The evaporator designed to recover heat from the EGR is placed on the original location of the EGR cooler and the evaporator designed to recover heat from the exhaust gases replaces the original muffler of the vehicle. The heat exchangerselected for the condenser has concentric tubes with axial flow and is positioned vertically on the rear wall of the cab. 9
The proposed system achieves a reduction of fuel consumption and CO2 emissions of 5.5% in most of the ICE regimes and 6.6% at high speed. The mechanical power increases by about 6% in most of the ICE regimes and 7% at high speed. The proposed system requires an investment of about 10,000 and has payback of 5 years. 9. Refererences [1] Azevedo, J. (2005). "Apontamentos de Equipamentos Térmicos". Textos de apoio, edição AEIST. [2] Bao, J., Zhao, L. (2013) A review of working fluid and expander selections for organic Rankine cycle.renewable and Sustainable Energy Reviews, 2013, vol. 24, issue C, pages 325-342. [3] Edwards, S., Eitel, J., Pantow, E., Geskes, P. et al., (2012) "Waste Heat Recovery: The Next Challenge for Commercial Vehicle Thermomanagement," SAE Int. J. Commer. Veh. 5(1):395-406. [4] Lopes, J. (2003). "Motores de Combustão Interna - uma abordagem termodinâmica". Textos de apoio, edição AEIST. [5] Seher, D., Lengenfelder, T., Gerhardt, J., Eisenmenger, N., Hackner, M., Krinn, I. (2012) Waste Heat Recovery for Commercial Vehicles with a Rankine Process. 21st Aachen Colloquium Automobile and Engine Technology 2012, Stuttgart, Germany. [6] Xin, Q. (2011). Diesel Engine System Design.Woodhead. 10