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The implementation of the RC for vehicle waste heat recovery was originally considered in relation to heavy-duty long-haul diesel trucks. Cummins in 2014 showed the “Super Truck” program, sponsored by the US Department of Energy, where made much progress. The test results on road by a RC system installed on a diesel engine showed that the brake efficiency of diesel engine was increased from 47.5% to over 51%. Recently, a greater number of research groups [132,133,135] and vehicle manufacturers (e.g.

Toyota, Honda, BMW, etc. [129,131]) are investigating RC applications to passenger vehicles and an increased application potential has been shown.

RC System Architectures

Different layouts are possible depending on the heat source: engine coolant and exhaust gas (including EGR), which carry a quite similar amount of thermal energy. However, exhaust gas shows a much greater recovery potential in terms of exergy because of a much higher temperature range. The preferred architectures for passenger car applications are mainly three:

 Architecture 1 - the RC system utilizes the exhaust gas as the only heat source to evaporate the working fluid (Figure 6-59).

 Architecture 2 - compared to the previous RC layout a recuperator is added before the evaporator, using the steam from the expander to preheat the working fluid (Figure 6-60).

 Architecture 3 - in the RC system the waste heat from the engine coolant is used to preheat the working fluid (Figure 6-61).

Figure 6-59 Architecture 1- the RC system utilizes the exhaust gas as the only heat source to evaporate the working fluid.

138 Figure 6-60 Architecture 2 of the RC in which a recuperator is added before the evaporator using the steam from the expander to preheat the working fluid

Other layouts are possible, for instance with dual loops using exhaust and cooling sources (see [132]), but for cost, complexity and packaging issues they are not preferred for passenger cars. The regenerative preheating of the architecture 2 requires a very complex liquid-gas heat exchanger with high surface area, while the preheater in architecture 3 is only required. For some working fluids, e.g. organic fluids, architecture 2 (with a recuperator) is needed to cool down the vapor exiting from the expander, which is still superheated, to reduce the cooling load of the condenser.

Figure 6-61 Architecture 3- in the RC system waste heat from the engine coolant is used to preheat the working fluid A novel integration of the RC is proposed and analyzed in this research activity, see Figure 6-62, with the aim to overcome the packaging and cost issues. In case of electric hybrid powertrain, the expander is linked to electric moto-generator (e.g. BSG) by means of the clutch CLT1. GB is a gearbox the can be used in case of high speed expander (e.g. Turbine). The clutch CLT2 connects the electric machine with engine shaft. In this architecture the Motor/Generator performs multiple functions, without the addition of a specific electric generator for the RC system.

139 Figure 6-62 Proposed RC system integration in electric hybrid powertrain

Different operation modes are possible by means of the commad of the clutches:

- with CLT1 and CLT2 closed the mechanical power from expander is transmitted to the wheel in parallel to engine or alternative both thernal machiens generates elctric power;

- with CLT1 closed amd CLT2 open the power form RC system is used to generate only electric power, for istance if the energy harvested is higher than the load requested for the traction;

- with CLT1 open and CLT2 close the motor is used to delivery power without the inertia of RC system, for instance during boosting monouvers.

The benefits of this architecture will be studied by means of simulation on standards cycle and presented in Chapter #12.

RC efficiency

The thermodynamic efficiency of RC system can be expressed with the following equation:

𝜂_𝑅𝐶 =𝑊̇_𝑒𝑥𝑝− 𝑊̇_{𝑝𝑢𝑚𝑝}

140 The following Figure 6-63 clarifies the meaning of the variables in the ηRC definition and it illustrates the typical values of the power flows.

Figure 6-63 Example of Energy balance in RC System, adapted from [130]

In Figure 6-64 the comparison between an ideal and a real RC cycle is illustrated. The main not ideality causes are:

 the finite size of the evaporator, that limits its efficiency;

 the evaporator pressure drop and maximum pressure/temperature constraints;

 the expander losses;

 the condenser pressure drop, size and low pressure constraints.

Figure 6-64 Ideal vs. real RC cycle in TS diagram

Another way to measure the effectiveness of RC system in passenger cars, commonly used in the literature is the thermal efficiency 𝜂_𝑡ℎ defined by the equation

Ideal Real

141 𝜂_𝑡ℎ = 𝑊̇_𝑒𝑥𝑝

𝑚̇_{𝑓𝑢𝑒𝑙}∙ 𝐿𝐻𝑉

(6.7) in which ṁfuel is the engine fuel consumption rate and LHV is the fuel low heating value. Table 6-11 summarizes the pros and cons of the three different RC architectures and the typical ranges of RC thermal efficiency ηth for passenger vehicle applications, as presented in [132].

Table 6-11 Comparison among the RC architectures diesel engine is presented, demonstrating that with a proper RC sizing it can be contained.

Working Fluid -When designing an RC system, special attention should also be paid to the working fluid selection, according to the heat source temperature, which has a significant effect on the system thermal and exergetic efficiency. The choice of the working fluid used in the RC depends on a number of factors including thermodynamic efficiency, environmental, safety, and process-related economic issues. For safety reason, alcohols and hydrocarbons, in spite of their good thermodynamic efficiencies, are not ideal candidates. Instead, refrigerants, which are used in automotive air conditioning systems, are usually better options. Generally, according to the slope of the saturation curve, the working fluid may be categorized into three different types: 1) wet fluid, 2) dry fluid and 3) isentropic fluid, see Figure 6-65.

Water is a preferable working fluid for high exhaust gas temperatures ranging from 500 to 800 ℃, for this motive it is suitable for SI engines. In terms of the disadvantages of water, there is the requirement for superheating to avoid turbine blade erosion if such a machine is selected for the expander. Also, the high degree of superheating makes water less practical for automotive applications due to the variation of exhaust temperature at different load conditions. Also, water high freezing point (0 ℃) cannot meet the standard automotive working temperature range (−40 ÷ 85 ℃). Therefore, practical solutions to solve these issues need to be developed if water is chosen as the working fluid for RC vehicle applications.

Most organic fluids are either dry fluids (e.g. R113, R245fa, R245ca, etc.) or isentropic fluids (e.g. R11, R134a, R123, etc.). Associated CFCs/ HCFCs have been rejected due to environmental concerns and phased out according to international protocols. Dry/isentropic refrigerants are widely used in small-scale RC applications because of their good heat transfer properties, excellent thermal stability and low viscosity. They are generally non-flammable, which is a great advantage for automotive application and

142 compatible with most materials. Under low temperature ambient conditions (−40 ÷ 0 ℃) they also do not freeze, which is a major benefit relative to water.

Figure 6-65 Three types of working fluid: dry fluid, wet fluid, and isentropic fluid.

It was also found that the heat exchanger effectiveness for R123 and R245fa is higher than that for water, and consequently when the exhaust temperature is relatively low, organic fluids can be considered appropriate for vehicle RC application. Organic fluids usually exhibit a lower power output than water, require an extra recuperator to reduce the cooling load of the condenser, have relatively low thermal instability temperatures. The selection of working fluid should be considered together with the expander for the specific vehicle RC application.

Expander - The expander can be mechanically plugged to the engine or connected to an electric generator or to both, as illustrated in Figure 6-62. In the case of connection with engine the harvested energy is used with high efficiency, without other conversions, but the system doesn’t allow a versatile energy management, furthermore the expander speed is linked to the engine with not optimal operation. The four main types of expanders (turbine, scroll, screw and piston types) have their own advantages and disadvantages for passenger vehicle applications.

The turbine expander has a compact structure, is light weight, and has high efficiency. However, design and manufacturing is very difficult, thus leading to a higher cost. The turbine expander also has lower efficiency under off-design conditions and cannot tolerate two-phase conditions. For the high rotational speed (50 ÷100 krpm), the turbine requires a gearbox for the direct connection and it can more suitable to combine with electrical generators for energy conversion.

The scroll expander is characterized by lower flow rates, higher pressure ratios and much lower rotational speeds than turbomachines. It has a compact structure, fewer moving parts, lower levels of noise and vibration, and the most cost efficient design.

A screw expander is highly efficient in off-design conditions. However, lubrication is required to avoid direct contact while achieving a seal between the lobes of the two rotors; this feature makes the screw expander relatively more expensive to fabricate than scroll expanders.

Piston expanders have larger built-in volume ratio, high achievable operating pressures and temperatures, and low rotational speeds. All of the three displacement types of expanders (scroll, screw, and piston types) have the ability to tolerate two-phase flow. Table 6-12 from [132] summarizes the advantages and disadvantages of the different types of expanders for RC application to passenger vehicles.

143 Table 6-12 Comparison of different types of expanders for RC application to passenger vehicles

Heat Sink- The heat sink can be the engine coolant or the ambient air. The firs one is the most stable source in temperature (80÷100°C) and capacity flow (3÷10kW/K), whereas air as heat sink allows lower temperatures, but higher temperature drops. The state-of-arts of RC systems is presented in the review study [132], from which the following Table 6-13 is extracted.

Table 6-13 Summary of the works about the application of Rankine Cycle to passenger vehicles

Rankine Cycle System Cost Estimation

The economic evaluation is derived from data reported in [87,135,175] and it is based on estimations for the cost of the utility components: pump, expander, heat exchangers (condenser and evaporator).

The cost value for every component is coming from the cost analysis of parts already known and with a large scale application in the automotive industry. The reference elements for the pump and the heat

144 exchangers are the components used for the air cooling installation in the vehicle, engine cooling and EGR. For the expander cost the engine turbo machines are benchmarked.

The total utility cost is defined by the following equation:

Cost RC= Cost condenser +Cost evaporator + Cost pump + Cost expander + Cost pipes + Cost fluid in [€]

(6.8) The following table summarizes the range of cost for each components considering a system in a range of power 5÷10 kW.

The task of the transmission is to transfer the power of the ICE and the ETD to the traction wheels at a proper speed. The transmission reduces the high propulsion machine speed to the lower wheel speed by increasing the torque. The use of automated transmission is key to implement management logics that lead the engine and electric motor to operate in high efficiency points and it will be mandatory for the autonomous driving implementation.

Transmission Technology Options

Three technologies are the main options for the future ICE based powertrains: the Automatic Transmission, the Dual Clutch Transmission and the Continuous Variable Transmissions.

The Automated Manual Transmission (AMT) is applied in low cost market and is not considered of wide diffusion due to the issue of the torque gap during gear-shift and the limit in number of implementable gears.

Automatic Transmission (AT) - This is a (globally) wide-spread solution for the pickup and SUV segment, due to torque overshoot at start-up. The gear ratios of an automatic transmission are realized via planetary gears. Synchronization is realized via multi-plate clutch, multi-disk brakes, etc. The start-up element is usually a torque converter.

The main benefits are: good launch at vehicle take-off and the comfort, by means of a smooth ride. These transmissions are widespread for the heavier vehicles, especially in the US-market. It also enhances hybridization potential.

Dual Clutch Transmission (DCT) - It combines almost the comfort of a conventional AT with the dynamics of a Manual Transmission (MT). The DCT comprises two independent and separate transmissions. The dual clutch connects both transmissions with the engine via two driving shafts in a force-locking manner. Furthermore, shift events are realized without interruption of traction. The