Lino Guzzella · Antonio Sciarretta Vehicle Propulsion Systems

These vehicles are responsible for a large part of the world's consumption of primary energy carriers, mainly liquid fossil hydrocarbons. Only those aspects of the lateral and horizontal dynamics of the vehicle that affect energy consumption are briefly mentioned.

Motivation

The last section of this first chapter lists the main options available for reducing the energy consumption of passenger vehicles. In the face of these trends, it is clear that new fuel sources must be developed and that the fuel consumption of passenger cars must be significantly reduced.

Objectives

At least three steps of energy conversion are important for a comprehensive analysis of the energy consumption of passenger cars. The following section presents an overview of the most important approaches to energy conversion.

Fig. 1.2. The main elements of the energy conversion scheme.

Upstream Processes

However, if the other borderline case (coal-fired steam turbines) is taken into account, the well-to-miles carbon dioxide emissions levels of a battery electric car become even worse than those of the worst ICE-based propulsion system. .9 In addition, it will be shown in the next paragraph that the energy density of batteries is so small that battery electric vehicles cannot meet the specifications of a passenger car as defined in art. The compressors required for this consume about 20 to 25 percent of the electrical energy produced by the fuel cell [212].

Fig. 1.3. Different paths to convert a primary energy source to mechanical energy needed to drive a car in the MVEG-95 test cycle

Energy Density of On-Board Energy Carriers

Not surprisingly, none of the several battery electric vehicle prototypes that have been developed in the past have ever developed into a mass-produced alternative to existing solutions. Systems based on pneumatic air stored at 300 bar in carbon fiber bottles can achieve a net energy density of the order of 20 Wh/kg.

Pathways to Better Fuel Economy

This chapter contains three main sections: First, the dynamics of the longitudinal movement of a road vehicle is analyzed. The influence of the driving pattern on the fuel consumption is analyzed in the second section.

Vehicle Energy Losses and Performance Analysis

Hence, the contribution of the wheels to the vehicle's overall inertia is given by the following term. From the first-order differential equation (2.1) the vehicle speed v can be calculated as a function of the force Ft.

Fig. 2.1. Schematic representation of the forces acting on a vehicle in motion.

Mechanical Energy Demand in Driving Cycles

8 In the MVEG–95 cycle, the vehicle is in traction mode for approximately 60% of the total cycle time. Comparison of the energy demand in the MVEG–95 cycle for a full-size car (left) and a lightweight car (right).

Fig. 2.5. US test cycle FTP–75 (Federal Test Procedure), length: 11.12 miles (17.8 km), duration: 1890 s, average speed: 21 mph (9.43 m/s).

Methods and Tools for the Prediction of Fuel ConsumptionConsumption

The average operating point method can provide reasonable estimates of the fuel consumption of simple powertrains (IC engine or battery electric propulsion systems). The slower effects important for fuel economy optimization will be discussed in the following chapters.

Figure 2.13 shows such a map. Note that η e strongly depends on engine torque, whereas the engine speed has less of an influence

IC Engine Models

The main components of IC motor-based propulsion systems are the motor and the gearbox. When the engine is operating at steady state, two normalized variables describe its operating point.

Fig. 3.1. Engine input and output variables in the quasistatic and in the dynamic system description.

Gear-Box Models

An example of model-based numerical optimization of gear ratios is shown in Appendix I in Case Study 8.1. With these preparations, it is easy to determine the efficiency of the torque converter in towing mode.

Fig. 3.4. Engine map 3.2 combined with two vehicle resistance curves (fourth and fifth gears on horizontal road) for the vehicle specified in Sect

Fuel Consumption of IC Engine Powertrains

Ancillary equipment, including the electric power generator and the friction clutch, consume a portion of the power produced by the engine. Distribution and frequency of motor operating points (square root of the number of counts for each non-zero motor load/speed point).

Fig. 3.11. Top layer of the QSS model of the powertrain analyzed in this section.

Electric Propulsion Systems

While in conventional ICE-based vehicles the energy carrier is fossil fuel, electric and hybrid-electric propulsion systems are characterized by the presence of an electrochemical or electrostatic energy storage system. Model representations of the electric drive train, torque clutch, and planetary gear are added as separate sections due to the importance of these components in most hybrid-electric powertrains. The key component is the battery, which is usually of the nickel metal hydrate or nickel cadmium type, although newer technologies (eg lithium-ion) have already been proven.

Hybrid-Electric Propulsion Systems

Engine operation is unrelated to the vehicle's power requirements (4) and the engine can therefore be operated at a point of optimum efficiency and emissions (3). An additional advantage may be that the transmission does not require a clutch, i.e. that the motor is never disconnected, as it is mechanically disconnected from the drive shaft (5). As will become clear in the following (see section 4.8), the pure ICE operation is often associated with a current flow through the generator and the engine.

Fig. 4.1. Basic series hybrid configuration. B: battery, E: engine, G: generator, M:

Electric Motors

82 4 Electric and Hybrid-Electric Drive Systems. mutual) inductance,σ= 1−L2m/(Ls·Lr),pis the number of pole pairs,ω(t) is the frequency of the stator voltage (i.e. the speed of the d-q frame), and p·ω2( t) is the frequency of the magnetic field induced in the rotor. In the following, a special but general case is considered, i.e. the so-called common operating mode of the three-phase inverter, such that Ud(t) = 0. Evaluation of the Willans parameters for a 32 kW permanent-magnet synchronous motor (top) and a 30 kW induction motor (bottom) of the ADVISOR database [272]. whereVr=π·r2·is the rotor volume, i.e. the equivalent to the swept volume of engines.

Fig. 4.10. Flow of power factors for a parallel hybrid configuration, with the qua- qua-sistatic approach (a) and the dynamic approach (b)

Batteries

After a certain time tf (called the discharge time), the voltage reaches a value (eg 80% of the open circuit voltage), called the cut-off voltage, at which the battery is considered discharged. The first is the ohmic resistance Ro, which is the sequence of ohmic resistance in the electrolyte, in the electrodes, and in the interconnections and terminals of the battery. 4.32 we distinguish ohmic voltage drop from non-ohmic overvoltage or polarization.8 The latter is

Fig. 4.24. Schematics of a battery cell. A: anode, C: cathode, E: electrolyte. The example illustrates a lead–acid cell

Supercapacitors

5 Wh/kg, depending on the material of the electrodes (carbon, metal oxides) and of the electrolyte (aqueous, polymer) [76]. A basic physical model of a supercapacitor can be derived from a description of the system in terms of equivalent circuit. The basic equivalent circuit model, (4.96) can be used to calculate the terminal voltage as a function of the terminal current.

Fig. 4.34. Schematic of a supercapacitor.

Electric Power Links

In the general case (in most practical cases = 1,2), an equation is given by the power balance along the link,. 4.112) Of course, in the usual case of m= 1 (parallel EVs and HEVs), this model simply implies the equality of the power supplied by the battery and the power absorbed by the motor and electrical loads. The output variables of the model are the current at the main source port, I1(t), and the voltage at the other ports, Uj(t),j= 2,.

Torque Couplers

In the general case (in all practical cases m= 2,3), equations are given by equating rotational speed. In the case of direct coupling on the same crankshaft, all the transmission ratios are uniform. This can be represented by m−1 control variables suj which are conveniently defined as the torque distribution relations.

Power Split Devices

In the nominal operating range of K, that is, between K1 and K2, the power distribution ratio of the simple PSD is always greater than that of the compound PSD. Variator ratio (top) and power split ratio (bottom) as a function of speed transmission ratio. Consequently, the curver(t) = r(K(t)) results from the superposition of the two curves labeled A and B in Figure 2.

Fig. 4.44. Schematics of a planetary gear set arrangement (as in Toyota Prius [250]).

Short-Term Storage Systems

The introductory section of this chapter describes several devices, all of which can be classified as short-term storage systems. First, the use of short-term storage systems in powertrain applications and the possible powertrain configurations are discussed. Usually, the vehicle concepts with short-term storage systems are series hybrids, according to the classification of chapter.

Flywheels

The specific energy of the flywheel is calculated as the ratio of stored energy to the mass of the flywheel. The output variable is the flywheel speed ω2(t), which is sometimes thought of as the "state of charge" of the flywheel. With the same data, it is possible to obtain a value for the timing range of the flywheel, i.e. the time that the flywheel speed remains above a certain threshold without external torque.

Fig. 5.4. Flow of power factors for a hybrid-hydraulic propulsion system, with the quasistatic approach (a) and the dynamic approach (b)

Continuously Variable Transmissions

In "slow" gearing, the overall transmission ratio is given by the product of the CVT ratio and the gear ratio. In the "fast" gear arrangement, the power flow through the CVT is reversed, so the overall transmission ratio is proportional to the reciprocal of the CVT ratio. CVT efficiency increases with torque at constant speed and ν, exhibiting a maximum that is more pronounced at higher speeds.

Hydraulic Accumulators

The output variable is the state of charge of the battery, i.e. the volume occupied by the gas, Vg(t). The mass of liquid to be taken into account in the calculation is the mass that occupies the maximum volume left behind by the gas, i.e. the volume VA−VB. The mass of the loading gas is the constant mg already introduced in the previous section.

Fig. 5.15. Schematic of a hydraulic accumulator.

Hydraulic Pumps/Motors

For a bent-axis unit, x(t) is related to the angle of rotation, i.e. the control input of the machine. The volumetric efficiencyηv(t) accounts for several phenomena, including leakage (turbulent and laminar) and fluid compressibility [197]. The overall machine efficiency is the product of the volumetric efficiency and the torque efficiency.

Fig. 5.21. Schematics of piston pumps/motors: swash-plate (a), bent-axis (b).

Pneumatic Hybrid Engine Systems

The braking torque can be varied by simultaneously changing the timing of the tailgate and intake valve. The thermodynamic performance of the pneumatic pump operation is described by the coefficient of performance (COP). The pneumatic engine mode allows for quick engine starting and even launching of the vehicle.

Fig. 5.25. Double logarithmic p–V diagram of the conventional engine mode. IVO:

Fuel-Cell Electric Vehicles and Fuel-Cell Hybrid VehiclesVehicles

The first section briefly describes the application of fuel cell systems as stand-alone energy sources for power lines or, in combination with a storage system, as fuel cell hybrid propulsion systems. The second section introduces some thermodynamic and electrochemical models of fuel cells, as well as some fluid dynamic models of the complete fuel cell system. The integration of a fuel cell into a propulsion system seems simple in principle, and several prototypes have already been developed [280].

Table 6.1. Comparison of fuel data for hydrogen and gasoline.

Fuel Cells

Hh·ρh·γht, (6.1) kumht is the mass of the fully charged hydrogen tank, practically corresponding to the mass of the containermm, H is the lower heating value (120 MJ/kg or 33.3 kWh/kg) of hydrogen , ρhit the density (depending on the pressure in the vessel), and γht is the storage capacity of the vessel, i.e., the stored volume per unit mass. It is sometimes necessary to modify the theoretical coefficients of the Nernst equation to account for other phenomena, e.g., liquid water flooding the cathode [211]. The circulation mass flow rate m∗h, rec(t) as well as the pressure levels pan, out (t) and pan, in (t) are related to the operation of the hydrogen circulation pump.