Sigla UPB
Sigla CNCSIS
Sigla AR
University POLITEHNICA of Bucharest The National University
Research Council
Automotive Engineering Departement

 
 
INNOVATIVE HYBRID (THERMAL - ELECTRIC) PROPULSION SYSTEM FOR AUTOMOBILES


THE NEAR INTRODUCTION OF EURO 5 NORMS, IN 2009, AND, ESPECIALLY, THE REQUIREMENTS OF EURO 6 LEGISLATION, IN 2012-2014 ?, IMPOSE THE CONCEPTION AND CREATION OF NEW POWERTRAIN SYSTEMS FOR MOTOR VEHICLES, AND, ESPECIALLY, AN OPTIMAL ENERGY MANAGEMENT. IN EUROPE, AS WELL AS IN THE U.S. AND JAPAN, REGENERATIVE TRANSMISSIONS HAVE BOOSTED AUTOMOTIVE INDUSTRY AND STIMULATE THE INTEREST OF BENEFICIARIES AND OF THOSE INVOLVED IN ENVIRONMENT AND RESOURCE PROTECTION. IN THE FRAME OF THE PROPOSED PROJECT, WE ARE DEVELOPING A COMPARATIVE ANALYSIS OF REGENERATIVE TRANSMISSIONS THAT ARE ELIGIBLE FOR MOTOR VEHICLES, BY SIMULATION VIA COMPLEX DYNAMIC MODELS AND EVOLVED PROGRAMMING ENVIRONMENTS. THE MOST PROMISING SOLUTIONS, ALSO ENVISAGED BY CAR MANUFACTURERS, WILL BE STUDIED. THE BEST HYBRID TRANSMISSION - WITH THE BEST DYNAMIC, ECONOMICAL AND ECOLOGICAL BEHAVIOR - WILL BE PRACTICALLY DEVELOPED. WE INTEND TO REPLACE THE CONVENTIONAL (MECHANICAL, 5 SPEED) TRANSAXLE WITH A SPECIALLY ONE, ENTIRELY CONCEIVED, DESIGNED AND REALIZED BY TEAM MEMBERS. THE INNOVATIVE POWERTRAIN SYSTEM THAT IS PROPOSED BY THIS PROJECT WILL ALSO WORK IN REGENERATIVE MODE, RECUPERATING VEHICLE KINETIC ENERGY IN COASTING OPERATION. MOREOVER, THE TRANSMISSION WILL WORK BOTH IN STEPPED AND IN CONTINUOUS MODE, WHICH WILL CREATE A REAL TESTING FACILITY (MOBILE LABORATORY), WITH POSITIVE IMPLICATIONS IN SCIENTIFIC RESEARCH ACTIVITIES.
 
 
Actual aspects and trends
 
Global warming, mineral resources exhaustion and urban pollution are problems to be solved in the new century, triggering new standards for automotive propulsion systems. By fuel consumption reduction the automotive manufacturers also obtain CO2 reduction, as an important component of the Green House effect.
European automotive manufacturers, grouped together in ACEA, accepted a reduction with 25 % of CO2 emisson levels by 2008, as compared to the 1990 level of 140 g/km. Correspondingly, a fuel consumption of 5.8 liters/100 km (petrol) or 5.2 liters/100 km (Diesel) is obtained in the NEDC European Drive Cycle. Until 2012, a bigger step ist o be done: reducing the CO2 emission levels until 120 g/km, which would translate in approximately 5 l/100 km – petrol and 4.5 l/100 km – Diesel fuels, respectively [3]. At the same time, car owners require improved safety and confort, although achieving these results leads to an increased vehicle mass and, therefore, increased fuel consumption. Additionally, better dynamic performances are imposed, without additional price tag.
It is important to emphasize that an optimum working of an internal combustion engine – at a set of specified operating points, on an engine dyno or other kind of testing rig – is not sufficient, because the engine functions, in a motor car, under a continuous domain of operating points, and, therefore, its entire range should ideally be optimized (in terms of brake-specific equivalent fuel consumption or, equivalently, in terms of energetic efficiency). It has been appreciated that efficient operation of internal combustion engine (ICE) would be possible only in the presence of an additional electric drive system. The latter would supply the required wheel power, during short periods of time, generally during transient vehicle operations, as is the case of city driving or traffic jams. In such a configuration (ICE, adaptive and smart transmission, electric motor/generator), the engine would work solely at its minimum BSFC (brake-specific fuel consumption) point – or in its vecinity –, the rest of the time it would be either stopped or idling.
 
 
Hybrid propulsion systems
 
Any propulsion/traction system that includes, besides the conventional internal combustion engine, at least a second mechanical power source (capable to create mechanical torque at the driving wheels) is named hybrid propulsion/traction system. Typically, the hybrid propulsion systems realize a so-called regenerative braking function, which recovers some part of vehicle kinetic energy while braking/coasting (decelerating). A significant part of the overall fuel economy associated with hybrid powertrain systems is due to this regenerative braking function. A typical hybrid powertrain layout is depicted in figure 1, which shows the fundamental architectures behind hybrid propulsion/traction systems for motor vehicles. While figure 1, (a) shows the most abstract (and, thus, the most general) hybrid propulsion structure, the other figures offer a more particular insight in energetic transfer between systems, in function of the nature of power links: series hybrid if the link between I.C. engine and wheels is done electrically (there is no kinematic link between engine shaft and driving wheels); parallel hybrid (when there is a kinematic link between ICE and wheels, a case when usually the electric machine is of lower power than the IC engine), and finally mixt (dual) hybrid systems (when there is a combination of the two above – mentioned types).
Most frequently, the second drive system is of electric nature, but it can be of virtually any other nature, generally hydraulical, pneumatical or even mechanical (flywheel systems). The second most important characteristic of hybrid drive systems ist hat they require at least two energy storage systems. The most familiar of them is, of course, the fuel reservoir, having the non-negligible advantage of storing the energy in a very concentrated form (high J/kg and J/m3 values). The second energy storage system (ESS) has the characteristics of both energy supplier and energy recovery device, in function of instantaneous vehicle driving requirements. The most suitable were proven to be the electric batteries, but also super-capacitors, kinetic (flywheels) or hydraulic accumulators. One of the most successful candidates is the Lithium – based battery, which have moderate storing capacity and exceptional life (>240,000 km).
Until 2020, high energy density batteries will impose on the market (as Lithium – Ion batteries, reaching 120-200 Wh/g, or NiMH, achieving 60-100 Wh/g, better than lead-acid – with only 30-45 Wh/g).
The advantages of hybrid propulsion systems are:
- zero local emissions (electric vehicle, or all electric range –AER, or even zero-emission vehicles ¬– ZEV);
- low fuel consumption and low CO2 emissions, because of: (i) braking energy recuperation, (ii) start/stop operation, (iii) moving average engine operating points towards economic pole, and (iv) engine downsizing – as short transients are assured by the electric motor.
There are three basic ways to store energy on a motor vehicle: (a) setting the I.C.E. operating points towards the BSFC area; (b) stopping the thermal engine when its usability is no longer required (as in the case where the car is stopped at a red light); and (c) storing and re-using the kinetic energy of the vehicle (which otherwise would have been wasted by dissipation in the conventional braking system), through the use of a reversible electric machine.
In function of the modality through which mechanical power is transmitted to driving wheels, one can identify two large families of hybrid powertrain systems (hereafter also called hybrid transmissions and which form, together with the vehicle, the so-called hybrid electric vehicle, or HEV), fig. 2 [4]: (a) hybrid systems with the ICE being assisted by an electric machine (e-machine), and (b) hybrid drive systems with the drive wheels connected with an e-machine supplied with electric power from a generator.
 In function of e-machine power, we can distinguish micro-hybrid systems (42 V and start/stop operation, as is the case with Toyota Crown), medium hybrids (100-250 V electric system – based, which operate by assisting the thermal engine with an Integrated Starter Generator or ISG, ex. Honda Insight), and finally full hybrid powertrain systems (with 250+ V, driving the wheels simultaneously or alternatively with the ICE – as in the case of Toyota Prius). The main difference between these drive systems is given by electric power, as compared with the total installed power (which define the so-called hybridization degree).
 
Micro and medium hybridization is the simplest version. A small e-motor equips the car, helping propulsion to some extent but without an independent motor-to-wheels transmission system. The e-machine can replace the IC engine totally, for small speed vehicle operation, but only if the installed electric power is relatively small, from 2-10 to 25 kW. An integrated starter/generator system replaces the alternator and the starter. Start/stop systems are either of belt-driven type (B-ISG) – when the electric machine drives via a belt – or of crankshaft-mounted type (C-ISG) – when the e-machine acts directly on the crankshaft. Micro-hybridization is most useful at standstill acceleration regime, using previously recovered energy, while medium hybridization uses the electric motor for power (and torque) boosting of the conventional system, but cannot drive the car solely. It is well adapted to small speed vehicle operation regimes, intense traffic with frequent stop&go situations, when fuel consumption and emissions are bad and have to be minimized. Stop&go operation allows for 5-10 % CO2 reduction. Also performances are improved, as statistics stated that the extra 10-25 kW of (electric) power is useful not only at breakaway, but also during accelerations and overtaking maneuvres (speeds between 80 and 120 km/h), or even at hill climbings.
Despite the relatively modest influence of medium hybridization on general vehicle performance, however the cost is significantly smaller than the one associated with total hybridization.
Full hybrid systems are characterized by the fact that the e-machine can drive itself the vehicle, without thermal engine assistance. That is to say, full hybrid propulsion systems are powerful enough to propel the vehicle on a certain distance without consuming fossile fuel from the reservoir. CO2 reduction is significant, reaching figures of up to 25-30 %.
 
Also, in function of engine/motor(s) inter-connection architecture (layout), we can distinguish two basic configurations of total hybridization systems [4], [2]: (a) series transmission systems (the name comes from the way subsystems are connected), where the ICE sends mechanical power to a generator – together forming an auxiliary power unit (APU). It is mostly a small car/commuter solution, more rarely applied to normal sedans or compact cars. The main reason is the large size and cost of the electric motor (and the whole electric system); and (b) parallel hybrid transmission systems, with both mechanical power sources (IC engine and EM) being connected to the driving wheels via adequate mechanical links (mechanical transmission system). Whether the two (at least) motors drive a single axle or both axles (via the so-called through-the-road, or TTR, layout) is another classification of parallel hybrid systems.
Regardless of the configuration (layout), the major (key) components are the same: IC engine, electric machine(s), electronic power converters (power electronics) and storing systems (batteries, super-capacitors, flywheels etc.). Through adequate (and intelligent) connection of these components (motors, energy storage systems) – via clutches, belts, chains, gears, planetary transmissions, universal joints – there is the possibility to realize smart hybrid transmissions that have, in the same construction, series, parallel and mixed (dual) hybrid architectures are achievable.
 
Mixed (dual) systems – also named power split systems – use electro-mechanical or electro-magnetic systems to optimally control the power flow parameters from the mechanical power sources (always more than a single one, in this case) and its transmission to driving wheels, in function of driving requirements and desired performances. The most representative system configurations are depicted in figure 4 [4]. Each design (layout) can be a solution thaty satisfies the most demanding environment requirements, as well as dynamic, cost and quality needs. The criteria that help select a hybrid transmission design are determined by several aspects, like: (a) the purpose (dynamic performances, unlimited mobility and exploitability), (b) requirements of today’s standards (emissions and CO2 levels, recyclability degree and environment-friendliness), (c) market situation (lifecycle cost and maintenance, available infrastructure) and (d) user perception (comfort, drivability/dynamicity).
Reaching a single goal (ex. zero emissions) should not exclude other important aspects that are related to the potential large penetration of these transmissions on the market. That is the reason why an alternative to mass production of hybrid transmission is a market niche, by adopting sophisticated, expensive technologies but sometimes crucial for certain purposes.
Parallel hybrid transmissions are much more flexible from the utilization point of view. Power, size and weight of electric machine(s) can be chosen in that way that a trade-off between cost, complexity, efficiency and drivability is to be found. Engine optimization can yield supplementary advantages, but this does not constitute the first priority. Performances and functions can be gradually increased, in function of electric power (and, subsequently, degree of hybridization, DoH) increase (36 V) and auxiliary systems power (air conditioning, catalyst warming system, control valves, drive-by-wire systems). A large number of potential layouts (architectures) exist, in the case of parallel hybrid systems [7], [2]. Choosing the optimal solution largely depends on the type of vehicle and system requirements. The combination between an e-machine and a continuously-variable transmission (CVT) allows for a compact construction with a large power density, if PMSM (permanent magnet synchronous motors) or BLDC (brushless DC motors) are employed.
 
There are some representative examples of parallel hybrid electric systems – one can notice the kinematic relative dependence between rotational regimes (angular velocities) of thermal and electric motors. The first case corresponds to the Honda Insight (with a stepped transmission version; there is also a CVT-based version) and is characterized by engine decoupling in part load operations – ZEV operation. Regenerative braking is achieved by putting the e-machine (ME) in generator mode, so as, with the aid of power electronics (inverter, DC/DC converter) the accumulator batteries are charged. The variant (c) is ingenious by the elimination of supplementary control algorithms (the one-way clutch automatically unlocks during braking and coasting regimes, so that all available kinetic energy is recovered by the electric system). The (d) ... (f) variants are so-called double-shaft architectures, having the e-motor downstream transmission. Sometimes, EM can be mechanically decoupled at high vehicle speeds, reducing total equivalent system inertia and improving global system efficiency. The CVT version employs the great advantage to send the IC engine (MT on figure 5) in the BSFC region, having (theoretically) an infinity of eligible operating points on engine characteristics (engine map). During braking/coasting operation, EM again takes a large part of kinetic energy, while clutch A decouples the engine, which is shut off for fuel savings reason. Finally, figure 5, (f) presents the scheme of a so-called through-the-road (TTR) hybrid – in the sense that kinematic link between engine and e-machine shafts is done by the road. Using an EM for each rear driving wheel is another advantage of the system, but drawbacks are equally present (increase in construction complexity, increased unsprung mass, need for embedded planetary reduction gear etc.).
The Honda Insight hybrid traction system is organized by simple shaft torque addition architecture [8] [9]. The advantages of flat electric motor of large diameter are mostly related to the big power density and satisfaction of axial size requirements.
 
Mixed (dual) hybrid transmission systems combine positive aspects of both series and parallel transmission systems, while avoiding oversizing (and over-costing) of the series design, but keeping its smoothness. The e-machine can drive the wheels either alone or in conjunction with the IC engine, in order to insure maximum efficiency at any time. Additionally, the system can be used to drive wheels and produce electricity at the same time (through the use of an additional – but small powered – electric machine that acts as a generator). Very low emissions are the outcome of this design.
These systems allow good dynamic performance, drivability, smoothness and optimum energy management, but the complexity and big battery pack voltage require high performance electric machines, and the costs keep most such designs off mass production. Such split-hybrid transmissions are characterized by the fact that only a part of engine power is transmitted mechanically to the wheels (in a parallel way), while the other is sent to the wheels via the electric system (in a series way). Thus there is a parallel and a series path at any time, but the percentage of these paths vary with driving operation, vehicle speed, driver demand etc. An outcome is the high efficiency, even at low speed (city driving).
 
Two dual (mixed) hybrid systems are briefly discussed in the following – the THS (HSD, nowadays) Toyota Prius system is the first one. The essential effect of power combination via the planetary mechanism MP (here associated with a power summator device, of speed-addition type) is given by the operation character – which is mostly of series type in part load and low speed operation, becoming mostly of parallel type at full load and highway cruising. Moreover, the system is continuous, shock-free, and achieves a smooth shift feel (as with a conventional, mechanical CVT system). However, control-linked complications fully compensate the mechanical simplicity (the lack of rigurous, advanced algorithms – based control of MT, ME1, ME2 motors yielding parasite power paths and, finally, the malfunction of the whole system). The system has proved to be a real success in the Toyota Prius, and now now it is at the basis of the Ford Escape Hybrid. Curiously, the latter achieves better fuel consumption in city driving – because of intense use of regenerative braking and series hybrid operation – than on the highway! It has to be noted that the traction motor (ME1 on the schematic) has about 50 kW, comparable to the thermal engine power – the latter working via Miller-Atkinson cycle. If parallel HEV decoupled (kinematically) the engine when coasting for regenerative braking, in the dual system case this does not occur (IC engine is shut off, and ME2 acts as a generator, charging the battery pack). Moreover, ME1 does the same thing, so the system is very efficient.
The powertrain of a prototype created at the Valencienne University, France, is a system of speed-addition type, but without a planetary power-split device. Instead, the speed addition is done by controlling the relative angular velocity between the stator and the rotor of the electric motor. Figure 10 shows the Lexus RX-400h powertrain schematic, using a double planetary mechanism. The difference (with respect to the THS system that equips Toyota Prius) consists in the larger M1 speed range (which was therefore reduced in size, without affecting dynamic performances).
 
The hybrid transmission from Toyota
 
An exceptional achievement in the field is represented by Toyota hybrid system (THS), launched in 1997 on the Prius [3]. Three goals have been taken into account: 1) using an IC engine with high efficiency; 2) using an advanced control system to insure all-time working of the system at the optimum regime; 3) energy loss reduction and energy regeneration. The compact class – belonging Toyota Prius has a maximum speed of 140 km/h, 30% (17 degrees) maximum climbing angle, 105 km/h hill of 5% (6 degrees).
The energy characteristics of THS (as compared with the design theme) are given in table 2.
The two reversible machines (motor and generator) are PMSM constructions. The powerful electric motor assures main tractive power at low speeds, yielding smooth acceleration and seamless shifting (identically to a CVT). It also provides regenerative braking.
The electric generator (or secondary e-machine) mainly constitutes an electric CVT device, together with the (simple) power-split device. The inverter transforms DC current into AC current to feed the motor and generator (working as a motor), when the e-machines are in driving mode, and AC current to DC current to store energy into batteries, when the electric machines operate in regeneration mode. The 1.5 litre engine has an increased expansion ratio (13.5:1) – working following the Atkinson cycle –, variable compression ratio (4.8 – 9.3) and intelligent variable valve timing (VVT-i). When the car slows down (coasting or braking operation), the motor acts as a generator. The regenerative braking facility is essential mainly in city driving, characterized by frequent traffic jams. Maximum braking efficiency is achieved by actuating both systems (regenerative braking, as well as mechanical braking), with different weighs.
 
When the car is standing still, the rpm figures of the three motors are zero (A). In normal driving conditions, the generator (G/M or M2 machine) the engine generates sufficient power to propel the car alone (C). In cases when a rapid speed increase is required, at a specific speed, both IC engine speed and generator speed are increased, producing more electricity, so the traction motor can produce more electric power to propel the car faster and fulfill the transient requirements (D). The traction characteristics of the THS system is given in figure 15.
 
The combined fuel consumption, in Japanese 10-15 test cycle, is only 3.57 l/100 km.
 
Toyota also launched in 2001 the THS-M (Toyota Hybrid System – Mild) which could be applied on numerous Japanese cars. A 3.0 litre petrol engine (V6) with automatic transmission was the base point of this parallel hybrid powertrain solution. The main three components are: a powerful yet compact three-phase synchronous starter/generator, a 36 V battery pack and an advanced control unit. When the car stops, the controller automatically shuts down the IC engine. As in the case of virtually all stop&go systems, the integrated starter/alternator (42 V) insures both engine cranking and vehicle driving in the very first moments when the car is beginning to move. The fuel economy is 15% - as compared to the conventional model – and emissions are half the ones stated by Japanese regulations of 2000 year.
 
References
1.         Gott, Philip, Linna, Jan-Roger and Mello, J.P. – The Evolution of Powertrain Technology 2008 and beyond: Engines, Hybrids, Battery Electric, Fuel Cells and Transmissions. FISITA World Automotive Congress, 23-27 May, 2004, Barcelona, Spain, Paper F2004F335;
2.         Noreikat, K.E. – Antriebstechnik, die die Welt (nicht) braucht. VDI-Berichte, nr.1704, 24-25 Oktober, 2002, p.143-160;
3.         Oprean, I.M. – Automobilul modern. Cerinte, Restrictii, Solutii. Editura Academiei Romane, Bucuresti, 2003;
4.         Rovera, Giuseppe, Vittorio, Ravello – Scenario and Trends on Hybrid Propulsion Technologies. ATA, 56, 3 / 4, 2003, p. 78-89;
5.         Walzer, Peter – Progress in Car Powerplant Technologies. FISITA World Automotive Congress, 23-27 May, 2004, Barcelona, Spain, Paper F2004F020;
6.         *** - Hybrid Hype – Future Drive Technologies. AutoTechnology nr.1, 2006;
7.         Andreescu, Cristian, Cruceru, Dragos, Recuperarea energiei cinetice a autovehiculelor, Revista AutoTest, nr. 115, mai 2006;
8.         Badin, Francois, Hybrid vehicles: realizations and potentials, Conferinta Nationala CAR 2005, Pitesti 2-4 noiembrie 2005.
9.        Cruceru, Dragos N., Contributions at modeling and simulation of hybrid automotive powertrain systems, Ph.D. Thesis, 2007;