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Electric Vehicles

Gyuláné Vincze, Gergely György Balázs

Budapest University of Technology and Economics Department of Electric Power Engineering

Hybrid-electric cars

Hybrid-electric cars

In Hybrid-Electric Vehicles (HEVs) there is an internal combustion engine (IC) and one or more electric motor for traction. Drive of the wheels is purely electric or combined with the internal combustion engine. The main aim of the design is to combine the advantages of internal combustion engines and electric motor drives, exploiting the advantages of electric motors and high unit (specific) energy of fossil fuels. Energy source is fuel stored in vessels, just as in traditional internal combustion cars. Accumulator, ultracapacitor or both are used to store electric energy temporarily. Pollution is characterized by the internal combustion engine.

Internal combustion engine and electric motor can interact in several ways. We distinguish several hybrid solutions: serial, parallel, simple and intelligent.

Table 9‑6. Technical data of some well-known hybrid-electric cars.

Manufacturer

Honda

Toyota

Toyota

Model name

Insight

Prius

Lexus

Hybrid type

simple hybrid, IMA

(Int. Motor Assist)

Pinion gear

intelligent hybrid

Two-axle, pinion gear

intelligent hybrid

Type of internal combustion motor

petrol motor+VTEC control

Atkinson-cycle petrol motor +VVT-i control

Atkinson-cycle petrol motor +VVT-i control

Cylinder volume

1ℓ

1,5 ℓ

3,3 ℓ

Power of ICM

50kW

(5700 f/min)

52kW

(4500 f/min)

155kW

(5600 f/min)

Torque of ICM

89Nm

(1000 f/min)

111Nm

(4200 f/min)

288Nm

(4400 f/min)

Type of electric drive

PM brushless DC

PM synchronous

PM synchronous

front+rear

Power of electric motor

10kW

(3000 f/min)

33kW

(1000…5600 f/min)

Front:123 kW

Rear: 50kW

Torque of EM

48Nm

(0…1000 f/min)

344Nm

(0…400 f/min)

Első: 333Nm

Hátsó: 130Nm

Type of battery

NiMH / Li-ion

NiMH / Li-ion

NiMH / Li-ion

Main voltage

144 V

288 V

288 V

Capacity of battery

6,5 Ah

6,5 Ah

6,5 Ah

Table 9.6 summarizes some technical data of well-known hybrid-electric car types. Besides, almost every car manufacturer develops hybrid cars. These developments show wide variety. There are cars with petrol and diesel engines, serial and parallel hybrid, synchronous or induction electric motors. New development is the PHEV (plug-in) hybrid-electric vehicle where energy refill can be made not only by fuel but electric charge, too.

Serial hybrid-electric cars

In a serial hybrid-electric vehicle the drive (traction) is purely electric. The internal combustion engine IC with electric machine ISG and inverter-type converter acts like a controlled electric energy source (aggregator) and is not included in the drive directly. ISG (integrated starter/generator) stands for its function; its role is to start internal combustion engine as well as to generate electricity. Starter motor used in traditional cars is not used here. ISG electric machine can be synchronous or induction type. The structure of the main circuit with voltage-source inverter fed AC drive is shown in Figure 8.23.

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Figure 8-23. Serial hybrid-electric vehicle.

Electric drive connected to u a DC voltage can besingle or multi.motor solution. Figure 8.23 shows a single-motor vehicle drive with motor EM.

Electric power of energy converter with internal combustion engine is p=u a i t where u a is voltage and i t is current. Internal combustion motor is connected mechanically only to ISG electric machine but not to the wheels. Because of this, required power can be provided in a rotational speed range where fuel consumption, efficiency and pollution are optimal. We also have to prevent the internal combustion engine from transient load for optimal operation, is possible. If momentary current i differs from current i t, then differential current (i-i t) must be provided by an interim energy storage which can be battery or ultracapacitor. Power required for average load is provided by the internal combustion engine, interim energy storage serves for transient momentary load so it can be designed for relatively low energy storage.

The direction of the differential current i a =i-i t can be charging or discharging. During acceleration, current of battery i a is added to current i t. In contrast, current i flowing into the DC circuit in opposite direction charges interim energy storage during regeneration braking. During electric brake energy regeneration is possible only into the accumulator, the direction of current i t cannot change because regeneration towards the internal combustion engine is notpossible. Reusing the regenerated energy is limited to the amount that the interim storage can store. This energy can be reused during acceleration.

Main characteristics of serial hybrid-electric systems are:

  1. Advantage is that its structure is simple, drive is purely electric.

  2. Energy regenerating brake can be realized depending on the size of the electric energy storage.

  3. Wheel hub motors can be used (multi motor drive).

  4. Optimal operation of internal combustion engine can be realized independently of the operating state of the vehicle.

  5. Disadvantage is that vehicle power must be installed more times. Taking into account the efficiencies, internal combustion engine, the ISG generator and the EM electric machine must be designed to total vehicle power. The design rating of the electric energy storage depends on the vehicle type, it can be in the 0-100% power range.

Parallel hybrid-electric cars

The main characteristic of parallel hybrid-electric vehicles is that traction drive is an internal combustion engine, similarly to traditional vehicles. The additional electric drive assists in torque development, usually. Drives can be separated in some cases. There are three types of parallel hybrid solutions:

  1. Selection of drive is realized with a mechanical switch and several clutches in traditional parallel hybrid vehicles.

  2. In simplified hybrid vehicle, selection is realized with controlling an electric machine installed to the driven shaft.

  3. Controller can select the driving motors in two-shaft parallel hybrid vehicles.

Traditional parallel hybrid vehicles

Traditional parallel hybrid vehicle is realized so that both internal combustion engine and electric motor can be connected mechanically to the wheels of the vehicle. Different operating modes can be selected with three clutches. Gear-box must remain because of the internal combustion engine. The structure of a traditional parallel hybrid type vehicle can be seen in Figure 8.24.

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Figure 8-24. Traditional parallel hybrid type vehicle.

Drive realized with EM electric machine is fed by a battery and can serve different functions:

  1. It can act as autonomous electric drive with CL0 and CL 2 clutches. This no-polluting and high efficiency mode can be used in urban traffic, as long as the battery discharges completely.

  2. Starter function can be realized with clutches CL 2 and CL 1.

  3. If the internal combustion engine is already working, then drive is realized with the internal combustion engine and all three clutches work. Electric machine charges the battery or helps accelerating the vehicle, if required.

Inverter feeding connected to the electric machine has to be able to operate in both power flow directions, as described above, and we have to control it according to the selected mode.

Electric drive can take part during problematic operation modes of the internal combustion, at starting or accelerating the car when high dynamic stress arises. Regenerating brake mode is also possible. Electronic control system harmonizes optimal internal combustion and electronic drive modes and continuous transition between the modes.

Main characteristics of traditional parallel hybrid-electric system are:

  1. Power of electric drive can be much lower than the power of the internal combustion engine. Electric drive must be designed for required power of the urban traffic only, starting and acceleration is short and electric motor can be overloaded for this short time.

  2. Power of internal combustion engine can be designed to highway traction, which is more economical, if acceleration torque is provided by the electric motor.

  3. Disadvantage is that mechanical system is complex.

Simplified parallel hybrid vehicles

Simplified parallel hybrid vehicle is different than traditional internal combustion vehicles because it contains higher power, electric machine functioning as integrated ISG (integrated starter-generator) and higher power high-voltage accumulator. Gear-box remains. The degree of hybridization can be measured with nominal power of electric machine and internal combustion engine:

γ=P el /(P IC +P el ).

Based on this value, we can distinguish minimal, mild, middle hybrid vehicles. If γ is higher, electric motor can provide more additional power for acceleration and can take more regenerated energy. IMA, „Integrated Motor Assist” is used for ISG electric machine drive, because of its increased assistance significance. The structure of simplified parallel hybrid type vehicle can be seen in Figure 8.25.

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Figure 8-25. Simplified parallel hybrid type vehicle.

ISG electrical machine connects to the main shaft of the internal combustion engine with fix or no gears, it cannot be disconnected with a clutch so the vehicle cannot be driven with autonomous electric drive. The result of the simpler construction is that drive can be done only by the internal combustion engine. In contrast, additional torque with electric machine and brake energy regeneration can improve efficiency of the internal combustion engine and the whole vehicle significantly. Gear-shifting can be more economic and rapid with ISG electric machine and start-stop function in urban traffic can be realized more easily.

The best-known simplified parallel hybrid vehicle is the Honda Insight „mild-hybrid” car, but several other developments exist. Electric machine used in Honda Insight is integrated with the internal combustion engine, has the same shaft as the driving shaft, disk-shaped, multi-pole synchronous machine (BLDC brushless DC motor) with permanent magnet rotor. The sketch of the motor is shown in Figure 8.26.a.

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Figure 8-26. Honda Insight hybrid-electric car a.) electric machine, b.) data of the driven haft .

In Figure 8.26.b., measurements of NREL laboratory in 2001 can be seen. This figure indicates how the torque and power data of the 50kW internal combustion engine IC of the examined vehicle can be improved with a 10kW ISG integrated machine. The time period of torque boost is limited by the battery, which has 6,5Ah capacity and 144V voltage in this vehicle.

Field oriented controlled induction machine drive can also be used for ISG integrated machine.

The main characteristics of the simplified parallel hybrid vehicles are:

Two-shaft, parallel hybrid vehicles divided to front and rear axle drives

In two-shaft parallel hybrid vehicles divided to front and rear axle drives parallel operation of internal combustion engine and electric motor drive is realized on different shafts of the vehicle. In contrast with traditional parallel hybrid-electric vehicles, not torques but traction forces on wheels are summarized.

The structure of such a drive can be seen in Figure 8.27. One shaft is driven by internal combustion engine IC with electric machine EM1, similar to Figure 8.25., and the other is driven purely by the electric motor EM2. The two electric drives are connected to the same battery.

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Figure 8-27. Two-shaft hybrid-electric vehicle.

Disadvantages of simple parallel drive can be eliminated with this solution and purely electric drive can be realized on one shaft. Gear-box and clutch must remain in the internal combustion drive.

Intelligent hybrid-electric vehicles

Advantages of serial and parallel type drives are combined in intelligent hybrid-electric vehicles (Full-hybrids). Vehicle can be driven with purely electric, purely internal combustion or combined modes. Continuous cooperation of the two drives are set that the operating point and rotational speed of the internal combustion engine be optimal relating to fuel consumption, operation modes with high consumption and low efficiency be eliminated.

Main parts of the intelligent hybrid-electric vehicle system are:

  1. internal combustion engine,

  2. two (or more) electric machine with power comparable to the internal combustion engine,

  3. interim electric energy storage (battery, ultracapacitor),

  4. smooth, changeable rotational speed transmission controlled electrically between the main shaft of the inrenal combustion engine and the driving shaft of the vehicle.

Main characteristics and control aspects of intelligent hybrid-electric vehicles are:

The role of the electric energy storage is to provide energy and take energy for transient loads. These roles are, that has to be taken into account during design, are:

Intelligent hybrid-electric vehicles use different technics to optimize rotational speed of internal combustion engine. Based on this, three types of intelligent hybrid-electric vehicles can be distinguished:

Intelligent hybrid-electric vehicle with planetary gear mechanical drive

The structure of intelligent hybrid-electric vehicle planetary gear mechanical drive can be seen in Figure 8.28.

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Figure 8-28. Scheme of a planetary gear hybrid-electric vehicle and the wheel itself.

Main shaft of the internal combustion IC engine is connected mechanically to the drive consisting of three or four planetary gears. Electric machine ISG, which function is generator and to start and set operating point, is connected to the inner wheel called sun-wheel. Electric motor EM is connected to the outer, so-called ring-wheel of the drive. The electric motor is also connected to the wheels of the vehicle with fixed gear-wheels. So, rotational speed ω EM of electric motor is proportional to the speed of the vehicle. Electric motor is part of the hybrid drive system, but purely electric drive is also possible with it.

Continuous variable transmission (CVT) can be realized with planetary gears between the rotational speed of internal combustion ω IC and driving shaft ω EM. This provides that internal combustion engine can provide the traction power at optimal rotational speed, where efficiency (relative fuel consumption) is the best. Control of variable transmission, i.e. ratio of ω IC and ω EM, together with setting optimal rotational speed, can be reached with controlling rotational speed ω ISG of electric machine ISG. The equation between rotational speed of sun i and ring o wheels of the planetary gears wheels is:

8-3

where Zo/Zi is the teeth ratio of outer (ring) and inner (sun) wheels. From this equitation we can calculate the rotational speed of the internal combustion engine:

8-4

From this, we get that gear ratio r=ω IC EM can be changed electrically with the ω ISG rotational speed of the generator:

8‑5

One of the realizations of planetary gear drive in Toyota Lexus hybrid-electric car can be seen in Figure 8.29. Change regarding to Figure 8.28 is that machine EM is not connected to the ring wheel directly but through another planetary gear drive, where wheels are standing. Also, we can see that there is a wheel reductor between the ring wheel and the wheels of the vehicle with a fix r fix ratio.

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Figure 8-29. Planetary gear drive used in Toyota Lexus hybrid-electric car.

Utilization of power from internal combustion engine can be optimized electrically with a planetary gear. Power appearing on the shaft of the internal combustion engine can be separated into two components:

8-6

The first part of (8.6) is mechanical power P M transferred to the wheels of the vehicle:

8-7

If the vehicle stops ω EM =0, then P M=0.

The second part of (8.6) is power P ISG transmitted to the shaft of ISG machine:

8‑8

Main function of power P ISG is to supply main electric circuit in generator mode. Electric motor drive EM is fed from the main circuit, supply for auxiliary devices and charge for additional energy storage (battery or ultracapacitor) is connected here. A different operation mode is the starting of the internal combustion engine when machine ISG has to work as starting motor with consumed electric power P ISG.

Torque required for traction can also be controlled electrically with planetary gear drive. In this case torque on drive shaft is the sum of torques of internal combustion engine IC and electric machine EM:

8‑9

Assuming that wheel drive efficiency is ideal 100%, component M M is a torque going through the wheel drive and proportional to the torque of the internal combustion engine M IC, appearing in equation (8.7). Torque M EM is produced by electric machine EM, its value can be controlled electrically, and can be in both directions (additional acceleration or brake). The limits of M EM vs. rotational speed ω EM are determined by the short-time and steady-state load limit curves of the selected controlled electric drive. In motor mode M EM >0, and this creates additional torque on the drive shaft in the same direction as torque M M, according to Figure 8.30.

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Figure 8-30. Block diagram for taction torque development.

Resulting torque M creates traction force. Dynamic behavior (acceleration) of the vehicle is determined by the inertia Θred reduced to the shaft of the electric machine and by the traction resistance of the vehicle.

Output power of electric machine EM is P EM = ω EM M EM . There can be operation mode when P EM >P ISG, so power taken from the main electric circuit is higher than supplied. The difference of the power is supplied by the battery. This mode can exist only for a limited time until the battery discharges completely.

Brake operation mode of hybrid-electric vehicles

With respect to the internal combustion engine, brake mode means reduced fuel feeding or stop similarly to no-load running, according to the motor control. Internal combustion engine provides the traditional engine brake torque in brake mode which comes from the fuel compression and mechanical friction and cannot be controlled. In engine brake M IC ≤0, so is M M≤0.

Well-controlled energy regeneration brake can be realized with the generator mode of electric machine EM, which corresponds to M EM <0 on the output shaft, decelerating the wheels. Direction of the current i in the main circuit (Figure 8.28) changes because of the regenerating brake. This mode can last only for a certain period of time until charging of battery is allowed without the risk of overcharge.

Only those wheels can be decelerated with regenerating brake which are in mechanical connection with the electric drive EM, so only the (partially) electrically driven wheels. Regenerating electric brake is always extended with traditional electrohydraulic brake system for the sake of security, where controlled ABS (anti-blocking system) can be also realized. Wheels not driven by electric drive can be decelerated only with friction and with loss.

Control diagram of intelligent hybrid-electric vehicle can be seen in Figure 8.31.

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Figure 8-31. Control diagram of intelligent hybrid-electric vehicle.

The signal of accelerator pedal AP starts three processes. On one hand, it provides reference signal to feed control of internal combustion engine, and sets torque reference signal to electric machine EM on the other. And, in the same time, it initializes a control process to set optimal operational rotational speed ω IC with machine ISG. Optimal operation point can be calculated with shorter, partially steady-state operations. Signal of brake pedal BP sets reference brake torque of electric drive EM. When pressing brake pedal powerfully, electric brake switches to brake operation provided by traditional electrodynamic brake system, continuously and without steps.

Another control task, not indicated in Figure 8.31., is to control the current (power) i t of main electric circuit supplied by machine ISG, to control the charge of the DC-link energy storing battery or ultracapacitor. As an example, the SoC state of the battery in a Toyota Prius hybrid car is set to about 55% of full charge from different starting points, as indicated in Figure 8.32. (purple, red, blue lines). This measurement was taken in NREL Laboratory. With this control, we can provide enough energy for additional acceleration and prevent overcharge during brake.

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Figure 8-32. Control of charge for the battery in a Toyota-Prius hybrid car

The state of the battery was examined with UDDS standard acceleration-deceleration cycles.

To realize the operation modes described above, complex control of electric drives is required. Only electric drives with intelligent rotational speed and torque control can be used. The best solutions for this purpose are inverter-fed field-oriented current vector controlled synchronous or induction motor drives.

Intelligent hybrid-electric vehicle with Strigear drive

Intelligent hybrid-electric vehicle with Strigear drive was developed with the idea of Stridberg Powertrain AB. An internal combustion engine and two electric machines are connected to a special “Strigear” propulsion unit which enables both serial and parallel hybrid operation with a traditional gear-box. The scheme of the driving system and the picture of the whole propulsion unit, with electric machines ISG and EM on the two sides, can be seen in Figure 8.33.

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Figure 8-33. Structure of a Strigear drive hybrid-electric vehicle and the picture of the drive gear.

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Electric machine ISG is on the same shaft as the internal combustion unit and electric machine EM connects to the output shaft through a gear-box. Disadvantage of Strigear system is that gear-box used in traditional vehicles remains, and clutch CL also remains, which is placed in between the two electric machines. Advantages of serial and parallel connected hybrid drives can be utilized with Strigear system. When selecting different operation modes, the state of clutch is important.

Operation modes available when clutch CL is open are:

  1. Operation modes available when clutch CL is closed are:

Traditional gear-box is required for the last two modes.

Intelligent hybrid-electric vehicle realized with double rotor electric machine

Intelligent hybrid-electric vehicle realized with double rotor electric machine is a new development of Swedish Royal Institute of Technology, KTH/EME. Its operation is similar to planetary gear driven hybrid vehicle but planetary gear is replaced with a double rotor, double fed electric motor. Figure 8.34 shows this driving system. On the right side, the picture of an experimental model can be seen.

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Figure 8-34. Structure of a hybrid-electric vehicled realized with dual-rotor electric machine.

There is electric transmission with controlled torque and rotational speed between the output (driving) shaft and the shaft of the internal combustion engine IC. Electric transmission is realized with a 4-quadrant transmission 4QT unit which is indicated with dashed lines in the figure. This unit is an electric machine consisting of two rotors and one stator.

The inner rotor (in red) has the same shaft as the internal combustion engine and is 3-phase wounded. Windings are fed through slip rings by the inverter INV-1. The outer rotor (in blue) is connected mechanically to the driving shaft of the wheels directly or through a fixed mechanical gear. The outer rotor has multi-pole magnets, and it has specially arranged magnet on the inner rotor side and on the stator side also, as shown on the right side of Figure 8.34. The stator of the machine (in green) also has three-phase windings and is fed through the inverter INV-2. Both inverters are connected to the common battery storage in the DC-link.

The functions available with the 4QT double rotor machine can be understood more easily if we divide it to two machines.

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Figure 8-35. Deviding 4QT double-rotor electric machine to two separated machines.

4QT can be described as two machines, as can be seen in Figure 8.35, and it can be constructed with two electric machines with permanent magnet rotors where there are magnets only one-one side of the rotors. (Magnets are on the inner part of the rotor on the left and outer part of the rotor on the right.) 4QT is implemented by current vector controlled synchronous drive for both machines, so the position of the current vector is synchronized electrically to the position of the permanent magnet rotor.

On the inner rotor (in red), a controlled rotating field with rotational speed ±∆ω can be produced with current vector control by the inverter INV-1. This rotational speed ∆ω is added to or subtracted from the rotational speed of the internal combustion engine. Because it is a synchronous machine ωoutIC±∆ω determines the rotational speed of the outer (blue) rotor and the resulting driving rotational speed of the vehicle. With double rotor machine, like with planetary gears, continuous changeable gear can be realized between the rotational speed of the internal combustion engine and that of the driving shaft.

Additional electric torque ±∆M can be realized with current vector control in the 3-phase winding of the stator (in green) so the torque of the output shaft can be controlled electrically. Additional acceleration can be realized with positive torque ∆M, and brake and energy regeneration to the battery can be realized with negative torque. Torque on the driving shaft of the vehicle is the sum (with correct signs) of the torques of the internal combustion engine and electric machine, Mout=MIC±∆M. Power required for additional acceleration ±∆Mωout is provided by inverter INV-2.

Every operating modes can be realized, like for the planetary gear solution. Internal combustion engine can provide power at optimal rotational speed. 4QT can be considered as an electric shaft between the internal combustion engine and driving shaft. Regenerating brake is also possible. Controlled starting of the internal combustion engine can be solved by current vector torque controlled inverter supply of the inner (red) rotor,  for both standing and moving vehicles. Also, energy supply for the DC-link circuit can be realized with this method if it is in generator mode.

Dynamic behavior of the vehicle, like acceleration and deceleration, can be improved with 4QT. Purely electric drive is also available. State of the DC-link circuit can also be monitored easily.

Disadvantage of the solution is that slip rings are required for connecting to the inner wounded rotor. Slip ring-brush system decreases reliability of the device, requires maintenance, wears and  sensible to dust.

According to Royal Institute of Technology, designation 4QT indicates that double rotor electric machine has to operate in four quadrants, it has to work with reversed rotational speed and torque.

(References used in this chapter:  [45]…[57])