Case Study of Toyota
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Transcript of Case Study of Toyota
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2. DIFFERENT TYPES OF HYBRID SYSTEM
2.1. SERIES SYSTEM:
The system supplements electricity generated by the engine (Fig.1.1). It is most
commonly used as a range extenderfor electric vehicles. Since the engine is not
mechanically connected to the drive wheels, this system has an advantage of controlling
the engine independently of the driving conditions. Accordingly the engine is used in its
optimum efficiency and low emission range. This system is particularly suited to engines,
which are hard to mechanically connect to the wheels such as gas turbine engines.
However, include large energy conversion losses because of the necessity of full
electricity conversion of the engine output. Further, a generator large enough to convert
the maximum engine output is required.
2.2. PARALLEL SYSTEM:
With the parallel system, an electric motor that supplements the engine torque is
added to the conventional driveline system of the engine and transmission (Fig.1.2).
Accordingly, operations of the engine are quit similar to those of an engine in normal
vehicle. This system requires no generator, and there is a mechanical connection between
the engine and the drive wheels, providing an advantage of less energy being lost through
conversion to electricity.
On the other hand, this system requires a transmission because no speed
adjustment mechanism is installed, though the motor supplements the torque. When an
automatic transmission is used, a torque converter, oil pump, and other auxiliary
components can reduce the transmission efficiency. Although the engine torque can be
controlled by the motor, the engine speed is determined by gear ratios like a conventional
vehicle. Accordingly the engine operation is linked to the driving conditions.
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2.3. SERIES-PARALLEL COMBINED SYSTEM:
This combined type, having a generator and a motor, features characteristics of
both the series and parallel system, and the following two systems are possible
2.3.1. SWITCHING SYSTEM:
Engagement & disengagement the clutch switches between the series or parallel
systems (Fig.1.3). For driving as by the series system, the clutch is released, separating
the engine and the generator from the driving wheels. For driving with the parallel
system, the clutch is engaged, connecting the engine with the driving wheels.
For example, the city driving requires low loads for driving and low emissions;
the series system is selected with the clutch released. For high speed driving where the
series system would not work efficiently due to higher drive loads and consequently
higher engine output is required, the parallel system is selected with the clutch applied.
2.3.2. SPLIT SYSTEM:
This system acts as the series and parallel systems at all times (Fig.1.4). The engine
output energy is split by the planetary gear into the series path (from the engine to the
generator) and the parallel path (from the engine to the driving wheels). It can control the
engine speed under variable control of the series path by the generator while maintaining
the mechanical connection of the engine and the driving wheels through the parallel path.
2.4 DUEL SYSTEM ADVANTAGES :
Free control of engine while keeping a mechanical connection between the engine
and drive wheels.
Compact design of transaxle integrating two motors requires little modification
for current production vehicle.
Use of generator as motor and its combination with traction motor permits the
engine and driveline to flexibility adapt to driving condition.
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Fig.2.1 Series System
Fig. 2.2 Parallel System
Fig. 2.3 Switching System
Fig. 2.4 Split System
( ___ ) Mechanical Connection, EG: Engine,
(- - - -) Electrical connection, C: Clutch,
G: Generator, PG: Planetary gear,
TM: Transmission, M: Motor, B: Battery.
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3. COMBINATIONS OF MAIN AND AUXILIARY HYBRID DRIVE
TECHNOLOGIES
Fig.3 shows that there are many possible combinations of main and auxiliarydrive, some being more practical than others. The more probable combinations are
marked A with the possible but unlikely ones marked B. If we consider only the probable
combinations it can be seen that there are 11 of these. The number of options is further
increased since those having a heat engine as the main drive can be operated in either the
series or parallel configuration. This adds three further options and makes a total of 14.
Two combinations are discussed here. First with flywheel and then hydraulic accumulator
(Fig.3.1 &Fig. 3.2)
Both flywheel and hydraulic accumulator are capable of supplying, or absorbing
during regeneration, more than 500 W/kg during acceleration or braking and typically of
storing up to 0.5 kWh of energy. Turnaround energy efficiency of these mechanically
storage devices is high about 98% compared to 75-80% for batteries, and as a result the
energy recovered during braking can be as high as 15% of the total energy used.
However problem exists of providing protection from disintegration of the flywheel in an
accident, and this together with the possible requirement for two contra-rotating
flywheels to overcome gyroscopic effects makes the flywheel a potentially expensive
solution. It is perhaps more suited to high rotational speed operation.
The hydraulic accumulator requires a pressure vessel in which a highly
deformable membrane separates high-pressure oil pumped into it by the pump/ hydraulic
motor from a compressible gas. This also requires protection to avoid any risk of failure
in crash conditions. It is, however, potentially a cheaper solution for auxiliary energy
storage in a hybrid of this type than the flywheel and has no vehicle stability problems.
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Fig.3 Combinations of main and auxiliary hybrid drive technologies.
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Fig.3.1 Regeneration and energy storage using a flywheel
Fig. 3.2 Regeneration and energy storage using a hydraulic accumulator
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4. THE TOYOTA PRIUS HYBRID CAR
The five-seater Toyota Prius hybrid operates with more equal sharing of the
power between the gasoline heat engine and the electric motor (Fig.4a & 4b). The battery
in the Prius is charged by regenerative braking, and when necessarily, directly from
gasoline engine power. An interesting thing of this car is that under light load conditions
such as initial acceleration, the Prius is operated solely on electric power from the
temporary battery storage. Depending on how fast you're accelerating and the battery's
state of charge, the Prius's gasoline engine will start when speed reaches between about
21 and 40 kmph. By waiting until this point to start the gasoline engine, this means that
the Prius doesn't operate the gasoline engine under very light power demands, when the
gasoline engine is less efficient. (At zero power demand, such as descending a hill,
braking or sitting at a stop, car can entirely stop operation of the gasoline engine.)
The Prius uses two motor/generators, which split the jobs done by one
motor/generator. The motor/generator "M" is connected to the wheels (via differential
and reduction gear), and is used for:
Providing propulsion to the wheels.
Charging the battery from the wheels during regenerative braking.
The Prius uses a planetary gear as a power-split device that provides a three-wayconnection between the wheels (and motor/generator "M"), the gasoline engine, and
generator/motor "G". Together, this system also forms the Prius's continuously variable
automatic transmission. The generator/motor "G" is used for:
Charging the battery from the gasoline engine.
Starting/stopping the gasoline engine.
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Fig. 4a Power train layout of the Toyota Prius
Fig.4b General layout of the system
4.1 MAIN COMPONENTS OF TOYOTA PRIUS:
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Atkinson cycle engine :
The Priuss 1.5 liter Atkinson cycle engine (Fig.4.1.1) provides an increased
expansion ratio for more efficient energy extraction. Variable intake valve timing reduces
cylinder pressure, to eliminate knocking. Engine speed is limited to 4,500 rpm, allowing
engineers to use smaller, lighter components for improved overall fuel economy.
Nickel-metal hybrid battery :
The sealed nickel-metal hydride (Ni-MH) battery pack (Fig.4.1.2), mounted
behind the rear seats, and provides a total voltage rating of 273.6V. Its temperature is
maintained by a cooling fan that draws air in from vents mounted over the rear parcel
shelf.
Inverter :
The inverter (Fig.4.1.3) converts the battery's DC current to AC for the electric
motor/generators, and vice versa. An intelligent power module provides precise current
and voltage control. A built-in transformer converts some of the hybrid battery's power
into 12V power for vehicle accessory operation.
Hybrid Transaxle (Power split device) :
In the Prius, a "planetary gear" is used as a power split device (Fig.4.1.4),
providing a three-way connection between the wheels (and by extension motor/generator
"M"), the gasoline engine, and generator/motor "G". The easiest way of thinking of the
planetary gear is that the rotation of the wheels is always equal to the sum of the gasoline
engine rotation and the rotation of generator/motor "G".
This means that the gasoline engine may be stationary, with any rotation of the
wheels being directed towards rotation of generator/motor "G". It also means that if the
gasoline engine is turning at a fixed speed, the faster the car is moving, the slower
generator/motor "G" will turn.
In a typical driving situation, the output from the Prius's gasoline engine is split
between the wheels and generator/motor "G". If the batteries are sufficiently charged, all
energy coming from generator/motor "G" will also be routed to the wheels, by using it to
power motor/generator "M". This means that power is taking two separate paths from the
gasoline engine to the wheels, one entirely mechanical, and the other partially electrical.
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Fig.4.1.1 Atkinson cycle engine Fig.4.1.2 Nickel-metal hybrid battery
Fig.4.1.3 Inverter
Fig.4.1.4 Hybrid Transaxle (Power split device)
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Fig. 4.2.1:Energy flow when starting up Fig. 4.2.2:Energy flow when
accelerating
Fig. 4.2.3:Energy flow when cruising Fig. 4.2.4: Energy flow when
regenerative braking
4.3 ENGINE PERFORMANCE :
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Fig.4.3 shows the modeled equivalent fuel economy for the Prius under the
various test cycle. It shows that the Prius achieves the best fuel economy (63 MPG) under
the Japanese 10/15 cycle. Its worst economy is about 35 MPG under the NYCC. Since
the Prius has an electrical CVT and Atkinson cycle engine, its engine peak efficiency,
transmission efficiency, and overall vehicle efficiency are all significantly higher than
that of CVs.
4.3.1 PERFORMANCE FEATURES THAT TOYATA HYBRID SYSTEM GIVES
TO PRIUS:
Double the fuel economy and half the CO2 emissions of a gasoline engine.
CO, HC, and NOx reduced well below Euro4 and California SULEV levels.
Seamless integration of power sources for smooth, powerful acceleration and
response.
Convenience equal or better than that of a CV. Just top it up with gas-no need
to worry about charging the batteries.
Reduced CO2 emissions -The engine shuts off automatically when the car
comes to stop (Fig4.3.1).
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Fig. 4.3 Fuel economy for Toyota Prius under Driving Cycles
Fuel consumption
(L/100 km, EC mode)
CO2
gm/km
Corolla (1.6L) 8 210
1997 Prius 6 140
2000 Prius 5 120
Fig. 4.3.1 CO2 Emissions
Fig.4.4 Cut-way model of engine
4.4 THE TOYOTA HYBRID SYSTEM (THS) ENGINE SPECIFICATIONS:
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Key Data
Project type Special Purpose Vehicle
Engine Size 1.5 litre 16 Valve 4 cylinder
Fuel Consumption 29 km/litre
Transmission ECVT
Power 70bhp at 4500 rpm
Torque 112 N-m at 4200rpm
Bore/Stroke 75.0/84.7 (mm)
Suspension - Front/Rear Independent MacPherson strut with stabilizer bar/ Torsionbeam with stabiliser bar
Steering Rack and Pinion with electro-hydraulic power-assist
Brakes Power-assisted ventilated front discs and rear drums with
ABSLaunch Date 2001
Key Players
Sponsor Toyota
Contractors MacPherson
Key Specifications
Length 4308(mm)
Width 1695(mm)
Height 1463(mm)
Wheelbase 2550(mm)Weight 1255(kgs)
Capacity 44.6 (lit.)
Tires P175/65R14
Electric Motor/Generator/Power Storage:
Motor type: Permanent magnet
Power output: 33 kW/44 hp @ 1,040 - 5,600 rpm
Torque: 350 N-m @ 400 rpm
Battery type: Sealed Nickel-Metal Hydride (Ni-MH)
Output: 273.6 V (228 Nos1.2-V cells)
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5. ADVANTAGES & DISADVANTAGES OF HYBRID VEHICLE
TECHNOLOGY
5.1 ADVANTAGES:
5.1.1REGENERATIVE BRAKING:
It is particularly valuable in the city where one continually slowing down and
speeding up again. Normally, each time you slow down by applying brakes, a lot of
energy is lost. Regenerative braking takes advantage of the fact that an electric motor can
also operate as generator. During regenerative braking, the electric motor operates as a
generator, slowing the vehicle down and turning some of the energy of forward motion
back into electricity that recharges the batteries. This energy that would otherwise be
wasted can now later be used to help propel the car.
5.1.2FUEL ECONOMY:
Improved fuel economy due to following:
Operation of the engine in optimum efficiency range.
Transmission efficiency between the engine and the driving wheels is improved.
Regeneration of deceleration power.
5.1.3 SMALLER ENGINE SIZE:
Smaller engines are more advantageous than bigger engine for the following
reasons:
The big engine is heavier than the small engine so the car uses extra energy every
time it accelerates or drives up the wheel.
The piston and other interior components are heavier, requiring more energy each
time they go up and down in the cylinder.
Bigger engines usually have more cylinders and each cylinder uses fuel every
time the engine fires, even if the car isnt moving.
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5.2 DISADVANTAGES:
The major inhibiting factor in making hybrid vehicles acceptable to the consumer
remains the high cost of a car with two separate propulsion systems. It is difficult to see
how this can be overcome in view of the actual production costs.
6 FUTURE DEVELOPMENT OF HYBRID VEHICLE
TECHNOLOGY
Hybrid electric technology is a leading technology for increasing vehicle fuel
economy, reducing greenhouse gas emission, and reducing criteria pollutant emissions
when equipped to have battery-only range. The mean values of responses by industry
experts to a 1998 survey appear to provide a realistic and technically consistent view of
the future HEVs. Forecast statistics were prepared based on Delphi Study.
The fuel cell is projected to be the most likely power plant for HEVs in 2020;
instead of hydrogen, however, such liquid fuels as gasoline and methanol will likely be
used. Projected HEV fuel economy ranges from 1.7 to 2.6 times the conventional vehicle
fuel economy. Thus, even with the fuel cell as its power plant, the HEV is not likely to
have a fuel economy three times that of conventional vehicle (a PNGV goal). The futures
HEVs are projected to emit significantly less NOx and particulate matter than CVs. Thestudy shows that the cost of a HEV will drop from 66% to 33% more than a $20,000 CV
by 2020. The typical respondent, however, characterized by the median and modal
statistics, expected the cost penalty to drop to 15% (median) or zero (mode) by 2020.
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7. CONCLUSION
From present study it can be concluded safely that Hybrid Electric Vehicle
(HEV) provides better fuel economy and offers advantages of smaller engine size along
with regeneration of deceleration power and less pollution. So fulfilling all the
requirements of a modern car this technology will surely prove itself as a platform for
development of Next-Gen cars.
Only limiting factor is the high cost in making hybrid vehicles acceptable.
However efforts are now being concentrated to reduce the cost.
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