4th-Generation Toyota Prius Teardown (Part 1)
Powertrain units miniaturized and lightened to achieve 40km/liter fuel economy
In January 2016, the Hiroshima Industrial Promotion Organization performed a teardown analysis of the all-new Toyota Prius (launched in December 2015). The Prius has been reborn as a completely new car through significant changes such as the adoption of a newly-designed hybrid transaxle, and the use of the new TNGA (Toyota New Global Architecture) platform, all of which was done with the aim of achieving a JC08 mode fuel economy of 40.8km/liter (E grade) and improving the driving performance. (The analysis was performed on the A grade Prius, which has a fuel economy of 37.2km/liter.)
Part 1 of the teardown report will focus on the powertrain units and other technologies that contribute to increasing the fuel economy. Part 2 will look at the TNGA, and the technical innovations that changed the Prius from what was merely a fuel-efficient car, to a fuel-efficient and power-efficient car.
Previous teardown reports:
Powertrain units improved to achieve a JC08 mode fuel economy of 40km/liter
Fuel economy improving effects of powertrain units (Source: Toyota)
The 4th-generation Prius has had collective updates to its fuel efficient technologies, including a redesigned engine; high-optimization and miniaturization of its battery, power control unit (PCU), and motor; and an evolved hybrid system. As shown in the chart above, the vehicle has been revised entirely and its various units improved for higher efficiency and performance, and because of this the new Prius has achieved a JC08 mode fuel efficiency of 40.8km/liter (E grade).
High performance engine with up to 40% thermal efficiency
|Comparison of maximum thermal efficiency of engines (Source: Toyota)|
The 4th-generation Prius is fitted with a 2ZR-FXE 1.8-liter four-cylinder engine, which is the same basic structure as the 3rd-generation Prius, but the new model has achieved a maximum thermal efficiency of 40% through the addition of various improvements.
The exhaust system has a rear-exhaust layout, which is also the same as the previous model. Some of Toyota's small cars have front exhaust systems, but all of the recent FF (front engine front drive) Toyota cars are being standardized to a rear exhaust system layout. A rear exhaust layout is more advantageous because the distance from the engine to the catalyst is shorter, and the exhaust gas temperature is kept at an optimal level that doesn't lower too much. On a separate note, this contrasts with recent Honda vehicles, which have front exhaust, possibly due to layout restrictions.
Stronger airflow in combustion chambers
|The re-designed intake port shape (Source: Toyota)|
The first improvement was changing the shape of the intake port based on an analysis of the "tumble flow," which is the vertical swirl of air-fuel mixture entering the combustion chamber. As illustrated in the image on the right, (1) the airflow was made straight and, (2) the inverse tumble component that blocks the tumble flow was reduced. As a result, the tumble ratio was improved from 0.8 to 2.8, the combustion velocity increased, and induction of a large volume of EGR (exhaust gas recirculation) gas became possible.
The shapes of the combustion chamber and the piston-top surfaces account for the tumble flow described above.
|The shape of the combustion chambers||The shape of the piston-top surfaces|
Large-volume cooled EGR
The EGR distribution passage in the intake manifold has been restructured to a tournament design. This allows a larger amount of EGR gas to enter evenly into each cylinder.
|The intake manifold cross-section (Source: Toyota)||The aluminum pipe at the left connects to the EGR passage|
|An external view of the intake manifold||The throttle valve (center)|
The intake manifold that directs air into the intake port is shaped so that it branches into two passages from the throttle valve, and supplies intake air into the respective ports after converging once at the collector located immediately before the cylinder head. This design ensures a smooth flow of intake air by increasing the cross-sectional area that the incoming air passes through, which minimizes air resistance and interference between cylinders.
|The EGR cooler configured at the back of the engine||The EGR cooler unit|
Adoption of a two-way cooling system
Improving the engine's thermal efficiency is key to raising fuel economy, but this leads to a dilemma where the performance of the heater declines because the heat supplied to it is reduced. The 4th-generation Prius incorporates innovations that maintain sufficient heater performance while increasing the thermal efficiency.
In conventional engines coolant flows through two courses; one to the engine itself and the other to the exhaust heat recovery and heater units. However, the new Prius is fitted with a flow shut-down valve that controls this coolant flow. While previous Prius models coolant did not flow to the heater during warming, heater performance during warm up is improved in the new model by allowing coolant to flow to it as needed.
The new Prius is also fitted with a grille shutter in front of the radiator, which is opened and closed automatically in accordance with driving and warm-up conditions. The grille shutter closes while the coolant is still cold. This reduces the amount of air going through the radiator, which accelerates warm up and reduces heat loss. Moreover, the updated exhaust heat recovery system recovers heat from the exhaust system, which also improves warm-up performance and minimizes heat loss. In this way numerous means are used to reduce heat loss and increase the overall thermal efficiency.
Two-way cooling system (Source: Toyota)
A newly-designed water jacket spacer optimizes cylinder bore wall temperature
|The structure of the water jacket spacer (Source: Toyota)|
In order to achieve optimized cooling by means of the cylinder bore region, a water jacket spacer consisting of stainless steel and foam rubber has been adopted. This involves a foam rubber with a brand name of EXPAD being placed in the coolant passage at the bottom of the cylinder bore. This allows a large amount of coolant to flow over the upper part of the cylinder bore, which becomes very hot, and ameliorates knocking by raising the cooling effect. At the same time, allowing a smaller amount of coolant to flow to the lower half portion of the bore to maintain a high temperature. With these techniques, fuel efficiency is achieved by making the expansion factors of the upper and lower cylinder bore even, and piston action friction is standardized at a low level.
|The water jacket spacer that is inserted in the cylinder block|
New structure for the hybrid transaxle (loss reduced by about 20%, length shortened by 47mm)
The most significant change in the 4th-generation Prius is the hybrid transaxle. Models until the 3rd-generation Prius used hybrid transaxles with two planetary gears, but in the new Prius, the reduction gear mechanism has been changed to parallel gears, which improves the transmission efficiency. The new design has achieved a reduction in mechanical loss of approximately 20%.
Moreover, the unit had a long length in the previous structure because the drive motor, charging generator, and two planetary gears were arranged in series coaxially. However, this has been shortened in the new Prius by changing to a dual axle structure, with the drive motor and reduction gear fitted separately on parallel shafts, which reduces the unit length by 47mm from 409mm to 362mm. This change makes it possible to layout the engine room framework in an efficient cross-sectional shape.
|The structure of the hybrid transaxle (Source: Toyota)|
|An external view of the hybrid transaxle connected to the engine||The hybrid transaxle seen from the engine connection side|
|Motor (left) and generator (right) housing||The motor has been miniaturized and made to have output density (Source: Toyota)|
|Generator (left) and motor (right)||The planetary gear (upper right) and differential gear (bottom) on the opposite side of the motor housing|
|The power split device planetary gear||The parking lock mechanism that locks the outer circumference of the planetary gear|
Miniaturized power control unit
An additional loss reduction of around 20% has also been achieved in the power control unit (PCU) through the use of low-loss components and other improvements. The unit size has been reduced 33% from 12.6L in the previous model to 8.2L. This created extra space in the engine room and allowed the auxiliary battery to be shifted there from the luggage space, where it had been placed previously.
|Internal structure of the PCU (Source: Toyota)||External view of the PCU after removal|
|Multi-layered circuitry in the PCU||Under the multi-layered circuitry|
|The 3rd-generation Toyota Prius The auxiliary battery was stored in the luggage compartment||The 4th-generation Toyota Prius The auxiliary battery is stored in the engine compartment|
Compact, high-performance lithium-ion drive battery pack
|Lithium-ion (left) and nickel metal hydride (right) variants are available for the high-voltage battery (Source: Toyota)|
Two types of drive battery packs (lithium-ion, nickel metal hydride) are available for the 4th-generation Prius, and this time the teardown was done with a Prius that used a lithium-ion drive battery. The battery is manufactured by Panasonic, and has a capacity of 3.6Ah, which is a lower specification than the 6.5Ah nickel metal hydride battery, but it is estimated that the actual charging from the generator and energy supply to the drive motor may be done at the same level. The lithium-ion battery has been significantly miniaturized to 30.5L from the 39.4L nickel metal hydride battery in previous models. The new nickel metal hydride battery has also been miniaturized to 35.5 liters, and while it was located behind the rear seats in the previous models, both battery types are now stored snugly under the rear seat cushion in the 4th-generation Prius. This results in increased luggage room for ease of use. The battery service plug for shutting off the high voltage circuit is located behind the finisher under the rear seat so that it is easy to remove when the vehicle is being serviced.
|External view of the high-voltage battery pack||Internal structure of the battery The service plug is colored orange The high-voltage circuit is shut off when it is removed.|
|The high-voltage battery stored under the rear seats||The high-voltage battery is stored under the rear seats to create more space for luggage|
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