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Powersplit or Parallel - Selecting the Right Hybrid Architecture.

INTRODUCTION

Drivers for Electrification

Global vehicle emission and fuel economy regulations are becoming more stringent than ever before. In the US, under the One National Plan, C[O.sub.2] emissions are forecasted to reduce by 5% annually. EU and China regulations require similar rates of reduction going forward. While conventional engine technology has made significant headway in improving emissions and fuel economy without compromising performance, going forward the need for electrification is becoming exceedingly apparent in order to meet future emission and fuel economy standards. Having an onboard secondary electrical power source enables engine downsizing, start/stop functionality, efficient engine operation, and regenerative braking that helps improve fuel economy and emissions.

While the emission and fuel economy standards are tightening, customers want a no-compromise solution. Increasing fuel economy at the expense of some other attribute, like performance or towing/payload, may not be an option. Furthermore, electrification can no more be confined to a certain class of vehicles. Traditionally, small and medium sized vehicles have been the beneficiaries of electrification. Going forward, other vehicle segments will need to be electrified.

Customer expectation varies considerably with the class of vehicles. Figure 1 shows the top purchase reasons for the CD car segment of vehicles while figure 2 shows the same information for full size pickup trucks. While Value for the Money and Gas Mileage are the top reasons for the CD car segment, Trailering/Towing is the top reason for the full size pick-up truck segment. Gas Mileage does not even figure in top 10 purchase reason. This vastly different customer want is driving the need for different electrification strategy and architecture.

The main types of electrified architectures currently in the market place are parallel and powersplits. Table 1 shows the major players for each type of architecture currently in the US market.

Parallel Architecture

Depending on the placement of the motor, there are many different implementation of parallel architecture. By industry convention, they are identified using the Pn nomenclature where n is a number corresponding to the placement of the motor in the system. The numbering convention is as follows -

P0 - motor located at the input to the engine

P1 - motor located at the output of the engine

P2 - motor located between the engine and transmission

P3 - motor located at the transmission output

P4 - motor mounted on axle (eRAD, eFAD)

In this paper, references to a parallel hybrid refer to the P2 parallel hybrid system. This implementation is the most prevalent in the industry today.

A single motor, parallel P2 hybrid architecture layout is shown in figure 3. It consists of conventional vehicle architecture - comprising of an engine connected to a step-ratio or dual clutch (DCT) - with an electrical motor and battery added in parallel. A disconnect clutch is used to disengage the engine.

The power flow shown in figure 4. The mechanical power from the engine along with the electrical power from the battery (via the motor) is delivered to the wheels. During EV operation, with the engine off, electrical power is delivered to the wheels via the motor. In this mode, the clutch is open. In blended mode, the clutch is closed with the engine and the battery, via the motor, providing power to the wheels. In this mode, the engine speed is governed by the wheel speed and transmission gear ratio. Since part of the power to the wheels is provided by the motor, the engine may be operated at a torque point for best efficiency. Parallel hybrids therefore operate with two degrees of freedom - one continuous (torque) and one discrete (speed). During regenerative braking, the motor operating as a generator, recovers energy and stores it in the battery.

Detailed power flows for each of the modes of operation are shown in the appendix.

Powersplit Architecture

While there are different implementations of the powersplit hybrid architecture, a simple, cost-effective layout using a planetary gear set is shown in figure 5. This is a single mode configuration that uses a single planetary gear set without any clutches or brakes. This paper only assesses this powersplit particular configuration. In this configuration, the sun gear of the planetary gear is connected to the generator, the carrier is connected to the engine with the ring gear is connected to the wheels. The power from the engine is transmitted partly to the wheels (mechanical path) and partly to the battery via the generator (electrical path).

The power flow diagram for a powersplit is shown in figure 6. During EV operation, the electrical power from the battery is delivered to the wheels via the motor. In blended mode, the engine power is "split" in the planetary into the mechanical and electrical paths. The amount of the "split" is optimized for maximum efficiency. Due to the nature of the planetary, the engine operating point can be varied in the speed and torque domains allowing powersplits to also have two degrees of freedom but with both being continuous. During regenerative braking, the motor operates like a generator to store power in the battery.

Detailed power flows for each of the modes of operation of a powersplit are shown in the appendix.

Powersplits have generally better efficiency than the parallel hybrids. Figure 7 shows the relative efficiency of a powersplit and parallel architecture for EV launch operation. For the powersplit, the electrical power passes through the Variable Voltage Controller (VVC) and the Inverter System Controller (ISC) and Motor where the electrical power is converted to mechanical power at 87% efficiency. The mechanical power then goes through the counter gears before it is delivered to the wheels with an overall efficiency of 82%. For the parallel hybrid, the ISC/Motor power output is sent to a conventional transmission which typically operates at an average efficiency of 89%. The path then goes through a Final Drive Ratio (FDR) before being delivered to the wheels.

A similar efficiency compare is shown for regenerative braking operation in figure 8. The path for regenerative braking is similar to the EV launch operation except in the opposite direction with friction brake losses added. The decreased efficiency in both directions of the power flow is multiplicative which results in even lower overall efficiency for the parallel system.

Note that this comparison is for a typical, generic vehicle to illustrate the relative efficiencies of powersplit and parallel hybrids. Specific applications of these architectures may have different relative efficiencies.

COMPARISON BETWEEN PARALLEL AND POWERSPLIT

Customer Attribute Comparison

Table 2 shows the customer attribute comparison between parallel and powersplit architectures with the conventional being the baseline. The comparison is hypothetical for identically sized vehicles using the same engine and battery.

Fuel Economy

As illustrated above, powersplits generally have better fuel economy than parallel hybrids. The absence of a conventional transmission in a powersplit contributes to this. This is especially the case in stop-and-go city driving where the regen energy available at the wheels takes an efficiency hit while being stored in the battery and another efficiency hit when being used to drive the wheels. Also, the continuous degrees of freedom in engine operation in both the torque and speed domains that the powersplit provides, helps optimize engine operation for peak system efficiency.

Performance

Unlike internal combustion engines, electric motors can produce torque at zero speed. This characteristic is what drives the superior performance of electrified vehicles, especially during launch. This seamless torque, available at zero or low speeds, helps boost the acceleration performance of both powersplits and parallel hybrids. For a given motor torque, parallel systems can achieve significantly higher peak accelerations than powersplit due to the first gear ratio of the conventional transmission. During higher power operations, the battery and motor power, supplements the engine power for improved performance for both parallel and powersplits.

Driveability/Smoothness

The driveability/smoothness attribute for a parallel hybrid is governed by the transmission shifting characteristics. It is only marginally improved to no better than the characteristics of the corresponding conventional powertrain. A significant drivability challenge associated with parallel hybrids is managing gear shifts during regen decel events. This is because power is being transmitted through the transmission during decel must be interrupted during down shifting. For a powersplit, however, the driveability is greatly improved owing to the lack of gear shifting. Powersplit operation is seamless due to the "eCVT" nature of the powertrain.

Max Vehicle Speed

Typically, in a simple powersplit hybrid there is fixed gear ratio from the wheels to the motor. This means that the max wheel speed can sometimes be limited by the maximum operating speed of the motor. Hence, the maximum vehicle speed may be kinematically limited by the maximum speed of the electric motor. Parallel hybrids do not have this restriction due to the different gear ratios available in the transmission. Also, for a powersplit, the generator has to provide a reaction torque in order for the engine power to be transmitted to the wheels. There is always a mechanical and an electrical path for the engine power to reach the wheels. The electrical path generates losses in the form of heat. The amount of heat generated can become a limiting factor in determining how long the max vehicle speed can be sustained before de-rating. Conventional powertrains and parallel hybrids also have thermal constraints. The thermal management system needs to reject the heat generated by the powertrain (engine, transmission, driveline) in order to sustain maximum speed. The thermal constraints and challenges are different for the electrical side of powersplit than those for conventional powertrain. Assuming that the challenges have been solved for the conventional powertrain systems on a given vehicle application, those solutions will also typically work for parallel hybrids. There are unique challenges for powersplit that have to be addressed when implementing the system that are not covered by the conventional powertrain work on the same vehicle.

Continuous Grade/Tow

Towing is typically characterized as a sustained high power operation. For the same reason outlined above for sustained max speed operation, powersplits typically have reduced towing capability. The amount of heat generated in the electrical path during sustained towing can be a limiting factor for the tow load. Parallel hybrids operate as effectively as conventional powertrains during sustained towing. Hence they have the same towing capability as the conventional powertrain.

Electric Drive Efficiency

Parallel hybrids have lower EV drive efficiency due to the presence of a conventional transmission in the power path. The transmission typically has more losses than the intermediate gears of the powersplit. This results in lower EV drive efficiency of the parallel hybrid. The improved efficiency of powersplits compared to parallel hybrids make powersplit architecture better suited for PHEV applications.

System and Functional Attribute Comparison

While not apparent to the customer, system and functional attributes are important considerations during the architecture selection process. Table 3 highlights some of the system and functional level attributes and compares the relative standing of each for parallel and powersplit hybrid architectures with the conventional vehicle as the baseline. As before, the comparison is for identically sized vehicles with the same engine and battery.

Package Transmission and eDrive

Parallel hybrids need to package an additional motor and associated power electronics along with the conventional transmission already present. This adds length to the existing powertrain and presents a significant package challenge especially for a front wheel drive vehicle. Powersplits replace the existing transaxle with the planetary and intermediate gear set thereby posing a less of a challenge.

CVT Function

Powersplits offer "eCVT" functionality due to the lack of a step-ratio transmission. This results in superior driveabilty and smooth operation of the vehicle.

EV and Regen Efficiency

In EV mode, the battery power flow in a powersplit is through the motor and intermediate gears to the wheels. The losses in this path are lower than the parallel hybrid where the motor power is subjected to the losses of a conventional transmission. This result in higher EV efficiency for the powersplit compared to the parallel hybrid. The same holds true for the regen path. The loss in EV and regen efficiency becomes a critical consideration for Plug In Hybrids (PHEVs) where it can have a significant impact to the range. Also, due to the increased transmission losses, parallel hybrids will have less EV drive capability compared to powersplits (for the same battery power).

Reverse Capability

The single mode input powersplit system discussed here does not have a mechanical reverse gear. Reverse operation is achieved by the electrical path only by running the motor in the opposite direction. This causes the reverse operation to the limited by the motor capability and state of the battery. Parallel hybrids have a mechanical reverse gear, similar to conventional vehicles, and can use full engine torque and power for reverse operation.

Engine Power to Wheels

Fundamentally, in a simple powersplit operation, in order for the engine power to reach the wheels, the generator has to provide a reaction torque. This means that the engine power is always delivered to the wheels via both the mechanical and electrical paths. It is not possible to deliver power to the wheels only via the mechanical path. Parallel hybrids do not have this constraint since the mechanical and electrical paths are in parallel and thereby independent of each other.

Powerpack Commonality

The components (generator, motor, etc.) in a powersplit are designed and sized for a given engine. For example, the generator torque capability is set to match the engine torque; increasing the engine torque capability would either require the generator to be resized or the engine to be operated at less than its full capability. This make it difficult to cost optimize a powersplit architecture across multiple engines or vehicle segments. Parallels do not have this limitation as the electrification is added on as a parallel path to existing conventional powertrains. Increasing the size of the engine would have no significant impact to the motor sizing.

Selecting the Right Architecture

As electrification spreads to more vehicle segments, customer expectations change. C/CD segment customers buy electrified products for fuel economy and total cost of ownership reasons and have little need for towing or increased payload. SUV and Pickup truck customers tend to buy these vehicles for towing and hauling loads. Hence, towing and payload requirements supplant fuel economy as design considerations for vehicles in the heavier segments. Figure 9 shows a possible architecture coverage by vehicle Equivalent Test Weight Class (ETWC) and Towing/Payload Requirements. With existing known design approaches, the advantages of the powersplit architecture can be fully realized for vehicles less than 4750 lbs. ETWC with less than 3500 lbs. towing/payload requirement. As ETWC starts rising over 5000 lbs. and towing/payload requirements increase, parallel architectures start becoming more attractive because the thermal management challenges associated with sustained high power operation have been addressed by the underlying conventional powertrain design. The intent of figure 9 is to show an approximate delineation between the two system architectures based on constraints associated with current design approaches. To extend the scope of powersplit applications beyond what is shown here, further work is needed to resolve electric machine thermal management challenges that are unique to the powersplit system operation.

Variable Cost Comparison

While satisfying customer requirements is critical to the success of any electrified vehicle, OEMs do need to focus on costs to deliver the most effective solution. Investment costs are excluded from the comparison as reuse of capital equipment and intellectual property could vary on a case to case basis. Table 5 shows the directional variable cost differences between parallel and powersplit powertrains. For the sake of comparison, the vehicle and engine are assumed to be the same.

The complete transmission in the parallel hybrid costs significantly more than the corresponding planetary and intermediate gear set of the powersplit. However, the 2 motor powersplit system has more eDrive costs compared to the single motor parallel system. If the power electronics are remotely mounted from the transmission then the HV EDS costs will be higher than if the power electronics are integrated with the transmission. In this analysis the power electronics are assumed to be directly mounted to the transmission.

The vehicle, engine and HV battery are assumed to be the same.

The HV battery, DC/DC converter, and power electronics cooling systems are assumed to be the same between the parallel and powersplit architectures which is generally the case. Hence, the cost differential is negligible.

Some OEMs make use of a separate motor to crank the engine on parallel hybrids. If this cranking device is needed, it would add a significant cost to the parallel system. Powersplits do not need this as the generator is always used to start the engine.

Weight Comparisons

Both Parallel and Powersplit architectures add weight compared to a conventional vehicle. Both add a common HV battery and DC/DC. The HV EDS and Power electronics weights will be similar. The major weight differentiator is within the transmissions. The Parallel motor is a pure add to the transmission whilst the Powersplit motor and generator weight is partially offset by the elimination of most of the gear train.

DESIGN CONSIDERATION FOR POWERSPLITS VS. PARALLEL HYBRIDS

The sizing process, regardless of the hybrid architecture, is generally an iterative process. The process starts with defining key attribute targets based on customer wants and marketing input. The key attributes are calculated, typically using CAE modelling, for the various powertrain system components (like engine, transmissions, eDrive, etc.). Gaps to customer targets for these attributes are analyzed and necessary changes are made to the system components. The process is iteratively repeated until targets are met. Figure 9 illustrates this process.

Battery and Motor Sizing

For any electrified architecture, properly sizing the battery and motor is key to delivering the right balance of fuel economy and performance. One of the main considerations for battery and motor sizing is regen collection which impacts fuel economy. Typically, the battery charge power limit is matched so as to maximize the regen energy collected not only over the regulatory fuel economy cycles but also in real world usage. There is also an opportunity to improve fuel economy by optimizing the motor loss map. The motor loss map provides the power losses of the ISC and motor at different operation speed and torque points. If the most common operating points of a vehicle are known, the motor loss map can be optimized at those points.

The other main consideration for battery and motor sizing is meeting the vehicles performance targets. The peak torque of the motor is sized for launch performance or low speed accelerations. The max power that the motor and battery is capable of delivering is typically matched to deliver passing maneuvers (like 45 mph to 65 mph time).

Engine Selection

One of the main reasons for using parallel hybrid architecture is to enable towing and meet maximum vehicle speed targets. Towing capability target is the main driver for engine sizing. Peak engine power must be capable of meeting the towing/payload target including passing maneuvers at high speeds while towing. Engine commonality with the non-electrified vehicles may also be a consideration.

Fuel economy is highly dependent on engine efficiency. Atkinsonizing [3] or Millerizing [4] the engine may be used to increase engine efficiency. These designs involve manipulating the valve timing for better fuel economy at the expense of torque/power. This is usually not a concern since the electrical powertrain can be used to compensate the torque/power that the engine is lacking.

Lastly, as mentioned previously, the engine must be capable of delivering enough power for max vehicle speed operation.

Engine Starting Considerations

The closing of the clutch during engine starting results in the motor torque being diverted away from the wheels. This causes a torque disturbance at the wheels which may lead to driveabilty concerns. In order to avoid this, many OEMs use a separate motor, typically connected in a B-ISG or P0 configuration, during engine starts. A BISG or P0 configuration is where a motor is connected to the engine via the FEAD similar to a conventional starter. A conventional starter motor can also be used for this but it may not provide enough durability over the life of the vehicle. This motor, usually much smaller in size compared to the traction motor, adds cost and complexity to the system but may be essential for smoothness during power-on engine start events.

Transmission Selection

Parallel hybrids are generally built upon conventional vehicle powertrains. The obvious choice for the transmission is to carry it over from the conventional powertrain. However, there may be certain factors that may warrant a unique transmission. One of the factors may be increased efficiency. A traditional torque converter, step-ratio transmission may be replaced by a more efficient Dual Clutch Transmission (DCT).

Number of gears in the transmission also needs to be carefully considered. A higher number of gears enables more engine speed control opportunities, but can also mean more transmission losses and increased shift busyness.

In a parallel hybrid, the transmission needs to function with the engine off. This may drive the need to add special hydraulic pump and/or clutch actuator for clutch control that are operational without engine power. This further adds cost.

Packaging Considerations

Parallel hybrids architecture typically adds the electrical system components to an existing conventional powertrain. This is different compared to a powersplit. In a powersplit, the transmission is replaced completely with a transaxle which consists of the gear set and e-machines. Hence parallel architectures tend to pose a significant packaging challenge.

Industry trends in vehicle design in recent years have been the compaction of the front overhang. This leads to minimal under hood space for the addition of electrical system. Recent trends to reduce space between front rails, leaves little latitudinal space to package the transmission with the additional electric motor. This is especially prevalent for Front Wheel Drive (FWD) vehicles due to the additional space needed for turn circle clearance.

CONCLUSIONS

Current production powersplit system designs have been primarily developed to deliver maximum fuel economy within smaller passenger car vehicle segments. Using current powersplit design approaches, critical attribute requirements of larger vehicle segments, including towing capability, performance and higher maximum vehicle speeds, can be difficult and in some cases impossible to meet. Further work is needed to resolve the unique challenges of adapting powersplit systems to these larger vehicle applications.

Parallel architectures provide a viable alternative to powersplit for larger vehicle applications because they can be integrated with existing conventional powertrain systems that already meet the additional attribute requirements of these large vehicle segments.

Going forward, electrification can no longer be confined to a few vehicle segments. The spread of electrification across the spectrum of vehicles is inevitable. With this "democratization" of electrification, the focus is shifting from fuel economy to other attributes like towing and performance. The inherent advantages of the parallel architecture to deliver a higher top speed and towing capabilities make it an appropriate choice for some applications. This, however, comes at the expense of fuel economy, cost, and driving smoothness where powersplit is superior.

Each architecture has its advantages and drawbacks. Different tradeoffs between architectures need to be carefully considered in order to make pick the optimal solution. Customer wants need to be balanced with cost and weight considerations.

REFERENCES

[1.] Kozarekar, S., Sankaran, V., and Newman, K., "Study of Suitability of Hybrid Architectures for Different Market Requirements," SAE Technical Paper 2008-21-0016, 2008, doi:10.4271/2008-21-0016.

[2.] Kim, Seok-Joon, "Development of Hybrid System for Mid-size Sedan," SAE Presentation, 2011.

[3.] Cordier, M., Laget, O., Duffour, F., Gautrot, X. et al.,"Increasing Modern Spark Ignition Engine Efficiency: A Comprehension Study of High CR and Atkinson Cycle," SAE Technical Paper 2016-01-2172, 2016, doi:10.4271/2016-01-2172.

[4.] Miller, R, Supercharged Engine, US patent 2817322, 1957.

CONTACT INFORMATION

The author for this work is Jimmy Kapadia, Electrified Powertrain Engineering, Ford Motor Company. Any questions or comments can be sent by email to

jkapadia@ford.com.

ACKNOWLEDGMENTS

The authors wish to acknowledge the support of colleagues at Electrified Powertrain Engineering, Ford Motor Company for their assistance in research and analysis behind this paper.

DEFINITIONS/ABBREVIATIONS

DCT - Dual Clutch Transmission or Twin Clutch Transmission

VVC - Variable Voltage Controller.

ISC - Inverter System Controller

FDR - Final Drive Ratio

PHEV - Plug In Hybrid Electric Vehicle

EVs - Electric Vehicles

EV Mode - Mode in which the vehicle behaves likes an electric vehicle

EDS - Electrical Distribution System or Wiring

BISG - Belt Integrated Starter- Generator

FEAD - Front End Accessory Drive

APPENDIX

A. Parallel Power Flows

B. Powersplit Power Flows

Jimmy Kapadia, Daniel Kok, Mark Jennings, Ming Kuang, Brandon Masterson, Richard Isaacs, Alan Dona, Chuck Wagner, and Thomas Gee

Ford Motor Company

doi:10.4271/2017-01-1154
Table 1. Hybrid architecture and their usage by OEM in the US market.
Data is

Architecture   Major Players

Parallel       BMW
               Hyundai/Kia
               Renault/Nissan
               Mercedes Benz
               Porsche
               VW/Audi
               Volvo
Powersplit     Ford/Lincoln
               Chevy/Cadillac
               Toyota/Lexus
Series         BMW (i3 range extender)

Table 2. Powersplit vs. Parallel Hybrid customer attribute comparison
assuming conventional vehicle as baseline. Comparison is for
identically sized vehicles and same engine and battery.

Customer               Conventional   Parallel P2   Powersplit
Attribute

Fuel Economy -         Baseline       + +           + + +
City
Fuel Economy -         Baseline       +             +
Hwy
Fuel Economy -         Baseline       +             + +
Combined
Performance -          Baseline       ++            +
launch
Performance -          Baseline       +             +
power
Driveability/Driving   Baseline       =             + + +
Smoothness
Max vehicle speed      Baseline       =             -
Grade/Towing           Baseline       =             --
Electric drive         N/A            +             + +
efficiency

Table 3. Powersplit vs. Parallel Hybrid system/function attribute
comparison assuming conventional vehicle as baseline. Comparison is for
identically sized vehicles and same engine and battery.

Attribute        Conventional   Parallel P2   Powersplit

Package
Transmission &   =              --            -
E-Drive
CVT function
for shift        N/A            N/A           + +
smoothness
EV efficiency    N/A            +             + +
(Range)
Regen Braking    N/A            +             + +
Efficiency
Reverse
Capability
Engine Power
to Wheels
Powerpack        =              =             -
Commonality

Table 5. Directional variable component cost comparison between
Parallel and Powersplit Hybrid. Assumes similarly sized powertrains for
the same sized vehicle.($ sign indicates that the cost of that
component for that architecture is more than the corresponding cost for
the comparator).

Component       Relative     Reason
                Cost

Engine          Equal        Same engine is assumed for this
                             analysis.
Trans           Parallel     Conventional transmission gear set vs.
                $$$          single planetary cost.
eDrive          Powersplit   Single motor in a parallel vs. motor and
                $            generator in a powersplit.
HV Battery      Equal        Assumes same high voltage battery.
DC/DC           Equal        No change in the DC/DC converter.
                             Similar LV electrical content.
HV Battery      Equal        No change in HV battery cooling is
Cooling                      assumed. Common battery in common
                             location
Power           Equal        No change in Power Electronics
Electronic                   cooling is assumed.
Cooling
Total           Parallel     Transmission increase is not offset by
Powertrain      $            eDrive reduction
Cranking        Parallel     An additional cranking system may be
Device (if      $$$$         needed for smoother engine starts
needed)                      which could add substantial additional
                             cost if BISG solution chosen.
Total           Parallel
Powertrain      $$
with Cranking
Device
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Article Details
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Author:Kapadia, Jimmy; Kok, Daniel; Jennings, Mark; Kuang, Ming; Masterson, Brandon; Isaacs, Richard; Dona,
Publication:SAE International Journal of Alternative Powertrains
Article Type:Report
Date:May 1, 2017
Words:4450
Previous Article:Analysis and Control of a Torque Blended Hybrid Electric Powertrain with a Multi-Mode LTC-SI Engine.
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