Editing Hybrid Synergy Drive (section)

== Principle ==
[[File:Toyota 1NZ-FXE Engine 01.JPG|thumb|Toyota 1NZ-FXE engine (left) with early HSD, sectioned and highlighted (right). Generation 1/Generation 2, chained, ICE-MG1-MG2 Power Split Device HSD is shown.]]

Toyota's HSD system replaces a normal geared [[transmission (mechanics)|transmission]] with an [[electromechanical]] system. An [[internal combustion engine]] (ICE) delivers power most efficiently over a small [[speed]] range, but the wheels need to be driven over the vehicle's full speed range. In a conventional automobile the geared [[transmission (mechanics)|transmission]] delivers different discrete engine speed-torque power requirements to the wheels. Geared transmissions may be manual, with a [[clutch]], or automatic, with a [[torque converter]], but both allow the engine and the wheels to rotate at different speeds. The driver can adjust the speed and torque delivered by the engine with the [[accelerator (car)|accelerator]] and the transmission mechanically transmits nearly all of the available power to the wheels which rotate at a different rate than the engine, by a factor equal to the gear ratio for the currently selected gear. However, there are a limited number of "gears" or [[gear ratios]] that the driver can choose from, typically four to six. This limited gear-ratio set forces the engine [[crankshaft]] to rotate at speeds where the ICE is less efficient, i.e., where a liter of fuel produces fewer joules. Optimal engine speed-torque requirements for different vehicle driving and acceleration conditions can be gauged by limiting either [[tachometer]] RPM rate or engine noise in comparison with actual speed. When an engine is required to operate efficiently across a broad RPM range, due to its coupling to a geared transmission, manufacturers are limited in their options for improving [[engine efficiency]], reliability, or lifespan, as well as reducing the size or weight of the engine. This is why the engine for an [[engine-generator]] is often much smaller, more efficient, more reliable, and longer life than one designed for an automobile or other variable speed application.

However, a continuously variable transmission allows the driver (or the automobile computer) to effectively select the optimal gear ratio required for any desired speed or power. The transmission is not limited to a fixed set of gears. This lack of constraint frees the engine to operate at its optimal [[brake-specific fuel consumption]]. An HSD vehicle will typically run the engine at its optimal efficiency whenever power is needed to charge batteries or accelerate the car, shutting down the engine entirely when less power is required.

Like a [[Continuously variable transmission|CVT]], an HSD transmission continuously adjusts the effective [[gear ratio]] between the engine and the wheels to maintain the engine speed while the wheels increase their rotational speed during acceleration. This is why Toyota describes HSD-equipped vehicles as having an '''e-CVT''' ('''electronic continuously variable transmission''') when required to classify the transmission type for standards specification lists or regulatory purposes.

=== Power flows ===
In a conventional car design the [[alternator (auto)|separately-excited alternator with integral rectifier]] (DC generator) and [[starter motor|starter]] (DC motor) are considered accessories that are attached to the internal combustion engine (ICE) which normally drives a transmission to power the wheels propelling the vehicle. A battery is used only to start the car's internal combustion engine and run accessories when the engine is not running. The alternator is used to recharge the battery and run the accessories when the engine is running.

The HSD system replaces the geared transmission, alternator, and starter motor with:
* '''MG1''', an AC [[Motor-generator#Modern use of the term|motor-generator]] having a [[permanent magnet synchronous motor|permanent magnet]] rotor,<ref>All electric motors with excited fields, either by a (separately-excited) electro–magnet rotor or a (integrally-excited) permanent–magnet rotor, can be used as generators (and vice versa), so the term motor–generator is normally used only when the same device is being used for both purposes, although ''not'' simultaneously.</ref> used as a motor when starting the ICE and as a generator (alternator) when charging the high voltage battery
* '''MG2''', an AC motor-generator, also having a permanent magnet rotor, used as the primary drive motor and as a generator (alternator), which regeneration power is directed to the high voltage battery. MG2 is generally the more powerful of the two motor-generators
* '''[[Power electronics]]''', including three DC-AC [[inverter (electrical)|inverter]]s and two DC-DC [[buck-boost converter|converter]]s
* '''Computerized [[control system]]''' and [[sensor]]s
* '''HVB''', a high voltage [[battery pack|battery]] sources electrical energy during acceleration and sinks electrical energy during regeneration braking

Through the power splitter, a series-parallel full hybrid's HSD system thus allows for the following intelligent power flows:<ref name="Burress (2006)">{{cite web|last=Burress|first=Timothy Adam|title=Vector Control and Experimental Evaluation of Permanent Magnet Synchronous Motors for HEVs|url=http://power.eecs.utk.edu/pubs/tim_burress_thesis.pdf|publisher=University of Tennessee|access-date=29 September 2012|year=2006|page=16}}</ref>

*'''Auxiliary power'''
**HVB -> DC-DC converter -> 12VDC battery
**12VDC battery -> [[automotive electronics|12V vehicle electronics]]
*'''Engine charge''' (Recharging and/or heating catalytic converter and/or interior comfort HVAC)
**ICE -> MG1 -> HVB
*'''Battery or EV drive''' 
** HVB -> MG2 -> wheels
*'''Engine & motor drive''' (Moderate acceleration)
** ICE -> wheels 
** ICE -> MG1 -> MG2 -> wheels
*'''Engine drive with charge''' (Highway driving)
** ICE -> wheels
** ICE -> MG1 -> HVB 
*'''Engine and motor drive with charge''' (Heavy power situation such as in steep hills)
** ICE -> wheels
** ICE -> MG1 -> HVB 
** ICE -> MG1 -> MG2 -> wheels
*'''Full power or gradual slowing''' (Maximum power situations)
** ICE -> wheels
** ICE -> MG1 -> MG2 -> wheels
** HVB -> MG2 -> wheels
*'''B-mode braking'''
** Wheels -> MG2 ->HVB
** Wheels -> MG1 -> ICE  (ECU - Electronic Control Unit - uses MG1 to spin ICE which drains battery – allowing more charge from MG2, and also links ICE to wheels causing "engine braking"; ICE RPM increases when charge level of HVB is too much to accept regen electricity from MG2, or increasing effort from driver pushing the brake pedal)
*'''[[Regenerative braking]]'''
** wheels -> MG2 -> HVB
*'''Hard braking'''
** Front disk/rear drum (rear disk in UK) -> wheels
** All disk -> wheels (2010 and newer, except 2012-current Prius c, which uses front disk, rear drum).

[[File:ToyotaOpenHSD.jpg|thumb|Power electronics from Prius NHW11 "Classic"]]

==== MG1 and MG2 ====
* '''MG1''' (Primary motor-generator): A motor to start the ICE and a generator to generate electrical power for MG2 and to recharge the [[EV battery|high voltage traction battery]], and, through a [[DC-to-DC converter]], to recharge the 12 volt auxiliary battery. By regulating the amount of electrical power generated (by varying MG1's mechanical torque and speed), MG1 effectively controls the [[transaxle]]'s [[continuously variable transmission]].
* '''MG2''' (Secondary motor-generator): Drives the wheels and regenerates power for the HV battery energy storage while braking the vehicle. MG2 drives the wheels with electrical power generated by the engine-driven MG1 and/or the HVB. During regenerative braking, MG2 acts as a generator, converting kinetic energy into electrical energy, storing this electrical energy in the battery.

=== Transmission ===
[[File:Toyota electronic continuously variable transmission (2010-10-16) 03.jpg|thumb|Late Toyota HSD, sectioned and highlighted. Generation 3, chainless, ICE-MG1 Power Split Device/MG2 Motor Speed Reduction Device HSD is shown. This is a P510 transaxle, from a 2012– Prius c; a P410 transaxle, from a 2010–2015 Prius, is similar, but is physically larger; a Generation 4 P610 transaxle from a 2016– Prius is 47mm narrower than a P410 by implementing side-by-side motors rather than end-to-end motors.]]

The mechanical gearing design of the system allows the mechanical power from the ICE to be split three ways: extra torque at the wheels (under constant rotation speed), extra rotation speed at the wheels (under constant torque), and power for an electric generator. A computer running appropriate programs controls the systems and directs the power flow from the different engine + motor sources. This power split achieves the benefits of a [[continuously variable transmission]] (CVT), except that the torque/speed conversion uses an electric motor rather than a direct mechanical gear train connection.  An HSD car cannot operate without the computer, power electronics, battery pack, and motor-generators, though in principle it could operate while missing the internal combustion engine. (See: [[Plug-in hybrid]]) In practice, HSD equipped cars can be driven a mile or two without gasoline, as an emergency measure to reach a [[filling station|gas station]].

An HSD [[transaxle]] contains a [[planetary gear]] set that adjusts and blends the amount of torque from the engine and motor(s) as it's needed by the front wheels. It is a sophisticated and complicated combination of gearing, electrical motor-generators, and computer-controlled electronic controls. One of the motor-generators, MG2, is connected to the output shaft, and thus couples torque into or out of the drive shafts; feeding electricity into MG2 adds torque at the wheels. The engine end of the drive shaft has a second [[differential (mechanics)|differential]]; one leg of this differential is attached to the internal combustion engine and the other leg is attached to a second motor-generator, MG1. The differential relates the rotation speed of the wheels to the rotation speeds of the engine and MG1, with MG1 used to absorb the difference between wheel and engine speed. The differential is an [[epicyclic gearing|epicyclic gear set]] (also called a "power split device"); that and the two motor-generators are all contained in a single transaxle housing that is bolted to the [[engine]]. Special couplings and sensors monitor rotation speed of each shaft and the total torque on the drive shafts, for feedback to the control computer.
<ref>{{cite web
|url=http://autos.yahoo.com/green_center-article_24/
|title=Synergy Drive: Why Toyota's Hybrids Rock
|publisher=[[Yahoo]]
|author=Bill Siuru
|work=Green Car Journal
|access-date=2008-03-12
|archive-date=2009-09-28
|archive-url=https://web.archive.org/web/20090928014036/http://autos.yahoo.com/green_center-article_24/
|url-status=dead
}}</ref>

In Generation 1 and Generation 2 HSDs, MG2 is directly connected to the ring gear, that is, a 1:1 ratio, and which offers no torque multiplication, whereas in Generation 3 HSDs, MG2 is connected to the ring gear through a 2.5:1 planetary gear set,<ref>In 2007 and later Camrys, this ratio is 2.636, and in 2010 and later Priuses, this ratio is 2.478, for an average ratio of roughly 2.5</ref> and which, consequently, offers a 2.5:1 torque multiplication, this being a primary benefit of the Generation 3 HSD as it provides for a smaller, yet more powerful MG2. However, a secondary benefit is the MG1 will not be driven into overspeed as frequently, and which would otherwise mandate employing the ICE to mitigate this overspeed; this strategy improves HSD performance as well as saving fuel and wear-and-tear on the ICE.

===High voltage battery===
[[File:Ni-MH Battery 02.JPG|thumb|High voltage [[nickel-metal hydride]] (NiMH) battery of [[Toyota Prius (XW20)|second generation Toyota Prius]].]]
The HSD system has two principal battery packs, the High Voltage (HV) battery, also known as the traction battery, and a 12 volt [[lead-acid battery]] known as the Low Voltage (LV) battery, which functions as an auxiliary battery. The LV battery supplies power to the electronics and accessories when the hybrid system is turned off and the high-voltage battery main relay is off.<ref name=HSDCase/><ref>{{cite web|url=http://www.alabc.org/publications/lead-acid-batteries-in-hevs |title=Do Hybrid Electric Vehicles Use Lead-Acid Batteries? Yes! Here's why. |author=The Advanced Lead-Acid Battery Consortium (ALABC) |publisher=ALABC |access-date=2014-11-23 |url-status=dead |archive-url=https://web.archive.org/web/20140506082429/http://www.alabc.org/publications/lead-acid-batteries-in-hevs |archive-date=2014-05-06 }}</ref>

The traction battery is a [[seal (mechanical)|sealed]] [[nickel-metal hydride]] (NiMH) [[rechargeable battery|battery]] pack. The battery pack of the first generation Toyota Prius consisted of 228 cells packaged in 38 modules, while the second generation Prius consisted of 28 Panasonic prismatic nickel metal hydride modules, each containing six 1.2 volt cells, connected in series to produce a nominal voltage of 201.6 volts. The discharge power capability of the second gen Prius pack is about 20 [[kW]] at 50% [[state of charge]] (SoC). The power capability increases with higher temperatures and decreases at lower temperatures. The Prius has a computer that's solely dedicated to keeping the battery at the optimum temperature and optimum charge level.<ref name=HybridBatteries/>

Like the second generation Prius, the third generation Prius battery pack is made up of the same type of 1.2 volt cells. It has 28 modules of 6 cells for a total nominal voltage of only 201.6 volts. A boost converter is used 
to produce 500 volt DC supply voltage for the inverters for MG1 and MG2.<ref name=HSDCase/> The car's electronics only allow 40% of total rated capacity of the battery pack (6.5 ampere-hour) to be used in order to prolong the battery life. As a result, the SoC is allowed to vary only between 40% and 80% of the rated full charge.<ref name=HSDCase/> The battery used in the [[Toyota Highlander Hybrid|Highlander Hybrid]] and the [[Lexus RX 400h]] was packaged in a different metal battery casing with 240 cells that deliver high voltage of 288 volts.<ref name=HybridBatteries>{{cite web|url=http://www.hybridcars.com/hybrid-car-battery/|title=The Hybrid Car Battery: A Definitive Guide - Today's Hybrid Car Battery: Nickel Metal Hydride - Toyota Prius Hybrid Battery|author=Brad Berman|publisher=HybridCars.com|date=2008-11-06|access-date=2014-11-22}}</ref> 
[[File:Camry Hybrid 2012 07 VA 4177.JPG|thumb|EV mode button in the 2012 [[Toyota Camry hybrid]].]]
A button labelled "EV" maintains [[electric vehicle]] mode after being powered on and under most low-load conditions at less than {{Convert|25|mph|abbr=on}} if the traction battery has enough charge. This permits [[all-electric mode|all-electric driving]] with no fuel consumption for up to {{convert|1|mi|abbr=on}}. However, the HSD software switches to EV mode automatically whenever it can.<ref>{{cite web|url=http://www.toyota.com/prius/#!/features/efficiency/three-drive-modes|title=Toyota Prius - Three drive modes|author=Toyota |publisher=Toyota01|access-date=2014-11-23}} ''EV Mode works under certain conditions at low speeds for up to a mile.''</ref><ref>{{cite web|url=http://www.tomshardware.com/reviews/camry-hybrid-xle-technology-review,3290-8.html|title=2012 Toyota Camry Hybrid XLE: Technology In A Mid-Size Sedan|author=Anh T. Huynh|publisher=Tom's Hardware|date=2012-10-15|access-date=2014-11-23}}</ref> Only the [[Toyota Prius Plug-in Hybrid]] has a longer driving [[all-electric range]] in [[blended mode|blended operation]] electric-gasoline of {{convert|11|mi|km|0|abbr=on}} ([[EPA]] rating) until the battery is depleted.<ref name=PiP>{{cite web|url=http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=34516&id=33335&id=32484|title=Compare Side-by-Side - 2012/2013/2014 Toyota Prius Plug-in Hybrid|publisher=Fueleconomy.gov|author=[[U. S. Environmental Protection Agency]] and [[U.S. Department of Energy]]|date=2014-11-21|access-date=2014-11-21}}</ref> The Prius PHEV is outfitted with 4.4 [[kWh]] [[lithium-ion battery|lithium-ion batteries]] co-developed with [[Panasonic]] that weighs {{convert|80|kg|abbr=on}} compared with the [[nickel-metal hydride battery]] of the [[Toyota Prius (XW30)|third generation Prius]], which has a capacity of only 1.3 kWh, and weighs {{convert|42|kg|abbr=on}}. The larger battery pack enables all-electric operation at higher speeds and longer distances than the conventional Prius hybrid.<ref name=GCC122009>{{cite web|url=http://www.greencarcongress.com/2009/12/prius-phv-20091202.html|title=2010 Prius Plug-in Hybrid Makes North American Debut at Los Angeles Auto Show; First Li-ion Battery Traction Battery Developed by Toyota and PEVE|publisher=[[Green Car Congress]]|date=2009-12-02|access-date=2010-02-03}}</ref><ref name=Toyota091611>{{cite press release|url=http://pressroom.toyota.com/releases/toyota+introduces+2012+prius+plug-in+hybrid.htm|title=Toyota Introduces 2012 Prius Plug-in Hybrid|publisher=[[Toyota]]|date=2011-09-16|access-date=2014-11-21|archive-date=2014-10-14|archive-url=https://web.archive.org/web/20141014072355/http://pressroom.toyota.com/releases/toyota+introduces+2012+prius+plug-in+hybrid.htm|url-status=dead}}</ref>

The following table details the HV battery capacity for several Lexus and Toyota vehicles.<ref name=HEVbatteries>{{cite web|url=http://cta.ornl.gov/vtmarketreport/spreadsheets/T33_Batteries_for_Selected_HEV_Vehicles_2013_2014.xls|title=Table 33. Batteries for Selected Hybrid-Electric Vehicles, Model Years 2013-2014|author=Josh Pihl|publisher=[[Oak Ridge National Laboratory]]|date=January 2014|access-date=2014-11-21|url-status=dead|archive-url=https://web.archive.org/web/20141129052920/http://cta.ornl.gov/vtmarketreport/spreadsheets/T33_Batteries_for_Selected_HEV_Vehicles_2013_2014.xls|archive-date=2014-11-29}}</ref>

{| class="wikitable sortable"
|-
!Vehicle!!Model<br>Year!!Battery<br>Capacity<br>([[kWh]])<ref name=HEVbatteries/>!!Battery Type!!Battery Charge Limit<br>([[kW]])<ref>Based on Min a Max values from Hybrid Assistant App (High Voltage Battery Statistics)</ref>!!Battery Discharge Limit<br>([[kW]])<ref>Based on Min a Max values from Hybrid Assistant App (High Voltage Battery Statistics)</ref>
|-align=center
|align=left|[[Lexus CT 200h]]||2011||1.3||NiMH||||
|-align=center
|align=left|[[Lexus ES 300h]]||2013||1.6||NiMH||||
|-align=center
|align=left|[[Lexus ES 300h]]||2021||1.6||Li-ion||||
|-align=center
|align=left|[[Lexus GS 450h]]||2013||1.9||NiMH||||
|-align=center
|align=left|[[Lexus IS 300h]]||2013||1.6||NiMH||-28,5||24
|-align=center
|align=left|[[Lexus LC|Lexus LC 500h]]||2018||1.1||Li-ion||||
|-align=center
|align=left|[[Lexus LS 600h L]]||2008||1.9||NiMH||||
|-align=center
|align=left|[[Lexus RX 450h]]||2014||1.9||NiMH||||
|-align=center
|align=left|[[Lexus NX 300h]]||2015||1.6||NiMH||-27||25,5
|-align=center
|align=left|[[Toyota Avalon Hybrid]]||2013||1.6||NiMH||||
|-align=center
|align=left|[[Toyota Auris Hybrid]]||2014||1.3<ref name=HSDCase>{{cite web|url=http://www.ae.pwr.wroc.pl/filez/20110606092430_HEV_Toyota.pdf|title=Case study: Toyota Hybrid Synergy Drive |author=Politechnika Wrocławska - Inżynieria Pojazdów|publisher=[[Wrocław University of Technology]]|access-date=2014-11-22}} ''See Auris HSD specs in pp.17: 201.6V x 6.5Amp/hr = 1.310kWh''</ref>||NiMH||-25||21
|-align=center
|align=left|[[Toyota Camry (XV50)#Camry Hybrid|Toyota Camry Hybrid]]||2014||1.6||NiMH||-27||25,5
|-align=center
|align=left|Toyota Camry Hybrid||2018||1.6 / 1.0||NiMH / Li-ion||||
|-align=center
|align=left|Toyota C-HR Hybrid||2016||1.3||NiMH||-31,9||21
|-align=center
|align=left|Toyota Corolla Hybrid||2019||1.4 / 0.75||NiMH / Li-ion||-31,9||21
|-align=center
|align=left|[[Toyota Highlander#Third generation (XU50; 2013)|Toyota Highlander Hybrid]]||2014||1.9||NiMH||||
|-align=center
|align=left|[[Toyota Mirai]] ([[fuel cell vehicle|FCV]])||2015||1.6<ref>{{cite web|url=http://www.cnet.com/products/2016-toyota-mirai/|title=Toyota Mirai: The 300-mile zero-emission vehicle|author=Wayne Cunningham|publisher=[[CNET]]|date=2014-11-19|access-date=2014-11-21}} ''The Mirai has a 245-volt nickel-metal hydride battery pack, similar to that in the Camry Hybrid. 245V x 6.5Amp/hr = 1.59kWh''</ref>||NiMH||||
|-align=center
|align=left| [[Toyota Prius (XW30)|Toyota Prius]]||2010||1.3||NiMH||-25||21
|-align=center
|align=left| Toyota Prius||2016||1.2 / 0.75||NiMH / Li-ion||-31,9||21
|-align=center
|align=left|[[Toyota Prius c]]||2014||0.9||NiMH||||
|-align=center
|align=left|[[Toyota Prius v]]||2014||1.3 / 1.0||NiMH / Li-ion||||
|-align=center
|align=left|[[Toyota Prius PHV]]||2014||4.4<ref name=Toyota091611/>||Li-ion||||
|-align=center
|align=left|[[Toyota Prius Prime]]||2016||8.8||Li-ion||-40||65
|-align=center
|align=left|[[Toyota RAV4]]||2015||1.6||NiMH||-27||25,5
|-align=center
|align=left|[[Toyota RAV4]]||2019||1.6||NiMH (2020- Li-ion)||-38||24||
|-align=center
|align=left|[[Toyota RAV4|Toyota RAV4 Prime]]||2020||18.1||Li-ion||
|-align=center
|align=left|[[Toyota Yaris Hybrid]]||2014||0.9<ref name=YarisHSD>{{cite web|url=http://toyota.co.za/Media/Default/brochures/Yaris.pdf|title=Yaris & Yaris HSD brochure |author=Toyota  |publisher=Toyota South Africa|access-date=2014-11-22}} ''See specs table: 144V x 6.5Amp/hr = 0.936kWh''</ref>||NiMH||-17,5||15
|-align=center
|align=left|[[Toyota Yaris Hybrid]]||2020||0.76||Li-ion||-35||20
|-align=center
|align=left|[[Toyota Innova (AG10)|Toyota Innova]]/[[Toyota Kijang Innova Zenix|Kijang Innova Zenix Hybrid]]||2022||1.31||NiMH||||
|}