Hybrid Synergy Drive system (HSD), also known as Toyota Hybrid System II, is the brand name of Toyota Motor Corporation for the hybrid car drive train technology used in vehicles with the Toyota and Lexus marques. First introduced on the Prius, the technology is an option on several other Toyota and Lexus vehicles and has been adapted for the electric drive system of the hydrogen-powered Mirai, and for a plug-in hybrid version of the Prius. Previously, Toyota also licensed its HSD technology to Nissan for use in its Nissan Altima Hybrid. Its parts supplier Aisin offers similar hybrid transmissions to other car companies.
HSD technology produces a full hybrid vehicle which allows the car to run on the electric motor only, as opposed to most other brand hybrids which cannot and are considered mild hybrids. The HSD also combines an electric drive and a planetary gearset which performs similarly to a continuously variable transmission. The Synergy Drive is a drive-by-wire system with no direct mechanical connection between the engine and the engine controls: both the gas pedal/accelerator and the gearshift lever in an HSD car merely send electrical signals to a control computer.
HSD is a refinement of the original Toyota Hybrid System (THS) used in the 1997 to 2003 Toyota Prius. The second generation system first appeared on the redesigned Prius in 2004. The name was changed in anticipation of its use in vehicles outside the Toyota brand. It was implemented in the 2006 Camry and Highlander, and would eventually be implemented in the 2010 "third generation" Prius, and the 2012 Prius c. (Lexus; the HSD-derived systems used in Lexus vehicles have been termed Lexus Hybrid Drive.) The Toyota Hybrid System is designed for increased power and efficiency, and also improved "scalability" (adaptability to larger as well as smaller vehicles), wherein the ICE/MG1 and the MG2 have separate reduction paths, and are combined in a "compound" gear which is connected to the final reduction gear train and differential; it was introduced on all-wheel drive and rear-wheel drive Lexus models. By May 2007 Toyota had sold one million hybrids worldwide; two million by the end of August 2009; and passed the 5 million mark in March 2013. , more than 7 million Lexus and Toyota hybrids had been sold worldwide. The United States accounted for 38% of TMC global hybrid sales .
Principle
Toyota's HSD system replaces a normal geared 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 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 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 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 separately-excited alternator with integral rectifier (DC generator) and 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 having a permanent magnet rotor,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. 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) when 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 inverters and two DC–DC converters Computerized control system and sensors HVB, a high-voltage 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:
Auxiliary power HVB → DC–DC converter → 12VDC battery 12VDC battery → 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–present Prius c, which uses front disk, rear drum).
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 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
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 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; 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 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.
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,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 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