The new Yaris Hybrid: Downsized full hybrid powertrain for fuel and space efficiency
— Substantially re-engineered 1.5 litre petrol unit engine combined with downsized hybrid system components
— Compact battery, relocated under the rear seats to preserve boot space
— Total system output of 100 hp, with low, 3.5 l/100 km fuel consumption
— Best in class CO2, NOX and Particulate Matter emissions
— Frequent all-electric operation with no fuel consumed and zero emissions
— Driving range of 1,000 km on one tank of fuel
The new Yaris Hybrid introduces the first downsizing of Toyota's proven Hybrid Synergy Drive® (HSD) technology. It represents a significant engineering breakthrough in the packaging of the full hybrid powertrain.
The new powertrain has been optimised for installation within the vehicle's compact, extremely efficient packaging design, without detriment to either system quality and performance, or passenger accommodation and load space.
Every key HSD component has been reduced in size and weight, and both the fuel tank and battery are installed under the rear seat. As a result, the Yaris Hybrid maintains identical occupant space and the same 286 litre luggage capacity as that of the standard model.
The downsized full hybrid system combines a 1.5 litre petrol engine unit with a lighter, more compact electric motor, transaxle, inverter and battery pack. The total system weight is 201 kg, 20% (42 kg) less than that of an Auris Hybrid.
Compact, Lower Displacement Petrol Engine
The 1,497 cc, 16 valve, DOHC Atkinson Cycle petrol engine with VVT-i is based on the engine block of the second generation Prius, but has been completely re-engineered. It is 50 mm shorter and 17 kg lighter than the 1.8 litre unit of existing Toyota full hybrid powertrains, equating to a 10% reduction in length and a 17% reduction in weight.
The new engine generates a maximum 55 kW/74 DIN hp at 4,800 rpm and maximum torque of 111 Nm from 3,600 to 4,400 rpm. With a focus on lower emissions and the minimisation of mechanical friction for improved fuel efficiency, some 70% of all engine components are either new or redesigned.
Overall, the thermal efficiency of the Yaris Hybrid petrol engine has been improved by 6% over that of the second generation Prius, contributing to improvements in both fuel efficiency and environmental performance.
Newly adopted engine technologies include a cooled Exhaust Gas Recirculation (EGR) system, an electric water pump, the elimination of the auxiliary drive belt, a low-friction timing chain, a lightweight, resin-based intake manifold and a compact exhaust manifold.
When used in combined with a cooled EGR system, the Atkinson cycle engine offers significant gains in fuel efficiency and the reduction of emissions.
In the Atkinson cycle, compression and expansion are asymmetrical, and the inlet valves close late, delaying compression. This creates a high expansion ratio for less compression, converting combustion energy to engine power more effectively. As a result, the exhaust temperature is lower than that of conventional engines. The cooled EGR system reintroduces cooled exhaust gas into the intake system, further reducing engine operating temperatures.
Allied to a small, lightweight, multi-function radiator, the adoption of an electric water pump both enhances engine warm-up performance and reduces cooling friction loss, improving fuel efficiency. The elimination of a mechanical pump drive belt reduces friction, further improving fuel economy.
A new, more compact, hybrid-exclusive resin-based intake manifold combines light weight with improved quietness through the reduction of both vibration and noise. And the use of highly efficient, 12-hole atomizing fuel injectors reduces both fuel consumption and emissions.
The exhaust system features a new, low heat capacity, maniverter-type exhaust manifold. By positioning the catalytic converter much closer to the engine exhaust port, it decreases the catalyser warm-up time by 24% and contributes to a marked improvement in emissions during shorter journeys.
Downsized Hybrid Transaxle
Housing the electric motor, generator, power split device and double-motor reduction mechanism in one lightweight, highly compact transmission casing directly comparable in size to that of a conventional gearbox, a new transaxle lies at the heart of the Hybrid Synergy Drive® system.
The use of a smaller E-CVT electric continuously variable transmission and a more compact electric motor effects a 6% reduction in the length of the Yaris Hybrid's hybrid transaxle and an 11 kg weight saving over that of the Auris Hybrid, creating the most compact hybrid transmission yet engineered by Toyota.
Generating an immediate maximum of 45 kW and 169 Nm torque during acceleration, the new electric motor has been made smaller and lighter through the optimisation of the magnet material, and the winding of the coils with flat, rather than round, wire. The motor cooling structure has also been improved, allowing for a smaller, yet more efficient and powerful motor.
The transaxle now incorporates a multifunctional motor reduction gear within the power division mechanism, and a new gear deceleration mechanism abolishes the use of chains.
Gear tooth grinding and shaving technology similar to that used on the Auris Hybrid effects a notable reduction in transmission noise and vibration. This is further lowered by the adoption of a newly designed torsional vibration damper. Its smaller diameter made possible by the use of a three spring structure. The new, lightweight damper helps suppress the transmission of engine vibrations to a minimum level.
The optimisation of lubrication both improves transaxle efficiency and minimises energy losses. A new lubrication mechanism is adopted, in which each gear and bearing is lubricated by combing up the Automatic Transmission Fluid (ATF) with the rotation of the final driven gear and counter driven gear, reducing driving force losses.
The use of a split construction oil catch tank which supplies oil for both the motor/generator and gear lubrication lowers the dynamic oil level, reducing agitation losses.
More Compact Hybrid Battery
The new, downsized full hybrid powertrain also features a smaller, 144 V nickel-metal hydride battery with a more efficient vehicle electric power management system.
The number of cells has been reduced from the 168 of the current, Auris Hybrid battery to only 120, composed of 20 modules connected in series. This effects a decrease in volume of 20% and a weight saving of 11 kg, allowing for the installation of the new battery entirely under the rear passenger seats, with no intrusion into the cargo loadspace.
In addition to this reduction in size and weight, battery charging efficiency has been improved, and the pack state-of-charge recovery time has been decreased by 67% compared to that of the Auris Hybrid.
New Power Control Unit with Smaller Inverter and Converter
Due to a reduction in hybrid system current and voltage, the system's Power Control Unit has also been made smaller and lighter through the development of a more compact inverter and switching unit.
Smaller and lighter than that fitted to the Auris Hybrid, the new inverter features a newly structured capacitor, reduced energy losses and improved cooling, enhancing fuel efficiency.
By optimising the layout of internal components, the size and weight of the DC-DC converter have also been reduced. And the composition of the noise filter has been revised to enhance unit quietness.
Compared to the Auris Hybrid, unit output current has been reduced from 120 to 80 A and output voltage lowered from 650 to 520 V, saving system energy with no loss of performance or efficiency.
Hybrid System Performance
Combining the power of a 55 kW petrol engine and battery powering the electrical motor, the Yaris Hybrid's highly efficient powertrain generates a maximum 74 kW/100 DIN hp system output. It offers the best balance of performance and fuel consumption/CO2 emissions in the B segment.
The Yaris Hybrid will accelerate seamlessly from 0-100 km/h in 11.8 seconds, and on to a maximum speed of 165 km/h. Conversely, it delivers an average fuel consumption of just 3.5 l/100 km and segment-best CO2 emissions of only 79 g/km, whilst allowing customers frequent driving in modes that emit zero NOx, PM or CO2 emissions. Its exceptional fuel efficiency gives the Yaris Hybrid a driving range of 1,000 km on a single tank of fuel.
Hybrid Synergy Drive in Operation
Over the course of any journey, Toyota's Hybrid Synergy Drive® system operates in several different modes to maximise the Yaris Hybrid's' overall efficiency. At rest, the engine stops automatically to save fuel. Under operating conditions of low engine efficiency such as start up and low to mid-range speeds, the vehicle runs on the electric motor alone, thus eradicating CO2, NOx and PM emissions.
Under normal driving conditions, power allocation is constantly adjusted between engine and electric motor to combine optimum performance with maximum fuel efficiency.
Via an Electronically Controlled Braking system, the electric motor acts as a high-output generator during deceleration and under braking to effect regenerative braking, optimising energy management in the Hybrid Synergy Drive® system by recovering kinetic energy (normally wasted as heat under braking and deceleration) as electrical energy for storage in the high performance battery.