Series – Parallel Hybrid Electric Vehicle / Power Split HEV

Power-split hybrid or series-parallel hybrids are parallel hybrids. They incorporate power-     split devices allowing for power paths from the engine to the wheels that can be either mechanical or electrical. The main principle behind this system is the decoupling of the power supplied by the engine (or other primary source) from the power demanded by the driver.

A combustion engine's torque is minimal at lower RPMs and in a conventional vehicle, a larger engine is necessary for acceleration from standstill. The larger engine, however, has more power than needed for steady speed cruising. An electric motor, on the other hand, exhibits maximum torque at standstill and is well-suited to complement the engine's torque deficiency at low RPMs. In a power-split hybrid, a smaller, less flexible, and highly efficient engine can be used. The conventional Otto cycle (higher power density, more low-rpm torque, lower fuel efficiency) is often  modified to a Miller cycle or Atkinson cycle (lower power density, less low-rpm torque, higher fuel efficiency). The smaller engine using a more efficient cycle contributes significantly to the higher overall efficiency of the vehicle.

Toyota Prius is a series-parallel hybrid, technology used for Prius is called Hybrid Synergy Drive by Toyota. It is having a single power-split device, since the power of the engine is split at the input to the transmission so it is known as input-split. General Motors, BMW, and DaimlerChrysler have developed in collaboration a system named "Two-Mode Hybrid" as part of the Global Hybrid Cooperation. The technology was released in 2007 on the Chevrolet Tahoe Hybrid. The Two-Mode Hybrid can operate in two power-split modes, the design can be referred as a multi-regime design. The Two-Mode Hybrid powertrain design can be classified as a compound-split design, the addition of four clutches within the transmission allows for multiple configurations of engine power-splitting. The objective of the design is to vary the percentage of mechanically vs. electrically transmitted power to cope both with low-speed and high-speed operating conditions. This enables smaller motors to do the job of larger motors when compared to single-mode systems. The four fixed gears enable the Two-Mode Hybrid to function like a conventional parallel hybrid under high continuous power regions such as sustained high speed cruising or trailer towing. 

Parallel Hybrid Electric Vehicle

In parallel hybrid electric vehicle, the electric motor and the internal combustion engine (ICE) are installed so that they can both individually or together power the vehicle.  

The drive train for a parallel hybrid is more complex than that of a series hybrid as both the electric motor and the APU must be mechanically linked to the driveshaft. Since parallel hybrids only works with APU’s that produce a mechanical output, fuel cells cannot be used for this option. In the parallel hybrid configuration, an APU capable of producing motive force is mechanically linked to the drive train. This approach eliminates the generator of the series approach. Then the APU is on, the controller divides energy between the drive train (propulsion) and the batteries (energy storage). The amount of energy divided between the two is determined by the speed and driving pattern. When the vehicle is under acceleration, more power is allocated to the drive train than to the batteries. During periods of idle or low speeds, more power goes to the batteries than the drive train.

In parallel hybrid electric vehicle, in addition to the battery drive, transmission is required to allow the engine to drive the wheels. Moreover there must be a means of coupling the engine, motor and transmission. Further, the batteries in parallel hybrids can be recharged through regenerative braking. Since, parallel drive trains typically use small battery packs, much of the recharging can be done by regenerative breaking.  The main advantage of this configuration is the ability to turn the engine off during idling.

Since motor speed is linked to the engine, the vehicle cannot operate in electric mode other than for extremely low speed. When the APU is off, the parallel hybrid runs like an electric vehicle. The batteries provide electricity to the electric motor which drives the vehicle. The batteries also provide additional power to the drive train when the APU is not producing enough power.

Most designs combine a large electrical generator and a motor into one unit, often located between the combustion engine and the transmission, replacing both the conventional starter motor and the alternator. Accessories such as power steering and air conditioning are powered by electric motors instead of being attached to the combustion engine. This arrangement is good for hiway driving in comparison of city driving. Examples of parallel hybrid electric vehicles are Honda’s Insight, Civic, Accord and Saturn VUE Aura Greenline a, Chevrolet Malibu hybrids etc. The schematic diagram of Parallel Hybrid Vehicle is given in Fig. (a), (b). 

Series Hybrid vehicles

A series hybrid is similar to an electric vehicle with an on- board generator i.e. range extended electric vehicle. The vehicle runs on battery power like a pure electric until the batteries reach a predetermined discharged level.  In a series hybrid electric vehicle, an electric motor is the only means of driving the wheels. The motor obtains electricity either from a battery pack or from a generator powered by an engine (APU-Additional power Unit). The engine does not need to speed up or slow down as load varies. As a consequence, the engine can run at its optimum efficiency (best engine efficiency zone), thereby greatly reducing fuel consumption. To gain most advantage in efficiency from using a small engine, series drive train typically use relatively large battery packs. But batteries and motor cost more than engines for the same amount of power, so series hybrids are more expensive. The generator needed to produce electricity from the engine also adds to the cost. Series hybrids are advantageous under slower operating conditions characterizes by stop –and- go driving.  For this reason, most of the serried hybrids currently under development are for buses or other heavy duty urban vehicles. The batteries in a series hybrid are recharged both by the engine/generator set and by storing some of the energy that is normally lost during braking. Examples of this kind of arrangement are Series-Hybrid bus by Toyota in JAPAN, General Motor’s Chevy Volt, AFS Trinity SUV etc. The schematic diagram of Series Hybrid Vehicle is given in Fig. (a), (b).

Advantages and Disadvantages of Battery Electric Vehicle

Air pollution -EVs do not produce any tailpipe emissions. They are cleaner than conventional vehicles, even when power-plant emissions are included. If battery recharges using renewable energy sources like wind, solar, or hydropower, they cause no air pollution at all.

Global-warming pollution - As with air pollution, electric power plants are the only source of global-warming pollutants from EVs. When EVs recharge using renewable energy sources, they do not cause any global warming emissions at all. Even if EVs are recharged using fossil fuels, they can cut global warming emissions reasonably.

Costs - EVs cost significantly more than gasoline vehicle, mostly because the battery packs. Higher vehicle prices are partially offset by the fact that fuel costs for battery electrics are about one-third those of a gasoline-powered vehicle. And because EVs have fewer moving parts than gasoline cars, they require less maintenance.

Performance - To the driver, EVs offers a quiet, smooth, and high-performance driving experience. The first-generation EVs had a real-world driving range of 50 to 80 miles. Battery advances now permit EVs to travel over a 100 miles on a charge, with future increases still possible.

Battery Electric Vehicles - Some models

Mahindra Reva Electric Vehicles Private Limited, formerly known as the REVA Electric Car Company (RECC), is an Indian company based in Bangalore, designing and manufacturing electric vehicles. The REVA Electric Car Company was a joint venture between Maini Group of India and AEV LLC of California and venture backed by lead US investors Global Environment Fund and Draper Fischer Jurvetson. It is manufacturing the very popular electric vehicle REVA. It is an urban electric micro-car seating two adults and two children. RECC currently produces two versions of the REVA one is REVAi equipped with lead-acid batteries, which has a nominal range of 80 km (50 mi) per charge and a top speed of 80 km/h (50 mph). Another model is REVA L-ion, equipped with Lithium-ion batteries, which has range of 120 km (75 mi) per charge. The REVA has been sold in India since 2001 and in the UK since 2003. RECC also built REVA-NXR & REVA-NXG electric cars. The production of NXR is expected in 2011 and of NXG in 2012. NXR has a top speed of 104 km/h (65 mph) and a range of 160 km (99 mi) and NXG has range of 200 km (124 mi) per charge and a top speed of 120 km/h (75 mph). RECC has constructed a 30,000 capacity assembly plant in Bangalore. It is currently the world's largest operational plant specially dedicated to the assembly of battery electric vehicles. 

Nisan is working on electric car named as Nissan Leaf (Leading Environmentally Friendly Affordable Family Car). Limited sale of its will start in 2010-11 but its full production is expected in 2012.Its expected all-electric range is 100 miles (160 km) on the EPA city driving cycle. The Leaf uses an 80 kW (110 hp) 280 N-m (210 lb-fts) synchronous motor which is powered by a 24 kilowatt-hours (86 MJ) lithium ion battery pack rated to deliver power up to 90 kW (120 hp). It is having an on-board 3.3 kW charger and 7.5 m (25 ft) cable. Leaf can be fully recharged from empty in less than 20 hours from a standard household outlet (120 volt, 15 amps). It can be charged in 8 hours from a 220/240 volt 30 amp supply. Using quick charging it can be charged to 80% capacity in about 30 minutes, Nissan has developed its own 500 V quick charger. It can go upto the speed of 90 mph. It is also having feature of regenerative braking.

In June 2009 BMW began field testing in the U.S. of its all-electric car Mini E through the leasing of 500 cars to private users. Another field test was launched in the U.K. in December 2009, where more than forty Mini E cars were handed to private users for a two consecutive six-month field trial periods. Field testing in Paris is scheduled to begin in 2010. A total of 100 trial vehicles were assigned to Germany for field testing, which is scheduled to begin in Munich in September 2010. Field testing in Beijing is scheduled to begin in 2010. This car is driven by a synchronous electric motor of 150 kW (204 hp) 220 N-m (160 lb-ft). The Mini E employs a lithium-ion battery pack with an overall capacity of a 35 kilowatt-hours (130 MJ). The car’s range is 156 miles (251 km) on a single charge under optimal conditions. Top speed is electronically limited to 95 mph (153 km/h). Estimates of normal driving conditions put ranges at 109 miles (175 km) city and 96 miles (154 km) highway. The Mini E can be charged through 120-volt 12 amp supply in approximately 26.5 hours. It can be charged with 240-volt 32amp supply in approximately 4.5 hours or in 3 hours with 240 volt 48 amp supply.

Renault is working on an electric car Renault Fluence Z.E., it is expected to launch in 2011. It is fitted with a 22 kWh lithium-ion battery which allows a total all-electric range of 160 km (99 mi) according to Renault, with speeds up to 135 km/h (84 mph). Battery can be charged by 16 A 220 V house hold supply in 6-8 hours or at fast charging station by using 32A 400V supply in approximately 30 mins. It is having synchronous electric motor of peak power 70 kW (94 hp) at 11,000 rpm and maximum torque of 226 N-m. 

The Tesla Model S is an electric car developed by Tesla Motors. The prototype vehicle was displayed on March 26, 2009. Production for the retail market is expected to begin in early 2012. The base model will have a range of 160 miles (260 km) with a 42 kW·h Lithium ion battery pack when fully charged, and a 0 to 60 mph (0 to 97 km/h) acceleration of 5.6 seconds. There will also be larger battery packs available of 65 kW·h & 85 kW·h with ranges of 230 and 300 miles (370 and 480 km). Normal charging times will be 3 to 5 hours, depending on the battery capacity and a 45-minute Quick Charge will be possible when connected to a 480 V outlet.  Top speed will be 195 kmph.

The Mitsubishi i MiEV (Mitsubishi innovative Electric Vehicle) is an electric car launched for sales in Japan in April 2010 and in Hong Kong in May 2010. Sale of i MiEV in UK and USA is expected in 2011. The car has a range of 130 kilometres (80 mi) for the 16 kW·h lithium-ion pack and 160 kilometres (100 mi) for the 20 kW·h pack with Japanese 10-15 mode driving pattern. Top speed is 130 kilometres per hour (80 mph). It is having permanent magnet synchronous motor of 47 kW 180 N-m. It can be fully charged with on board charger by house hold supply of 200v 15 A in 7 hours and with 100V 15 A in 15 hours. This can also be quickly charged for 80% charging level by quick charging system of 3-phase 200v-50KW supply in 30 minutes only.