The Toyota Prius is the world's first commercially mass-produced and marketed hybrid automobile. It went on sale in Japan in 1997, and worldwide in 2001. By the end of 2003, nearly 160,000 units had been produced for sale in Japan, Europe, and North America. In Latin, prius means "before" or "first". The Prius (2000 to 2003 model years) is certified as a Super Ultra Low Emission Vehicle (SULEV) by the California Air Resources Board (CARB). With the 2004 model, the Prius was redesigned as a midsize hatchback, and certified as an Advanced Technology Partial zero-emissions vehicle (AT-PZEV). For North America, it used to slot between the Echo and Corolla in the lineup, but now slots beween the Corolla and Camry since the second generation of this vehicle was released. Prius has won numerous awards including Car of the Year awards for Europe, Japan and North America.
How it works
The Prius has been called a true hybrid vehicle, designed from the bottom up. Toyota's goal for the Prius was to reduce the amount of pollutants it produced and to be as energy-efficient as possible. They used several methods to try to achieve this goal, including:
More efficient use of the internal combustion engine (ICE), reducing gasoline consumption. The 1NZ-FXE engine uses the more efficient Atkinson cycle instead of the more common Otto cycle;
Two electric motor/generators, providing 50 kW (67 hp) @ 1,200 to 1,540 rpm and 400 N·m (295 ft·lbf) torque from 0 to 1,200 rpm, which significantly contribute to performance & economy;
50 kW IGBT inverter controlled by a 32-bit microprocessor, which efficiently converts power between the batteries and the motor/generators.
Lower coefficient of drag at 0.26 (0.29 for 2000 model), reducing air resistance, especially at higher speeds;
Lower rolling-resistance tires on the 2000 model, reducing road friction;
Regenerative braking, a process for recovering kinetic energy when braking or traveling down a slope and storing it as electrical energy in the traction battery for later use while reducing wear and tear on the brake pads;
Sealed 168-cell nickel metal hydride (NiMH) battery providing 201.6 volts;
Continuously variable transmission — the Prius does not use a typical CVT; Toyota calls it the Power Split Device. The electric motors and gasoline engine are connected to a planetary gear set which is always engaged, and there is no shifting.
Flexible resin gasoline tank, reducing the amount of hydrocarbon emissions in the form of escaped gasoline vapor;
Vacuum flask coolant storage system that stores hot engine coolant, then reuses it to reduce warm-up time.
Weight reduction — for example the hatch and hood are made of aluminium instead of steel.
Touted advantages of the Prius over previous energy-efficient designs include never needing to be plugged in, as all energy ultimately comes from the gasoline engine. This means it drives like a traditional ICE automobile, with the onboard computer taking care of shifting power to and from the engine and motors, and automatically determining when to charge the battery, as well as the most efficient use of the engine or the electric motors (or both) based on driving conditions. This also means that one cannot use electricity from external sources; some consider this to be a serious disadvantage. The Prius recovers energy that would be otherwise wasted: when braking, the car's kinetic energy is partly recovered to recharge the battery instead of being wasted as heat. This also reduces brake wear.
The engine can shut down once it has warmed up and the catalytic converter in the exhaust system has reached operating temperature. The Prius can then be driven on electric power only; this is sometimes referred to as "stealth mode" due to the lack of engine noise. This further reduces gasoline consumption and wear and tear on the engine. When driving conditions demand additional power from the engine, it starts up automatically.
The on-board computer ensures that the engine runs under the most efficient conditions. Typically, a gasoline engine runs inefficiently when throttle is half-open, creating a choking operation conditions. This effect, called pumping loss, is a major reason for the inefficiency of gasoline engines compared to diesel engines. The Prius minimizes the pumping loss by running the gasoline engine at high torque range with throttle fully open. Drive-by-wire throttle control technology and what Toyota calls Hybrid Synergy Drive (a simple torque combiner, relatively strong electric drive, and computer control) are essential in this engine control.
As well as the immediate benefit of reducing fuel consumption and emissions, stopping the engine also improves the performance of the catalytic converter, as in a normal vehicle the exhaust gases from an idling engine tend to cool the catalysts below their optimal temperature.
Unlike in conventional automobiles, frequent starting up and shutting down of the engine causes neither additional wear and tear nor emission problems, because the drive motors have enough power to quickly and smoothly spin the engine to optimal rpm (around 1,000) before the engine actually begins to "fire up". This avoids wear when the engine is "running" (with fuel and spark) at very low rpm, as happens in most vehicles.
For any car, aerodynamic losses due to drag are much greater on the highway than in low speed city driving. A non-hybrid car nonetheless gets worse mileage in city driving because its engine is far less efficient at low power, such as when stopped in traffic, and because it must frequently dump its kinetic energy into the brakes during stop-and-go driving. The Prius gets better fuel efficiency in city dynamometer cycles because the engine can shut down instead of running at low power, and run solely off the battery at low speeds and when stopped (including the cabin air heating/cooling system and the power steering). Also, the car's kinetic energy can be captured when braking, and stored in the battery. According to EPA tests, the 2004-2006 Prius gets 60 mpg (US) in the city dynamometer tests, compared with 51 on the highway, and Natural Resources Canada estimates 58.8 mpg (US) (4.0 L/100 km) in the city and 56.0 (4.2 L/100 km) on the highway. (Owing to pecularities of these tests, few drivers obtain these mileage values in typical suburban circumstances, but "real world" performance with careful driving can come close.) This means the greatest advantages of a hybrid are mainly in city driving, though factors including driving style, air conditioning use, and short trips may offset some of this advantage. The hybrid has less of an advantage in higher speed open road driving typical of intercity driving, yet obtains improved mileage under these circumstances since it can use a smaller and more efficient engine than would otherwise be required (because the battery and electric motor can provide the extra peak power needed for passing and limited hill climbing).
2004 to 2006 Prius
The Prius 2004 model was a complete redesign of the previous generations of Prius. The new model is based on the new (second generation) Hybrid Synergy Drive (HSD or also known as THSII), replacing the earlier Toyota Hybrid System (THS) technology. The first generation (1997 to 2003) could not run its air conditioning 100% of the time unless the engine was running. While idling, the engine would need to start every minute or so if the air conditioning was running. In contrast, the 2004 model introduced an all-electric compressor for cooling. This not only allowed the use of air conditioning without the engine starting from time to time when the car was completely still, it also allowed more extensive use of the "stealth mode" (operation on electric motor only).
Its drag coefficient of 0.26 was then the second-lowest in the industry, after the smaller Honda Insight at 0.25.
Regenerative braking was greatly improved, relying so little upon the friction-type brakes (except for panic stops) that some Toyota technicians quipped that the original brake pads might well last for the life of the car.
The 2004 Prius emerged as a midsize car with more room than the previous compact, redesigned as a "five-door" model (four doors plus a rear hatch). In general the car remained conveniently narrow on the outside, rather flat sided, and relatively tall (several inches taller than a Camry). The profile was much more continuous, with a short sloping nose transitioning to a highly sloped windshield and an arcing roofline ending in a cut-off Kammback. The additional height allowed a more erect seating position and a higher eye point, giving a better view of the road to the driver. There was also a surprising amount of additional rear-seat leg room, resembling that available in a much larger vehicle. Fold down rear seats with a 60/40 split made for easy carriage of most parcels.
With a smaller and lower voltage NiMH battery and a boost converter to step the voltage up to 500 V, the 2004 model was more powerful (2 seconds faster in 0 to 96 km/h acceleration) and is 15% more fuel efficient than the previous generation Prius, with 60 miles per US gallon (4.3 L/100 km) city and 51 miles per US gallon (4.7 L/100 km) highway (according to the EPA). When driven appropriately, commuting and mixed suburban drivers are reporting fuel consumption of 45 to 50 miles per US gallon (5.2 to 4.7 L/100 km).
The multi-function-display (MFD) shows fuel consumption bars for each five minute segment of driving and this can encourage economical driving. The display also indicates instantaneous fuel consumption, which is useful for detecting when the vehicle has switched from electric-only to electric + ICE. At this time it can usually be advantageous to accelerate quickly rather than lug the vehicle, in an attempt to get to a more favorable location or speed range for electric-only mode.
An option called the Smart Key System (SKS) eliminates the traditional ignition key. The "smart key," which resembles a conventional keyless entry remote, has buttons to remotely lock and unlock the door, as well as trigger the alarm system. It is not necessary to use any of the buttons to utilize the SKS system. Approaching to within one metre of the car while in-possession of the key (even inside a pocket or purse) causes the interior dome lights to fade in (if the switches are at the DOOR position). When the person with the key in their possession touches either of the front doors, or the rear hatchback door, that door is immediately unlocked and can be opened. There are three settings in which the Smart Key System (SKS) can operate with: driver's door unlocking mode, single door unlocking mode, and all doors unlocking mode. Once inside, the car may be started by pushing the "POWER" button while depressing the brake pedal, with the fob still safely tucked away inside your pocket or purse. To stop the car, push the "POWER" button again. Once parked, the car can be locked by a pushing a black button on the exterior door handle. The SKS system will detect keys left inside the vehicle when powered-off, and will not allow the vehicle to be locked in this situation.
There is no conventional ignition switch which needs to be turned to start the Prius, that function having been replaced by a simple round "POWER" button on the dashboard. With the SKS models, the driver only needs to sit down inside the vehicle (with the fob in their possession), press the brake with their foot and push the "POWER" button to start the vehicle. On cars which are not equipped with SKS, the driver uses the lock/unlock buttons on the fob, and puts the remote fob into a slot in the dash to allow the car to start.
The vehicle is classified as a SULEV (Super Ultra Low Emissions Vehicle), 90% cleaner than conventional gasoline-only automobiles. It comes with an Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) certification by CARB (California Air Resources Board).
As an interesting side note, an option package offered in Japan and Europe gives the Prius the ability to perform parallel and reverse parking assisted by the on-board computer — the first time such a capability has been offered on a production vehicle. Eighty percent of the Prius buyers in Japan have chosen this option. The system is not intelligent though, particularly lacking machine vision which would allow avoidance of obstructions or pedestrians, so it is very limited.
Passengers of the Prius (TSpirit model only, not T3 or T4) can use their Bluetooth-enabled cellphone via the car's audio system without taking the phone out of their pocket. However, not all Bluetooth phones are compatible with the Prius.
The 2006 Prius introduced some minor cosmetic changes, along with advanced airbags, a rear-view camera, an upgraded audio system, and a new higher-resolution LCD display.
Aftermarket upgrades
Various upgrades for the Prius include extra batteries, chargers, stealth mode and solar panels.
Stealth Mode
When the vehicle is turned-on with the "start" button, it is ready to drive immediately with the electric motor, while electric pumps warm the engine with previously saved hot engine coolant, before the internal combustion engine is started. The delay between starting the car and starting the internal combustion engine is approximately seven seconds. The Asian and European versions of this vehicle provide a button labeled "EV" that maintains Electric Vehicle mode after startup, under most low-load driving conditions. The North American model does not have the "EV" button, although the "EV" mode is still supported internally by the Prius high-voltage management computer. While some have speculated that EV mode was not included on North American models in the interest of retaining the warranted battery life of 100,000 miles (160,000 km) in the US—150,000 miles (240,000 km) in California and several other states—engineers note that EV mode is automatically overridden when the battery requires recharging from the internal combustion engine. The Prius is designed to protect the battery from extreme discharge as well as over-charge and will utilize the engine as-necessary to maintain the optimum conditions for a long battery life. In addition to information at online discussion groups, the PRIUS+ Project offers instructions for do-it-yourselfers who wish to enable the button, and after-market components provider Coastal Electronic Technologies offers a kit.
Extra batteries
Evolving from the button project, The California Cars Initiative (CalCars) converted a Prius in 2004, adding larger batteries, and private companies EDrive Systems in the USA and Amberjac Projects in the UK announced plans to sell conversion kits in 2006. The most recent versions use a new, patented advanced safe-format Lithium Battery from Valence Technology (VLNC, NASDAQ) called Saphion. The energy density and far more efficient utilisation of these batteries provide around 20x the available power of the standard NiMH battery pack but at only double the weight. Tests show it capable of acheiving >30 miles (50 km) in all electric mode at speeds below 34mph (55 km/h). The internal combustion engine starts only at higher speeds or when extra acceleration is needed. Fuel economy (as demonstrated for several journalists) is improved to 100-160mpg (US) (40-68 km/liter) source article depending on driving conditions. City driving may be possible without using the ICE at all, eliminating the use of almost all liquid fuel in exchange for electrical energy.
Chargers
CalCars initiated efforts to promote the idea of plug-in hybrids to be built by automakers, and documented the emissions benefits of plug-in hybrids not only on California's clean power grid but also on the national (50% coal-fueled) power grid. This addition would be relatively ineffective without addtional battery capacity or reprogramming to more completely discharge the batteries (which would perhaps be detrimental to their service life).
Recently Toyota has announced that it intends to build a plug in hybrid vehicle at an unspecified future time.
Solar Panels
Solar panels provide additional energy to the batteries while the vehicle is in motion or otherwise away from the grid. Solar panels are also useful when the Prius is parked for extended periods of time in a sunny location if it doesn't have a charger or is not connected to a power outlet. Some drivers have claimed up to 10% mileage improvements with the addition of their solar panels, although individual results are dependent on the size and efficiency of the solar panels as well as weather conditions. Drivers in Australia and the southwestern states for instance, see the some of the largest mileage improvements on their Prius. One advantage of solar panels is that they do not wear out or depreciate like ICE components.
How it works
The Prius has been called a true hybrid vehicle, designed from the bottom up. Toyota's goal for the Prius was to reduce the amount of pollutants it produced and to be as energy-efficient as possible. They used several methods to try to achieve this goal, including:
More efficient use of the internal combustion engine (ICE), reducing gasoline consumption. The 1NZ-FXE engine uses the more efficient Atkinson cycle instead of the more common Otto cycle;
Two electric motor/generators, providing 50 kW (67 hp) @ 1,200 to 1,540 rpm and 400 N·m (295 ft·lbf) torque from 0 to 1,200 rpm, which significantly contribute to performance & economy;
50 kW IGBT inverter controlled by a 32-bit microprocessor, which efficiently converts power between the batteries and the motor/generators.
Lower coefficient of drag at 0.26 (0.29 for 2000 model), reducing air resistance, especially at higher speeds;
Lower rolling-resistance tires on the 2000 model, reducing road friction;
Regenerative braking, a process for recovering kinetic energy when braking or traveling down a slope and storing it as electrical energy in the traction battery for later use while reducing wear and tear on the brake pads;
Sealed 168-cell nickel metal hydride (NiMH) battery providing 201.6 volts;
Continuously variable transmission — the Prius does not use a typical CVT; Toyota calls it the Power Split Device. The electric motors and gasoline engine are connected to a planetary gear set which is always engaged, and there is no shifting.
Flexible resin gasoline tank, reducing the amount of hydrocarbon emissions in the form of escaped gasoline vapor;
Vacuum flask coolant storage system that stores hot engine coolant, then reuses it to reduce warm-up time.
Weight reduction — for example the hatch and hood are made of aluminium instead of steel.
Touted advantages of the Prius over previous energy-efficient designs include never needing to be plugged in, as all energy ultimately comes from the gasoline engine. This means it drives like a traditional ICE automobile, with the onboard computer taking care of shifting power to and from the engine and motors, and automatically determining when to charge the battery, as well as the most efficient use of the engine or the electric motors (or both) based on driving conditions. This also means that one cannot use electricity from external sources; some consider this to be a serious disadvantage. The Prius recovers energy that would be otherwise wasted: when braking, the car's kinetic energy is partly recovered to recharge the battery instead of being wasted as heat. This also reduces brake wear.
The engine can shut down once it has warmed up and the catalytic converter in the exhaust system has reached operating temperature. The Prius can then be driven on electric power only; this is sometimes referred to as "stealth mode" due to the lack of engine noise. This further reduces gasoline consumption and wear and tear on the engine. When driving conditions demand additional power from the engine, it starts up automatically.
The on-board computer ensures that the engine runs under the most efficient conditions. Typically, a gasoline engine runs inefficiently when throttle is half-open, creating a choking operation conditions. This effect, called pumping loss, is a major reason for the inefficiency of gasoline engines compared to diesel engines. The Prius minimizes the pumping loss by running the gasoline engine at high torque range with throttle fully open. Drive-by-wire throttle control technology and what Toyota calls Hybrid Synergy Drive (a simple torque combiner, relatively strong electric drive, and computer control) are essential in this engine control.
As well as the immediate benefit of reducing fuel consumption and emissions, stopping the engine also improves the performance of the catalytic converter, as in a normal vehicle the exhaust gases from an idling engine tend to cool the catalysts below their optimal temperature.
Unlike in conventional automobiles, frequent starting up and shutting down of the engine causes neither additional wear and tear nor emission problems, because the drive motors have enough power to quickly and smoothly spin the engine to optimal rpm (around 1,000) before the engine actually begins to "fire up". This avoids wear when the engine is "running" (with fuel and spark) at very low rpm, as happens in most vehicles.
For any car, aerodynamic losses due to drag are much greater on the highway than in low speed city driving. A non-hybrid car nonetheless gets worse mileage in city driving because its engine is far less efficient at low power, such as when stopped in traffic, and because it must frequently dump its kinetic energy into the brakes during stop-and-go driving. The Prius gets better fuel efficiency in city dynamometer cycles because the engine can shut down instead of running at low power, and run solely off the battery at low speeds and when stopped (including the cabin air heating/cooling system and the power steering). Also, the car's kinetic energy can be captured when braking, and stored in the battery. According to EPA tests, the 2004-2006 Prius gets 60 mpg (US) in the city dynamometer tests, compared with 51 on the highway, and Natural Resources Canada estimates 58.8 mpg (US) (4.0 L/100 km) in the city and 56.0 (4.2 L/100 km) on the highway. (Owing to pecularities of these tests, few drivers obtain these mileage values in typical suburban circumstances, but "real world" performance with careful driving can come close.) This means the greatest advantages of a hybrid are mainly in city driving, though factors including driving style, air conditioning use, and short trips may offset some of this advantage. The hybrid has less of an advantage in higher speed open road driving typical of intercity driving, yet obtains improved mileage under these circumstances since it can use a smaller and more efficient engine than would otherwise be required (because the battery and electric motor can provide the extra peak power needed for passing and limited hill climbing).
2004 to 2006 Prius
The Prius 2004 model was a complete redesign of the previous generations of Prius. The new model is based on the new (second generation) Hybrid Synergy Drive (HSD or also known as THSII), replacing the earlier Toyota Hybrid System (THS) technology. The first generation (1997 to 2003) could not run its air conditioning 100% of the time unless the engine was running. While idling, the engine would need to start every minute or so if the air conditioning was running. In contrast, the 2004 model introduced an all-electric compressor for cooling. This not only allowed the use of air conditioning without the engine starting from time to time when the car was completely still, it also allowed more extensive use of the "stealth mode" (operation on electric motor only).
Its drag coefficient of 0.26 was then the second-lowest in the industry, after the smaller Honda Insight at 0.25.
Regenerative braking was greatly improved, relying so little upon the friction-type brakes (except for panic stops) that some Toyota technicians quipped that the original brake pads might well last for the life of the car.
The 2004 Prius emerged as a midsize car with more room than the previous compact, redesigned as a "five-door" model (four doors plus a rear hatch). In general the car remained conveniently narrow on the outside, rather flat sided, and relatively tall (several inches taller than a Camry). The profile was much more continuous, with a short sloping nose transitioning to a highly sloped windshield and an arcing roofline ending in a cut-off Kammback. The additional height allowed a more erect seating position and a higher eye point, giving a better view of the road to the driver. There was also a surprising amount of additional rear-seat leg room, resembling that available in a much larger vehicle. Fold down rear seats with a 60/40 split made for easy carriage of most parcels.
With a smaller and lower voltage NiMH battery and a boost converter to step the voltage up to 500 V, the 2004 model was more powerful (2 seconds faster in 0 to 96 km/h acceleration) and is 15% more fuel efficient than the previous generation Prius, with 60 miles per US gallon (4.3 L/100 km) city and 51 miles per US gallon (4.7 L/100 km) highway (according to the EPA). When driven appropriately, commuting and mixed suburban drivers are reporting fuel consumption of 45 to 50 miles per US gallon (5.2 to 4.7 L/100 km).
The multi-function-display (MFD) shows fuel consumption bars for each five minute segment of driving and this can encourage economical driving. The display also indicates instantaneous fuel consumption, which is useful for detecting when the vehicle has switched from electric-only to electric + ICE. At this time it can usually be advantageous to accelerate quickly rather than lug the vehicle, in an attempt to get to a more favorable location or speed range for electric-only mode.
An option called the Smart Key System (SKS) eliminates the traditional ignition key. The "smart key," which resembles a conventional keyless entry remote, has buttons to remotely lock and unlock the door, as well as trigger the alarm system. It is not necessary to use any of the buttons to utilize the SKS system. Approaching to within one metre of the car while in-possession of the key (even inside a pocket or purse) causes the interior dome lights to fade in (if the switches are at the DOOR position). When the person with the key in their possession touches either of the front doors, or the rear hatchback door, that door is immediately unlocked and can be opened. There are three settings in which the Smart Key System (SKS) can operate with: driver's door unlocking mode, single door unlocking mode, and all doors unlocking mode. Once inside, the car may be started by pushing the "POWER" button while depressing the brake pedal, with the fob still safely tucked away inside your pocket or purse. To stop the car, push the "POWER" button again. Once parked, the car can be locked by a pushing a black button on the exterior door handle. The SKS system will detect keys left inside the vehicle when powered-off, and will not allow the vehicle to be locked in this situation.
There is no conventional ignition switch which needs to be turned to start the Prius, that function having been replaced by a simple round "POWER" button on the dashboard. With the SKS models, the driver only needs to sit down inside the vehicle (with the fob in their possession), press the brake with their foot and push the "POWER" button to start the vehicle. On cars which are not equipped with SKS, the driver uses the lock/unlock buttons on the fob, and puts the remote fob into a slot in the dash to allow the car to start.
The vehicle is classified as a SULEV (Super Ultra Low Emissions Vehicle), 90% cleaner than conventional gasoline-only automobiles. It comes with an Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) certification by CARB (California Air Resources Board).
As an interesting side note, an option package offered in Japan and Europe gives the Prius the ability to perform parallel and reverse parking assisted by the on-board computer — the first time such a capability has been offered on a production vehicle. Eighty percent of the Prius buyers in Japan have chosen this option. The system is not intelligent though, particularly lacking machine vision which would allow avoidance of obstructions or pedestrians, so it is very limited.
Passengers of the Prius (TSpirit model only, not T3 or T4) can use their Bluetooth-enabled cellphone via the car's audio system without taking the phone out of their pocket. However, not all Bluetooth phones are compatible with the Prius.
The 2006 Prius introduced some minor cosmetic changes, along with advanced airbags, a rear-view camera, an upgraded audio system, and a new higher-resolution LCD display.
Aftermarket upgrades
Various upgrades for the Prius include extra batteries, chargers, stealth mode and solar panels.
Stealth Mode
When the vehicle is turned-on with the "start" button, it is ready to drive immediately with the electric motor, while electric pumps warm the engine with previously saved hot engine coolant, before the internal combustion engine is started. The delay between starting the car and starting the internal combustion engine is approximately seven seconds. The Asian and European versions of this vehicle provide a button labeled "EV" that maintains Electric Vehicle mode after startup, under most low-load driving conditions. The North American model does not have the "EV" button, although the "EV" mode is still supported internally by the Prius high-voltage management computer. While some have speculated that EV mode was not included on North American models in the interest of retaining the warranted battery life of 100,000 miles (160,000 km) in the US—150,000 miles (240,000 km) in California and several other states—engineers note that EV mode is automatically overridden when the battery requires recharging from the internal combustion engine. The Prius is designed to protect the battery from extreme discharge as well as over-charge and will utilize the engine as-necessary to maintain the optimum conditions for a long battery life. In addition to information at online discussion groups, the PRIUS+ Project offers instructions for do-it-yourselfers who wish to enable the button, and after-market components provider Coastal Electronic Technologies offers a kit.
Extra batteries
Evolving from the button project, The California Cars Initiative (CalCars) converted a Prius in 2004, adding larger batteries, and private companies EDrive Systems in the USA and Amberjac Projects in the UK announced plans to sell conversion kits in 2006. The most recent versions use a new, patented advanced safe-format Lithium Battery from Valence Technology (VLNC, NASDAQ) called Saphion. The energy density and far more efficient utilisation of these batteries provide around 20x the available power of the standard NiMH battery pack but at only double the weight. Tests show it capable of acheiving >30 miles (50 km) in all electric mode at speeds below 34mph (55 km/h). The internal combustion engine starts only at higher speeds or when extra acceleration is needed. Fuel economy (as demonstrated for several journalists) is improved to 100-160mpg (US) (40-68 km/liter) source article depending on driving conditions. City driving may be possible without using the ICE at all, eliminating the use of almost all liquid fuel in exchange for electrical energy.
Chargers
CalCars initiated efforts to promote the idea of plug-in hybrids to be built by automakers, and documented the emissions benefits of plug-in hybrids not only on California's clean power grid but also on the national (50% coal-fueled) power grid. This addition would be relatively ineffective without addtional battery capacity or reprogramming to more completely discharge the batteries (which would perhaps be detrimental to their service life).
Recently Toyota has announced that it intends to build a plug in hybrid vehicle at an unspecified future time.
Solar Panels
Solar panels provide additional energy to the batteries while the vehicle is in motion or otherwise away from the grid. Solar panels are also useful when the Prius is parked for extended periods of time in a sunny location if it doesn't have a charger or is not connected to a power outlet. Some drivers have claimed up to 10% mileage improvements with the addition of their solar panels, although individual results are dependent on the size and efficiency of the solar panels as well as weather conditions. Drivers in Australia and the southwestern states for instance, see the some of the largest mileage improvements on their Prius. One advantage of solar panels is that they do not wear out or depreciate like ICE components.