Monday, May 17, 2010

Design and technology



The Toyota Prius Hybrid Synergy Drive is a series-parallel full hybrid, sometimes referred to as a combined hybrid.

The Prius is a power-split or series-parallel (full) hybrid, sometimes referred to as a combined hybrid, a vehicle that can be propelled by gasoline and/or electric power. Components of the system include:

  1. Regenerative braking, using motor-generators, which converts kinetic energy of motion into electrical energy that is stored in the traction battery, giving better braking performance and reducing wear on the brake pads;
  2. A 1NZ-FXE internal combustion engine (ICE) using Atkinson cycle which is 12-14%more efficient than the more powerful Otto cycle. The gasoline engine normally shuts off during traffic stops and the accessories (including the air conditioning) are powered by the battery pack. The engine is used both to propel the vehicle and to recharge the batteries. Because of the availability of extra power from the electric motors for rapid acceleration (at lower speeds), the engine is sized smaller than usual for increased fuel efficiency and lowered emissions with acceptable acceleration (at lower speeds). The penalty of the Atkinson cycle is low specific power, so the battery and generator are needed to give extra power when accelerating, whilst allowing the vehicle to cruise on the low powered engine;
  3. Two electric motor/generators: MG1 and MG2. MG1, reversible and up to 10,000 rpm, starts the engine and provides counter torque for the Continuously Variable Transmission (CVT). MG2 provides 50 kW (67 hp) between 1,200 to 1,540 rpm and 400 N·m (295 lb·ft) torque between 0 to 1,200 rpm, contributing to performance and economy. These motor-generators are used as generators when in regenerative braking mode, and as motors are capable of powering the vehicle at low speed (<~30 mph (48 km/h), 41 mph (66 km/h) in NHW20) travel. Each electric motor requires 1 kilogram (2.2 pounds) of neodymium;[105]
  4. A Hybrid Synergy Drive (HSD) unit that combines a planetary gearset that behaves like a Continuously Variable Transmission (CVT) called the Power Split Device to increase efficiency. The computer controlled HSD transaxle adjusts and blends the amount of power from the gasoline engine and electric motor-generator(s) as needed by the front drive wheels and rechargeable batteries.
  5. A sealed 38-module nickel metal hydride (NiMH) battery pack providing 273.6 volts, 6.5 A·h capacity and weighing 53.3 kg (118 lb) is supplied by Japan's Panasonic. They are normally charged to 40–60% of maximum capacity to prolong battery life as well as provide a reserve for regenerative braking. Each battery uses 10 to 15 kilograms (22 to 33 pounds) of lanthanum;
  6. Wind resistance is reduced by a drag coefficient of 0.25 (0.29 for 2000 model) with a Kammback design to reduce air resistance. Lower rolling-resistance tires are used to reduce road friction;
  7. A vacuum flask for storing hot coolant when the vehicle is powered off for reuse so as to reduce warm-up time (US market);
  8. An EV mode that allows drivers to operate on electric power only under low-power conditions for a limited duration of time.
  9. Vehicle weight is reduced by the use of a flexible resin gasoline tank and by the use of aluminum instead of steel for the engine bay hood and hatchback;
  10. Starting with the 2010 model (3rd generation), the new 1.8 liter 2ZR-FXE gasoline engine will feature advanced technologies such as the Atkinson cycle, a cooled EGR (Exhaust Gas Recirculation) system, intake-side VVT-i, and an electric water pump, permitting an engine design with no serpentine belts, improving efficiency.

inverter unit

The computer program used in the Prius shuts off the engine when the car is stopped, is reversing, or is descending hills, thereby dramatically decreasing fuel consumption in city driving. The HSD's on-board computer program determines when and how to use the engine, motor, or both to power the car and recharge the battery so as to maximize efficiency. Typically, a gasoline engine runs inefficiently at half-throttle, creating a choking condition called pumping loss, a major reason for the inefficiency of gasoline engines compared to diesels. The Prius minimizes pumping loss by using a high torque range as much as possible with the throttle fully open. Drive-by-wire throttle control technology and Toyota's Hybrid Synergy Drive (a torque combiner, electric drive, and computer control) are essential to this engine control. In addition to the immediate benefit of reducing fuel consumption and emissions, stopping the gasoline engine also improves the performance of the catalytic converter, as the exhaust gases from an idling engine tend to cool the catalyst below its optimal temperature.


Energy monitor on the 2005 Prius multi-function display (MFD), showing energy flow to/from the engine, battery, and from the regenerative braking, as well as battery charge level.

Hybrid Synergy Drive

Typical gasoline- or diesel-engined vehicles consume fuel when the vehicle is stopped, as is common in city driving. The Prius typically shuts down the gasoline engine and runs solely off the battery pack at low speeds and when stopped. A non-hybrid car also gets worse fuel consumption in city driving because its engine is continually used to accelerate the car to driving speed and then this energy is lost to friction brakes during stop-and-go driving. The Prius recaptures some of this energy by using regenerative braking to charge its battery pack while slowing down with its motor-generators in generator mode. For acceptable acceleration, the internal combustion engine on a "standard" car typically is much larger than needed for constant-speed travel, and is a relatively high power-to-weight ratio Otto cycle engine. By using both an Atkinson cycle gasoline engine, which whilst more efficient has a much poorer power to weight ratio, and an electric motor for acceleration, the Prius engine can nevertheless be a smaller size and still provide acceptable acceleration. This means such a hybrid can have high fuel efficiency (but slower top speed than comparable non-hybrid vehicles) when using the Atkinson cycle engine for cruising or city driving, though factors including driving style, air conditioning use, and short trips may offset some of this advantage. The frequent starting and stopping of the Prius engine does not cause noticeable additional wear or emission problems because the electric drive motors have enough power to quickly spin the engine to optimal speed (around 1,000 rpm) before fuel is admitted.


Energy screen

For any car, aerodynamic losses, which vary roughly as the velocity squared due to air drag are much greater on the highway than in low speed city driving. The Prius hybrid has less of an advantage in higher speed open road driving, typical of intercity driving. Nevertheless, the Prius still obtains improved fuel consumption under these circumstances since it can use a smaller and more efficient engine than would otherwise be required. The electric motor(s), powered by the battery pack, can provide the extra peak power needed for acceleration, passing and limited hill climbing. With low weight and the lowest drag coefficient of a production car in the U.S. market, the Prius is a very efficient highway traveler, getting 48 mpg-US (4.9 L/100 km; 58 mpg-imp) on the highway according to the U.S. EPA.

The environmental impact of the car's batteries has been questioned, though Toyota offers a $200 "bounty" for recycled batteries to try and prevent any toxins from entering the environment. An independent commodities consultant called the Prius "the biggest user of rare earths of any object in the world."[111] To encourage battery recycling, dealers are paid a US$200 fee for return of the car's batteries.[112] Toyota has laboratory reports that some Prius battery packs have lasted the equivalent of 180,000 miles (290,000 km).[107] As the cars start to age, early reports have shown that in at least some cases the batteries can last in excess of 250,000 miles (400,000 km).[113] Individual battery longevity will vary depending upon the treatment and use history of the battery pack. A degraded battery pack will reduce performance and fuel economy, but not leave the car inoperable.

EV 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[citation needed], before the internal combustion engine is started. The delay between starting the car and starting the internal combustion engine is approximately seven seconds. A button labeled "EV" maintains Electric Vehicle mode after start up and under most low-load driving conditions. This permits driving with low noise and no fuel consumption, and is advertised as a "quiet" option for short journeys, for example in residential areas at night, in the Asia manual.[citation needed] The car automatically reverts to normal mode if the battery becomes exhausted. Prior to the 2010 model, the North American model did not have the "EV" button, although the "EV" mode is still supported internally by the Prius Hybrid Vehicle management computer.[citation needed]

Fuel consumption


Monitor on a 2005 Toyota Prius MFD, displaying accumulative fuel economy for 59 miles (95 km) since last gasoline fill-up.

Fuel consumption, or fuel-efficiency in cars is generally measured as distance travelled per unit volume of fuel, as in miles per gallon (mpg), or as the volume of fuel required to cover a given distance, as in liters per 100 kilometres (L/100 km). Diesel fuel does have about an 11% higher energy density than gasoline, so that needs to be taken into account in pure energy-efficiency calculations and comparisons. Fuel-price-efficiency calculations will vary between various parts of the world, with diesel fuel pump prices being higher than gasoline prices in some countries, whilst in others it is less expensive than gasoline.[114]

Official government data

United States

In the United States, the Environmental Protection Agency (EPA) test results must be posted on new vehicle windows, and are the only fuel consumption figures that can be advertised. EPA testing procedures for all vehicles were revised in 2007 in an attempt to better match what the average driver would achieve.[115] The following data is taken from these figures:[116]

2010 Prius:

  • 51 mpg-US (4.6 L/100 km; 61 mpg-imp) city driving
  • 48 mpg-US (4.9 L/100 km; 58 mpg-imp) highway driving
  • 50 mpg-US (4.7 L/100 km; 60 mpg-imp) combined

2005 Prius:

  • 48 mpg-US (4.9 L/100 km; 58 mpg-imp) city driving
  • 45 mpg-US (5.2 L/100 km; 54 mpg-imp) highway driving
  • 46 mpg-US (5.1 L/100 km; 55 mpg-imp) combined

2001 Prius:

  • 42 mpg-US (5.6 L/100 km; 50 mpg-imp) city driving
  • 41 mpg-US (5.7 L/100 km; 49 mpg-imp) highway driving
  • 41 mpg-US (5.7 L/100 km; 49 mpg-imp) combined

The Prius is the most efficient car powered by liquid fuel available in the U.S. in 2009, based on the official rating.[1] Only the first-generation Honda Insight equipped with a manual transmission attained higher mileage.

United Kingdom

The official fuel consumption data, provided by the Department for Transport, rates the Prius as: 56.5 mpg-imp (5.00 L/100 km; 47.0 mpg-US) urban, 67.3 mpg-imp (4.20 L/100 km; 56.0 mpg-US) extra urban and 65.7 mpg-imp (4.30 L/100 km; 54.7 mpg-US) combined.[117] Eleven diesel-powered vehicles with manual transmissions, most of them smaller cars have better fuel-efficiency (by fuel volume) than the second generation Prius for combined use.[118][119]

The third generation Prius is rated at 72.4 mpg-imp (3.90 L/100 km; 60.3 mpg-US) combined. Ten diesel powered cars match or exceed that figure.[118]

Independent comparison tests

Several organizations have tested the fuel economy of the Prius and compared it directly with other cars.

Consumer tests have shown that gasoline-electric hybrid cars in general are a little more fuel-efficient (by fuel volume) than top of the range manual transmission diesels.[120] However, diesel fuel has a significantly higher energy density than the gasoline used by the Prius, and in some countries diesel is less expensive than gasoline, so overall diesel with manual transmission may be competitive with hybrid technology in some cases.[114]

  • January 2010: Road and Track compared the redesigned Prius with the Volkswagen Golf TDI and Ford Fiesta on seven separate routes, and twice comparing the vehicles at a 70 mph (110 km/h) cruising speed either with or without the air conditioning. The Prius returned the highest mileage in all the tests with an overall average of 54.5 mpg-US (4.3 L/100 km; 65.5 mpg-imp) over 336 miles (541 km), the Golf returned 42.7 mpg-US (5.5 L/100 km; 51.3 mpg-imp) and the Fiesta 34.0 mpg-US (6.9 L/100 km; 40.8 mpg-imp).[121]
  • January 2009: Edmunds.com compared the 2009 Prius to the all-new 2010 Insight. The Prius returned 54.4 mpg-US compared to the Insight's 51.5 mpg-US.[123]
  • September 2008: A Popular Mechanics comparison shows that the 2009 Jetta TDI diesel has slightly better highway fuel economy than the 2008 Prius with 45.4 mpg-US compared to 44.8 mpg-US. The Prius did better than the Jetta in city fuel economy, however: 44.7 mpg-US compared to 32 mpg-US.[124]
  • August 2008: Edmunds.com test of Prius and Honda Fit, resulted in the Prius averaging 42 mpg-US (5.6 L/100 km; 50 mpg-imp). While the compact-sized non-hybrid Fit averaged 28 mpg-US (8.4 L/100 km; 34 mpg-imp).[125]
  • June 2009: The Prius is the most fuel efficient car that Consumer Reports has tested[126] since the discontinuation of the 3-door Honda Insight. Consumer Reports recorded 44 mpg-US (5.3 L/100 km; 53 mpg-imp) over its tests simulating real-world driving conditions for the Prius.[127]
  • May 2008: The Prius was the overall winner of the Edmunds.com "Gas-Sipper Smackdown" fuel economy test, winning three of the five tests including lowest overall fuel costs. The 2005 Jetta TDI won the remaining two tests, but factoring in the higher cost of diesel the Jetta ranked third in fuel costs behind both the Prius and Smart Fortwo. The other car tested was the North American Ford Focus.[128]
  • March 2008: The UK's Sunday Times did a comparison of the Prius and the BMW 520d SE with a 177 bhp (132.0 kW) diesel engine, 6-speed manual transmission and regenerative braking, which averaged 50.3 mpg-imp (5.62 L/100 km; 41.9 mpg-US) whilst the Prius averaged 48.1 mpg-imp (5.87 L/100 km; 40.1 mpg-US) over exactly the same route. The comparison was performed on a journey of 545 miles (877 km) from London to Geneva, including 100 miles (160 km) of urban driving, about 200 miles (320 km) of driving on rural roads, and about 200 miles (320 km) of motorway driving at speeds of up to 78 miles per hour (126 km/h) on one French autoroute.[129]
  • August 2007: UK's Auto Express magazine performed independent fuel efficiency tests, on public roads, on a number of hatchbacks, and in August 2007 published their list of the ten most efficient. The Prius achieved 10th place in the list, returning 41.5 mpg-imp (6.81 L/100 km; 34.6 mpg-US). A Citroën C4 Coupé 1.6 HDi got 1st place with 49.6 mpg-imp (5.70 L/100 km; 41.3 mpg-US).[130]
  • Motor Trend conducted a test comparing the Prius with a Civic Hybrid. The Prius had the better fuel economy with 43.8 mpg-US (5.37 L/100 km; 52.6 mpg-imp) compared to 39.9 mpg-US (5.90 L/100 km; 47.9 mpg-imp) for the Civic.[131]
  • December 2005: Edmunds.com test of the Prius and Civic Hybrid resulted in the Prius outperforming its competitor by an additional 4.5 mpg-US (52 L/100 km; 5.4 mpg-imp) at 48.3 mpg-US (4.87 L/100 km; 58.0 mpg-imp).[132]
  • February 2005: In a Popular Mechanics magazine diesel versus hybrid comparison, the Prius outperformed the Jetta TDI diesel in both city and highway fuel economy.[133][134]
  • September 2004: Car and Driver tested the Prius, the Civic Hybrid, Echo, and Jetta TDI over three separate routes. The Prius yielded the best mileage on the highway and town routes, while tying the Echo for top spot on the suburban route.[135]

Air pollution

In the United States the EPA measures a vehicle's air-borne pollution based on hydrocarbons, nitrogen oxides, carbon monoxide, particulate matter and formaldehyde before assigning them a score.[136] In most states the NHW20 Prius is rated Tier II Bin 3.[137] CARB also does its own emission scoring, the NHW20 Prius meets AT-PZEV certification in California and states that adopted CARB emission rules.[138]

CO2 emission

According to the UK Department for Transport, the third-generation Prius launched in the UK in August 2009 emits 89 g/km, overtaking other cars except the two seat Smart fortwo with its 0.8 liter turbocharged 45 hp diesel which emits 88 g/km.[139]

The previous Prius tied with the MINI Cooper D as the fourth least CO2-emitting vehicles at 104 g/km, behind the Ford Fiesta Econetic at 98 g/km and the tied Volkswagen Polo 1.4 TDI/SEAT Ibiza 1.4 TDI at 99 g/km - it is the least CO2-emitting gasoline powered car, since the Ford and the Polo/Ibiza (both of which are city cars) run on diesel.

Criticisms

CO2 advertising claims

In the UK, the Advertising Standards Authority, an independent body charged with policing the rules of the advertising industry, ruled that a television advert for the Toyota Prius should not be broadcast again in the same form, having breached rules concerning misleading advertising. The advert claimed the Prius "emits up to one tonne less CO2 per year," while on-screen text included: "1 tonne of CO2 less than an equivalent family vehicle with a diesel engine. Average calculated on 20,000 km a year." Points of contention were the vehicles chosen for comparison, whether "up to one tonne less" adequately communicated that reductions could be lower, and whether the distance used was appropriate: 20,000 km per year is around a U.S. car's average annual driving distance, while a UK car's is 13,440 km.[140]

An earlier complaint to the New Zealand Advertising Standards Authority, decision 03/360 2003 was also upheld. It was ruled that Toyota was in breach of the New Zealand advertising standards code of ethics, by claiming "up to 89% fewer smog forming emissions”. The figure was based on comparison with Euro 3 standards, which most new cars also met comfortably at the time.[citation needed]

Lifetime energy usage

The British Government and British media have requested Toyota release detailed figures for the energy use and CO2 emissions resulting from the building and disposal of the Prius. The UK Government Car Service runs 130 Priuses. Toyota has not supplied the detailed data they requested to support claims that the lifetime energy usage of the Prius (including the increased environmental cost of manufacture and disposal of the nickel-metal hydride battery) are outweighed by lower lifetime fuel consumption.[141] Toyota states that lifetime CO2 saving is 43 percent.

CNW Marketing Research published a study[142] in which they estimated that the total lifetime energy cost of a Prius was greater than that of a Hummer. The study is widely cited, and its contents have also been widely challenged: see for example "Hummer versus Prius: 'Dust to Dust' Report Misleads the Media and Public with Bad Science".[143]

Repair costs

According to a study by Audatex, Prius repairs are slightly more expensive than those for comparable non-hybrid cars, with a greater cost difference in the past because of the lack of availability of recycled parts.[144]

Political symbolism

The large number of Prius-owning progressive celebrities in 2002 prompted the Washington Post to dub hybrids "Hollywood's latest politically correct status symbol."[145] While conservative "Prius Patriots" were also cited in 2005,[146] the vehicle carries an image as being a car for politically liberal environmentalists. A 2007 San Francisco Chronicle article said "Prius Progressives" were becoming an archetype, quoting conservative U.S. pundit Rush Limbaugh opining that "these liberals think they're ahead of the game on these things, and they're just suckers."[147]

Some conservatives promote use of the Toyota Prius. For example, Jim Road from What Would Jesus Drive? encouraged people to drive hybrid cars because of the damage that large SUVs and/or faster cars can do to others.

Former Central Intelligence Agency chief R. James Woolsey, Jr. drives a Prius because of its low fuel consumption. Woolsey noted the volatility of the Middle East, coupled with anti-US sentiment in much of the region. Noting that the high percentage of oil drilled in the Middle East gives vast profits to Middle Eastern regimes, Woolsey believes that it is a patriotic obligation to drive more efficient vehicles. In a Motor Trend magazine article, Woolsey claimed that those oil profits find their way to terrorist groups like al-Qaeda, meaning that Americans who buy inefficient vehicles would, in effect, be indirectly funding terrorism. "We're paying for both sides in this war, and that's not a good long-term strategy," said Woolsey. "I have a bumper sticker on the back of my Prius that reads, 'Bin Laden hates this car.'"[14

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