How it works

The Prius is a series-parallel configuration hybrid, a vehicle that can run on just the engine, just the batteries, or a combination of both. Toyota’s design goals are to reduce the amount of pollution and to maximize fuel efficiency. To do this, it uses a gasoline/electric hybrid powertrain, incorporating large batteries that are charged by the gas (petrol) engine directly or by regenerative braking (cannot be plugged in as built). Either the engine or the battery (or both) can power the vehicle, depending on conditions. This gives it the acceleration and power of a standard car having a much larger gasoline-burning engine. Specifically, the Prius incorporates:

1. More efficient use of the internal combustion engine (ICE), reducing gasoline/petrol consumption. The 1NZ-FXE engine uses the more efficient Atkinson cycle instead of the more common Otto cycle;
2. Two electric motor/generators, MG1 and MG2 are in the transaxle. The MG2 provides 50 kW (67 hp) @ 1,200 to 1,540 rpm and 400 NA·m (295 ftA·lbf) torque from 0 to 1,200 rpm, which significantly contributes to performance and economy. The MG1, with rpm from -10,000 to +10,000 rpm, provides the engine starter and counter torque for the electronic Continuously Variable Transmission;
3. 50 kW IGBT inverter controlled by a 32-bit microprocessor, which efficiently converts power between the batteries and the motor/generators.
4. Lower coefficient of drag at 0.26 (0.29 for 2000 model), with a Kammback design reducing air resistance, especially at higher speeds;
5. Lower rolling-resistance tires on the 2000 model, reducing road friction;
6. Regenerative braking, a process for recovering kinetic energy when braking or travelling down a slope and storing it as chemical potential energy in the traction battery for later use while reducing wear and tear on the brake pads;
7. Sealed 168-cell nickel metal hydride (NiMH) battery providing 201.6 volts;
8. Continuously variable transmission aˆ” the Prius uses a computer-managed (rather than a mechanical) CVT; Toyota calls it the Power Split Device[2]. The electric motors and gasoline engine are connected to a planetary gear set which is always engaged, and there is no shifting.
9. Flexible resin gasoline tank, reducing the amount of hydrocarbon emissions in the form of escaped gasoline vapour; (US model only)
10. Vacuum flask coolant storage system that stores hot engine coolant when the vehicle is powered off, then reuses it to reduce warm-up time. (US model only)
11. EV mode (Europe and Asian markets only, aftermarket option in the U.S.) allows the driver to select electric-only mode in low-power conditions. The vehicle can only be driven a couple kilometers (depending on conditions) on battery power alone before the gas engine is needed.
12. Weight reduction aˆ” for example the hatch and hood/bonnet are made of aluminium instead of steel.

One reason that the Prius gets such good gasoline mileage is that the ICE is smaller than in most cars this size. The ICE usually shuts off at stop lights and when backing up and descending long hills. The Prius performance remains very good because the battery/electric motor booster automatically provides more than enough extra power for acceleration and hill climbing. This means it drives like a traditional ICE automobile, with the on-board 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. Advocates of ‘plug-in’ hybrids consider this to be a missed opportunity. The Prius also uses its electric motor to recharge the battery during braking, with kinetic energy normally wasted as heat being recaptured. This also significantly 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 operate solely on electric power under low energy loads. This is sometimes referred to as “stealth mode” due to the lack of engine noise. This further reduces gasoline consumption and engine wear. When driving conditions demand additional power, the engine starts up automatically.
A battery module used on the Prius
A battery module used on the Prius

The on-board computer ensures that the engine runs under the most efficient conditions. Typically, a petrol/gasoline engine runs inefficiently at half-throttle, creating a choking condition. This effect, called pumping loss, is a major reason for the inefficiency of gasoline engines compared to diesels. The Prius minimizes pumping loss by running the gasoline engine at a high torque range 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.
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.
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.
Consumption monitor on the 2005 Toyota Prius MFD, displaying mpg for highway driving, in the previous 30 minutes, as well as average mpg for 244 miles since last gas fill-up.
Consumption monitor on the 2005 Toyota Prius MFD, displaying mpg for highway driving, in the previous 30 minutes, as well as average mpg for 244 miles since last gas fill-up.

In addition to the immediate benefit of reducing fuel consumption and emissions, stopping the combustion engine also improves the performance of the catalytic converter. In a non-hybrid vehicle the exhaust gases from an idling engine tend to cool the catalysts below their optimal temperature.
Energy screen on the 2003 Prius
Energy screen on the 2003 Prius

The frequent starting and stopping of the engine does not cause additional wear and tear or emission problems because the drive motors have enough power to quickly spin the engine to optimal rpm (around 1,000) before the engine fires up. This avoids the wear that would occur if the engine were to run (with fuel and spark) at very low rpm.
Electric power steering
Electric power steering
THS inverter unit (from NHW11)
THS inverter unit (from NHW11)

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 is captured when braking and stored in the battery. According to the revised EPA tests[1], the Prius gets 48 mpg (4.9 L/100 km) in the city dynamometer tests, compared with 45 (5.2 L/100 km) on the highway, and Natural Resources Canada estimates 58.8 mpg (4.0 l/100 km) in the city and 56.0 (4.2 l/100 km) on the highway. (Owing to peculiarities of these tests, few drivers obtain these mileage values in typical suburban circumstances, but “real world” performance with careful driving can come close. Typical real-world drivers get about 46-50 mpg or 5.1 to 4.7 L/100 km, hypermilers can get up 100 mpg (42.5 km/L). 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).