What the hybrid
does in each mode

1. Two operating axes

The hybrid controller solves two problems at once. After power-on it warms the engine and catalytic converter. While driving it chooses whether torque comes directly from the engine, from MG2, from the battery or from a combination. “The engine is running” and “the engine is driving the wheels” are therefore not always the same statement. [1] [2]

2. Five warm-up stages

After a cold start, the system passes through S1, S2, S3a, S3b and S4. In S1 the engine runs continuously and the battery is readily used for light acceleration. In S2, after roughly 40 °C, the engine may stop when no torque is needed, but full electric driving is not yet available.

At about 73 °C the system enters S3a. The engine is reluctant to stop while moving and may shut down with a noticeable shudder only after a complete stop lasting 5–10 seconds. If the car reaches about 34 mph (55 km/h), S3b begins: it resembles S4, but braking back below that speed can return to S3a.

S4 is full hybrid operation: engine and motor-generators vary their contributions, and the controller may select electric driving. This is a useful model of one generation, not a promise that every Prius uses the same algorithm. [2]

3. Starting and launch

While stopped, MG1 spins the engine using electrical power. The engine is brought to roughly 1000 rpm before ignition, then MG1 becomes a generator. Unlike a conventional starter, the engine begins combustion at a speed where it can run smoothly.

At launch MG2 provides torque directly through the ring gear and reduction gears. Part of engine torque reaches the wheels mechanically; another part goes through MG1 and becomes electricity for MG2. The battery can add power to a gentle launch, while harder acceleration increases the engine contribution. [1]

4. Acceleration and hills

When power demand is high, the engine and MG2 work together. As MG2 spins faster it approaches its power limit, so available torque falls. The controller can add battery current and route MG1's electrical output directly to MG2 rather than necessarily sending it through the battery.

The display may show the engine driving the wheels and feeding a generator at the same time. These are two paths of one system joined by the planetary gearset.

5. Steady speed

Flat-road cruising needs little power, so the engine can run at low rpm. In the historical “heretical mode”, MG1 spins backwards as a motor and helps hold the engine at efficient speed. MG2 applies a little generator drag and supplies electricity to MG1. The name is historical; it does not indicate a fault.

This is why Energy Monitor arrows are not a one-to-one mechanical diagram. Electricity can circulate among MG1, MG2 and the battery while wheel power remains small.

6. Coasting and braking

When the accelerator is released, MG2 can act as a generator. Its drag imitates engine braking and returns some energy to the battery. Pressing the brake increases regeneration, then adds friction braking when the motor alone cannot provide the requested deceleration.

At high speed the controller may spin the engine without fuel to prevent MG1 overspeed. That does not mean the engine is providing wheel power.

7. Electric drive

At low speed MG2 can move the car from the battery alone while power demand, charge and temperature permit. Early accounts call this EV mode or “golf-cart mode”. When charge falls or more power is needed, the engine starts; MG1 changes direction and spins it up, while MG2 supplies a brief torque pulse to avoid a lurch.

Button-selected EV mode and automatic electric drive are different. The button's limits are stricter and market-dependent; automatic operation is chosen by the hybrid controller.

8. Reverse

This layout has no separate mechanical reverse gear. MG2 moves the car backwards by turning the ring gear in reverse; with the engine stopped, MG1 spins freely forwards. If the engine stays on because charge is low, the controller still limits reverse speed to protect MG1.

9. What not to generalise

If morning economy is low, the engine stays on or the display shows unexpected arrows downhill, first identify the warm-up stage and energy path. Do not diagnose a fault from one screen: it is a simplified visualisation, not every internal power loop.

Electric operation by generation and market

In early owner writing, Stealth means quiet travel without the engine burning fuel. The 2003 description mentions 42 mph and about 0.5 mile; these do not define every hybrid's speed or range. The engine can be stopped or turn without combustion, and those states need distinguishing. [4]

The personal account of 13 EV minutes and a climb at 57 mph concerns the author's Prius PHV in winter 2013. Conditions include just-above-freezing weather, the engine later starting on a 70 mph section, an 18.4% SOC display and 70 MPG in the first minute. It is one trip, not a switching threshold for every Prius. The ScanGauge account also notes speed under-reporting by 3 mph and slow rpm updates. [12] [13]

Regeneration and four different heating tasks

For Gen2 Prius, Toyota describes the traction motor generating electricity and ECB coordinating electrical and hydraulic braking. Releasing the accelerator and pressing the brake do not by themselves imply a switch from MG1 to MG2. The log's 10 kW claim during accelerator release does not establish MG1's rated output. The 72/28 shares concern idealised engine-input torques; power also depends on speed. [14] [15]

The hot-coolant tank in a US 2004 Prius is described in a 25 December 2003 owner account as retaining heat for up to three days. Gen3 Prius introduced exhaust heat recovery. The UK 2017 Plug-in separately documents battery warming while charging down to −20 °C outside and cabin heat-pump operation without the engine at −10 °C. These four tasks cannot be reduced to a single “warm-up stage” or extended to the whole fleet. [16] [17] [10]

The European 2023 PHEV has three Regeneration Boost levels. This is a version-specific feature; it neither replaces the historical B mode nor sets the controls of an early Prius. For plugging in and cabin preparation, see home charging. [18]

Sources and scope

1. Graham Davies. What's Going On as I Drive my Toyota Prius? Last revised 15 December 2001. A detailed author explanation for early Prius energy flow, not a Toyota service manual.
2. Bill Merchant / Daniel. The Five Stages of Prius Hybrid Operation, PriusChat, 11 December 2005. An observational warm-up description for second-generation Prius; the author states that his understanding is incomplete.
3. Toyota Motor Corporation. Toyota Launches All-New Prius HEV in Japan and Completely Redesigned Camry. Official examples of current series-parallel systems with M20A-FXS and A25A-FXS engines.
4. John1701a: Prius Stealth28.05.2003 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
5. Toyota: Leaner and Greener, Prius Gets Even Better25.05.2000 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
6. Toyota UK: Prius Plug-in Ownership Cost Performance, Restatement24.07.2012 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
7. Toyota UK: The New Prius Plug-in Hybrid13.09.2011 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
8. Toyota USA: окончательный EPA-рейтинг Prius Plug-in23.02.2012 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
9. Toyota USA: Introduces 2012 Prius Plug-in Hybrid05.12.2011 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
10. Toyota UK: New Toyota Prius Plug-in06.02.2017 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
11. Toyota USA: Prime Time for More EV Miles, Prius Prime05.04.2023 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
12. John1701a: Personal Log 60528.01.2013 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
13. John1701a: Personal Log 55820.03.2012 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
14. Toyota UK: Prius Paves The Way Ahead02.12.2003 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
15. John1701a: Personal Log 11229.03.2004 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
16. John1701a: Personal Log 9525.12.2003 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
17. Toyota: запуск третьего поколения Prius18.05.2009 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.
18. Toyota Europe: All-New Prius Plug-in Hybrid2023 · Selected document sections; vehicle, market and measurement limits accompany the cited claims.

Provenance and usage limits are recorded in the notices.