The Five Stages of Prius Operation -- The Five Stages of Prius Hybrid Operation â€” The computer that controls the operation of the Prius hybrid system (HSD) appears to have 5 distinct stages of operation, which I will call Stage 1, Stage 2, Stage 3a, Stage 3b, and Stage 4. (Abbreviated S1, S2, S3a, S3b, S4.) Briefly, they govern how the HSD will combine power from the Internal Combustion Engine (ICE) and to and from the battery via two motor-generators, MG1 and MG2. The following was drawn principally from my observation, after reading several posts on the subject. EV (electric vehicle) mode is distinct. I drive a 2004 "package AM" Prius. Disclaimer: I am not an engineer or a technician. My understanding of this is not complete. Definitions: ICE: The internal-combustion engine. SOC: The state of charge of the battery. (How much charge it has.) Shown by an icon with 8 bars on the MFD (multi-function display). MG1: Motor/generator 1. Most commonly used to crank the ICE for starting, and to generate electricity from the output of the ICE, which can then help drive MG2 or charge the battery. MG2: Motor/generator 2. Most commonly used to drive the car, alone or in combination with the ICE, and to provide regenerative braking. (Occasionally MG1 and MG2 can act in arcane ways as well.) Golf-Cart Mode (so called because the car is as quiet as a golf-cart): is my term for when the car is moving on electric power alone, with the ICE not running. Some have used the term â€œstealthâ€ which I think sounds too militaristic for such a peaceful car, and is misleading because the car is not completely silent when moving on electric power. There is the whirr of the motor, and the crunch of tires on pavement. EV Mode: EV Mode is when Golf-Cart mode is entered via an EV button or switch. The button is standard on European and Japanese Prius. The button or a switch can be installed on North American Prius. For DIY instructions, see Dr. Evan Fusco's article in the Knowledge Base. For a relatively easy-to-install kit, visit Costal Tech You cannot enter EV mode if: You are in S1; your SOC is very near or below 3 bars; you are accelerating; your battery is too hot; or you are moving at 34 mph or faster. EV mode will automatically disengage when: your SOC reaches 2 bars; you step too hard on the accelerator; you reach 34 mph; or your battery gets too hot. The upper limits of speed and acceleration, and possibly the lower limit of battery SOC are different for EV mode than when Golf-Cart Mode is chosen by the computer. In EV Mode the upper limit of speed is lower, but the permitted acceleration is greater. Normal Mode: Any time the car is not in EV Mode. In Normal Mode, the car may go in and out of Golf-Cart Mode. I have seen references that say the transitions mentioned below depend on coolant temperature. I have also seen references to the importance that HSD gives to keeping the catalytic converter warm, to reduce emissions. I will simply refer to the system temperature in the discussion that follows. The Stages: S1: In Stage 1 the ICE is cold. If not prevented by use of the EV switch, the ICE will start a few seconds after the car is powered On. Until the system reaches the S1/S2 transition temperature, the ICE will run continuously. This accounts for the anomalous observation of electric regeneration or no arrows (coasting) at the same time as a low instantaneous mpg reading. No power is being drawn from the ICE, but it is running. Also in Stage 1 you will notice that battery power is used for even mild acceleration. In Stage 1 the HSD prefers not to draw power from the ICE if it is not necessary. You cannot enter EV mode while in Stage 1. S2: When the system temperature reaches 40 degrees C. (104 F.) the computer goes to Stage 2 operation. The original Japanese poster gave more detail than I clearly remember, but it appears that in Stage 2 the ICE will shut off if it is not needed to provide power. At a stoplight, in S2, the ICE will stop immediately and fairly smoothly. However, the car will not go into golf-cart mode in Stage 2. It can draw power from the battery or it can recharge the battery, but will not use the battery without the ICE. I have not ascertained if you can enter EV mode from S2. S3: When the system temperature reaches 73 C. (163.4 F.) the computer goes into Stage 3a. In S3a the ICE will not shut off unless you come to a full stop for 5 or 10 seconds. Paradoxically, it is more reluctant to shut off than it was in S2, and in S3a it shuts off very roughly. You will feel the car shudder. But once it does shut off at a full stop in S3a, it will then be in Stage 4 operation. Once the car is in S3a, if you reach 34 mph without having come to a full stop for 5 or 10 seconds, the car will go into S3b operation. S3b is identical to Stage 4, except that if you slow down below 34 mph and apply the brakes, you will be back in S3a. In S3 you can engage EV mode providing that you meet all the conditions for EV mode. I have observed that at the full stop in S3a sometimes the ICE stops after 5 seconds, and other times runs for 10 seconds before stopping. I do not know what determines the difference. I have also observed more than one rough shut-down in a trip, suggesting to me that perhaps the ICE may sometimes stop roughly in S2, and may not stop immediately when you let off the accelerator in S2. I repeat that I do not clearly understand Stage 2 operation. Coming to a full stop while in EV mode does not seem to trigger S4. You must be in normal mode when you stop in order to enter S4 operation. S4: Stage 4 is normal, full hybrid operation. The Prius is most efficient in S4. The car will go into golf-cart mode at the computerâ€™s discretion. You can encourage it to do so by lifting your foot off the accelerator pedal for a moment, and then pressing on the pedal very lightly. This is known as â€œfeatheringâ€ the pedal. Feathering does not force the car into golf-cart mode, nor is it necessary for the car to go into golf-cart mode. It merely encourages the car to do so a little bit sooner than it would otherwise do on its own. The benefit of feathering the pedal is questionable, since the computer is deciding when it thinks Golf-Cart Mode is most desirable. Increasing the time spent in Golf-Cart Mode may only increase the efficiency losses involved in charging the battery and drawing power from it. On the other hand, encouraging Golf-Cart Mode when the SOC is very high may be useful, as it creates head room in the battery to accept charge wheen needed. Above 42 mph the ICE must spin, and will generally provide power. It is possible for the ICE to spin without providing power to the car above 42 mph, and this has been observed. But since the ICE is more efficient at these higher speeds, Golf-Cart Mode above 42 mph (Super Golf-Cart Mode) is unusual. So the maximum speed in EV Mode is 34 mph, cutting out at a tiny fraction over 34 mph, and the maximum common speed in golf-cart mode is 42 mph. You can go faster in non-EV golf-cart mode than you can in EV mode. Thanks to jfschultz for providing the transition temperatures given above. They were cited by a Japanese poster some time ago but I had forgotten them. â€” Sidebar on using EV mode. The following is my personal speculation: â€” EV Mode is inherently inefficient because not all the energy fed to the battery comes back out of it. Heat is generated when charging the battery and when drawing it down. However, there are situations when I believe EV mode is desirable: Short trips: Because the carâ€™s warm-up cycle burns extra gas to get the catalytic converter hot as quickly as possible to reduce emissions, if you have enough charge to complete a trip entirely in EV mode I believe you more than offset the efficiency losses mentioned above. End of day: This is very speculative, but I believe that the system is reluctant to charge the battery if it is at or above 6 bars. Since the ICE will insist on running until it reaches Stage 4, more power will be produced than is needed to drive the car, and it is desirable that this energy be used to charge the battery. I believe that leaving the SOC at around 4 bars (or even 3) at the end of the day will provide the head room necessary to assure that the most energy possible produced during the inefficient warm-up cycle (Stages 1 through 3a) gets into the battery. Thus if your first trip of the following day will not be short enough to make entirely in EV Mode, I think it is a good idea to use EV at the end of the last drive of the day, if necessary, to leave the SOC at around 4 bars. You burn no gas during that last half-mile or so, and you leave head room in the battery for the next morningâ€™s obligatory warm-up cycle. Very slow driving: I sometimes observe very low instantaneous mpg at slow speeds even with a high SOC. And I never observe very good instantaneous mpg at slow speeds except in Golf-Cart Mode, when itâ€™s at 99.9. I therefore believe that itâ€™s always a good idea to use EV Mode at very slow speeds, such as parking lots or creep-and-stop traffic jams. Showing off: Sometimes you just want to show off how quiet the car is. It may not be the most efficient thing for the circumstances, but itâ€™s fun. â€” End Sidebar â€” For some fascinating in-depth technical explanations of how the prius works, visit Graham's Prius Page and click on "Understanding the Prius." He wrote this for the Classic (2001 - 2003) Prius, so some of the gear ratios may be different for the 2004/2005 Prius, but the operation is the same.