Prius overview

Hello Folks. I am a retired electrical engineer and a lifetime car enthusiast. We bought a 2015 Prius V in March of 2019. This Prius is our first Toyota vehicle and it is also our first hybrid vehicle. I had a vague idea of how these hybrid systems work before we bought one. Now that we have one it gave me a reason to really dig in to their inner workings. Since I am retired, I have free time for reading, investigating, watching video’s and observing how this system works. There are so many things about the Prius that I wasn’t aware of before we purchased one. My wife just gets in and drives but I want to know more about how this thing works. The Prius is a good vehicle in either case. For others that are interested in more than just driving the car, I would like to share what I have learned.

Before getting started, I want to say that PriusChat is a great website and most of what I have written has probably already been covered in bits and pieces by others. I wanted to put together an overview of the whole system. If readers find any errors, feel free to let me know.

My search was for information on the Generation 3 Prius V. Most of this information applies to all generations and models but a few of the details, like temperature thresholds, may be a little different for other Prius models and generations.

I installed a ScanGauge in our car so that I could monitor parameters. If you are not familiar with ScanGauge it is a dash mounted instrument that plugs into the ODBII port under the dash and it allows you to choose 4 of about 50 vehicle parameters to display at one time. I chose to monitor engine RPM, battery state of charge (SOC), coolant temperature and battery current as my 4 displayed parameters.

Batteries: The 12 volt battery is located in the trunk. It is a lower ampere-hour rating than most non-hybrid cars because it is not used to start the internal combustion engine (ICE). It provides power to all the electronics (16 computers/microcontrollers) and the lights. The 12 volt battery is charged by the inverter.

The hybrid battery is located under the back seat. It is a 203 volt, 90 pound, Nimh (nickel metal hydride) battery pack in a metal box. It provides energy for propulsion and stores energy during charging or while braking. The Nimh battery is particularly well suited for the wide temperature range that these vehicles may be operated in, the wide range of charging/discharging currents that it will see and still yield a long battery life. They are warranted for 8 years and 100,000 miles.

Transmission: There is no transmission as such in a Prius. The internal combustion engine (ICE) is mechanically connected to what Toyota refers to as motor/generator (MG1). The wheels are mechanically connected to a second motor/generator (MG2). There is no mechanical connection between the ICE/MG1 and the wheels/MG2. In simplest terms, when the ICE is running, it spins MG1 as a generator. The inverter directs the electrical power from MG1 to MG2. MG2 operating as an electric motor applies the torque that turns the wheels.

Inverter: The inverter is located under the hood. Driver input such as throttle position or brake pedal position plus hybrid battery state of charge (SOC), engine coolant temperature, vehicle speed and other conditions determines where the inverter directs the high voltage power to/from MG1, MG2 or the hybrid battery.

Starting the engine: To start the ICE, the inverter draws power from the hybrid battery and sends it to MG1 using it as the starter motor. Once the ICE is running, MG1 becomes a generator. It provides electrical power that can either charge the hybrid battery or provide power to MG2 to propel the vehicle.

The details in the next few paragraphs may be too much information……….

The term motor generator is used in most Prius descriptions of operation. Generator is usually associated with D.C. power but MG1 and MG2 are actually used as motors or alternators. Alternators generate A.C. current (alternating current) whose frequency varies with the speed of the ICE in the case of MG1 or the speed of the vehicle in the case of MG2.

Conversion Losses: Mechanical energy (power from the ICE) is converted to A.C. electrical energy that is generated by MG1. This conversion is about 80% efficient and the remaining energy is lost as heat. Efficiency varies. It is higher at lower loads and decreases with increasing load on the alternator.

The A.C. energy from either MG1 or MG2 is converted to D.C. energy at the inverter. This conversion is 99% efficient.

If the D.C. energy from the inverter is used to charge the hybrid battery there is an additional 66% conversion loss in converting D.C. energy to chemical energy in a nickel metal hydride battery.

If the D.C. energy from the inverter is to be sent to MG2, it is converted back to A.C. at the proper frequency to match the MG2 motor RPM. This D.C. to A.C. inverter conversion could be as high as 95% efficient.

MG2 converts the A.C. energy back to mechanical energy to turn the wheels. MG2 as an A.C. motor could be as high as 90% efficient.

Because of these conversion losses, only about 50% of the energy produced by the ICE ends up being stored in the hybrid battery or about 70% of the energy produced by the ICE is available to power the wheels.

Inverter Cooling: The Prius has a separate cooling system for the inverter. It has a coolant reservoir under the hood, uses a separate small radiator and a small electric circulation pump.

Hybrid battery level: The electronics/inverter tries to maintain the hybrid battery at about 70% charged so that power is available for acceleration and there is sufficient capacity in the battery to receive and store recovered braking energy.

Bargraph: The battery bar graph display on the dash has 8 bars. 1 bar represents 40% SOC and 8 bars represents 80% SOC. Even with 1 bar, there is enough energy remaining in the hybrid battery to start the ICE.

Battery only propulsion: If the SOC is high enough and the temperature of the ICE is warm enough, a gentle acceleration up to about 44 mph (for a Gen 3 Prius) can be accomplished on the hybrid battery only for up to 2 miles. This is a great feature for stop and go traffic. If quicker acceleration is required or the SOC is low, the ICE will start and assist in the acceleration/charging. At freeway speeds, the ICE provides the propulsion.

Braking: Regenerative braking recovers the energy normally converted to heat in mechanical brakes. It converts the vehicle kinetic energy to chemical energy in the hybrid battery. The mechanical brakes may assist the regenerative braking above 8 mph. Below 8 mph, it is mechanical brakes only.

During braking, the electronics/inverter monitors brake pedal pressure and determines how much of the energy to extract from MG2 to achieve the desired level of braking. Heavier brake pedal pressure causes the inverter to increase the generation from MG2 up to a limit of about 110 amps. During harder braking that exceeds the 110 amp limit, the mechanical brakes provide the additional braking power. Hard slowdown/stops partly rely on regeneration and partly on the mechanical brakes. A more gradual slowdown/stop allow regeneration to capture most of the energy and return it to the battery. Since regenerative braking is the primary braking means for every stop, the mechanical brakes should last 100,000 miles.

Coasting: With throttle off coasting, MG2 charges the hybrid battery at approximately a 12 amp rate and gradually slows the vehicle down. To maintain a constant vehicle speed, you have to apply just enough throttle that that charging doesn’t slow the vehicle. If the Prius V is at operating temperature and you release the throttle below 44 mph, the ICE will shut off and you will truly coast.

Engine: Most automotive ICE engines use the Otto cycle. Modern Otto designs produce maximum power and decent fuel economy. They have a thermal efficiency (percent of energy from the gasoline that is converted to motive power) of about 30%-35%. The remaining 65-70% is lost as waste heat through the radiator and exhaust.

The Prius uses an Atkinson cycle engine. Atkinson engines have different valve timing and compression ratio’s than an Otto engine. Atkinson engines provide less power for a given engine size than an Otto engine but they have a higher thermal efficiency. The Prius V generation 3 engine is 41% efficient.

The Atkinson engine choice makes sense for a hybrid vehicle since it gives better highway mileage than an Otto engine could and the lower power of the Atkinson ICE can be assisted by the hybrid battery when more power is required.

Accessories: On most non-hybrid vehicles, the air conditioning, power steering and water pump are belt driven directly from the ICE and power brakes are vacuum assisted. In order for the Prius to be able to travel without the ICE running, the air conditioning, power steering, water pump and brakes have all been converted to electric and are driven from individual electric motors. One advantage of these electrically operated systems is the ICE can be off, at a traffic light for example, and the air conditioning compressor will still operate.

Pressing the Start button causes the various computers and microcontrollers to begin to boot up and perform some start-up tests which accounts for the delay before the chime sounds and the Ready light message is illuminated. One of the tests that is performed is that the ABS brake system pump has to pump up it’s reservoir to within the pressure limits.

There are several temperature thresholds with the Prius:

Stage 1A. This is the first 80 seconds after the ICE starts and the coolant temperature is below 104 deg F. The engine fast idles at about 1300 rpm and it does not charge the hybrid battery or propel the car. The ICE fuel air mixture is set to rich and ignition timing is retarded. These settings result in higher than normal exhaust gas temperatures which quickly heats up the catalytic converter and O2 sensor.

Note: Catalytic converters and O2 sensors need to be hot > 500 deg F to work.

If you try to move the vehicle during Stage 1, it will be electric only and depending on how hard you accelerate, the motor current could be as high as the inverter 110 amp limit.

Stage 1B. After the first 80 seconds and until 104 deg F. There ignition timing and the fuel/air mixture return to normal settings. The ICE will charge the hybrid battery and/or assist in propelling the car.

Stage 2. Greater than 104 deg F and less than 158 deg F. ICE will switch off when you are stopped if you are not using the cabin heater.

Note: Cabin heat is provided by using a loop that circulates engine coolant through a small radiator under the dash. When the ICE is cold there is a supplemental 1000 watt electrical heating element powered from the hybrid battery that will also provide heat.

Stage 3. After a vehicle restart at greater than 158 deg F and until the vehicle has exceeded 35 mph the ICE will not shut off.

Stage 4. Greater than 158 deg F and once the vehicle has exceeded 35 mph the Prius is in full hybrid mode.

It is a pretty amazing system.

I hope that you found this overview helpful and that it explains a few of the why and when’s of how a Prius behaves a little differently than an internal combustion only powered vehicle.

 

I took a look at the Hybrid Assistant app that you suggested. Wow. I can't believe that is a free app as someone has put a lot of work into it's development. As you said, it shows a dozen or more parameters simultaneously and not just the 4 that you are limited to with ScanGauge.

My only fear with that app is that it would be a distraction while I was driving. Too bad there is not a splitter for the OBDII port so that I could keep my ScanGauge while I am driving and still use the app as a diagnostic tool when I wanted more information.

 

I took a look at his video's. They are very good at explaining the operation of the drive. The best video is P410 Quick Look.

Gen 3 added a second planet so the two systems ICE/MG1 and MG2 drive a common ring gear. I still have difficulty visualizing the operation of the ICE/MG1 planet but it works.

I like that he called MG2 a traction motor. That is similar to what they are called in diesel/electric locomotives.