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Both the Insight and Prius are charge-sustaining gasoline-electric hybrids. This means that neither car is plugged in to charge. Instead, both cars charge their batteries from energy that would otherwise be wasted, using gasoline as their sole external source of energy.

The big benefit of hybrid electric technology is that the gasoline engine and electric motor are each used for what they do best. The gas engine allows for long range on a high-density power source (gasoline), while the electric motor allows efficient capturing and reusing of "excess" energy.

The end result is that the Insight & Prius get a lot more out of each drop of gasoline than a comparable non-hybrid car would do.

When is Each Power Source Used?

At most driving speeds, both the Insight and Prius are primarily powered by the gasoline engine.

In both cars, the electric motor and gasoline engine are used together during peak power demand, such as when accelerating or climbing a hill.

Both cars will also turn off the gasoline engine when it is not needed, such as when sitting at a stop light.

In both Insight and Prius, the battery is charged by regenerative braking, and when necessarily, directly from gasoline engine power.

A small but interesting difference between the two cars is that in the Prius, under light load conditions such as initial acceleration, the Prius is operated solely on electric power from the temporary battery storage. Depending on how fast you're accelerating and the battery's state of charge, the Prius's gasoline engine will start when your speed reaches between about 13 and 25 mph. By waiting until this point to start the gasoline engine, this means that the Prius doesn't operate the gasoline engine under very light power demands, when the gasoline engine is less efficient. (At zero power demand, such as descending a hill, braking or sitting at a stop, both cars can entirely stop operation of the gasoline engine.)

Some writers wanting a simple comparison between the two cars have extrapolated from this to say that the Prius is primarily "electric powered with gasoline assist", while the Insight is primarily "gasoline powered with electric assist". This is a misleading statement that has confused a lot of people! A more accurate summary of the two cars would be to say that:

"Both cars ultimately get all their energy from burning gasoline, but improve efficiency by using the electric portion of the powertrain for temporary storage of energy that would otherwise be wasted, such as by braking. The way the cars differ is when and in what combination they use this temporary storage. Both cars will often use both power sources together, splitting the total power supply between gasoline engine and battery storage. However, in some situations the Prius will use only power from the temporary battery storage."

Battery and Electric Motor Size

A sensitive balancing act in the design of these two cars is the size of the battery and electric motor. A larger battery and motor allow more reliance on energy temporarily stored in the battery. However, the increased weight of the car means that more energy is spent moving that extra weight around.

Toyota engineers chose a larger battery pack and electric motor, in part because the Prius's steel body and 5-seat format make it heavier, and in part to allow the initial acceleration on electric power alone.

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Under hood of Honda Insight
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Under hood of Toyota Prius

(The silver box contains the hybrid system control circuitry. In the Insight, these electronics are located at the rear of the car near the battery pack.)

A Closer Look at Why the Insight & Prius Use Hybrid Technology

In both cars, there are two major efficiency advantages that come from hybrid electric propulsion:

1. The first efficiency advantage of hybrid electric propulsion comes from regenerative braking. Regenerative braking is particularly valuable in the city where one is continually slowing down and speeding up again. Normally, each time you slow down by applying the brakes, a lot of energy is lost. Regenerative braking take advantage of the fact that an electric motor can also operate as a generator. During regenerative braking, the electric motor operates as a generator, slowing the vehicle down and turning some of the energy of forward motion back into electricity that recharges the batteries. This energy that would otherwise be wasted can now later be used to help propel the car.

Both cars also retain normal friction brakes that are used in addition to the regenerative braking system in more abrupt stops.

2. A second advantage of the hybrid powertrain in the Insight an Prius is that it allows the gasoline engine to be made more efficient. Unlike most cars on the road, where the gasoline engine is must larger and less efficient so that it can provide lots of extra power for accelerating, the gasoline engines in the Insight and Prius are optimized for constant-speed cruising.

Hybrid System Configuration

Honda and Toyota engineers chose different configurations of electric motor, generator and transmission:

Insight Prius
The Insight uses a single combination motor/generator, which serves the purposes of:
  • Providing propulsion to the wheels
  • Charging the battery from the wheels during regenerative braking
  • Charging the battery from the gasoline engine
  • Starting/stopping the gasoline engine.
  • Smoothing-out engine rotation speed to reduce vibration

This motor/generator is connected on one end to the gasoline engine, and on the other end to the transmission.

This allows two transmissions to be offered on the Insight, a 5-speed manual transmission, and a continuously variable (CVT) automatic transmission.

The Prius, on the other hand uses two motor/generators, which split the jobs done by the Insight's one motor/generator.

The motor/generator "M" is connected to the wheels (via differential and reduction gear), and is used for:

  • Providing propulsion to the wheels
  • Charging the battery from the wheels during regenerative braking

The Prius uses a planetary gear as a power-split device that provides a three-way connection between the wheels (and motor/generator "M"), the gasoline engine, and generator/motor "G". Together, this system also forms the Prius's continuously variable automatic transmission.

The generator/motor "G" is used for:

  • Charging the battery from the gasoline engine
  • Starting/stopping the gasoline engine

Understanding the Prius's Planetary Gear

The planetary gear in the Prius probably needs a little explanation, as it is at the heart of how the Prius allows the gasoline engine speed to be varied relative to wheel speed.

As mentioned above, the Insight is made in both manual transmission and CVT (continuously variable transmissions) versions. If you compare the CVT version of the Insight to the Prius, in both cases the continuously variable transmission allows the gasoline engine to run at its optimal speed regardless of vehicle speed. In both cases, it also provides smooth, stepless acceleration. However, the continuously variable transmissions in the two cars work in entirely different ways.

The Insight uses the same CVT automatic transmission design that Honda has used on its high-efficiency Civic HX. In this transmission, a belt and a pair of variable-diameter pulleys are used to allow any required engine speed to wheel speed ratio.

In the Prius, a "planetary gear" is used as a power split device, providing a three-way connection between the wheels (and by extension motor/generator "M"), the gasoline engine, and generator/motor "G". The easiest way of thinking of the planetary gear is that the rotation of the wheels is always equal to the sum of the gasoline engine rotation and the rotation of generator/motor "G".

This means that the gasoline engine may be stationary, with any rotation of the wheels being directed towards rotation of generator/motor "G". It also means that if the gasoline engine is turning at a fixed speed, the faster the car is moving, the slower generator/motor "G" will turn.

In a typical driving situation, the output from the Prius's gasoline engine is split between the wheels and generator/motor "G". If the batteries are sufficiently charged, all energy coming from generator/motor "G" will also be routed to the wheels, by using it to power motor/generator "M". This means that power is taking two separate paths from the gasoline engine to the wheels, one entirely mechanical, and the other partially electrical.

Energy Flow when Stationary

Insight Prius
In both cars, the gasoline engine is typically turned off when the vehicle is stationary, such as when sitting at a stop light. As such, there is no energy flow in the system.

The exception is when the battery charge is low, in which case both cars can run the gasoline engine and route its power to charging the batteries.

Energy Flow when Starting Up

Insight Prius
When it is time for the car to start moving (such as when you touch the accelerator pedal), the Insight instantly starts its gasoline engine.

This instant start is accomplished by using the electric motor as a powerful starter motor, with energy coming from the high-voltage battery pack.

When the Prius begins moving, energy from the battery is used by the electric motor "M" to drive the wheels, putting the car in motion. Since the gasoline engine is not yet turning, all movement of the wheel side of the planetary gear is directed towards the generator/motor "G".

Under initial start up when power demand is relatively low, the Prius will move on electric power from the temporary battery storage alone.

When power demand becomes higher, usually between 13 and 25 mph, the Prius's gasoline engine will be started. This is done by directing energy from the battery to motor/generator "G", to increase its speed. This, in turn, will cause the gasoline engine to start moving. When fuel supply and spark are turned on, the engine will then be running.

Since the Prius has no physical reverse gear, reverse is done on electric power alone. This is the same power flow as initial start up, but motor "M" is turning in the opposite direction.

Energy Flow when Accelerating

As soon as the gasoline engine is running, acceleration in both cars continues using both gasoline engine and electric motor power:

Insight Prius
During acceleration or high load conditions, both cars direct all power from the gasoline engine to the wheels. In addition, both cars use energy from the battery pack to provide addition propulsion. The amount of additional propulsion will vary depending on the amount needed.

Here, the only difference is the Prius's two-path power flow from the gasoline engine: some power flows mechanically through the planetary gear, and some flows electrically from generator/motor "G" to motor/generator "M".

Energy Flow when Cruising

Insight Prius
Under typical cruising conditions, both cars are powered entirely by the gasoline engine.

In both cars, if the battery charge level is low, some of the energy from the gasoline engine is directed to charging the battery.

Under light load conditions, such as going down hill, the Prius will turn off the gasoline engine and operate in the same electric only mode that is used during initial startup (above).

On the other hand, when going down hill, the Insight may enter its fuel cut mode where combustion in the gasoline engine is shut off, similar to what it does during regenerative braking mode (below).

Again, notice the Prius's two-path power flow from the gasoline engine: some power flows mechanically through the planetary gear, and some flows electrically from generator/motor "G" to motor/generator "M".

Energy Flow when Decelerating: Regenerative Braking

Insight Prius
During deceleration, both cars enter regenerative braking mode, where the motor/generators operates in generator mode to charge the battery.

In the Insight, the gasoline engine's crankshaft continues to move along with the wheels. However, the engine is off in the sense that fuel injection and spark are turned off, and so no gasoline is being burned. When the clutch is disengaged, the gasoline engine will typically enter idle stop mode, where it is not turning and again no gasoline is being consumed.

In the Prius, the gasoline engine will also typically be turned off as the car comes to a stop.