What's Going On as I Drive my Toyota Prius?

In this topic, I'd like to use the information already discussed in other topics to put together a description of how the car operates under various normal driving conditions.

StationaryWarmUp.gif - 48666 Bytes Engine Starts

To start the engine, MG1 (connected to the sun) is driven forward using electrical power from the high voltage battery, as shown in the animation at right.  If the car is stationary, the ring gear of the PSD will also be held stationary.  The rotation of the sun gear therefore forces the planet carrier to rotate.  This is connected to the ICE and turns it over at 1/3.6 of the rotation rate of MG1 (see equation 1 in The Power Split Device).

Whereas a conventional car passes fuel and sparks to the ICE as soon as the starter motor gets it turning, the Prius waits until MG1 gets the ICE up to around 1000 r.p.m.  This happens in less than a second.  MG1 is much more powerful than an ordinary starter motor.  To turn the ICE at this rate, it needs to spin at 3600 r.p.m. itself.  Starting the ICE from 1000 r.p.m. puts virtually no strain on it because this is a speed at which the ICE would be happy to run under its own power.  In addition, the Prius begins by sparking only a couple of cylinders.  The result is a very smooth start up, free from noise and lugging, that eliminates the wear associated with normal car engine starts.

Once the ICE has started to run under its own power, the computer controls the throttle opening to get a suitable idle speed during warmup.  Electricity is no longer fed to MG1 and, in fact, if the battery is low MG1 may be used to generate electricity to charge the battery.  The animation above illustrates this situation also.  The computer merely configures MG1 as a generator instead of as a motor, opens the ICE throttle a bit more and draws off electricity.

Cold Starts

When you start the Prius with a cold engine, its top priority is to warm up the engine and catalytic converter to get the emission control systems going.  The engine will run for several minutes until this happens (how long depends on the actual engine and converter temperature).  In the meanwhile, measures are taken to control warm-up emissions, including saving exhaust hydrocarbons in an absorber to be purged later and running the engine in a special mode.  The idle speed of the engine after a cold start will be around 1300 r.p.m.

Warm Starts

When you start the Prius with a warm engine, it will run for a short time and then stop.  The idle speed will be a little under 1000 r.p.m.

Unfortunately, it is not possible to prevent the ICE from starting when you turn on the car, even if all you want to do is move the car onto the driveway to wash it.  I don't think there is any reason for this.  If I'd been designing the car, I think I would have delayed starting the ICE until it is first needed.

Starting Off

Recall that the Prius engine is always in top gear.  This means the engine cannot alone supply all the torque you expect to move smartly off the line.  The torque for initial acceleration is supplemented by MG2 acting directly at the PSD ring gear, that is at the input to the reduction gear that drives the wheels.  Electric motors are best at supplying torque at low speed, so this is an ideal approach to getting the car moving.

Normal.gif - 61096 Bytes

Since the ICE is running (we assume here) and the car is stationary (at first), MG1 is spinning forwards (see the animation above in Engine Starts).  The control electronics begins to draw power from MG1 and passes it to MG2.  Now, when you draw power from a motor/generator, that power has to come from somewhere.  A force appears which would slow the shaft down and whatever is spinning the shaft has to resist that force to keep the speed up.  To resist this "generator drag", then, the computer opens up the ICE throttle a bit so as to supply the power.  So, the ICE is driving the planet carrier of the PSD harder and MG1 is trying to slow the sun gear down.  The result is a force on the ring gear which causes it, and the car, to start to move, as shown in the animation at right.

In the PSD, recall that the ICE torque is split 72% to 28% between the ring gear and the sun.  Until we pressed the accelerator pedal, the ICE was just idling and producing no output torque.  Now, however, the throttle has opened and 28% of the torque is running MG1 as a generator.  The other 72% of the torque is passed mechanically to the ring gear and hence the wheels.  Even though most of the torque will come from MG2, the ICE does apply torque to the wheels in this way.

Now we have to figure out how the 28% of the ICE torque that goes to MG1 can possibly lead to most of the effort of starting the car coming from MG2.  To do this, we have to be clear on the distinction between torque and power.  Torque is a rotating force, but like a straight-line force, there is no energy consumed in just maintaining the force.  Suppose you draw a bucket of water up from a well using a windlass.  That takes energy.  If you were to use an electric motor, you'd have to supply it with electrical power.  But, when you've got the bucket at the top, you can engage a pin or hook or something to hold it there.  The force that the bucket's weight exerts on the rope and the torque the rope exerts on the barrel of the windlass has not disappeared.  But, because the force is not moving there is no transfer of energy and the situation is stable without power.  Likewise, when the car is stationary, even though 72% of the ICE torque is passed to the wheels, no power flows in that direction since the ring gear isn't rotating.  The sun gear, however, is spinning frantically and although it receives only 28% of the torque, this enables it to generate a lot of electrical power.  A similar line of reasoning shows that MG2's task in applying torque to the input of a stationary reduction gear does not require much power.  A lot of current must be driven through the motor windings against their electrical resistance and this represents power lost as heat.  But, while the car is moving slowly, this power comes from MG1.

As the car begins to move and picks up speed, MG1 spins less quickly and might generate less power.  However, the computer can open the ICE throttle a little more.  Now more torque comes from the ICE and, because more torque must also pass through the sun gear, MG1 can keep up its level of power generation.  A reduced spin rate is compensated by more torque.

I've avoided mentioning the battery up to this point to try to be clear about how it is not essential to getting the car going.  In practice, however, most starts result in the computer drawing power from the battery and passing it directly to MG2.  There is a limit to how fast the ICE can spin when the car is moving slowly.  This is imposed by the need to prevent MG1 being damaged by spinning too fast.  This limits how much power can be produced by the ICE.  Also, it would be unpleasant for the driver to hear the ICE rev up too much for soft starts.  The harder you press the accelerator, the more the ICE revs up but also the more power is drawn from the battery.  If you floor it, about 40% of the power comes from the battery and 60% from the ICE in the region of 25 m.p.h..  As the car speeds up and the ICE spin rate increases, it supplies a larger fraction of the power, reaching about 75% at 60 m.p.h. if you still have the pedal floored.  Remember, the ICE power includes what is taken off by MG1 and passed as electricity to MG2.  At 60 m.p.h. MG2 is actually supplying more torque and hence more power to the wheels than comes directly through the PSD from the ICE.  But a lot of the electrical power it is using comes from MG1, and hence indirectly from the ICE, and not from the battery.

Accelerating and Driving Up Hills

When the power demand is high, the ICE and MG2 both contribute torque to drive the car in much the same way as described above for starting off.  As the speed of the car increases, the torque that MG2 is able to supply is reduced because it begins to operate at its 33kW power limit.  The faster it spins, the less torque it can apply using only this amount of power.  Fortunately, this is consistent with the driver's expectations.  As a conventional car speeds up, the step gearbox upshifts and the torque at the axle is reduced so that the engine can drop its rotation rate to a safe r.p.m. value.  Although it does so using a completely different mechanism, the Prius gives the same general feel as a conventional car accelerating.  The main difference is the complete absence of lurching as the step gearbox shifts, because there simply is no step gearbox.  (Cars with true CVTs, such as the CVT Insight, also have this smooth feel during acceleration.)

Normal.gif - 61096 BytesTo summarize, then, the ICE is spinning the planet carrier of the PSD.  72% of its torque goes mechanically through the ring gear to the final drive and the wheels.  28% of its torque goes to MG1 via the sun gear where it is turned into electricity.  This electricity powers MG2 which adds some extra torque at the ring gear.  The same animation as applies to starting off is useful for visualizing this situation, so here it is again at right.

The harder you press the accelerator, the more torque the ICE produces.  This increases both the mechanical torque though the ring and the amount of electrical power generated by MG1 for MG2 to use to add still more torque.  Depending on various factors such as the battery state of charge, the road grade and exactly how hard you press the pedal, the computer might draw extra power from the battery to boost MG2's contribution.  This is how highway passing acceleration is achieved with only a 70 horsepower ICE in such a big car.  On the other hand, if power demand is not that high, some of the electricity produced by MG1 may be used to charge the battery, even while accelerating!  The important thing to remember is that the ICE both drives the wheels mechanically and drives MG1 forwards enabling it to generate electricity.  What happens to that electricity and whether more electricity is taken from the battery depend on complex factors which may be beyond our ability to fully figure out.

Cruising at Moderate Speed

Once you've reached a steady speed on a flat road, the power that must be supplied by the engine drops to only what is needed to overcome aerodynamic drag and rolling resistance.  This is much lower than power during acceleration or hill climbing.  In order to operate efficiently at low power (and perhaps also to give you a quiet ride) the ICE runs at a low spin rate.  The following table shows the net road power needed to move the car at various speeds on a flat road (calculated by me) and the approximate spin rate at which the ICE runs (measured by me using the OBD-II port and a PC-based scan tool):

Vehicle Speed

Power at Road

ICE Spin Rate

MG1 Spin Rate

40 m.p.h.

3.6 kW

1300 r.p.m.

-1470 r.p.m.

50 m.p.h.

5.9 kW

1500 r.p.m.

-2300 r.p.m.

60 m.p.h.

9.2 kW

???? r.p.m.

???? r.p.m.

Heretical.gif - 57807 BytesNow, the higher speed and the low ICE rotation rate puts the power split device in an interesting situation.  MG1 must now spin backwards, as in the table and the animation at right.  By spinning backwards, it makes the planet gears rotate forwards.  The rotation of the planets is added to the rotation of the planet carrier and makes the ring gear move much faster.  To compare this situation with the previously discussed situation for acceleration and climbing hills, you may want to open two browser windows on this page and look at the animations side-by-side.  The rotation rate of the planet carrier (ICE) is the same in both, as far as I am able to make it.  The difference, once again, is that in the earlier case we were happy with a high ICE spin rate to get plenty of power even though we weren't moving that fast.  In this new case, we want the ICE to stay at a low spin rate even though we're up to a fair speed so as to deliver the lower power demand with high efficiency.

We know, from studying the power split device, that MG1 must exert a backward torque at the sun gear.  This is how the ICE achieves leverage to turn the ring gear forward.  Without the resistance of MG1, the ICE would just spin up MG1 instead of moving the car.  When MG1 was spinning forward, it was easy to see that this backward torque could be supplied by generator drag.  Hence, the inverter electronics needed to draw power from MG1 and the backward torque appeared.  But, now that MG1 is spinning backwards, how do we arrange to supply this backward torque.  Well, how would we make MG1 supply forward torque and spin forwards?  By being a motor!  It's the same in reverse - if MG1 is spinning backwards and we want torque in that same direction MG1 must be a motor and push using electrical power supplied by the inverter.

This is starting to look peculiar.  The ICE is pushing, MG1 is pushing, is MG2, then, pushing too?  There is no mechanical reason why it couldn't - for a while.  This may look attractive at first.  Two motors and the ICE are all contributing to motive force.  But, we need to remind ourselves that we got into this situation by reducing the ICE spin rate for efficient low-power operation.  This would not be an effective way to get a lot of power to the wheels; to do that we should increase the ICE spin rate and go back to the earlier situation of MG1 spinning forwards.  Having got that issue out of the way, we need to think about where we're going to get the power to run MG1 as a motor.  The battery?  For a time, we could do this, but after a while we'd be forced out of this mode of operation with no battery charge left for acceleration or climbing hills.  No; we need to get this power continuously, without drawing down the battery charge, so we're forced to the conclusion that the power must come from MG2, which must therefore be acting as a generator.

MG2 generates power for MG1 to be a motor?  If you think this sounds all wrong, you're in good company.  My efforts to model the Prius powertrain mathematically were stuck for weeks because I failed to realize that this could happen.  When I presented it to the Yahoo! discussion group Prius_Technical_Stuff, I was met with skepticism and received one response with the subject "No Way!".  As evidence was collected by people who could measure the ICE r.p.m. with OBD-II scanners and the mechanics were rehashed, this mode of operating the power split device slowly became accepted.  Because both the ICE and MG1 contribute power which is combined by the PSD, the name "power joiner mode" was suggested.  However, the idea of MG2 generating power for MG1 to be a motor was so counter to the way people believed the system worked that the name that has stuck is "heretical mode".

Let's run through it again and vary the point of view.  The ICE drives the planet carrier at low speed.  MG1 drives the sun gear backwards.  This causes the planets to spin forwards and adds more spin to the ring gear.  The ring gear still gets only 72% of the torque of the ICE, but the speed at which the ring spins is increased by MG1 motoring backwards.  Turning the ring faster allows the car to travel faster for a low ICE r.p.m.  MG2, incredibly, resists the car's motion slightly with its generator drag and produces electricity to be fed to MG1.  The car is moved along by the remaining mechanical torque from the ICE.

You can tell when you're in this mode if you have a good ear for engine r.p.m.  Before I had the OBD-II scan tool, this is how I convinced myself that the mode existed.  You are traveling along at a fair speed and you can barely hear the engine.  It may be completely masked by road noise.  The Energy Monitor display will show engine power to the wheels and the motor charging the battery.  It may also alternate between charging and drawing from the battery through the motor to power the wheels.  I interpret this alternation as the adjusting of the generator drag of MG2 to maintain steady road power.  Power flowing alternately into and out of the battery also prevents its charge state drifting away from optimal.  In my daily commute, which is in between highway and city driving, the car is in heretical mode for about a third of the time, more than in any other mode such as the "normal" accelerating mode, EV mode, coasting, stopped at lights, etc.

Before leaving this subject, several months after proposing "heretical mode" to the Yahoo! groups, I decided to have another try at converting the non-believers.  Whether or not you accept my explanations above, you might like to read about this mode of operation explained from a different perspective.


When you take your foot off the accelerator, you can be said to be "coasting".  The engine does not try to push the car forward.  The car slows down gradually due to rolling resistance and aerodynamic drag.  In a conventional car, the engine is still connected to the wheels by the transmission.  The engine turns over without fuel and therefore also tends to slow the car.  This is called engine braking.  Although there is no reason for this to happen in the Prius, Toyota have decided to give the car the same feel as a conventional car by simulating engine braking.  When you coast, the car slows faster than would be the case if only rolling resistance and aerodynamic drag were acting on it.  To produce this additional slowing force, MG2 is configured as a generator and charges the battery.  Its generator drag simulates engine braking.

EvMode.gif - 31708 BytesBecause the engine is not needed to power the car, it can stop.  The animation at right shows the planet carrier stationary and the ring gear still rotating.  MG2, remember, acts directly at the ring gear.  The planets spin forwards and MG1 spins backwards.  Power is neither used by MG1 nor generated by it; it just spins free.

However, from equation 1 in The Power Split Device, we know that MG1 spins backwards 2.6 times faster than the ring gear and MG2 spin forwards.  This situation isn't safe when the car is travelling at high speed.  At 42 m.p.h. and above, keeping the planet carrier still would result in MG1 spinning backwards at more that 6500 r.p.m.  So that this doesn't happen, the computer configures MG1 as a generator and begins to draw off power.  The generator drag created prevents MG1 from overspinning and instead the planet carrier begins to turn forwards.  By spinning the planet carrier and the ICE at 1000 r.p.m., MG1 is protected at speeds of up to 65 m.p.h.  At higher speeds, the planet carrier and ICE must spin faster.  The electrical power produced by MG1 in this process can be used to charge the battery.  To see an animation of high-speed coasting, refer to Cruising at Moderate Speed, but remember that the ICE is being forced to turn and is not producing power.


When you want to slow the car more rapidly than rolling resistance, aerodynamic drag and engine braking, you press on the brake pedal.  In a conventional car, this pressure is transmitted by a hydraulic circuit to friction brakes at the wheels.  Brake pads rub against metal disks or drums and the energy of motion of the car is converted to heat as the car slows down.  The Prius has this exact same braking mechanism, but it has something else as well - regenerative braking.  Whereas during coasting MG2 produces some generator drag to simulate engine braking, when the brake pedal is pressed, the electrical power generation of MG2 is stepped up and a much greater generator drag contributes to slowing the car.  Unlike friction brakes, which waste the car's kinetic energy as heat, the electrical power produced by regenerative braking is stored in the battery and will be re-used later.  The computer calculates how much deceleration will be produced by regenerative braking and reduces the hydraulic force transmitted to the friction brakes by an appropriate amount.

In a conventional car on a steep hill, you might decide to change to a low gear to increase the intensity of engine braking.  The engine spins more rapidly and holds the car back more, helping the brakes to slow it.  The same option is available in the Prius should you decide to use it.  If you move the mode selector lever to the "B" position, the engine will be used for engine braking.  Whereas normally the engine is stopped during braking, in this mode the computer and motor/generators arrange for it to turn over without fuel and with an almost closed throttle.  The resistance it offers slows the car, reducing brake heating and allowing you to ease up a little on the pedal.

Creeping and Electric Starts

A conventional car with automatic transmission will tend to move forward from a stop if you take your foot off the brake.  This is probably a side-effect of the torque converter operation, but has the benefit of preventing the car from rolling backwards on hills while you transfer your foot to the accelerator.  It is called "creeping".  As with engine braking, there is no reason why the Prius should behave this way, except that Toyota want the car to feel familiar to drivers.  Therefore, creeping is simulated too.  A small amount of power from the battery is applied to MG2 when you release the brake.  This moves the car gently forward.

EvMode.gif - 31708 BytesIf you apply a bit of accelerator pressure, the power to MG2 is increased and the car will move forward more positively.  Since MG2 is quite powerful and has high torque, you can "launch" the car on only electric power up to a fair speed as long as the traffic allows you to accelerate gently.  The more you press down on the accelerator, the sooner the ICE will start to help you along with its torque and the electricity generated by MG1.  If you mash the pedal to the floor, the ICE will fire up right away, although you'll be off the line before it gets up to speed and contributes much power.  But, for most around-town starts, you will find yourself moving off the line in almost total silence using just the drive from MG2 powered by the battery.  The ICE remains stopped and MG1 spins freely backwards, as in the animation at right, which you may recognize as being the same as for Coasting.

EvMode.gif - 31708 BytesSlow Driving and "EV Mode"

Above I described how the car will "launch" using only electric power to MG2 if you don't press too hard on the accelerator.  If you reach your target speed before the ICE fires up, you can continue to drive using electric power only.  This is called "EV mode", since the car is powered in exactly the same way as a pure electric vehicle (EV).  The same animation at right shows the ring gear turning as MG2 powers the car, the planet carrier and ICE stopped and the sun gear and MG1 spinning freely backwards.

Even if the ICE does start during acceleration, when you get up to speed and reduce accelerator pedal pressure, the power you need may drop to a level that MG2 can comfortably provide.  The ICE will then turn off and you'll find yourself in EV mode.  I find it hard to predict when this will happen, as it depends on factors such as how much charge is in the battery and other things I haven't figured out yet.  Sometimes, I will be driving at 35 m.p.h. and find the car staying in EV mode while gathering speed up a slight incline to 38 m.p.h. or even above.  At other times, at 30 m.p.h. the slightest accelerator pressure will cause the ICE to start.  My car seems to have a mind of its own in regard to EV mode!  However, after some time in EV mode the battery charge level is bound to decrease and the likelihood that the ICE will come on to power the car and recharge the batteries will increase.

The way in which the ICE is started in EV mode, when this becomes necessary, is similar to a warm start , but the ring gear and sun gear are not stationary to begin with.  The sun gear is turning backwards and must first slow down.  This may be enough to get the ICE up to a starting speed, depending on the speed of the car, or the sun may have to change direction and run forwards.  To slow the sun gear, MG1 is first configured as a generator and power is drawn off.  However, as MG1's speed drops close to zero, it has to be configured as a forward motor and supplied with power to quickly flip it through stationary and into forward rotation.  The result, as in the case of an engine start in a stationary car, is that the planet carrier turns forward and the ICE turns over along with it.  The PSD ring gear, which is rotating forward with the car under power from MG2, helps to get the ICE up to starting speed at a lower MG1 spin rate.  However, this is not entirely free, and starting the ICE tends to hold back the ring while the ICE spins up.  So that this is not felt as a lurch by the driver and passengers, not to mention the mug of coffee in the cup holder, an extra pulse of power is fed to MG2 to supply the extra torque needed to get the ICE turning.

On the Yahoo! discussion groups, EV mode is usually referred to with the more imaginative name "stealth mode".  I have not used that name here because it is less descriptive, requiring explanation to newcomers, and because I am nervous about promoting silent driving as a "fun" thing to do, having had a couple of near misses in parking lots with pedestrians who had no idea I was there.

Slowing Down and Downhill Driving

When you decelerate gently or go down a hill, the power you need to keep the car moving is reduced because inertia or gravity is helping you out.  You therefore release some accelerator pedal pressure.  If you decelerate very slightly or go fast down a shallow hill, the ICE power output and spin rate reduce somewhat but this may be hard to notice.  For greater deceleration or on a steeper hill, depending on speed, the ICE may stop delivering any power at all if MG2 can supply what is needed.

In Slow Driving, I described how MG2 can supply all motive power with the ICE stopped.  When accelerating and driving at constant speed on the flat, this EV mode operation is unlikely to occur at speeds above 40 m.p.h. because the power needed to overcome aerodynamic drag is enough to cause the ICE to start.  EV mode at higher speeds can occur, however, under some conditions and is quite likely to occur when slowing down or driving fast down a hill.  To operate in EV mode at speeds of 42 m.p.h. and above, the car has to protect MG1 from overspinning in the same way as for Coasting, above.  The only difference is that instead of the car's motion turning the PSD ring gear, it and the car are powered by MG2.  MG1 still generates power to resist excessive spin with the result that the ICE turns over.  No fuel is supplied and no sparks are generated.  Of course, by doing this, MG1 is drawing off power that otherwise would drive the car.  Some is lost in spinning the ICE, but some shows up as electrical power generated by MG1.  This is simply returned to the high voltage electrical supply to partly replenish the power being used by MG2.

I will call EV mode above 42 m.p.h. "high-speed EV mode" or "EV-plus" to distinguish it from EV mode at slower speeds when the ICE has no need to spin.  On the Yahoo! discussion groups, it is usually referred to with the more imaginative name "warp stealth".

If you take your foot completely off the accelerator pedal, you are, or course,  Coasting.

ReversingReverse.gif - 31708 Bytes

The Prius has no reverse gear that would allow the ICE to push the car backwards.  Therefore, it can only move backwards under electric power from MG2.  No direct help from the ICE is possible.  In most cases, the car will stop the ICE when you put the running mode selector lever in the R position.  As MG2 turns the input to the reduction gears backwards, the PSD ring gear will also move backwards.  With the ICE, and therefore the planet carrier, stopped, this simply means that MG1 will turn forwards, as shown in the animation at right.  It turns free, without using or generating power.  This is exactly like EV mode, but backwards.  The computer will not allow you to go so fast backwards that MG1 overspins.

Should the ICE continue running when the running mode selector lever is in the R position, for example if the battery state of charge is low, then MG2 still simply drives the car backwards as before.  [Note to self: next time I have several hours with nothing to do, make another animation.]  The only difference is that with the planet carrier running forward, the sun gear and MG1 spin faster forward and the computer must limit the backward speed of the car to a lower value to protect MG1 from over-spinning.  Power can be drawn from MG1 to supply MG2 and charge the battery [Note to self: this may give rise to questions beyond the scope of this explanation].

Last edited December 15, 2001.  All material Copyright © 2001 Graham Davies.  No liability accepted.