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Hybrid synergy drive emblem

The Hybrid Synergy Drive is made by a 1.8L Internal Combustion Engine (ICE) that is not connected directly to the wheels, rather via an electric motor (MG2) and a generator/motor (MG1; smaller, with way less torque than MG2 and much higher max rev speed). The MG2 drives the wheels directly at all times via an additional planetary gearing .

The Prius has no clutch, no hydraulic coupling, no traditional gearbox. All gears employed are at a fixed ratio that never changes.

The two motors and the engine are connected together via the Power Split Device (PSD) that is nothing but a planetary gearing. The “sun” is MG1, while MG2 is the outer ring. The ICE is the central planets. The computer calls on the power of MG2 and/or of the ICE according to driving conditions (accelerator pedal position, battery charge status, temperature, speed, etc.) and uses the MG1 as a “buffer” between the two.

The accelerator is not connected physically to the ICE, but rather to a computer that simply gets an electrical signal indicating how much the driver wants to accelerate (drive-by-wire). The accelerator is in fact a “torque requestor” to the HSD. The computer then decides, depending on a number of variables, from which engine the torque should be generated from, to give the necessary acceleration requested with maximum efficiency.

The behaviour of this drivetrain is in some way equivalent to a Continuously Variable Transmission (CVT), but it is not a CVT. The ICE/MG1&2 are connected with fixed ratio gears. Toyota calls the PSD, for regulation purposes, an E-CVT (Electronic controlled CVT).

While driving there is no shifting, no explicit revving up and down of the ICE like on a conventional car (with either manual or automatic gearshift).

The driver can influence to a certain degree whether to use the ICE or the MG2, but essentially this is not necessary and the computer will always select the most efficient combination depending on the driving conditions. You cannot, and do not have to, “switch” between engine type – they work together seamlessly without any user intervention.

In general, the computer will make sure that MG2 and the ICE are running at their most efficient rpms, where the most torque with the least energy can be generated. Typically this is for MG2 from zero to low/moderate rpms, while for the ICE, from moderate to high rpms. Any excess torque generated from the ICE is either used immediately from MG2, via MG1 that spins creating electricity that can be used directly from MG2, or saved in the High Voltage Battery (HVB) for later use.

How does the HSD work in daily usage

So how does this “E-CVT” feel to the driver? What happens when you turn on the car, when you accelerate and brake, when you are driving in city/highway/suburban areas? What implications/benefits and changes are there compared to a conventional drivetrain based car?
Let’s see in detail:

1. Getting in and turning on

You approach the car to enter it, touch the handle (no key necessary, as long as it is in your pocket) and as you open the door you hear a clear whir from the engine compartment. This is the first thing different from a conventional car. The whir is the brake pump starting up and creating enough pressure for the brakes to work correctly. Why? Simple: no accessories are tied to the ICE with a belt. They are all electrically driven and therefore usable to whatever degree is necessary, regardless of whether the ICE is running or not. So if you are running only with electricity, and need the hydraulic brakes to kick in, the car needs to prepare itself for that.

You turn on the car with the On/Off button and, compared to a conventional car, “nothing happens” – a series of clicks emanate from behind the rear seat as the 12 volt battery energizes the car's computers and assuming no errors are returned, the switch connecting the traction battery to the high voltage circuits is thrown -  the dashboard fires up, and you hear nothing else. Silence. No starter/alternator revving up and no ICE noise. All quiet.

If the A/C is on and it is warm/hot outside, you can also hear a mildly loud non-constant whine, coming from the electric A/C compressor. It cannot be heard when you drive, or when in traffic, or with closed windows, or if the ICE is on. Like the brake pump, this accessory is electrically driven and can be turned on/off and tuned at will by the car's control unit depending on needs. If the A/C is turned off, for example, the whine stops almost immediately.

2. Warm-up

After several seconds (depending on outside temperature, ICE coolant and engine temperature, battery charge, whether moving or not) the ICE enters a relatively complex warm-up procedure (more details can be found in Priuschat “Gen3 Warming Up Stages“).

First a very light whirring sound is heard, and then the ICE starts. The initial whirring is the MG1 acting as a motor/starter. Taking energy from the HVB, it speeds up the ICE to about 1000 rpm, at which point gas is injected in the ICE, that fires up. The ICE is therefore always started in a very smooth way, by which oil and coolant have already reached the engine before getting started.

Also the coolant pump is electrical and can work, according to needs, independent of the ICE on/off status & load. The oil pump is instead (and rightly so) connected directly to the ICE and therefore will work as soon as the MG1 will start revving the engine before it is turned on (i.e. gas is being injected).

The warm-up period can be 1 to 3-5 minutes long, based on outside temperature, battery charge status and coolant temperature. With a cold engine, the longest warm-up times are when the battery is empty (2 bars) and it is very cold outside. Shortest (about 1 min) is when the battery has at least 3 bars and it is warm to hot outside. If the ICE is still warm from a previous stop, there is simply no warm-up. The driver can essentially move out of the parking spot and get to the first traffic light/intersection electrically without a single drop of fuel being used.

The Prius does its best to make the shortest warm-up possible. If the coolant liquids are still warm, they are circulated before running the engine. And the exhaust gas is also used to more quickly warm up the engine by transferring heat to the coolant liquid. The Prius tries not to waste energy or produce any excess energy that can get wasted.

Moreover during warm-up, at low speeds (up to around 40 km/h), the ICE is not effectively driving the wheels, rather simply warming up with “no load” and essentially dragging only the MG1 which acts as a generator and charging the battery. MG2 is driving the car at low speeds, taking the electricity either from the MG1 (i.e. the ICE) or the battery. Because of this strategy, compared to a conventional car, the ICE can run at whatever necessary and most efficient rpm it requires to provide a load-free and faster warm-up, compared instead to conventional cars. It is not uncommon to notice that during warm-up the ICE noise is completely decoupled from the car’s speed. The driver can accelerate up to a certain speed (as long as MG2 can do the job efficiently in pushing the car) and hear practically no change in the ICE noise.

Once the engine is warmed-up or the speed is higher than 35-40 km/h, it can be felt distinctively that the reaction of the Prius is different – it simply means that the engine can or must now help effectively drive the car via the torque provided to the MG2.

Because no coolant temperature light or indicator is available on the dashboard, there is no way to know (except coming to a full stop) if the engine has warmed up completely or not. The only coolant related light is a warning when the coolant is too hot (i.e. there is a malfunction).

3. Driving around

When driving, depending on a number of factors (load, accelerator position, speed, external temperature, battery charge status, selected drive mode) the computer will either use the ICE and/or the MG2. MG1 either acts as generator to create electricity for the HVB, or for the MG2 (no transfer to the HVB is happening), or as a motor to rev up the ICE (if necessary) to the necessary 1000 rpm to turn it on. The ICE can be dragged from the MG2 (i.e. the wheels) if necessary or simply idle at 0 rpm, with the MG1 taking the extra torque from the MG2 (when not accelerating), thus creating electricity converted to the HVB.

4. Driving a Prius “ecologically”Edit

On the Prius Gen 3 instrument cluster there is an "Eco Driving Indicator Light" (“ECO light” for brevity) that helps being “eco” by indicating whether you are driving efficiently or not, that is, whether you are asking for accelerations that can be efficiently served by the power train or not (i.e. wasting fuel). The Prius Owners Manual reads that if the ECO light is off it:

“Shows that an environmentally friendly driving range is being exceeded (during full power driving etc.)”

When driving faster than 143 km/h the “ECO” light in the instrument cluster will remain off, regardless of gas pedal position (i.e. speeds above 143km/h are not deemed environmentally friendly). The same happens at all speeds when PWR mode is selected.

It is not known at this time how keeping the ECO light on at all times during driving exactly affects fuel efficiency or the HSD engine management.

5. “Driving like a granny will not help FE!”Edit

For most drivers, the best way to drive a Prius is to simply, drive it! The HSD will do its best, based on your driving patterns, to improve FE.

The best way to improve FE on the Prius (apart from using ECO mode when applicable) is to look for traffic and anticipate its behaviour. Avoiding strong braking, strong accelerations, and making sure to have an overall smooth ride will yield excellent fuel efficiency.

It is quite easy with the Prius to get a “smooth” ride, since there is no gear shifting going on, and therefore no jerking or jumpy acceleration "push-pulls".

Depending on the acceleration/torque requested, the ICE will always run at the most efficient rpms, independently from wheel speed. Several Prius drivers will actually advise to accelerate “briskly” without flooring the accelerator pedal (traffic permitting) at traffic lights (while keeping the ECO light on) and to brake as gently as possible to avoid using the hydraulic brakes (and thus wasting kinetic energy in heat).

A very nicely done video describing how to drive the Prius to get good FE is available here. Essentially, predicting traffic and by carefully “playing” with the accelerator pedal to “trick” the HSD, can guarantee very good fuel efficiency in all conditions.

6. Prius “Magic” & “Happy Mode” Edit

When starting from a stand still, the Prius will move forward (depending on the accelerator position), using only MG2 as this is the most efficient thing to do. (In PWR mode this requires a light foot).

When the speed increases above 20-30km/h and depending on accelerator position and battery charge status (including drive mode selected), the ICE will be turned on and either will assist MG2 or completely provide the torque to push the Prius forward. Once the desired speed is reached, one should completely or almost completely release the gas pedal and depress it again gently to keep the reached speed (as per user’s manual).

And this is when the Prius “magic” begins.

Depending on the desired speed, load (road steepness) and battery charge, the computer can decide to do one of the following 3 things:

a. Use only MG2 and the energy in the HVB to provide enough torque to keep momentum and let the Prius move forward while consuming no gas. This could be called “electric mode”, compared to the true “EV mode”. This mode can be achieved up to speeds of about 70-75km/h depending on various conditions and will average over several hundred meters (how many?) 0L/100km to whatever fuel consumption the driver has had so far.

b. The ICE is kept at an rpm at which it can 1) generate enough torque to push MG2 (i.e. the wheels), 2) generate via MG1 enough electricity to drive MG2 as well as 3) charging the HVB. This is especially true for higher speeds and with inclines or counter wind.

c. The ICE goes into what could be called “happy mode”: when the speed is appropriate (which range?) and the load low enough, the ICE can drive directly the wheels (MG2 will spin at the same rpms as the ICE, but not draining any electricity at all from the HVB or MG1) and at the same time giving torque to MG1 to charge the HVB. This is a high efficient state in which on flat roads the instantaneous fuel consumption will show between 2,5-5 L/100km depending on load. This mode is not documented in the Energy Monitor description pages on the USA Prius Owner’s Manual.

[1]

"Happy mode"

Mode a) in town pushes up FE drastically. The battery charge can be replenished by the ICE when starting at the next traffic light (this is why it is better to provide a brisk acceleration rather than crawl to try to move only electrically; flooring the accelerator though will not help FE).

Mode c) is the best mode in suburban areas as it provides the lowest fuel consumption, while at the same time charging the HVB. In mode c) you can truly appreciate the effort of Toyota engineers to make a super efficient ICE and highly aerodynamic car .

Mode b) is the most common when driving fast on the highway (more than 120km/h). In this case there is a “dance” of electricity going to and from the HVB to MG2 and MG1 to provide additional torque when accelerating and to make sure the battery is always full for extra acceleration or to keep battery charge to a safe level (overcharging the HVB battery will reduce its lifetime and therefore the control unit will use charge every now and then to keep the charge level under control).

7. Driving (fast!) with the Prius

Although the Prius 2010 is an “eco car”, not designed for racing on highways, it achieves a top max speed of 180km/h. This far better than the previous Prius Gen 2.

The engine is limited electronically to 5200rpm, which means that the gas pedal can be floored without any issue and the max speed can be reached without worrying about damaging the drivetrain. Prius drivers report a max displayed speed of 195 km/h. Considering the official max speed and factoring in a 10% error at the speedometer, a displayed speed of 198 km/h is theoretically possible. The speedometer can display only up to 199km/h.

At speeds around 150 km/h Prius drivers report that it is still somewhat responsive and very stable (with the stock 17" rims). The ICE does get noisy though.

An article about german Autobahn High Speed driving with the Prius can be found at this link: Driving the Prius 2010 on a German Autobahn…

8. (Regen-)BrakingEdit

Each time the gas pedal is released, the MG2 will work as a generator (like all electrical engines do) and will re-charge the HVB while slightly braking the car. Because the Prius does keep its momentum well (i.e. less drag on the drivetrain due to lack of directly driven accessories, no gearbox, excellent aerodynamics), a driver can slow down to a stop (e.g. to a distant traffic light) while averaging 0 L/km (like on conventional cars with injection cut-off) while at the same time re-gaining energy by charging the HVB. On a conventional car, the energy will be 100% lost into heat due to friction from the drivetrain or on the brakes while on the Prius a portion of this energy can be used instead to charge the HVB (NOTE: conversion to and from forms of energy is never 100% efficient).

Whenever actually braking (pressing the brake pedal, that is and not rolling to a full stop), the MG2 generates even more current, charging the HVB (until the HVB is full, or the current generated is more than the battery can handle), and the hydraulic brakes (always ready) are activated only depending on the brake pedal position and vehicle speed. The hydraulic brakes are most needed if the HVB is fully charged during braking, or the braking effort demanded by the driver is more than the HVB can be charged at - for example rapid stop from high speed.

At all times, the ECU can blend regen braking and friction braking to achieve the requested braking effort in the most efficient way. It's not unusual for Prius brake pads (friction material) to last over 100,000 miles before renewing.

Below 7mph, the regen is not efficient, and the hydraulic friction brakes are always used. Friction brakes can stop the car and keep it steady, an electric engine cannot – see trains or trams.

More information on regenerative braking here.

9. At traffic lights/stoppingEdit

When approaching a traffic light or a stop, most likely the ICE has already turned off (this means completely off, it is not even turning anymore). If the driver is in PWR mode the ICE will not necessarily turn off when releasing the accelerator, in order to provide a faster response if the driver accelerates again before coming to a full stop.

After coming to a full stop, the Prius is essentially silent.

One benefit of keeping the engine off, is also that the coolant fan is running far less than in a conventional car. When getting to a parking spot, the engine may already have been off for a few minutes, and there may be nothing to cool off: the air flowing past the engine while driving electrically has already been cooling the ICE off while approaching an empty parking spot.

10. Down-shifting/engine brakingEdit

If the battery becomes fully charged (all 8 bars lit) while the gas pedal is released (e.g. going down a long hill), the ICE will freewheel (no gas injected) acting as an air compressor (exhaust valves are kept closed), and releasing excess energy as heat. In this way the ICE offers resistance and therefore braking power, without involving the hydraulic brakes.

The same happens when the B mode gear is selected (similar to downshifting on a conventional drivetrain/gearbox). Using B mode gear in normal driving situations is not useful, since energy that could be used to re-charge the batteries is instead wasted in heat generated by the engine which is compressing air. No extra energy is actually recuperated in B mode, energy is just being wasted. The only benefit is to prevent the brakes overheating during long mountain descents.

Considerations about the HSDEdit

The ICE can theoretically spin (within limits of the MG1 max rev speed) at any rpm, independent of wheels' speed. The ICE is not directly connected to the wheels and the MG1 acts as a “clutch” or “buffer” to the MG2.

It is not clear as of today, whether the powertrain computer can freely set the rpms of the ICE so that it is always running at the most efficient range (dependent on a number of factors or not (e.g. speed, gas pedal position, etc.)

If the HSD can really run the ICE independently to wheel speed, then a Prius driver could really simply drive as required and still get the best fuel consumption according to the driving style adopted. Then, how much does acceleration style affect really FE? Many users on Priuschat claim/report that using for example PWR mode yields them the best fuel efficiency.

It has been observed, moreover, that the ICE will spin at different speeds, at same wheel speeds, with the same gas pedal pressure, depending on battery charge level - this is something a conventional car cannot do.

Resources

More info can be found on how the PSD works here with a simulation, here from Toyota and again here with an animation. The first and last link refer to the Prius 2nd generation, but the 3rd generation Prius works almost exactly the same way. Another video about the PSD (from the Prius Gen 2, but basically the same on the Prius Gen 3) can be found here . Developed by Temidayo Adewale, Nigeria

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