Although technically one of the most sophisticated cars ever offered for sale, the Toyota Prius is mechanically very simple. There is very little to go wrong, and indeed the maintenance requirement is lower than for a normal car.

It has no gearbox in the conventional sense, either manual or automatic. (It is sometimes said that the Prius has a CVT gearbox  ((Continuously Variable Transmission)) but this is not strictly true. A conventional CVT gearbox uses belts and pulleys to achieve different gear ratios; the Prius uses none of these.)

It has no reverse gear.

It has no starter motor. 

It has no alternator.

It has no clutch or torque converter.

Its engine does not use a timing belt, so this never needs to be changed.

Its brake pads are called into use far less than a conventional car, and so last longer.

It has low rolling resistance tyres. These heat up less than standard tyres and so contribute towards improved economy. They also last longer.

Its engine revs lower than a conventional engine, giving longer life.

Most engine wear occurs at start up when cold. Because the Prius has no starter motor it uses a motor generator to start the engine. This is far more powerful that a starter motor. It cranks up the engine to 1,000 rpm before switching on the ignition and fuel. Thus the engine fires up at its natural idle speed. In a conventional car the fuel and ignition are firstly switched on then the starter motor turns the engine to about 350 rpm. As soon as the engine fires it struggles under its own power to achieve it’s required idle speed. This results in cold engine wear. Although the Prius suffers some cold engine wear, it is greatly reduced.

 The Prius hybrid system consists of two electric motor generators, high voltage batteries, (in addition to a 12V standard battery) and a controller unit for the system. (computer). The two motor generators are 3 phase units. They have no brushes in contact with the rotor, no carbon build up, and no wear. They are maintenance free and designed to last the life of the car without ever needing attention.

  The hybrid system and batteries are maintenance free, and designed to last the life of the car.

  So how does it work? It was clear at the design stage that a hybrid petrol electric system could result in greatly reduced emissions and improved economy. The idea was to use the electric system to recover and store energy under braking, to allow the petrol engine to switch off during slow moving traffic or while stationary, and to enable the engine to operate within its most efficient power band.

  However the challenge facing Toyota (and this was a very significant challenge indeed!!) was to produce a system that would be small, light, compact, cost effective, easy to manufacture, efficient and above all reliable. (Sounds impossible doesn’t it!)

  And yet this is exactly what Toyota achieved, and within a short timeframe too. The Prius was offered on sale in Japan in 1997 and was an immediate success. Initially Toyota sold each car at a slight loss, but as with everything else, volume sales resulted in reduced costs, improvements to the Hybrid system, and eventual profit per unit. The system invented by Toyota is patented and now belongs to them. It can be fitted to any car, large or small and it is likely that we will see eventually see all models of Toyota offered with a hybrid option. It is also likely that other carmakers will pay Toyota for the right to use this technology. (Ford already do)

The system has very few moving parts (less than a normal car). If you look at the bonnet of a Prius you will notice that it is no larger than any average car. (in fact probably smaller) Nor is the car any heavier, despite the hybrid system and batteries. And eight years of heavy use in Japan (Since 1997) have proven just how reliable the Prius is. There have been no horror stories of breakdowns or expensive repairs. And cars are regularly achieving 350,000 and even more km without any trouble at all.

  In addition the hybrid system requires no knowledge or training to use. The car is just driven as a regular automatic, the cars computer working behind the scene to manage the electronics. In fact the car is most pleasant and effortless to drive, and is much quieter than a normal car.

  And although Toyota recommend that the Prius be taken to a Toyota dealer for maintenance, (well they would wouldn’t they!) in fact any good motor mechanic can perform a full service on the car without any special training.

Technically the system works as follows: 

The output from the petrol engine is fed to a power split device. This consists of a sun gear in the centre surrounded by planet gears within a ring gear. The ring gear has the gear teeth facing inward. The sun gear has the teeth facing outward. The planet gears rotate freely between the two (see diagram) The power from the engine is fed to the centre of the planet gears. (Note this is done in such a way as to rotate these gears as planets around the axis of the sun gear, not around their own axes) Therefore the torque from the engine can cause either the sun gear to rotate or the ring gear or both. It is much the same as a differential unit on a car, where the output from the gearbox can cause one or both wheels to rotate.

The hybrid system has two motor generator units, MG1 and MG2. Both are capable of acting as generators or motors, and of turning in either direction. MG1 the smaller of the two is connected to the sun gear. MG2 the larger is connected to the ring gear, which is connected to the driven wheels.

  To start the engine MG1 spins. Because the car is stationary, MG2 and the ring gear are stationary, and so this causes the engine to crank up and start. Now as the car idles without moving MG1 acts as a generator and charges the battery. If the throttle is depressed the engine speeds up and MG1 spins faster. Electricity from MG1 is fed to MG2 which turns the ring gear and the car begins to move forward.

  MG1 and MG2 working together act as an infinitely variable electrical gearbox. (This is quite similar to the system used in a Diesel Electric Locomotive. In this instance power from the diesel engine drives a generator, which provides electricity to feed traction motors located in the bogies. This allows the large diesel engine to maintain its operation within a narrow rev range, while allowing the train to travel at different speeds.)

  However in the case of the Prius the engine also provides mechanical torque directly to the wheels. The power split device sends 72% of the torque from the engine to the wheels through the ring gear and 28% to MG1 through the sun gear.

  As the car speeds up the rotation of  MG1 and hence the sun gear is reduced by the controller (hybrid computer) This has the effect of reducing engine speed for a given road speed. At a certain speed MG1 is made to rotate backwards, thereby acting as an overdrive. In this way the Prius can achieve a very tall gearing for a given road speed helping to reduce fuel consumption. In this situation MG1 and MG2 are still operating as an electrical gearbox, but MG2 is acting as a generator drawing torque from the engine, and MG1 acts as a motor spinning in reverse.

  At low speeds the engine can switch off altogether and power fed to MG2 from the batteries powers the car.

  Under braking MG2 acts as a generator converting kinetic energy into electrical energy, which is stored in the batteries. This is then used at a later stage to assist driving the car, thus saving fuel.

  Reverse is achieved by powering MG2 backwards. This causes the ring gear to rotate backwards driving the car in reverse. The engine is unable to provide power directly to the wheels in reverse, as there is no reverse gear. It usually switches off when reverse in engaged.

  From a technical point of view the whole system is quite fascinating. It is initially a challenge to fully grasp how it works, and it is then hard to imagine a more efficient or compact way of using a petrol engine to drive a car.

  The end result of all of this is low fuel consumption, low emissions and quiet operation. The Prius achieves truly staggering economy in slow moving traffic and around town, where the hybrid system really comes into its own. As much as 100mpg is possible in some situations. And in this respect the Prius is  the opposite of virtually every other car on the road. It gives its best mileage figures in town traffic. On the highway it is about the same as a diesel.

  Here of course an obvious question comes to mind. Why not make a diesel hybrid? There is no obvious reason why this should not be possible. However the prime motivation behind the Prius was the environment, not just good fuel economy, and petrol engines are cleaner than diesel engines. Also a diesel would tend to be noisy and vibrate under the stop start conditions of the hybrid system. That having been said it is likely we will see diesel hybrids in the near future, probably small capacity highly turbocharged units, giving amazing mpg under all conditions.

The above photo shows the display on the generation 1 Prius. MPG is shown as km per litre. The bar chart on the right hand side is the current fuel consumption. In theis case the car is at rest so there is no reading.

In this photo (Taken with difficulty by myself while driving!) the consumption is shown at 26 km per litre. (Note 10 km per litre is 28.4 mpg so in this case the car is achieving 76.7 mpg.) Every 5 minutes the computer calculates an average consumption achieved and displays this on the left hand bar chart, beginning from 0 to 5 and so on.

In the above case the consumption achieved during a 20 minute journey is shown. (From the Square in Tallaght to my house in Cabinteely using the M50) During the last 5 minutes of the journey 30 km per litre was achieved (85 mpg) In addition 25 WH of energy was saved under braking and stored as electrical energy in the batteries. This is shown as a half sun in the section at the top of the bar chart.

Although the above display may seem rather gimmicky it does serve a very useful purpose. It shows how to achieve the best mpg out of the hybrid system. For example it pays to brake earlier and more gently. In this way more energy is saved in the batteries instead of being wasted as heat in the brake pads. It is also surprising how wasteful short journeys are. The Prius gives poor MPG over the first few miles until it warms up. (All cars do) So try to plan your trips to avoid too many short ones! It is also surprising that brisk acceleration does not seem to give poor economy. This is probably because the engine runs very efficiently under high load conditions. So there is no need to accelerate gently away from the traffic lights. It seems best to briskly accelerate up to a moderate speed and then ease off the throttle.

It is also surprising how efficient the Prius is around town and in slow moving traffic. I once averaged almost 100mpg over a 30 minute period in rush hour traffic.

Arthur McCooey 13/10/05

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Last modified: July 21, 2006