Some years ago, the famous Porsche research facility at Weissach, near Stuttgart, was tasked with developing hybrid technology for itself, as well as the VW/Audi group. I’ve now been allowed to see the results of its efforts, and can report that Porsche has succeeded in developing a hybrid that behaves so smoothly in the different modes that it’s just about impossible to know which power unit is in operation.
In addition, it still has the Porsche qualities of performance and sharp handling that thousands of owners cherish. I was not allowed to drive either of the two prototypes, but was driven around in one by a Porsche technician at the Weissach test track, and found that the experience felt as if only the petrol engine was in use. This is because when only the electric motor is in use, it is as quiet as the petrol engine at part throttle, and when the throttle is floored, and both power units are working, a slight V6 throb dominates.
Being a latecomer to hybrid technology has many advantages. Weissach was able to examine the efforts of the major producers of these drivetrains, such as Toyota and Honda, as well as all the minor players in this game of guessing the future. The result is a hybrid that, at first glance, looks like a series layout, but is actually a parallel hybrid. It is undoubtedly the best layout I’ve seen, judged by the criteria of mechanical simplicity, familiarity of components, and versatility.
Why build hybrids?
When one thinks about the future of motoring, it’s worth asking why companies are spending money on hybrids when there’s no doubt that fuel cells are the power units of the future. The answer is that while a lot of time and money is being spent developing both types of vehicles, the fuel cell will only be a practical alternative to hydrocarbon fuel-powered vehicles when ways are found to produce hydrogen cleanly, and store it in a practical manner.
Hybrids are somewhat paradoxical creatures. Why would two engines be more economical to operate than one? The reason is that most cars are overpowered for the job they have to do. Very few cars need more than one-quarter throttle opening to cope with inner-city traffic flow and cruise at 120 km/h. It’s easy to prove this for yourself. You may find that on smaller cars the acceleration will be sedate, but you’ll be surprised by the excellent fuel consumption.
The only reason any car has the extra power is because we’ve got used to accelerating fast when we feel like it; and this explains why we pay extra to get more power. The ideal hybrid uses an internal combustion engine, usually petrol, to supply the basic power needed for sedate driving, and supplements this with an electric motor that can respond quickly to deliver extra torque. This arrangement means that the petrol engine can be tuned for economy instead of performance, and an electric motor forms an ideal partner because it can be designed to deliver maximum torque at a very low speed.
Hybrids can be classified as mild or full, depending on the amount of hybrid capability and the amount of assistance rendered by the electric component. In addition, the layout of the components can be described as parallel or series. A mild hybrid is one that offers additional power and torque via an electric motor that can be engaged or disengaged, but does not have the capability, to drive the car only on electric power. It may well offer a start-stop capability and regenerative braking. Toyota and Honda sell some mild hybrids in Japan that consist of a normal petrol engine aided by a flywheel generator/alternator, making them series hybrids.
A full hybrid offers all the above plus the capability to drive the vehicle on electric power only, for short periods. The Toyota Prius is the best-known example of such a hybrid, and it uses a parallel configuration. The petrol engine or the electric motor can be engaged separately via an epicyclic transmission.
The Porsche hybrid
Porsche has chosen to base a full hybrid on its Cayenne V6. The drivetrain is modified by interposing an electric motor between an electrically operated mechanical clutch and the standard automatic transmission. The usual flywheel, attached to the crankshaft, is just a thin plate to accommodate the clutch unit, so that the flywheel-shaped electric motor forms an additional mass to help damp the engine’s power surges. The gearbox end of the electric motor carries the normal flexplate that connects with a standard automatic transmission. This layout is very compact, and looks hardly any longer than the normal transmission, but has the advantage that it is as simple as a series layout. However, the addition of the clutch makes it a parallel hybrid, meaning that either power unit can operate on its own.
When the control unit determines that the electric motor should run on its own, the clutch is opened, enabling the petrol unit to remain stationary. The electric motor thus powers the vehicle through the automatic transmission, so that its already high torque is multiplied even further. It can do this up to 120 km/h, which is a lot faster than any other hybrid can run on electric power only.
When it is determined that the petrol engine can manage on its own, the clutch is closed, the electric motor starts the petrol engine in milliseconds, and is then deactivated, so that the petrol engine now drives the vehicle through the automatic transmission in the normal manner. In this mode, the electric motor can be activated whenever extra torque is required. This simple system needs a number of extra components and modifications to existing components in order to function seamlessly.
Electro-hydraulic power steering system
The steering system has had to be modified, so that the power steering still operates even when the petrol engine is not running. This means that an electrical power steering pump had to be fitted, drawing power from a low-voltage battery fed by the high voltage battery via a converter. This electro-hydraulic power steering system uses only 12 per cent as much energy as a standard mechanical-hydraulic power steering, resulting in a 1,5 per cent saving in fuel consumption, as measured by the new European driving cycle (NEDC). The Weissach engineers have made a thorough study of what they consider to be typical Porsche steering of the kind that enthusiasts enjoy, and have tailored the new system to have the same feel as previous mechanical-hydraulic systems.
Braking and air-con
A new under-pressure pump (the quaint, but delightful German translation for vacuum) has been developed to operate the vacuum brake servo when the petrol engine is not running. Recuperating some of the energy that goes to waste while braking or coasting is performed by a recuperative brake unit, which works with the ABS unit and the master control unit to ensure that part of the retardation energy is used to charge the main battery, and the other part is supplied by the brake system. For example, mild braking at up to 40 km/h will feed all the retardation energy into the main battery, but at 100 km/h only 66 per cent will be fed into the charging system. The brakes will have to supply 33 per cent. Likewise, the air-conditioning system also needed an electric compressor, allowing the air-conditioning to be used with the engines switched off, but not for long.
High-voltage battery
The main battery is a NiMH (nickel metal hydride) unit of 288 volt nominal capacity. It is able to operate between -30 and 40 degrees C, and is cooled by climate-controlled air from the interior of the vehicle, sucked in by a special fan. It is maintenance-free, gas tight, and mounted in the space normally occupied by the spare wheel, so luggage space is not affected. It weighs 69 kg, and has dimensions of 633 x 347 x 291 mm. A normal 12V battery is fitted for the usual automotive duties, but it doesn’t have to start the engine.
Power units
The electric motor is a three-phase synchronous unit capable of 34 kW at 288 volt, and 285 N.m at start-off and up to 1 000 r/min, gradually falling to 50 N.m at 6 000 r/min. The petrol engine is a direct-injection V6 Cayenne unit developing a maximum of 375 N.m at 3 000 r/min. The combined torque maximum of 555 N.m occurs at 1 600 r/min.
Performance
No claims were made for acceleration and maximum speed, because all the emphasis in a hybrid is on fuel economy. In the case of the Porsche, the improved fuel economy is due to:
1. Driving with an electric motor when possible, at speeds of up to 120 km/h.
2. Regeneration of energy during braking and coasting.
3. Allowing the petrol engine to charge the battery only when an additional load will make the engine run closer to maximum efficiency. In practice, this means that most battery charging will take place when the engine runs at large throttle openings.
4. Coasting with the petrol engine not running. Porsche engineers call this sailing, because the vehicle movement appears to be effortless.
5. The use of a start/stop function. This is particularly useful in town driving, where the electric drive can be used to eliminate using the petrol engine.
The claimed fuel consumption improvement amounts to a reduction of 3 litres/100 km, from the 12,9 litres/100 km measured on the new European driving cycle for the Cayenne V6, to a consumption of 9,9 litres/100 km for the hybrid. Porsche has given no indication when this model will be put into production.
