Why the future is hybrid describes the different kinds of hybrid “petrol-electric” car:
The simplest kind is the “stop-start” or “micro” hybrid, which is not generally regarded as a true hybrid because it relies solely on an internal-combustion engine for propulsion. As the “stop-start” name implies, the engine shuts off when the vehicle comes to a halt. An integrated starter-generator restarts the engine instantly when the driver steps on the accelerator. All of this increases fuel efficiency only slightly, typically by around 10%. But few modifications to a conventional design are required, so it costs very little. In Europe, PSA Peugeot Citroën has just introduced a stop-start version of the Citroën C3, which sells for roughly the same price as a similarly equipped conventional C3.Next come so-called “mild” hybrid designs, such as Honda’s Integrated Motor Assist (IMA) — the hybrid configuration found in the Insight, the Civic and the new Accord. In addition to a stop-start function, an electric motor gives the engine a boost during acceleration. During braking, the same motor doubles up as a generator, capturing energy that would otherwise be lost as heat and using it to recharge the car’s batteries. Since the electric motor is coupled to the engine, it never drives the wheels by itself. That is why this system is called a mild hybrid, much to Honda’s dismay. The design is less expensive than Toyota’s more elaborate approach, but can provide many of the same benefits, says Dan Benjamin of ABI Research, a consultancy based in Oyster Bay, New York. The hybrid version of the Civic achieves 48 miles per gallon, a 37% improvement over a comparable conventional Civic.
Toyota’s Hybrid Synergy Drive, a “full” hybrid system, is much more complex. (The Ford Escape hybrid uses a similar system; Ford licenses a number of patents from Toyota.) Using a ?power split? device, the output from the petrol engine is divided and used both to drive the wheels directly and to turn the generator, which in turn drives the electric motor and also drives the wheels. The distribution of power is continuously variable, explains David Hermance of Toyota, allowing the engine to run efficiently at all times. When its full power is not needed to drive the wheels, it can spin the generator to recharge the batteries. The batteries also get replenished when the car is coasting or braking. During stop-and-go traffic and at low speeds, when the petrol engine would be most inefficient, it shuts off and the electric motor, powered by the battery, takes over. That explains why the Prius has a better fuel economy rating for urban driving (60 miles per gallon) than for motorway driving (51 miles per gallon) — the opposite of a conventional vehicle.