Alternative hybrid concepts
The Toyota prius may be the most successful hybrid to date,but taking a different approach could produce a more efficient drivetrain
You have to feel sorry for powertrain departments. The pressure on them these days makes working in formula one seem comparatively laid back. Emissions regulations mean that existing engines have to be cleaned up.At the same time,product planners are demanding solutions for the mid-term.
A lot of interest is focusing on hybrids, fuelled mainly by the commercial success of the Prius.Toyota’s vehicle is routinely held up as being at the cutting edge of low-emission technlongies. The Prius may have the advantage of being the first to market,but immprovenent is always possible.
One criticism leveled at the Toyota is that its fuel economy figures are not as good in real life as in theory.On the highway,the added bulk of the hybrid system weighs on the vehicle.At higher speeds,when the engine is the sole source of motive power,the car’s fuel consumption can be higher than the official figures.
It is easy to forget that Toyota’s system is not the only way or achieving a hybrid drivetrain.Othetfirms are working on alternatives that may produce more efficient,more drivable vehicles. DaimlerChrysler and General Motors are collaborating on one such system.
The key to any hybrid system is how you add the electric motors without losing the benefits of the internal combustion enginge. Lennart Stridsberg,an engineer with a background in industrial automation and aircraft control systems ,may have hit upon something that the car companies have missed.
Inventors approaching the car industry with their innovations are often given short shrift.”Automotive engineers tend to assume that outsiders are missing something,” says Stridsberg.
But withcarmakers now taking hybrids much more seriously,they are keen to make up for lost time.Three carmakers have recently started taking a marked interest in STRIDSBERG Powertrain’s technology,confirming the firm’s simulation work checks out .
Unlike the Prius system,which is based on a planetary gearbox,the firm’s”Strigear”drivetrain rdlies on a conventional manual-type transmission under computer control.Both systems have two electric motors, one for generating and ong for supplying motive force. There is also an engine, a battery and some cogwheels.
The Strigear system marries a standard internal combustion enging with a gearbox and clutch.One electric motor sits before the clutch, one sits after. The electric motor after the clutch is used for low-power driving around town.When the clutch is closed,however,all the power from the engine is transferred mechanically to the tyres.It is highly dfficient.
The two motors can slso deliver fast gear shifts, unnoticeable to passengers. The torque interruption is just 55 milliseconds.
In a single-mode planetary gear hybrid, the power from the engine enters the planetary gear via the planetary wheels.Some of the power goes to the outer wheel ,which has low losses.But the remaining power passes through the sun wheel and into the generator-motor. It must then pass through the motor, its inverter and the propulsion-motor and its inverter before it can reach the wheels.”Because some power always passes through the entire system, you have unavoidable parasitic losses,” says Stridsberg.
The other main difference between the two systems is that single-mode hybrids can be more difficult to optimize. With a given speed and torque required by the transmission ,the control system may select an optimal speed and torque for the engine. But the laws of a planetary gear will then dictate the speed and torque for the generator-motor.
“These may turn out to be far from optimal.:says Stridsberg.”The,control system sometimes will have to make compromises between bad engine afficiency and bad generator-motor efficiency.”
Compared with the Strigear system, the first generation Prius’electric motors are larger and produce more torque.Strigear, however,is able to push the motors’torque through the gearbox and so multiply it. So even when the Strigear motors have the same peak power as those of the Prius,the available torque at low and medium speeds are much higher.The arrangement also makes optimization relatively straightforward.
The system’s layout is similar to the one used by Daimlerchrysler in its S400 hybrid. But instead of a manual.the prototype uses Merceded-Benz’secven-speed automatic.
DaimlerChrysler is pooling its resources with General Motors to develop the “two –mode”hybrid. “Two-mode system innovations provide performance and fuel economy improvements at highway speeds,”says Tom Stephens ,vice president or GMpowertrain.”they are better at towing trailers and packaging is more efficient.”
Stridsberg’s simulation work indicates there are some serious benefits to be gained from his drivetrain.”We achieved significantly lower fuel consumption or between 37 and 50 percent and farhigher acceleration when compared to a Prius-type design.”he says “A vehicle developed properly around the Strigear concept will show even better performance.”
As yet there is no physical prototype of the Strigear system using a manual-type gearbox. This could change quite quicdly if and of the manufacturers that have expressed and interest decide to license the technology.
“Not invented here” ued to be a serious obstacle to establishing credibility in the car industry. Finding solutions for emissios regulations is changing that. It is fast becoming the norm.