Issue: Feb 2007

A hybrid vehicle concept called Strigear from inventor and industrialist Lennart Stridsberg

Automotive Industries spoke to Lennart Stridsberg of Stridsberg Powertrain and asked about the commercial future of his parallel hybrid transmission.

by Lenny Case

The Swedish Stridsberg Powertrain AB develops ultra-efficient hybrid vehicles and high-power density electric motors. Together with the aircraft industry, it is also developing a wide set of proprietary designs to make electromechanical systems in aircraft’s much more reliable. So reliable, that an aircraft can be permitted to take off even after a single fault has been detected. The brainchild of inventor and industrialist Lennart Stridsberg, the company currently markets a hybrid vehicle concept called Strigear.

According to the company, a Strigear car with the body and weight of a Toyota Prius, would be able to do 101 mpg in US city traffic, 82 mpg on US highways, and is able to achieve 100 kilometres on 2.58 liters of fuel in the European Union plus 40 kilometre per liter of fuel in Japan.

Highly efficient hybrids have become much more important with the new EU limit of 130 g CO2 per kilometer. Presently, a Volvo V70 emits 231 g/km, far above the EU limit. With a Strigear hybrid powertrain, it would emit 78 g/km, a reduction by two thirds and far below the EU limit. The Volvo example would require the same amount of semiconductors and battery as in the first version of the Toyota Prius, so costs should not be prohibitive.

A Mercedes E200 NGT with its present diesel powertrain emits 173 g/km and makes 0 –100 in 9.9 seconds. With a Strigear hybrid, it could emit 76 g/km and make 0 – 100 in 9 seconds.

The company says that the Strigear uses two electric motors, an internal combustion engine and an actuator controlled gear box. Gear shifts are performed in some 0.03 to 0.07 seconds, which makes the gear shift unnoticeable for passengers. “When compared to a planetary (Prius) hybrid using equal power components, a gasoline Strigear gives 37% less fuel consumption and 50 - 55 % higher acceleration. In planetary systems like the Prius from Toyota or the Dual Mode from Daimler - GM - BMW, part of the engine power passes both electric motors on its way from engine to tire. This gives lower efficiency and lower accelerations than in a Strigear with similar components. Like the planetary hybrid systems, the Strigear hybrid has two electric motors. One may act as the engine fly-wheel and the other is connected to the input shaft of an actuator controlled gear box. Unlike planetary system, the engine power can reach the tires without passing any electric motor,” says a company statement.

Sophisticated control of the two electric motors permits very fast (30 - 70 ms) gear shifts. The company has made a sample transmission. It has a 6-stage gear box and two electric motors originally developed by Stridsberg Powertrain for use in SAAB aircraft. The motors are air cooled. This eliminates the cost and space normally required for a separate, lower temperature cooling circuit.

Stridsberg has a wide range of patented designs to make aircraft actuators lighter and dramatically more reliable. Some of these are already licensed to airframe suppliers and are designed into coming aircraft. Two Stridsberg companies are also Contractors in three projects in the Sixth Framework Programme of the European Community (2002-2006). Two of these concern aircraft actuators. In the third, a Stridsberg company “HDD Servo Motors AB” develops motor systems for a new basic topology in the robotics industry. This has given Stridsberg and his staff considerable experience in adopting and transferring proprietary concepts in cooperation with air frame suppliers and robot manufacturers. He is looking forward to do the same with auto industry suppliers and car makers.

Stridsberg Powertrain’s patented parallel hybrid vehicle systems supposedly have the advantage that it to some extent uses both the advantages of a serial hybrid vehicle system and a parallel hybrid vehicle system. The thermal engine has an electric generator/motor that is mechanically coupled to its output shaft. On the gear box input there is also an electric motor. The two motors can be connected together, for example by a clutch. The engine side motor can be used to permit very fast start of the thermal engine. During gear shifts, there is one electric motor on each side of the clutch, and these can be used to permit very fast gearshifts. There should be no need for mechanical synchronization hardware as the synchronization can be performed by servo control of the two motors.

Automotive Industries spoke to Lennart Stridsberg of Stridsberg Powertrain and asked about the commercial future of his parallel hybrid transmission.

AI: Is the Strigear concept engine the most fuel-efficient among competitors (you compared it to the Toyota Prius engine) and if so, why?

1.) Its hardware design eliminates the losses inherent in the electrically variable transmissions based on two motors and one or more planetary gear systems. In such systems part of the thermal engine power passes through two electric machines on its way to the tires. This is a high loss path. In a Strigear, the power will in most cases go from the engine to the tires over a purely mechanical path.

2.) The gear box permits the optimizing algorithm to select between for example five different topologies by selecting a suitable gear. In a planetary system such as the Prius, one of the engine and generator speeds can be selected to an optimal load, but the law of planetary systems will then force the remaining part to a speed and torque that may be far from optimal.

3.) The patented optimization algorithm seems to give better utilization of components.

AI: What have been the commercial implications of Strigear? Which companies have evinced an interest in the system?

So far the interest has been very limited. Initially, up to 2005, the performance we described seemed completely unrealistic. Thereafter the simulated performance has been confirmed by some large auto manufacturers. Today most developers are already engaged in heavy development programs regarding other solutions also using two electric motors and one thermal engine. What they eagerly are looking for is a single motor hybrid system that can cut the high costs. The new 130 grams limit from the EU is likely to change the priorities towards high efficiency systems.

AI: The Strigear concept claims almost dramatically higher acceleration than competing system even when using almost identical components. Can you explain why?

LS: During acceleration, there are two power sources, the thermal engine and the energy storage such as a battery. In planetary systems such as the Prius or the Dual Mode, part of the power from the thermal engine is absorbed by the first electric motor (the left motor in Figure 2). To be of use, this power must be used by the second motor (right motor in figure 2) This means that the second motor must be able to absorb both the electric power from the battery and electric power that the first motor has absorbed from the engine. To be able to do that, the second motor must have a power rating above the power of the battery. In the Strigear (figure 1), the two electric motors work in parallel. As long as the added power rating of the two Strigear motors are at least equal to the power of the battery, the acceleration will be given by the total power of the battery and the thermal engine.

AI: What are some of the challenges facing a concept like the Strigear?

LS: We are quite confident in our technology and our simulations. We have inhouse and with partners all the knowledge needed to develop prototypes for verifications. Our major challenge is to find an industrial partner that together with us is prepared to develop the technology into industrial components and bring those to the market either as licensees or supply of components.

AI: What in your opinion is the future of the hybrid powertrains?

LS: When politicians start to implement regulations that can significantly affect the CO2 emissions, there are not many alternatives to highly efficient hybrids using internal combustion engines. The IEA estimates that biofuels could cover between 4 to 8 % of the fuels used in the road transport sector in the year 2030. Biofuels are therefore scare resources and should be used to power high efficient hybrids. Fuel cells vehicles are far away in the future, and so far they do not seem to offer better well-to-wheel efficiency than a Strigear hybrid. As long as the do not improve the total efficiency, fuel cells will require gigantic investments but give no return. Therefore, the future of very efficient hybrids is bright if there are regulations to significantly reduce the consumption of fossil fuels.

AI: What are some of the other concepts you are developing for the automotive market?

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