Engineering firm designs innovative “HyDrid” engine solution using Simcenter Amesim

Developing a hydraulic hybrid

Every automotive maker is busy with hybrid vehicles. It is the current buzzword in the automotive industry, and it seems that consumers are putting their money behind this new technology. For instance, the electric hybrid Honda Insight ranked as the top-selling vehicle in Japan in April 2009, the first time a hybrid has secured such a lofty position. But these are early days for hybrid technology and there are many innovative ideas emerging to produce better, more intelligent and efficient hybrid solutions.

Innas is an independent engineering firm specializing in innovative product development. It has a different take on the standard gas-electric hybrid engine. During the past ten years, Innas has been devoted to the development of a hydraulic hybrid. They see it as a better solution since the hydraulic accumulator is far more efficient than an electric battery, especially at high power rates, and the power density of hydraulic motors is greater than electric motors. Moreover, the hydraulic system adds less weight to a vehicle than an electric system.

The fundamental principle behind hybrid vehicles is the storing of energy in an accumulator that can subsequently be used to power the vehicle. In the Innas “HyDrid” concept, a fixed displacement hydraulic pump, connected to the internal combustion engine, is used to charge the accumulator. When it is full, the engine is switched off. The car is then propelled on stored energy until it runs out, at which point the engine is engaged once more and the accumulator is refilled. A hydraulic transformer converts the pressure level in the accumulator to the level required by the hydrostatic wheel motors.

“A hydraulic transformer converts the input pressure into output pressure and functions as a clever integration of motor and pump,” says Titus van den Brink, a project leader at Innas. “The pressure in the accumulator varies as it is loaded and unloaded. Hydraulic motors are connected to the wheels and demand a certain pressure depending on car speed, road inclination and acceleration.

Retaining and returning energy

“That pressure is not necessarily equal to the accumulator pressure. The hydraulic transformer converts the accumulator pressure to the required pressure for the wheel motors. The transformer can even amplify the pressure, boosting the wheel torque with small, simple and robust constant displacement wheel motors.”

Another advantage of the transformer is that it uses the energy released when braking to load the accumulator. This is known as regenerative braking: “Normally, the kinetic energy generated from braking is lost in the heat of the brake discs, but with a transformer you can retain this energy and return it to the accumulator,” says van den Brink.

Managing complexity

Innas has worked on the development of its hydraulic transformer for more than 10 years, culminating in the recent design of a transformer built specifically for hydraulic hybrid drive trains. When it was time for the company to model its hydraulic transformer, Innas selected Simcenter Amesim™ software from Siemens Digital Industries Software.

“We needed a good solver for a complex hydraulic machine with a high number of pistons and shuttle valves,” says van den Brink. “For this we used Simcenter Amesim fluids modeling capabilities. By providing an easy-to-use framework, this 1D simulation platform was able to simplify multi-domain integration for us.”

Innas originally attempted to undertake its modeling by using another tool, but this proved to be difficult. “You had to create your own model in the other solution,” says van den Brink. “If you did it well it would function well, but it wasn’t easy to develop a robust system simulation model. Simcenter Amesim was a more specialized and better solver for the system. This was a big advantage for us. For example, with Simcenter Amesim, the oil model is done for you. You can simply say there is 0.2 percent air in the oil and that’s it.”

Saving time

Innas used the hydraulic component design (HCD) library of Simcenter Amesim, which has a collection of 3,500 components that can be used to build the machine. All users had to do was simply connect validated components in order to accurately predict multi-disciplinary intelligent system performance.

“We tried to model with the other solution, but it simply didn’t work, so we stopped,” says van den Brink. “There’s no doubt that the use of Simcenter Amesim saved us a considerable amount of time.”

Making the difference

In the past, hydraulic motors and pumps have not been regarded as appropriate for use in passenger cars due to high noise levels, high torque variations and poor efficiency, especially under part-load operating conditions. With the help of Siemens Digital Industries Software technology, Innas’ innovative new design overcomes such disadvantages.

“When we developed this transformer, we started with conventional displacement machines that have a limited number of pistons,” says van den Brink. “What we needed was a displacement machine with more pistons, which is why we developed one with 24, known as the ‘Floating Cup’ design. With this many pistons the flow pulsation is decreased, as is the noise.”

The engineer believes that Innas would not have been able to complete the Floating Cup design in the same timeframe without Siemens Digital Industries Software: “With Siemens Digital Industries Software (technology), we increased the development speed considerably,” says van den Brink. “Simcenter Amesim made all the difference.”