|The Hydrid drivetrain. Click to enlarge.|
At the Hannover Messe, 20-24 April, Innas BV and NOAX BV will introduce the latest design of the Innas hydraulic transformer (IHT) and the “Hydrid”, a hydraulic series hybrid drive train for passenger cars and off-road equipment. With this new design, the fuel consumption of a vehicle can be more than halved, with a corresponding reduction in CO2 emissions of 50%, without compromise on weight, size, traction or top speed, according to the companies.
In a Hydrid, the complete mechanical drive train of a car is replaced by a full hydrostatic transmission, allowing energy recuperation and more efficient operation of the engine operation. The backbone of the Hydrid is the hydraulic common pressure rail (CPR) system, which collects and distributes the power inside the vehicle. The accumulators determine the pressure levels in the system. On the high pressure side, the pressure varies between 200 and 400 bar (20 to 40 MPa).
The internal combustion engine powers a constant displacement pump. The engine torque is directly related to the pressure in the high pressure accumulator and can consequently only vary between 50% (at 200 bar) and 100% (at 400 bar) of the maximum torque. Operation of the engine at low loads is therefore completely avoided.
Each wheel has its own hydraulic motor. These motors act as a pump when braking. The recuperated brake energy is stored in the high pressure accumulator. The torque of the in-wheel motors is controlled with the hydraulic transformers, one for each axis. The system has a variable traction control for the front and rear axis. The Hydrid uses floating cup type pumps, motors and hydraulic transformers developed by Innas.
The Floating Cup is an axial piston principle for hydrostatic pumps, motors and transformers. “Floating Cup” refers to the cylinders—each piston gets its own cup-like cylinder, which are free-floating on a barrel plate. Floating cup machines typically have 24 pistons, compared to the 7 or 9 pistons of other axial piston pumps and motors. The pistons are pressed into a central rotor in a double, mirrored configuration.
|Components of a floating cup machine.
Click to enlarge.
|The new Innas variable floating cup pump.|
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The floating cup principle has an extremely high torque efficiency, even at low speed—more than 95% at 0.1 rpm and 350 bar. The principle has almost no torque losses at low-speed driving or during start-up when accelerating from standstill. The multi-piston design creates a smooth, almost constant torque output which is necessary for low noise, vibrations and harshness (NVH).
Compared to electric machines, or even to other hydraulic pumps and motors, the floating cup principle has a very high power and torque density. This is especially important for the in-wheel motors in order to minimize the unsuspended weight of the wheels.
The floating cup principle is designed for deep drawing, sintering and other production technologies which are familiar in the automotive world but relatively new for the production of hydrostatic machines, Innas says.
|Comparing transmission efficiencies. Click to enlarge.|
Transmission efficiency. The German Institute for Fluid Power Drives and Controls (IFAS) at RWTH Aachen University has built a simulation model of the Hydrid. In this model, the efficiencies of the hydrostatic components are derived from measurements on existing floating cup machines. Although the cycle analysis shows that the hydraulic components themselves create more losses than a comparable mechanical transmission applied in a mid-sized sedan, these losses are more than compensated for by the energy which is recuperated during braking.
Including the recuperated brake energy, the total efficiency of the Hydrid transmission is in the end somewhat better than the estimated efficiency of an all-wheel drive mechanical transmission.
|Engine efficiency. Click to enlarge.|
Engine efficiency. Comparing a conventional diesel-engined car with a hydrid car using the same engine (100 kW), the Hydrid delivered a specific fuel consumption of 3.1 L/100km (76 mpg US)—less than half the fuel consumption of the conventional vehicle.
For some 80% of the NEDC, the power demand of the vehicle is less than 10 kW. At these low power conditions, the engine coupled to a conventional mechanical drivetrain can only run in an area with poor efficiency. In the vehicle with the Hydrid transmission, the high-pressure accumulator forces the engine to run between the loads Tmin and Tmax (diagram at right). In this area the engine has the highest efficiency.
The engine is now in on/off operation and is only in operation during 11% of the cycle. For the other 89%, the engine is switched off, thereby completely eliminating idle losses. The hydraulic pump can be used as a starter to enable the frequent on-off operation of the engine.
Innas is an independent engineering company specializing in the fields of hydraulic components, hydraulic drives and combustion engines. Key technologies are the Floating Cup technology for hydraulic pumps and motors, the Innas hydraulic transformer, the Chiron free piston engine and the Hydraulic Common Pressure Rail. NOAX is responsible for the marketing and further development of the Innas technology. NOAX has the exclusive right to exploit the industrial rights and know-how of the Chiron Free Piston Engine, the Innas Hydraulic Transformer and the Floating Cup technology.