From the outside, the bright blue forklift truck looks like any other, but is equipped with Innas BV's Common Pressure Rail (CPR) system. Innas' CPR system, which integrates a Centaur free-piston engine and hydraulic transformers, replaces the existing forklift truck's driveline. The Innas Hydraulic Transformer (IHT) is a highly efficient, compact, hydraulic, continuously variable transmission.

The truck's drive system is the result of efforts by Innas BV, a Dutch engineering joint venture of IHC Holland, Innas Free Piston, and Triodos Deelnemingen, to demonstrate the workings of an alternative hydraulic motor/pump combination in a forklift truck. For this purpose, a Centaur free-piston engine and hydraulic transformers have replaced the driveline of an existing forklift truck. Innas is convinced that the only way that the hydraulics industry can maintain its market position in the face of competition from electric- or fuel-cell-driven mobile machinery will be the application of this newly developed Common Pressure Rail (CPR) system.

"More and more the hydraulic industry is being forced into a defensive position by developments in the electric drivelines, which are cleaner, more flexible, and more efficient than diesel/hydraulic drivelines," stated Peter Achten, Director of Innas. "If the hydraulics industry doesn't respond to these developments, it will end up producing only low-cost hydraulic cylinders and constant-displacement motors and pumps. Or it can recognize that there still is tremendous potential in hydraulics not utilized so far and find creative solutions to exploit this potential.''

According to Innas, complex hydraulic systems using many (expensive) load-sensing valves to control flow are no longer necessary. The engineering company developed a variable hydraulic transformer, which can substitute a complex hydraulic system for the simpler, cheaper, and more efficient CPR system. The Innas Hydraulic Transformer (IHT) is a compact hydraulic version of a continuously variable transmission. It converts an input flow at a given pressure to an output flow at any other pressure with very little power loss. Except for the small energy loss, the conversion is reversible—i.e., the product of pressure and flow at the input is equal to that at the output.

"The principle could best be compared to an electric transformer, where the product of voltage and current, in principle, remains constant. The hydraulic energy is supplied to the cylinders and motor directly. This indicates that the IHT has high efficiency," explained Achten.

The IHT can also act as a pressure amplifier and can transform hydraulic energy to a higher pressure than the input level. The transformation ratio between input and output can be varied manually or by means of a direct electric control. Of great importance for secondary control is the high dynamic response of the IHT. Said Achten: "The hydraulic transformer is the missing link in connecting hydraulic cylinders to a common pressure rail. But, also in combination with constant displacement hydraulic motors, the IHT has proven to be an efficient and cost-effective solution, combining a high efficiency with good controllability.''

Innas engineers replaced the existing forklift driveline with a Centaur free-piston engine and hydraulic transformers. The engine was designed specifically for hydraulic-system applications with a common pressure rail. The free-piston engine is an integrated combination of an internal combustion engine and a hydraulic pump. Its heart is the piston, which is a combustion piston on one end and a hydraulic plunger on the other. The piston is not connected to any mechanism but is free to move within the limitations of the cylinders.

According to Achten, "The advantages of a Centaur free-piston engine are exploited fully if it can be designed for operation at a constant or slightly varying output pressure. For this reason, when using the Centaur in a vehicle, a constant pressure rail—from which the wheel drive, implement systems, and auxiliary functions take their power—is the obvious choice.''

The resulting hydraulic circuit layout is often referred to as being "secondary controlled"—vehicle functions are controlled at the secondary side, directly by the energy users. The hydraulic power unit is controlled in such a way that it keeps the rail pressure above a minimum level.

In contrast, the more conventional type of hydraulic circuit layout is generally called "primary controlled." Here, flow from the power unit is controlled to produce the desired speed of the vehicle functions. The CPR system has several advantages over primary control, most importantly superior controllability and the ease with which energy can be recuperated to the constant pressure rail.

In spite of these advantages, a common pressure rail system has never gained much of a foothold in series-produced mobile machinery. Said Achten: "One of the main reasons for this is the lack of an efficient and inexpensive method of controlling varying cylinder loads from a constant pressure rail. So far, throttling down from the rail pressure has been the method used for obtaining the pressures required by the cylinders. This method results in excellent controllability, but at the cost of high losses. The use of hydraulic transformers works just as well, however, without these high throttling losses.''

Hydraulic system schematics comparing traditional primary-controlled (top) and CPR-controlled systems.

The CPR principle, combining IHT and Centaur technology, can be used for many applications. Noax BV, which obtained the right to exploit the technology, sees many possibilities. In addition to forklift trucks, it can be used in mobile machinery such as mini-excavators and skid-steer loaders—the market segments for which the IHT and Centaur were initially developed.

For more information from Innas BV, circle 413.

SAE Off-Highway Engineering April 2000