Improved fuel economy and performance from specifying hydraulic systems generally comes about indirectly. For example, load sensing hydraulics manages power by matching the output of the hydraulic pump to the load. Ultimately, this results in greater fuel economy. Another technique is to design the hydraulic system so the engine can operate at a speed for maximum fuel economy.
Hydraulic fan drives have also become widely used for improving fuel economy. They keep engine temperature within a narrow range, and because diesel engines exhibit maximum fuel efficiency within a fairly small window, tight control of temperature squeezes a little more fuel econo m y f r o m t h e engine. Conventional belt-driven and fixed-speed fan drives, on the other hand, allow the engine to operate over a wide range of temperatures. Consequently, the engine usually operates at a temperature above or below optimum, so most of the time it is not achieving maximum fuel economy. But fan drives also do not improve fuel economy directly; they simply allow the engine to operate more efficiently.
Hydraulic turbo control
However, Komatsu America recently introduced its PC390LC-10 excavator, which uses hydraulics to control the engine’s Variable Geometry Turbocharger, thereby improving engine performance directly. Standard turbochargers use a turbine in the engine’s exhaust channel to capture the kinetic energy of exhaust gas and convert it to rotational power. This rotational power is then transmitted through a shaft to a turbofan that pressurizes (charges) air in the engine’s intake manifold.
A limitation of standard turbochargers is poor low-speed performance. At low engine speeds, exhaust flow is not strong enough to provide sufficient charging of the intake air. However, Komatsu’s Variable Geometry Turbocharger uses a hydraulic cylinder to vary the effective area of the exhaust turbine. This causes the turbofan to rotate fast enough to charge the intake air at lower engine speeds.
The hydraulic cylinder controls the axial position of the exhaust turbine. At high engine speeds, the cylinder is fully retracted, which exposes the full width of the turbine to the stream of exhaust. However, at lower engine speeds, a directional control valve — commanded by the engine’s electronic control system — causes the cylinder to extend. This removes a portion of the turbine (the right side, as shown in the drawing) out of the exhaust flow. The narrower flow path of the exhaust passage increases laminar velocity of the exhaust flow, which causes the turbine and turbofan to rotate faster from lower exhaust flow. The result is cleaner exhaust and higher fuel economy without sacrificing engine performance.
Functionality with hydraulics
Of course, the PC390LC makes much more extensive use of hydraulics than just for the variable geometry turbo. Its 257-hp EPA Tier 4 interim engine drives a variable-displacement axial-piston pump rated for maximum flow of 141.3 gpm. This main pump powers cylinders for the machine’s boom, arm, and bucket, and three axial-piston hydraulic motors, one for a swing drive and a pair for the hydrostatic travel drive. Relief pressure for the cylinder and hydrostatic circuits is set at 5400 psi, and the swing motor circuit relief pressure is 4050 psi.
Two cylinders for actuating the main boom have a 5.5-in. bore, 58.3- in. stroke, and 3.9-in. rod diameter. The arm cylinder has a 6.3-in. bore, 71.9-in. stroke, and 4.3-in. rod diameter. The bucket cylinder has a 5.5- in. bore, 50.6-in. stroke, and 3.9-in rod diameter. The cylinders are controlled through dual-spool valves with Komatsu’s Proportional Pressure Control (PPC) feature. PPC allows fine control and reduces the effort and travel distance the operator must execute to control the machine.
The reservoir must be sized to accommodate return fluid from all these cylinders at once. The entire hydraulic system requires 96.4 gal of fluid, and the reservoir holds 49.7 gal. This is a dramatic departure from reservoirs designed for stationary (industrial) equipment, where reservoirs are usually sized to hold one minute’s worth of maximum pump flow. The larger volume of industrial hydraulic reservoirs provides an opportunity for fluid to “rest” before being drawn back into the pump. This rest allows sufficient time for cooling, for air to rise to the surface of the fluid, and for heavy contaminants to settle.
Mobile equipment generally cannot benefit from these luxuries because repeated motion of the machine would prevent contaminants from settling and air from rising. Therefore, reservoirs in mobile equipment typically use baffles to reduce fluid turbulence, diffusers to promote deaeration, and heat exchangers.
Just the right touch
Maximizing efficiency and controlling all this power is left to Komatsu’s HydrauMind (Hydraulic Mechanical Intelligence New Design System) feature. HydrauMind is a closed-center system that uses pressure compensation and load sensing to optimize operation and energy efficiency of multiple functions simultaneously. HydrauMind uses simple electronic controls with sensitivity that can be adjusted to match work conditions.
For example, when delicate control is needed when working close to obstacles, controls can be set for low response for the most precise position control. Conversely, controls can be set for maximum response to achieve high-speed motion when working in open work areas.
Six working modes match engine speed and pump output to load conditions based on the positions of operator controls. With load sensing, HydrauMind automatically matches pump output to work conditions. Unlike open-center hydraulic systems used in other machines, Hydrau- Mind’s closed-center system ensures that actuator speed is controlled solely by valve spool position — even when loads change or multiple actuators move simultaneously. This means less power will be used to move a lighter load — such as a bucket full of lowdensity soil — and heavy loads are just as maneuverable as lighter loads.
The PC390LC also takes advantage of well-proven design techniques to maximize performance and reliability of the hydraulic system. For example, boom, arm, and bucket cylinders have cast iron guard rings to protect piston rod-seal interfaces for long life. Also, hoses use flat-face O-ring seals at their end connections for leak-free performance. Furthermore, dual return hoses are used with implement cylinders to provide smooth operation of arm-out functions by returning a portion of return oil flow directly back to the reservoir.
For more information on Komatsu’s PC39LC-10 excavator, visit www.komatsuamerica.com.