Four primary power transmission subsystems work together in a wheel loader: the transmission system, the work functions system, the steering system, and the thermal management system. Optimizing any one of these without considering how it impacts the rest of the machine design can result in poor vehicle performance. On the flip side, looking at the machine as one cohesive system can result in true, overall optimization.
In particular, optimizing the thermal management system on a wheel loader—or any piece of construction equipment—can positively impact power management.
Strategies developed by companies like Danfoss Power Solutions can help address many of the challenges facing machine designers, including peak engine power, engine overspeed and challenging ambient conditions.
Creating short windows of increased power availability
During the dig cycle on a wheel loader, peak engine power can be reached when filling the bucket. This means engine speed torque demanded causes engine rpm to drop below an acceptable level. Effective engine power management is key to achieving operating efficiencies.
In these conditions, it can be particularly advantageous to utilize a variable ratio cooling fan system. Utilizing the capabilities of such a system, the cooling fan speed can be reduced significantly (or even turned off) to partially offset the effects of engine droop. As fan power is typically about 10 to 15% of engine power, the increase in available work power can be significant.
The challenge with this approach is the charge air cooler (CAC) performance must be maintained at all times. One method to accomplish this is by implementing a split cooling system with two separate fan drives—one with a larger fan responsible for the fluids (including the hydraulic oil, radiator coolant, transmission oil, etc.) and one with a smaller fan dedicated to the charge air system. In times where peak engine power may be reached, the machine can be programmed to trim the larger fan system while keeping the smaller system running, effectively managing power, cooling demands, and emissions compliance.CAC cooling power is typically around 10% of the liquids’ cooling fan power demands; most of the total installed cooling fan power can be saved. This means power needs are being managed most efficiently during times when the demands are highest. In addition, a smaller fan drive provides the flexibility for consideration of a hydraulic or electric drive solution.
Thermal mass, or the ability of a material to absorb and store heat energy, enables this capability. The liquids in the first system typically have a large thermal mass, therefore temperature changes typically occur over several seconds (or minutes). The charge air system, on the other hand, is a much more dynamic, faster responding system—meaning its cooling needs cannot be suspended. Plus, changes in temperature in the charge air system directly correlate to the ability to remain emissions-compliant. In short, the liquids cooler can accommodate brief periods of undercooling that the CAC cannot.
Our studies have shown liquids cooling systems can be suspended for up to a minute while still maintaining the fluids’ temperatures within acceptable working levels. This is well over the typical time period needed during the duty-cycle peak power demand, which is usually around 5 to 10 seconds per instance.
When the machine returns to a situation with less-demanding engine power requirements, the liquids can then be overcooled to reduce the fluids’ temperature if necessary.
Preventing engine overspeed
Hydrostatic and hydromechanical (CVT) transmissions offer an advantage with dynamic braking behavior—reducing the need for service brake use. During the dynamic braking event, machine energy is dissipated by resistance and losses in the driveline system as well as the available braking torque from the engine.
When wheel loaders experience dynamic braking, engine overspeed can become a concern due to the high travel speed and operating weight of the vehicle. There are already various control system architectures in place that can help avoid an overspeed condition. However, a typical approach is to dissipate the energy into non-usable heat energy within the hydraulic system, which then needs to be cooled. This increases risk of excess heat in the system while reducing overall efficiency due to the additional cooling required.
One method to reduce this waste is by commanding the hydraulic fan drive to run at full speed when dynamic braking. This reduces the amount of torque the engine has to dissipate, as it’s being reallocated to run the fan drive at full speed—subsequently reducing the overspeed behavior.
Enabling better engine performance in cold environments
For construction equipment operating in cold ambient conditions, maintaining acceptable engine operating temperatures can become problematic. In these situations, reducing the minimum fan speed (potentially even zero rpm) can be of significant value. Even if the fan drive is running at a low speed (around 30% of maximum), it can be difficult to get enough heat into the engine in order to achieve optimal performance.
By reducing the fan speed in these conditions, machine uptime can be enhanced as it may no longer be required to install radiator grille covers or other restriction devices used to maintain engine temperatures.
Minimum fan speed can be challenging for larger fan drives that use a piston pump due to minimum margin pressure—or low-pressure standby. This pressure is directly associated with minimum fan speed. Therefore, the ability to achieve a lower operating pressure will result in a lower minimum fan speed, which contributes to better engine performance.
Other fan drive configurations offer additional options and flexibility. Closed-loop fan drive systems, and similar types, coupled with a closed loop control make it possible to manipulate the control input to create a zero-stand speed condition for the fan drive. This basically puts the fan motor on standby, achieving the lowest fan drive operation possible. Implementing new technologies can directly increase wheel loader operability in climates affected by cold operating temperatures.
When engineering teams look for new ways to streamline power management and boost engine performance, they should examine how the thermal management system is being applied. Considering the wheel loader as a complete, cohesive system (rather than a combination of autonomous systems) can result in unexpected improvements in efficiency.
Aaron Becker, market development manager at Danfoss Power Solutions, wrote this article for Truck & Off-Highway Engineering. Becker has a diverse background in product engineering, manufacturing engineering, quality and sales in both ISO9000 (APQP framework) and AS9100 manufacturing environments.Continue reading »