Big performance in small packages

  • 15-Aug-2014 08:45 EDT

Of the three cooling loads, the charge-air inter-cooler usually requires the least fan power but determines the fan speed.

Optimizing fan control strategies enables engineers to achieve a machine performance greater than expected from limited power sources. Additional power savings are available by controlling the fan in response to transient power demands, as well as effective integration of hydraulics and electronic control strategies.

Achieving maximum useful work from a limited power source is a step that should be addressed early in the planning strategy for the machine designer. This requires a thorough understanding of the application by the system integrator, including communication with the end user to define their expectations.

Since applications rarely operate at both continuous peak power and extreme environmental conditions simultaneously (typically less than 5% of the duty cycle over the vehicle’s operating life), this creates an opportunity for the designer to tailor the machine performance to the power available and the operating conditions. Once defined, the cooling fan’s implementation strategy may be cost effective and relatively easy to achieve.

One area where the machine designer can see an immediate performance boost is through coolant fan control. By taking advantage of the “thermal inertia” of the systems being cooled, additional power savings can be achieved. There are essentially two strategies to achieve additional power savings: fan modulation based on current system heat rejection and current ambient temperature, and transient power demands on the engine.

Fan modulation

A cooling system is sized to provide the necessary cooling to protect the engine and hydraulics at their maximum defined conditions. When the machine operates at a cooling power demand less than the maximum required, the difference is power that is available for useful work.

While the rate of air moved is linearly proportional to the cooling demand and fan speed, the fan power requirement is proportional to the flow rate cubed. As an example of power savings, if an engine rated at 55 kW requires a cooling power of 6 kW at maximum design ambient conditions, the net available for work and other loads is 49 kW.

If the current ambient fan speed requirement, at constant engine speed, is 63% of the maximum design fan speed, the fan power savings at this condition is 75% or 4.5 kW. The actual power available for work is 53.5 kW, an increase of 9% with no additional input from the operator. In actual practice, the average fan speed can be less than 50% of the maximum design requirement, or a greater than 11% average increase in power available for reallocation.

Distributed cooling

A machine’s cooling system consists of three parts: engine coolant, hydraulic oil, and turbo-charged air inter-cooler. Often these sections are bundled together to use one fan. Of the three cooling loads, the charge-air inter-cooler usually requires the least fan power but determines the fan speed.

The engine coolant, hydraulic oil, and charge-air inter-cooler temperatures are read by sensors. These are compared to set points in the microcontroller and the fan drive software determines the highest fan speed requirement, signaling the variable pump to go to that flow.

The charge-air system has less thermal capacity, or “inertia,” than the engine coolant or hydraulic fluid, and is more dependent on fan speed to minimize temperature fluctuations.

Since the demand extremes on the charge-air inter-cooler are greater than those of the engine coolant and hydraulic oil, it can be advantageous to separate the cooling fans for the engine and hydraulic system and the charge-air cooler. If a single pump is used to drive both fan motors, compensated or bypass proportional valves (depending on whether the motors are in parallel or series) will optimally set each fan speed according to its cooling demand.

Transient power requirements

Transient, or “peak,” power requirements are short duration events. The duration is determined by the difference in load demand and available power.

If a “downsized” engine is desired, then an action is required to overcome the engine power limitations. These limitations might include engine anti-stall and engine over-speed. The strategy may be to shed accessary and/or cooling fan loads for short intervals.

Using the concepts from modulated fans and distributed cooling, a few more lines of code in the fan drive software can turn the fans (for systems with thermal capacity) completely off (for power boost) or completely on (for power absorption.

Power boost

The last figure in this article is representative of an actual test where the engine is loaded to rated power and governed speed, “1,” where the coolant temperature is 80°C, “3.” An additional load is applied causing the engine speed to droop, “2,” below the desired minimum engine speed. At “4,” the coolant fan has been turned off allowing the engine speed to stabilize at 2000 rpm. The coolant temperature climbs to 90°C at “6,” the maximum allowed, over a period of 60 seconds where the coolant fan is ramped back up to prevent engine overheating. The engine speed droops more due to this additional load.

In this test sequence the commanded fan speed is only 50% of maximum when the additional load request is made and the fan is turned off.

Similar tests on other vehicles have demonstrated a 10% increase in additional power available to the system for transient peak loads by shedding accessary loads (in this case, the cooling fan) for short intervals—without compromising system integrity.

Where a cooling system must be sized for more extreme ambient conditions, greater savings may be achieved. Even considering an engine that was downsized by 18%, through optimizing the cooling system and by introducing additional power saving control strategies, the machine can maintain its original productivity.

Conversely, if over-hauling loads (that would tend to over-speed the engine) are present, the fan system can be commanded to maximum speed—automatically, and without involving the operator. The additional power demand of the cooling system is typically equal to the engine’s friction power. In most systems this will essentially double the available power to limit the maximum engine speed during this transient event.

Again, once the proportional fan drive is added to the system, this function can be provided “with the addition of a few more lines of code” to the controller and without increasing cost for additional sensors or valves.

Overall, variable speed hydraulic fans can help the light-duty engine system integrator meet customer’s expectations. This might be measured as power savings from modulated and distributed fans over the life of the vehicle, or by meeting transient peak demands with a downsized engine by taking advantage of thermal inertia. Understanding the application, defining the customer’s expectations, and planning a control system strategy are key steps. As always, testing for confirmation is required.

This article was written for SAE Off-Highway Engineering by Robert Harris, Systems and Application Engineer, Danfoss Power Solutions.


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