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HOT AND COLD


Furthermore, even with validated software models, there could be powertrain hardware limitations that remain undiscovered until the vehicle undergoes comprehensive testing.


While electronically controlled powertrains have greatly simpli- fied the responsibilities of the operator, they provide challenges to the Army test community when attempting to evaluate mobility performance at desert operating conditions (120 degrees). Teir ability to self-regulate their critical operating limits occurs in two primary methods: engine deration and modulated powertrain cooling. (See “What Is Engine Derate?”)


Both of these regulation methods often have a nonlinear response to the change in ambient temperature. In order to definitively evaluate a vehicle’s mobility performance of an electronically controlled powertrain, the vehicle must be immersed in the targeted environment to produce a representative response. Tis becomes particularly challenging for ambient temperatures at 120 degrees as these temperatures are rarely occurring within the United States.


OUT OF THE DESERT AND INTO THE OVEN Even though permitted by test operating procedures, linear extrapolation of powertrains fluids based on ambient temperature should be heavily discouraged when considering electronically controlled powertrains as this can practice introduce significant error or invalidate the test results.


If linear extrapolation is not an option for electronically controlled powertrains, what is the solution?


Since the 1950s, the Army has invested hundreds of millions of dollars in vehicle laboratories that have environmental control. Tese facilities can heat or cool the air from 50 below to 120. Furthermore, they are equipped with dynamometers to exercise the vehicle’s powertrain. A dynamometer is essentially a treadmill for vehicles. Tese vehicle laboratories have the capability to test every tactical or combat vehicle from a Humvee to an Abrams, all with the vehicle operating at top speed but standing still.


To study the dangers of linear extrapolation on electronically controlled powertrains, test data was analyzed by test engineers from the Ground Vehicle Systems Center (GVSC). Tis test data came from various sources including engine, power pack (engine plus transmission plus cooling system), or full-vehicle test programs on vehicle platforms such as Abrams, Stryker, the M109A7 self-propelled howitzer and the Heavy Equipment Transporter.


78 Army AL&T Magazine Fall 2022


WHAT IS ENGINE DERATE?


Though both operate via a simple accelerator pedal, electronically and mechanically controlled powertrains are vastly different under the hood, so to speak. In a mechanically controlled powertrain, the force of the driver’s foot on the accelerator pedal is physically transferred via linkage or cable to the engine’s fuel injection pump. More force on the pedal, more gas to the engine. While this simplicity has a predictable outcome, it comes with some disadvantages. In the event the vehicle is operating in a hot environment or under heavy load, a driver may unintentionally surpass critical limits for engine oil temperature or coolant temperature, which could result in severe damage, like blown head gaskets, failed main bearings, scored cylinder walls and other heat-induced failures. That means that a mechanically controlled powertrain will meet the driver’s demands even to the point of total catastrophic failure. The only restraint might be a driver’s own response to indicators like temperature gauges or warning lights.


In contrast, electronically controlled powertrains inher- ently possess the ability to self-limit—and effectively override the driver’s demand for more. They monitor and control their own condition from the feedback of the sensor network and prevent operation beyond crit- ical limits. If the driver floors demands 100 percent, the system won’t honor the demand if the driving conditions are too severe. That is engine derate.


What’s important to understand here is that error resulting from extrapolation combined with electronic controls introduces a bias toward failure—the error can fail a perfectly good vehicle because the test data is wrong.


Due to these operating principles, linear extrapola- tion will always produce a false failure, but never a false pass. It’s important that this one-way bias is recognized as it presents significant risk to a vehicle acquisition program.


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