Due to non-disclosure agreements with Ford, I am unable to directly share drawings, data, and documentation that I produced during my tenure there. Instead, I will outline a few problems I encountered and explain my methodology in solving them.
Cold Weather Efficiency: All vehicles, especially electrified vehicles, experience worse fuel economy in cold weather until they reach their nominal operating temperature. The period where the powertrain is warming up to operating temperature is known as a Cold Start. I was asked to investigate how preconditioning various systems affected cold weather, cold start fuel economy. The two systems I chose to investigate in detail were the high voltage (HV) battery, and the transmission. All tests were conducted with the same EPA drive cycle and at the same ambient temperature within a climate-controlled dynamometer cell in an effort to eliminate as many noise factors as possible from the data.
When the high voltage battery is cold, the amount of power that it can deliver is limited to protect the longevity of the battery cells. This drives more engine use, which increases emissions. The theory was that preconditioning the high voltage battery to its nominal operating temperature prior driving would allow more electric power to propel the car, reducing the amount of engine power needed, thus improving fuel economy. Consultations with lithium battery technical experts confirmed that the train of thought here was valid, and that it would be worthwhile to test.
Since using vehicular systems present on the test vehicle would slowly heat up electrical components and drain the HV battery, I needed to develop a method to circulate hot air through the HV battery while the car was turned off. Since the high voltage battery on the vehicle I tested is air-cooled, I was able to supply hot air into the pack with a household hair dryer and some light ductwork. Additionally, I developed a small circuit to interface with the HV battery cooling fan to externally power it, using it to draw the hot air through the battery pack without needing to turn on the vehicle. HV battery temperature (among many other parameters) was monitored with a data acquisition tool called ATI Vision, and data was recorded during both the preconditioning phase, and the drive cycle phase. This was repeated for approximately twenty trials. I analyzed and organized data for presentation to my management, showing that preconditioning the battery to a nominal temperature prior to performing a cold start did not meaningfully affect fuel economy. It turned out that the climate control system was the dominating driver of engine-on time, due to engine coolant temperature targets which were set by Ford’s climate team to meet internal trustmark requirements.
In cold start conditions, especially in cold weather, transmission oil is extremely viscous, causing temporarily amplified power losses due to the gears spinning through and flinging the cold, thick oil. To reduce this loss at cold transmission oil temperatures, there are two main way to lower the viscosity (power losses); precondition the transmission oil so that it is closer to its normal operating temperature, and use a different formulation of oil. I tested both of these solutions in a full factorial fashion (every possible combination was examined). This involved working with a variety of personnel at Ford Motor Company, ranging from transmission technical specialists, to fabricators, to dynamometer lab technicians.
To heat the transmission oil, a new coolant loop was designed and fabricated with the help of workshop and fabrication personnel. A set of engine block heaters were used to heat a water/glycol solution, which in turn was routed through a liquid-liquid heat exchanger to transfer the heat into the transmission oil. In conjunction with a transmission auxiliary oil pump and some custom plumbing, the transmission oil could be heated without using any of the vehicular systems, improving the accuracy of the experiment. By measuring transmission oil pressure, temperature, and heating circuit flow rate, one could calculate power consumption of the system. By comparing the oil temperature to its temperature-viscosity chart, I selected a test start temperature where the transmission oil was within 10% of its nominal viscosity at operating temperatures. This heating setup also allowed for the transmission oil to be drained and replaced with an ultra-low viscosity (ULV) blend without disassembling the heating circuit.
Approximately 10-15 trials of each test scenario, cold transmission oil, cold ULV, preconditioned transmission oil, and preconditioned ULV, data was analyzed and tabulated for management review. The number of trials per experimental setup varied due to equipment difficulties, dynamometer downtime, and other unavoidable circumstances. As expected, the cold standard transmission oil fared the worst, with the lowest average fuel economy. Preconditioned standard transmission oil and cold Ultra-Low Viscosity transmission oil faired similarly, showing ~5% fuel economy benefit on this specific EPA drive cycle. The preconditioned ULV transmission oil fared the best, showing ~8% fuel economy gain in this specific use case. A statistical analysis (t-test) showed that these results were statically significant with a high level of certainty. Since the results were valid, an energy analysis was performed, comparing the amount of energy used to preheat the transmission oil to the amount of fuel saved, taking into account local energy prices, revealing that preconditioning the transmission oil cost approximately 10x more energy than it saved through lowering viscous losses within the transmission.
As a result of these experiments, all variants of the Ford Fusion hybrid and C-Max hybrid are now shipped with ultra-low viscosity transmission oil. This is because the change in oil specification essentially cost nothing in exchange for a tangible fuel economy boost in certain operating conditions.