Ford's HF35 Transmission: A Hybrid Powerhouse

The automotive landscape is constantly evolving, and at the forefront of this transformation is Ford's commitment to hybrid technology. A key component in this evolution is the HF35 transmission, a sophisticated piece of engineering that underpins the impressive performance and fuel economy of vehicles like the 2022 Ford Maverick. This transmission isn't just a gearbox; it's a testament to Ford's dedication to innovation, efficiency, and delivering a superior driving experience. Let's explore what makes the HF35 transmission a standout in the world of hybrid powertrains.

Understanding the HF35 Transmission

The Ford HF35 transmission is a hybrid automatic transmission that plays a crucial role in enabling the impressive fuel efficiency and smooth power delivery of Ford's hybrid vehicles. It's a complex system that integrates electric motors and a traditional engine to provide optimal performance across a variety of driving conditions. The "HF" in HF35 likely stands for "Hybrid Function" or "Hybrid Ford," signifying its specialised design for hybrid applications. The "35" might refer to a specific generation or series within Ford's transmission nomenclature.

The Maverick's Hybrid Heart: HF45 and its HF35 Heritage

The 2022 Ford Maverick compact pickup truck is a prime example of the HF35's capabilities, featuring a standard hybrid powertrain that achieves a remarkable 42 mpg in city driving. This achievement was made possible through the integration of a new, Ford-designed electric machine into the two-motor HF45 hybrid transmission. While the Maverick specifically uses the HF45, it's important to understand that the HF45 is an evolution of the earlier HF35. As Chris Mazur, MCA (Maverick) program chief, noted, the development team integrated and validated the new electric machine on an "aggressively accelerated" timetable. This was facilitated by leveraging learnings and base calibration from the Ford Escape Hybrid powertrain, which utilized a similar transmission architecture.

Manny Barberena, the hybrid powertrain supervisor, highlighted the time constraint as the biggest challenge. "Go-fast programs make engineers nervous, but we were able to overcome it by being efficient," he stated. By carrying over and adapting the Escape Hybrid's powertrain, the development team significantly minimised re-development efforts. "We did have to do some tuning to make things work properly with this new transmission but it sure saved a lot of time in going from Escape to this program on the common C2 architecture. It enabled us to hit the ground running." This approach underscores the importance of building upon existing, proven technologies.

Engineering Excellence: The HF45's Advancements

The HF45 transmission in the Maverick represents a significant leap forward in hybrid transmission design. Abdul Hajiabdi, the E-drive system and applications supervisor, elaborated on the enhancements. His team focused on upgrading the power electronics for higher voltage and current and enhancing the generator's performance. "pushing its boundaries," he said.

A key innovation was the design of the in-house electric motor, manufactured by Toshiba according to Ford engineering sources. This motor shares the same footprint and transmission interfaces as the one used in the Escape, a crucial synergy that helped keep costs down without compromising performance. "That was one of the major enablers, the synergies, in keeping the cost down without affecting other components," Hajiabdi explained.

Further refinements were made to the motor's rotor and stator. The stator was revised from round distributed winding to flat-wire, hairpin-type windings. This hairpin design results in higher current density and torque density, allowing for a slightly smaller motor with approximately a 20% mass saving. On the rotor side, magnet orientation and tension were re-engineered, with magnets now molded into the rotor core rather than adhesively bonded. This close collaboration between Design and Manufacturing engineering groups was instrumental in achieving these improvements.

In-House Design and Manufacturing Synergies

The development of the in-house motor for the Maverick program marked a shift in Ford's approach to component manufacturing. Hajiabdi noted that prior to this, the relationship between design and manufacturing often involved suppliers. However, with the in-house program, Ford gained full design and manufacturing ownership. "So now we have a full in-house team that works with what we call PTME, the manufacturing-engineering team, producing the parts together." This integrated approach allowed for rapid prototyping, process checks, and quality validation. "We were able to come up with the design, take it to the shop, prototype build it, check it for process and quality prove-out," he continued. This process revealed that some designs, while theoretically sound, were not easily manufacturable at scale. The result was finding the "sweet spot" between high function and manufacturability, a hallmark of true engineering innovation.

Thermal Management and Towing Capability

The Maverick's 2,000-lb (907-kg) trailer-tow capability presented a challenge for the transmission's thermal management. Integrating a new electric machine with its own heat signature into the existing HF45 architecture required careful consideration. "taking an all-new electric machine with its own heat-rejection signature and cooling it in the carryover HF45 'was not a straight plug-and-play,'" Barberena recalled. Through clever internal "tuning" and optimisation of lubricant flow, the team ensured the hybrid pickup could withstand SAE J2807 tow testing, helping to close the towing capacity gap with the 4,000-lb (1814-kg) capacity of the Maverick's turbo gas-engine models.

The HF35's Heritage: A Foundation of Success

The HF35 transmission has an interesting lineage, tracing its roots back to an agreement between Ford and Toyota. Craig Renneker, VP Product Engineering at American Axle Manufacturing, explained that the HF35 was based on the Aisin AW PowerSplit architecture. This architecture, itself inspired by early-1970s TRW technology, allowed Ford to utilise the power flow of the first-generation Aisin AW models HD-10 and HD-20. Renneker proudly stated that the HF35 was "one of the great successes" among the 18 transmissions launched by Ford between 2000 and 2018. The power flow of the Aisin and Ford HF35 are described as "nearly identical," as are the motors and Denso inverter architecture.

The durability of the underlying Aisin design was proven by Ford Escape taxi cabs operating in New York City. Renneker recalled a hybrid Escape cab that had accumulated 450,000 miles (724,205 km) and was "still running fine with the original battery." A subsequent teardown revealed transmission internal parts that were in perfect condition, indicating they would have lasted much longer.

Debunking CVT Misconceptions

Renneker also clarified a common misconception regarding the HF35/HF45 design. Contrary to some media reports, it is not a traditional Continuously Variable Transmission (CVT) where the torque multiplication ratio between the engine and wheels can be varied continuously. In the PowerSplit concept used by Ford, the gear ratios themselves are fixed. However, one of the electric motors is employed to control the engine speed, optimising for either fuel economy or power. This speed control is continuously variable, managed by the software without driver input, which is why the "CVT terminology is still OK," he explained. This intelligent control system is key to the transmission's efficiency and performance.

Maintenance and Fluid Changes for HF45

For owners of vehicles equipped with the HF45 transmission, understanding basic maintenance is crucial. A common query is regarding transmission fluid changes. According to a user who performed the task, the recommended fluid is Mercon ULV. The procedure involves accessing the fill and level plug, typically on flat ground. A pump with a small plastic hose is used to extract the old fluid. It's important to note that not all of the transmission's capacity (around 4.76 quarts) will be drained; typically, around 2.5 quarts are removed in this "half-change" process. This helps remove some of the metal particles from break-in wear.

After draining, approximately 2.5 quarts of new Mercon ULV fluid are added. The fill and level plug is then temporarily replaced. Driving the vehicle for a short distance (about a quarter mile) is recommended to allow any air pockets to escape, as per Ford's procedure. After this, the plug is removed again to check the fluid level. The ideal ambient and fluid temperature for checking the level is between 68°F and 100°F. When no more fluid flows out on level ground, the correct level has been achieved. The fill and level plug is then torqued to 26 lb-ft.

Performing this "half-change" every 30,000 miles can help maintain the transmission's health and potentially avoid the need to access the main drain plug. It's also worth noting that a high transmission temperature warning light on the dashboard could indicate a low fluid level, as insufficient fluid can impair cooling. Early detection of such issues is vital to prevent damage.

Available Parts for HF35/HF45

For those requiring repairs or seeking to understand the components of the HF35/HF45 transmission, a range of parts are typically available. These include, but are not limited to:

The availability of these parts ensures that the HF35 and its successor, the HF45, can be maintained and repaired, extending the life and reliability of Ford's hybrid vehicles. The engineering behind these transmissions is a complex interplay of mechanical and electrical components, designed for both efficiency and durability.

Conclusion

The Ford HF35 transmission, and its evolution into the HF45, represents a significant achievement in automotive engineering. By combining proven Aisin architecture with innovative in-house electric motor technology and intelligent control systems, Ford has created a hybrid powertrain that delivers exceptional fuel economy, smooth performance, and impressive capability. The development process, marked by ambitious timelines and close collaboration between engineering teams, highlights Ford's commitment to pushing the boundaries of hybrid technology. As the automotive industry continues its shift towards electrification, transmissions like the HF35 and HF45 will undoubtedly play a vital role in shaping the future of transportation, offering drivers efficient, capable, and enjoyable vehicles.

Frequently Asked Questions

Q1: What is the main difference between the HF35 and HF45 transmissions?
The HF45 is an evolution of the HF35, featuring a new, Ford-designed electric machine, upgraded power electronics, and refined thermal management, specifically developed for vehicles like the Ford Maverick.

Q2: Is the HF35 transmission a CVT?
While it uses a power split device and offers continuously variable control of engine speed, it's not a traditional CVT with continuously variable torque multiplication ratios. Ford calls it a PowerSplit automatic transmission.

Q3: What type of fluid does the HF45 transmission use?
The recommended fluid is Mercon ULV.

Q4: How often should I check the transmission fluid in my HF45?
Performing a "half-change" of the fluid every 30,000 miles is a recommended maintenance practice.

Q5: Can the HF35/HF45 transmission tow heavy loads?
Yes, the transmission is designed to handle towing, as demonstrated by the Maverick's 2,000-lb tow rating, with thermal management systems in place to cope with the increased load.