Other Articles - May - 2018

Are More Gears the Answer to Improve MPG?

In case you haven’t noticed, many automotive manufacturers now offer transmissions with eight to ten different gear ratios. This has been a major change in the past decade where four-, five-, and six-speed transmissions dominated the market.

The additional transmission gearing is mainly due to the increased demand for fuel economy, where the automotive manufacturers are challenged to achieve a corporate average fuel economy (CAFE) of 54.5 mpg by 2025.

Just for reference, the combined CAFE rating in 2011 was 24.1 mpg. The 54.5 mpg requirement seems like a lofty goal, but it isn’t a hard-set number. There’s an advanced formula for calculating CAFE for 2012 through 2025 vehicles, which involves the vehicles’ actual fuel economy and their “footprint,” or size.

Nevertheless, this fuel economy requirement has automotive manufacturers scrambling to design transmissions that can place the engine at the most efficient speed for maximum fuel economy.

This is in addition to the other trends we’re seeing in the industry, such as plug-in hybrids, electric vehicles, turbocharged engines, gasoline direct injection, variable compression (Nissan), and even compression-ignition gasoline engines (Mazda).

To improve efficiency through the transmission, automotive manufacturers are using continuously variable transmissions, dual clutch transmissions, and transmissions with many available gear ratios. With these transmissions, the torque converter clutch can engage at lower speeds so energy isn’t wasted through needless heat generation in the torque converter.

But how much have the increased gearing and shift strategies affected today’s vehicles’ overall fuel economy? And how has the increased gearing affected the reliability and stress of the internal transmission components? I’ve been curious about these questions ever since Lexus offered the first production eight-speed in the 2007 LS460.

One thing to consider is that the source of information for this article isn’t very scientific. I used the published data from fueleconomy.gov to determine the fuel economy changes when comparing models with different transmission options.

I tried to pick vehicles that simply had a transmission change, but most manufacturers release new powertrains with new model launches. So, even though every example has the same displacement engine, there’s no guarantee that the rest of the vehicle hasn’t changed to affect the fuel economy significantly.

As far as reliability, it’s difficult to forecast the failure rate of a transmission based on the number of gears and how often the clutches, solenoids, and valves cycle on and off. But I looked at the clutch and solenoid activity between the lower-speed and higher- speed transmissions. Maybe this information will provide an outlook to how much the additional transmission shifting and hydraulic actuation will have on the wear and tear of the internal transmission components.

First, let’s look at Fiat Chrysler Automobile’s (FCA) six-speed RFE transmission compared to the ZF eight-speed.

FCA started to phase out the 65RFE, found in the half-ton RAM truck in 2014, in favor of the ZF8HP70. Incidentally, ZF makes up about 50% of FCA’s entire transmission lineup in model year 2017. They’ve placed over 3.5 million ZF transmissions on the road since 2013!

When reviewing the fuel economy increases in table 1, a modest one mpg in the city and two mpg on the highway doesn’t sound like much, but since the vehicle doesn’t get the greatest fuel economy to begin with, the improvements are seven and nine percent, respectively.

When comparing the 65RFE to the ZF8HP, the clutch element cycling increased 29%, which is expected, given the additional gears. Clutch element cycling is simply the number of times the clutches shifted from released to applied or from applied to released, while upshifting from first gear to high gear.

The number of clutch cycles is divided by the number of clutches in the transmission. So, in the case of the six-speed, there are five clutches that cycled a total of ten times, for a clutch-cycling ratio of 2.0. The ZF eight-speed transmission cycled its five clutches 14 times for a cycle ratio of 2.8. Tables 2 and 3 show the range reference charts for the two transmissions.

Regarding electronic and hydraulic activity, we can take a similar approach by looking at the solenoid activity and how often the electronics and valves are shuttled on to off and vise-versa.

As shown on tables 4 and 5, since both the FCA and ZF transmissions use solenoids that control clutches, the solenoid activity is the same as the clutch cycling ratio. The only exception to this is that the L/R solenoid on the 65RFE is also used to apply the torque converter clutch in second through sixth gears. This activity isn’t factored into the solenoid-cycling ratio.

The Toyota Highlander has seen an upgrade from the tried-and-true U660E six-speed transaxle to the new UA80E eight-speed transaxle. They call this transaxle a direct shift transmission; don’t confuse that with a dual clutch-type transmission, such as the one found in VW vehicles, which is also called a direct shift gearbox (DSG).

A comparison of the fuel economy, shown on table 6, isn’t likely fair. Even though both vehicles have a 3.5L V-6 engine, the 2017 eight-speed equipped engine has slightly higher compression ratio, direct injection, and advanced variable valve timing (VVT-iW). The VVT-iW allows up to 75 degrees of intake valve retard, which lets the engine mimic the Atkinson’s Cycle, allowing the intake valve to remain open longer to increase efficiency.

As you can see in table 6, the fuel economy has improved slightly.

A review of the range reference charts (table 7 and 8) for these two transaxles shows that the clutch cycling for the U660E is 2.25 clutch cycles across the four clutches while upshifting from first to sixth gear.

The UA80E has 2.6 clutch cycles across the five clutches when upshifting from first to eighth gear, which is about 13% more active than the six-speed. Like the previous Ram truck example, the solenoids are responsible for applying clutch pressure, so the solenoid cycling increased the same amount.

Honda has explored different transmission options across the past few years. The Honda Odyssey has a six-, nine-, and ten-speed available for the 2018 model year. They’re all found behind the 3.5L V-6 engine and surprisingly there’s hardly an improvement in fuel economy between the three.

As you can see in table 9, the city fuel economy didn’t change at all from the six-speed to the nine and 10-speed, and there’s only a 4% improvement on highway MPG.

These three transmissions operate completely differently from each other. The Honda six-speed is the traditional Honda design with helical mesh gears and clutch assemblies that lock the speed gears to their respective shafts. The ZF9HP is a planetary gearset type transmission that has two dog-clutch mechanisms that allow the transmission to stay relatively small, yet offer nine speeds.

The new Honda 10-speed has four planetary gearsets. Yes, you read that correctly. Honda built their own transmission with planetary gearsets, which is a big step considering the only transmission built with a planetary gearset in the past was their CVT, and it was only for reverse operation. Look for articles on this unit in the coming months.

The Honda 6-speed looks massive when sitting next to its 10-speed replacement (figure 1). Just in size and weight alone, one would expect fuel economy improvement. The six-speed’s hydraulics have been dramatically simplified when compared to the five-speeds that are common in transmission shops, such as the BAYA.

The six-speed uses three clutch pressure control (CPC) solenoids to apply hydraulic pressure to each gear’s clutch. The only gear that doesn’t actively shift is the first clutch, since it receives line pressure from the manual valve and can stay applied because of a one-way sprag.

As you can see on table 10, the three solenoids operate the remaining five clutches and gears with shift solenoids A and B shuttling shift valves to allow CPC A to either control the 2nd or 5th clutch, and CPC B to control the 3rd or 6th clutch. The clutch cycling is 1.67 and the linear (CPC) solenoid-cycling rate is 1.5.

The ZF9HP is basically the same unit as the ones used in various Jeep, Chrysler, and Land Rover vehicles. Following the trend with most modern electronics, it also uses solenoids that control pressure to their respective clutches, as shown on table 11.

The ZF9HP cycles its clutches and solenoids 2.67 times when shifting between first and ninth gear. The clutches are 37% more active and the solenoids are 44% more active than the six-speed.

The Honda 10-speed has 15 solenoids in the transmission (figure 2). Clutch pressure control (CPC) solenoids B through G control pressure to specific clutches as you can see in table 12.

This transmission also has a two-way clutch that’s actuated by shift solenoids A and B. The two-way clutch is a mechanical ratchet-type clutch that uses a lever to lock the clutch from rotating in either direction, or just one direction.

Like many of the previous units described, the solenoids are assigned to specific clutches, so their cycling rates are the same as the clutches, which average 2.83 cycles when shifting between low and high gear. This accounts for a clutch and solenoid cycling increase of 41% and 47% respectively. Not bad, considering the 10-speed has 67% more gear ratios than the six-speed!

Ford and GM broke barriers once again and came together to design a 10-speed transmission. For the F150, the 10-speed replaced the ZF6HP and represents modest-to-no fuel economy change as shown in table 13.

Ford’s application of the ZF transmission uses five clutches and four solenoids to provide six forward speeds. The solenoid cycling is higher than the clutch cycling, because ZF uses the VFS4 solenoid to operate both clutch E and brake D. From low gear to high gear, the clutch cycling is 2.0 and the solenoid cycling is 2.5. Table 14 shows the clutch and solenoid application.

The new 10-speed uses six hydraulic clutches and a mechanical one-way clutch. The solenoids and clutches cycle an average of 2.83 times when shifting from low to high gear.

As shown in table 15, the shift solenoids A through F are proportional and control their respective clutches. Even though the 10-speed has 40% more gear ratios available than the six-speed, the clutch activity only increased by 29% and the solenoid activity only increased by 12%.

Without a doubt, performance and marketing also play a role in the movement to higher-speed transmissions, but after reviewing these modest fuel economy improvements, it goes to show how difficult and expensive it is to squeeze out an additional mile or two per gallon.

It also shows the enormous task to achieve the 54.5 CAFE requirements of 2025. It’s going to be interesting to see what comes out in the next seven years as the quest to high fuel economy continues.