Which transmission is best for the Actros in off-road terrain? A twelve-speed transmission by Powershift or the standard 16-speed automatic transmission? We have tested both versions.
The Powershift family offers a twelve-speed transmission with a maximum input torque of 2433 lb-ft (3300 Nm) for the Actros. This is also sufficient for hard off-road missions. However, customers who nonetheless prefer to have 16 speeds in their dump truck can still order this option. The 16-speed equivalent to the Powershift automatic transmission provided by Mercedes is the G 240-16 automated transmission that even has two overdrive gears at the end of the gear range. The twelve-speed Powershift can only offer one overdrive. A strong argument for the conventional 16-speed solution is the transmission spread of 16.99, which is impressively high and promises a first gear with high traction and low clutch wear as well as economic rpm in the highest gear.
Gear Ratios: 3.33 versus 3.52
The maroon 2646 LK test candidate with a 233-millimeter crown gear and 16 speeds has a gear ratio of 3.33. This results from a rear axle drive ratio of 4.83 and a gear ratio of 0.69 for the highest gear. The first gear therefore extends up to 3.9 mph (6.2 km/h). The small reverse gear brings the Mercedes Actros to a maximum of 4.2 mph (6.8 km/h). This offers advantages under off-road conditions. The tachometer read 1450 rpm at a freeway speed of 53 mph (85 km/h). It dropped to under 1100 rpm at a country road speed of 65 km/h. This is only a hair above the point of maximum torque, and driving is no longer very comfortable when the track is even slightly inclined.
The gold-colored Powershift dump truck is different. It has a crown gear with a diameter of almost 12 inches (300 mm). This corresponds to a rear axle drive ratio of 4.57, instead of the 4.83 used in the Bordeaux-colored Telligent dump truck. But the highest gear of the unsynchronized twelve-speed transmission—an overdrive with a 0.77 gear ratio—moves the overall gear ratio to 3.52. The test truck with the Powershift had therefore slightly shorter-gear ratios than its competitor with the Telligent transmission. When it comes to off-road abilities, the gear arithmetic is not a reliable predictor. The first gear reached 4.2 mph (6.7 km/h), and the first reverse gear had a maximum speed of 3.8 mph (6.1 km/h), which is just the other way around than for the Telligent dump truck. The Powershift transmission spread of 15.10 is narrower overall, which has an impact when driving faster. Country road speed is reached at 1160 km/h. Minimal inclines are handled without Powershift having to hunt for another gear. On the freeway, at 53 mph (85 km/h), the tachometer goes towards 1520 rpm. This is acceptable, even when the engine room sounds quite lively.
Powershift Has No Weight Advantages
Although the Powershift transmission is not synchronized, it does not have an overall-weight advantage. The theoretical advantage is immediately negated by the weight of the significantly more solid gear wheels. After all, ruggedness does appear out of thin air. 2433 lb-ft (3300 Nm) needs to be handled. The Powershift G 330-12 transmission is actually almost 29 lbs. (13 kg) heavier than the G 240-16 16-speed transmission. On the other hand, the 16-speed transmission did not have fuel-consumption advantages on mountain routes either, in spite of its finer grading. The editors drove both dump trucks, each with a total weight of 40 tons, as a tandem set through the steep mountain passes of the Swabian Alb. The evaluation even showed a very small fuel-consumption advantage for the Powershift dump truck. This was in the range of half of a percent, a theoretical value.
This advantage shrinks to a tiny quarter of a percent on hilly or flat roads. This is partially due to the longer overall gear ratio of the Telligent dump truck. It implies better fuel consumption with partial load. That leaves the question why the Telligent dump truck cannot take advantage of the finer grading of its transmission when driving up hills. The answer is simple: the numerous gears create the disadvantage. The Telligent transmission simply shifts too often. There is no lack of rolling-resistance changes on this test route, with the steep, hair-pin-packed climbs up the Swabian Alb. The finely graded Telligent transmission tried to get things right all the time and ultimately shifted gears so often that the high number of traction-force interruptions negated the theoretical advantage of shifting gears.
Electronic System Not Always Right
Telligent dump truck shifted gears exactly 26 times between the (low) Unterland and the (high) Albtrauf sections. The gearshift result in differences of 250 to 300 rpm. That challenges the computer. The electronic system has no way to look ahead, so it misinterprets the situation sometimes. It works well on some test inclines, but clearly makes the Telligent transmission shift too often on other climbs. It can therefore not match the overall results of the Powershift dump truck. The Powershift competitor handled the same challenge with only ten gear shift processes between gears eleven and six. Its gearshifts result in differences of approx. 400 rpm, which can sometimes necessitate unfavorable outcomes.
On an eight-percent incline, the twelve-speed dump truck will stubbornly persist in sixth gear, with nominal engine rpm. Shifting to the next-higher gear would not be possible on such an incline. However, the truck is economical. Instead of drinking unduly as a result of the high rpm, it even saves overall half a percent of fuel on mountain routes, due to its low number of gearshifts. Most of all, it saves time. The Powershift dump truck covers the same mountain route an impressive three percent faster than its Telligent competitor, i.e. it provides a clearly better driving performance with a lower number of gears.
The test results are clear. Twelve gears in the Powershift transmission beat 16 gears in the Telligent transmission. The Powershift not only shifts gears more comfortably and, if needed, faster than the Telligent transmission, it also offers special functions such as maneuvering, power, and rocking modes, and can also provide an off-road mode (at a surcharge). How many gears are really correct is also pondered by other manufacturers than Daimler. From Scania to Volvo to ZF, all seem to agree on twelve gears.