An Innovative Idea For RV Transmission Control

November 12, 2012 by Lug_Nut · 16 Comments 

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Steep gradeAn innovative product idea from the Lug_Nut file. Automatic transmissions are quickly replacing the mechanically clutched manual gearbox in the world’s high performance automobiles. While many are equipped with single or dual action hydraulic clutches, all are capable of shifting gears automatically. For the positive control of spirited performance shifting, these newer systems employ steering wheel mounted paddle switches. These spring loaded momentary contact levers are located behind the wheel at about nine and three o’clock. They are easily operated with your fingertips while your hands are comfortably gripping the steering wheel. The left one toggles the downshift while the right the upshift. The main transmission control provides a selection of either fully automatic or paddle control shifting. The automatic or manual modes can also be toggled back and forth on some models using just the paddles.

Generally the paddle shift feature is not available on the average automobile, other than as a sales gimmick, as it would have little operational value. But what about a large vehicle like a truck or motor home? The ability to manually select the gearing, in this type of unit, is often a need when operating in very hilly or mountainous terrains.

Currently, diesel pusher owners control user input shifts by depressing one of two buttons on the Allison keypad. The control keypad is generally located to the driver’s left, often near waist level about equal to the operator’s position. This requires looking down and ninety degrees to the left. This is certainly not an ideal place for the driver to have to look during an event that requires such driving strategy.

Jaguar-XKR_2012_800x600_wallpaper_01To understand the value and just how they would work on an RV application, we will use the same principle employed on the ZF six speed transmission as found in a new Jaguar XK. Unlike the XK, an RV will not need the dedicated manual selection that maintains a selected gear regardless of engine speed. But the automatic/manual toggling feature would fit the requirement perfectly.




So let’s see how it could be used and the benefits it would provide.

Ascending a Mountain Grade: (Scenario for a diesel pusher)

The vehicle is approaching a 6% mountain grade and will be climbing for about 4 miles. The ambient temperature is 90 degrees F. To maintain reasonable engine and transmission temperatures, the RPM will have to be fairly high, say about 90% of the engine governed speed. If you allow the transmission to do all the shifting, you will need to operate the engine at, or near, WOT (Wide open throttle) otherwise the transmission will upshift and the RPM will fall. This WOT climb is probably the most common that operators use in this scenario. It does, however, consume the maximum amount of fuel during the ascent. The other method, which is far more fuel efficient and cooler running on the driveline, is to manually downshift. The operator finds the right gear to deliver a steady climb at a constant higher RPM, with only half to three quarter throttle position. While this may take slightly longer to crest the grade, the fuel savings far outweigh the difference. So, why don’t more people use the latter method? Well, it is awkward depressing the shift pad and observing the gear selection. Also the transmission will not automatically up shift as you gain speed at the top of the hill. This also runs a risk of the operator not returning the transmission to a state capable of full upshifts to high, only later realizing he has been driving in a lower gear for the past ten miles.

In this application, the Jaguar designed shifting would allow non-distracting downshifting with the flick of the left paddle. In addition, in this mode, automatic upshifts would occur at the gear’s top RPM. A squeeze on the right paddle exceeding two seconds automatically returns the transmission to fully auto.

Descending a Mountain Grade:

The vehicle is approaching a 6% down grade. As the speed starts to increase the operator applies the Jake Brake, or PacBrake, if applicable. The engine brake automatically selects a lower gear, usually 2nd , and the transmission shifts to the lowest gear it can without over speeding the engine. As the grade eases at the foot of the hill, the vehicle’s speed reduces. At this point either the operator cancels the engine brake or the transmission may continue downshifting. If the operator chooses to cancel the auxiliary brake, as this is most common in this scenario, the vehicle is now coasting, but in a lower gear. The Allison transmission will not upshift while coasting, like that of an automobile, unless either the engine RPM reaches the maximum speed for that gear, or the throttle is depressed at least briefly. Almost all drivers do the latter and depress the throttle to cause an upshift. Unfortunately this wastes more fuel as it restarts the fuel flow which has been cut off some eight seconds after starting the descent. Granted, the fuel burn will only occur for ten seconds or so if the coasting continues, but it is still a waste.

Now with the paddle shift set up. The engine brake is applied in the normal fashion. The operator then flicks the appropriate paddle. He now has full operational control as to which gear will be used. The engine braking can be more varied now as the operator can allow the RPM to go below that where it would have shifted in the automatic mode. Additionally, this feature can be more safely used on a wet, possibly slippery surface as no sudden downshifts will occur. Once to the foot of the hill with the engine brake off, the operator can force the shift to high gear without the need to touch the throttle.

Driving on Flat Terrain

Well it is hard to imagine that the manual shift would be of benefit while crossing the plains, but it can. How often are you travelling on a highway and because of traffic, or person comfort, are hovering around 55 MPH? At this speed, an Allison transmission will more than likely operate in 5th gear as a slightly higher speed will be needed to cause an upshift to 6th. In many cases drivers will briefly accelerate to the higher speed, get the upshift to 6th and slow again to 55 MPH. The high gear will hold generally down to about 50 MPH providing rolling resistance is minimal. But, with traffic and the odd raise in the road, the high gear running time will be short lived in most normal cases. Again, you will find it necessary to increase speed briefly to get the higher more fuel efficient gear.

The manual selected gear shifts via the paddles would prove very beneficial here. At 55 MPH, just flick it into high, done. Even with this forced shift, the Allison will monitor the engine speed to assure it stays within a specified operating range and will downshift, if needed, automatically.

Paddle Shift Design


Now, there is another feature found on the Jaguar ZF paddle shift controlled transmission that would also benefit the RV. During downshifts, the control module sends a message to the ECM that causes the engine RPM to spike briefly and sync the correct RPM of the selected gear. This makes the downshift extremely smooth and helps avoid any deceleration traction loss.  A sample of a paddle shift wiring diagram is shown on the left.


So in summary, think of it this Way. You would have a manual mode that would allow you to operate your coach in any gear within a specified engine speed. If you exceed the high engine RPM or fall to low in RPM, the Allison will make the shift for you. But your operating RPM scope would be a wide window that would offer a wide variety of driving strategies. If operated correctly, this could increase vehicle safety while decreasing fuel consumption. Additionally, a dash readout would display either the Auto or the current manual gear selection. This could be a winning design and a benefit to the heavy vehicle market today.

With An Innovative Idea – Lug_Nut – Peter Mercer


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