The Future of Motor Coach Transmissions

What may be the next innovation in transmissions for heavy coaches?  To best understand what technology may be next, we will inevitably need to look where we’ve been.  Typically heavy vehicles used manual transmissions or, what was termed as power shift transmissions.   One of the issues with “power shift or automatic shifting transmissions” is internal heat control.  A great deal of heat build up occurs within the torque converter pump assembly and the clutch pack group.  This heat, combined with other internal stress factors, pretty well limit the prime mover’s (main engine) output of both horse power and torque.  The accepted horse power rating of such hydraulic power transfer seems to be about 1,000.  It may be even questionable of the actual efficiency of being over 80% of that number.

Additionally, we look at the total number of gear ratios to best be able to use our available power output.  In this case, more is usually better, as it will better allow the engine to be operated at the most efficient speed for the job at hand.   Early automatic transmissions delivered as little as two gears, with some offering three and four speeds.  In motor homes today we expect five or more gears, and in diesel pushers, mostly six.  European transmissions, specifically Germany’s ZF, offers 10 speed models.  This transmission is quite different from what we in North America are used to.  It is essentially an auto shifting manual transmission.  That is it requires, and looks after, decelerating the engine, engaging the clutch, shifting, disengaging the clutch and returning the throttle to the operators setting.    This sounds simple, and is, but for an operator not used to this, it feels terribly awkward.   For myself, when I had the opportunity to drive such an equipped vehicle, I did not like the auto throttling down when I have my foot into it.  I will stay with the Allison feel and operation, anytime, regardless of less total ratio choices.

So, what could the next generation of transmission be for a motor coach?  We need something that will offer multi-gear like ratio steps, be easy to service or replace and not be dependant on horse power/torque efficiency limits.  It must also deliver the power with as little energy loss as possible to attempt to run more fuel efficient.  It would also be great if it could somehow fit into possible adaptation to any future Hybrid electric vehicle technology. 

Diesel electric, or at least combustion engine to electric generator, may be the solution. This generator would be in place of what we have today, a hydrostatic transmission.  With this system, electric motors would take the place of drive-lines and could even operate directly at the drive wheels reducing all the energy losses associated with 90 degree drive configurations, gear sets, and other inherit heat losses.  Electric motors also offer high torque at low revolutions per minute while capable of delivering very high rev’s without trade off.   It is not surprising to know that Allison is already developing and marketing this technology (Link ) and will be ready to fill the future needs.

Electric driven units may lead to serious fuel savings in the future for all sizes of vehicles. They could employ battery power storage cells and captured reserve energy during grade descents or braking, and we all know that coaches have tons of spin-off energy plowing down a 7% grade.

Well, whether electric drive solutions will prove to be the next generation or not, there is one thing for sure, we will see design changes to power transfer in the coming decade.

Something To Think About,    Lug_Nut