780688 Development ofan Automotive Hydromechanical Transmission William E.Weseloh, Kenneth E.Rudolph, James G.Pratt, and Peter Huntley Orshansky Transmission Corp. THERE ARE SEVERAL IMPORTANT POTENTIAL APPLICATIONS ofacontinuously variable transmission (CVT) inthegeneral field of automobile developments; incorporation into thedrivetrain with aflywheel orwith a single-spool turbine are examples which are well recognized and other examples such as the application with anelectric traction motor are receiving attention. Theability ofthe CVT totransmit power efficiently at atorque ratio which may beindependently controlled (within aspecified range) pre sents anopportunity toimprove the matching ofprime-mover characteristics with the instantaneous drive-line load. Currently available vehicle transmissions have either alimited number ofdiscrete ratios orhave arange ofratios which iscontinuously variable but theratio cannot beindepen dently controlled. Wewould expect the CVT tobeanattractive alternative tocurrent production vehicle transmissions where the prime-mover characteristics show sensitivity totorque orspeed. Fig. 1,for example, shows thebrake thermal efficiency ofa stock 1975 six-cylinder gasoline engine (certified for 49states). Data from themore usual "fish-hook" fuel flow curves was replotted, using aheating value of 20,230 BTU/lb(4.705E 07J/Kg) and shows, for various levels ofpower atthe flywheel ,the sensitivity ofthe engine efficiency tothe speed-torque combination with which that power isdelivered.Ifthe variation oftransmission effi ciency with the input speed issmall enough tobeconsidered asecond order effect, then the engine power required ateach point in time over agiven driving cycle isprescribed and the transmission may improve the driving cycle fuel economy ifitisable tomaintain its input speed such that, ateach required power level, the engine isoperating mostefficiently. Clearly, ifthere ismismatching due tolack oftransmission flexibility, the fuel penalty isnotvery great atthehigher power levels (inexcess of60hp,say), but isrelatively severe when theengine is lightly loaded such asunder vehicle cruising conditions. The CVT mayalso beadvantageous in comparison tocurrent production vehicle transmissions when considering vehicle per formance, orability toaccelerate. Onthe ABSTRACT The development ofanautomotive hydro mechanical transmission isdescribed and the results ofin-vehicle testing presented. Fuel economy and performance results are encouraging; emissions increase, however.Asecond generation transmission isinthe design stage; critical components areunder going development andtesting. Significant progress has been made inreduction ofsize, weight andlevel ofnoise generation. 2649 0096-736X/79/8703-2649$02.50 Copyright ©1979 Society ofAutomotive Engineers, Inc. Downloaded from SAE International by Purdue University, Monday, August 20, 20182650 W. E. WESELOH, ET AL. one hand, if the prime-mover's maximum power versus speed has the flatness of a high- torque-rise turbocharged diesel (over the upper 30% of its operating speed range), the flexibility of a CVT contributes little to the vehicle's acceleration capability over results achieved with a transmission which permits engine speed to depart from the speed giving peak power. If, however, we consider the other extreme, that is a prime-mover showing strong sensitivity of maximum output power to speed (such as the single-spool gas turbine), this requires the CVT's ability to provide any ratio (within a certain range) for the vehicle to attain a practical level of performance. Intermediate cases such as Otto cycle engines and d.c. traction motors will, in general, benefit from operation with a CVT versus a current production transmission, and if the two transmission efficiencies are comparable, the improvement in vehicle per­ formance attributable to the CVT depends primarily on the size of the ratio steps used during acceleration in