SOCIETY OFAUTOMOTIVE ENGINEERS, INC. TwoPennsylvania Plaza, New York, N.Y.10001 THE ALLISON HT-740T AUTOMATIC TRANSMISSION FOR GAS TURBINE POWERED VEHICLES M. E.FISHER Detroit Diesel Allison Division General Motors Automotive Engineering Congress Detroit, Mich. January 11-15, 1971710201 Downloaded from SAE International by University of Edinburgh, Sunday, August 26, 20181 710201 THE ALLISON HT-740T AUTOMATIC TRANSMISSION FOR GAS TURBINE POWERED VEHICLES M. E.FISHER Detroit Diesel Allison Division General Motors Allison transmissions have been adapted for usewith avariety ofgasturbine engines ina number ofvehicle applications. For instance, between 1957 and 1968, 26different installations were made; these included: OnandOff-Highway Trucks Transit Coaches Crawler Tractors Switcher Locomotives Military Tracked andWheeled Vehicles Atleast tendifferent engine models were en countered inthis continuing program ofcoopera tion with engine and vehicle manufacturers. This experience provided theknowledge necessary to achieve optimum compatibility between transmis sion design parameters andfeatures andgastur bine engine characteristics.Automatic range shifting was found very desirable intransmissions operating with free spool gas turbine engines, primarily tocontrol power turbine overspeed andeliminate bulky and costly components and controls from thedrive train. Completely automatic control oftransmission shifting further optimized vehicle and engine operation by. Preventing overspeed duetoinadvertent oper ator action. Maintaining apositive connection between theengine andthewheels atalltimes. Achieving optimum performance and fuel consumption atallpower levels through useof programmed range changes. ABSTRACT Thevehicle industry ingeneral andthetruck ingindustry inparticular have found thatmechan icalpower conversion ofgasturbine engines for prime mover needs requires theuseofanautomatic transmission. Multiple gear ratios arerequired to keep theengine running within itsmost efficient operating range. Power shifting isrequired to prevent engine runaway during shifts andelimin atetheattendant difficulty inmatching ofengine andtransmission speeds forsmooth shifting. The HT-740T automatic transmission wasdesigned toprovide optimum usage ofgasturbine engines intrucks andbuses. Inthispaper adescription ofthetransmission gearing arrangement andcontrols ispresented to show how thetransmission functions and utilizes signals from thegasturbine engine. Other trans mission features peculiar togasturbine applica tions arealsopresented. Inaddition, theextensive development program towhich this product has been subjected isdescribed. Downloaded from SAE International by University of Edinburgh, Sunday, August 26, 20182 Experience hasshown that thegeneral per formance characteristics oftwin spool "free tur bine" engines fallinto a definable pattern. Thus they canbegiven special emphasis during the design anddevelopment oftheautomatic trans mission. The characteristics are: The sharp riseinoutput torque asoutput speed isdecreased gives high output power over abroad usable operating band. This means that wider transmission ratio steps canbeused togood advantage. Specific fuel consumption isgenerally ata minimum inthehigher output speed range. This canbeconsidered inchoosing ashift point modulation program. Inertia oftheengine power turbine andoutput gear reductions isgenerally higher than the inertia ofdiesel engines ofthesame power output. High durability transmission clutches which canbeengaged smoothly against this inertia, with nofluid member tohelp, must beprovided. Generally, thetwin spool gasturbines have a specific limit placed ontheamount ofpower which canbedirected from thegasifier section todrive accessories. Hence, high accessory loads such asairconditioning must bedriven bythepower turbine output. However, with adirect drive mechanism between thetrans mission andengine, when thewheels stopthe power turbine stops. Therefore, inapp