National HistoricMechanical EngineeringLandmark

The American Society ofMechanical Engineers

Jacobs® Engine Brake RetarderBloomfield, ConnecticutOctober 17, 1985

Clessie L. Cummins at work in his Sau -

salito shop.

A History of The Jacobs ®

Engine BrakeBy C. Lyle Cummins, Jr.

Long steep downhill grades not onlyprovide anxious moments for truckd r i ve rs , bu t t hey a l so add ex t rahours behind the wheel. The adventof the Jake Brake ® compression en-g i n e r e t a r d e r b r o u g h t a n o p p o r -tunity to end this fear almost entirelyand at the same time to safely raiseaverage speeds.

To a motorist or a bystander, theJ a c o b s E n g i n e B r a k e , c o m m o n l yk n o w n a s t h e “ J a k e B r a k e , ” iSsometimes associated with a char-acter is t ic “popping” exhaust noiseheard when a truck is deceleratingor descending a h i l l . To the t ruckdriver, it is a device that turns hisengine in to a re tarder , g iv ing h imabout the same braking effort at thewheels as his engine puts out whenit is producing power.

How does this work? Quite sim-ply, the Jake Brake retarder turns adiesel engine into an air compressor.Rather than store the energy of thepressurized air in the cylinder, cre-ated by the piston coming up on thecompression stroke, the Jake Brakeretarder mechanism hydraul ica l lyopens the exhaust va lve near theend of the upward piston stroke. Thestored energy in the cylinder iS r e -leased to the a tmosphere so thatwhen the piston descends on what

would normally be the power stroke,no pressure remains in the cylinderto act on the piston.

Thus , t he ene rgy t r ans fe r r edf r o m t h e d r i v i n g w h e e l s i n t o t h ecompressed and highly heated air(through gravity or forward momen-tum of the vehicle) goes mostly outthe exhaust pipe, with a portion en-tering the cooling water from heattransfer through the cylinder walls.This has two results: on long down-grades the engine cooling system re-m a i n s a t n e a r n o r m a l o p e r a t i n gtemperatures, and in the case of tur-bocharged engines there is sufficientexhaust energy to allow the turbineof the turbocharger to power thecompanion compressor and main-tain a high enough degree of boostto increase the retarding effort.

A diesel, being a free-breathingengine by virtue of having no valvingor ventur i rest r ic t ions (carburetor)offers inherently less braking effortthan a gasoline (spark ignition) en-gine even though the diesel enginehas about tw ice the compress ionrat io o f the spark ign i t ion engine(which obviously means that i t re-quires more effort to force up thepiston on the compression stroke),that work is g iven a lmost ent i re lyback (less friction and heat transferlosses) when the air is allowed to ex-pand on the next outward s t roke.Conversely, the spark ignition engineb e c o m e s a v a c u u m p u m p w h e nb e i n g m o t o r e d w i t h t h e t h r o t t l eclosed. This is a basic reason whythe spark ignition engine is so muchless efficient than the diesel or com-pression ignition engine when oper-a t i n g a t i d l e o r l i g h t l o a d i n g . O fcourse this assumes that the fuel isessentially shut off and no combus-t ion can occur–which is t rue wi ththe diesel engine but not so with thespark ignition engine.

A l t h o u g h t h e J a c o b s E n g i n eBrake has been on the market since1961, the need for it was terrifyinglydemonstrated to its inventor, ClessieL. Cummins, some thirty years ear-lier. In those times, Clessie was try-i n g t o p r o v e t h e w o r t h o f t h eautomotive diesel engine as well asto learn firsthand the kind of enginethe trucking industry most needed.In August of 1931, one of these com-binat ion test and publ ic i ty s tuntsfound him, Ford Moyer, and DaveEvans dr iv ing a Cummins Diesel -p o w e r e d I n d i a n a t r u c k f r o m N e wYork to Los Angeles in an attempt to

set a new truck speed record acrossthe continent.

One of the f irst things provenwas that the diesel engine does in-deed offer less braking than a gas-o l i n e e n g i n e o f e q u i v a l e n t s i z e ,placing a greater burden on the vehi-cle’s braking system. With the higherspeeds being driven on this test run,plus the load on board that includedt h e C u m m i n s D i e s e l 1 9 3 1 I n d i -anapolis race car and extra equip-ment, the intrepid trio soon found itnecessary to place more reliance onprayer than on brake linings.

All went reasonably well until thedescent of Cajon Pass, on old U.S.66 leading into San Bernardino, Cal-i fornia. A long, winding and steepgravel road, criss-crossed by a busymainline railroad, almost led to thedemise of the truck, its driver ClessieL. Cummins (who was to become the“father of the U.S. automotive die-sel”), and a frantic crew who weretossed around the i r bunks in thec a r g o b o x b e h i n d t h e c a b . I nClessie’s words:

“About dusk on the fifth day, wereached the top of Cajon Pass west ofBarstow, California. Before retiring tothe sleeping compartment, Dave hadwarned me against this thirty-f ive-mile stretch of mountainous down-grade. ‘Wake up Ford and me whenyou get to Kayhone Pass,’ I had un-derstood him to say, ‘I don’t want tobe in this box when you start downthat twister with the kind of brakeswe’ve got.’ I had heard but not seen,my Spanish being nonexistent, theword Cajon failed to register when thesign appeared. Soon, however, I real-ized my error. The brakes wouldn’thold. Now running in third gear, I trieddesperately to get into a lower speed.Nothing doing. I saw I would just haveto ride it out.

Well down the long grade by now,I suddenly saw something movingacross the road ahead. There was along dark shadow and then a redglow flared in the sky. I realized withnew alarm that a freight train wascutting across our path. The truckroared on. Dave and Ford screamedbloody murder in the compartmentbehind me. And I clung to that steer-ring wheel like a madman. Had MackSennett been on hand with a moviec a m e r a , h e w o u l d h a v e g o t t e nenough footage for one of his famousKeystone Kops features.

As we raced inexorably towardthe crossing and doom, the train’s

caboose loomed out of the darkness.Its red lights cleared the highway justas we reached the tracks We hadescaped certain death by Inches.”

The memory of the dark nightwas never forgotten. The venerablediesel-powered lndiana did set a newcoast-to-coast truck speed record of97 hours, 20 minutes running timefor the 3,214 miles, and one of itsdrivers vowed someday to make hisengine work when going downhill justas well as going uphill.

Twenty-four years later, in 1955,Clessie Cummins’ inventive geniusagain addressed the engine retardingquestion. In retirement in Sausalito,California, he began studying whatmight be done to turn his engine intoan effective “brake.”

It was also in 1955 that Clessieent iced h is youngest son Ly le , agraduate mechanical engineer, tojoin forces and form the large con-cern which he named Centco (shortfor Cummins Enterprises Company),a name chosen to keep the house-h o l d a c c o u n t s s e p a r a t e f r o mClessie’s ventures in the basement.The basement was actually an areathat included an office with a view, asoon to be equipped machine shopwith the usual lathes, mil l ing ma-chine, etc. and a four-car garage. An-other member of the Centco team,added in 1957, was Ray Hansen.H a n s e n w a s a n a b l e m a c h i n i s t ,

An American Automobile Association

official (with flag at left) begins count-down for a December, 1931 endurance

run at Indianapolis Speedway. The Cum-mins Diesel-powered Indiana truck had

been used in an August, 1931 cross-

country trip which demonstrated the

need for an engine brake. Clessie L.

Cummins, Ford Moyer, Dave Evans andT. E. Myers (Speedway Vice President)are poised in readiness for the run.

The Engine without a Jake BrakeDuring the piston’s normal compression

stroke (2), air compressed in the cylin-ders raises internal heat to almost

1,000°F. At this point, fuel is injectedand combustion occurs, raising temper-

ature even further. Pressure from this

expanded gas forces the piston down(3). The engine has thus produced thepositive power necessary to turn the

crankshaft.

w e l d e r , a n d s h e e t m e t a l b e n d e rwhose hobby was building and rac-ing sports cars.

Lyle’s initial effort was expendedon thermodynamic analyses to provethe Clessie’s idea of turning the en-gine into a compressor really wouldprovide sufficient retarding potential.T h e a n s w e r w a s a n u n q u a l i f i e d“yes,” but how to do it? One schemewas to c lu tch- in a h igh pressurep u m p a n d a t i m e d d i s t r i b u t o r t oca r r y hyd rau l i c f o r ce t o a s l avepiston, which in turn would act on theexhaust valve to open it at the properpoint on the compression stroke. Asecond method considered was tou s e a m u l t i - p l u n g e r p u m phydraul ica l ly t imed to act on thes lave p is ton. Whi le both of thesemethods could be made to work, nei-ther was considered a best solution.

An idea for a practical method,emphasizing the heat of the inven-tion, came to Clessie in 1957 duringa sleepless night in a Phoenix, Ari-zona hotel room. The idea that hitClessie revolved around taking ad-vantage of perfectly timed motion al-ready built into Cummins and DetroitDiesel engines; these engines have athird cam on the main camshaft thatactivates the fuel injector of each cyl-inder. A simple retrofit mechanismshould be able to transfer this motionto open the exhaust valve. The ideaw a s j o t t e d d o w n o n a b e d - s i d e

scratch pad and was te lephonedback to son Lyle in Sausalito early thenext morning. By the time the vaca-tioning Stella and Clessie Cumminsreturned home, layouts of possibledesign solutions were waiting, albeitin retrospect, they were more compli-cated than proved to be necessary.

An important subtlety of the in-vention made it novel enough that avery broad patent was u l t imate lygranted, affording strong patent pro-tection dating from the time of i tsapplication. The application was inthe U.S. Patent Office for seven yearsdue to new pertinent additions beingtwice added after the original filing in1957.

A very thorough patent searchmade prior to filing Clessie’s patentshowed that the idea of opening avalve, exhaust or otherwise, at ornear the end of the compress ionstroke was not a new one, some re-lated patents dated back to 1918.These were either for the purpose ofeasing the cranking load during start-ing or for actual engine retarding.

A vital point had always beenoverlooked, however; this preventedany of the old braking patents frombecom ing commerc ia l l y f eas ib l eproducts. None of the prior art madethe conversion to the braking modewithout altering in some way the ex-isting valve train, i.e., cam, pushrod,rocker lever, etc. Clessie’s idea was to

The Engine with the Jake BrakeThe key to the Jake Brake’s perfor-mance is the slave piston. Hydraulically

actuated, the slave piston opens the en-gine’s exhaust valve (2A) near the end

of the compression stroke. The com-pressed air, which normally forces thepiston down even though no fuel is

added, is vented through the exhaustsystem (2A). By releasing this energy,the Jake prevents any positive power

being exerted on the piston. The vehi-cle’s forward momentum provides theenergy needed to return the piston toits bottom position (3A), thus complet-ing the process of generating maximum

retarding power with a Jake Brake.

super impose onto the existing ex-haust valve motion an already avail-able and correctly timed motion (theinjector cam, for example, but notlimited to this cam) without prevent-ing the exhaust cam from acting toopen and close the exhaust valve as itnormally would during non-brakingt imes.

How best to transfer the injectorcam motion was developed duringthe next two years. The s implestmethod was to tie together mechan-ically the injector and exhaust rockerlevers with a one-way locking con-nec t i on so t ha t t he i n j ec to r camcould open the exhaust valve, but theexhaust cam could not actuate theinjector plunger. The experimentalcons t ruc t i on cons i s ted o f b l ockswelded to s tandard rocker leversover the rocker shaft bosses. The in-j e c t o r r o c k e r b l o c k h a d a d e e pcurved slot open to the rear. Engineoil pressure, controlled by a three-w a y s o l e n o i d v a l v e a n d a c t i n gthrough a hydraulic piston, forcedthe pin into the exhaust rocker leverslot. Engagement occurred duringthe exhaust, intake, and part of thecompression strokes on a catch-as-catch-can basis.

Because the Cummins fuel sys-tem at the time tended to inject somefuel into the cylinder, even duringcoasting conditions, it was necessaryto add another hydraulically-oper-

a t e d p i s t o n t o h o l d t h e i n j e c t o rplunger seated whenever the lockingpin was engaged. This piston had aw e d g e - s h a p e d o u t e r e n d a c t i n gagainst an extension to the injectorplunger spring retainer and bridgingaround the injector rocker level.

The vehicle selected for first fieldtests of the completed device was a1 9 5 5 G M C “ S u b u r b a n ” s t a t i o nwagon repowered with a CumminsJN-6 diesel engine. The purchaseand conversion were made in early1956 when the vehicle was used forsome investigative work prior to thefinal “assault” with the engine brakeprototype. The Cummins JN-6 dieselengine had a displacement of 401cubic inches and was rated at 125BHP at 2,500 rpm. Because of addi-tional engine weight and length, aswell as accessibility reasons, heaviersprings were added in the front andthe Suburban’s frame, fenders, andh o o d w e r e l e n g t h e n e d b y s o m eeleven inches. The sheet metal workand modifications were done in thebody shop of Br i t ish Motors , theRolls Royce dealer in San Francisco.Cless ie ’s f r iend Kje l l Qvale, whoowned the dea lersh ip , suggestedthat the shop would be a good placeto perform the “surgery.” Needlessto say the work was beautiful, butexpens ive! The longer wheelbaseand add i t iona l we ight he lped thesuburban’s riding characteristics, yet

handling was far from that of a sportscar!

Downhill braking performance ofthe 6,500-pound test wagon with itscylinders acting in the mode of aircompressors was most encouraging.Second gear allowed the diesel en-gine to slow down from governedspeed to an idle in less than 200yards on a thirty percent grade. Con-siderable retardation was also easilyobtained on direct drive on six per-cent hills. After recovering from theirfright at the way the vehicle began itsdownhill test runs in San Francisco,riders witnessing the operation of theinvent ion as inv i ted guests cameaway pale but visibly impressed.

Since the major market potentialwas in the Cummins NH series die-sels at that time, the engine brakem e c h a n i s m w a s s c a l e d u p a n dadapted to a Cummins NHRS super-charged model of 300 BHP at 2,100rpm. The chief reason for selectingthis model was simply because a pairof these diesel engines were installedin Cless ie ’s 96- foot yacht C a n i mb a s e d a t t h e S a u s a l i t o h a r b o r !Clessie saw no reason to invest largesums of money in a test facility with amotoring dynamometer when one ofhis boat engines could tell almost asmuch! By powering the front four cyl-inders to drive the rear two, continu-ous downhill braking conditions weresimulated on the two acting as com-

were barely warm to the touch. Billsaid he never wanted to drive a truckagain unless it was equipped withClessie Cummins’ new invention!

Marketing of the engine brakebegan while the design and patentp rocesses we re s t i l l be i ng com-pleted. As a result of a prior contrac-tual arrangement, Clessie was obli-gated to show his ideas first to Cum-mins Engine Company. This was donethree times during the engine brakedevelopment because of the patentre f i l ings. The novel ty o f the idea,which broke into untried mechanicalareas, p lus the uncer ta in ty of i tscommercial merit caused it to be re-jected by Cummins Engine Company.Lest incorrect surmises drawn, a l lo f the other major manufacturerscontacted, who either built or used“three cam” diesel engines in theirtrucks, also turned down the oppor-tun i ty to l icense or buy the idea.Nonetheless, Cless ie was not d is-c o u r a g e d , b e c a u s e h e a l s o w a sthinking about starting a small com-pany to manufacture the brake him-self. At seventy years of age, he wasjust get t ing h is second (or tenth)w ind!

The story of how Clessie Cum-mins’ engine brake became the Jac-obs Brake retarder is the tracing of animprobable and complicated set ofcircumstances. Follow this through ifyou wi l l : C less ie ’s brother DelossCummins, once service manager ofCummins Engine Company, lived inPhoenix, where he and a partner rana successfu l Cummins engine d is-tributorship. He visited Clessie oc-cassionally in Sausalito and knew ofthe ongoing “basement” act iv i t ies.Deloss’ son Don was serving in theU.S. Coast Guard, with one of his laststations being in New London, Con-necticut. The Cummins Engine Com-pany distributor in nearby Hartfordwas a good friend of Deloss and hadknown Don Cummins for years. Thisresulted in an invitation to Don tocome to Hartford; naturally the visitwould be more pleasant if Don couldalso spend some time there with aperson more his age (especially aperson of the opposite sex). Thus ab l ind date was ar ranged wi th thedaughter of a good friend. The girl’sfather, Bob Englund, also happenedto be a Vice President of Jacobs Man-u fac tu r i ng Com pany , t he wo r l d ’ sleading manufacturer of drill chucks.There was a mutual attraction be-tween Don Cummins and Rober ta

The Jake Brake today. pressors, without the boat (truck!)even leaving the dock. (In the Cum-mins NH series six-cylinder engine,each head covers two cylinders.)

A l t hough t he p r i nc i p l es we reproved by mechanically transferringthe injector motion, a more practicalmethod was to use a fully hydraulicmotion and force transfer. A test wasnext made with the rudiments of afinal hydraulic design that later be-came the production prototype. Thefirst retarder housings of the proto-type design were installed on a Cum-mins diesel engine in a truck ownedand operated by the Sheldon Oi lCompany. The work was done a tSheldon’s shop in Suisun, Californiadur ing August 1959. She ldon Oi lCompany trucks were then hauling400°F asphaltic oil to “batch plants”making highway paving material.

The init ial run with the enginebrake was to one of these plants justat the eastern base of the grade,down the Sierras on U.S. Highway 50near Lake Tahoe. Bill Hill, an eigh-teen-year veteran driver for Sheldon,said he normally was forced to passthe turnoff onto the job site becauseof faded brakes; he would come backwhen he could slow down enough toturn around! With the engine brakehe needed to use his wheel brakesbriefly only two times on the descentf rom the summit . The turnof f waseasily made, and the brake drums

Englund, and in due course weddingbells were heard.

Don knew through his dad thatUncle Clessie was working on somekind of engine brake as one of hisret i rement pro jects , and the wordeventually reached the ears of JacobsManufacturing Company’s PresidentLouis Stoner and his brother Arthur,then head of engineering at Jacobs(The description of what was goingo n i n C a l i f o r n i a b y t h i s t i m e w a srather remote from any relationshipto the actual device!) Since Jacobswas actively seeking new projects inorder to diversify its operations, the“air compressor” brake that Bob En-glund’s son-in-law’s uncle was work-ing on might be worth looking into!Just weeks before the Sheldon OilCompany tes t began, a le t ter wasreceived in Sausalito asking if theStoner brothers and Page Wodel, acapable young Jacobs sa lesman,could come to California to see whatinvention Clessie had under develop-ment. A demonstration of the brakeon the boat engine, plus the infec-tiousness of Clessie Cummins’ deter-mination and foresight experiencedduring a yacht ride on San FranciscoBay, excited the visitors to the pointthat agreement was soon reached tobuild brake assemblies for ten en-g ines as quick ly as poss ib le f romdrawings that Lyle provided.

An opt ion agreement betweenClessie Cummins and Jacobs Man-u f a c t u r i n g C o m p a n y w a s e n t e r e dinto in December of 1959. Based onthe resu l t s o f Cen t co - conduc tedtests with trucks using the Jacobs-made units, along with market sur-veys and independent tests, Jacobswould then be allowed to make theirdecision as to whether or not theywould exercise the option. Consoli-da ted F re i gh tways , Pac i f i c I n t e r -mountain Express, Wil l ig, Sheldon,O - N - C , a n d o t h e r t r u c k i n g f i r m skindly furn ished test t rucks. Whi lesevera l minor prob lems appeared,the tests demonstrated conclusivelythe engine brake’s improved, saferoperation, as well as a large potentialcost sav ings in brake l in ings andd r u m s .

Increased engine life, due to thefact that the engine remained hot onlong downhill runs, was an advantagep r o v e n l a t e r , t h o u g h a s s u m e d b yClessie from the very beginning. InApril of 1960, Jacobs Mfg. Companymade the decisions to establish itsnew Cless ie L . Cummins Div is ion

(now named Vehicle Equipment Divi-sion) for manufacture of the enginebrake.

Although there was never anydoubt on Cless ie ’s par t as to thequality of the product that Jacobswould build, he did have reservationsthat they could succeed in a newmarket totally foreign to their pre-vious experience. Thus, while he wasselling Jacobs on the value of his en-g i ne r e ta rde r , t hey i n t u rn we rewooing him on their ability to make ita commercial success.

The “Clessie L. Cummins Divi-sion” title was chosen to show that ithad a tie to the man whose name wason the diesel engines for which theproduct made by that division was tobe used. The final contract betweenClessie and Jacobs Mfg. Companyspecified that he would also be will-ing to act in an advisory capacitywhen either party felt the need. Thisclause was actively exercised duringthe f i rs t few years. Others in theCummins fami ly a lso par t ic ipated.D e l o s s C u m m i n s a s s i s t e d P a g eWodell in establishing a dealer net-work. (For a number of years, a l lsales were on an aftermarket basis.)Don Cummins, now a mechanical en-gineer, went to work for Jacobs, and,in addition to his official title of salesand service engineer, provided prac-t ica l know-how for many years inmore areas than his title indicated

An early Jake Brake.

Lyle Cummins was also involved withJacobs Mfg. Company, first while onloan from Centco, and later in theJacob engineering department work-ing in product design. Methods usedby Lyle and Don were sometimes un-orthodox, but sudden situations aris-ing with a new product, in a companyunfamiliar with the market where theproduct was used, often called fordirect and “interesting” approachesto problem solving!

Nevertheless, the Jacobs organi-zation, particularly under the direc-tion and dedication of Page Wodell insales and Gerry Haviland (Chief Engi-neer a f ter Ar thur Stoner ’s re t i re-ment), rose to the occasion and acompetent team was the result.

The first production units for theCummins NH series engines left thefactory in 1961, followed shortly by abrake for the Detroit 71 series. TodayJake Brake retarders are available forthe major U.S. heavy duty dieseltruck engines; Caterpillar, Cummins,Detroit Diesel and Mack. Military ap-plications have also become signifi-cant with Jacobs retarders, beingstandard on several U.S. mil i taryvehicles.

Clessie Cummins’ idea for mak-ing a diesel engine work downhill aswell as uphill has thus been imple-mented. The Jake Brake retarder hasb e c o m e a m a j o r c o n t r i b u t o r t ogreater control and safer operation ofheavy trucks worldwide.

The American Society ofMechanical Engineers345 East 47th StreetNewYork NewYork 10017 H108

The text of this brochure isbased on an article by C. LyleCummins, Jr., from Diesel CarDigest, Vol. 6, No. 4, Winter,1981, reprinted here with thekind permission of its editor,Robert E. Flock, Diesel CarJournals, P.O. Box 160253,Sacramento, CA 95816.

The Jacobs Engine Brake is the81st National Historic Mechan-ical Engineering Landmark des-ignated since the program be-gan in 1973. In addition 18International and 9 RegionalLandmarks have been recog-nized by the Society. Each rep-resents a progressive step inthe evolution of mechanical en-gineering, and each reflects aninfluence on society. Forfurtherinformation contact ASME,Public Information Dept., 345East 47th St., New York, N.Y.