FEV’S vari­able com­pres­sion con-rods

Ultimate Diesel Builder's Guide - - Contents -

It has been years, over a decade in fact, since re­duc­ing emis­sions or im­prov­ing fuel econ­omy was ei­ther easy or cheap. In the early days of en­gine ef­fi­ciency up­grades you could get a one-per­cent im­prove­ment at low cost. An early ex­am­ple: Sim­ply chang­ing pis­ton ring de­sign for less blowby dra­mat­i­cally im­proved hy­dro­car­bon emis­sions.

To­day we’re at a point in en­gine tech­nol­ogy and ad­vance­ments where ev­ery sin­gle per­cent­age is hard won and costly. Yet there are still so­lu­tions that re­sult in over five per­cent im­prove­ment in CO2 on the New Euro­pean Driv­ing Cy­cle, like this con­nect­ing rod that de­liv­ers vari­able com­pres­sion ra­tio.

FEV is an engi­neer­ing con­sul­tant com­pany based in Aachen, Ger­many, with an of­fice in Auburn Hills, Michi­gan. We’ve been fol­low­ing devel­op­ment of this con­nect­ing rod for five years, but the idea of vari­able com­pres­sion ra­tio goes back more than a decade.

There was an at­tempt to con­tin­u­ously vary com­pres­sion ra­tio by mov­ing the en­tire crank­shaft up and down. Mov­ing the crank ver­ti­cally meant the trans­mis­sion had to move along with it, or in­volve some en­ergy trans­fer mech­a­nism. The last straw in that devel­op­ment was that there was no way to eas­ily adapt a fixed cen­ter line change with an ex­ist­ing trans­mis­sion (think about it). A later so­lu­tion moved the head it­self, but both had sig­nif­i­cant chal­lenges. Why con­tin­u­ously vari­able com­pres­sion? At the time, en­gi­neers thought it nec­es­sary to lin­early move the com­pres­sion ra­tio across the en­tire en­gine map.

The next evo­lu­tion was to look at the con­nect­ing rod. Ear­lier de­signs were based on an ec­cen­tric bear­ing at the crank­shaft. Now FEV’S con-rod de­sign uses an ec­cen­tric bear­ing at the wrist pin to move the pis­ton up and down from its peak in the cylin­der bore. If you are us­ing an ec­cen­tric bear­ing you de­liver what be­comes a two-step sce­nario.

While you could force con­tin­u­ously vari­able com­pres­sion, this re­quires en­ergy in­put. If you re­quire en­ergy in­put to ac­tu­ate the thing, what­ever the so­lu­tion is, it’s not go­ing to work out. The whole ob­jec­tive is to get bet­ter fuel econ­omy, not cre­ate par­a­sitic losses and re­duce the over­all ben­e­fits you could achieve.

So, back to two com­pres­sion ra­tios. FEV’S R&D and en­gine test­ing fa­cil­i­ties, and their cus­tomers and their en­gine devel­op­ment, have sug­gested that most of the ben­e­fit of in­fin­itely vari­able com­pres­sion ra­tio can be achieved with only two ra­tios. We spoke to Dean To­mazic, Vice Pres­i­dent of FEV, about this con­nect­ing rod. He says you can achieve most of the ben­e­fit, ap­prox­i­mately 80-85% of the more com­plex fully vari­able sys­tem, with only two ra­tios.


The pri­mary ben­e­fit of com­pres­sion ra­tio changes is a sig­nif­i­cant re­duc­tion in fuel use and ev­ery truck maker is scram­bling to meet 2025 emis­sions and ef­fi­ciency re­quire-

ments. Be­cause you can set the com­pres­sion ra­tio across the en­gine map, tak­ing full ad­van­tage of the part of the map you’re op­er­at­ing in, you can get torque im­prove­ments, es­pe­cially at the low end.

The ben­e­fit of be­ing able to change the com­pres­sion ra­tio, de­pend­ing on where you are in the map, opens the en­gine’s op­er­at­ing area sig­nif­i­cantly. You can take ad­van­tage of any fuel’s char­ac­ter­is­tics and ad­just the com­pres­sion ra­tio for the char­ac­ter­is­tics of the fuel you’re run­ning.

In a diesel en­gine, when op­er­at­ing un­der low load con­di­tions you can use high com­pres­sion ra­tio, which pro­vides a dra­matic in­crease in ef­fi­ciency. Vari­able com­pres­sion has an­other ad­van­tage for diesel en­gines: de­creas­ing hy­dro­car­bon emis­sions. As you move to­wards high load you can set the com­pres­sion ra­tio lower, which helps re­duce par­tic­u­late emis­sions with­out sig­nif­i­cantly in­creas­ing NOX out­put. If you were to leave com­pres­sion ra­tio sta­ble, to get higher ef­fi­ciency you’d have to in­crease the process tem­per­a­ture. As we all know, when you in­crease tem­per­a­ture, NOX goes up. By be­ing able to play with com­pres­sion ra­tio you now have the abil­ity to nearly flat-line your NOX emis­sions and still re­duce par­tic­u­lates. With vari­able com­pres­sion you can in­crease your full load power and have the abil­ity to get bet­ter ef­fi­ciency with­out chang­ing peak fir­ing pres­sure in the en­gine.


Div­ing into the de­tails, this so­lu­tion has its mech­a­nism driven by gas forces in the cylin­der and re­cip­ro­cat­ing mass forces. There are oil gal­leries in the con­nect­ing rod, but the oil pres­sure is only used to lock the con-rod into one set­ting or the other. Ac­tual ec­cen­tric move­ment is done by com­bus­tion pres­sure or kine­mat­ics—the pis­ton mov­ing up or down. The ad­van­tage to this is, we're not throw­ing away en­ergy to move the pis­ton up or down, all we're do­ing is us­ing the oil pres­sure com­ing out of the jour­nal side as a way to hold the con­nect­ing rod in po­si­tion. Ac­ti­va­tion time is 0.2-0.7 sec­onds.

While the rod ap­pears nor­mal at first glance, in­side are two sup­port rods with small pis­tons and oil gal­leries. At the bot­tom, a small shift valve di­rects oil in the proper di­rec­tion and holds in that po­si­tion.

We asked the af­fa­ble Mr. To­mazic about the cost of com­plex parts and if there are con­cerns with mak­ing them. He tells us: “We’ve col­lected a lot of ex­pe­ri­ence and know the dos and don’ts about fab­ri­ca­tion and ma­te­ri­als. We would sug­gest to any man­u­fac­turer or sup­plier that we be heav­ily in­volved in pro­duc­tion and ma­chin­ing. We didn’t make a lot of them, but un­der­stand the tol­er­ance, ma­chin­ing, as­sem­bly, and ma­te­ri­als. We want to use our ex­pe­ri­ence to en­sure suc­cess.”

We con­cluded our chat by ask­ing the in­evitable—how Soon? and How Much? To­mazic re­sponds: “Sev­eral man­u­fac­tures are look­ing at tak­ing this into pro­duc­tion and the time from con­tract to im­ple­men­ta­tion de­pends on the sup­plier. Is that rod sup­plier al­ready work­ing with FEV and knows what’s be­ing done or start­ing at zero? So, if you look at val­i­da­tion test­ing on the road it would take a few years de­pend­ing on how quickly a sup­plier can ramp up pro­duc­tion.

“With a po­ten­tial im­prove­ment of 4-7% on the New Euro­pean Driv­ing Cy­cle, this is quite re­mark­able. All the low hang­ing fruit, $20 for 1% has been picked. Ev­ery man­u­fac­turer is now look­ing at more ex­pen­sive so­lu­tions and vari­able com­pres­sion ra­tio rep­re­sents a big ham­mer in CO2 re­duc­tion and bet­ter fuel econ­omy.” Oh, and it’s not pie-in-the-sky. There are val­i­da­tion ve­hi­cles out test­ing to­day. Does this en­sure we’ll see this sys­tem? No, but the chances are very good. UDBG

This is the cur­rent two-stage com­pres­sion ra­tio rod, now in test­ing by FEV. Eas­ily seen are the rods that se­cure, not drive, bear­ing ro­ta­tion.

This com­plex group of parts, a highly tech­ni­cal con­nect­ing rod, rep­re­sents one way to in­creas­ing diesel power and ef­fi­ciency.

Con­nect­ing rod blue­prints. There's a lever, two sup­port rods with small pis­tons in them and you can see the oil gal­leries. There's a shift valve at the bot­tom that di­rects oil in the proper di­rec­tion.

By en­larg­ing an oil gallery to al­low two small pis­tons, an ec­cen­tric bear­ing can move swiftly to ei­ther raise or lower pis­ton height to change com­pres­sion ra­tio. The pis­tons and low pres­sure oil only lock the po­si­tion.

At the bot­tom end a small but­ton (top of photo) is the me­chan­i­cal switch for chang­ing oil pres­sure to keep the cur­rent CR.

The key to eas­ily chang­ing com­pres­sion ra­tio is an ec­cen­tric bear­ing at the pis­ton pin. Ma­chined pis­tons are moved by kine­mat­ics—mo­tion, not oil pres­sure.

By vary­ing the con­nect­ing rod length, the pis­ton height in the bore is var­ied, which in turn will re­sult in two dif­fer­ent com­pres­sion ra­tios within the en­gine to help re­duce emis­sions while pro­vid­ing more torque in the lower rpm band and more...

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