The sim­ple sci­ence be­hind the art of car­bu­ra­tion

Cycle World - - Fundamentals - By KEVIN CAMERON / Pho­tog­ra­phy by JEFF ALLEN

TThe car­bu­re­tor flick­ered out of ex­is­tence in mod­ern Amer­i­can au­to­mo­biles in 1990, but nearly 30 years later, those of us with the in­cli­na­tion can walk into a mo­tor­cy­cle deal­er­ship and ride out on a brand-new car­bu­reted ma­chine. De­spite the de­vice’s longevity, it isn’t widely un­der­stood. To this day, when peo­ple talk of ac­cel­er­at­ing, they say, “I gave it the gas.” That’s a mis­nomer.

Whether our ve­hi­cle’s en­gine has mod­ern dig­i­tal fuel in­jec­tion or car­bu­re­tors, the ac­tion we take to ac­cel­er­ate is to open the throt­tle, which con­trols only the flow of air into our en­gine’s cylin­ders. Some other sys­tem then adds fuel in cor­rect pro­por­tion to that air­flow, re­sult­ing in a com­bustible air-fuel mix­ture be­ing drawn into our en­gine. Be­fore the dig­i­tal-fu­elin­jec­tion era, be­gin­ning in the late 1970s and com­pleted in the 1990s, that sys­tem was a car­bu­re­tor.

Not all mix­tures of gaso­line and air can be ig­nited by the hot spark that jumps across the elec­trodes of spark plugs. Only mix­tures be­tween 10 parts air to one part fuel and 18 parts air to one part fuel fit that bill. We want fuel to burn com­pletely to get all the chem­i­cal en­ergy it con­tains. That hap­pens when all its car­bon atoms join with oxy­gen from the air to form car­bon diox­ide, and all its hy­dro­gen atoms com­bine with oxy­gen to form wa­ter. This oc­curs at a mix­ture of about 14 parts air to one part fuel—the so-called chem­i­cally cor­rect mix­ture.

De­liv­er­ing this cor­rect mix­ture across the range of en­gine speeds and throt­tle open­ings from idle to max­i­mum is no easy prob­lem. The ear­li­est car­bu­re­tors were sim­ple evap­o­ra­tors, which passed air­flow across a wick kept wet with fuel. Be­cause evap­o­ra­tion is a cool­ing process, wick car­bu­re­tors had to be heated. The ve­hi­cle oper­a­tor had to ad­just how fast fuel was dripped onto the wick, and then cor­rect the mix­ture by con­trol­ling an air bleed. It was dif­fi­cult to get smooth, steady op­er­a­tion.

Wil­helm May­bach in Europe and Os­car Hed­strom at In­dian in the U.S. had the idea of the spray car­bu­re­tor. It uses Bernoulli’s Prin­ci­ple, which ob­serves that when air moves, its pres­sure drops. The essence of their in­ven­tion was to place one end of a small ver­ti­cal tube in a cup of fuel, and then place the other end in­side the pipe car­ry­ing en­gine air­flow. Be­cause pres­sure in the fast-mov­ing

in­take flow was less than pres­sure out­side it, fuel was driven up the small tube to spray out of its end, mix­ing with en­gine air­flow.

But May­bach and Hed­strom both found that the fuel-flow rate de­creased as the gaso­line level in the cup fell. The fuel had to be lifted farther up the pipe. That one was easy—just pro­vide a float-con­trolled valve like the one that keeps a con­stant level of wa­ter in toi­let tanks. As the level of fuel fell, the float dropped, open­ing the fuel valve and restor­ing the fuel level. The lit­tle cup and its float-con­trolled valve are the car­bu­re­tor’s float bowl, which keeps the liq­uid fuel at a con­stant height.

There was another prob­lem. The faster our en­gine runs or the more we open the throt­tle, the faster air moves through the in­take pipe, and the air pres­sure drops. Why does air pres­sure fall as it moves faster through a pipe? In still air, all the mo­tions of its mol­e­cules con­trib­ute to pres­sure by their con­stant ran­dom col­li­sions. Air pres­sure is the sum of all those col­li­sions. But when the air be­gins to move, some of its molec­u­lar mo­tion be­comes or­ga­nized in the di­rec­tion of the air­flow, leav­ing less-ran­dom molec­u­lar mo­tion to cre­ate pres­sure.

This loss of pres­sure as air moves faster and faster is also a loss of den­sity. That means that the ra­tio of air to fuel changes, be­com­ing steadily richer (con­tain­ing more fuel in pro­por­tion to air) as in­take air­flow speeds up. All sim­ple car­bu­re­tors en­rich as en­gine air­flow moves faster through them. If un­cor­rected, this nat­u­ral en­rich­ing ten­dency wastes fuel, and even­tu­ally the mix­ture be­comes too rich to ig­nite.

A great many in­ge­nious de­vices were in­vented to cor­rect this nat­u­ral en­rich­ment—things such as spring-loaded air valves that let in ex­tra air, or mul­ti­ple fuel noz­zles ex­posed in se­ries as the air throt­tle opened—each one set leaner than

This is Kei­hin’s ven­er­a­ble CR, which has ex­isted since the 1960s. As you see them here, four CRS have been “racked” into a unit, keep­ing them aligned, and greatly sim­pli­fy­ing the task of mak­ing a sin­gleor dual-throt­tle ca­ble ac­cu­rately con­trol all four throt­tle slides. It was Gil­era in Italy that first mounted car­bu­re­tors this way. The CR, with its in­te­gral float bowl, was a huge step for­ward from the no­tal­ways-pre­dictable fuel flow of carbs with sep­a­rate float bowls.

the one be­fore it. But the sim­ple con­cept that worked best was to bleed some air into the fuel flow. The faster the fuel flowed, the more bleed air flowed along with it, and by siz­ing ev­ery­thing cor­rectly, a con­stant ra­tio of fuel flow and air­flow could be achieved.

If the oper­a­tor sud­denly opens the throt­tle to ac­cel­er­ate, the fuel is mo­men­tar­ily left be­hind be­cause it is 640 times denser than air, re­sult­ing in mo­men­tary lean­ness. The en­gine ei­ther stum­bles or cuts out com­pletely. To pre­vent this, some car­bu­re­tors in­cluded an ac­cel­er­a­tion pump, whose lit­tle pis­ton, mov­ing with the throt­tle, forcibly squirted fuel into the air­flow to pre­vent mo­men­tary lean­ness.

Gaso­line is a mix­ture of dif­fer­ent hy­dro­car­bons hav­ing a range of volatil­i­ties (abil­ity to evap­o­rate). At the tem­per­a­ture of a very cold en­gine, only about 10 per­cent of the fuel can evap­o­rate, while the rest re­mains in­com­bustible in liq­uid form. The re­sult is a mix­ture too lean to fire. For cold start­ing, the mix­ture is en­riched enough (by a choke, or start­ing car­bu­re­tor) that there is enough volatile ma­te­rial evap­o­rat­ing to al­low the en­gine to fire and start. As the en­gine warms up, its in­take sys­tem be­comes warmer, evap­o­rat­ing more and more of the fuel, al­low­ing this tem­po­rary en­rich­ment to be re­duced un­til, with the en­gine at op­er­at­ing tem­per­a­ture, it is able to evap­o­rate all the fuel flow­ing to it.

Car­bu­re­tors at their best weren’t very good be­cause, be­ing pas­sive de­vices, they could not com­pen­sate for changes in at­mo­spheric den­sity from weather, al­ti­tude, or hu­mid­ity. They ran lean in win­ter and rich in sum­mer—un­less tuned for ex­ist­ing con­di­tions by a hu­man, ad­just­ing fuel de­liv­ery by chang­ing the sizes of fuel me­ter­ing ori­fices, called jets. To­day’s dig­i­tal fuel sys­tems are ac­tive, mak­ing such ad­just­ments au­to­mat­i­cally. But car­bu­re­tors are cheap and well-un­der­stood by the engi­neers who still em­ploy them. They are also rel­a­tively sim­ple and ro­bust, and while they might not al­ways op­er­ate at peak per­for­mance, they will func­tion well enough to get a ma­chine down the road even while far out of tune. They pro­pelled the motoring world for nearly 100 years, the small de­vices that took a sim­ple prin­ci­ple and car­ried it far into the fu­ture, and us along with them.

In 1977, Lec­trons sud­denly ap­peared on Kenny Roberts’ TZ750. Rid­ers praised their part-throt­tle re­sponse and fine mix­ture for­ma­tion. Yet they were a clas­sic mass-pro­duc­tion job, made of die cast­ings as­sem­bled with self-tap­ping screws. The func­tion was in the cylin­dri­cal nee­dle, which in­stead of be­ing ta­pered had one or more slop­ing flats ground onto its down­stream side. With the nee­dle lo­cated on the vac­uum side of the slide, Lec­trons had the strong­est Ven­turi vac­uum in the busi­ness. Trans­par­ent float bowls re­vealed the ef­fects of carb vi­bra­tion.

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