Mo­tor­ing

Jake Ven­ter takes a look at the mad world of drag­ster rac­ing, where ac­cler­a­tion is all that mat­ters.

Farmer's Weekly (South Africa) - - Contents - FW

Please close your eyes and wait for four sec­onds be­fore you read an­other word. If you had been sit­ting in what is known as a top fuel drag­ster, you would now be trav­el­ling at just over 500km/h.

These ma­chines con­sist of a 6 000kW engine fit­ted into what looks like a long bed-frame car­ry­ing large rear wheels and small front wheels. They are built to race in pairs from a stand­ing start to cover a quar­ter-mile (0,4km). The first across the line is the win­ner, and the fastest at the end of the day takes the tro­phy.

There is no time to change gears, so the car has no gear­box. In­stead, a con­trolled-slip, multi-plate clutch com­bined with rear tyre di­am­e­ter growth (con­trolled by wheel­spin) are used to ac­cel­er­ate the car from a stand­ing start to just over 500km/h with a sin­gle gear ra­tio!

the car that melts

The fol­low­ing note­wor­thy events oc­cur dur­ing the run: • Just be­fore the start, the driver spins the rear wheels to make the tyres and track sticky for bet­ter grip. • The driver ac­cel­er­ates when the flag comes down and the car takes one sec­ond to reach 180km/h. The com­puter now re­tards the ig­ni­tion from 56° to 27° be­fore top dead cen­tre to curb wheel­spin. • Af­ter 1,5 sec­onds, the speed is 260km/h. As the car speeds up, the rear tyre di­am­e­ter grows from 914mm to 1 117mm. These are spe­cial wrin­kle-wall tyres with a side­wall thick­ness of only 3,8mm, which en­ables them to de­form at the road sur­face un­der ac­cel­er­a­tion and thus pro­vide more grip. They last eight runs (3,2km) and cost about US$700 (R9 800) each.

• The clutch plates have been slip­ping all this time, but the disc pres­sure is now in­creased to de­liver more power to the rear wheels.

• Af­ter two sec­onds, the car is trav­el­ling at 340km/h.

• Af­ter 2,5 sec­onds, the speed is 400km/h. Some of the clutch plates are now welded to­gether be­cause of the heat. At this point, the engine speed has dropped to its low­est value of 6 800rpm. • Af­ter three sec­onds, the car has reached 440km/h. Most of the spark plugs have now burned away but the com­bus­tion cham­ber is hot enough to ig­nite the in­com­ing mix­ture.

• At the fin­ish­ing line, just un­der four sec­onds later, the speed is slightly over 500km/h. This equates to an av­er­age ac­cel­er­a­tion of 3,6g, but at the lower speeds the ac­cel­er­a­tion ex­ceeded 5g.

the power be­hind the glory

A typ­i­cal drag­ster engine is based on the V8 Chrysler Hemi, but the ig­ni­tion sys­tem em­ploys two spe­cial mag­ne­tos that fire two spark plugs per cylin­der with sparks that are gen­er­ated from a 44 amp cur­rent. This is 10 times more than the am­per­age needed to fire the spark plug in the av­er­age car. The spark is so strong that you could weld with it.

a top fuel drag­ster takes one sec­ond to reach 180km/ h

A su­per­charger sup­plies a 3,2 bar boost (about three times the boost on a typ­i­cal tur­bod­iesel) to the mix­ture in­side the in­take man­i­fold and in the process robs 440kW from the engine. The out­put at the fly­wheel is es­ti­mated to be about 6 000kW.

The engine runs on nitro­meth­ane, which con­tains oxy­gen, and burns at an air:fuel ra­tio of 1,7:1 in­stead of the 14,7:1 ra­tio used in or­di­nary cars. The evap­o­ra­tion of this rich fuel mix­ture cools the engine dur­ing the run, so that a ra­di­a­tor is un­nec­es­sary.

The clutch, pis­tons, con­rods, rings and spark plugs are re­placed or re­fur­bished af­ter ev­ery run.

• Jake Ven­ter is a jour­nal­ist and a re­tired en­gi­neer and math­e­ma­ti­cian. Email him at ja­cob­ven­[email protected] Sub­ject line: Auto en­gi­neer­ing.

Getty im­ages

LEFT:And they’re off! Mas­sive power, max­i­mum trac­tion, and a con­trolled­slip, multi-plate clutch are used to ac­cel­er­ate a drag­ster to a blis­ter­ing speed in sec­onds. There is no gear­box.

Jake Ven­ter

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