Farmer's Weekly (South Africa)
Motoring
Jake Venter takes a look at the mad world of dragster racing, where accleration is all that matters.
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 jacobventer77@gmail.com. Subject line: Auto engineering.