• How it works: Top-fuel dragster

    1: Fuel; 2: Clutch; 3: Tyres; 4: Engine.
    Illustration by Axel De Roy
    Date:1 March 2011 Tags:, , , ,

    Anatomy of a top-fuel dragster. By Larry Webster

    No other ground vehicle can out-accelerate a top-fuel dragster. These cars can rocket to 500 km/h in less than 4 seconds. That seemingly physics-defying performance is why Jim Oberhofer, the vice-president of operations at Kalitta Motorsports, calls top-fuellers violence on wheels. Add expensive, too. One run down the strip consumes fuel, bearings, valve springs and other parts to the tune of R40 000. Stand nearby when a “rail” car lights off and you can feel the ground shake. It’s unnerving. These machines are so fast – topping 530 km/h – that the quarter-mile track (402 metres) was shortened to 1 000 feet (about 305m) after Scott Kalitta was killed in 2008 when his car exploded at the finish and crashed into the catch fence. That tragedy did not deter the Kalitta crew. As the team prepares for the 2011 season, we peel off its top-fueller’s bodywork to learn the secrets of its insane acceleration.

    1. Fuel
    Top-fuellers burn a mixture that’s 90 per cent nitromethane and 10 per cent alcohol. Interestingly, a litre of nitromethane, which costs R104, has less energy content than petrol (3,7 kilowatt-hours versus 8,9). But nitro is an oxygen-rich compound that requires less air to burn, so the engine can consume more of it, thereby producing greater power. The 58 nozzles in the intake tract are always open, dumping about 20 litres of fuel in a 4-second run. That kind of flow requires a firehose-like fuel line.

    2. Clutch
    A five-disc dry clutch is the only link between the engine and the locked rear end – there’s no transmission. “The clutch is the lifeline of the car,” Oberhofer says, because it regulates wheelspin by gradually engaging and slipping as the car moves down the track. A hydraulically motivated throw-out bearing operates off a simple timer (computer controls are illegal). The clutch is tuned according to track conditions. If it engages too quickly, the tyres will spin, but if it’s too lazy, the car won’t accelerate as fast as possible. The discs get so hot that at least two of them are welded together by the run’s end.

    3. Tyres
    Specially constructed Goodyears have the intimidating task of transferring all that rotational energy to forward speed. The bias-ply tyres also dramatically change diameter over a run, which has the effect of altering the overall gearing. At the start, the 91 cm-diameter tyres squat as the sidewalls wrap around the wheels’ bead locks. With increasing vehicle speed, the tyres expand to a final diameter of 111 cm. The special blend of tread rubber is designed to adhere to the adhesive that’s applied to the track. The resulting grip is akin to driving on flypaper.

    4. Engine
    A top-fueller’s exact power is a mystery – there isn’t a dynamometer that can handle one. Current estimates are in the 6 000 neighbourhood, and, no, we didn’t mistakenly add a zero on the end. The basic layout is very similar to the 1964 Dodge Hemi 426 V8 – 16 pushrod-activated valves – but with two spark plugs for each cylinder and a total displacement nearly one-fifth greater at 8,2 litres. The supercharger, which is just a belt-driven air pump that force-feeds the engine, is so massive that it takes 520 kW to run it. The extreme internal forces literally flatten the crankshaft bearings and destroy valve springs during a pass. So the engine is rebuilt after every run by a team of eight mechanics. They perform this harried rebuild in just 40 minutes.

    The race
    On 13 March, 2010, in Gainesville, Florida, Kalitta driver David Grubnic set a top speed record on the 1 000-foot (about 305 m) track. Here’s a breakdown.

    0.00 sec
    0 m | 00,00 km/h
    After a burnout to clean and scuff the tyres, Grubnic hits the gas at the green light, instantly accelerating at over 4 g’s.

    0,50 sec
    3,13 m | 118,89 km/h
    The motor screams at its 8 300 r/min redline, and the tyres buckle under the load of getting the 1 044-kilogram dragster moving.

    1,00 sec
    15,83 m | 183,14 km/h
    To reduce power and the acceleration of the driveshaft and tyres, the ignition advance is retarded from 56 degrees to 27 – a strategy to keep the tyres gripping the track surface instead of spinning.

    1,50 sec
    38,13 m | 261,38 km/h
    The ignition timing is advanced back to the maximum value. The clutch’s throw-out bearing has moved through three of its five stages, increasing the pressure on the discs.

    2,00 sec
    70,68 m | 342,86 km/h
    The fuel flow ramps to 360 litres per minute. Aerodynamics increase tyre traction, so the throwout bearing tightens its grip. The engine slightly bogs to 7 200 r/min.

    2,50 sec
    115,78 m | 399,84 km/h
    In the time required to take a sip of coffee, Grubnic’s car passes 400 km/h. Some of the clutch plates begin welding together, pulling the engine to its lowest revs, 6 500.

    3,00 sec
    172,56 m |  437 km/h
    Thanks to 2 270 kilograms of aero downforce, the tyres have incredible traction, but wind resistance slows acceleration from the maximum 5 g’s to about 4.

    3,50 sec
    238,85 m | 473,09 km/h
    Late in the run, many spark plugs have burned away, so the engine is dieselling, and some cylinders simply don’t fire. “This is the engine’s toughest zone,” Oberhofer says.

    3,83 sec
    305 m | 517,42 km/h
    The mechanical abuse finally ends as the dragster clears over 150 metres per second. Next, the driver pulls the chute and the crew then feverishly prepares for the next run.

    Related video:
    * Scott Kalitta’s fatal crash

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