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CFD Used in Design of Land Speed Vehicle
Posted Thu July 13, 2006 @01:48PM
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Application The Autospeed Magazine article Diesel Speed Demon describes how JCB, the fifth largest manufacturer of construction equipment in the world, will make a land speed record attempt this August in a bid to earn its purpose-designed streamliner car the title of "World’s Fastest Diesel". To reach speeds of over 300 mph (480 km/h), JCB has developed the world’s most powerful automotive diesel engine, with a specific power of 150hp per litre.

The aim is to set a new record in excess of 300 mph. The current FIA mark stands at 235.756 mph to Virgil W Snyder and the Thermo King Streamliner, a record that dates back to 1973.

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The JCB DIESELMAX streamliner will be driven by Wing Commander Andy Green, who set the first-ever supersonic world land speed record at 763.035 mph in ThrustSSC on the Black Rock Desert on 15 October 1997.

The innovative car has been designed by a team led by JCB Group Engineering Director Dr. Tim Leverton. Richard Noble, the former land speed record-holder, has acted as a consultant to the project, and JCB has worked with long-term technology partner Ricardo plc to develop the JCB444-LSR engine.

JCB’s purpose in creating the world’s fastest diesel automobile is to prove the versatility of the standard JCB444 engine, and to validate its inherent excellence in a totally different – and extremely demanding – engineering environment.

"Our intention all along with the speed record project was to use a standard engine block, cylinder head and bedplate," explained Dr Leverton. "I set that task at the very beginning. I wanted it to be the standard design block and have exactly the same fundamental architecture. It had to be recognisably the JCB444 engine."

But how do you take what is basically a bulletproof industrial engine and turn it into a record-breaking powerplant?

"The JCB444 has been designed with a very stiff bottom end. It’s designed to sit there for hours and hours and hours, chugging out the required horsepower. It’s an incredibly tough, long-life engine and therefore has the inherent strength to cope with the very high cylinder pressures generated when harnessing two-stage turbocharging to boost power to 750hp."

The land speed record engine develops almost 1500 Nm of torque at 2500 rpm, with a rev limit of 3800 rpm. Each engine will be laid over in the car - inclined at an angle of 10 degrees from the horizontal - to minimise the frontal area of the machine.

The DIESELMAX engines have been designed using Ricardo’s High Speed Diesel Race (HSDR) direct-injection combustion technology. Fuel is delivered via two parallel high-pressure pumps to a common-rail system delivering an injection pressure of 1600 bar. The cylinder head has been modified to enable the larger injectors required for the HSDR system. However, demonstrating the robust design of the original JCB444 engine, the valvetrain is carried over substantially in its original form, with the exception of high-temperature specification exhaust valves, up-rated valve springs and a modified camshaft profile.

It’s not only the body shape of a record-breaker that needs to be highly aerodynamically efficient but also the underside, because the air flowing under the car accounts for about one-half of the total aerodynamic drag.

Project aerodynamicist Ron Ayers believes that the interaction between tyre and salt can significantly affect aerodynamic efficiency: salt and debris thrown up by the car’s passage slow it down. To minimise this drag, he has very carefully shaped not just the spats around the lower section of the wheels, but also the flow of air through the choke points between the wheels. Spray beneath the front of the car is deflected outwards, ensuring the rear wheels and tyres run on as clean a surface as possible.

For very practical reasons, all of the aerodynamics study was done via computational fluid dynamics (CFD), not in a wind tunnel.

"Even at the speeds we envisage," Ayers explained, "compressibility effects are beginning to become significant. Indeed, in the region near the wheel/ground contact points, the local airflow actually goes supersonic. We could not simulate such effects in a low-speed wind tunnel with a rolling road.

"The second reason is one of scale. To fit our long, slender vehicle into a tunnel with a rolling road would have meant restricting ourselves to a model scale of about one-sixth and the errors would have been too great."

The main changes as the shape evolved were to lengthen the nose and round it off, to lengthen the tail and to minimise the frontal area. At every stage Ayers had to achieve the optimal balance between aerodynamic drag, skin drag (the larger the surface area, the higher the skin drag) and downforce. If the car is envisaged as an arrow or a dart, it is the tail fin that acts as the flights to maintain stability at maximum speed.

The overall result is an outstandingly beautiful and effective car with a drag coefficient of 0.174 Cd and a CdA of 0.153m2 – extraordinary even by land speed record standards.

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