Going Nowhere Fast: The Chassis Dynamometer’s Rise to Crucial Development Tool

From the January 2014 Issue of Car and Driver

The dyno’s rise in modern car culture is well chronicled on YouTube. Here, car guys while away hour after mesmerizing hour watching dyno pulls that depict unbelievable power, catastrophic consequences, or sometimes both. But the dynamometers that most enthusiasts have access to are crude instruments compared with what these machines are capable of. With the hardware and software to raise prices from less than $25,000 to more than $1 million, the dyno becomes a real-world driving simulator and a crucial development tool.

Used for emissions testing, the Horiba Vulcan dynamometer illustrated here is one example of just how much an OEM’s or major supplier’s dyno differs from what you’d find at your neighborhood shop. The Vulcan is so precisely tuned that different finishes and textures are available for the rollers to simulate different road surfaces in various markets. An auto-pilot system is also optional.
Rollin’
The steel roller connects the tires to the power absorber and provides the inertia that makes transient behavior more realistic.

Take A Load Off
Prior to the use of these vibration-isolating footers, up to eight tons of concrete had to be poured to securely anchor a chassis dynamometer. The entire dyno assembly can weigh up to 20 tons.

Extra Absorbent
The power absorber, typically an eddy-current brake or an electric machine, simulates the power lost to wind and rolling resistance.

In its most basic form, a dynamometer is a rotating steel drum weighing about 3000 pounds. Inertia dynamometers measure torque at the wheels by tracking the time it takes to accelerate the rotation of a known mass. Horsepower is calculated from torque and rpm. An inertia dyno’s simplicity brings durability, repeatability, and a reasonable price, but it is only good for sweep tests over a wide rpm range. And altering the driveline mass with a lighter flywheel, driveshaft, or wheels would be wrongly interpreted by an inertia dyno as an increase in torque.

A dynamometer becomes much more than an output gauge when it utilizes a power-absorption unit. By altering the dyno’s resistance in sync with road speed, the absorber enables part-throttle runs, transient tests, and road-load simulation outside the influence of traffic, weather, and terrain. An eddy-current brake—a kind of no-contact disc brake—uses a magnetic field to increase the load at the car’s tires. Affordable prices make eddy-current dynos popular with tuning shops that use them to calibrate and test modified cars.

Automakers favor AC or DC electric machines as absorbers for their ability to simulate downhill coasting and to improve control over resistance. Emissions and fuel-economy applications demand less torque than most other dyno tasks but require greater accuracy, making an AC motor ideal. Low inertia is desirable, and Horiba, which supplies the Bentley of dynos to automakers and the EPA for emissions testing, claims the lowest with a drum weight of 2535 pounds. The run-out (outer diameter variation) on its four-foot-diameter roll is less than 0.01 inch. With the most complex setup—a four-wheel-drive unit that can endure minus 40 degrees Fahrenheit in a temperature-controlled cell—the cost for one dyno swells from $500,000 to as much as $1.5 million. But for automakers, that’s just one chip of the huge ante required to be in the game.

Today’s single-roll dynamometers are far more sophisticated than the twin-roll units that were common in the 1980s, with dynos now designed for specific tasks. NVH engineers often rely on a 60-inch drum to reduce tire roar as well as a DC power absorber, which can be located outside the test cell to isolate the noise it generates. A 72-inch roller maximizes traction for torquey, heavy-duty vehicles. Specialized engine, transmission, and axle dynamometers can sniff out friction losses in individual links of the chain by evaluating components outside the chassis.