OLDSMOBILE RACERS TAKE IT TO THE LIMIT Rev Limiters Put the Brakes on Engine Costs and Speed in the Indy Racing League Las Vegas; October 9, 1998 - The most famous words in motor racing are "Gentlemen, start...
OLDSMOBILE RACERS TAKE IT TO THE LIMIT
Rev Limiters Put the Brakes on Engine Costs and Speed in the Indy Racing League
Las Vegas; October 9, 1998 - The most famous words in motor racing are "Gentlemen, start your engines" - but the most frequently heard words in the 1998 Pep Boys Indy Racing League may be "I ran my engine on the rev limiter."
The meaning of this second sentence can be a mystery to casual motorsports fans. The "rev limiter" is a device that both reins in speed and puts the brakes on runaway engine costs. It achieves these results by restricting the maximum speed of the IRL's naturally aspirated racing engines to 10,500 rpm (10,300 rpm in 1999).
The rev limiter is a cornerstone of the IRL's commitment to contain costs and enhance safety. It is the "enforcer" that prevents engine manufacturers (and the independent builders who assemble IRL Aurora V8 motors) from pursuing power through higher engine speeds. By eliminating the incentive to turn the engines faster, the rev limiter compels engine tuners to seek higher levels of performance by improving efficiency and reducing internal losses. Limiting engine speed also improves reliability and longevity by reducing the stresses on engine components.
The Delphi Automotive Systems Gen IV electronic engine management system used by Oldsmobile teams in the IRL has an integral rpm limiting function. "When the engine reaches the programmed rpm limit, the engine control module shuts off the signal to the ignition module," explains GM Motorsports engineer Ned Baker. "The rev limiter effectively turns off the engine just like flipping an ignition switch - but the interruption lasts only a thousandth of a second.
"The rev limiter is very decisive and very accurate," Baker adds. "The ignition system is on at 10,499 rpm, and it's off at 10,501 rpm."
When a driver exceeds the preset rpm limit, the engine note changes abruptly. "The change in pitch is the natural ringing frequency as the ECM switches the ignition on and off as quickly as it can calculate the engine speed," Baker explains. "The system shuts off cylinders randomly. Each cylinder still receives a load of fuel, but since there is no spark, the unburned fuel goes out the exhaust. On the next cycle, the ignition lights off the fuel charge."
Drivers seek to avoid "running on the limiter" because the device cuts engine power dramatically. "When the engine is on the limiter, it's effectively running at half-power," Baker reports. "As the ignition cycles on and off, the horsepower output alternates between zero and 700. You can see exactly what happens on the torque meter when you test an engine on a dynamometer, and the driver can definitely feel the car lose power when the limiter is activated. When the engine hits the limiter, the car loses momentum."
Racing with an rpm limiter is a challenge for both drivers and crew chiefs. The objective is not to run the engine at the maximum rpm, but rather to maintain the highest possible engine speed through the corners. "Even at the tracks where a driver can run flat-out all the way around, the cornering forces scrub off speed," Baker notes. "A crew chief wants the engine to be in the 'sweet spot' of its power curve when the driver accelerates out of the corners. This strategy usually requires turning the engine close to the limit at the end of the straightaways."
An IRL race engineer must select transmission gear ratios that keep the engine below the cutoff point. Fourth, fifth, and sixth gears are closely spaced to allow the driver to choose a ratio that keeps the engine running as fast as possible without exceeding the rev limit. The driver needs a safety margin, however, because the engine speed can "spike" momentarily if the tires lose traction as the car encounters bumps in the racing surface. Other environmental factors also influence engine rpm - running in the draft behind another car or getting a push from a favorable tailwind can increase speed and activate the rev limiter. In these instances, the driver must shift into a gear that will maintain maximum speed without exceeding the limit.
Engine rpm limiters have been used successfully in other racing series to slow the seemingly inevitable upward spiral in both cost and speed. "Historically there have been three routes to improving engine performance," notes Ed Keating, lead development engineer for the IRL Aurora V8. "You can increase engine speed, enlarge the valve area, or make the engine physically smaller to produce a more aerodynamic package. All of these approaches tend to drive up the cost of racing by making engines obsolete.
"The IRL rules do not allow manufacturers to continually develop smaller, higher-revving engines which have little relationship to production engines," Keating continues. "The rpm limit prevents the constant escalation in engine speeds, the maximum cylinder bore diameter limits valve size, and the minimum block dimensions define the physical size of the engine package. In order to achieve a competitive advantage in the IRL series, an engine manufacturer has to focus on fundamentals such as improving efficiency and minimizing parasitic power losses - topics that have relevance to production engines.
"The IRL rules also require that all approved engines be available to teams at a specified price," Keating continues. "This imposes a disciplined approach to engineering because we have to continually evaluate the cost and the value of any new components we might develop."
Speeds on the IRL circuit have climbed this season as teams have become more familiar with their new-formula open-wheel cars and engine builders have found more power. For example, the fastest lap at Charlotte Motor Speedway was five mph faster than last year, and speeds jumped ten mph at Texas Motor Speedway. League officials will lower the rpm limit to 10,300 rpm in 1999 to maintain safety on the speedways.
"This reduction will control engine costs and significantly improve engine reliability while providing even more competitive racing for the fans," said IRL Executive Director Leo Mehl.
It's a familiar maxim in motorsports that speed costs money. The IRL's rev limiter rule shifts the emphasis from the size of a team's budget to the skill of its driver and the expertise of the crew.