Chevy Indy V8 insights: The hard facts on soft rev limiters. NASHVILLE, Tenn., July 18, 2002 -- It is the nature of racers to seek speed. Without the limits imposed by rulebooks and sanctioning bodies, modern race cars could easily achieve...
Chevy Indy V8 insights: The hard facts on soft rev limiters.
NASHVILLE, Tenn., July 18, 2002 -- It is the nature of racers to seek speed. Without the limits imposed by rulebooks and sanctioning bodies, modern race cars could easily achieve levels of performance that would breach the boundaries of prudence, safety and expense. As a result, virtually every form of organized motorsports imposes limitations on engine performance. These range from displacement limits and fuel allocations to carburetor restrictor plates and turbocharger pop-off valves. In the Indy Racing League, the final arbiter of horsepower is an electronic rev limiter.
IRL teams will be running at the limit this weekend in pursuit of victory on Nashville Speedway's distinctive 1.33-mile concrete oval, the site of Saturday night's Firestone Indy 200. The championship-winning Chevy Indy V8 engines that compete in the IRL are governed by a trio of rpm limiters. Two "hard" limiters effectively shut down the engine's ignition system when it reaches the prescribed 10,700 rpm maximum, while a third "soft" limiter progressively cuts spark to the cylinders as engine speed approaches the legal limit.
"The IRL limiter is called a hard limiter because when it is activated, it shuts down the entire ignition system," explained GM Racing engineer Ned Baker, the group's electronics specialist. "It's like an on-off switch. When it determines that the engine has exceeded 10,700 rpm, it effectively shuts off the ignition until the rpm falls below the specified speed. When the limiter is activated, engine output drops to zero. The limiter doesn't restore current to the ignition system until it has determined that the engine is below the rpm limit, so there is a period when the car is essentially without power."
This stand-alone rev limiter that is supplied, maintained and inspected by IRL officials is the policeman that enforces the series' rpm rule. It calculates engine speed by measuring the intervals as four teeth on the flywheel pass a block-mounted sensor. The circuitry can measure engine speed with remarkable accuracy. At 10,000 rpm, for example, there are 666 pulses per second, permitting the limiter to detect changes in engine rpm continuously.
"The IRL limiter also has a powerful data-logging capability," Baker noted. "It records engine rpm in real time, so officials can examine engine speed at any point during an event -- in practice, qualifying and the race.
"The IRL hard limiter is extremely harsh, so there is a second rpm limiter incorporated in the engine control module (ECM)," he continued. "This limiter is set just below the threshold of the IRL limit. The ECM calculates engine speed just as the IRL limiter does, but uses its own sensors and its own logic. When it is activated, the internal ECU limiter shuts off the signal to the ignition module. The interruption may last only a thousandth of a second, but it is usually enough to keep the engine speed below the point where the IRL limiter intervenes.
"The third limiter, which is also contained in the ECU software, is called a 'soft' limiter because it is reduces engine power progressively by cutting the spark to selected cylinders," Baker said. "Instead of shutting down all eight cylinders like a hard limiter does, it drops one, two or more cylinders per revolution. When the soft limiter is engaged, the engine can still produce a significant percentage of its potential power instead of going to zero as it does when either of the hard limiters is activated.
"With the hard limiter, it's all or nothing; with a soft limiter, you might still have 90 percent of engine power available to drive the car. There is still the absolute boundary of 10,700 rpm that you can't cross, but the soft limiter allows you to walk a little closer to the line while maintaining a relatively high performance level."
It is instantly apparent when a driver steps over the line because the engine's exhaust note changes abruptly. When the ignition system is shut down, fuel is still injected into the cylinders. The unburned methanol goes out the exhaust system, where it is ignited by subsequent firings. The result is a distinctive sound that can be heard in the grandstands.
IRL teams must carefully balance gear ratios, aerodynamics and track conditions to maximize vehicle speed without exceeding the engine rpm limit. Fourth, fifth, and sixth gears are closely spaced to allow the driver to choose a ratio that will keep the engine running as fast as possible without exceeding the rev limit. In spite of a team's best efforts to keep the engine below 10,700 rpm, the limiter can be activated when engine speed spikes momentarily as the rear tires skip over 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 night races like this weekend's Firestone Indy 200, changes in temperature and track conditions between daylight and darkness also impact speed and engine rpm. In these situations, the driver must select a gear that will produce maximum speed without exceeding the mandated rpm limit.
The rpm limit has been revised several times since the series implemented its naturally aspirated engine formula in 1997. Initially set at 10,500 rpm when the series introduced 4.0-liter engines, the limit was lowered to 10,300 in 1998 and to 10,000 rpm in 1999 to rein in rapidly escalating speeds. The move to smaller 3.5-liter engines in 2000 saw the limit raised to the current 10,700 rpm standard. In 2003 the second-generation 3.5-liter engines will run initially at a maximum of 10,300 rpm.
"The rev limiter is a key element in the IRL's commitment to affordable technology," said GM Racing Group Manager Joe Negri. "Along with rules that specify maximum cylinder bore diameter, minimum engine weight and key engine dimensions, the rev limiter encourages manufacturers to focus on fundamentals such as improving efficiency and minimizing parasitic power losses. These are topics that have great relevance to GM production engines."
While racers invariably push the limits of speed and performance, the IRL's electronic rev limiters provide a reality check that keeps the focus on close competition and appropriate technology.