Traditional rev limiters work by controlling the ignition and/or fuel.
- Retarding the spark (smoother)
- Completely cutting the spark (more aggressive)
Fuel Control - Amount of fuel thrown in is controlled. This is used very carefully as effects of running lean at such high RPM can be detrimental to the engine health. In fact, lot of controllers completely shy away from it for good reason.
Cylinder Control - Allows for one more parameter to play with to smooth-en out the total power produced.
For the sake of categorizing, the above parameters are controlled to achieve the desired output
- Hard Cut - With hard cut, the power is abruptly reduced once it reaches the SET RPM
- Soft Cut - power is progressively reduced as it approaches the set RPM
The above was a crude generalization while the actual strategy employed could be thought of as a fader between the above two.
While revving in nuetral/clutched - There is very little rotational inertia and so as the rev's reach the SET limit soft control becomes less effective and hard-cut is initiated to the rev's within range. This causes the rev's to bounce off the limit repeatedly.
While in gear - as the vehicle approaches the rev-limit, the above mentioned bounce is lesser. One will still feel the limiter.
For passenger cars the limiter is a lot softer and smoother at the expense of losing power.
For race cars, the all or nothing approach works well (wouldn't want to compromise power) and consequently (almost as a side-effect) works as a quick feedback that the limit is reached.
Rev- limiters are also used for launch control, flatshifting (multistage limiters) and for antilag
Launch Control - Engine RPM is held at a particular RPM which is optimum for launching (right balance point between RPM, torque, wheelspin etc.)
Flatshifting - Allows for the driver to keep his leg floored while the limiter automatically brings the revs to optimum RPM for the shift.
Anti-Lag for turbo - By using ignition retard + fuel control allows for hotter exhaust gases at the set RPM which helps reduce turbo lag (between gear-shifts and during launch). An extreme setting for the same causes backfires and the loud pops commonly heard in race cars with massive turbos.
DBW Systems allow for complete control of the throttle body and makes for a more accurate and smoother control. Depending on the strategy and setting, it has the option of pre-emptively reducing the throttle response as it approaches the limit to control the offshoot. And even when it does, the ability to choke the engine allows for engine braking control. So we're not just letting the engine slow down by cutting power, we have brakes to some extent.
So, in a way , the traditional system is reactionary while DBW is active , in some sense at least.