Abstract
Keywords - CO2, brake, boost, vacuum, regenerative
Powertrains are challenged by reduced CO2 and other exhaust emissions. Thus mechanical auxiliary drives are vanishing including mechanical vacuum pumps. The creation of vacuum at SI engines may disappear temporarily or permanently due to start/stop functionality, intake valve control replacing throttle or due to hybrid/electric drives. The increasing demand for safety improving driving assistance functions calls for control-by-wire ability of powertrain but also of brake systems.
Today´s conventional hydraulic brake systems with vacuum boosters and hydraulic units for ESP® offer a high performance/cost ratio but may be pushed to their limits by above mentioned requirements. Adaptive Cruise Control Systems featuring Stop & Go functionality or effective regenerative braking functions ask for high duty and low noise solutions that can exceed the possibilities of hydraulically controlled brake interventions by enhanced hydraulic units like ESP®plus or similar solutions.
A huge multitude of solutions is created or at least concepts are discussed that may help to overcome the described short comings of enhanced conventional solutions.
Electro-hydraulic Brake Systems are in mass production and can cope with a. m. requirements but have proven to be rather costly and hardly scalable. Brake-by-wire systems with electromechanical actuators with or without self-energizing functionality (wedge brakes) suffer from complexity and thus reduced reliability as well as high cost.
Systems with actively controlled vacuum boosters can be used for blending functions for regenerative braking but still rely on vacuum supply. The same is valid for all solutions that use ESP® hydraulics for brake force blending.
Some proposals are made that have pedal decoupled electronic control on the rear axle only either by using electro-mechanic rear brakes or controlling them electro-hydraulically. Those approaches offer a limited regeneration only since the front brakes always are engaged by pushing the pedal and by that generating friction braking instead of generating electrical energy for battery recharging. In addition those systems still require vacuum boost for the front hydraulic brakes whereas the rear axles are controlled electro-hydraulically or electrically with already mentioned issues.
There are several more or less complex solutions with full or limited blending functionality but all of them are lacking a reasonable scalability in case of hybrid and conventional drives on the same vehicle platform.
Therefore a system that could replace the standard vacuum booster offering control-by-wire and independence from vacuum would be the most favourable one. In the paper the electromechanical booster (iBooster) is presented.