Mazda ‘i-ELOOP’ – world’s first capacitor based regenerative braking system

by hans | November 25, 2011 8:48 pm

• Recovers otherwise lost energy during braking or engine over-run.
• Energy stored in a capacitor
• Improves fuel economy by 10% in urban stop-go driving environment
• Works with Mazda’s ‘i-stop’ engine auto-stop function to extend the period that the engine can be shut off while vehicle is idling.

Mazda created another world’s first by launching the ‘i-ELOOP’ capacitor based regenerative braking system. In regular cars, energy is wasted every time the driver brakes. A regular alternator is permanently running to charge the vehicle’s battery once the engine starts, irrespective of the battery’s state of charge. Because of this permanent operation, the alternator is constantly generating a small amount of drag on the engine, contributing to overall friction. Thus, the vehicle’s alternator is one area where car makers can squeeze a little more mileage for their cars, contributing to reduced fuel consumption and exhaust emission.

There are currently three main methods of (kinetic brake) energy recovery – one is electro-chemical, commonly used by hybrid models, whose traction motors will go into ‘generator’ mode to charge the vehicle’s traction battery (battery to powers a driving motor, not the regular 12V battery) every time the driver lifts off the throttle or brakes. The recovered energy is stored in a dedicated traction battery. Another less commonly used method involves a mechanical flywheel spinning at very high rpm within a vacuum sealed housing. Formula One KERS (Kinetic Energy Recovery System) and Porsche’s one off race car concept 911 GT3 R Hybrid are two working examples. Both of these methods involve very complex and expensive compoents. A third solution, involves a special high efficiency alternator with a special clutch that only engages on-demand, i.e. when the driver brakes or lifts off the throttle. Some car makers term this as micro-hybrid. Consistent with Mazda’s SKYACTIV umbrella[1], which seeks to reduce fuel consumption without the use of expensive hybrid solutions, the ‘i-ELOOP’ belongs in the third method.

Under the ‘i-ELOOP’ solution, the regular battery-alternator link is replaced by three components, a 12-25V variable voltage alternator, a low-resistance electric double layer capacitor and a DC/DC converter.

‘i-ELOOP’ engages as soon as the driver lifts off the throttle. The variable voltage alternator produces up to 25V of electricity for maximum efficiency, sending it to the Electric Double Layer Capacitor (EDLC) for storage. The capacitor, specially developed for automotive powertrain application, can be fully charged in seconds. Because the vehicle’s battery is only rated at 12V, a DC/DC converter is required to steps down the voltage from a maximum of 25V to 12V, before it can be directed to the battery.

This way, the alternator does not drags the engine unnecessarily as it only engages and charges the battery on demand. Mazda claims this will translate to around 10% improvement in fuel economy under stop-and-go driving environment, like in a typical urban commute.

The first i-ELOOP equipped Mazda will hit the market by 2012.

Mazda is not the only car maker to develop a non-battery dependent energy recovery system. The concept of Mazda’s ‘i-ELOOP’ is similar to BMW’s EfficientDynamics. But a regular lead-acid battery is not able to sustain the frequent but high voltage and rapid burst of charge. In BMW’s case, all EfficientDynamics equipped models use a special absorbed glass-mat (AGM) battery in place of regular lead-acid types. From an end user’s perspective, AGM battery is more complicated to maintain. In the case of a EfficientDynamics equipped BMW models, a simple battery replacement job needs to be done by a trained technician with a proper diagnostic tool. Because the battery management computer tailors the battery charging based on the battery’s condition. A new battery needs to be ‘registered’ into the battery computer, telling it that a fresh battery has been installed. Else, premature battery failure will happen as the battery is subjected to excessive charging.

At the moment, Mazda has not release any further details aside from the press release briefly explaining details of the i-ELOOP’s function. But from our limited knowledge, we speculate that an AGM battery may not be necessary, as the capacitor is already shielding the battery from any intensive direct charging by the alternator. From this author’s limited knowledge on capacitor discharge characteristics, capacitors do not discharge in a linear fashion, but discharges progressively slower (negative exponential) over time. The capacitor should shield the battery from receiving sudden and rapid burst of energy everytime the alternator kicks in, so a regular lead-acid should be sufficient. Rapid burst of power from the alternator only serves to charge the capacitor. Power will be stored in the capacitor until a voltage drop is applied (i.e. low charge on the parallel linked battery). Moreover, a capacitor’s rate of discharge can be regulated with application of resistance, in this case, the engine management / battery computer should be able to handle that. Thus removing any need to replace a regular lead-acid battery with an AGM unit.

Mazda is not the only company working on capacitors to store recovered kinetic energy, although Mazda is the first to put it on a production car. Honda has been experimenting with ultra-capacitors since the late-90s. Before the Honda Insight (first generation) model was launched, the car was previewed as the JV-X Concept (below) at the 1997 Tokyo Motor Show. The JV-X featured ultra-capacitors to store recovered kinetic energy. Again, from our limited knowledge, we believe this is the first working concept of a capacitor based regenerative braking system. Initially, Honda explored the petrol-electric hybrid concept by employing ultra-capacitors to store recovered energy. But in the end, Honda went down the NiMh battery route because it was obvious that capacitor technology still required more development work.

Source URL: http://www.motorindustry.org/2011/11/25/mazda-i-eloop-worlds-first-capacitor-based-regenerative-braking-system/

---