The first automotive 12-V regenerative braking system to use a "supercapacitor" will make its debut on Mazda's new Mazda6. Called i-ELOOP for intelligent energy loop, the long-promised system is scheduled for May production and is expected to arrive in U.S. dealerships by July. The system itself is in line with the intelligent simplification approach to advanced engineering that the company has demonstrated with its Skyactiv engine lineup and new engine block machining system (view article here).
The automaker has not released “window sticker”-effect numbers for a model with the system, but its in-house tests shows it delivers 10% better fuel economy in stop-and-go operation, while adding just 9.3 kg (20.5 lb) to a 12-V car. The supercapacitor sidesteps the usual hybrid vehicle approach of a specific battery and larger alternator. However, it fits the Mazda engineering intent, which is to reduce as much as possible the need for alternator output to power electrical accessories, rather than provide acceleration assist or electric vehicle operation, as in a hybrid.
The only storage capacity specification Mazda is releasing is for the maximum used capacity, and in joules: 25,000 J. As performance of the supercapacitor is improved, it offers potential future electrification (such as of currently belt-driven accessories) for still better fuel economy without having to go the more expensive high-voltage hybrid route.
The cylindrical supercapacitor, which is of a type also called a "double-layer capacitor" or "ultracapacitor," is mounted under hood on the driver’s side. It is 350 mm tall (13.8 in), is 120 mm in diameter (4.72 in), and weighs just 6.0 kg (13.2 lb). Mazda has “crushed and crashed it in every imaginable way with no issues,” a company spokesman told AEI. The circuit requires a heavier wiring harness, which adds 1.5 kg (3.3 lb).
Why a supercapacitor instead of a battery? Supercapacitors accept and release charge much more quickly and can be discharged and recharged many more times—and with far less deterioration than a battery. The Mazda unit can accept a full charge in just 8-10 s. And although it can discharge in as quickly as 40 s (at a maximum rate of 50 A/14.5-V), the capacitor may take up to about 113 s when the load is at the minimum—about 18 A. The battery capacity is unchanged, because a primary factor is the requirement for worst-case cold start, which would not include a charged supercapacitor.
Varying voltage alternator
The alternator, although about the same weight as its 12-V predecessor, is a varying-voltage design that operates in a 12- to 25-V range. So the circuit also requires a dc-dc converter (which weighs 1.8 kg/4.0 lb) to provide 12-V power for electrical accessories. As soon as the driver lifts his foot off the accelerator, the regenerative mode begins, and the alternator uses the kinetic energy of deceleration to produce electricity at maximum possible voltage for efficiency. The “free” electricity goes through the dc-dc converter, and if there is any available electricity beyond the car’s electrical load, it goes to charge the 12-V battery.
There obviously is a conversion loss through the dc-dc converter, but the 12- to 25-V range of the alternator means the supercapacitor may be able to take in a larger “gulp” of electricity during "regen." At 50 A vehicle demand and below, with a fully charged supercapacitor, the alternator is allowed to freewheel, and the capacitor and dc-dc converter supply the electricity.
When the supercapacitor is discharged, but the electrical load is at 50 A and below, and the car is being driven in cruise or on an upgrade, no regenerative braking energy is available. The alternator then will charge but through the dc-dc converter. The smart charging system may compensate for transmission losses in this type of operation by running the alternator at higher voltage with reduced current, a more efficient approach. An example cited by Mazda: the charging system is more efficient at 25 V/ 25 A than 12.5 V/ 50 A. The smart system is always looking for the "sweet spot," Mazda spokesman David Coleman explained, and that also improves battery durability.
The dc-dc converter has a maximum throughput of 50 A at 14.5 V. Headlamps and other exterior lighting, HVAC, wipers, and the audio system account for about 40 A, so in "normal" use the supercapacitor system is well within range. However, on a cold day, particularly right after vehicle start, the driver may be using the rear window defogger and perhaps the optional seat heaters. These could push the electrical load over the dc-dc converter's capacity. If so, the smart charging system triggers a relay that bypasses the capacitor system completely, and for that period (likely to be brief) the car's electrical system reverts to conventional alternator-powered operation.
Although other idle-stop systems draw a lot of battery power for heavy-duty starters, Mazda's system ("i-Stop") would not be significantly involved in the picture, even when it eventually reaches the U.S. market. i-Stop is sold in Japan and some other countries but has been withheld from the U.S. as there is no window-sticker fuel-economy benefit on the U.S. EPA drive cycle.
The novel system incorporates a high-resolution crank position sensor with an electronic strategy that uses the alternator to stop the engine so the piston of one cylinder is in an optimum position at the start of the power stroke. A precisely timed squirt of fuel and spark creates some downward force, which combines with just a quick boost from the starter motor to restart the engine in under 0.4 s. Idle-stop systems may not help the window sticker numbers on a significant number of vehicles; but, they are eligible for a corporate average fuel economy (CAFE) credit, and Mazda reportedly will introduce its system in 2016.