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Do start-stop systems, which smartly switch off the gasoline (or diesel) engine at stoplights and restart it as soon as you lift off the brake pedal, produce significant gains in gas mileage?
In real-world stop-and-go driving, the answer is certainly yes; stop-start systems can produce mpg gains of up to ten percent—a very significant several miles per gallon for a compact car. But that’s not how the feds see it; in U.S. EPA test cycles, start-stop systems have shown them to provide no appreciable boost in mpg. Nevertheless, start-stop is available in a handful of premium-brand cars, and even in the U.S. it will soon be offered in the 2012 Kia Rio.
Almost a mild-hybrid system
Although Mazda doesn’t offer any of its U.S.-market vehicles with start-stop, it’s a global forerunner in the technology, which it terms i-Stop. And now Mazda has supplemented i-Stop with a capacitor-based regenerative-braking system that should provide some of the benefits of a mild-hybrid system without added bulk. Mazda says that this new system, which it’s calling i-ELOOP, can improve fuel economy by up to ten percent in stop-and-go driving—and we’re assuming that’s on top of the ten-percent improvement Mazda has already said i-Stop can achieve in some conditions.
With i-ELOOP—set to go into an upcoming production model that Mazda hasn't yet named—the automaker has added regenerative braking, like in a mild-hybrid, but it doesn’t rely on weight-adding bolstered battery systems—instead it uses capacitors to more aggressively recover the energy lost in deceleration and braking. Mazda claims that the new i-ELOOP system starts to recover energy the moment the driver lifts off the accelerator.
The results are almost like a mild-hybrid system, only without the added weight and complexity of a system that might in a limited sense supplement motive force.
Unlike batteries, which are very limited in how rapidly they can charge, or discharge, due to heat and chemistry constraints, capacitors can very rapidly store and discharge energy. The capacitor in Mazda’s system, the automaker says, can be fully charged in seconds. Other strengths of capacitors is that they’re easier to package, and they don’t deteriorate appreciably over time, as batteries do.
Lets engine stay off longer
The i-ELOOP system rapidly charges the capacitor with regenerative braking when the driver is coasting, decelerating, or braking the vehicle, recovering more energy than a battery-based system could. That energy then contributes to the power used in climate control and audio functions, as well as ancillaries like lighting, and —this is really the key—it works with Mazda’s existing ‘i-stop’ system and allows the engine to stay off longer, whether that be for extra-long traffic-light cycles in inclement weather, or when stopped in gridlock.
The new system adds a 12-25-volt variable-voltage alternator, a low-resistance, electric double-layer capacitor, and a DC/AC converter.
Capacitors have been examined for more than a decade as an energy-storage alternative to batteries, but the downfall of capacitors—or larger-scale ultracapacitors—is that they’re essentially one-trick ponies, with discharge rates that can’t be significantly extended. Many automakers are examining them as buffers to help increase efficiency in hybrids, and some have used them with fuel-cell vehicles—in earlier development versions of the Honda FCX fuel-cell vehicle, for example.
Perfect for U.S. stop-and-go driving patterns
Mazda’s configuration makes sense for congested city-driving patterns, where rapid braking is typically followed by idling; and it’s in those following seconds that ancillaries can best use the extra power.
The new i-ELOOP system fits under the automaker’s new Skyactiv initiative, which aims to extensively reengineer core body, chassis, and powertrain components for greater efficiency, without losing the brand’s characteristic driving personality. Those technologies, as a suite, are making their U.S. debut in the 2013 Mazda CX-5, but it's still possible (as the schematic up above hints) that we might see some extended technologies in a completely redesigned Mazda6 that's due later next year.
Will we see a system like i-ELOOP in the U.S.? It’s unlikely in the very near future, although recently one firm’s analysis anticipated that 25 percent of all U.S. vehicles will have the technology by 2015. As of yet, there’s still no sign of EPA window-sticker and corporate fuel economy (CAFE) ratings accounting for the gains of start-stop systems—even though they would make more efficient real-world picks.