RESEARCH SNAPSHOT: Review of start-stop engines
Bosch provides batteries designed specifically for vehicles featuring start-stop systems.
Continuing just-auto's series of research snapshots - pulled from its automotive research platform, QUBE - Matthew Beecham reviews the blossoming market for start-stop applications and some implications for batteries and lubricants.
Start-stop is a technology applied to a car that automatically shuts the engine off during idle at, say a red light and restarts in a jiffy when the driver engages the clutch or releases the brake pedal. The urban myth that you'll save fuel by letting your car run idle rather than switching it off no longer applies. Start-stop technology has been proven to reduce fuel use and emissions by 5 to 12 percent.
Since the first start-stop system went into series production in 2007, Bosch alone has sold more than four million systems. The technology was initially fitted to hybrid engines but has gradually spread to cars powered by a conventional internal combustion engine.
Motorists' appetite for such applications shows no signs of easing, either. Johnson Controls estimates that the global annual market for new start-stop vehicles will reach 35 million by 2017, and in this time frame over 70 percent of new cars being built in Europe will be based on this technology. It has therefore become something that car buyers can expect, not hope for. That further explains why Johnson Controls is investing some US$280 million in Germany to expand its start-stop battery manufacturing capacity there to 11 million units. It is also investing US$140 million in the US and US$100 million in China, to grow its annual start-stop manufacturing capacity to 6 million and 2 million, respectively, in these countries.
Elsewhere, the market for start-stop applications is gathering pace. Ford has introduced start-stop technology across all its vehicles in North America. Chevrolet is also increasingly fitting start-stop; the automaker made it standard on the 2014 Malibu and 2015 Impala.
In Japan, Toyota and Honda have offered cars with start-stop technology for a number of years. In India, Mahindra & Mahindra was the first to launch stop-start technology based on a micro-hybrid system.
Kia is also among those offering start-stop but with a novel twist. For instance, the Kia Cee'd ISG (which stands for ‘idle stop and go') model features a fail-safe system where if the car's battery power drops below 75 percent of maximum for any reason, or there is insufficient energy available for the next start, the system will temporarily suspend stop and go.
Taking the technology to the next level, Jaguar's XF model comes equipped with an ‘intelligent' start-stop system, halting the engine only when justified and restarting instantly. That, Jaguar says, is because it has a tandem solenoid starter motor with secondary battery, bi-directional crank sensor and fuel rail pressure holding system. The smart bit is a 'change of mind' system; release the brakes before the engine stops and the injection system is refuelled and returns quickly to idle speed which also brings CO2 emission reductions of 5 to 7 percent.
Given that starting the engine is the most demanding aspect for a car's battery, a start-stop car needs a heavy duty and more expensive Absorbed Glass Mat (AGM) battery to do the job. Indeed, AGM batteries for start-stop cars differ from conventional batteries in a number of ways; they have higher output power, input power (charge acceptance) and endurance against frequent high-rate discharge. Masato Furumi, General Manager, Business Promotion Department, Automotive Battery Business Promotion Division, International Business Unit, GS Yuasa International Ltd told just-auto: "This is typically achieved by producing cells with an increased number of thinner plates with the appropriate type and amount of carbon additive in the negative plates, higher density positive active material and higher endurance positive grids. [Collectively] these measures protect the batteries from stratification, sulphation and prevent grid corrosion due to lower electric potential."
Johnson Controls offers customers two battery technologies to enable start-stop functionality in vehicles, namely Enhanced Flooded Battery (EFB) and AGM. "EFBs provide twice the cycling performance and can be discharged more often without any loss of functionality," said Craig Rigby, Advanced Market and Technology Strategist, Johnson Controls Power Solutions during an interview with just-auto. The supplier manufactures more than one million EFBs annually in its European facilities located in Burgos, Spain and Hannover, Germany.
"AGM batteries have four times or more the cycling durability of a conventional battery," continued Rigby "and efficiently manage high accessory loads such as heated seats and personal devices, as well as electronic systems such as brakes, steering and HVAC [heating, ventilation and air conditioning]. A glass mat separator in the battery, in combination with a high degree of compression, consistently maintains the electrolyte around the positive plate to extend the battery's life and reduce the effects of vibration."
While start-stop applications have triggered battery innovations, special lubricants are appearing too. Not so long ago, engine oil was pretty much a standard product, to which most of us seldom gave much thought. But nowadays, with alternative powertrains, it is being brought increasingly to the consumer's attention. For stop-start engines in particular, UK-based Millers Oils has developed a special lubricant. "There's a potential issue with temperature fluctuation in stop-start operation," said Nevil Hall of Millers Oils during an interview for just-auto's QUBE. "Most people are aware of the effect of oil viscosity, in that efficiency is reduced at lower temperatures because the oil is thicker, but few realise that film strength is also affected by temperature. We have seen some alarming shortfalls in film strength at certain temperatures when benchmarking competitor oils. Many oil additives only react chemically from around 70 degrees C upwards, so they don't offer the same protection when cold. To formulate oils suitable for engines with stop-start, we have developed additive packages that ensure consistent film strength across a wide temperature range."