RESEARCH ANALYSIS: Review of stop-start systems

22 October 2009 | Source: just-auto.com editorial team

The gradual emergence of eco-friendly vehicles on our roads has featured, among other technical wizardry, systems that automatically switch off vehicle engines during idling, for instance at red traffic lights. Matthew Beecham pauses to review the latest advances in stop-start systems.

According to various research studies, vehicles are at a standstill for one-third of the time while in urban areas.  Stop-start systems could help make cities quieter, boost fuel efficiency and reduce exhaust pipe emissions. Stop-start systems operate by cutting the engine when the driver comes to a complete standstill.  The engine is switched back on when he/she releases the brake pedal.  Such systems are a good route for automakers to reduce emissions and meet CO2 reduction targets. 

Integrated starter-alternator systems aim to balance out irregularities in the running engine and in the drivetrain, providing more comfort. Clever electronics do the trick here. They help to control the starter-alternator operating state, depending on the load and battery charge status. Features such as the stop-start function are proving to be a major attraction for car buyers.  There are two main alternative design concepts:
 

• belt-driven ISG device—shaped like an alternator, this electronically controlled machine replaces the starter motor and alternator, providing an instant stop-start function for fuel savings up to 5%. Various refinements of this belt-driven device can boost economy further - such as brake energy regeneration and mild engine boost.
• in-line units—mounted directly on the crankshaft between the engine and gearbox. Although the in-line systems are more expensive, they offer more power, regenerative power from braking and even engine-assist ability under heavy load.  This integrated crankshaft-starter alternator (ICSA) effectively increases the efficiency of the electrical system, says Bosch.  The electric motor is designed as a permanent-field synchronous machine and is located between the internal combustion engine and transmission. When the vehicle is in operation, the ICSA generates electrical energy.  The high efficiency of the starter alternator contributes to a reduction in fuel consumption by about 0.5%.  This potential for fuel savings can be increased to 4 - 6% if the ICSA is equipped to include a start-stop function.

While developing stop-start systems is a noble cause, it takes a lot of engineering effort simply to obtain a stop-start function. "Setting aside the additional cost," said Mike Dowsett, senior manager for micro-hybrid systems at Controlled Power Technologies, "the critical issue for automotive engineers developing stop-start is product refinement and how much confidence the driver has in the system.  Above all, stop-start cannot afford to irritate the motorist by being slow or faltering.  Unfortunately, there were too many reports of early systems that were less than perfect causing their drivers to opt for the off switch and it is interesting to note that virtually all manufacturers still retain a disengage button.  First generation automated manual transmissions received similar criticism.  Drivers want peace of mind; they expect modern cars to be inherently reliable and similarly will have an expectation that a stop-start system will be robust and work reliably and without hesitation." Controlled Power Technologies Ltd was founded in early 2007. Asset and technology acquisitions from Visteon Corp, together with the signing of associated licensing and collaboration agreements with Switched Reluctance Drives Ltd, gave the company immediate access to a portfolio of production-ready and near-term solutions to the problem of automotive CO2 reduction.

Dr Gerhard Wagner, member of the board of management, ZF Group and responsible for the group's car driveline technology division, told us: "The savings potential of this start-stop system is solely due to the fact that the combustion engine is switched off once the vehicle is stopped. If even greater savings effects are to be generated, then more complex systems are required; systems that enable recuperation or shifting of the combustion engine's operating point."

Over the past few years, a number of automakers have offered start-stop technology on at least one model in their line-up.  Some examples of stop-start systems applied to vehicles over the past 12 months include:

• Late last year, Toyota added a new 1.33-litre Dual VVT-i gasoline engine with stop-start technology to its Auris model. Toyota claims that its new powertrain with stop-start technology results in 17% lower CO2 emissions (135g/km) and a 19% improvement in fuel consumption (48.7mpg combined cycle) compared to the previous 1.4-litre VVT-i unit. Auris is the first model in Europe to benefit from Toyota's stop-start technology.
• Earlier this year, Kia's assembly plant in Zilina, Slovakia, began producing six new gasoline-powered Ceed ISG (idle stop and go) models claimed to deliver up to a 15% fuel economy improvement in city driving. 
• In March, Fiat presented its first model to feature a stop-start system: the Fiat 500. The system is being supplied by Bosch.
• In May, Land Rover began offering stop-start technology on the manual diesel Freelander models. The system is available on all manual versions of the 2.2 litre diesel TD4 models. Phil Wiffin, manager of stop-start systems at Land Rover, said that, on a typical 40-minute journey in London, a vehicle's engine is only needed for about 19 minutes. "We're saying this is the world's first intelligent stop-start SUV. Fuel consumption in these conditions is improved by 12% which is a saving of £13 on a tank of diesel at today's prices on a typical London test route."
• In June, Mazda launched its redesigned Axela, sold in export markets as the 3, in Japan.  The automaker claims that its new model offers improved fuel economy from new idle stop technology. Japanese-specification four-door sedan and five-door models come with 1.5, 2.0 and 2.3 litre engines. The two-litre front wheel drive models have a new so-called 'i-stop' system which automatically stops the engine whenever the car idles and restarts it in a split second while at the same time cutting down engine noise and vibration.
• Valeo's so-called StARS micro-hybrid system is being fitted to the new Mercedes-Benz A and B Class. Valeo's stop-start system is already equipped to the Citroen C2 and C3 and the Smart MHD (micro-hybrid drive). Valeo has also won a contract from PSA Peugeot Citroen to equip more than a million vehicles with StARS by 2011. 

For its part, Controlled Power Technologies has been developing technology, known as SpeedStart.  The company claims SpeedStart represents the development of the world's first bespoke belt-driven Integrated Starter Generator (B-ISG) capable of starting a 2-litre diesel engine with a conventional 12-volt electrical system.  The system has been proven in a gasoline and diesel engine.  Dowsett told us: "To demonstrate its capability and showcase the technology, CPT has installed SpeedStart in a Volvo S40 equipped with a 2.0-litre common-rail diesel engine to represent a fairly typical European powertrain.  The 12-volt system is powerful enough to start the engine in almost half the time required by a normal starter motor - 0 to 750rpm in less than 400 milliseconds compared with typically 750 milliseconds - and it also avoids the need for expensive super capacitors.  SpeedStart not only offers a comfortable and powerful engine cranking capability, but also is a very efficient low speed high power generator.   The system consistently delivers an almost silent as well as fast starting event and will repeatedly crank a 2.0-litre diesel powertrain under a wide range of operating conditions.  To deliver this performance, the SpeedStart belt drive integrated starter generator (B-ISG) has been optimised for powertrains with petrol engine displacements up to 3.0-litres and diesel engines up to 2.5-litres, depending on the application.   It's also an easily installed and well packaged solution."

As far as market demand is concerned, Denso expects stop-start systems to grow rapidly from around 2015, partly because of European CO2 emission regulations. The European carmakers are leading the market for stop-start technology, supported by their supply base. 

Wagner added: "Today, almost all manufacturers already offer models with start-stop systems that - for the audience - excel through low fuel consumption and low CO2 emissions. For manufacturers, start-stop systems are an opportunity to lower fuel consumption of their models at rather modest technical expenses. Consequently, the segment-spanning spread of "simple" start-stop systems will presumably continue."

Bosch is said to be the market leader in supply such stop-start systems, closely followed by Valeo.  However, manufacturers point out that competition from Chinese sources is increasing.  Stefan Asenkerschbaumer, president of Bosch's starter motors and generators division, believes that the start-stop market will grow rapidly in the next five years.  He said: "In 2008, roughly 5% of all vehicles in Europe are equipped with a start-stop system. By 2012, we estimate this will be [in] every second newly registered vehicle - most of them with Bosch technology."

Finally, while stop-start systems have evolved, manufacturers of rotating electrics have also pushed back the technical boundaries of starter motors and alternators.  Indeed, the increasing use of power-hungry options in modern passenger cars is placing unprecedented demands on automotive charging systems. "Modern vehicles are not only demanding more electrical power to satisfy customer requirements, but in pursuit of lower fuel consumption engine idle speeds are also being reduced, which further compounds the problem," said Simon Roberts, chief commercial officer, Antonov plc. "Legislators too are placing more demands on vehicle electrical power, with the requirement for reduced carbon emissions meaning smaller petrol and diesel engines and more stop-start systems. Even daytime headlight legislation is increasing the electrical load, further increasing the need for more efficient charging systems." Such trends are leading to ever larger alternators with consequent problems both for their efficiency and the need to package them into the limited space available in an engine bay. "The two-speed solution proposed by Antonov enables an alternator to be driven faster at low engine speeds for increased charging, particularly when the engine is idling. At higher engine speeds the alternator can be run slower. The alternator operates more often in an optimal speed range, thereby improving efficiency."

Matthew 'Beechy' Beecham

See also: Global market review of starter motors and alternators - forecasts to 2016

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