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DEVELOPMENT

Mercedes-Benz CLA

Industry-leading aerodynamics and downsized engines give this compact coupe good fuel efficiency, and its exterior design should attract new buyers to the brand By James Scoltock

F or mainstream OEMs, the compact class is not only very competitive but also lucrative, Vehicles such as the Volkswagen Golf have sold in

huge numbers for decades. Premium brands have been reluctant to compete in the class, instead producing larger luxury sedans with higher margins.

But consumers' buying habits are shifting as they look to downsize, making the segment more attractive to the premium manufacturers. The challenge then for engineers is to develop vehicles that offer high levels of fuel efficiency, ride and handling, comfort and safety - but in smaller packages.

Daimler has invested heavily to increase its presence in the market, developing an architecture that will eventually underpin five C-segment vehicles. The A-Class hatchback and B-Class MPV have already benefited from the technology, and they will be joined by the GLA SUV as well as one further model, likely to be an estate.

Daimler has also seen a space in the market for a small four-door coupe - the CLA, The hope is that the new car will capture sales not only in Europe but also further afield. The company has been successful with larger four-door coupes such as the CLS,

"The idea was to take the CLS concept and transfer it to a lower segment," says Jörg Prigl, vice-president of programme management for compact cars, "Most cars are hatchbacks. But by bringing a sedan- type car to the market it might also attract interest from China, where people prefer this type of vehicle,"

Development of the CLA was made easier by the use of modules and systems common to other cars. The transfer of knowledge acquired on other vehicle programmes, such as the CLS, also helped,

"We started defining what the market expected from a compact car, and we already clearly knew what it wanted from a midsize car," says Prigl, "Values for areas such as stiffness, NVH, driving comfort and other parameters had already been defined and transferred to the modules and components,"

Engines and transmissions were developed in parallel with the new architecture, which made packaging simpler. And although the CLA is longer

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Got it covered: Almost the entire underfloor area is shielded, helping todelivertheO.E2Cd

than the A-Class - 4,630mm compared v\iith 4,292mm - the wheelbase is identical at 2,699mm,

"Commonality between these cars is about 75%," says Prigl. "You need to be smart with how you integrate five cars onto one platform,"

Although high levels of commonality made certain areas of development simpler, others, such as weight reduction, were more challenging because of cost sensitivity - margins are particularly tight in the C-segment.

So aluminium couldn't be widely adopted. Instead, high-strength and ultra-high-strength steel were used,

"In Formula One they have the resources to go with carbon fibre and aluminium but in my area of engineering I am stuck with steel," he says, "Customers won't pay for weight saving."

Engineers from Daimler and its suppliers were asked to take the design of the original concept and come up with smart solutions to minimise weight without breaking the budget,

Prigl says: "Functional integration, minimisation, all the stuff that forces people to think twice - do I really need that? With suppliers it's the only way in the compact segment to cut weight."

Close attention to aerodynamics helped to keep down CÔ emissions from the CLA. With the A-Class, engineers achieved a drag co-efficient of 0,26, a significant achievement for a hatchback. But the CLA manages an industry benchmark for a volume vehicle of just 0,22.

"The benefit is tremendous - the equivalent would be 35kg in weight savings," says Prigl.

The CLA has an advantage over the A-Class insofar as the longer body helps the airflow to remain attached to the vehicle's surfaces for longer. The starting point for engineers was creating a small frontal area - 0.49m2 - and improving airflow over the front and rear.

The A-pillar's geometry has been designed to lower its shoulder, while the exterior mirrors have also been optimised.

Front and rear wheel-arch spoilers with serrated edges have been introduced to reduce turbulence. Extensive use of underbody panelling, with special attention paid to the rear axle, has also aided airflow.

Prigl says: "We started with a higher Cd target but during the design phase we soon recognised that achieving 0,24 was quite easy. Everybody became hungrier, and said how can we integrate the underbody covering, giving us another 0,02,"

Prigl is rightly proud of the achievement - especially as even some of his own engineers thought it might not be possible. He says that his peers from other OEMs ask him how the team managed such a low Cd value,

Prigl's simplistic answer is that it required engineering and design to work together, "It only works if the styling guys, the aerodynamics guys and the component guys are willing to work together to achieve a result. If, for example, styling fights the aerodynamics you end up making compromises," he says,

AU this work means that carbon emissions are only 109g/km on the NEDC test cycle in the CLA's most efficient form - using a 2,2-litre 125kW/350Nm diesel engine mated to a seven-speed dual-clutch transmission. And 4,2 litres of fuel are consumed to travel 100km,

The most efficient gasoline engine is a

1,6-litre, four-cylinder 90kW/200Nm unit, which gives figures of 118g/km and 5 litres/lOOkm,

The improvements to airflow also reduce the noise level in the cabin, making it a calmer place to drive,

Volkswagen's XLl plug-in hybrid manages an even lower aerodynamic drag factor of just 0,19, but was developed with achieving the best fuel efficiency in mind, rather than sales,

Prigl suggests that reaching a similar level in volume vehicles would require too many compromises that consumers would be unwilling to accept,

"Of course you can achieve 0,20 but we wouldn't go in that direction for a premium car because it would start to impact on the styling," he says, "And for cars like this styling is a priority,"

Aesthetics may be important but consumers also demand practicality in a vehicle, Prigl is fully aware that, no matter what the impact on efficiency, all-wheel drive is essential for some drivers - especially in parts of North America and Europe - even if it adds 60kg and increases emissions by lOg/km,

"In Formula One they have the resources to go with carbon fíbre and aluminium but in my area I am stuck with steel"

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DEVELOPMENT

"Of course there's a weight penalty that can't be entirely compensated for and there's an impact on the fuel side, but in Switzerland or Austria customers need all-wheel drive for everyday driving," he says.

But the impact of an all-wheel drive system on efficiency can be minimised. The CLA uses an electrohydraulic system that can send 50% of the torque to the rear wheels in more challenging driving conditions.

In normal driving scenarios, torque is only sent to the front axle.

Daimler says the system is 25% lighter than those used in competitor vehicles.

Getting a grip: The modular architecture was designed from the start to deliver the all-wheel drive version of the CLA as well as the CLA SUV

This is partly achieved by integrating the power take-off unit into the dual-clutch transmission and using its oil circuit to lubricate the system.

At €29,000, the CLA is half the price of the larger CLS but consumers will still expect the same levels of connectivity and convenience features, while active safety has become a prerequisite. The CLA shares technologies with the A- and B-Class, including the radar-based collision warning system, and the pre-safe system with reversible seatbelt pretensioning, which closes the windows and sunroof and moves the seats into a safer position in the event of a crash.

Smartphones can be integrated into the CLA's infotainment system, giving access to the internet and social media sites including Facebook and Twitter.

Daimler's investment in the compact class, and the number of such vehicles it is bringing to the market, should mean that it is better able to compete with the established manufacturers in the sector.

Linking attractive design with technologies generally found in much larger vehicles should give Daimler an important competitive edge.

And with the CLA it should attract consumers who want a different option from the typical hatchback. •

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