Silence is Golden at Renault
The French put traffic noise at the top of their list of types of noise pollution, ahead of other neighbourhood noises. So it’s no surprise that Renault has been very serious about the issue of vehicle noise and had made reducing it one of the focal points of its brand image.
In 1997, the company took the decision to go further than the European Union (EU) legal requirements in keeping down noise, a policy that led to the Laguna II and Vel Satis becoming the only European vehicles to have received 71 dB(A) approval (all engine sizes combined). Current EU regulations set the limit at 74 dB(A) and the 3 dB(A) difference is significant because it actually represents a reduction of 50% in noise pollution – although it seems to only be 4.1% less in dB(A) terms.
The story of how Renault’s Acoustics Department reaching the objective is a story worth writing about. The department was involved in the two models right from the start and allocated a ‘quota’ for each of the main sources of external noise (engine, exhaust system, intake area and tyres) in the total noise level produced by the cars. From this starting point, the engineers set out to meet the individual targets while trying to obtain the bets possible balance between cost and benefits. Simulation tools were used to experiment with various approaches to meet the targets.
As engineers, they already understood certain facts. Like the quietest engine is one that is stiff and whose walls, which act like the diaphragms of loudspeakers, vibrate only moderately. “The engine vibrates under the effect of the forces of combustion and forces of inertia of the rotating assembly,” explains Serge Lacaze, head of Renault’s Acoustics Department. “The resonance of the crankshaft adds to its vibration spectrum, as do the various repetitive noises coming from the engine – piston/cylinder, crankshaft/bearing, etc.”
In the case of the Vel Satis, the Powertrain Department dealt first with the primary excitation sources to cut down engine noise, optimized pre-injection for diesel engines, reduced play of the pistons, valves and crankshaft bearings, as well as profiling the gear teeth. Through all the measures taken, the noise level was lowered by more than 40 dB(A).
The exhaust system, on the other hand, can be worked on later in the development process. The pipes themselves do not make noise but the acoustics specialist works on volumes (silencers can be noisy!) and outlet noise (as the exhaust gases leave the pipes).
“It is possible to deal with exhaust noise by changing the design of the external surfaces. We can also insert reinforcements to make the internal surfaces stiffer, change the route taken by the gases or add materials such as mineral wool to deaden air flows inside the silencer. All our efforts have already cut exhaust noise by 3 dB(A),” says Lacaze.
Tyres make a relatively minor contribution to keeping noise levels within the norms. “We sign contracts with tyremakers which put a ceiling on the amount of noise their tyres can make. Then we check that the tyres supplied actually meet those specifications,” reveals Lacaze.
Tyres emit noise by acoustic radiation. Contact with the road surface causes the tyre to vibrate, starting in the tread and moving out to the sidewalls. The phenomenon of alternative compression and decompression of air in the tread is a second source of noise. Thus the tread has an essential part in keeping down noise levels. As manufacturers obviously cannot make smooth tyres because that would be incompatible with roadholding, they are now incorporating better acoustics into the design process.
Noises produced at low speeds and during idling are not included in legal norms but the level involved is relatively low anyway. However, they are fairly important in determining the perceived quality of a vehicle because this is what people hear more often. This is especially so with diesel engines which are well known for their clatter.
“The specifications for diesel models in the Renault range include the quality of sound when the engine is idling. We try as far as possible to get rid of the typical diesel sound,” said Lacaze.
Clattering noises, which are extremely sensitive to piston/cylinder play, can also be dealt with at a later stage, when the fuel injection is being calibrated. This forms part of programming the software that controls the injection process, and means continually striking a balance between two requirements that can sometimes be contradictory: cutting noise and controlling pollution.
Contrary to what many motorists may think, the engineers do not stop giving attention to cars they have worked on once those cars are in production and running on the streets around the world. According to Lacaze, there are monthly surveys which measure customer satisfaction regarding the quality of soundproofing in their vehicles. All Renault models are covered and every version is included.
“If we deviate from our targets, then appropriate action is taken to ensure that any developments made during the life cycle of the vehicle are in the interests of customers. In this way, we have observed that customer reaction to the improvements brought to the second phase of the Clio 2 has been positive, for both the petrol and diesel versions,” says Lacaze.
In addition, cars are often picked at random as they leave the production line by various authorities responsible for monitoring standards. They conduct checks to see that the vehicles comply with regulations.
“Interestingly, our cars are usually about 1 dB(A) quieter at this stage then when they are officially homologated. That process [of homologation] usually takes place 8 to 12 months before the car goes on sale and before development work is completed,” Lacaze says.
