People living in built-up areas will be able to sleep undisturbed at night, thanks to a ‘silent’ vehicle developed by DAF. Just by pushing a button, delivery drivers will be able to switch their trucks into ‘whisper mode’ when entering a highly populated place.
The vehicle’s software controls limit the maximum engine speed to 1,100rpm, and use delayed fuel injection to reduce the noise output. This changes the diesel engine’s injection timing, delaying the opening of the injector’s electromagnetic valve, increasing the temperature and pressure of air in the cylinder, and thereby reducing the noise made from the combustion process, said Marenus Klaase, senior engineer at DAF.
The ‘silent’ truck, still at concept stage, has been developed in response to new legislation in the Netherlands. This states that noise from delivery vehicles must not exceed 65dBA after 9pm, or 60dBA after 11pm until 7am the following day.
‘In the Netherlands large shops are in the centre of towns, and can be close to living quarters, so there have been protests against the noise from deliveries in the early morning and late evening,’ he said.
To further reduce noise from the engine in whisper mode, it is almost fully enclosed in an absorbing material, such as pressed cotton fibres, said Klaase. An extra silencer has been fitted into the exhaust system, through which the gas is directed during whisper mode, while part of the air supply to the engine compartment behind the grille is acoustically closed off by a shutter.
This increases the risk of the engine overheating, but is only used at low speeds in towns, when an emergency device can open the shutter if the temperature is too high.
The use of the accelerator pedal when changing gear manually is also too loud to meet the regulations, so the prototype vehicle has been fitted with an automatic gearbox. A conventional gearbox with automated control could be used in future. DAF is now investigating demand for the truck, before deciding whether to take it into full production. The technology would not be suitable for cars, as encapsulating the engine adds weight, while delayed injection increases fuel consumption, and the system is not effective at high speeds, due to road noise. But the technology could be applied to buses.