It is not just users of powerful sports cars that want a distinctive aural engine signature; Ford has listened to what buyers of regular and environmentally responsible compact family sedans, hatchbacks, and wagons want to hear—and it's a special sound, although not a continuous snarl; so it is introducing a system to provide it.
The company's gasoline 1.6-L EcoBoost engine, available in the Focus, has been given what the company describes as an impressively sporty growl under hard acceleration.
For this level of the market, the engine has comprehensive technology including high-pressure direct injection with up to 200 bar (2.9 ksi) and droplet size typically less than 0.02 mm (0.0008 in); a low-inertia turbo, spinning at more than 200,000 rpm under full load; and twin independent variable cam timing.
It also has a sound symposer. Essentially, this is a sound box normally used for high-performance cars that “harvests” what specialist engineer Ralf Heinrichs describes as “good” engine-generated frequencies from the intake systems and transmits them to the passenger compartment. “Typically, the exhaust system determines engine sound at low revs, whereas higher up the rev range the intake system gives an engine its unique voice,” he said.
However, with turbocharged engines, the turbo, as an integral part of the intake system, tends to mask the sound of induction. “That is because its ‘whoosh’ noise becomes dominant when the turbo is spinning at speed,” stated Heinrichs.
He and his team have been refining a symposer solution to negating the turbo’s sound while enhancing that of the intake: “We needed to recapture or recreate that exciting induction noise, so what we have done with our sound symposer is to capture engine oscillations in the intake system downstream from the turbo and then direct them straight into the cabin for a good sporty growl.”
Measuring 60 x 50 x 40 mm (2.4 x 2.0 x 1.6 in), the symposer comprises four internal chambers, two of which are on the engine side and two on the outlet side. Heinrichs explained that the chambers are separated by a movable flap. Under hard acceleration, the pulsations generated in the intake meet the pivoted flap, producing sound waves that are then transferred to the outlet chambers.
In effect, the symposer’s role is to rebuild the engine note by revising the signal-to-noise ratio when oscillation is anywhere between 250 and 450 Hz. If necessary and depending on engine application, the frequency range can be modified and the sensitivity of the feedback adjusted.
A sound pipe is used to take the resultant “good” pulsations through the front bulkhead into the car’s cabin.
Originally, Ford developed the basic system for use on high-performance versions of the Focus, but with a change in end-user expectations, it revisited the technology. Engineers developed a dedicated system for less exotic models, ensuring that under most conditions engine noise is kept quiet with the distinctive sound signature only manifesting itself on wide throttle openings.
“Our philosophy has been that the car should be refined at low/medium acceleration but sporty/powerful sounding at high acceleration," Heinrichs said. "Even at motorway speeds the engine is never intrusive.”
The regular 1.6-L EcoBoost engine forms the basis of a highly tuned version for the Ford Fiesta RS WRC, competing in the 2011 World Rally Championship.