2011 Kia Optima SX Turbo: Cold Start Jingle
September 20, 2011
I've noticed that our longterm 2011 Kia Optima Turbo occasionally makes a muted but distinct rattle upon cold start. After a short time, ten seconds or so, it goes away.
I'm quite confident this is the same characteristic for which Hyundai issued a TSB for the Sonata 2.0T (the Optima Turbo shares its powertrain and most of its genes with this car). Click the jump for the gory details of what's going on here.
It's all related to lighting off the catalyst. In this engine the turbo's wastegate is actuated electronically, which allows for some unconventional tricks that would be impossible with a conventional pressure-based pneumatic actuator.
One big trick is this -- during a cold start the wastegate is commanded to an open position to allow hot exhaust gas from the exhaust manifold to bypass the turbo's turbine stage. The turbine housing and wheel are chunks of metal that, when cold, suck heat out of the exhaust gas. By opening the wastegate, the hot gas stays hot, resulting in more rapid catalyst lightoff, which reduces emissions (cold starts constitute the vast majority of a car's emissions profile).
When the turbocharger is cold, so is the wastegate. There's a bit of clearance incorporated among the the wastegate's valve and swingarm to allow for thermal expansion. Enough so that during a cold start, exhaust pulses from the turbo patter against the valve that's hanging in the breeze, and the result is the jingly-rattle noise we hear.
--Jason Kavanagh, Engineering Editor