Original article by Aaron Brzozowski for GM Authority Oct 18, 2018: http://gmauthority.com/blog/2018/10/why-some-gm-exhaust-tips-become-blackened-with-use/
As governments around the world continue to ratchet up the pressure on automakers to produce ever more fuel-efficient automobiles, direct injection has gone mainstream. That’s where fuel is injected directly into the cylinder, rather than being sprayed in at the intake port or further upstream, allowing for more precise control over the fuel timing and mixture.
It helps the 6.2-liter V8 small-block LT1 engine achieve up to an EPA-estimated 25 miles per gallon (highway) in the 2019 Chevrolet Corvette.
But like any automotive technology, direct injection has its shortcomings, and some owners surely will have noticed that many modern GM exhaust tips become covered in filthy, black soot as time goes on. We’re used to associating that soot with a fuel-rich air/fuel mixture, as that’s typically been the culprit on cars fueled by carburetors and port injection; blackened, unburned hydrocarbons are scavenged from the engine and go on to coat the inside of the exhaust system as they’re pushed toward the exhaust outlet.
But why would new direct-injected engines, factory-calibrated to run as efficiently as practical, and using three-way catalysts to clean up the exhaust as much as possible, so quickly develop the same tell-tale blackened tailpipes?
According to Car and Driver, which spoke to U-M Mechanical Engineering Associate Research Scientist John Hoard, the answer is simply that direct-injection engines have the same tendency as diesel engines to produce particulate matter. This is especially true when the engine is running at low loads, as there’s often inadequate swirling of the intake charge to evenly distribute the fuel particles, resulting in small, rich pockets.
The low intake-charge velocity also quite often leads to gasoline pooling on the cylinder walls, Hoard told Car and Driver. Both phenomena can lead to the production of soot, despite the air/fuel ratio being just right for safe, efficient combustion.
Countless GM exhaust systems are at risk of being coated by charred, sooty filth as a result of direct injection. The list includes those used by the LT1 and its high-performance derivatives, the 6.2-liter supercharged V8 LT4 and LT5; GM’s EcoTec3 small-block light truck engines, including the 2019 Chevrolet Silverado 1500‘s new 5.3-liter V8 EcoTec3 L84 engine; and the new GM 3.6-liter V6 LGX engine used in the Cadillac ATS, Cadillac CTS, and Chevrolet Camaro from 2016.
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