The science of rain-sensing wipers

Q: How do rain-sensing wipers work? I love mine — it’s like magic!

— Susan

A: In most of these systems, a rain sensor is located on the inside of the windshield behind the rear view mirror. Infrared light is projected outward through the windshield glass at a 45 degree angle. Depending on how much light reflects back to the sensor (rain, snow or dirt scatters the light, reducing reflect-back), the wipers are turned on at an appropriate speed and delay frequency. Vehicle speed is often another input for automatic wiper operation.

Q: I have a 2018 GMC Sierra 3500 HD with the 6.6 liter diesel engine and Allison transmission. The truck generally gets quite respectable mileage, getting about 22 mpg on the highway. From time to time, though, the fuel consumption abruptly increases from this to 14 mpg for about 20 minutes and then returns to normal. This is on level roads at the same steady speed and wind conditions. Fuel consumption will even remain at 52 mpg when coasting downhill where it normally drops to near zero. Why does this occur?

— Len

A: I believe what you are noticing is the truck is performing an active regeneration procedure, which involves the spraying of additional fuel into the exhaust at certain times to create significant heat so as to burn off accumulated soot (particulate matter) in the diesel particulate filter.

Duramax and other diesel engines built since 2011 employ a sophisticated and sometimes problematic exhaust emissions treatment system including (from front to rear on Duramax): hydrocarbon injector (HCI), diesel oxidation catalyst (DOC), diesel exhaust fluid (DEF) injector, selective catalyst reduction (SCR) catalyst, and diesel particulate filter (DPF). The parts we’ll focus on are the ninth injector (HCI) and the DPF.

Your diesel particulate filter traps soot and requires periodic cleaning (oxidation) on a regular basis. A combination of heat added due to fuel being injected (about 1,000-1,300 degrees F) and precious metals within the DPF create this reaction. If the truck is driven on the highway frequently, some soot cleaning occurs naturally (passive regeneration). Active regeneration occurs when the system either senses a level of differential pressure before and after the DPF indicating about 42 grams of soot has built up or 700 miles have accrued since the last active regeneration process. Raw fuel is injected into the exhaust via the HCI to raise exhaust temperature for a period of about 20 minutes, as you have noticed on your fuel consumption readout. Other clues active regeneration is occurring are a different exhaust sound, elevated idle speed, an odor, and more noticeable heat beneath the truck as it is stationary.

For informative articles: that help explain the process, go to:

— https://blog.duramaxtuner.com/blog/how-doesregen-work-in-an-lml-duramax

— Diesel Particulate Filter Breakdown and Operational Theory at www.knowyourparts.com/technical-resources/diesel-engines/diesel-particulate-filter