What Are Those Silver Cylinders On The Side Of A Semi-Truck For?

If you've ever studied the outside of a large big rig, you may have noticed some shiny components bolted below the side mirrors on both sides of the truck (along with a massive front grille). These silver canisters are air cleaners, and they offer a critical function to the most powerful semi-truck engines.

In order to get the most out of a diesel engine, it needs clean air. Given the sensitive nature of the compression process, some engineers estimate that a mere 8 ounces of dirt could be enough to wreck a diesel engine. Inside each of those silver air cleaner assemblies is a massive filter, designed to trap particulates and impurities from entering the engine. If a truck driver fails to properly maintain the rig's air cleaners, it can negatively affect several aspects of the engine, including the fuel timing, pressure, and NOx conversion.

The large silver air cleaners are considered a more classic or vintage look. Many newer semis don't have them, though in some cases you can add external air cleaners to big rigs that didn't come with them. Modern-styled big rigs only feature one air cleaner instead of two, but it's concealed under the hood and built into the engine compartment. 

This offers some benefits over the older configuration. For example, without external components creating additional drag, you get enhanced aerodynamics. In addition, maintenance is an easier process because it's less work to gain access to the filter. Simply lift the hood of the truck, locate the air cleaner housing, undo the clips, and carefully remove the cap. Looking at the most popular semi-truck brands in America, there's an absence of external components and a greater emphasis on a streamlined shape to reduce drag and improve efficiency.

External air cleaners require a bit more effort to access, but also require less frequent maintenance. For example, you may be required to remove bolts on the supports holding the mirrors just to gain access to the air cleaner assembly top. Other designs are held together with one giant bolt underneath the canister or feature several bolts along the top that you need to remove to get access to the filter.

However, two massive air filters mounted externally on the cowl don't require changing as often as one smaller unit inside. They can also move larger volumes of air, which is particularly handy when air quality is bad or a trucker's route takes them through a thick section of sand-swept desert or a dusty construction area. Speaking of dust, one of the hazards of carelessly removing an internally mounted air filter is accidently "dusting" the engine, which is essentially spreading particulate matter deeper inside. In addition to being sensitive to dirt, there may be other things you might not know about diesel engines that may surprise you.