Why Diesel Truck Exhaust Systems are More Complex than Small-Car Exhausts

A vehicle’s exhaust system plays the critical role of removing harmful gases from the engine and minimizing noise. Newer cars have advanced exhaust units that convert the engine gases into less harmful compounds before being released to the environment.

While exhaust systems perform the same function, they differ depending on the car type. For instance, you will find that diesel exhaust systems are more complex than those in small cars.

There are several differences between the two exhaust systems, and an in-depth look at the diesel unit will reveal its add-ons, which make it distinct. 

Difference between a Gasoline Catalytic Converter and a Diesel DOC

In both cases, the catalytic converters are found on the exhaust pipe and turn toxic compounds in the exhaust gases into less harmful ones. Gasoline and diesel engines work differently, and they emit different compounds.

Gasoline engines burn a nearly stoichiometric air-fuel mixture. This produces nitrogen oxides, carbon monoxide, and unburned hydrocarbons as by-products.

Gasoline converters use a simple three-way catalyst, handling the three pollutants at once. The converters use precious metals, mainly rhodium, palladium, and platinum. Rhodium is a key player in the reduction reactions, which convert nitrogen oxides to nitrogen and oxygen.

On the other hand, diesel engines run lean and emit oxides of nitrogen and soot. Both are hard to handle with a single converter. The diesel oxidation catalysts (DOC) are built to handle hydrocarbons and carbon monoxide, converting them to water, while separate units handle soot and nitrogen oxides.

DEF delete kits are used to tweak diesel catalytic systems for various purposes, especially in off-road or motorsport vehicles. 

What Does DPF Do?

The diesel particulate filter (DPF) works like a filter in diesel engines to capture soot. They come in various variants, mostly depending on the material. Cordierite wall flow filters are among the most common and are made from cordierite. Others are silicon carbide wall flow, ceramic fiber, metal fiber flow-through, paper, and partial filters.

DPF delete kits, such as the 2017 F350 delete kit, are designed to bypass the factory-installed DPF. Typically, they come with high-flow exhaust pipes to reduce backpressure and soot accumulation, block-off plates, and tuning software. 

 The SCR and DEF

Most modern diesel engines come with selective catalytic reduction (SCR) to deal with nitrogen oxides. The SCR works by injecting diesel exhaust fluid (DEF), a urea solution, into the exhaust system. The fluid converts nitrogen oxides into nitrogen and water vapor.

The SCR and DEF require various supporting components for their functionality. They include a DEF tank, a heater, a pump, an injector, an SCR catalyst, and nitrogen oxide sensors. All these parts add to the complexity of diesel exhaust systems.

 Why isn't EGR Considered a Standard Exhaust System?

The exhaust gas recirculation (EGR) unit is found on both diesel and gasoline engines. However, it is bigger and sturdier in diesel engines due to the workload. This system reduces the emission of nitrogen oxides by recirculating the emitted exhaust gas back to the engine to cool it.

While plugged into the exhaust system, it is considered separate, as its primary role is to cool the engine. Additionally, most of it lies in the intake part of the engine instead of the outflow section, as with most exhaust components.

Bottom Line

Diesel and gasoline exhaust systems differ in many ways, with the former being more complicated. This piece shows the many add-ons that diesel exhaust systems have, especially in emission controls. The components play numerous roles, due to diesel engines having more complicated by-products that need specialized catalysts to meet set environmental standards.