Diesel Particulate Filters

What is a DPF (Diesel Particulate Filter)?

A DPF is used in diesel vehicles that comply with Euro-6 emission standards. The manufacturers use the DPF to filter, store and burn the soot particles that are emitted as a result of the combustion process of the diesel engine. DPF’s come in a common cylindrical unit. The DPF consists of silicon carbide. It can filter about 99% of solid particulate matter from the exhaust of a diesel engine. The soot particles or the carbon particles deposit on the filter channels are oxidised into carbon dioxide at exhaust temperatures above 600degC. Basic DPF’s are the single use type. You need to dispose or replace them when they get full, after accumulating ash and when regeneration is no longer possible.

 

What are Particulates?

Particulates are a form of carbon that accumulates in the exhaust system of the vehicle’s engine, originating from various leftovers that define what type they are. In term of vehicles, particulates are the minute solid particles of exhaust gases emitted from the engine. The engine emits these particulates mainly in the form of carbon or soot. The particulate matter forms a layer of carbon inside the exhaust system of the engine. This is the main reason why taking care of the exhaust system of the engine is so important. It is necessary to limit the quantity of emitted carbon particulates to prevent environmental pollution.

 

How does a DPF work?

The unfiltered exhaust flows through the DPF’S channels that open at the inlet end. The core contains porous walls of a ceramic honeycomb structure made of silicon carbide. The exhaust gases then enter into the channels that are open at the outlet end. The exhaust system takes away the exhaust gases. The DPF core retains the soot particles and later burns them off during the regeneration process. The ECU computes the amount of soot and ash accumulation in the DPF with the help of the DPF differential pressure sensor.

 

Components of a DPF

The temperature sensor upstream of the DPF detects the temperature of the exhaust before it enters the DPF. The integral resistor changes its electrical resistance according to the exhaust temperature and then sends a corresponding voltage signal to the ECU control unit. The control unit uses the voltage signal to monitor the rise in exhaust temperature before and during the regeneration process. The DPF differential pressure sensor detects the difference between the exhaust upstream and downstream of the DPF. The exhaust pressure sampling pipes – upstream and downstream of the DPF – detect the exhaust pressure difference. The pressure difference between the exhaust pressures upstream and downstream of the DPF acts on the piezo electric pressure sensor element. This produces a voltage which is passed into the ECU as a voltage signal.

 

What is Regeneration?

Regeneration is the process that burns off the soot particles accumulated in the DPF as C02. The regeneration takes place when the ECU detects a certain pressure difference in the DPF, then at a certain speed usually above 80kph the DPF temperatures will raise to about 600 degrees Celsius, causing the soot build up to burn away. This will continue until the pressures in the DPF fall back into specification. That is why it is recommended that a vehicle with a DPF be driven on a freeway at least 20 minutes every week to initiate a regeneration (please consult vehicle owner’s handbook).

Variable Nozzle Turbine (VNT) diesel turbo

Problems with Variable Nozzle Turbine (VNT) Turbos

Since the late 1990s many diesel vehicles have a Variable Geometry Turbine (VGT) or Variable Nozzle Turbine (VNT) turbo fitted. These turbos minimise turbo lag, improve throttle response at low speed and provide much improved torque.

Most commonly, the design utilises a ring of moveable vanes around the turbine wheel to change the speed and direction of the exhaust gases acting on the turbine wheel. At low speeds the vanes move closer together which accelerates the gas flow onto the turbine wheel. At higher speeds the vanes open wider to prevent the turbo over boosting.

 

Despite these benefits, turbos can be prone to problems, which include: