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towards cleaner diesels

The legislative efforts on both sides of the border to control tailpipe emissions on heavy duty vehicles is something all maintenance managers have had to become familiar with over the past few years. Given the impact of such legislation,...

The legislative efforts on both sides of the border to control tailpipe emissions on heavy duty vehicles is something all maintenance managers have had to become familiar with over the past few years. Given the impact of such legislation, however, on the performance, longevity and price of truck engines, it’s important that all C-level executives have at least a basic understanding of what is involved. Included below is a primer on the move towards cleaner diesels.

Beginning with the 2007 model year, diesel engines were legislated to reduce NOx emissions by 60% and particulate matter by 90% in order to meet Environment Canada’s emissions standards. NOx emissions are created by high combustion chamber temperatures and are controlled by using Exhaust Gas Recirculation (EGR) as well as Selective Catalytic Reduction (SCR), which involves injecting urea into an oxidation catalyst.

A diesel engine’s exhaust particulate matter is composed of unburnt hydrocarbons, organic fractions from the engine oil, sulfates from sulfur in the fuel as well as trace elements of many chemicals. The particulate matter is known to result in public health issues along with causing environmental damage. Diesel Particulate Filters or DPF are designed to physically trap, store and then burn-off or oxidize the particulate matter in a safe and effective manner. The by-product of the burn-off process, known as DPF regeneration, is ash. This is the non combustible residue left from the engine oil.

Vehicles equipped with DPF exhaust systems require ultra low sulfur fuel (ULSD) and low phosphate engine (CJ-4) oils. Using other grades of engine oil or fuel with higher sulfur and phosphate content will rapidly foul the DPF-causing a loss of engine performance and reduced DPF life.

The DPF is installed after the catalytic converter and contains a substrate of porous material that all exhaust gases must pass through. There are different types of DPFs but the most common is the double walled flow through design which uses a cordierite core. The core is similar to a full flow honeycomb catalytic converter but has the channel blocked at the rear of the passage which forces the exhaust gases to flow through the channel walls.

When the exhaust gas passes through the walls the particulate matter is trapped and remains there until it is burned off during the regeneration process. When the regeneration process is complete a small amount of ash remains. Over time ash will build up and it too will require removal. This can only be done by removing and either replacing or cleaning the DPF.

The Engine Control Unit (ECU) uses a differential pressure sensor to monitor the pressure difference between the DPF inlet and outlet. Most systems use a single sensor system with two pressure lines (inlet and outlet). The sensor will provide a voltage signal to the ECU that will correspond to the pressure differential between the inlet and outlet of the DPF. The ECU uses this information to calculate the amount of soot loading in the DPF and determine when regeneration is required. If the DPF becomes excessively loaded with soot it will cause excessive back pressure which may lead to engine and DPF damage.

The regeneration process is designed to restore the DPF’s ability to allow the exhaust gasses to flow through it while maintaining engine performance. The process involves elevating the DPF temperature above 600° C (1112° F). There are different methods used to increase the exhaust temperature but most diesel road and highway vehicles use an active type of regeneration.

The different types of regeneration are:

Passive – The regeneration takes place while the vehicle is driving where the engine load is used to elevate the exhaust temperatures to high enough levels to burn the soot. This method requires no driver input or engine management intervention.

Active – The process involves using engine management software and takes place with the vehicle running at idle. Active regeneration is used to burn-off large amounts of soot when the DPF pressure sensor informs the engine management system that the DPF has a high particulate loading condition. The active system also requires no driver input.

Passive/Active – This type of regeneration is a combination of the active and passive methods. Due to varying engine loads and inconsistent exhaust temperatures, the passive regeneration alone will not be capable of burning off all of the soot deposits. Active regeneration is required to control the exhaust temperatures to complete the process.

Manual – manual regeneration involves using factory compatible software and a diagnostic tool.

The two processes used to increase the exhaust temperature are dosing and after treatment device. Dosing involves increasing the exhaust temperature by introducing fuel into the exhaust where it will react with the oxidation catalytic converter before entering the DPF. The exhaust gases are fuel enriched by injecting fuel during the exhaust stroke or by injecting fuel directly into the exhaust.

An after treatment device or ADR is the use of an exhaust component that is used to create heat for the regeneration process. The ADR is a self-contained device, controlled by the ECU that meters fuel with an injector and has its own supply of air for combustion. The ADR systems are very complex but allow exhaust regeneration to take place under any driving conditions.

DPFs are designed to last approximately 880,000 km. or 550,000 miles before the DPF has to be replaced.

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