Normal Performance Overview

Before talking about failures and changes in performance, a baseline for normal needs to be established.

Normal performance is expected from a truck or car on a daily basis. These expectations are set by manufacture expectations and how the vehicle drives on a daily basis.

Below will walk through how diesel engine combustion affects normal diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and selective catalytic reduction (SCR) operating during start-up, operation, and shutdown.

Powertrain Startup

When starting the engine from “cold iron” – meaning the engine has been off for several hours and was not simply shutdown recently at normal operating temperature (NOT). At the initial startup all exhaust components (DOC,DPF, and SCR) were heating up. Consult manufacture specifications to find the NOT for the vehicle in operation.

Thre are two important pieces to recognize at this stage

1. Exhaust system components will not work properly until they are NOT
2. Excessive idling will adversely affect the exhaust system

For this walk-through, consider the unit is at NOT when coolant is in the NOT and the vehicle is operating at power (not Idling).

Normal Driving/Operation

During normal operation, the largest challenge of the exhaust system is the oxidation of diesel soot produced by unburnt fuel. This is a challenge because soot particles are large and when not oxidized properly will quickly plug an exhaust system.

Soot is produced from unburnt fuel. Fuel is not burnt when excessive fuel is added to the engine or there is insufficient air to burn the fuel.

These conditions exist during normal operation and can best be seen in an older pre-DPF diesel rig by watching the “smoke” coming out of the stack during normal operation.

When first starting to move, fuel is added to the engine to move the vehicle. At low engine revolutions per minute (RPM) there is more fuel than air and soot can be seen coming out of the exhaust stack. As engine RPM increases the turbocharger begins to engage forcing additional air into the engine. At high RPM the turbo typically provides sufficient ait that allows the extra fuel being dumped in during acceleration to burn fully and the soot “plume” doing out the exhaust stack starts to thin and disappear. This is because at high RPM the turbo provides sufficient air to allow the fuel to be fully burnt in the engine, reducing soot that escapes the stack.

The cycle repeats when the operator shifts to the next high gear. Engine RPM drops reducing the air introduced by the turbo, creating more soot that escapes the stack until RPM and air get high enough to fully burn fuel in the engine.

At Highway speed on flat ground, fuel addition is minimal and engine RPM and air induction are high when within the turbo’s “power band.” At this point, there is little or no unburnt fuel visible out of the stack.

The balance between RPM, air induction, and fuel addition is the constant challenge that a diesel after-treatment system faces during normal operation.

Ash generation should not be a concern until later in a vehicle’s life. Ash generation in normal powertrain operation happens slowly over time. Ash accumulation, if left untreated, effectively lowers the capacity of the DPF and/or DOC. The primary impacts of ash accumulation include; higher back-pressure, reduction in filtration capacity, loss of fuel economy, loss of power.

Shutdown

The best time to shut down is after a run at highway speed. In this state, the majority of soot that has accumulated in the tilter will have oxidized (burnt) and the filter will be as clean as possible.

Shutdown after stop-and-go driving, particularly under heavy load and acceleration will leave higher levels of soot in the DPF that will cause higher back-pressure the next time the vehicle is started.

Coolant – a liquid or gas that is used to remove heat from something (like an engine)

Diesel Oxidation Catalyst (DOC) – promotes chemical oxidation of carbon monoxide (CO), and hydrocarbon (HC), as well as the organic fraction (OF) of diesel particulates. Is an exhaust after-treatment device that reduces emissions.

Diesel Particulate Filter (DPF) – is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine

Normal Operating Temperature (NOT) – Most experts agree that your engine should run between 195 degrees and 220 degrees.

Oxidation – a process in which a chemical substance changes because of the addition of oxygen. The loss of electrons during a reaction by a molecule, atom, or ion. Oxidation occurs when the oxidation state of a molecule, atom, or ion is increased.

Revolutions Per Minute (RPM) – measures how many times the engine’s crankshaft makes one full rotation per minute, and how many times each piston goes up and down in its cylinder.

Selective Catalytic Reduction (SCR) – an advanced active emissions control technology system that injects a liquid-reduction agent through a special catalyst into the exhaust stream of a diesel engine.