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Emissions controls are complex and expensive to maintain.

It’s not so long ago that we were all in fear of the imminent arrival of exhaust gas recirculation (EGR) on 4WD diesels. The latest systems have DPFs and SCR as well.

All new diesels have high-pressure, common-rail fuel injection, turbocharging, intercooling and exhaust after-treatment that uses a combination of several emissions control devices.

The ‘bogey’ emissions from diesel engines include oxides of nitrogen (NOx) that cause acid rain and particulates (Pm) that are linked to cancer. Carbon dioxide is also emitted and it’s one of the major greenhouse gases (GHGs).

It’s possible to design an engine that produces low levels of nitrogen oxides (NOx), but higher particulate quantities (Pm), or a low-Pm engine, with higher NOx levels.

Unfortunately, it’s not possible with current or foreseeable internal combustion engine technology to have an engine produce low levels of both NOx and Pm in the combustion chamber.

All modern diesel engines rely on exhaust after-treatment to reduce harmful tailpipe emissions and they are fitted with some or all of the equipment described below.


Exhaust gas recirculation (EGR)

NOx is produced when nitrogen
in the air mixture in the cylinder combines with oxygen at high combustion temperatures. The obvious way to reduce NOx is to lower the peak combustion temperature.

Combustion temperature is a function of the amount and timing of fuel injection and the volume of oxygen in the combustion chamber to burn that fuel.

By routing some of the exhaust gas back into the combustion chamber during certain engine operating conditions this incombustible gas displaces some of the oxygen that would otherwise flow in.

Exhaust gas recirculation (EGR) has been with us in petrol and diesel engines for many years.

Recirculation is most commonly done by a pipe that runs between the exhaust manifold and the inlet manifold, with flow control by a valve in the circuit.

This type of EGR is called external EGR and most systems incorporate a cooler, to drop the temperature of the burnt gas. That cooling work is done by a heat exchanger between the manifolds and adds to the cooling system’s load.

Internal EGR relies on variable valve timing to retain some exhaust gas inside the cylinder by not fully expelling it during the exhaust stroke and is much less common than external EGR.

Unlike the lower-compression, spark-ignited engine the higher-compression, injection-ignition diesel always has more combustion air than it needs. Much of the excess air can be replaced by exhaust gas without affecting combustion stability and so diesels can use EGR rates as high as 50 percent at idle.
Once under load, the EGR rate is much less and in full-load conditions there may be no EGR flow at all.

Since diesel engines don’t have a throttling butterfly, EGR does not lower throttling losses in the way that it does for petrol engines, but it lowers peak combustion temperature and heat rejection in the combustion chamber.

For EGR to work there needs to be more pressure in the exhaust system than there is in the inlet manifold. To provide that pressure advantage throughout the entire engine speed range most diesel engine makers have adopted variable geometry turbochargers (VGTs) or variable nozzle turbochargers (VNTs) that provide constant, higher boost pressures from idle to peak revs.


Selective catalytic reduction (SCR)

Selective catalytic reduction (SCR) uses urea (AdBlue) injection in a catalytic converter to eliminate NOx from the exhaust gas. Urea is a nitrogenous crystalline compound that is found in mammal urine.

As weird as it sounds urea has been used for the past 30 years as a means of ‘scrubbing’ exhaust gases in many stationary engine applications and is now being used in vehicle applications.

The SCR efficiency improvements over an EGR-only engine come from timing and injection rate changes that make an SCR engine behave like a pre-emission engine. The engine actually produces higher emissions at the exhaust valves than its predecessors, but the gases are scrubbed in the SCR units.

To ‘scrub’ the NOx that is being emitted, urea from an on-board tank is injected into the exhaust pipe, just upstream of a catalytic converter where the NOx is converted into water and nitrogen.

The chemistry looks like this: the urea (NH2)2CO is converted to ammonia (NH3) in the exhaust stream, then, in the catalytic converter:

4NO + 4NH3 + 3O2 + (catalyst) = 4N2 + 6H2O


Diesel particulate filter (DPF)

Modern emissions-compliant diesels are high-maintenance machines, with compromises that older diesels just don’t have. The most discussed after-treatment item is the diesel particulate filter (DPF).

Various types of ‘honeycomb’ filtration media are used to trap particulate material and convert it to harmless ash.

It’s no secret that diesel particulate filters are troublesome. Even behind a well-tuned diesel engine a DPF needs regular ‘regeneration’ and will eventually plug with ash residue, requiring a thorough out-of-vehicle clean or replacement.

In the USA, where DPFs have been used for the past decade, exhaust after-treatment maintenance cost is second only to tyres in the overall R&M task.

DPF design is aimed at ensuring that combustion-produced, carcinogenic particulate matter is trapped, rather than being emitted from the vehicle’s tailpipe. That’s obviously great intent, but DPF operation in the real world leaves a lot to be desired.

Soot that builds up in the filter medium needs to be burnt off in the DPF, but unfortunately, diesel exhaust gas temperature (EGT) is often too cool for that process to take place without additional heat input.

EGT is often increased in a diesel oxidation catalytic converter (DOC) before entering the DPF, so it becomes hot enough to purge the particulate matter from the filter material in what is known as a ‘passive’ regeneration.

Unfortunately, that doesn’t always work, because diesel exhaust gas temperature (EGT) varies, depending on the vehicle’s duty cycle, engine output and gearing. The common low-EGT villains are low speed operation and excessive idling.

When the DPF pressure sensor indicates that the DPF is starting to block up, the engine electronic control unit (ECU) programs an ‘active’ regeneration. That process involves increasing EGT, by post-injection in the engine combustion chambers that adds a fuel burst to the departing exhaust gases; by using a dedicated injector in the exhaust plumbing, or, more rarely, a small diesel burner in the DOC or DPF.

All three regeneration systems increase fuel consumption.

Post-injection runs the risk of engine oil dilution, caused by unburnt fuel passing by the piston rings and a dedicated exhaust-system injector or burner can block with soot.

Oil dilution can cause serious problems, particularly in the case of extended oil drain engines. For that reason some dipsticks have a third level, indicating excess fuel level in the sump and the need for an oil change, regardless of mileage or fuel burn.

In some cases, active regeneration doesn’t work well enough to burn soot out of the DPF material, so a ‘manual’ regeneration is signalled to the driver, who instigates it via a dashboard button.

This process requires the vehicle to be parked and let run through a computer-controlled burn process that can take up to 45 minutes.

Cutting short a regeneration cycle or ignoring a DPF regeneration warning light will eventually lead to a plugged DPF, at which point the engine may derate and go into limp-home mode. (If there’s a fault in the DPF pressure sensor this plugging may not register and turbo failure from excessive back pressure
is possible.)

Other life-shortening factors are myriad, because the DPF is at the end of the ‘food chain’ and, consequently, what is swallowed upstream of the DPF finishes up in its bowels: an unpleasant analogy, but a clear one!

Any over-supply of fuel causes heaps of engine soot to be dumped in the DPF and it blocks rapidly.

Other engine complexities that can cause DPF failure include: dirty fuel that gets past the fuel filters; engine oil diluted by fuel from post-injection; leaking injectors; air leaks in intercooler and manifold plumbing; exhaust gas recirculation (EGR) valve blockage; minor coolant leaks inside the engine; incorrect engine oil; sensor malfunction anywhere in the exhaust system (there are several sensors); positive crankcase ventilation (PCV) oiling up the inlet manifold and oil leaks in the turbo housing.



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