Diesel engine emissions - All you need to know!

James Whittonin Technical

The "new" low emission diesel engines have been around for a few years and, I think it's fair to say that, end users and maybe even some dealers still don't fully understand this technology and how it affects them in their day to day work. In the second article of his series, James Whitton attempts to demystify this subject.

I am not surprised by this, since this technology is super technical especially when it comes to the complex chemistry involved with diesel engine combustion. In this article, I will attempt to demystify and simplify this subject and concentrate only on what we "need to know" to get along with this new technology in our jobs. Hopefully, the information will help to prevent some costly engine failures because these engines do need different maintenance to "conventional" diesel engines.

I am a firm believer in knowing the background to a subject in order to understand it more fully. So firstly, let's look at why this technology has been introduced. Unlike many new systems and technologies in our industry, "low emission diesel engines" have not been developed by the manufacturers to give them a commercial advantage in the market. It's not a marketing ploy! This new technology is driven purely by legislation. The legislation itself is driven by recognition that diesel engines do in fact emit some real nasty compounds in their exhaust gas, even though they generally emit far less carbon dioxide (CO2) than the equivalent capacity petrol engine. Government environmental bodies have therefore introduced very stringent emission limits which aim to dramatically reduce the "nasty" compounds emitted by diesel engines into the atmosphere. This legislation has been in force for diesel road vehicles for over 10 years, so now it's being rolled out to include "non-road" equipment. For engine manufacturers redesigning their engines to meet the lower emissions, it must have initially appeared almost impossible to achieve.

However, radical changes to engine design, electronic control and advanced exhaust after treatment systems now sees diesel exhaust emissions, in some countries, cleaner than the surrounding air quality. Equipment manufacturers have also had their work cut out to design their machines around these new engines. Machine and engine electronics have to "talk" to each other, and the packaging of the power unit in the machine chassis with its bulky exhaust after treatment unit has been a considerable challenge. The significant additional cost of the new type engine has also created issues for machine manufacturers, adding around 30% to the cost of the power plant compared to conventional engines, a cost that has to be passed on to the end user. The cost challenge has also triggered development of machine transmission and hydraulic systems in an effort to increase their efficiency. This would allow them to be powered by conventional engines which fall below the power threshold (37kw 50hp) affected by the new emission regulations. This however is only a short term solution since the regulations in the UK and Europe will soon include engines below 37kw (50hp) as well. In the USA, they already do.

Governments have given out mixed messages over the last decade regarding diesel engine powered road vehicles, initially seeming to support a swing to diesel power based on their lower CO2 emissions compared to petrol. Global car manufacturers, on the back of this, spend millions developing their diesel engines. Now, having convinced the car buying public that diesel engines are cleaner than petrol, and supporting this by offering low road and company car tax for diesel cars, it seems that now the diesel engine has been demonised by various governments. They have no doubt realised that exhaust emissions are not just about CO2 when it comes to health and the environment. So just to be clear, this article is focused on diesel engines between 37kw and 56kw (50-75hp) fitted to non-road mobile machinery such as mowers, that have been manufactured after 1st January 2013.

The Legislation

The diesel engine emissions legislation has to be a good thing, since it aims to protect human health and the environment, which is something that we all should welcome.

Most legislation regarding engine emissions (petrol and diesel) originated during the 1950s in the USA and, in
particular, in California in a response to massive problems with air pollution and harmful smog caused by the explosion of motor car use.

European legislation has always tended to follow. In America, the governing body is the EPA (Environment Protection Agency) or CARB (California Air Research Board) and in Europe, this legislation is enforced by the European Commission. It will come as no surprise then that we have ended up with two standards, although their content up to now is substantially the same.

In the USA, the diesel engine emission levels are described as Tiers; Tier 1, 2, 3, 4, etc. To complicate matters slightly Tier 4 is sub divided into Tier 4 interim and Tier 4 final. Tier 4 final is the current emission level in force for machines that we are concerned with.

In Europe, the emissions levels are described as Euro Stages; Euro Stage I, II, III etc. Here again, Euro stage III is split into stage IIIA and stage IIIB. Stage IIIB is the current emission level in force. So, in your machine specifications you will see the engines emission level described as a particular Tier or stage.

Over the past 10-15 years, the enforcing bodies have gradually reduced the acceptable emission levels to give manufacturers time to develop their engines. The levels are also phased in by the engine power output and, currently in the UK and Europe, the new limits only affect engine outputs of over 37Kw (50hp). This process is ongoing and still lower emissions are already on their way that will encompass engines below 50hp as well.

The current levels of Euro stage IIIB and Tier 4 final probably posed the greatest challenge to engine manufacturers and resulted in the most radical changes to the engines.

Now we understand why the changes to diesel engines had to be made, let's look at the exhaust emissions themselves and then the changes made to the engines to reduce these emissions. At this point, it will be useful to understand the many acronyms associated with this subject, given in the list on the right.

The two main exhaust emission compounds we are most concerned with reducing are:

  • NOx Nitrogen oxide
  • DPM Diesel Particulate Matter (soot)

Other acronyms used in the engine emission reduction systems:

  • EGR Exhaust Gas Recirculation
  • DPF Diesel Particulate Filter, (DPF is a registered trademark)
  • Re-Gen Regeneration or self- cleaning process for the DPF
  • ECM Engine control module (black box)!
  • CR Common rail injection system (controlled by the ECM)
  • DOC Diesel oxidisation catalyst

Firstly, let's look at NOx. NOx is produced when nitrogen in the air charge burnt in the combustion process is turned into nitrogen oxide. Now this is the stuff that makes your eyes water and is considered harmful. So here's the thing. The higher the combustion temperature, the higher the level of NOx in the exhaust gas. The engine designers' strategy to lower the combustion temperature in most cases is to use Exhaust Gas Recirculation (EGR). The theory being that by introducing controlled volumes of exhaust gas back into the inlet tract of the engine, the combustion temperature will be lowered by reducing the mass of oxygen in the fuel / air mix. In most engines, this recirculated gas is also cooled by the EGR cooler unit to increase its effect.

So, job done then for lowering NOx emissions? Well unfortunately not. This is where the problem gets complicated because as combustion temperature is lowered to reduce NOx, more DPM (soot) is produced as combustion becomes less efficient. The soot being incompletely burnt fuel. So this is where electronic diesel engine control was adopted as a way of constantly adapting the engine systems to optimise both NOx and DPM emissions. The modern electronic common rail diesel was born and, although the common rail fuel injection system is only one of the systems utilised to reduce emissions, common rail has become the generic name for these new breed of diesel engines. For you "engine techies" out there, "Common rail" diesel injection is not new. It has been in use for over 50 years and adopted most famously by Cummins engines where the injectors were "fired" mechanically. The very latest common rail engines are fully electronic with solenoid or piezo fired injectors.

Next, we will look at the other main "nasty" in the exhaust, "Diesel Particulate Matter" (DPM), or soot to you and I. DPM is worrying since the particles are extremely small and, being airborne, can easily be inhaled. Reducing DPM in isolation is fairly straightforward; just make the combustion process more efficient and hotter. In these latest engines, one of the ways this is accomplished is by increasing the fuel injection pressure. Let's say most conventional engines use around 2000 psi (138 bar) injection pressure. Most common rail injection engines are using around 29,000psi (2000 bar). Crazy pressure! The theory here is that, as injection pressure is increased, the fuel is atomised into smaller particles which are able to mix more readily with the charged air and so burn more completely. Pressure alone though is not enough. To enable the emissions to be controlled for all engine load conditions (which is impossible to do with conventional engines), the injection process had to come under computer control. In simple terms, a variety of sensors on the engine report back to the ECM (engine control module) the various engine parameters such as speed, air intake temperature, fuel temperature, coolant temperature, intake air pressure etc., etc. The ECM then adapts the injection timing, injected quantity and is also capable of initiating multiple injections to optimise power and keep emissions within limits. So, super high pressure electronic fuel injection eliminates DPM (soot), job done? Well no, remember what I said earlier about reducing NOx.

For optimum control of both NOx and DPM, the engine is constantly running a compromise controlled by the ECM, and since the legislation calls for DPM control to be a minimum of 99.1% efficient, engine design required some additional system to ensure a very high percentage of soot is trapped before exiting the exhaust pipe. This is where the exhaust after treatment system comes into play. This system generally comprises three components arranged in a single unit, these being the Diesel Oxidisation Catalyst (DOC), the Diesel Particulate Filter (DPF) and the muffler.

This device is largely as described. It traps soot particles in a filter located in the exhaust system. Now, this DPF is a science in its own right so I don't intend to cover the chemistry involved here. However, because the DPF is constantly trapping soot particles, it will come as no surprise that without some form of intervention it would eventually start to restrict the exhaust flow and affect the performance of the engine. The DPF therefore has an automatic system which monitors the soot levels in the filter and, when required, will initiate a self-cleaning process. The Diesel Oxidisation Catalyst's (DOC) main job is to generate enough heat to promote the DPF self- cleaning. This is accomplished by the ECM adding "post combustion" fuel injection pulses into the engine, effectively providing raw fuel down the exhaust to assist the DOC raise the exhaust gas temperature to over 500C. More of this later in the tips and warnings section. What I will also say is that, if the engine is constantly working hard enough and the exhaust gas temperature is therefore high enough, the DPF will be in a state of "passive" regen, oxidising the soot particles and keeping itself clean. This is an important fact when considering the specifications of a new machine. Choosing a machine with excessive power may not be a good plan in this case because the engine may then be running at low load and therefore the exhaust temperature may be too low to allow the DPF to "passive" regenerate and keep the filter clear of soot. The result would then be that the DPF would go through many more "active regen" cycles which would shorten the service life of the filter.

From my experience, the DPF unit is likely to be the device that causes the most problems for end users if the manufacturer's maintenance and operation procedure is not followed. Thankfully, all these complicated electronics and magic black boxes need not cause too much worry because these very systems have been in diesel road vehicles for over 10 years, are very well developed and very reliable.

Tips, warnings and advice: Now we know a little bit about how these new engines work and why, it's time to take a look at some tips, warnings and advice on operating and maintaining them.

1. Fuel

These engines MUST run on good quality Ultra-low sulphur diesel fuel. A typical engine manual states the following: Ultra-Low Sulphur to meet ASTM D975 S15 or EN590:2009 or equivalent fuel with a Sulphur content of less than 15ppm. Bio content does not exceed 5% as described in ASTM D6751 or EN 14214.
You should contact your fuel supplier to ensure compliance. Poor quality fuel will lead to problems down the line with the DPF and fuel injection system.

Make sure your fuel storage tank is clean. Diesel engines don't like contaminated fuel and especially water. The fuel injection equipment is made to very fine tolerances (0.5microns) which is about 100 times smaller than the width of a human hair! NEVER wash these engines when they are running because heat shock could destroy the injector pump and I for one would not like to foot the bill for a replacement. It's not good practice anyway to wash any engine when running and I have seen a number of failures as a direct result. It goes without saying that this type of failure is not covered under warranty!

2. Engine oil

Now getting this right is VITAL. Again, a typical engine manual states the following for the oil specification:

API CJ-4 Spec engine oil (specifically for DPF engines). Running these engines on your normal fleet engine oil (unless it also meets the spec) is going to cost you money, especially if you don't adhere to the oil change interval. Why? Well, there are a number of issues. Firstly, this oil spec is "low ash" and therefore helps the DPF stay cleaner. Secondly, engines with EGR systems are known to increase the soot in the engine oil over time. This of course would be exaggerated if the change interval was extended. Excessive soot from EGR can thicken the oil and lead to small oil supply passageways becoming restricted, resulting in a possible catastrophic engine failure. You have been warned! The correct oil is ABSOLUTELY ESSENTIAL.

3. Other maintenance operations

The DPF unit has a limited service life before it needs to be replaced. Even though it has a self-cleaning system, eventually the ash which remains after the "regen" process builds up in the filter until only cleaning by a specialist (off the machine) or replacement is the solution. The service life of the DPF unit will depend on a number of factors including the work profile of the machine and how well it has been maintained. A typical replacement interval would be 3000 hours. Some engine suppliers run a service exchange programme to limit the cost to the end user of replacing the DPF unit. However, the rebate charge is only applicable if the customer's unit can be successfully recovered. Any internal damage to the filter would mean a brand new unit would be required. The new cost
of replacement DPF units can be near £1000 with a rebate on the old unit of £500, so this needs to be factored into the maintenance budget for machines equipped with these engines.

I know what some users are already thinking. When the DPF needs replacing, we will just by pass the unit and fit a suitable cheap aftermarket muffler in its place. Well good luck with that! Because the DPF unit is an integrated part of the engine management system and I doubt that it would run at all with the DPF removed. Also, because you would be defeating the emissions system I guess you could be breaking the law, even though there are currently no M.O.T tests for non-road
diesel engines.

Onboard regeneration of the DPF

The diesel particulate filter (DPF) is equipped with sensors that detect when the soot level inside reaches a certain point. The ECM will then initiate the on-board cleaning process known as regeneration (Regen). Without going into too much detail, the system basically raises the temperature of the exhaust gas passing through the Diesel Particulate filter which burns off the soot and leaves behind a very small amount of ash. During this Regen process, the exhaust gas temperature can reach 600 C. There is a warning light on the machine instrument panel to warn the operator that the system is in Regen mode. At this point, the machine can still be used normally without problems. When the process is complete, the machine returns to normal mode. This is known as "Active Regen."

Warning! This is where most problems could occur. When the engine has entered Regen mode, it must NOT be stopped until the process has completed. So, if the machine is for example low on fuel, there is an over-ride switch on the instrument panel to postpone the regen sequence. This would allow the machine to be refuelled. If however the operator over-rides the regen request too often, the machine will request a "Parked Regen," which involves parking the machine in a safe area to avoid fire risks and allowing the machine to run through the regen sequence. Make sure there is sufficient fuel in the machine before the start. Don't forget that the exhaust gas will now be around 600oC, so be very careful where the machine is parked for this operation.

If this request is also ignored, the machine will eventually go into "Limited Operation Strategy" (LOS). The "check engine" warning light on the instrument panel will be ON and the engine power reduced by the ECM.

Recovery from this situation will require the dealer to perform a "service regen" using a special computer service tool. Be warned this is NOT a warranty issue, and it will be a chargeable procedure.


Finally, some advice regarding general maintenance on these engines which will render them beyond the average DIY mechanic. Don't forget the fuel injection pressure on the common rail engine is very high, 25,000 - 30,000 psi and pressure is held in the fuel rail after the engine has stopped. Even if you are tempted to replace a fuel injector, maybe due to an external leak, each injector has a unique correction code allocated in the Engine Control Module (ECM). Fitting a new injector would require the dealer computer service tool to change the correction code in the engine ECM. Some common rail injectors are triggered by voltages up to 110v, so there is a new risk of electric shock to consider as well.

There is also good news though; the dealer diagnostic tool is really excellent, I have used one and found it to be extremely useful when diagnosing faults. It saves a lot of technician time and customers' money.

My advice then would be to leave maintenance on these engines to the dealer. This will save money in the long run and keep you safe. Also make sure you have a stamped service history for your machine. This is super important when selling machines fitted with these engines, and on the flip side, beware if you are considering a used machine with a common rail engine fitted. Ask for the service history or you could be in for a world of pain!

Overall, this new engine technology is "just different" and not to be feared, as long as maintenance is carried out to the manufacturer's instructions and the correct oil and fuel is used, but don't park the machine next to a haystack when a regen is in progress unless you have the local fire brigade on speed dial!!

Look out for the next article in Jim's series in the February/March issue of Pitchcare.