Tier Technologies

The next stage of emissions legislation is looming and its imminent arrival is driving considerable change in the diesel engine industry. The challenge of successfully meeting the new regulations is much greater than just the simple addition of new technology to the engine. It is demanding investment in new manufacturing processes and techniques. The new, more complex technology demands a solid approach to product support with, for instance, service technicians needing new skills to diagnose, repair and maintain equipment.  

It is also driving a new way of working with customers. Engine manufacturers are now working in greater detail and much sooner with customers than has previously been the case. By taking this longer-term cooperative view, diesel engine companies like Perkins see a real opportunity to deliver machines of higher quality, durability, fuel economy and productivity than currently exist.

When we talk about “Tier 4” we are actually talking about two stages of emissions legislation. The first, known as “Tier 4 Interim” in North America and  “Stage IIIB” in Europe, demands a moderate reduction in the levels of NOx (oxides of nitrogen) of between 15 and 50 percent, depending on rating. The reduction in the levels of particulate matter (PM) of up to 95 percent is a significant move. It is intended to drive the use of exhaust after-treatment to achieve this. This will take effect in January 2011 for engines greater than 130 kW and one year later for engines from 56 to 130 kW. Three years on, in 2014 and 2015, Tier 4 Final/Stage IV will demand a further 80 to 90 percent reduction in NOx, which will also drive additional new technology.

However, there is a raft of other legislative requirements apart from attaining the required emissions levels. These are intended to ensure that compliance is not only adhered to at the production stage, but also in the field under real operating conditions. These areas include not-to-exceed requirements, expanded ambient conditions, transient testing, in-use testing and crankcase blow-by control. The current emissions legislation specifies a steady state test at eight different points on the engine operating curve, using a fully warm engine. This is a reasonably simple test to implement, and it is relatively easy to predict how an engine will perform and what emissions it will produce.

But at Tier 4 Interim/Stage IIIB, there is the introduction of the “non-road transient cycle.” This is a standardized test test for all engine manufacturers, designed to represent the operating cycle of a typical off-highway machine. It includes a period of running when the engine is cold, but also uses many rapid changes of engine speed and load. This test requires a more sophisticated engine test facility, and engine performance is considerably more challenging to predict.

Measuring very low levels of particulate matter also provides an additional technical challenge. The traditional method of weighing a filter paper to determine PM emissions during a test becomes more difficult when the particulate matter that you are measuring could feasibly be small relative to condensation or to the weight of a smudge from an operator’s fingerprint.

The emissions must be met not only at normal ambient temperatures (20 to 30 C) and altitudes (sea level), but also over a wider range of operating conditions such as higher and lower ambient temperatures, high altitudes and over almost the whole operating range of the engine.

There will be in the future a requirement for in-use testing of machines, to ensure that they meet emissions after they have been sold. This is likely to involve a measurement program requested by the emissions legislators that is the responsibility of the engine manufacturer. This will take a sample of a particular machine model, on which a portable emissions measurement system (PEMS) is installed that will measure the emissions output over a short period.

In addition, gases and oil mist from the crank case ventilation must also be considered as part of the emissions output of the engine and must be controlled. In some cases, the crank case gases may be filtered, and in other cases, it may need to be routed back to the air intake (closed circuit breather).

In many cases, the engine will not be shipped to the OEM customer already connected to the after-treatment in its final configuration. The engine and after-treatment may be shipped as two separate packages and it may even arrive from different manufacturing locations. The customer may need to install the two components at two different points in their manufacturing process.

The emissions of the engine are certified by the engine manufacturer, but the system will not meet these standards until it has been assembled by the customer. Obviously, this will require a level of close collaboration between the engine manufacturer and the OEM to ensure that the correct after-treatment has been fitted to the correct engine and to ensure that it has been assembled correctly.

The emissions’ picture is developing fast. As new countries announce their plans for air quality improvement, the map continuously changes with the general trend being that emissions regulations set in Europe, North America and Japan are subsequently adopted by other major economies. By 2016, there will be up to five different levels of emissions globally, and these will be certified by many more different national and regional authorities.

Tier 4 Technology

There are a number of technologies available for the management of NOx. Previous emissions’ tiers were managed successfully with in-cylinder technologies, changing the dynamics of the combustion process to achieve a cleaner burn, but this alone is insufficient to meet the levels of NOx reduction required for Tier 4 Interim/Stage IIIB.

After considerable research, Perkins reached the conclusion that there were two viable technology paths: One is to use exhaust gases to cool the combustion process and the second is to use a system called selective catalytic reduction (SCR). NOx is formed at very high temperatures within the engine’s cylinder, so a small proportion of exhaust gas can be cooled and introduced into the cylinder. As exhaust gas is high in water vapor (a byproduct of the combustion process) this will act to cool the combustion and reduce NOx formation. It is relatively cost-effective, can be packaged tightly onto the engine and is expected to give a best specific fuel consumption improvement of around 5 percent over current Tier 3 products of around 3 percent.

In the past, Perkins was disinclined to use such technologies as soot and even sulphuric acid could collect on intake components and affect engine durability. Today, lower sulphur fuels and low levels of particulate output means this technology is robust and appropriate for the industrial engine market.

SCR is a technology that is starting to be seen on-highway, especially in Europe. The principle difference is that a second fluid is required in addition to the diesel fuel. Typically, a second tank is fitted on the machine that the operator needs to fill regularly, perhaps every third or fourth time that the main fuel tank is refilled with diesel. This fluid, known as Adblue in Europe or Diesel Emissions Fluid in North America, contains a chemical called urea.

The urea is injected in small quantities (typically four to five percent of diesel fuel) into the exhaust system, where it is mixed with the exhaust gases. The hydrolysis catalyst converts the urea into ammonia, which reacts with the NOx in the SCR catalyst to produce nitrogen, water and carbon dioxide. A final stage of catalyst (oxidation catalyst) is required to clean up any ammonia left in the output gases, as it is potentially harmful and has an unpleasant odor.

SCR does enable a savings in diesel fuel consumption of approximately 3 percent over other technologies. Although the total fluid consumption is actually extremely close. Whether the operator saves money through SCR will depend on the relative cost of urea and diesel fuel in a particular country. In the United Kingdom, for example, Adblue is currently a little more expensive than the red diesel used by many operators of off-highway equipment, but cheaper than on-highway fuel. The production of urea is energy intensive so is likely to closely track diesel fuel prices in the future.

The Diesel Particulate Filter

For particulate reduction, Perkins has chosen a cordierite diesel particulate filter (DPF). This porous ceramic material is highly efficient at removing particulate matter — 90 percent as a minimum — and often much higher.  The exhaust gases actually flow through the porous walls of the material, depositing the particulate and leaving the exhaust gases clean.

Diesel Oxidation Catalyst

The diesel particulate filter on its own cannot remove all the legislated gases. Hydrocarbons, carbon monoxide and the “soluble organic fraction” must also be managed. So the DPF is used in combination with another device called a diesel oxidation catalyst (DOC). The DOC is a similar cordierite material but uses a through-flow principle. The gases pass straight through the device rather than through the walls. The DPF and DOC are combined together in the same canister in the machine.

As the diesel particulate cleans the particulate matter from the exhaust gases, soot will start to accumulate in the filter. This needs to be cleaned out through a process called regeneration. The rate at which the filter fills depends on how clean the engine is and it is certainly desirable to make the particulate output of the engine as low as possible. The operating cycle of the machine also has some effect on the rate of soot accumulation. There are basically two forms of regeneration that can clean the soot from filters — a continual process called low temperature regeneration or an occasional process called high temperature regeneration.

Low Temperature Regeneration

For low temperature regeneration, a catalyst of precious metal helps create NO2 in the exhaust gas which oxidises carbon at temperatures of around 250 C or higher. One limitation of this technology is that it requires some NOx to make the process work; usually a minimum ratio of 25:1 NOx to particulates is required, with 40:1 being desirable. Even though the particulate output of the engine is very low, it is only really viable in engines up to 130 kW where a little more NOx is allowed by the legislation.

High Temperature Regeneration

High temperature regeneration is an occasional process that is used to burn off the accumulated soot after a few hours of operation. There are several different methods, but the one preferred by Perkins, for engines over 130 kW, is to use a burner in the exhaust stream, which heats the exhaust gases to over 600 C, directly oxidising the accumulated soot in a well controlled manner. This system is robust and highly controllable compared to some competitive systems, as it is possible to start and stop the regeneration at any time. The process is most fuel efficient at lower speeds, and once again from the measurement of machine cycles it is clear that there will be plenty of opportunities in almost all machine types for this to occur without operator intervention.

Ash

Although almost all the particulate matter in the filter will oxidise completely in regeneration, there are tiny quantities of minerals (e.g. phosphates) in engine lube oil that do not combust. These result, over many hours of operation, in the accumulation of ash in the channels of the diesel particulate filter. These eventually reduce the volume of the filter and increase the backpressure, resulting in deterioration in fuel economy and performance.

Ash does not regenerate so it needs to be cleaned out using a special machine. In North America the Environmental Protection Agency (EPA) specifies that the first ash service should not be sooner than 3,000 hours for engines less than 130 kW, and 4,500 hours for engines above 130 kW. In Europe, although it is not specified by the legislation, Perkins feels that ash service is an inconvenience to the machine owner and should be avoided as far as possible. By a small increase in filter size at the lower power end, Perkins has managed to avoid ash service altogether for some of its engine range.

Sulphur

Sulphur in fuel is a problem not only for Perkins but the whole industry. Sulphur reacts with precious metal catalysts and prevents them from working correctly. The legislators recognise this and new Ultra Low Sulphur Diesel standards are being introduced for off-highway fuel. Typically this will mean sulphur levels of less than 15 parts per million, similar to those of modern on-highway fuels.

But there are issues with this change in fuel. For example, will storage tanks really be completely empty before new ultra low sulphur diesel is added? With this in mind Perkins is making it an imperative that its technologies are designed to be robust in the face of occasional mis-fueling.

Ben Reed is a technical manager with Perkins and Julia Flatters is a senior certification engineer with Perkins Global Emissions Regulation and Conformance, both based Peterborough, England.