Emissions reduction in diesel engines

The desire for future generations to breathe clean air, a commitment to regulatory compliance, plus the aspiration to be seen as a ‘green’ organisation, is driving a change in technology in the power generation industry. Diesel engines are becoming cleaner and greener.

Cummins Power Generation is at the forefront of emissions reduction in diesel engines and has invested heavily in developing new in-cylinder technology that reduces them at source, before being exhausted from the engine, thus enabling customers to comply with new directives. Richard Payne, manager European regulatory affairs, Cummins Power Generation comments:

“Unlike the automotive industry, which has been heavily regulated since the early 1990s, industrial power generation - in terms of stationary, non-stationary, mobile and standby generators - has come later. Political efforts have concentrated on areas, like automotive, where the greatest results can be quickly visible. The industry is pushing for consistent regulation. At Cummins, we know that any refinements in emissions result in overall improvements and efficiencies in engine design – and that’s good for end customers and owner/operators.”

Tackling the cause of the problem

The subject of emissions is about air quality. It makes sense that in countries and areas of the world that are heavily populated and industrialised it should matter.

“In terms of technology, the drive is to lessen the volume and harmfulness of emissions that are actually the fruits of combustion before they leave the exhaust.” Payne adds. “Treating emissions afterwards (referred to as after-treatment) is expensive and should only be used where the rewards are greatest: high load and high hours operation. For emergency standby use it makes sense to minimise there production in the first place: in the cylinder”.

Through in-cylinder design improvements and precision control of the combustion process, Cummins has developed technologies that reduce primary pollutants in the diesel generator set exhaust by around 80 per cent (since 1996 when USA emissions regulations for non-road diesel engines first went into effect).

Using a tier system to bring emissions under control

The USA and EU are leading the way in terms of setting standards and regulation. In the USA, the EPA has established a tier system (Tiers 1 – 4) for introducing regulations for non-road and stationary diesel engines. This establishes uniform federal standards for emissions for mobile and stationary generator sets. The EU has implemented a similar scheme comprising stages I-IIIA for mobile generators. In both schemes, each Tier (or stage) specifies lesser amounts of four specific pollutants: NOx (nitrogen oxide), HC (hydrocarbons), CO (carbon monoxide) and PM (particulate matter).

As of January 1^st, 2007, requirements for stationary generator applications (including standby generators) came into line with prevailing non-road regulations in the USA, taking account of the fact that it does not make sense to apply after-treatment to emergency standby generator sets. The target is for NOx and PM levels to have dropped by 98% below unregulated levels by 2014. Tier levels are applicable to the power rating of the engine but only apply to new engines.

Other countries too (Germany, France and China) have their own versions. In Germany, TA Luft sets the standard for NOx and PM reductions, while in France, Directive 2910 controls emissions of NOx, CO, SOx (sulphur dioxide), NMOC (non methane organic compounds) and PM.

Payne explains: “Development of low emissions technology is expensive and requires a big market to pay for it. The USA and EU are highly populated and interested in controlling air quality because it gives a better quality of life and saves on health care.”

Emissions reduction without compromising performance

“The real challenge, however, for us has been designing cleaner diesel engines to reduce NOx and PM emissions while maintaining performance.”

Historically, modifications in engine design to reduce NOx have tended to increase particulates, fuel consumption and reduce performance.

Cummins has developed a system for controlling combustion through new bowl geometry and improved fuel injection. Power ratings are maintained and improved, without increases in engine displacement or fuel consumption, which also decreases mechanical stress and thus reduces maintenance requirements and costs.

Fuel injection improvements equate to greater efficiency and less fuel being consumed when the engine is idling, starting up from cold or performing load changes. Cummins’ new technology uses high-pressure common rail fuel systems and provides fast response consistently.

The introduction of microprocessor-based (computerised) control has enabled generator set manufacturers to greatly improve fuel efficiency and power output while decreasing emissions. This is what maintains optimum combustion efficiencies; fuel quantity is more tightly controlled, as is injection timing and turbo-charger boost pressure.

The issue of after-treatment

In terms of the schedule for emissions reduction, the EPA’s Tier system, Tiers 1 and 2 are passed and Tier 3 will be completed throughout 2010. Advanced in-cylinder combustion technology provides emissions solutions for all three Tiers. Tier 4, which has entered an interim stage, is concerned primarily with exhaust after-treatment. Selective catalytic reduction (SCR) systems have been gaining ground here as well as after-treatment technology that uses advanced regenerative filters, but dealing with emissions at this stage comes at a high price.

States Payne: “After-treatment strategies add considerable costs and complications for end users. It makes no sense to spend money on them when the problem can be addressed much earlier on. Our philosophy is to discharge the problem in the first place.”

There are many applications out there that utilise diesel generator sets only for emergency, standby. The EPA defines emergency standby as an internal combustion engine whose operation is limited to emergency situations - and up to 100 hours per annum for testing and maintenance. It is understood in the US that it does not make sense to force after-treatment for this kind of operation. It is assumed engines in this category hardly ever run and so emissions, by definition, are not a major issue.

“There is another more obvious motivator to invest in low emissions technology.” Says Payne, “As we make improvements we enhance engineering in other ways too, such as smaller, lighter components and fuel efficiency, all of which are worthwhile. The challenge is to find the right technology to meet the customers’ needs - not just emissions targets.”

Pollutants

Nitrogen Oxides (NOx) – gases composed of Nitrogen and Oxygen that react together during combustion. These create smog in the atmosphere. The EPA estimates that diesel engines emit 25% of all NOx in the USA. NOx is controlled by reducing combustion temperature inside the cylinder.

Hydrocarbons (HC) and Carbon Monoxide (CO) – minor constituents of diesel exhaust, which react with NOx in sunlight to form ground-level ozone. They are also involved in the formation of smog and acid rain. Controlled by improving combustion efficiency.

Particulate matter (PM) is composed of soot particles in diesel exhaust from unburned carbon. The EPA estimates diesel engines emit 15% of all PM in the USA. This is controlled by implementing the above (optimising combustion temperature and efficiency).

Legislation in the rest of the world

China – legislation exists for all non-road engines up to 560kWm.

India – regulations divided into two categories: engines below 800kWm and those above. Engines smaller than 19kWm must comply with USA EPA, Tier 1. The next level of regulations in India will be based on EU stage II and are likely to come into effect in 2011.

Japan – Japan does not regulate emissions at extreme ends of the scale (very small or very large engines) but specifies permissible levels, similar to EU stage I and USA EPA, Tier 1.

Singapore – bases its regulations on EU stage I and USA EPA, Tier 1.

Central and South America – Mexico, Argentina and Brazil have no controls on non-road power generation engines, although many local authorities institute emissions regulations if warranted by air quality. Chile is adopting its own version of Germany’s TA Luft standard.

The emissions issue is an instigator for continuous improvement in power generation engines. However, the desire for clean air in an increasingly populated world is a very real problem that cannot be dealt with whilst still using the very equipment that first created it. It requires new solutions and new technology. It’s good to know that the industry is taking responsibility.