DIESEL ENGINE EMISSIONS

Diesel fuel is injected under pressure into the engine cylinder, where it mixes with air and combustion occurs. Compared to gasoline-powered engines, the lean nature of the diesel-air mixture results in cooler combustion with smaller volumes of carbon monoxide (CO) and hydrocarbons (HC) being produced. Their good fuel efficiency also results in lower carbon dioxide emissions. However, diesel emissions do contain relatively high levels of nitrogen oxides (NOx) and Particulate Matter (PM).

A brief overview of the gaseous emissions is included in Issue 6, but what is particulate matter?

Particulate Matter (PM) PM is a complex mixture of dry carbon particles (soot), ash from engine oil and a soluble oil fraction, SOF (hydrocarbon condensed on carbon particles), as well as sulphates (from combustion of the sulphur in the fuel) adsorbed on the soot particles. Being very small (with diameters measured in microns or fractions of a micron one micron being one thousandth of a millimetre) diesel particulates penetrate deep into lungs when inhaled and have significant health impacts. Particulates from diesel engine exhaust are known to increase the risk of heart and respiratory disease, and have been labelled as a "probable human carcinogen" by several regulatory agencies.

NOx and particulates are traded against each other in many aspects of diesel engine design. Very high temperatures in the combustion chamber help reduce the emission of soot but produce higher levels of NOx. Lowering the peak temperatures in the combustion chamber reduces the amount of NOx produced but increases the likelihood of soot formation. The way forward is a combination of close control of the combustion process, better mixing of the air and fuel coupled with good after-treatment. Here are some of the main developments used to reduce diesel emissions.

Fuel Injection Systems

In the past, fuel systems were mechanical, and used injection pressures of 200-300 bar, with one fuel injection per power stroke. The resulting plume of fuel in the combustion chamber had a wide temperature range, due to poor mixing with the air. The combustion in the rich region of the flame produced soot, and the lean regions produced NOx. To overcome this, systems today operate at pressures up to 1500 bar and have up to 8 holes per injector. The temperature profile across the plumes is far more limited; this reduces emissions and offers better air utilisation within the cylinder.

Mechanical pumps are still used in modern systems to generate the pressures, but the injection timing is now computer-controlled, and delivers very precise amounts of fuel. This has enabled the development of homogeneous charge compression ignition (HCCI) engines, which operate with up to six injections per power stroke (pulse injection). This combustion technology further reduces both NOx and smoke.

Engine Design and Lubrication

As the combustion process becomes cleaner, the emissions caused by the lubricating oil become more significant. Diesel fuel is a very efficient solvent for lubricating oil, and careful design is needed to prevent the fuel from dissolving the oil on the cylinder walls. This is particularly relevant to the small diesel engines that are becoming more popular with advances in diesel technology. This has led to changes in the shape of the combustion bowl within the piston and in the fuel injector configuration.

Turbochargers

A turbocharger increases the charge pressure, that is, the pressure of air in the cylinder before compression begins. Increasing the charge pressure allows the engine to operate on a leaner mixture resulting in lower particulate emissions.

Exhaust Gas Recirculation (EGR)

EGR recycles some of the exhaust gases into the intake air. This reduces the peak combustion temperature and hence the formation of NOx.

Diesel Emission Control Devices.

After-treatment of diesel emissions can be divided into two categories, particulate filters and catalytic converters.

Diesel particulate filters effectively remove particulate matter (PM) from diesel engine exhaust . Oil consumption is a big issue for particulate trap manufacturers, as the additives in oil produce particles of ash, which can collect in the particulate trap. These filters have to be regenerated periodically by burning off the trapped soot particles, various systems being employed to achieve this.

Catalytic converters minimise toxic gases and odour. The diesel oxidation catalyst (DOC) is effective for the control of carbon monoxide (CO), hydrocarbons (HC), odour causing compounds, and the soluble organic fraction (SOF) of particulate matter. Unlike the 3-way catalysts commonly used on petrol vehicles, however, these simple oxidation catalysts have no effect on NOx emissions.

After-treatment devices are not presently used to any significant extent on diesel vehicles in RSA. However, the reduction of diesel sulphur content to 500 ppm from January 2006 will enable the application of sufficient after-treatment to meet the Euro-2 emission standards which are to be introduced. More stringent standards such as Euro-4 and 5 require more advanced after-treatment technology to further reduce PM and NOx. Combinations of catalytic reduction systems for NOx conversion with oxidation catalysts and more sophisticated particulate traps are needed. These more advanced technologies, which are still undergoing continuing intensive development, are typically much more sensitive to fuel sulphur content and require less than 50 ppm sulphur for satisfactory operation. It is to enable the future introduction of vehicles equipped with these advanced systems that a 50 ppm sulphur diesel will be introduced in RSA next year, although initially only as a "niche" grade alongside the standard 500 ppm grade.

Conclusion

The diesel engine is the most efficient power plant among all known types of internal combustion engines. Heavy trucks, urban buses, and industrial equipment are powered almost exclusively by diesel engines all over the world. Diesel powered cars are becoming increasingly popular. The diesel engine is a major candidate to become the power plant of the future. Before that happens, however, further progress in diesel emission control is needed as increasingly tighter environmental regulations worldwide call for advanced emission controls and near-zero diesel emission levels in the years to come.