Diesel engines are amongst the largest emission sources in urban air. The emissions consist of soot particles of about 40-80 nm in diameter. The engines emit also nanoparticles at less than 20 nm or so in diameter, consisting of various hydrocarbons and possibly also sulphur (in case of sulphur-containing fuel).
Why is it so attractive to switch to diesel instead of a less emitting gasoline engine? The answer is in fuel economy. In Finland, the national policy towards the diesel fleet has changed over the last couple of years. Today the price per liter is almost equal for both diesel and E10 gasoline. The owner of the diesel car needs to pay a monthly tax, and the savings must come from the more economical engine.
The emissions of diesel engines are rather well know and there is a continuous effort to limit the emissions even more. In 2013, the new EURO VI limits for heavy-duty vehicles will be 0.01 g/kWh for PM (particulate matter) and 0.4 g/kWh for NOx. If you compare these to the EURO I values just 20 years ago the change is dramatic: EURO I for PM was 0.36 g/kWh and for NOx 8 g/kWh. Unfortunately, the diesel engines tend to be strong and last forever, so we have to wait for a while until the fleet has majority of EURO VI level engines.
In order to cut down the emissions to EURO VI level, changes in engine technology alone will not be sufficient. This emphasises the importance of the after-treatment of the exhaust gases. Several different types of catalysts and filters exist. How well do these work? What is their efficiency? Answering these questions requires intensive research.
What makes the development towards lower emissions even more challenging is the need to consider also the effects of new type of fuels. It is possible to produce diesel fuel from biomass-based renewable energy sources, and research towards this goal is already going on. These new-generation fuels aim for a lower CO2 footprint.
I am acting as an opponent for a recent PhD thesis from Tampere Technical University that has focused on all of these aspects in a wide set of experiments. The overall message of the thesis is clear: with new technology, one can fight against particle and gaseous emissions successfully together with lower greenhouse gas emissions.
By Prof Kaarle Hämeri (aerosol physics, Division of Atmospheric Sciences)