Particle emission is always the biggest headache of diesel engines. While diesel engines emit considerably less pollutant CO and NOx as well as green house gas CO2, the only shortcoming is excessive level of particles. These particles are mainly composed of carbon and hydrocarbons. They lead to dark smoke and smog which is very crucial to air quality of urban area, if not to the ecology system of our planet.
Since the '80s particle emissions from cars have been reduced by three-quarters, thanks to the Government’s legislation as well as the advances in diesel technology such as direct injection. However, diesel engines still emit more particles than petrol, and it seems that it is the nature of diesel. Therefore the largest diesel car maker, PSA, developed a particle filter for use in next year’s (2000) HDi common-rail diesel family.
Basically, PSA’s particle filter (PF) is a porous silicon carbide unit, comprising passageways which has a propensisity to easily trap and retain particles from the exhaust gas flow. Before the filter surface is fully occupied, these carbon / hydrocarbon particles should be burnt up, becoming CO2 and water and leave the filter accompanying the exhaust gas flow. We call this process as regeneration.
Normally regeneration takes place at 550° C. However, the main problem is: this temperature is not obtainable under normal conditions. PSA tells us normally the temperature varies between 150° and 200°C when the car is driving in town, as the exhaust gas is not in full flow.
Luckily, the new common-rail injection technology helps solving this problem. Gifted by its high-pressure, precise injection during a very short period, the common-rail system can introduce a "post-combustion" by injecting small amount of fuel during expansion phase. This increases the exhaust flow temperature to around 350°C.
Then, a specially designed oxidising catalyst converter locating near the entrance of the particle filter unit will combust the remaining unburnt fuel coming from the "post-combustion". This raise the temperature further to 450° C.
The last 100°C required is fulfilled by adding an addictive called Eolys to the fuel.
Eolys lowers the operating temperature of particle burning to 450° C, now regeneration occurs. The liquid-state additive is store in a small tank and added to the fuel by a pump. The PF unit needs to be cleaned at dealerships every 80,000 km by high-pressure water, to get rid of the deposits resulting from the additive.
One more thing to be solved is the influence of "post-combustion". It increases engine torque when the driver doesn’t expect. Therefore the engine management system has to regulate the torque by adjusting the amount of normal fuel injection, pre-injection etc. and turbocharger’s boost pressure to compensate.