Exhaust emissions of diesel engines operating under transient conditions with biodiesel fuel blends

Abstract

The transient operation of turbocharged diesel engines can prove quite demanding in terms of engine response, systems reliability and exhaust emissions. It is a daily encountered situation that drastically differentiates the engine operation from the respective steady-state conditions, requiring careful and detailed study and experimentation. On the other hand, depleting reserves and growing prices of crude oil, as well as gradually stricter emission regulations and greenhouse gas concerns have led to an ever-increasing effort to develop alternative fuel sources, with particular emphasis on biofuels that possess the added benefit of being renewable. In this regard, and particularly for the transport sector, biodiesel has emerged as a very promising solution.The target of the present work is to review the literature regarding the effects of diesel-biodiesel blends on the regulated exhaust emissions of diesel engines operating under transient conditions (acceleration, load increase, starting and transient cycles). The analysis focuses on all regulated pollutants, i.e. particulate matter, nitrogen oxides, carbon monoxide and unburned hydrocarbons; results are also presented for combustion noise and particle size concentration/distribution. The most important mechanisms of exhaust emissions during transients are analyzed based on the fundamental aspects of transient operation and on the impacts the physical and chemical properties of biodiesel have relative to conventional diesel oil. Biodiesel feedstock, transient cycle and fuel injection system effects are also discussed.For the majority of the reviewed transients, a decreasing trend in PM, HC and CO, and an increasing trend in NOx emissions is established when the biodiesel ratio in the fuel blend increases. Irrespective of driving cycle type, the NOx emission penalty and the PM benefit with biodiesel seem to increase for more aggressive cycles/driving patterns. Moreover, biodiesels produced from unsaturated feedstocks tend to increase the NOx emission liability, at least for older production engines; no such correlation has been established for the emitted PM, HC or CO. Since the research so far stems from engines optimized for diesel fuel, application of a revised engine calibration (e.g. EGR, injection system) can prove very useful in eliminating, at least in part, any inefficiencies caused by the use of biodiesel.Based on a large amount of published data over the last twenty years, best-fit correlations are deducted for quantification of biodiesel benefits or penalties on all regulated pollutants during various transient/driving cycles. Also, a detailed list is provided summarizing data from all published works on the subject during the last two decades.