buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

142 Anca Elena Eliza Sterpu and Anca Iuliana Dumitriu
1. Introduction
Diesel engines are the axis of world industry, dominating sectors like
road transport, agricultural, military, construction, maritime propulsion and
stationary electricity production. In recent years, the concern over the depleting
world reserves of fossil fuels and more stringent emission regulations for
harmful pollutants have led to the resolute efforts in searching for renewable
alternative fuels and ultra-low emission combustion strategies. Biodiesel fuel,
which is identified as a clean alternative fuel, has become commercially
available in a number of countries (Zheng et al.., 2008),. In addition, this
biofuel is completely miscible with petroleum diesel, allowing the blending of
these two fuels in any proportion. Biodiesel can be used neat or blended in
existing diesel engines without major modification in the engine hardware.
However, differences in the chemical nature of biodiesel (mixture of monoalkyl
ester of saturated and unsaturated long chain fatty acids) and conventional
diesel fuel (mixture of paraffinic, naphthenic and aromatic hydrocarbons) result
in differences in their basic properties, affecting engine performance and
pollutant emissions. Biodiesel, produced from any vegetable oil or animal fat,
generally has higher density, viscosity, and cetane number, and lower volatility
and heating value compared to commercial grades of diesel fuel (Benjumea et
al., 2008). Biodiesel most commonly are made from soybean oil in the United
States and from rapeseed (Canola) oil in Europe using methanol.
1.1. Biodiesel as Engine Fuel
Since most modern diesel engines have direct injection fuel systems, and
these engines are more sensitive to fuel spray quality than indirect injection
engines, a fuel with properties that are closer to diesel fuel is needed (Canakci,
2007). The best way to use vegetable oil as fuel is to convert it in biodiesel.
Biodiesel is made from natural, renewable sources such as new or used
vegetable oils and animal fats. The resulting biodiesel is almost similar to
conventional diesel in its main characteristics. Biodiesel is an oxygenated fuel
and leads to more complete combustion; hence CO emissions reduce in the
exhaust. Biodiesel contains no petroleum products, but it is compatible with
conventional diesel and can be blended in any proportion with mineral diesel to
create a stable biodiesel blend. The level of blending with petroleum diesel is
referred as Bxx, where xx indicates the amount of biodiesel in the blend (i.e.
B20 blend is 20% biodiesel and 80% diesel). It can be used in compression
ignition (CI) engine with no significant modifications to the engine (Agarwal,
2007).
1.2. Combustion Characteristics of Biodiesel
It is important to know the combustion properties of biodiesel–diesel
blends. Some of these properties are required as input data for predictive and