Course: BFM 202 Biodiesel Production

Lecture

The Fuel Properties of Biodiesel
Credits
BFM 202 Biodiesel Production Unit 2 Lecture has been adapted from "ENGY 131 Unit 2 Commentary: The Diesel Engine" by Southeastern Illinois College.

ENGY 131 Biodiesel Production course content by Southeastern Illinois College is licensed under a Creative Commons Attribution 3.0 Unported License. Except where indicated otherwise.

The Fuel Properties of Biodiesel

Auto-ignition and the Cetane Number

The cetane number is an indicator of one of the most important properties of diesel and biodiesel fuels.
ASTM D 613 is the approved laboratory test used to measure the readiness of a diesel fuel to auto-ignite at the operating pressures and temperatures in the cylinder where the fuel's injected.
The test compares the auto-ignition of the test fuel with a blend of two reference fuels, cetane (hexadecane) and heptamethylnonane.
High CN numbers indicate shorter ignition delays. Most biodiesel has higher CNs than petroleum-derived diesel and biodiesel from higher saturated feedstocks have higher CNs. For example, biodiesel from soybean oil normally has a CN of 48-52. Biodiesel from yellow grease contains more saturated esters and CNs of 60-65 are normal.
Lubricity

Diesel engines rely on the fuel itself to lubricate the closely-fitted moving parts of the injection system. Lubricity is described as the ability of a fuel to lubricate. As diesel fuel sulfur content was reduced, so was the ability of the fuel to self- lubricate.
Diesel fuel lubricity is measured by two methods:
- SLBOCLE: ASTM D 6078-99 known as the Scuffing Load Ball On Cylinder Lubricity Evaluator
- HFRR: ASTM D 6079-99 known as the High Frequency Reciprocating Rig

ASTM D 975 now includes HFRR lubricity test method. The lubricity of biodiesel is excellent and blending as little as 1-2% biodiesel fuel can raise poor fuel to an acceptable level.
Engine operation at low temperatures

The long-chain hydrocarbons found in diesel fuel, referred to as waxes, crystallize at normal diesel engine operating temperatures. If temperatures are low, the wax crystals form a mass that plugs fuel filters preventing the engine from normal operating. At even lower temperatures, the diesel fuel solidifies.
This also occurs with biodiesel fuel where saturated fatty acids produce methyl esters which begins crystallization at 0°C for soybean oil and up to 13-15°C for frying oils and animal fats. This crystallization is measured by a fuel's cloud point or pour point.
ASTM D 2500 provides standards for a fuel's Cloud Point (CP) indicating the temperature at which crystallization appears as cloudiness.
ASTM D 97 sets standards for the Pour Point (PP), referring to the lowest temperature at which fuel pours.
PP depressants are additives used to reduce crystallization and help prevent reaching the CP or PP.
Viscosity

Viscosity describes a fluid’s resistance to flow in a fluid or semifluid state. The higher the viscosity the greater its resistance to flow, and the more difficult it is to atomize the fuel for injection into the combustion chamber. Most diesel fuel injection systems compress the fuel by using a plunger and barrel. The high pressures are achieved by using clearances of ~0.0001” (0.0025 cm). This clearance still allows fuel leaks around the plunger during compression. If viscosity is low, the leakage directly correlates to the engine's power loss. If the viscosity is high, insufficient fuel is provided to the chamber resulting in power loss.
Biodiesel's viscosity range overlaps the diesel fuel with some biodiesel viscosities being above the limit. Biodiesel from vegetable oil can be excessive, resulting in poor atomization of the fuel spray to the cylinder, lube oil contamination, and black smoke.
ASTM D 975 is the standard for diesel fuel viscosity (1.9 to 4.1 cSt) and hydrocarbon fuels meet this viscosity standard.