Environmental Engineering Reference
In-Depth Information
Various reasons have been reported to explain the torque and power recovery at
full load (corresponding to the loss of heating value) of biodiesel with respect to
diesel fuel.
The higher viscosity of biodiesel may affect the engine brake effective power,
especially under full-load operating conditions. The increased injected volume has
also been attributed to the increase in viscosity [150].
The higher bulk modulus and sound velocity of biodiesel, together with its higher
viscosity, lead to an advanced start of injection [151]. This fact, together with an
increase in the cetane number, may slightly advance the start of combustion. To
reduce pressure and temperature peaks in the combustion chamber, and thereby
nitric oxide formation, current diesel engines need to have delayed combustion.
This delay involves a loss of thermal efficiency and consequently of brake effec-
tive power. If the start of injection, and thus that of combustion, is advanced, the
combustion process is then re-centered and the power output increases [149, 152,
153].
The higher lubricity of biodiesel could also reduce the loss of friction leading to
an increased brake effective power. Several researchers have used this argument to
explain the increased thermal efficiency or power recovery in spite of the unknown
origin of this improvement (reduction of mechanical losses in the injection pump
and cylinder walls) [153]. In any case, it seems very unlikely that the lubricity can
contribute to the torque and power recovery.
The concept of thermal barrier coatings may be useful to limit the effect of
the high viscosity of biodiesel. Engines with thermal barrier coating are called
low heat rejection (LHR) engine. The LHR concept is based on suppressing heat
rejection to the coolant and recovering the energy in the form of useful work.
Insulating the combustion chamber components of LHR engines can reduce heat
transfer between in-cylinder gas and cylinder liner, thus enhancing engine power
and torque due to the increased exhaust gas temperatures before the turbine inlet
[143, 154].
Brake-specific fuel consumption (BSFC)
. BSFC is the ratio between mass fuel
consumption and
Brake effective power
, being inversely proportional to the thermal
efficiency. According to literature reports, the biodiesel specific fuel consumption is
expected to increase around 10-20% in relation to diesel fuel, corresponding to the
increase in heating value in mass basis. In other words, the loss of heating value of
biodiesel has to be compensated with a higher fuel consumption. An indicator of the
loss of heating value is the oxygen content in the fuel [144]. A correlation between
BSFC and oxygen content has been found and the conclusions are the increase in
BSFC is due to the oxygen enrichment from the fuel, but not from the intake air
[155, 156].
Fuel consumption seems to behave proportionally to the loss of heating value,
whether heavy-duty or light-duty engines were tested. Turrio-Baldassarri et al.
tested a 6-cylinder 7.8 L engine with a 20% rapeseed-oil biodiesel (with a glyc-
erin content of 1.15%)/diesel fuel blend [157]. They measured a BSFC increase of
2.95% with 95% statistical confidence. A similarly sized engine (6 cylinders and
170 kW of rated power) was tested by Hansen and Jensen with pure rapeseed-oil
biodiesel measuring a 14% increase in BSFC [135].
