Brake thermal efficiency evaluates how efficient the engine
transforms the chemical energy of the fuel into useful work. This
parameter is determined by dividing the brake power of the engine
to the amount of energy input to the system. The amount of energy
cannot be converted into mechanical energy is discharged by the
system through friction losses, heat transfer through the engine
cylinder and exhaust gases. The friction losses are highly sensitive
to the engine speed regardless of the fuel. Whereas the heat transfer
losses not only depends on the engine speed, but also varies
with the combustion characteristics of the fuel.
The experimental results show that no general correlation can
be confirmed between the BTE and biodiesel blend ratio, however,
it can be observed that the BTE values of B100 are consistently lower
compared to diesel under lower load conditions and the percentage
reduction increases as the engine load decreases. At
higher load conditions, B100 even has a better BTE than diesel.
Fig. 4a–c compare the BTE of different tested fuels under 100%,
50% and 10% loads. As demonstrated in the figures, the average
BTE value of B100 increases by 5.1% at 100% load, but decreases
by 10.7% at 10% load, leaving 50% load as a transition where no increase
or decrease in average BTE is observed. This is because at
higher load conditions, the fuel injection pressure reaches as high
as 1400 bars such that the viscosity effect of B100 is negligible,
on the other hand, the fact that B100 contains oxygen results in
a more complete combustion, and hence a higher thermal efficiency
is achieved. Another possibility goes to the fact that the exhaust
temperature of B100 is found to be lower compared to diesel
which reduces the heat transfer losses, increasing the thermal efficiency.
At lower loads, as explained previously, the viscosity effect
of biodiesel becomes the dominating factor, resulting in a poor
combustion process and reduced BTE. Besides, the release of oxygen
atoms from B100 may have leaded to an extra lean combustion
process under partial load conditions and reduced the thermal
efficiency.