With the rapid growth of biodiesel production, it is prudent to
research ways to improve its operation and performance in an
engine, especially concerning fuel economy and exhaust emissions.
This requires a thorough understanding of both the biodiesel
production and engine operating processes. Completion of a
published study of the impact of biodiesel fuel properties on
engine operation indicated that it is difficult to draw conclusions
about the exact causes of increased NOx emissions with
respect to biodiesel properties without the capability of measuring
engine cylinder pressures. As improvements were made to the
authors' laboratory, a system to monitor and record pressure
inside a diesel engine during operation was constructed to test
dissimilar fuels. In the current work, three different fuels were
tested in order to investigate combustion phasing, emissions, and
fuel consumption as a function of fuel properties such as density,
viscosity, Cetane Number, and energy content. The fuels tested were
Ultra Low-Sulfur Diesel (ULSD), Used Cooking Oil (UCO) biodiesel
produced on campus, and jet propellant number 8 (JP-8). ULSD serves
as a control fuel for the test since the engine and injector are
designed to utilize this specific fuel. JP-8 was included in order
to investigate the feasibility of using this fuel in diesel engines
as a part of the United States Military's Single Fuel Forward
policy. This policy involves a simplification of battlefield
logistics through use of a single fuel across all combustion
platforms and will result in the use of JP-8 in older engines that
were not intended for this fuel.
The engine used for this study was a single-cylinder
direct-injection diesel engine, originally packaged as part of an
electric generator. The engine was loaded by applying resistance
loads while simultaneously recording in-cylinder pressures, engine
speed, torque, and air and fuel flow rates at steady-state
conditions. The emissions of nitrogen oxides (NOx),
carbon monoxide (CO), and total hydrocarbons (HC) were also
measured. The experimental results indicate that fuel chemistry
differences between JP-8, UCO biodiesel, and ULSD result in
distinctive combustion characteristics affecting both performance
and emissions. Specifically, it was found that UCO biodiesel usage
advances injection and combustion timing due to an increased bulk
modulus and Cetane Number. This leads to improved combustion
efficiency, but also higher NOx emissions due to higher
in-cylinder pressures and temperatures (among other factors). In
addition, JP-8 combustion occurs later in the engine cycle than
ULSD due in part to a lower bulk modulus and resultant later
injection. This results in lower combustion efficiency and
NOx emissions than both UCO biodiesel and ULSD. However,
the fuel consumption of ULSD and JP-8 were found to be similar
across all loads, indicating a possible reduction in fuel
consumption for JP-8 to levels below those of ULSD through
optimization of injection timing.
