NREL strives to more deeply understand how fuel properties impact engine operation.
We accomplish this by relating fuel properties to fuel chemistry and molecular structure.
We evaluate a broad range of renewable gasoline and diesel fuels ranging from currently
available ethanol and biodiesel to future products such as dimethyl furan and hydrotreated
biomass pyrolysis oils.
For gasoline boiling range hydrocarbons, NREL applies detailed hydrocarbon analysis
(DHA), which is a high-resolution gas chromatography method for identifying and quantifying
more than 98% of the components of a petroleum refinery gasoline. This rich data set
of chemical information can then be used to calculate useful properties.
Fuel Chemical Properties Data and Analysis
Our analytical and data science prowess plays a vital role in accelerating the pace
of fuels and combustion research advancements.
High-resolution gas chromatogram showing the components of petroleum refinery gasoline.
About the Metric
The DHA-derived parameter particulate matter index (PMI) is a widely used metric for
ranking the particulate matter formation tendency of gasolines, including gasolines
containing biofuels. Particulate matter consists of fine particles that have negative
impacts on human health. Their emissions from cars and trucks are regulated by government
agencies worldwide.
PMI is calculated from the DHA, considering the properties of each individual component.
NREL’s fuel chemistry and engine combustion research has shown that particle formation
from biomass-derived oxygenates is not accurately predicted by PMI because some oxygenates
have low energy barrier reaction pathways to soot formation. For more information,
see engine combustion research.
Additionally, alcohols such as ethanol have a much higher heat of vaporization (HOV)
than gasoline, and when blended into gasoline the increased evaporative cooling can
cause more particles to form from the aromatic compounds in gasoline under some conditions.
The DHA can also be used to calculate the HOV of complex mixtures such as gasoline-ethanol
blends. Current research is exploring how to predict the distillation curve of gasoline
from the DHA, as well as other properties.
NREL has developed methods for analysis of sodium and other metals to well below 1
part per million (ppm) (sodium detection limit of 0.023 ppm) using microwave-plasma
atomic emission spectroscopy and inductively coupled plasma atomic emission spectroscopy.
This research has shown that sodium levels in biodiesel on the market are typically
well below 0.5 ppm. However, a small percentage of samples were as high as 3 ppm.
Durability Testing and Physico-Chemical Characterization
In collaboration with Cummins, Inc., and Oak Ridge National Laboratory, NREL examined
the impact of sodium on diesel emission control systems in a 1,000-hour accelerated
durability test followed by detailed physico-chemical characterization of the emission
control system components.
It showed that at the currently allowed level of 5 ppm in 100% biodiesel the sodium
doubles the rate of ash accumulation in the DPF (the balance is ash from the engine
lubricant), increasing engine back pressure and resulting in increased NOx emissions.
A typical biodiesel contains sodium at less than 1 ppm. Nevertheless, industry stakeholders
are considering measures to significantly reduce the sodium content in biodiesel.
Fuels can contain metal impurities, such as sodium and calcium, that end up in the
engine exhaust. They can also deposit on emission control system components—such
as diesel oxidation catalysts, diesel particle filters, and NOx reduction catalysts—resulting
in catalyst deactivation and filter clogging.
Biodiesel—a biofuel produced from vegetable oils, animal fats, and waste cooking oil—can
contain sodium as a residue from its manufacturing process. Sodium is potentially
present at levels below 1 ppm, making accurate analysis of the sodium content of a
fuel a significant challenge.
Innovative Gasoline Heat of Vaporization Measurement Method
NREL has developed a method for measuring heat of vaporization (HOV) as the fuel evaporates
using a differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) instrument.
The method has been applied to ethanol blends into the Fuels for Advanced Combustion
Engines (FACE) A research gasoline.
Results show that adding ethanol increased the heat flow until the ethanol has evaporated,
so there is less cooling during the later phase of evaporation. Ongoing research is
examining more complex gasolines and the impact of azeotropic interactions between
ethanol and gasoline boiling range hydrocarbons.
Fuel Direct Injection and Heat of Vaporization
Gasoline engines that use direct injection (DI) of the fuel currently make up approximately
half of new cars sales in the United States. One of the advantages of DI is that the
fuel evaporates in the engine cylinder, which lowers the temperature of the fuel-air
mixture because of the fuels HOV. This evaporative cooling has several beneficial
effects, including reducing pumping loss for induction of air into the engine and
increasing the fuel’s effective knock resistance allowing increased compression ratio—both
effects significantly improve engine efficiency. Alcohols such as ethanol have a much
higher HOV than gasoline hydrocarbons (923 kilojoules per kilogram [kJ/kg] for ethanol
versus 350 to 400 kJ/kg for gasoline). Thus, blending of ethanol increases HOV and
results in even lower fuel-air mixture temperature.
While the total HOV of a gasoline ethanol blend can be calculated from the DHA, engine
developers and combustion researchers need to understand how the HOV evolves as the
fuel evaporates.
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