Aviation-Grade Ethanol for Improved Performance and Safety in Civilian and Military Aircraft
Aviation-grade ethanol (AGE) represents an economic, lead-free, high-performance alternative
to lead-containing aviation gasoline known as “100LL.” The University of North Dakota John D.
Odegard School of Aerospace Sciences (UND Aerospace), UND Energy & Environmental
Research Center (EERC), South Dakota State University, ConocoPhillips, and other members of
the American Society of Testing and Materials (ASTM) Ethanol Aviation Fuel Development
Task Force are working to develop an ASTM-approved specification for AGE. UND Aerospace
and EERC propose to demonstrate the value of AGE in civilian and military piston-engine
aircraft applications based on:
x
Improved performance in turbocharged engines.
x
Improved safety through prevention of water-related engine stoppage.
x
Reduced levels of exhaust emissions.
x
Reduced exhaust emission temperature.
Because ethanol contains less energy per gallon than 100LL, normally aspirated aircraft engines
designed to burn 100LL typically experience a range reduction of 20 to 30% when burning AGE.
However, because ethanol has a significantly higher resistance to detonation than 100LL,
a
turbocharger can deliver a higher-percentage performance improvement with ethanol. For
example, the Saab automobile company recently introduced the first commercially available
ethanol turbo. A computer samples the fuel mixture and adjusts boost pressure—from 5.8 pounds
per square inch (psi) for pure gasoline to 13.8 psi for “E85,” a fuel comprising about 81%
ethanol and 19% gasoline. On gasoline, the engine produces 148 horsepower, but on E85 it
produces 184 horsepower—an increase of 24%—with no penalty in fuel economy. Because
AGE is about 90% ethanol and 10% pentane isomerate (a branched paraffinic gasoline
blendstock), similar power increases should be achievable for turbocharged aircraft engines
running on AGE versus 100LL.
With its ability to absorb water contamination, AGE offers the potential to eliminate the fuel
system presence of water and/or ice as a combustion-quenching separate phase from fuel. With
octane provided by ethanol, AGE will eliminate exhaust emissions of lead, and based on
extensive data acquired for automotive E85 emissions, AGE offers the potential to significantly
reduce emissions of hydrocarbons and carbon monoxide, while producing a neutral or slightly
positive impact on NOx emissions. Because of ethanol’s higher latent heat of vaporization versus
100LL, AGE combustion yields a lower exhaust emission temperature than 100LL combustion.
In addition to environmental benefits, the ability to effect reduced levels of pollutant emissions
and reduced exhaust gas temperature may represent a significant benefit to military aircraft,
including unmanned aerial vehicles (UAVs), since it would translate to a reduced potential for
detection by enemy-operated aircraft targeting systems.
Key objectives of the proposed project are to:
x
Assess the overall performance impact of AGE in a turbocharged aircraft—including
potential power and fuel economy benefits achievable with AGE versus 100LL.
x
Demonstrate the flight safety improvement achievable with AGE by virtue of its ability to
prevent water-related engine stoppage.
x
Preliminarily assess the extent to which AGE can reduce exhaust emission levels of
hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx), versus 100LL.
x
Assess the extent to which AGE can reduce exhaust temperature versus 100LL.
To accomplish the key project objectives, the following statement of work is proposed.
Task 1 – AGE Performance Impact Assessment – Using a twin-engine turbocharged aircraft,
UND Aerospace will evaluate the extent to which engine power output can be safely increased
with AGE versus 100LL. Each engine will be configured with appropriate instrumentation to
enable real-time data acquisition on power output, fuel economy, manifold pressure, exhaust gas
temperature, and other performance indicators during engine operation. Following installation of
all instrumentation, the aircraft will undergo shakedown procedures to ensure reliability and
safety of all flight and data acquisition systems under a range of aircraft operational modes
including idle, taxi, take-off, cruise, descent, and approach power settings. A series of flight tests
will be conducted to measure the maximum safe power output achievable with AGE and 100LL.
The ability to simultaneously operate two engines on two different fuels and switch fuel flows
between engines will enable accurate and meaningful comparison of fuel performance. Because
of the high detonation resistance of ethanol, the project team will consult with appropriate
representatives of the engine manufacturer to establish any structural integrity-based manifold
pressure limitations.
Task 2 – Assessment of AGE Ability to Prevent Water-Related Engine Stoppage – In addition to
lack of need for octane-providing lead, another key advantage of AGE versus 100LL is its ability
to dissolve water, which should help prevent engine stoppage and crashes caused by the presence
of water or ice in aircraft fuel systems. According to the National Transportation Safety Board,
from 1983 to 2001, water- or ice-related fuel system problems were responsible for 431
accidents, resulting in 88 deaths. It is likely that that many or all of these accidents could have
been prevented by the use of AGE. In a tank filled with 100LL, water removal is dependent on
detection. Detection requires drainage to a sump, which is not always assured, especially with
certain bladder-type tanks. With AGE, low-level water contamination is a non-issue, since water
is absorbed into the fuel and cannot exist as a potentially engine-stopping separate phase.
As part of an ongoing FAA-funded research program, EERC is assessing the water-absorbing
capacity of AGE and AGE–100LL blends, and developing an accurate, simple, quick,
inexpensive method for conducting a preflight check on fuel water content. The water content
analysis method is based on the use of a cobalt chloride-impregnated paper indicator that
changes color in the presence of water dissolved in fuel at a concentration exceeding a specific
threshold value. In addition to assisting with completion of this work as necessary, Task 2 will
utilize data from the AGE water-absorbing-capacity determination work to establish a flight
safety-assuring fuel water content maximum value for incorporation into the under-development
ASTM specification for AGE, and then conduct aircraft engine tests to assure the validity of the
value in prevention of water contamination-related engine performance degradation. The tests
will comprise engine operation with fuel containing water added at the established maximum
contamination level, and monitoring of power output, fuel economy, and other parameters to
assess overall performance impact. Based on recent EERC work in which the occurrence of
ethanol–water phase separation from petroleum was measured as a function of water content in
mixtures of 100LL and AGE, it appears likely that the proposed water content maximum for
AGE will not exceed 1%, and may be 0.5% or lower.
Task 3 – Preliminary AGE Emissions Impact Assessment – To enable comparison of AGE and
100LL based on exhaust emissions per unit fuel consumption, the test aircraft will be
appropriately instrumented for acquisition of real-time data on aircraft speed, fuel flow, intake
manifold mass air flow, cylinder head and exhaust gas temperatures, and other parameters as
required. Because designing and installing a system for empirically measuring lead exhaust
emissions from a 100LL-fueled aircraft would require the development of a new test protocol
and likely be cost prohibitive, grams-per-mile lead emissions will be calculated based on real-
time aircraft speed and fuel consumption data, and analyzed fuel lead content. Following
installation of all instrumentation, the aircraft will undergo shakedown procedures to ensure
reliability and safety of all flight and data acquisition system under a range of power settings.
Exhaust, engine and fuel system, and performance data will be acquired for each test fuel while
operating at cruise power and possibly one or two more power settings. Appropriate atmospheric
data will also be acquired for use in evaluating test results. Because of the importance of
replicability to acquisition of meaningful data, special attention will be paid to developing
procedures that are easily and accurately reproducible for each power setting. To ensure
availability of statistically significant and accurate data, at least four separate data sets will be
acquired for each fuel–power setting combination. Each emission and accompanying engine and
fuel system data set will be reduced to an average value, and each resulting average value set will
be analyzed for statistical significance using procedures described in Statistical Methods for
Environmental Scientists (by R.E. Stecker, J.S. Powell Publishing, Boulder Colorado, 1995).
Using average emission and fuel and air flow values that meet a “95% confidence interval”
precision acceptability threshold, grams-per-liter-fuel-consumed (at specified air speed)
emissions of hydrocarbons, carbon monoxide, and NOx will be calculated for each fuel–power
setting combination. Grams-per-liter-fuel-consumed lead emissions will be calculated for each
fuel–power setting combination based on analyzed fuel lead content and average speed measured
over each power setting test sequence.
Task 4 – Assessment of AGE Ability to Reduce Exhaust Temperature – As described above,
exhaust gas temperature data will be acquired during Task 1 and 3 activities to assess AGE
impact on performance and emissions, respectively. These data will be analyzed to look for
significant correlations between exhaust gas temperature and the engine operational parameters
evaluated. Any significant correlations will be further explored, with the objective of establishing
the maximum exhaust gas temperature reduction achievable under safe and economic operational
conditions.