15-10-2012, 01:20 PM
AIRCRAFT PISTON ENGINE EMISSIONS
AIRCRAFT PISTON ENGINE.pdf (Size: 1.48 MB / Downloads: 43)
ABSTRACT
According to Swiss aviation law (SR 748.0, LFG Art. 58), emissions from all engine powered aircraft
have to be evaluated and tested. The legal requirement also incorporates aircraft engines that are
currently unregulated and do not have an ICAO1 emissions certification – like piston, helicopter, turboprop
and small jet engines. Credible emissions data are necessary for emission and immission inventory
purposes, environmental impact assessments and for issues directly related to environmental
protection.
Up to now, there have only been few emissions data available for aircraft piston engines. This report
tries to fill this gap of knowledge in a comprehensive approach. The following aspects are documented
in this report and in its Appendices:
• Emissions performance for a wide range of existing aircraft piston engines.
• A methodology for cost effective standardized on ground emission measurements of aircraft
piston engines.
• A methodology for the calculation of aircraft piston engine emissions.
• General principles of aircraft piston engine combustion and the effect of pilot operations on
emissions.
• Research for emission reduction through optimization of pilot operations, new aircraft piston
engine concepts, technological improvements and the use of cleaner AVGAS.
General information
Background
There are basically three propulsion concepts used for today’s engine powered civil aircraft:
• Propulsion with „Turbofan“ (In everyday language, this is usually called “jet engine”),
• Propulsion with „Turboprop“ (Turbojet engine, driving a propeller) and
• Propulsion with piston engines (driving a propeller).
Turbofans (jet engines) and turboprops burn jet fuel (kerosene), aircraft piston engines primarily burn
aviation gasoline (AVGAS). The highest portion of aviation fuel is consumed by large aircraft with large
engines (mostly turbofans). Large turbofans and jet engines are certified for compliance with tight
emissions standards.
Aircraft piston engines are mostly used for what might be called “small propeller aircraft”. On a global
scale, such aircraft consume a relatively minor share of fuel. In Switzerland, the annual fuel consumption
of aircraft piston engines is less than the fuel consumption for gardening activities2. Basically,
emissions from aircraft piston engines have not been seen as a significant problem when looking at
the broad picture of total emissions, and when looking at measured pollutant concentrations in a country
like Switzerland. This can vary when looking at the aviation sector only, as we will see later (section
2.2.3 of this report). But assuming little to no influence, emissions from aircraft piston engines have not
raised any interest, so far. Internationally, there have not been any efforts to consider work towards an
emission certification for such engines. The disadvantage is that information about aircraft piston engine
emissions performance is practically nonexistent. In Switzerland, this has sometimes caused
problems, e.g. to produce a complete aircraft emissions inventory, or to provide credible data for an
environmental impact assessment.
Main scope and objectives of the project
- Development of a cost-effective ground based in-field measurement technique for aircraft piston
engines (gaseous and non-volatile particle emissions) by using state of the art automotive
testing equipment and by comparing in-flight to ground measurements.
- Gather absolute emissions data of a variety of piston engines for emissions inventory purposes
(some helicopter – and small jet engines will follow at a later step).
- Development of operational guidelines for piston engines with manual mixture control which
take engine durability and emission performance into account.
- If feasible, use the basic research for the development of an emission certification for small
engines, bearing in mind that certification costs should be reasonable.
- Support basic research in order to replace leaded by unleaded AVGAS for aviation piston engines
and to improve emissions performance.
Cruise Emission Factors
Most of the piston engines which are dominating the market have manual air/fuel mixture control to
adjust the engine to different altitudes (in fact to density altitude). This air/fuel mixture adjustment has
to be done by the pilot during flight whenever the aircraft changes its power configuration and altitude.
This adds a particular degree of complexity to representative emissions measurements, because
emissions can vary strongly even if the same aircraft is flying at the same mass, same density altitude,
same speed, configuration and attitude. The variation comes from different “leaning techniques” and is
also dependent on the pilot’s experience, training and available engine cockpit instrumentation (sections
2.2, 2.3 and Appendix 2). For this reason and as a result of in-flight measurements,
Data Quality and Accuracy
FOCA data sheets are primarily based on the "low cost" STARGAS 898 measurement system and the
MEXA 1170 HFID (see Appendix 1). For each engine on ground static tests, a minimum of three
measurements have been performed for every power mode. Data variability has been statistically
checked with a T-test and 90% confidence interval (Example in Appendix 3).
The particle emission measurements have been performed in collaboration with German DLR. Primary
focus were the nanoparticle and carbonyl emissions. There is presently no "low cost" for such measurements.
First measurements have been made with HBKEZ. The measurement results have been
reproduced one year later with the DLR measurement system and the same aircraft (see Appendix 4
for details).
Swiss FOCA sees the primary purpose of the data collection in their application in emissions inventory
calculations. The measurement system that has been used so far (STARGAS 898) does clearly not
represent certification standard. However, the measurement quality achieved by requirements described
in Appendix 1a/b) is considered sufficient for emissions inventories. This conclusion is based
on the following comparisons:
FOCA has started to use a total HC FID in parallel to the "low cost" STARGAS NDIR HC measurement
and has done crosscheck measurements with a chemiluminescence (CLD) NOx analyzer. From
comparative measurements with different systems, correction factors for the "low cost" gas analyzer
with NDIR HC and electrochemical NO probe have been derived. (See Appendix 5 for details.)
In addition to that, FOCA has been given the chance to use HORIBA OBS 2200. This system meets
the requirements for high quality certification emissions measurements (THC FID, NO/NO2 CLD,
heated sampling line at 191°C, exhaust flow measurement etc, see Appendix 1.g) and is very compact,
originally designed and tested for on board measurements in cars and trucks under real operating
conditions. The system has been used for comparative ground static tests in the ECERT project. It
has then been installed for the first time in an aircraft and flown successfully, providing high quality real
time mass emissions data at one second intervals, together with position, altitude, speed and flight
time. Complete, 4-dimensional resolved emissions inventories have been measured and recorded,
from taxi out to taxi in. Those real world data have been compared to the STARGAS data and to the
existing LTO-modelling data. (Details are given in Appendix 2.)
Investigated manual mixture setting procedures
Setting the air/fuel mixture without EGT: Many “old tech” piston engine aircraft with fixed pitch propeller
do not have any exhaust gas temperature instrumentation. Therefore no information about exhaust
gas (and internal combustion) temperatures is available. To handle the mixer of such aircraft,
pilots are often instructed to adjust the mixture with a "rule of thumb": At a fixed throttle setting, the
pilot pulls the mixture lever slowly back and leans the mixture until a slight RPM drop of the fixed pitch
propeller is recognized. At this condition, the engine is running slightly lean. After that, the pilot pushes
the mixture lever slightly forward (about 1 cm) and the engine is running slightly rich. After a while,
the pilot checks cylinder head and oil temperature.