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This work describes an investigation of measurement techniques for the indicated mean effective pressure (IMEP) on a 55 cc single-cylinder, 4.4 kW, two-stroke, spark ignition (SI) engine intended for use on Group 1 and Group 2 remotely piloted aircraft (RPAs). Three different sensors were used: two piezoelectric pressure transducers (one flush mount and one measuring spark plug) for measuring in-cylinder pressure and one capacitive sensor for determining the top dead center (TDC) position of the piston. The effort consisted of three objectives: to investigate the merits of a flush mount pressure transducer compared to a pressure transducer integrated into the spark plug, to perform a parametric analysis to characterize the effect of the variability in the engine test bench controls on the IMEP, and to determine the thermodynamic loss angle for the engine. The results indicate that as a spark plug, the measuring spark plug is not statistically different from the stock spark plug at the 95% confidence level. The results indicate a statistically significant, 4% difference in the measured IMEP between the pressure transducer in the measuring spark plug and the flush mount transducer. The results also suggest a statistically significant difference in performance between the modified and unmodified engine heads, verifying the suppositions of other researchers who suggested that even a small modification to a combustion chamber this size could measurably affect the engine performance. While run-to-run variation resulted in a 2% to 5% variation in IMEP, a sensitivity analysis determined that 1% to 3% of that variation arose from variability in the control variables, while the remainder was caused by variation in other engine operating parameters. Between 1000 rpm and 2000 rpm, where the engine was typically motored to determine the TDC, the thermodynamic loss angle was 0.3 crank angle degrees (CAD) to 0.7 CAD, larger than loss angles observed in automotive-sized gasoline engines. The results indicate that using tabulated thermodynamic loss angles to set the TDC location of the engine using a mono-directional peak pressure method would lead to a −1% to −2.5% bias in the IMEP.

Small internal combustion engines, particularly those ranging in power from 1 kW to 10 kW, propel many remotely piloted aircraft (RPA) platforms that play an increasingly significant role in the Department of Defense. Efficiency of these engines is low compared to conventional scale engines and thermal losses are a significant contributor to total energy loss. Existing thermal energy loss models are based on data from much larger engines. Whether these loss models scale to the engine size class of interest, however, has yet to be established. The Small Engine Research Bench (SERB) was used to measure crank angle resolved gas temperature inside the combustion chamber of a small internal combustion engine (ICE). A 55 cc, twostroke, spark-ignition ICE was selected for this study. The engine was modified for optical analysis using sapphire rods 1.6 mm in diameter on opposite sides of the combustion chamber. The engine modification was found to have no measurable impact on indicated mean effective pressure or heat rejection through the cylinder. FTIR absorption thermometry was used to collect mid-infrared absorption spectra. The FTIR was allowed to scan continuously while simultaneously recording the scanning mirror position and crank angle associated with each data point, then data was re-sorted by crank angle. Measured spectra were compared with lines generated using CDSD-4000 and HITEMP line list databases. The line of best fit corresponded to the mean gas temperature through the combustion chamber. In this way temperature was determined as a function of crank angle for three operating conditions: 4,300, 6,000, and 7,500 revolutions per minute, all at wide open throttle. High cycle-to-cycle variation in the regions of combustion and gas exchange degraded temperature measurements at the affected crank angles. Future research will attempt to improve signal to noise in these measurements.

Measurements of temperature and CO2 concentration using dual-pump coherent anti-Stokes Raman scattering, (CARS) are described. The measurements were performed in laboratory flames,in a room-temperature gas cell, and on an engine test stand at the U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base. A modeless dye laser, a single-mode Nd:YAG laser, and an unintensified back-illuminated charge-coupled device digital camera were used for these measurements. The CARS measurements were performed on a single-laser-shot basis. The standard deviations of the temperatures and CO2 mole fractions determined from single-shot dual-pump CARS spectra in steady laminar propane/air flames were approximately 2 and 10% of the mean values of approximately 2000 K and 0.10, respectively. The precision and accuracy of single-shot temperature measurements obtained from the nitrogen part of the dual-pump CARS system were investigated in detail in near-adiabatic hydrogen/air/CO2 flames. The precision of the CARS temperature measurements was found to be comparable to the best results reported in the literature for conventional two-laser, single-pump CARS. The application of dual-pump CARS for single-shot measurements in a swirl-stabilized combustor fueled with JP-8 was also demonstrated.

As internal combustion (IC) engines decrease in displacement, their cylinder surface area to swept volume ratio increases. Examining power output of IC engines with respect to cylinder surface area to swept volume ratio shows that there is a dramatic change in power scaling trends at approximately 1.5 cm-1. At this size, thermal quenching and friction losses are expected to dominate , so power production and efficiency characteristics suffer. Furthermore, small IC engines (100cc displacement) have limited technical performance data compared to IC engines in larger size classes. Therefore, it is critical to establish accurate performance figures for a family of geometrically similar engines in the size class of approximately 1.5 cm-1 in order to better predict the phenomena that contribute to lower efficiencies in small ICEs. A series of three two stroke, single cylinder, spark ignited, air cooled, and carbureted IC engines were tested in this study. They had a displacement of 28cc, 55cc and 85cc corresponding to cylinder surface area to swept volume ratios of 1.81 cm-1, 1.46 cm-1, and 1.28 cm-1 respectively. The engines share design features like compression ratio, gas exchange port design, and scavenging method.

Three analysis techniques were used to measure the heat rejection of a 55 cc air-cooled two-stroke engine. This study was performed as part of a larger e ort aimed at extending range and endurance limitations of Group 1 & 2 Remotely Piloted Aircraft (RPA). The engine selected for the study was a 55 cc gasoline-fueled, carbureted, spark-ignition engine made by 3W-Modellmotoren and is representative of RPA engines in these groups. With a surface area to volume ratio of 1.48 1 cm, the engine is in a size region where thermal losses begin to dominate engine e ciency and thermal e ciencies of less than 20% are common. The rst measurement method was an energy balance between the fuel energy entering the system and the various avenues for energy to leave the system. The second method used an enclosure around the engine and measured the enthalpy increase of the air owing past the cooling ns. The third method used heat ux gauges placed on the cylinder head to measure the heat ux at those locations. The energy balance method estimated heat rejection at approximately 30-40% of the total fuel energy for full and partial power settings. As part of the energy balance method, the engines tested achieved a maximum thermal e ciency of 13.7% and a maximum brake power value of 2.9 kW. The engine enclosure method measured heat rejection values to be approximately 2 kW at full and partial power settings. This equates to 8-11% of the fuel energy at full power and 20-26% at a 25% power setting. The heat ux gauge method measured heat ux values of up to 33 kW m2 . Applying the heat ux values over the surface area of the cylinder resulted in 1.2-2.1 kW of heat loss. As a percent of total fuel energy this represents 7-13% at full power and 22-30% at a 25% power setting.

A simple time-resolved fluorometer is constructed with an all-solid-state-based, frequency-tripled, Q-switched, diode-pumped Nd:YLF laser as the excitation source. Signal processing is accomplished with a digital oscilloscope. Simplicity of operation and applicability to trace analysis and in time-resolved spectroscopy are demonstrated with this new instrument. The laser produces 2.5-ns pulses at 349 nm and is capable of kilohertz repetition rates. For every shot of the laser, the oscilloscope collects an entire fluorescence decay at a 1-ns digitizer resolution and can average these data at the maximum laser repetition rate. When one is operating at 1 kHz and signal averaging for one second, detection limits (S/N = 3) in the 10–100 pM region are obtained. Excited-state decays are collected for several enzymatic probes and quinine sulfate, providing lifetimes consistent with those obtained by established instruments.

Diode-laser-based sensors were implemented to measure the concentrations of nitric oxide (NO) and hydroxyl (OH) radicals in the vitiated inlet airflow of a model scramjet combustor. The sensors utilized sum-frequency-mixed sources consisting of a fixed frequency 532-nm laser and a tunable diode laser to generate ultraviolet radiation for absorption spectroscopy with electronic transitions of OH and NO. Sensitive, interference-free, absolute measurements were possible, enabling the first measurements of both species in a model scramjet combustor using diode-laser-based sensors. With wavelength-modulation spectroscopy, no absorption by OH was evident in the vitiated airflow, verifying that the OH concentration was below the 0.2-ppm detection limit of the sensor.

This paper reports for the first time the combination of a dual-pump coherent anti-Stokes Raman scattering system with an interferometric Rayleigh scattering system (CARS - IRS) to provide time-resolved simultaneous measurement of multiple properties in combustion flows. The system uses spectrally narrow green (seeded Nd:YAG at 532 nm) and yellow (552.9 nm) pump beams and a spectrally-broad red (607 nm) beam as the Stokes beam. A spectrometer and a planar Fabry-Perot interferometer used in the imaging mode are used to record the spectrally broad CARS spectra and the spontaneous Rayleigh scattering spectra, respectively. Time-resolved simultaneous measurement of temperature, absolute mole fractions of N2, O2, and H2, and two components of velocity in a Hencken burner flame were performed to demonstrate the technique.

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Unfortunately, these studies represent a tremendous research challenge owing to the inherent complexity of such flows. In an effort to reduce the complexity of these systems while capturing the essential features that define the physics and chemistry of turbulent reacting flows, we have been studying the interaction of a vortex with a laminar flame. The experimental apparatus includes a piston-cylinder device configured to provide a controlled toroidal vortex. The generated vortex/jet interacts with a nonpremixed hydrogen-air flame supported in a counterflow burner. The counterflow configuration permits precise selection of the flame and the associated strain field. Vortex characterization is essential to interpreting the experimental observation and accomplishing numerical modeling of vortex-flame interactions. Two-color particle- image velocimetry (PIV) has been employed to characterize the vortex and to describe the underlying counterflow velocity field. The hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and PLIF measurements of OH are performed simultaneously. A distinct annular extinction of the OH layer is observed, in good agreement with previous computational modeling predictions for the apparatus.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Spark-ignition systems play a critical role in the performance of essentially all gas turbine engines. These devices are responsible for initiating the combustion process that sustains engine operation. Demanding applications such as cold start and high-altitude relight require continued enhancement of ignition systems. To characterize advanced ignition systems, we have developed a number of laser-based diagnostic techniques configured for ultrafast imaging of spark parameters including emission, density, temperature, and species concentration. These diagnostics have been designed to exploit an ultrafast- framing charge-coupled-device (CCD) camera and high- repetition-rate laser sources including mode-locked Ti:sapphire oscillators and regenerative amplifiers. Spontaneous-emission and laser-shlieren measurements have been accomplished with this instrumentation and the result applied to the study of a novel Unison Industries spark igniter that shows great promise for improved cold-start and high-altitude-relight capability as compared to that of igniters currently in use throughout military and commercial fleets. Phase-locked and ultrafast real-time imaging strategies are explored, and details of the imaging instrumentation, particularly the CCD camera and laser sources, are discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.