A new internal combustion engine configuration: opposed pistons with crank offset

[Abstract]: Theoretical and experimental performance results for a new internal combustion engine configuration are presented in this paper. The engine is a piston ported, spark ignition petrol engine which consists of two opposed pistons in a single cylinder controlled by two synchronously timed crankshafts at opposite ends of the cylinder. It makes use of crank offset to create the required piston motion aimed at engine efficiency improvements through thermodynamic performance gains. In particular, the engine employs full expansion in which the power stroke displaces a larger volume than the compression stroke, thereby allowing the expanding gas to reach near atmospheric pressure before the exhaust port opens. This allows more work to be done by each thermodynamic cycle. It also features a greater rate of volume change after combustion than a convention 4-stroke engine for the same crank speed. This reduces the time that the temperature difference between the gas and the cylinder is high relative to a conventional engine which in turn, should reduce the heat lost from the combustion products. Thermodynamic and friction modelling of the engine indicated that efficiencies around 38% might be achieved. However, experiments with a prototype engine demonstrated that friction losses in the engine exceeded that predicted in the original modelling

Impact on a water filled cylinder

The computational and experimental results of impact loading a water filled cylinder with a high speed piston are presented. Computational simulation of the impact process is performed by means of DIANA, a commercial finite element software package. In this simulation, water is modeled as a solid with very small shear modulus compared to the bulk modulus of water. The efficiency of the simulated impact is evaluated by the time dependent water pressure in the vicinity of the cylinder. Also, the shock pressure resulting from impact is detected by using a pressure transducer located in the middle of the water tube. Comparison of the computational and experimental results shows that the impact process on a water filled cylinder is well modeled. It is shown that the best way to increase the pressure peaks of the pressure profile curve is to increase the piston’s impact velocity

Eroding ribbon thermocouples: impulse response and transient heat flux analysis

We have investigated a particular type of fast-response surface thermocouple to determine if it is appropriate to use a one dimensional transient heat conduction model to derive the transient surface heat flux from the measurements of surface temperature. With these sensors, low thermal inertia thermocouple junctions are formed near the surface by abrasive wear. Using laser excitation, we obtained the impulse response of these commercially available devices. The response of particular sensors can vary if new junctions are created by abrasive wear. Furthermore, the response of these sensors was found to deviate substantially from the one dimensional model and varied from sensor to sensor. The impulse response was simulated with greater fidelity using a two dimensional finite element model, but three dimensional effects also appear to be significant. The impact of these variations on the derived heat flux is assessed for the case of measurements in an internal combustion engine. When the measured impulse response is used to derive the surface heat flux, the apparent reversal of heat flux during the expansion stroke does not occur

A Robust Method for Tuning Photoacoustic Gas Detectors

Detection of gases in industrial contexts is of great importance for ensuring safety in storage and transport, so as to limit atmospheric pollution and precisely control industrial and agricultural processes. Although chemical sensors are in widespread use, solid-state infrared detectors for gas sensing promise numerous advantages over conventional catalytic detectors in terms of sensitivity, calibration requirements, and lifetime. The laser-modulation photoacoustic approach is an alternative. Compared to other approaches, it provides more precise measurements with a stable zero baseline, as well as having significantly less complicated optics than cavity ringdown approaches. One enduring problem, though, is the relatively long time required to make photoacoustic measurements. The key contribution of this paper to the industrial context is twofold: first, we show how a sensitive dual-buffer acoustic resonator may be fabricated using 3D printing, and secondly we describe a method for localizing the peak absorption more rapidly than stepping a laser through the gas absorption profile. Modelling of the proposed approach demonstrates its potential, and the expected results are confirmed using an extensive experimental setup for the detection of methane in air

Time-resolved stagnation temperature measurements in hypersonic flows using surface junction thermocouples

Fast-response coaxial surface junction thermocouples have been used to measure time-resolved stagnation temperature of the Mach 6 flow produced by the University of Southern Queensland’s hypersonic wind tunnel, TUSQ. The piston compression and the nozzle expansion of the test gas were found to be approximately isentropic for the first 65 ms of flow. Thereafter, the stagnation temperature reduces from due to the heat lost to the cold barrel, and this process can be modelled based on the measured barrel pressure history to simulate the stagnation temperature in TUSQ to within 2% of the actual value for the first 150 ms of flow. By operating the thermocouples at the flow stagnation temperature, the fluctuations of the flow stagnation temperature were investigated. A 3–4 kHz narrowband stagnation temperature fluctuation appearing after was measured, and found to be correlated with the transition to turbulence of the flow in the barrel

Measurements of freestream density fluctuations in a hypersonic wind tunnel

Density disturbances in the freestream of the University of Southern Queensland’s Mach 6 wind tunnel (ρ∞≈34gm−3) have been measured using a focused laser differential interferometer (FLDI). The direct contribution of the turbulent shear layer from the Mach 6 nozzle to the FLDI signal was largely eliminated by mechanically shielding the FLDI beams from these effects. This improvement significantly enhanced the low wavenumber FLDI spectra which allowed a von Kármán spectrum fit and demonstrated a −5/3 roll-off in the inertial subrange and enabled the identification of the integral length scale (28–29 mm). The normalised root-mean-square density fluctuations were found to change over the flow duration (typically between 0.4 and 0.6%) for the 1–250 kHz frequency range which corresponds to the wavenumber range of 6– 1600 m−1 in this Mach 6 flow. Previous disturbance measurements using intrusive methods have identified a narrowband 3–4 kHz disturbance that is first measured in the core flow about 65 ms after the flow begins and remains until the flow terminates. The onset of this narrowband disturbance was previously correlated with transition-to-turbulence in the subsonic test gas supply to the nozzle. This correlation was investigated further herein, and the 3–4 kHz feature was inferred to be entropy mode disturbances by showing the departure of the FLDI measurements from Pitot pressure measurements. Through the comparison of FLDI and Pitot pressure data, Pitot pressure probes were demonstrated to produce a poor estimate of the static pressure fluctuations when non-isentropic disturbances are non-negligible

Laser-Induced Diaphragm Rupture for Improved Sequencing and Repeatability in a Hypersonic Facility

For hypersonic facilities where the flow conditions are established through the rupture of a diaphragm, such as in the University of Southern Queensland’s hypersonic wind-tunnel facility, the variability in the flow conditions is related to the uncertainty of the pressure at which the diaphragm ruptures. Variability in the diaphragm rupture pressure also results in uncertainty of the time at which the diaphragm will rupture. For experiments that require knowledge of when the test flow will be initiated, the sequencing of events relative to the flow onset is difficult when the flow is initiated using the natural rupture of a diaphragm. The challenge of experiment sequencing that arises due to rupture pressure variability is addressed by introducing a laser for rapid thermal weakening of the diaphragm. Event sequencing challenges are discussed in the context of free-flight testing, including model release strategies for such testing. The work proceeds through a review of Ludwieg tube flow initiation strategies and a discussion of the present context, which requires a reliable method for sequencing the retraction of the free-flight model holder. The natural variability of strength of the Mylar diaphragms in the present work is found to result in around ±6% uncertainty in rupture pressure. This rupture pressure variability is demonstrated to have a significant temperature dependence through empirical results and engineering models. Implementation of the laser-induced diaphragm rupture method is demonstrated to enhance repeatability in generating the flow conditions; the variability in rupture pressure was reduced to ±2% when the laser method was used. Based on the remaining sequencing uncertainties with the laser-induced rupture method and practical speeds for model platform retraction, uncertainty in the positioning of the free-flight models at the time of flow onset is shown to be ±2  mm

Super-orbital re-entry in Australia - laboratory measurement, simulation and flight observation

There are large uncertainties in the aerothermodynamic modelling of super-orbital re-entry which impact the design of spacecraft thermal protection systems (TPS). Aspects of the thermal environment of super-orbital re-entry flows can be simulated in the laboratory using arc- and plasma jet facilities and these devices are regularly used for TPS certification work [5]. Another laboratory device which is capable of simulating certain critical features of both the aero and thermal environment of super-orbital re-entry is the expansion tube, and three such facilities have been operating at the University of Queensland in recent years[10]. Despite some success, wind tunnel tests do not achieve full simulation, however, a virtually complete physical simulation of particular re-entry conditions can be obtained from dedicated flight testing, and the Apollo era FIRE II flight experiment [2] is the premier example which still forms an important benchmark for modern simulations. Dedicated super-orbital flight testing is generally considered too expensive today, and there is a reluctance to incorporate substantial instrumentation for aerothermal diagnostics into existing missions since it may compromise primary mission objectives. An alternative approach to on-board flight measurements, with demonstrated success particularly in the ‘Stardust’ sample return mission, is remote observation of spectral emissions from the capsule and shock layer [8]. JAXA’s ‘Hayabusa’ sample return capsule provides a recent super-orbital reentry example through which we illustrate contributions in three areas: (1) physical simulation of super-orbital re-entry conditions in the laboratory; (2) computational simulation of such flows; and (3) remote acquisition of optical emissions from a super-orbital re entry event

Microwave technique for liquid water detection in icing applications

The partial melting of ingested ice crystals can lead to ice accretion in aircraft compressors, but accurately measuring the relatively small fraction of liquid water content in such flows is challenging. Probe-based methods for detecting liquid water content are not suitable for deployment within turbofan engines, and thus alternatives are sought. Recent research has described approaches based on passive microwave sensing. We present here an approach based on active microwave transmission and reflection, employing a vector network analyzer. Utilization of both transmission and reflection provides additional data over and above emission or transmission only, and permits a more controllable environment than passive sensing approaches. The paper specifically addresses the question of whether such an approach is viable within the context of representative icing wind tunnel and engine flow conditions. A quasi-thermal equilibrium approach is presented herein to estimate the melting ratio during microwave analysis of samples at 0 °C. Experimental results using microwaves in the 2.45GHz region are presented, and post-processing methods investigated. This is followed by an investigation of detection limits for ice accretion in the sub-gram range. The results indicate the potential of the technique, with a number of avenues evident for further research