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

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 finite difference routine for the solution of transient one dimensional heat conduction problems with curvature and varying thermal properties

The implicit finite difference routine described in this report was developed for the solution of transient heat flux problems that are encountered using thin film heat transfer gauges in aerodynamic testing. The routine allows for curvature and varying thermal properties within the substrate material. The routine was written using MATLAB script. It has been found that errors which arise due to the finite difference approximations are likely to represent less than 1% of the inferred heat flux for typical transient test conditions

Heat transfer during transient compression: measurements and simulations

Experiments have been performed to assess the utility of unsteady one-dimensional heat conduction modelling for the calculation of heat losses during a free piston compression process. Heat transfer measurements have been obtained within a gun tunnel barrel using surface junction thermocouple instrumentation. The gun tunnel was operated with a relatively heavy piston such that the shock waves induced by the piston motion were weak. Peak heat transfer values are estimated reasonably well by the unsteady one-dimensional model. However, overall quantitative agreement between the measurements and calculations has not been achieved at this stage, principally because the development of turbulent heat transport was not properly modelled

Cranz-Schardin visualisation of a hypersonic cone with gas injection

We have developed a five-picture Cranz-Schardin system for Schlieren flow visualization on a gun tunnel facility at the University of Southern Queensland to aid the study of unsteady shock systems in nominally steady hypersonic flows. The system produces useful images at framing rates up to about 1 MHz even though the system development was constrained by a very modest budget. The system uses multiple LED light sources driven by an in-house designed device that delivers a high current pulse to each LED with a programmable time delay between each pulse. The images are captured using four separate, black and white video devices and one digital still camera. The utility of the system is demonstrated by imaging gas injection from an annulus on a 10 degree half angle cone positioned at the exit of the contoured Mach 7 nozzle. Visualisation of the cone without gas injection demonstrates that the half angle of the conical shock is approximately 13.9 degrees (the Taylor-Maccoll conical shock angle at Mach 7 for an inviscid cone half angle of 10 degrees is 12.9 degrees). The gas injection condition used in these experiments disturbed the flow field upstream of the injection point to such an extent that the thickness of the shocked flow at the point of injection was larger than the no-injection case by a factor of approximately two. The conical shock angle in the case of injection increased to approximately 19 degrees, and a variation in this shock angle of approximately 1 degree was observed during the nominally steady, facility run time

Computational simulation of the wind-force on metal meshes

[Abstract]: Estimating the drag force on a metal mesh is important if the installed metal mesh area is large and wind speeds are moderate. Such a situation may arise if a metal mesh is used to protect a property from ember attacks in a bushfire prone area. In this work, computational simulations are used to correlate the drag coefficient of a metal mesh in terms of its porosity and the Reynolds number for 10 ≤ Re ≤ 1000. A benchmarking exercise suggests that the computational simulations may be in error by up to 13 % error for the level of discretization that could be achieved due to computer memory limitations. The drag coefficient correlation we have obtained has a maximum error of only 6.5 % with respect to the results from the simulations

Premature ignition in scramjets with intake injection: a preliminary laminar mixing layer simulation

Injecting fuel on the intake of scramjet engines is one strategy that might be used to minimize the required length of scramjet combustion chambers. Premature ignition must be avoided for the strategy to be viable. Premature ignition is not normally observed in shock tunnel experiments with compression ignition scramjet configurations even though local regions of elevated temperatures sufficient to support combustion would have been present on the model scramjet intakes. However, for full scale flight vehicles, we cannot conclude that ignition will generally be delayed until the combustion chamber based on limited empirical results from shock tunnel ground-testing. Reliable intake/injection design correlations for premature ignition avoidance in a flight scramjet are yet to be developed. Numerical simulation offers an approach for the investigation and identification of premature ignition regimes which should be avoided in compression-ignition scramjets. A particular case of hydrogen injection in the presence of a laminar boundary layer is simulated numerically. The location of the stoichiometric mass fraction of hydrogen occurs very close to the peak mixing layer temperature which is also within the lowest speed region of the mixing layer. An ignition delay correlation is used to demonstrate that ignition will almost certainly occur. This case is offered as an example to highlight the potential problem and perhaps stimulate further study in the area of premature ignition with intake injection

Concentration probe measurements in a Mach 4 nonreacting hydrogen jet

A new probe technique is introduced for the measurement of concentration in binary gas flows. The new technique is demonstrated through application of the probe in a Mach 4 nonreacting jet of hydrogen injected into a nominally quiescent air environment. Previous concentration probe devices have mostly used hot wires or hot films within an aspirating probe tip. However, the new technique relies on Pitot pressure and stagnation point transient thin film heat flux probe measurements. The transient thin film heat flux probes are operated at a number of different temperatures and thereby provide stagnation temperature and heat transfer coefficient measurements with an uncertainty of around ±5K and ±4% respectively. When the heat transfer coefficient measurements are combined with the Pitot pressure measurements, it is demonstrated that the concentration of hydrogen within the mixing jet can be deduced. The estimated uncertainty of the reported concentration measurements is approximately ±5% on a mass fraction basis

Hardware-based engineering problem solving for on-campus and external teams

We contend that engineering analysis and design will continue to rely on the synthesis of experimental observations and theoretical analyses. For the past three years, we have been providing teams of on-campus and external students the opportunity to work with actual engineering hardware as a focus for engineering analysis and problem solving. Providing external teams of student with the opportunity to problem solve with actual engineering hardware represents a number of challenges. By focusing on initial value problems and requiring the teams to design the parameters necessary to achieve the desired system performance, we have been able to expose both on-campus and external teams to problem solving with testable physical systems and actual engineering hardware

Simulation of Oxford University gun tunnel performance using a quasi-one-dimensional model

The performance of the Oxford University Gun Tunnel has been estimated using a quasi-one-dimensional simulation of the facility gas dynamics. The modelling of the actual facility area variations so as to adequately simulate both shock reflection and flow discharge processes has been considered in some detail. Test gas stagnation pressure and temperature histories are compared with experimental measurements at two different operating conditions – one with nitrogen and the other with carbon dioxide as the test gas. It is demonstrated that both the simulated pressures and tem-peratures are typically within 3% of the experimental measurements