Ray Malpress

Investigation into the flow physics of hypersonic variable geometry inlet starting

A coupled numerical-experimental investigation studying the applicability of a Sliding Doors variable geometry inlet starting mechanism for a 2D SCRAMjet inlet at Mach 6 has been undertaken. Door opening speeds ranging from 3 to 20ms repeatedly produced started inlet conditions when tested within the TUSQ Low-Enthalpy Wind Tunnel. Transient flow during door retraction was captured both quantitatively and qualitatively via surface pressure measurements and Schlieren visualization techniques. A blunt wedge protruding upstream into the combustor from the rear of the model was also installed, simulating a large pressure rise within the combustor at the centerline. Inlet unstart was encountered for a wedge position protruding 7mm upstream into the combustor, with door opening speeds as low as approximately 1ms were seen to have no visible effect on inlet startability. A viscous, transient, time accurate numerical investigation was conducted, recreating the experimental conditions by solving the Reynolds-Averaged Navier-Stokes equations. These simulations provide insight into pockets of the flowfield for which optical access was not available, as well as also allowing faster door opening speeds to be employed which were not mechanically possible in the tunnel. It was found that even extreme door opening speeds of 0.1ms were not able to start the more aggressive forward wedge positions. Interactions between shock structures around the leading edge of the wedge and the boundary layer at the rear of the combustor were seen to supersede the positive effects of employing unsteady effects.

Simulation of instantaneous heat transfer in spark ignition internal combustion engines: unsteady thermal boundary layer modeling

Simulation of internal combustion engine heat transfer using low-dimensional thermodynamic modeling often relies on quasisteady heat transfer correlations. However, unsteady thermal boundary layer modeling could make a useful contribution because of the inherent unsteadiness of the internal combustion engine environment. Previous formulations of the unsteady energy equations for internal combustion engine thermal boundary layer modeling appear to imply that it is necessary to adopt the restrictive assumption that isentropic processes occur in the gas external to the thermal boundary layer. Such restrictions are not required and we have investigated if unsteady modeling can improve the simulation of crank-resolved heat transfer. A modest degree of success is reported for the present modeling, which relies on a constant effective turbulent thermal conductivity. Improvement in the unsteady thermal boundary layer simulations is expected in the future when the temporal and spatial variations in effective turbulent conductivity are correctly modeled.

Simulation of Instantaneous Heat Transfer in Spark Ignition Internal Combustion Engines: Unsteady Thermal Boundary Layer Modelling

Simulation of internal combustion engine heat transfer using low-dimensional thermodynamic modelling often relies on quasi-steady heat transfer correlations. However, unsteady thermal boundary layer modelling could make a useful contribution because of the inherent unsteadiness of the internal combustion engine environment. Previous formulations of the unsteady energy equations for internal combustion engine thermal boundary layer modelling appear to imply that it is necessary to adopt the restrictive assumption that isentropic processes occur in the gas external to the thermal boundary layer. Such restrictions are not required and we have investigated if unsteady modelling can improve the simulation of crank-resolved heat transfer. A modest degree of success is reported for the present modelling which relies on a constant effective turbulent thermal conductivity. Improvement in the unsteady thermal boundary layer simulations is expected in future when the temporal and spatial variation in effective turbulent conductivity is correctly modelled.© 2009 ASME

Air motor for improved engine brake efficiency Design and preliminary experiments

The brake efficiency of a throttled internal combustion engine is reduced at low load operation because of the engine work required to drop the intake manifold pressure. These throttling losses are experienced by all throttled engines operating at less than wide open throttle (WOT). By replacing the throttle plate with a suitable air motor, work can be recovered in an expansion process that reduces the induced air pressure to the same intake manifold pressure as the throttled engine. To maximize the benefits from coupling the air motor to the engine cycle, the air should be returned to a thermal state identical to that of the throttled case at some point prior to combustion. This might be achieved either: (i) prior to cylinder compression via regenerative heat transfer to the inducted air; or (ii) through cylinder compression at an increased compression ratio. The work generated by the Induction Air Motor (IAM) can be directly applied to the engine output thereby increasing the brake efficiency for the same indicated work. This paper reports on the performance of an IAM designed to reduce intake pressure of an engine for low load operation. Increased brake efficiency will be achieved. The IAM design specifications are explored using a numerical model including isentropic efficiency, friction and service life considerations. A prototype has been constructed and was bench tested at flows and pressures comparable to a throttled engine. These tests indicated that the modelled friction was lower than the friction measured during the experiments. From the experiments performed with the prototype, the net performance of an IAM will give efficiency improvements in excess of 5% for an equivalent throttled engine operating at loads in the range up to 10 % of its WOT power.

A Comparison Between Two-Position Variable Compression Ratio and Continuously Variable Compression Ratio Engines Using Numerical Simulation

Fuel consumption for the New European Driving Cycle (NEDC) is assessed via numerical simulation for a vehicle operating with two types of variable compression ratio device: i) a continually variable compression ratio (VCR) device that optimises efficiency at all loads, and ii) a VCR device that allows the engine to operate at one of two discrete compression ratios. The simulated engine configuration uses late intake valve closing (LIVC). A maximum geometric compression ratio (GCR) of 17:1 is adopted in the simulations resulting in a constant effective compression ratio of 10.2:1 in all configurations. Reduction from full load is achieved in the simulation with LIVC until the maximum GCR is reached after which lower loads are achieved through throttling. In the two-position VCR engine simulation, the full load range is achieved through throttling in combination with LIVC. At part load, in combination with LIVC, the VCR devices increase the geometric compression ratio to return the effective compression ratio to that for full load in each case. The simulations indicate that the increase in net fuel consumption over a driving cycle is effectively no different for the two-position VCR engine relative to a continually variable CR and this justifies further research into two-position VCR technology. Net fuel consumption can also be improved by the use of a limited acceleration that maintains the engine in the reduced compression stroke configuration. An acceleration rate with a driver feedback mechanism is proposed which, in combination with a two-position VCR engine, shows potential for significant reduction in fuel consumption of greater than 15% relative to the full compression, fixed CR configuration for the NEDC.