On Gas Dynamics of Exhaust Valves

With increasing effects of global warming, efforts are made to make transportation in general more fuel efficient. When it comes to internal combustion engines, the most common way to improve fuel efficiency is through ‘downsizing’. Downsizing means that a smaller engine (with lower losses and less weight) performs the task of a larger engine. This is accomplished by fitting the smaller engine with a turbocharger, to recover some of the energy in the hot exhaust gases. Such engine systems need careful optimization and when designing an engine system it is common to use simplified flow models of the complex geometries involved. The exhaust valves and ports are usually modelled as straight pipe flows with a corresponding discharge or loss coefficient, typically determined through steady-flow experiments with a fixed valve and at low pressure ratios across the valve. This means that the flow is assumed to be independent of pressure ratio and quasi-steady. In the present work these two assumptions have been experimentally tested by comparing measurements of discharge coefficient under steady and dynamic conditions. The steady flow experiments were performed in a flow bench, with a maximum mass flow of 0.5 kg/s at pressures up to 500 kPa. The dynamic measurements were performed on a pressurized, 2 litre, fixed volume cylinder with one or two moving valves. Since the volume of the cylinder is fixed, the experiments were only concerned with the blowdown phase, i.e. the initial part of the exhaustion process. Initially in the experiments the valve was closed and the cylinder was pressurized. Once the desired initial pressure (typically in the range 300-500 kPa) was reached, the valve was opened using an electromagnetic linear motor, with a lift profile corresponding to different equivalent engine speeds (in the range 800-1350 rpm). The results of this investigation show that neither the quasi-steady assumption nor the assumption of pressure-ratio independence holds. This means that if simulations of the exhaustion process is made, the discharge coefficient needs to be determined using dynamic experiments with realistic pressure ratios. Also a measure of the quasi-steadiness has been defined, relating the change in upstream conditions to the valve motion, i.e. the change in flow restriction, and this measure has been used to explain why the process cannot be regarded as quasi-steady.QC 20170306