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.