The Influence of Sound Speed on the Thermodynamic Data of Diesel Engines Obtained From p, V Measurements

The pressure-time function p(t) of the gas enclosed in the cylinder volume of a piston reciprocating engine can be measured easily, even in the field, with a time resolution better than one degree of crank angle. By proper processing of the p(t) function, knowing the geometry of the engine, it is possible, among other values, to obtain the temperature T(t), the heat exchange q(t), the entropy s(t), the heat release rate dq(t), the heat transfer coefficient h, the power output, the fuel consumption etc. In order to calculate this values, apart from the measured pressure function p(t), its time derivative dp(t)/dt is of great value. Smooth and accurate dp(t)/dt function is very important in obtaining the accurate results. In the field measurements the pressure sensor is placed with the help of a Tompson adapter after the cylinder gas cock while in laboratory measurements some times it is possible to achieve for a flush mounted sensor. Depending on the geometry imposed by the design of each type of engine, the pressure acting on the piston head is not the same with the one being measured at the same time by the pressure transducer. This difference is mainly imposed by the limited gas sound speed and the path between piston head and pressure sensor, the pressure wave has to travel, and may change considerably the shape of the measured indicators diagram. In large long stroke marine propulsion engines, the sound path length can be as long as two meters. In addition to that, sometimes, during a small part of the thermodynamic cycle, a sonic choking may appear at the “throat” where the combustion chamber connects to the tube of the pressure sensor. By this phenomenon the gas flowing to and from the pressure sensor is metered and an additional pressure phase lag is superimposed. The problem described is present even in small spark ignition engines where usually the sensor used includes a conventional spark plug pressure transducer mounted through a small hole to allow for the gas flow. An experimental study has been undertaken in the Laboratory to estimate the order of magnitude of these deviations and the influence on the measured pressure function p(t). Some of the results are given on the present paper.