CALCULATION OF CRANK MECHANISM GEOMETRIC DIMENSIONS ACCORDING TO THE RESULTS OF PERFORMANCE DIAGRAM

Abstract

The article proposes a method of joint calculation of the length of the diesel connecting rod and the average index of polytropic compression n when performing the algorithmic synchronization of indicator diagrams of internal combustion engines. In the author’s previous publications, it was shown that the angle that corresponds to the maximum rate of compression pressure growth depends on the geometric dimensions of the crank-andconnecting mechanism parts, the volume of the compression chamber, and the polytropy index. The latter value varies depending on the conditions of the compression process. In addition, strictly speaking, a situation with a shortage of data regarding the geometric dimensions of the crank-connecting mechanism, namely, the length of the connecting rod, is quite likely. Therefore, to perform algorithmic synchronization in conditions of data scarcity, it is necessary to search for the length of the connecting rod and the value of the polytropic index together. To do this, it is suggested to first approximate the section of the experimental compression curve with a fourth-order polynomial. Experimental data are used to calculate its coefficients. The end of the approximating section, which corresponds to a larger value of the angle of rotation of the crankshaft, lies at the point of the fastest increase in compression pressure. The accuracy of the determination of unknown quantities as a result of the solution of the proposed equations is acceptable for engineering calculations. This makes it possible to clarify the preliminary calculation of the position of the top dead center of the indicator diagram in conditions of a lack of data on the engine, since the obtained value of the length of the connecting rod and the correspondingly calculated value of the polytropic compression coefficient more fully reflect the current conditions of the compression process in the cylinder of the internal combustion engine.