Mechanical efficiency of diesel engine based on pressure measurements of all cylinders

Abstract: The mechanical efficiency of a diesel engine is of great importance as an indicator to evaluate the mechanical loss. It also has a significant impact on economic performance and dynamic performance for the diesel engine. Many scholars have done a lot of work on the measurement of the mechanical efficiency of the multi-cylinder engine. In addition to the numerical calculation with computers, the traditional measurement method of mechanical efficiency common used by scholars can be summarized into four types as follows: an indicated diagram based on one cylinder pressure data, power measured under a motoring condition, the fuel cut method, and a fuel consumption characteristic curve. For the non-uniformity on the structure, intake airflows, fuel injection, and combustion process on the multi-cylinder engine, some errors, without considering the difference of indicated work between different cylinders on the condition of different engine speeds and loads, can be seen for the traditional measurement method of mechanical efficiency based on the pressure data of one cylinder. In this paper, the research object for mechanical efficiency chosen was a V-type turbocharged diesel engine with eight-cylinders. The pressure data of all cylinders was measured at different engine speeds and loads respectively by using the instantaneous pressure sensor of KISTLER and the combustion analyzer of DEWETRON. All cylinder pressure data was measured at the speeds of 1100、1300、1500、1700、1900、and 2100 r/min with relevant loads while the diesel engine was working in the steady state. To ensure the accuracy of the measurements, the pressure data were collected once each 1 crank angle. In each case, the combustion analyzer would average the cylinder pressure data that was collected continuously over 50 cycles. Then the indicated work of all cylinders and the total indicated power for the engine would be obtained by computing the average cylinder pressure data which can largely eliminate the effects of any cycle by cycle variation. Furthermore, the non-uniformity of performed work of the engine was analyzed on the basis of all of the cylinders' indicated work. Meanwhile, the mechanical efficiency and its fluctuation rules under different speeds and different loads could be obtained with the effective power tested by the eddy current dynamometer.It can be concluded that the indicated work of different cylinders at the same case is largely different from each other, which means the non-uniformity of the performed work of the engine changed significantly. Based on the analysis of the data, it can be induced that the non-uniformity of the performed work of the engine was reduced with effective power increasing. The application of the full cylinder pressure measurement and the calculation method of mechanical efficiency can avoid measurement errors caused by the non-uniformity of the engine and improve the accuracy of the results. At the same speed, the mechanical efficiency of an engine increases as the power increases, which can be curved as a cubic polynomial relationship between the two factors. In the case where the power remains constant, the mechanical efficiency of the diesel engine increases as the speed decreases, which can be curved as an approximate linear relationship between the factors.