Abstract
The phenomena that take place during the functioning of diesel engines having a classic type fuel injection are complex ones, therefore the corresponding mathematical models can be elaborated mostly based upon the experimental results and data (known as system identification).
In order to improve the performances for vehicles (cars and tractors) equipped with this kind of engine, during the last years, researches have been conducted to obtain some specific mathematical algorithms. These researches are based upon the advantages offered by the analytical solution from the viewpoint of experimenting with compression ignition engine. Thus in the case of mathematical simulation, defining an engine performance map (considering as variables engine torque, engine speed and specific fuel consumption) involves up to 20-25 points (useful as input data in the case of analytic solution) against 250-300 points (as necessary if the map is experimentally made).
For the studied engine (D115, used for Romanian made off road vehicles and tractors), it was considered the specific fuel consumption as a multinomial function of engine torque and speed with aij type coefficients. The determination of these coefficients was made so that the values obtained with the use of the theoretical function would be as closer as possible from the ones that are experimentally determined (the smallest square method).
The input data used to solve the mathematical model were the values obtained by summing the data drawn while doing the characteristics for speed engine on stand. Stationary conditions were applied then the differential equation systems were solved and the mathematical model for the specific fuel consumption was this way obtained.
The ensemble of the points indicating the minimum specific fuel consumption gives the optimum working curve of the studied engine. To obtain the analytic expression of this curve, the equal slope theorem was used, thus resulting a engine torque-engine speed function for whom the fuel consumption is minimum for each level of constant power.
Based on these studies and also taking account of the values of the multiple correlation coefficient R2, the mathematical simulation is mainly made upon a second degree multinomial function.
In order to outline the curve, the izoconsumption curves have to be drawn as well as the speed engine characteristic at full load and the curves of power at different constant levels.
The results offered by the mathematical simulation have been compared with the ones experimentally obtained in real conditions (traction tests on circuit) and with the help of the values of R2 (greater than 0,95) the accuracy of the mathematical simulation was proven (determinist relation).
The elaboration of the optimum working curve for a diesel engine constitutes the most important step in conceiving of an on-board informational system which helps reducing the engine fuel consumption.
Keywords: diesel engine, optimum working curve, on-board informational system