8. TORSIONAL STRESS CALCULATIONS
Stress calculations for the proposed shafting configuration were performed with the original excitation (standard engine settings & propeller law, Fig. 3), and the new optimised excitation for +25 deg. ATDC and the propeller law (Fig. 12). The additional torsional stress within the barred speed range due to the I/6th harmonic component of the engine torque and the synthesis also, showed a dramatic decrease. The following results were obtained for the most critical operational point of the installation at the resonance with the I/6th harmonic at 48 rpm:
These values of the additional torsional stress are well below of Classification Societies prescribed limits and in this case the installation would work without any barred speed range.
9. REFINED INJECTION LAW - PILOT INJECTION AND DELAYED COMBUSTION
The good results from the previous investigations in the torsional vibrations control should be supported by a functional injection law. An important setback of the proposed solution is the low cylinder temperature at the time of combustion at +25 deg. ATDC. A new solution was searched in order to solve this problem. There are two principal interdependent goals:
a) To increase the cylinder temperature at the start of combustion.
b) To maintain the cylinder pressure trend as close as possible to the optimum that was found, in order to preserve the advantageous tangential pressure Fourier analysis spectrum.
At refined injection law that preserved the pressure trace close to the previously determined optimum, and increased in the same time the in cylinder temperature for the self-ignition of the fuel was proposed. A pilot injection before TDC of a proper amount of fuel kept the gas temperature at the required level (over 600 ℃), the main injected quantity of fuel staring to burn at +25 deg. ATDC as previously shown. The higher equivalent load (20%) enabled to have enough fuel reserve for this pilot injection, the required load for the engine operation in this point (48 rpm) on the propeller law being only 8.5%. The obtained pressure trace and the tangential pressure are plotted in Fig. 13 and Fig. 14. The heat release law and cylinder temperature are presented in Fig. 15. As can be seen the temperature is over 900K at the time of ignition. Also can be seen that the I/6th harmonic component amplitude (excitation) still remained at a very small level 0.044 N/mm2, (at 20% load) compared with standard engine settings of about: 0.129 N/mm2, (at 8.5% load) despite of a load increase of over 2 times! The stress calculation for the new injection law at 20% load, 48 rpm, showed a 10 N/mm2 stress in the crankshaft and around 35 N/mm2 in the intermediate shaft, values well below of dangerous limits (Fig. 16). No barred speed range is necessary. Impressive results are also obtained for a 7RTA62 engine. The excitation (the I/7th order harmonic amplitude) was reduced by this pilot injection and delayed combustion from 0.0931 to 0.0156 N/mm2, for the same conditions of load and speed this being a 6 time decrease (Fig. 14).
