The favourable firing order would permit that. The engine torque variation would be higher, the engine unbalances too, but from the point of view of torsional vibrations some advantage will be gained.

The calculations show a slight decrease of the intermediate shaft maximum torsional stress due to the 6/I harmonic at the resonance (48 rpm). This achievement would be possible with an electronic fuel injection able to cope with very small changes in the load and engine speed. After careful judgement was not considered as an interesting solution due too small excitation decrease for the relatively complex change in the injection management in order to achieve this result.

　

Fig. 4 Cylinder pressure: 6RTA62 engine, 44 rpm, propeller law, combustion starts at TDC

　

Fig. 5 Cylinder pressure: 6RTA62 engine, 44 rpm, propeller law, combustion starts at +5 deg. ATDC

　

Fig. 6 In Cylinder pressure, 6RTA62, 48 rev/min., combustion starts at +25 deg. ATDC

　

5. COMBUSTION TIMING VARIATION FOR EXCITATION AMPLITUDE REDUCTION

　

The decrease of any of the engine torque's harmonic components means for the same dimensions of the engine, bore & stroke, the decrease of the correspondent excitation. The novel idea is to decrease the 6^th order only, and to keep the rest of the harmonic components of the engine torque to the required level in order to operate the engine with the same m.i.p. on the propeller law. The interest was focused on the M₆ because it was responsible for the dangerous resonance within the barred speed range. The timing interval in computer simulations for the combustion start was chosen between [- 30 deg. Before Top Dead Center (BTDC), up to +30 deg. After Top Dead Center (ATDC)] with a 5 deg, step. For three operational points of the engine, within the barred speed range, careful investigations were performed [3]:

　

・ 44 rpm - the left resonant flank for the 1/6^th harmonic component;

・ 48 rpm - the critical speed and resonance with the first mode of vibration 1/6^th.

・ 53 rpm - the right resonant flank for the same harmonic.

The 44-53 rpm interval is the barred speed for this engine in the specified application. The cylinder pressure and based on it, the Fourier analysis of the tangential pressure (amplitudes and phases) for 24 harmonics were analysed and presented for all three points.

　

6. ANALYSIS OF THE CYLINDER PRESSURE

　

The load for the three points on the propeller law was as follows:

　

Table 2 Propeller's law

　

For all three points the top cylinder pressure is reached at TDC for early injection - BTDC, the normal (standard) combustion start being at TDC, Fig. 4. The top pressure increases as the injection (combustion) starts earlier, reaching the values:

　

・ 44 rpm: approx. 79 bar, for -30 deg. BTDC combustion start;

・ 48 rpm: approx. 87 bar, for -30 deg. BTDC combustion start;

・ 53 rpm: approx. 97 bar, for -30 deg. BTDC combustion start;

　

For late injection and combustion starting (ATDC) there are two pressure maxim within the same cycle (Fig. 5) and from +15 deg. ATDC for all speeds, the combustion pressure is lower than the compression pressure. These _"special" conditions suggested a poor possibility in the self-ignition of the injected fuel (Fig. 6).

A theoretical remedy for this phenomenon would be:

BACK　　　CONTENTS　　　NEXT