“Tough Gear”and “High Quality”for New X-TRAIL - 3. Engine Evolution of e-POWER Fig. 7 Engine fuel efficiency and engine operating frequency3.2 Maximum output point and quietnessFig. 8 Maximum output speeds at different compression ratiosFig. 9 Comparison of vertical piston system and piston forceConventionally, the fixed-point operation of the e-POWER system is set at around 2350 rpm to balance energy management. However, it is now possible to perform energy management at the best fuel-consumption point of engine operation through the increased options for the engine operating point attributed to the large increase in torque.Changing the engine speed of this fixed-point operation (2350 → 2000 rpm) improves quietness considerably, which reduces engine noise by about 2 dBA. This helps eliminate the annoying engine noise while driving, which makes it possible to provide customers with the EV-ness feeling.Now, more than 80% of the operating time can be provided in “WLTC + Extra High” mode at the best fuel efficiency point by achieving high efficiency and quietness while satisfying energy management requirements (Fig. 7).The C/R of a general engine requires a design that considers the balance between fuel economy demand and output requirements. The VCR mechanism allows the C/R to be selected freely (ranging from 8.0 to 14.0) because of the VC-TURBO engine, which allows for performance-based design exclusively for the maximum output point; therefore, “C/R = 8.0” is selected. The maximum output of 106 kW is achieved at 4400 rpm (for domestic market).This C/R setting and engine sound are compared with those of a general engine; the comparative engine setting is assumed to be C/R = 10.5. At this C/R, the maximum output point needs to be set at 5600 rpm to avoid knocking (abnormal combustion), whereas the VC-TURBO engine allows the maximum output point to be set at 4400 rpm. The engine speed can be set lower by 1200 rpm, and therefore, the engine noise is reduced by approximately 3.5 dBA under high-load conditions such as during continuous high-speed driving.3.3 Sound quality of 3-cylinder VC-TURBO engineThe sound pressure level of the engine at the maximum output point can be reduced considerably; however, it is not possible to completely block the engine noise from the driver. Thus, the VC-TURBO engine is designed to create a pleasant sound unlike that generated by a conventional 3-cylinder engine(3).19In a conventional 3-cylinder engine, the main component of engine speed is the 1.5th order attributed to torque fluctuations, and therefore, the tone of the engine sound is full of low frequencies.In contrast, the VC-TURBO engine has a high-order component in the vertical inertial excitation force of the piston because the piston motion (Fig. 9) has different ascending and descending strokes unique to the multilink mechanism.Fig. 9 shows the waveforms of the pistonʼs vertical inertia force of each cylinder and the graph of the resultant force at each crank angle.
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