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IV.2- Compression ratio and maximum cylinder pressure
The choice of a compression ratio 16 to 1 was led by the high specific power output required at high engine speeds for the 127 kW engine. It was obtained thanks to an increase of the combustion bowl volume.
As depicted on figure 11, a trade-off exists between specific power level and in-cylinder maximum pressure, for each compression ratio. The lower is the compression ratio, the lower is the maximum cylinder pressure level, as far as pressure increase during the compression
stroke is reduced. Besides, for a given compression ratio reduction, the shift in maximum pressure increases with specific power:

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Figure 11: Maximum cylinder pressure level as a function of compression ratio and specific power at 4000 rpm

In order to reach the target of 63.7 kW per liter, two evolutions were necessary with regard to
the 1.9 dCi Euro3 reference engine (47 kW/l):
• a 30 bar increase of the maximum cylinder pressure level : ! ! "
• a volumetric compression ratio decreased to 16 (instead of 18) : " ! #

At 63.7 kW per liter, the maximum cylinder pressure reduction with a compression ratio of 16 to 1 is close to 30 bar. This was a determining factor to limit the additional cost on the engine base (in particular lower parts) and to keep engine frictions low (with a favorable impact on
fuel consumption).
The reduction of compression ratio was also favorable for full load torque at low engine speeds. Indeed, the increase of the combustion bowl volume is advantageous on such engine operating conditions since the combustion phase is mainly located within the combustion bowl. The fuel efficiency at part load was not affected by this reduction of compression ratio.
The increase of HC and CO emissions inherent to the reduction of compression ratio, were managed via the application of multiple injection strategies, the improvement of exhaust catalyst and the optimization of engine calibration during cold operating conditions (intake throttling,…).