buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 113
4. Theoretical and Experimental Investigations Results
The results of theoretical and experimental investigations presented in
Fig. 1-6, show good correlation between them.
Supercharging pressures used are in the range of 1.4…1.8 bar.
Comparative to the aspirated engine, the maximum pressure increases with
almost 100 % for a supercharging pressure (ps) of 1.8 bar, Fig. 1. High value of
maximum pressure leads to the limitation of supercharging pressure at 1.4 bar
when the increase of maximum pressure value (~ with 50 % comparative to
classic solution) is acceptable for the engine reliability.
In order to avoid the knocking and to limit the maximum pressure, the
ignition angle value optimization was achieved in such a way that the BSFC is
maintained almost constant for different supercharging pressures (Fig. 2).
For 1.4 bar supercharge pressure the increase BMEP is significant
(~22%) and increase is more at much higher supercharging pressure because of
the influence of ignition timing value (Fig. 3). Also from this reason the
limitation of the supercharging pressure at 1.4 bar is justified.
Due to combustion process improvement the level of CO and HC
emissions decreased (~24% for CO and ~11% for HC) (Fig. 4 and Fig. 6). The
NOx emission level decreases significant for 1.4 bar supercharging pressure
(Fig. 5) fact also influenced by the reduction of spark ignition timing.
Supercharging represents an efficient method of engine downsizing.
Thus, in order to obtain the same power as the reference engine, the engine
displacement must be reduced by 1.5 times for 1.4 supercharging pressure. This
engine displacement reduction can be assured by the limitation of the engine
cylinders number, which also leads to the increase of the engine mechanical
efficiency. Another advantage of supercharging is the reduction of the
maximum power/maximum torque speeds – downspeeding concept. Thus, the
same power of 66 kW at 4800 rpm of the normal aspirated engine was obtained
for supercharging at 1.4 bar at 3900 rpm, and the maximum torque of 137
Nm/3600 rpm was reached at the speed of 2500 rpm. The reduction of the fuel
consumption and engine wear are the main advantages of this concept.
Spark ignition timing modification directly affects the combustion
process variability evaluated by coefficient of variability in maximum pressure.
For 4800 rpm the coefficient of variability in maximum pressure reach the value
of (COV) = 6.574% for λ= 0.85 and 1.4 bar supercharging pressure.
p max
The influences of combustion process on engine running are reflected by
the values of the coefficients of cycle variability in indicated mean effective
pressure. The values of variability coefficient in IMEP are (COV)IMEP=1.986%
for λ= 0.85 at ps = 1.4 bar.