Design and Simulation of Two Stroke Engines

Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
Eq. 1.5.22 predicts higher power and thermal efficiency for increasing compression ratio.
Thermal efficiency is inversely related to brake specific fuel consumption, which can be seen
from Eqs. 1.6.4 and 1.6.5, i.e., the higher the thermal efficiency, the lower the specific fuel
consumption. That this holds true in practice is seen in Fig. A7.1. The bmep rises and the bsfc
falls with increasing compression ratio, although the rate of change is beginning to decrease
between a CRt of 7.5 and 8.0. The power output has increased and the bsfc has decreased by
about 8% over the CRt simulation range from 6.5 to 8.0.
Eq. 1.5.22 predicts that the change of thermal efficiency for the same change of compression
ratios, i.e., from 6.5 to 8.0, is 7.2%. Thus even the most fundamental of thermodynamics
does give some basic guidance. However, it should be pointed out that Eq. 1.5.22 forecasts a
thermal efficiency of 56.5% for a compression ratio of 8.0, and 52.7% for a CRt of 6.5, when
the actual values are less than half that!
The effect of compression ratio on nitric oxide (NO) emissions
Fig. A7.1 shows the brake specific nitric oxide emissions, bsNO, and the total nitric oxide
emissions, NO, in g/kWh and g/h units, respectively. A somewhat dramatic increase with
compression ratio is observed in both cases. The bsNO value has increased by 26.7% from
4.5 to 5.7 g/kWh. The total NO emission has gone from 18 to 25 g/h, which is an increase of
39%. The word dramatic is not too strong a word to employ, as those engineers who have to
meet emissions legislation usually struggle with gains or losses between 1 and 2% during
R&D experimentation, and will visibly blanch at a simulation indicating changes of the order
shown above.
The origins of the change are to be found in Figs. A7.2 and A7.3. In Fig. A7.2 the peak
temperatures in the burn zone are shown to rise with respect to CRt from 2340°C to 2390°C.
As commented on in Appendix A4.1, the formation of NO is exponentially connected to these
temperatures, so that even an apparently modest rise of only 50°C above 2340°C has the
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537