Downsizing has a significant effect on friction reduction potential. This effect can be explained as follows with the calculation of brake specific fuel consumption (BSFC) at a typical Federal Test Procedure (FTP) cycle operating condition, assuming a constant indicated specific fuel consumption (ISFC).
|BSFC = ISFC × IMEP/BMEP = ISFC × (BMEP + FMEP)/BMEP||(1)|
The BSFC that would result with a typical baseline friction level at a typical FTP cycle operating condition would be as follows, using Equation 1:
BSFC = ISFC (38 + 11)/38 = 1.289 ISFC
Where: BMEP (brake mean effective pressure) = 38 psi (typical FTP cycle operating condition)
FMEP (friction mean effective pressure) = 11 psi (typical baseline friction at FTP operating condition)
Achieving a 10 percent reduction in friction would provide the following improvement in BSFC:
BSFC = ISFC (38 + 0.9 × 11)/38 = 1.261 ISFC
Therefore, a 10 percent reduction in friction will provide a 2.2 percent reduction in fuel consumption. Consequently, a 25 percent reduction in friction will provide a 5.6 percent reduction in fuel consumption, as discussed in the SI Efficiency Fundamentals section of Chapter 2.
For a 50 percent downsized, high BMEP engine, a BMEP level twice that of the naturally aspirated engine would be required for the same operating condition of the vehicle. The BSFC for this engine with the baseline friction is as follows:
BSFC = ISFC (2 × 38 + 11)/(2 × 38) = 1.145 ISFC
Applying the same 10 percent reduction in friction would provide the following improvement in BSFC:
BSFC = ISFC (2 × 38 + 0 .9 × 11)/2 × 38 = 1.130 ISFC
Therefore, a 10 percent reduction in friction in the downsized engine will provide only a 1.3 percent reduction in fuel consumption, which is approximately half the reduction in fuel consumption shown for the naturally aspirated engine. Consequently, a 50 percent downsized engine will require nearly twice the reduction in friction relative to that required in a naturally aspirated engine to achieve the same reduction in fuel consumption.
Even though friction reductions are not as effective in the downsized engine, the downsizing itself provides a significant reduction in friction. Friction power is calculated as follows:
|Friction Power =
K × RPM × Displacement × FMEP
Equation 2 indicates that friction power would be reduced by 50 percent when the engine displacement is reduced by 50 percent, assuming constant FMEP (although FMEP would be expected to show a moderate increase due to the engine redesign to withstand higher BMEP levels).
The effect of 50 percent downsizing can be calculated by comparing the baseline conditions shown previously for both the naturally aspirated and downsized, high BMEP engines as follows:
Naturally Aspirated Engine: BSFC = 1.289 ISFC
Downsized Engine: BSFC = 1.145 ISFC
This comparison indicates that 50 percent downsizing could potentially provide a 11 percent reduction in fuel consumption, with the simplifying assuming of constant FMEP in both engines.