The Otto Cycle: Compression Ratio vs
The Otto Cycle: Compression Ratio vs. Efficiency
| Increased compression, by a change in displacement, chamber volume, or both, increases torque throughout the RPM range, and is especially helpful at |
|part-throttle. However, the increase is most effective in cases where the original compression ratio is relatively low. Adding 1 point of compression |
|(e.g., 5.0-1 becomes 6.0-1, a change of +20% in ratio, increases efficiency by 7.79%), is more effective than the same percentage increase beginning with|
|a higher ratio (10.0-1 becomes 12.0-1, also +20%, but only increases efficiency by 4.65%). |
| “Efficiency” refers to the relative use made of the energy contained in the fuel, not the amount or percentage of fuel or air consumed. The |
|efficiency assumes that no other changes are made when the compression ratio is altered, such as quench, valve shrouding, piston dome masking, etc. and |
|that fuel of sufficient octane is used to permit the best ignition advance. |
| To use this table: locate the original compression ratio (“C-Ratio”) in the 1st, 3rd or 5th row across. The table cell directly underneath it will |
|show the efficiency calculated from the formula below, where E = efficiency, R = static (nominal) compression ratio, and K = 1.4 (the ratio of variable |
|heats for air). |
| To compare ratios for efficiency, divide the Efficiency value of the new ratio by the Efficiency value of the original. |
|[pic] |
|C-Ratio |
|4.75 |
|5.00 |
|5.25 |
|5.50 |
|5.75 |
|6.00 |
|6.25 |
|6.50 |
|6.75 |
|7.00 |
|7.25 |
|7.50 |
|7.75 |
|8.00 |
| |
|Efficiency |
|.464 |
|.475 |
|.485 |
|.494 |
|.503 |
|.512 |
|.520 |
|.527 |
|.534 |
|.541 |
|.547 |
|.553 |
|.559 |
|.565 |
| |
|C-Ratio |
|8.25 |
|8.50 |
|8.75 |
|9.00 |
|9.25 |
|9.50 |
|9.75 |
|10.00 |
|10.25 |
|10.50 |
|10.75 |
|11.00 |
|11.25 |
|11.50 |
| |
|Efficiency |
|.570 |
|.575 |
|.580 |
|.585 |
|.589 |
|.594 |
|.598 |
|.602 |
|.606 |
|.610 |
|.613 |
|.617 |
|.620 |
|.624 |
| |
|C-Ratio |
|11.75 |
|12.00 |
|12.25 |
|12.50 |
|12.75 |
|13.00 |
|13.25 |
|13.50 |
|13.75 |
|14.00 |
|14.25 |
|14.50 |
|14.75 |
|15.00 |
| |
|Efficiency |
|.627 |
|.630 |
|.633 |
|.636 |
|.639 |
|.642 |
|.644 |
|.647 |
|.650 |
|.652 |
|.654 |
|.657 |
|.659 |
|.661 |
| |
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