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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