The effect of bore-to-stroke (B/S) ratio on indicated specific fuel consumption (ISFC) and emissions of a gasoline-fueled, spark-ignited, single-cylinder engine was studied while holding compression ratio and bore diameter constant. From the baseline B/S of 1.1, B/S was increased to 2.2 and 3.3 by shortening the stroke length. The top dead center (TDC) piston position was raised with increasing B/S, since clearance volume must be decreased with stroke for a constant compression ratio. Test results based on lean air/fuel ratios, a wide open throttle, and maximum brake torque spark advance showed that increased ISFC and hydrocarbon exhaust emissions were primary deterrents to operation at a larger B/S in screening tests of B/S ratios (1.1, 2.2, and 3.3) at a 16.5:1 air/fuel ratio and 1500 rpm. A study of combustion chamber variations at 1.1 and 2.2 B/S ratios, an 18:1 air/fuel ratio, and engine speeds from 750 to 3000 rpm showed that ISFC was about 18% higher at 2.2 B/S than at 1.1 B/S. Cylinder heat loss at a larger B/S, expressed as a percent of fuel energy, could be minimized by reducing the combustion chamber surface-to-volume ratio at TDC and by increasing engine speed. Combustion duration could be reduced at a large B/S by more compact chamber geometries, more centralized or dual ignition, and increased mixture motion. The hydrocarbon emission index increased with surface-to-volume (S/V) ratio, decreased with increased engine speed, and was relatively independent of combustion duration. The nitric oxide emission index decreased with increased S/V ratio and decreased with increased combustion duration. The carbon monoxide emission index was relatively independent of S/V ratio, decreased with increased engine speed, and increased with combustion duration.

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  • Supplemental Notes:
    • Presented at SAE International Fuels and Lubricants Meeting, Toronto, 13-16 November 1978.
  • Corporate Authors:

    Society of Automotive Engineers (SAE)

    400 Commonwealth Drive
    Warrendale, PA  United States  15096
  • Authors:
    • Siewert, R C
  • Publication Date: 1978

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Subject/Index Terms

Filing Info

  • Accession Number: 00399225
  • Record Type: Publication
  • Source Agency: National Highway Traffic Safety Administration
  • Report/Paper Numbers: SAE 780968, HS-025 911U
  • Files: HSL, USDOT
  • Created Date: Oct 31 1985 12:00AM