Effects of Compression Ratio of Bio-Fueled SI Engines on the Thermal Balance and Waste Heat Recovery Potential

• Main Authors: Ali Qasemian, Sina Jenabi Haghparast, Pouria Azarikhah, Meisam Babaie
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Sustainability
• Subjects: internal combustion engine; energy balance; compression ratio; ethanol biofuel; waste heat recovery
• Online Access: https://www.mdpi.com/2071-1050/13/11/5921

In internal combustion engines, a significant share of the fuel energy is wasted via the heat losses. This study aims to understand the heat losses and analyze the potential of the waste heat recovery when biofuels are used in SI engines. A numerical model is developed for a single-cylinder, four-stroke and air-cooled SI engine to carry out the waste heat recovery analysis. To verify the numerical solution, experiments are first conducted for the gasoline engine. Biofuels including pure ethanol (E100), E15 (15% ethanol) and E85 (85% ethanol) are then studied using the validated numerical model. Furthermore, the exhaust power to heat loss ratio (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula>) is investigated for different compression ratios, ethanol fuel content and engine speed to understand the exhaust losses potential in terms of the heat recovery. The results indicate that heat loss to brake power ratio (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>W</mi><mo>˙</mo></mover><mi>b</mi></msub></mrow></semantics></math></inline-formula>) increases by the increment in the compression ratio. In addition, increasing the compression ratio leads to decreasing the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula> ratio for all studied fuels. According to the results, there is a direct relationship between the ethanol in fuel content and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula> ratio. As the percentage of ethanol in fuel increases, the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula> ratio rises. Thus, the more the ethanol in the fuel and the less the compression ratio, the more the potential for the waste heat recovery of the IC engine. Considering both power and waste heat recovery, the most efficient fuel is E100 due to the highest brake thermal efficiency and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula> ratio and E85, E15 and E00 (pure gasoline) come next in the consecutive orders. At the engine speeds and compression ratios examined in this study (3000 to 5000 rpm and a CR of 8 to 11), the maximum efficiency is about 35% at 5000 rpm and the compression ratio of 11 for E100. The minimum percentage of heat loss is 21.62 happening at 5000 rpm and the compression ratio of 8 by E100. The minimum percentage of exhaust loss is 35.8% happening at 3000 rpm and the compression ratio of 11 for E00. The most <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>e</mi><mi>x</mi></mrow></msub><mo>/</mo><msub><mover accent="true"><mi>Q</mi><mo>˙</mo></mover><mrow><mi>h</mi><mi>t</mi></mrow></msub></mrow></semantics></math></inline-formula> is 2.13 which is related to E100 at the minimum compression ratio of 8.