Download Iowa State Univ. - Agri. & Biosystems Eng. Homework 2: Engine Analysis and more Assignments Business Management and Analysis in PDF only on Docsity! Agricultural and Biosystems Engineering, Iowa State University Fall 2007 I:\TSM335\HWK2.wpd Homework 2 Question 1: For the following engines: 2 cylinder, 4-cycle; 2 cylinder 2-cycle; 3 cylinder, 4-cycle; 4 cylinder, 4-cycle; 6 cylinder, 4-cycle; 8 cylinder 4-cycle; (a). Determine the average firing interval. (b). Identify those engines with an uneven power strokes. (c). Identify those engine with power strokes that overlap. Assuming engines of equal power were available for each of the engine types determine the order of relative size of flywheel required for each engine. (i.e. From largest flywheel to smallest flywheel). (d). For a 5cylinder, 4 cycle engine, determine the crank angle degrees when intake and exhaust valves would open and close for any 3 of the five cylinders. Assume that a crank angle of 0 degrees corresponds with the piston 1 at TDC at the beginning of the intake stroke. Assume that intake valve opens 15o before TDC and closes 45o after BDC; and that exhaust valve closes 15o after TDC and opens 45o before BDC. The firing order of the engine is 1,5,2,4,3 Question 2: (a). Is the minor diameter (Measured parallel to the piston pin) and the major diameter (measured perpendicular to the piston pin) of a piston the same when the piston is cold? If not, explain the reasons for this difference. (b) If the exhaust valve clearance is too small, would this cause the valve to prematurely fail? If this is the case, give two different reasons this could cause valve failure. (c). After an engine is dismantled the following observations were made on only one of the four engine cylinders (other 3 appeared to have no damage). (1). The front top and back bottom edges of the piston were badly worn in a plane parallel to the crankshaft. Give potential causes for this uneven wear and explain how the wear occurred. Question 3: The stoichiometric A/F ratio for gasoline engines is 15.05 for complete combustion, producing carbon dioxide and water. (a). If the A/F ratio was decreased to 12.04, would the air/fuel mixture be considered a rich or a lean mixture, and would complete combustion occur. (b). Name two additional combustion products that may be produced under these conditions. (c). Determine the mass of all combustion products at the A/F ratio of 12.04, for gasoline (C8H18) (c). An engine is tested with the following A/F ratios, (10, 15, 20, 25, 50, and 75). It was found that the engine would run satisfactorily at A/F ratios above 25. Is this a SI engine or diesel engine? Explain your answer. (d). When would you use a high cetane fuel in a gasoline engine? Explain your answer. Question 3b: Blended fuels are sometimes used in engines. Different ethanol blends can can be used for automobiles. Given the following information: Chemical Formula Density Heating Value Ethanol C2H5OH 0.780 kg/L 21 000 kJ/L Gasoline (Octane) C8H18 0.740 kg/L 35 000 kJ/L Note: For every mole of an oxygen molecule in air, there are 3.76 moles of nitrogen molecules. a). For a 15% Ethanol Blend (by vol) Determine the composite fuel molecule CxcHyc0zc which can be used to represent blended fuel. b). Show the balanced Stoichiometric combustion equation for the composite molecule. c). Calculate the Stoichiometric air to fuel ratio for the fuel blend. Question 4: The point’s T10 , T20 , and T90 refer, respectively to the temperatures on the fuel distillation curve at which 10, 50 and 90 % of the fuel has been distilled. (a). Why is it important that the T10 temperature is not too high? What would the effect of increasing the T10 temperature be on starting an engine in cold weather?
