Problem 27
How is the rpm (revolutions per minute) of an actual four-stroke gasoline engine related to the number of thermodynamic cycles? What would your answer be for a two-stroke engine?
Problem 28
Why are high compression ratios not used in sparkignition engines?
Problem 30
What is the difference between fuel-injected gasoline engines and diesel engines?
Problem 31
An ideal Otto cycle has a compression ratio of \(10.5,\) takes in air at \(90 \mathrm{kPa}\) and \(40^{\circ} \mathrm{C},\) and is repeated 2500 times per minute.
Problem 33
Using constant specific heats at room temperature, determine the thermal efficiency of this cycle and the rate of heat input if the cycle is to produce \(90 \mathrm{kW}\) of power. An ideal Otto cycle has a compression ratio of \(8 .\) At the beginning of the compression process, air is at \(95 \mathrm{kPa}\) and \(27^{\circ} \mathrm{C},\) and \(750 \mathrm{kJ} / \mathrm{kg}\) of heat is transferred to air during the constant-volume heat-addition process. Taking into account the variation of specific heats with temperature, determine \((a)\) the pressure and temperature at the end of the heat-addition process, \((b)\) the net work output, \((c)\) the thermal efficiency, and \((d)\) the mean effective pressure for the cycle.
Problem 36
A six-cylinder, four-stroke, spark-ignition engine operating on the ideal Otto cycle takes in air at 14 psia and \(105^{\circ} \mathrm{F},\) and is limited to a maximum cycle temperature of \(2400^{\circ} \mathrm{F}\). Each cylinder has a bore of 3.5 in, and each piston has a stroke of 3.9 in. The minimum enclosed volume is 9.8 percent of the maximum enclosed volume. How much power will this engine produce when operated at 2500 rpm? Use constant specific heats at room temperature.
Problem 37
A spark-ignition engine has a compression ratio of 8 an isentropic compression efficiency of 85 percent, and an isentropic expansion efficiency of 95 percent. At the beginning of the compression, the air in the cylinder is at 13 psia and \(60^{\circ} \mathrm{F} .\) The maximum gas temperature is found to be \(2300^{\circ} \mathrm{F}\) by measurement. Determine the heat supplied per unit mass, the thermal efficiency, and the mean effective pressure of this engine when modeled with the Otto cycle. Use constant specific heats at room temperature.
Problem 38
An ideal Otto cycle with air as the working fluid has a compression ratio of \(8 .\) The minimum and maximum temperatures in the cycle are 540 and 2400 R. Accounting for the variation of specific heats with temperature, determine ( \(a\) ) the amount of heat transferred to the air during the heat-addition process, \((b)\) the thermal efficiency, and \((c)\) the thermal efficiency of a Carnot cycle operating between the same temperature limits.
Problem 40
When we double the compression ratio of an ideal Otto cycle, what happens to the maximum gas temperature and pressure when the state of the air at the beginning of the compression and the amount of heat addition remain the same? Use constant specific heats at room temperature.
Problem 41
In a spark-ignition engine, some cooling occurs as the gas is expanded. This may be modeled by using a polytropic process in lieu of the isentropic process. Determine if the polytropic exponent used in this model will be greater than or less than the isentropic exponent.
