Thermo Review

What is the total compressed volume in an ideal Otto Cycle, which starts at 27 C and 90 kPa, has a compression ratio of 7, and 0.004181 kg of air? Props: Cp=1.005 [kJ/kg K], Cv=0.718 [kJ/kg K], K=1.4, R= 0.287 [kJ/kg K]

• 0.9567 m^3

• 0.1366 m^3

• 0.0005714 m^3

• 0.003999 m^3

Answer explanation

Use the ideal gas law to calculate the initial specific volume, the compression ratio to find the final specific volume, and then multiply the specific volume by mass to get total volume

Tags

NGSS.HS-PS3-2

NGSS.HS-PS3-4

How much work is done during the power stroke of an ideal Otto Cycle with a maximum temperature of 1127 C, compression ratio of 7, and 0.004181 kg of air? Props: Cp=1.005 [kJ/kg K], Cv=0.718 [kJ/kg K], K=1.4, R= 0.287 [kJ/kg K]

• 1.831 kJ

• 2.273 kJ

• 2.241 kJ

• 1.029 kJ

Answer explanation

Use an isentropic process relationship to find the gas temperature at the end of the power stroke, then use the temperature difference, Cv, and the mass to calculate the work done.

Tags

NGSS.HS-PS3-1

An ideal Otto cycle running with air rejects 327 kJ/kg of waste heat each cycle. What is the pressure of the gas before heat rejection if at the end of the heat rejection process the gas is 27 C and 95 kPa. Props: Cp=1.005 [kJ/kg K], Cv=0.718 [kJ/kg K], K=1.4, R= 0.287 [kJ/kg K]

• 239 kPa

• 755 kPa

• 534 kPa

• 4392 kPa

Answer explanation

Because this is a constant volume process,

Pv/T = R

allows you to relate both states (through R) where v cancels on both sides (constant volume process) creating P2/T2 = P1/T1. You have enough information to solve for P1 using this information if you also calculate T1.

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

An ideal Otto cycle running with air starts its cycle at 27 C and 95 kPa. After the compression stroke, the gas state is 416 C and 1745 kPa. What's the compression ratio (r) of this cycle?

• 1.2

• 8

• 7

• 10

Answer explanation

You can arrange the ideal gas law as,

v = RT/P

giving you enough information to calculate the specific volume at both states. The compression ratio is then r =v1/v2

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

How much mass is in an ideal Otto Cycle which starts at 27 C and 90 kPa, has a compression ratio of 7, a maximum cycle temperature of 1127 C, and 1.029 kJ of total heat rejection per cycle? Props: Cp=1.005 [kJ/kg K], Cv=0.718 [kJ/kg K], K=1.4, R= 0.287 [kJ/kg K]

• 0.004181 kg

• 0.04646 kg

• 0.00283 kg

• 0.002988 kg

Answer explanation

Use r to get efficiency, use efficiency to calculate Q_in based on Q_out, then use Q=m*Cv*ΔT to calculate mass based on the heat addition to the cycle.

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

What will increase the efficiency of an Otto Cycle: Increasing the compression ratio or increasing the amount of air in the cycle?

• Increase Compression Ratio

• Increase Air in Cycle

Increasing the amount of air in a cycle will not increase the total net work produced by a cycle.

• True

• False

The compression ratio is calculated based on: specific volumes 1 and 2 (v1 and v2), total volumes 1 and 2 (V1 and V2), either

• Specific Volumes - v

• Total Volume - V

• Either

Heat addition in an ideal Otto Cycle is a(n):

• Constant Pressure Process

• Isentropic Process

• Isothermal Process

• Constant Volume Process

Tags

NGSS.HS-PS3-4

The compression stroke in an ideal Otto Cycle is a(n):

• Constant Pressure Process

• Isentropic Process

• Isothermal Process

• Constant Volume Process