Otto Cycle and Thermal Efficiency

Flashcards covering key concepts from the lecture on the Otto cycle, including energy conservation, specific vs. total internal energy, individual process equations, net work output, thermal efficiency, compression ratio definitions, and the impact of compression ratio on thermal efficiency through T-S diagram analysis.

According to the energy conservation principle for a closed system, what is the final energy equation discussed?

Mass * (specific internal energy at final state - specific internal energy at initial state) = Energy in - Energy out.

In thermodynamics, what is the key difference between 'u' (lowercase) and 'U' (capital) when referring to internal energy?

'u' represents specific internal energy (internal energy per unit mass), while 'U' represents the total internal energy of the system.

How is the work input per unit mass (w12) for the isentropic compression process (1-2) of the Otto cycle calculated?

w12 = u2 - u1 (specific internal energy at state 2 minus specific internal energy at state 1).

What is the equation for heat addition per unit mass (q23) during the constant volume heat addition process (2-3) of the Otto cycle?

q23 = u3 - u2 (specific internal energy at state 3 minus specific internal energy at state 2).

How is the work output per unit mass (w34) for the isentropic expansion process (3-4) of the Otto cycle expressed in relation to specific internal energies?

w34 = u3 - u4 (specific internal energy at state 3 minus specific internal energy at state 4).

How can the net work output of the Otto cycle per unit mass be expressed in terms of heat transfers per unit mass?

Net work per unit mass = qin - qout (heat addition per mass - heat rejection per mass).

What is the primary definition of thermal efficiency for the Otto cycle?

Thermal efficiency = (Net work output / Heat input) or (qin - qout) / q_in.

How is the compression ratio (r) defined for the Otto cycle, considering its different states?

The compression ratio is the ratio of the maximum volume to the minimum volume (V1/V2), which is also equal to V4/V3 due to the constant volume heat rejection and addition processes.

On a T-S (Temperature-Entropy) diagram, what does an increase in the compression ratio for an Otto cycle indicate about the temperature at the end of the isentropic compression (T2')?

An increased compression ratio leads to a higher temperature at the end of the compression process (T2' > T2), assuming the same initial state.

When comparing two Otto cycles with different compression ratios that share the same heat rejection process (4-1) on a T-S diagram, how does the amount of heat rejected (q_out) compare between them?

The heat rejected (q_out) remains the same for both cycles because they share the same heat rejection process, implying identical areas under the process line on a T-S diagram.

How does an increase in the compression ratio in an Otto cycle affect the amount of heat added per unit mass (q_in) during the constant volume heat addition process (2-3)?

A higher compression ratio results in increased heat addition per unit mass (q_in) during the second process.

What is the key relationship between the compression ratio and the thermal efficiency of an Otto cycle?

The thermal efficiency of an Otto cycle increases with an increase in the compression ratio.