Engineering Thermodynamics
Otto cycle efficiency is higher than Diesel cycle efficiency for the same compression ratio and heat input because in Otto cycle

Combustion is at constant volume
Heat rejection is lower
Expansion and compression are isentropic
Maximum temperature is higher

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Engineering Thermodynamics
According to First law of thermodynamics,

Work-done by a system is equal to the heat transferred by the system
Total internal energy of a system during a process remains constant
Total energy of a system remains constant
Internal energy, enthalpy and entropy during a process remain constant

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