An ideal gas, CP $=(7/2)$R and CV $=(5/2)$R$,$ undergoes a cycle consisting of the following mechanically reversible steps:

$$ An adiabatic compression from P$1,$ V$1,$ T$1$ to P$2,$ V$2,$ T$2$

$$ An isobaric expansion from P$2,$ V$2,$ T$2$ to P$3=$ P$2,$ V$3,$ T$3$

$$ An adiabatic expansion from P$3,$ V$3,$ T$3$ to P$4,$ V$4,$ T$4$

$$ A constant$-$volume process from P$4,$ V$4,$ T$4$ to P$1,$ V$1=$ V$4,$ T$1$

Determine its thermal efficiency if T$1=250\backslash$deg C$,$ T$2=1000\backslash$deg C$,$ and T$3=1700\backslash$deg C$.$

The thermal efficiency is

$.$