I was instructed to create a program that computes the efficiency of an engine based on the specific heat ratio of a gas, for different compression ratios.

We were told that we need sub-functions that do a bunch of different things, and then to link them into the main function. I've got what I think is most of the code to be done, with obvious additions that I still need to put in, but the stuff I have isnt working properly. It will run, but when I enter an option to choose the specific heat ratio, it doesn't return the calculation properly. I tried putting all of my code in a separate program, and running it all in one main function, but the same things happen.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double get_k()
{
double k;
int option;

/* Introduction*/
printf("Welcome to the Engine Efficiency Calculator! To begin, enter the number to \n");
printf("show a list of engine efficiencies for the gas selected. If you wish to use \n");
printf("a different specific heat ratio than those shown, enter 5. \n");
/* List options. */
printf(" 1. Hydrogen (k = 1.416)\n 2. Oxygen (k = 1.398)\n 3. Carbon Dioxide (k = 1.314)\n 4. Air/Fuel Mixture (k = 1.35)\n 5. Other\n");
printf("Enter your option:\n");
scanf("%d", &option);

if (option == 1)
{
k == 1.416;
return k;
}

else if (option == 2)
{
k == 1.398;
return k;
}

else if (option == 3)
{
k == 1.314;
return k;
}

else if (option == 4)
{
k == 1.35;
return k;
}

else if (option == 5)
{
printf("Please enter a different specific heat ratio: \n");
scanf("%f", &k);
return k;
}

}

double calculate_eta(double k)
{
double eta1;

eta1 = 1/pow(10, (k-1));
printf("%lf \n", eta1);
return eta1;
}

int main()
{
int z = 1;
double k = get_k();
double eta = calculate_eta (k);

while (z = 1);
{
get_k();
calculate_eta (k);
}

system("PAUSE");
return 0;
}