# What variables affect a Heat engine's efficiency? (and more)

I'm writing a Minecraft mod and I'm trying to create the equations for the engines players can build. I'm a highschool sophomore, so my knowledge of physics and mathematics is limited. I asked first a question on the Chemistry site of stackexchange about how do we get the energy output of combustion engine. The answer was

$E = \kappa \Delta H°_{comb}V$

where $\kappa$ is the engine's efficiency,

$\Delta H°_{comb}$ is the enthalpy of combustion of the fuel

$V$ is the volume of fuel burned.

$\kappa$ was limited by several factors such as

1. Compression Ratio (internal combustion only)
2. Gear Ratio (for high torque applications)
3. Working Fluid (external combustion only)
4. Ambient Temperature (mainly external combustion only)
5. Heat Exchanger Material (external combustion only)

From there I continued researching on my own. I found there is an upper limit for $\kappa$ that is known as Chambadal-Novikov efficiency which is

$\eta = 1 - \sqrt\frac{T_L}{T_H}$

I also learned that heat engines that are used to produce electricity can have combined cycles: The principle is that after completing its cycle (in the first engine), the working fluid of the first heat engine is still low enough in its entropy that a second subsequent heat engine may extract energy from the waste heat (energy) of the working fluid of the first engine.

My questions are:

How can I define variables 2 to 5 and how can I approximate how much of $\kappa$ each of them account for?

How arbitrary can the temperatures of the hot and cold reservoirs in the Chambadal-Novikov efficiency equation be? I assume the cold reservoir is the environment, but what dictates $T_H$'s upper limit?

Considering a steam engine with combined cycles, how high must the temperature of the steam be as it exits the first engine to make it useful for a second engine?

• Why don't you make use of reversible engines? Then everything else apart from temperatures would not matter. – Diracology Jul 20 '16 at 12:05
• because during my research I read reversible engines are impossible in reality. – FinnTheHuman Jul 20 '16 at 12:32