Thermodynamics is the branch of physics that looks at the relationships between heat and other forms of energy. The thermodynamic relationships are key to our understanding of so many areas of engineering, if not all areas.

Adiabatic Process

A process in which the system does not exchange heat with the surroundings.

  • Adiabatic Compression – Compressing a gas without removing or adding heat.
  • Adiabatic Cooling – A method in which paramagnetic salts are pre-cooled, and then demagnetized, thereby producing further cooling.
  • Adiabatic Expansion – The expansion of a gas, vapour, or liquid stream from a higher pressure to a lower pressure, with no change in enthalpy.
  • Throttling – An irreversible adiabatic steady flow process in which the fluid is caused to flow through an obstruction in a pipe with a resulting drop in pressure.

Very fast processes can often be considered adiabatic with respect to heat exchange with the surroundings, because heat exchange is not instantaneous.

Brake Thermal Efficiency – the ratio of Brake Power to Heat of Fuel for a heat engine.

Carnot Engine – An idealized reversible heat engine working in a Carnot cycle.

Carnot Limit – a theoretical limit on the efficiency of an engine based on the flow of heat between two reservoirs.

Carnot’s Theorems – no engine can be more efficient than a reversible engine working between the same limits of temperature. All reversible engines working between the same two limits of temperature have the same efficiency.

Clausius Statement – No process is possible whose sole result is the transfer of heat from a colder to a hotter body.

boiler cooling curve

Cooling Curve – When measuring any thermal properties of an object, one of the first things to measure is the cooling curve. This will help to establish a number of factors. This is also a good curve to use to compare design options.

Newton’s Law of Cooling states that the rate of temperature of the body is proportional to the difference between the temperature of the body and that of the surrounding medium. If we have a body at temperature T and the surrounding ambient temperature of TA then we can describe the rate of change of temperature (dT/dt) according to Newton as:

dT/dt = -k(T – TA)

where k is a positive proportionality constant.

cooling fins

Cooling Fin Design – the design of the cooling fins of an air cooled engine is critical to getting the heat out of the engine cylinder and for ensuring that there are no hot spots.

Entropy – Measure of the disorder of a system. Any spontaneous change disperses energy and increases entropy overall. Example: when water evaporates, the internal energy of the water is dispersed with the water vapour produced, corresponding to an increase in entropy.

First Law of Thermodynamics – simply states that energy cannot be created nor destroyed but can only be converted from one form to another.

Many equivalent thermodynamic relationships / statements are possible, including:

  • Internal energy changes depend only on the initial and final states of the system, not on the path taken.
  • The work done during an adiabatic process depends only on the initial and final states of the system, and not on the path taken.
  • The internal energy change for any cyclic process is zero.
  • Energy may change its form, but it cannot be created nor destroyed.
  • When work is transformed into heat, or heat into work, the quantity of work is mechanically equivalent to the quantity of heat.
  • The heat entering a system is equal to the increase in energy of the system plus the external work done by the system during the entry. ng

Kelvin-Planck Statement – No process is possible whose sole result is the absorption of heat from a reservoir and the conversion of all of this heat into work.

heating silver steel with blow torch

Heat – If a temperature difference exists between two points heat will flow from the point at the higher temperature to the point at the lower temperature. This flow will continue until equilibrium has been reached. The transmission of heat may be due to three different processes.

multi-layer insulation

Insulation – Can be applied to reduce radiation and conduction. Most common materials such as rockwool or polyurethane foam form an insulating barrier. These materials all have a low thermal conductivity.

Multi Layer Insulation takes the approach of insulation and reflection into account for multiple layers, achieving a very low overall thermal conductivity.

Otto Cycle – An idealized reversible cycle of four operations occurring in a perfect four-stroke petrol engine.

Rankine Cycle – A mathematical model that is used to predict the performance of steam engines. The Rankine cycle is an idealised thermodynamic cycle of a heat engine that converts heat into mechanical work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid.

Steam – The gas phase of water.

steam indicator diagram

Steam Indicator Diagram
the pressure in the cylinder is plotted versus the cylinder volume. This was developed by James Watt and John Southern to improve the efficiency of the steam engine. The work done is the area within the red curve.

Super Heated Steam – Steam heated above its saturation temperature.

Thermal Conductivity – A measure of the ability of a material to conduct heat.

thermal insulation of a boiler

Thermal Conductivity of Wood

Plotting the thermal conductivity across the grain versus the density of the wood shows a strong relationship. For fibreboards [ref 4] it has been shown that at 85% RH the thermal conductivity increases rapidly with increasing temperature. For wood fibreboards thermal conductivity is linearly proportional to the temperature.

Thermal Efficiency

Quantity of heat produced in relation to fuel input. In the case of a heat engine this is described more precisely as: The percentage of the total chemical energy in the fuel consumed that is converted into useful work.

Thermal Efficiency, η = heat converted into mechanical work / heat supplied

Thermal Efficiency, η = (T1 – T2) / T1

where: T1 = heat source [K], T2 = heat sink [K]

Thermal Gradient – The rate at which the temperature changes with position.

Thermal Interface Material – a material used as a joint between two surfaces to improve the conduction of heat between the two. This material needs to form a good thermal interface as well as have good thermal conductivity properties itself.

Thermodynamic Equilibrium – A system is at thermodynamic equilibrium if the energy it gains from its surroundings is exactly balanced by the energy it loses, no matter how much time is allowed to pass.

Thermodynamic Free Energy – the energy in a physical system that can be converted to do work.

Whippet coolant tank - thermosiphon


A thermosiphon is a method of passive heat exchange between the heat source and the heat sink. In the case here the engine is the heat source and the water tank is the heat sink.

Vapour Pressure – the partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance.

Water Cooling System – Water passes through the system or through a water cooling jack, this is then sent to a radiator where air is passed over the radiator to remove the heat from the water before it is sent back to the system to be cooled.

Working Fluid – A fluid used as the medium for the transfer of energy.

Zeroth Law of Thermodynamics – If two bodies are each in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other.