Before you can understand how to build, modify, improve or maintain your engine properly, it’s best to understand the fundamentals of WHY your engine works to begin with. This article will teach you those fundamentals, which are part of a scientific field called “Thermodynamics”. When you finish reading this lesson in the thermodynamics of the car engine, you will be able to answer questions like, “why does a car engine have 4 strokes?” and “why is compression ratio important?”

This article is the first of a multi-part series! Each part of the series will go into further depth and apply the fundamentals you learn to real-world applications, like oil changes and engine design. If you are you building or rebuilding your engine for your daily driver or project car, then start here!

If you are a bit shady on how a combustion engine is put together and how it works, it would be best to check out our article How A Car Engine Works before continuing. It includes 3D animations of a real compression engine to help you visualize the pistons, valves, fuel system and more.

Background

In 1860, a German inventor named Nikolaus Otto observed a compression engine made by Jean Joseph Etienne Lenoir operate. The 18 liter engine produced only 2 horsepower. Otto correlated that an increase in gas pressure (compression) would increase the combustion engine’s efficiency, and in 1876 he invented the first “Otto Engine”, which utilized a compression stroke to increase the engine’s horsepower and efficiency. This is the engine we know of today as the internal combustion engine, or four stroke engine.

Otto needed an excellent understanding of thermodynamics to be able to theorize the benefits of a compression stroke. One way he visualized the principles involved was by creating a diagram of the “Otto Cycle”, which shows each stage of the four stroke engine in a Pressure vs. Volume diagram. Let’s walk through it. Hold on, this gets technical, but you’ll understand by the end. His idea worked, but why?

The Laws of Thermo

First, let’s cover a few basic principles that make a heat engine possible. A compression engine is only one type on heat engine. Other designs exist as well, such as the Sterling Engine or Carnot Engine, however we won’t discuss those here.

The First Law of Thermodynamics – Energy can neither be created nor destroyed; however it can change forms and can flow from one place to another. No smart remarks about complex physics and interdimensional characteristics of the universe please! For our application, this means any energy that our engine produces must come from somewhere, and can’t simply be created out of thin air!

The Second Law of Thermodynamics – This law can be worded many different ways. For the purposes of this article, it means that any natural process tends to move towards disorder or higher Entropy. Put more plainly, heat must move from areas of high temperature to areas of low temperature, and this direction is never reversible. Even your air conditioner follows this law.

The Ideal Gas Law – A relationship between pressure, volume, temperature and the number of molecules of a gas. PV=nRT

• P = Pressure
• V = Volume
• n = number of molecules of the gas
• R = Ideal Gas Constant
• T = Temperature

This equation is important to understand relationships such as, if the temperature of a gas goes up, then either the volume or the pressure must also go up.

Definitions:

We might use these words. Here’s what they mean!

Adiabatic – No theoretical heat loss occurs, no heat leaves the system. In this case, for example, this means no heat is lost from the combustion chamber to the engine coolant.

Isentropic – Reversible, frictionless adiabatic process. For example, we will not account for the friction loss of the piston rings against the cylinder wall.

Isochoric – Constant volume process.

The Otto Cycle

Now that we got the background stuff out of the way, let’s get started with the fun parts.

Each of the numbers shown above (1 through 6) represent one stage of the Otto cycle, or one step of the internal combustion process. To see each of these stages in action, check out our the animation below. The visualization below shows each of the 4 “Strokes”, which occur during a stage when the piston is moving up or down. The common automobile engine is called a “4 Stroke Engine” because it has 4 strokes, which are the Intake Stroke, Exhaust Stroke, Compression Stroke, and Power Stroke. The “Combustion Process” shown in the Otto Cycle above occurs between Strokes 3 and 4, and the “Heat Rejection” shown above occurs between Strokes 3 and 4.

Have you ever heard someone mention “compression ratio”, or “High Compression Engine”? Well, “compression ratio” on a four stroke engine is simply V2/V1 in the Otto diagram above, or a value of about 8 to 10 on most street cars.

This Otto Cycle Diagram represents idealized conditions that don’t exist in real life. These idealized conditions also represent the highest possible thermodynamic efficiency possible, because they don’t take into account any friction or heat losses. In actuality, the Otto cycle looks more like this:

Our task in each of the next articles in this series is to make our engine operate as close to this idealized Otto engine as possible. If you can do that, then you will maximize the horsepower and fuel economy of your engine. Each of the next articles in this series will discuss in detail each of the 6 Steps shown in the Otto Cycle Diagram above, and apply that information towards real life design and maintenance concepts in a way that the every day driver can understand.

What’s really neat is that if you read each article in this series, at the end you will actually be able to calculate the theoretical horsepower on any engine you want, simply by knowing a few easy to find facts, including:

• Engine RPM, of course
• Cylinder head air flow (This information is provided by aftermarket testers or the OEM manufacturer)
  • Alternatively, if unavailable, you can use the bore x stroke volume calculation and assume an air pressure
• Fuel heat value (this is gasoline, easy to find)
• Air to fuel ratio (stoichiometric is 14.7)
• compression ratio of the engine.

All of this added together means you can troll your friends by telling them how much power their engine really makes. For an enthusiast/designer, you can play with air/fuel ratio, stroke length, camshaft profiles and more to estimate theoretical horsepower impacts, with no cost.

See you in the next article!

This concludes Part 1 of the Teaching Engine Thermo Series! Check out the next part to learn more about The Intake Stroke!

Thank you for reading!

LEFTLANEBRAIN

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