Welcome to Part 3 of the Teaching Engine Thermodynamics Series. In this edition, it’s all about COMPRESSION!

What is the Compression Stroke?

The compression stroke occurs after the intake stroke is completed. The cylinder is now full of air from the intake stroke, and the piston is at Bottom Dead Center (BDC). During the compression stroke, the piston starts at BDC and moves upward while the intake and exhaust valves stay closed. The fuel-air mixture is compressed, causing a rise in temperature and pressure within the cylinder. The compression stroke is complete when the piston reaches Top Dead Center (TDC). The ratio of the volume of the cylinder at TDC to the volume at BDC is called the Compression Ratio of the engine, an important and commonly publicized metric.

If the compression stroke wasn’t present, for example on the Lenoir engine, the 2-3 stroke in the Otto Diagram wouldn’t increase in pressure. Rather it would simply move back to the left and reduce the cylinder volume at constant pressure.

WHY does increased compression ratio increase the engine’s thermal efficiency and power?

The compression ratio, in a theoretical adiabatic process, is equal to the “expansion ratio.” The expansion ratio is basically exactly the same thing as the compression ratio, but occurs during the power stroke rather than the compression stroke. Why are they equal? Well, the volume of the cylinder, size of the piston, or shape of the crankshaft and cylinder heads never change once the engine is assembled, so if the cylinder was 0.5 liters when the piston was moving up, it still will be when the cylinder moves down. The power stroke is responsible for doing work (making power), and a larger expansion ratio allows for more work to be done per stroke. So the goal is to increase the expansion ratio as much as possible for a given amount of fuel. Since the expansion ratio (in a theoretical system) is equal to the compression ratio, then all we have to do is increase the compression ratio. Easy!

Here’s why the power stroke makes power:

If V2=V1, then then amount of work done is smaller. This smaller amount of work is why the Lenoir engine only made 2 horse power with 18 liters of displacement.

If you’re brushed up on your calculus skills, it’s easy to see in the formula that increasing the difference in V2 and V1 will increase the amount of work done. This difference between V1 and V2 is what we express as an “expansion ratio” or “compression ratio”.

Another reason a higher compression ratio is more efficient is that it leads to an increased adiabatic flame temperature, or peak flame temperature for a given amount of fuel. This means you get more temperature per gallon of gas. The gasoline engine is a type of “heat engine” which means that a difference in temperature is what actually produces the work. More temperature means more heat available to do work.

A higher compression ratio also causes the flame to burn faster, because flames propagate more quickly in high pressure than low pressure. A flame that can burn more quickly allows the combustion process to be more ideal, and requires less ignition advance. We’ll talk about this more in the next section.

If high compression is so great, why isn’t everybody doing it?

Higher compression ratios create more heat in the cylinder with each compression stroke. From an efficiency perspective this is a good thing, but from a fuel management perspective this can be a bad thing. If the cylinder temperature is hot enough, the fuel-air mixture in the cylinder from the intake stroke can self-combust, which is known as spark knock, pre-ignition or pinging. Because the fuel ignited on its own, without the precision timing of the spark plug firing, the combustion process will likely occur at a non-ideal time and lead to poor thermodynamic efficiency. Most likely, the combustion process will occur too soon, and create pressure while the cylinder is still trying to complete the compression stroke, rather than during the power stroke. This causes the energy from the fuel to actually work against the engine, rather than for it. This issue limits the compression ratio, and creates a need for high-octane fuels. High octane fuels have higher self-ignition temperatures, which helps prevent pre-ignition. We’ll discuss that more in the next part.

Higher compression engines also require beefier engine components. Higher pressures and temperatures require thicker cylinder heads, pistons, connecting rods, crankshafts, and so on, which adds cost and weight. Heavy rotating assemblies within the engine also reduce engine responsiveness.

There are also smaller negative impacts to high compression ratios that are less significant. For example, a high compression ratio engine has larger peak cylinder pressure, so a larger starter motor and battery are required to start the engine and overcome peak cylinder pressure. Thicker motor oils may also be required to provide the necessary protection for the engine internals, and thicker motor oil will reduce engine efficiency by increasing friction losses. More expensive exhaust valves that can cope with the higher ignition temperatures may also be required. High compression ratios may also prevent the use of certain materials of construction within the engine, such as aluminum, and instead require materials that are stronger at higher temperatures, such as steel. This will add machining costs during construction and add weight to the engine.

In the end, it’s a balance between performance and efficiency, versus manufacturer cost and fuel costs.

What can I do with my new found knowledge?

The bad news is that once an engine is built, you can’t easily change the compression ratio. There are however maintenance tasks you can do to keep the compression ratio in the right place.

If you ever need to replace a cylinder head gasket, make sure you use a gasket with the correct thickness. A thinner head gasket will move the cylinder head closer to the engine block, which reduces the vapor space above the piston at the top of its stroke. This increases the compression ratio. Also, carbon buildup in the combustion chamber will increase the compression ratio, because it reduces the volume at the top of the piston’s stroke (V2 mentioned above). An easy way to test for carbon buildup is to remove the spark plug and look at it. For this reason, increased carbon buildup and cause pre-ignition.

Worn out piston rings, valve seat leaks, or cylinder head leaks will reduce peak cylinder pressure and temperature. For an ideal gas (mentioned in Part 1), pressure and temperature are related. If the cylinder is leaking during it’s stroke, then a reduction in pressure will also lead to a reduction in temperature. Since this is a heat engine, a reduction in temperature or pressure will reduce the work done by the cylinder. Looking at the Otto cycle diagram, picture the pressure at point 3 and point 4 moving down since the pressure was allowed to escape the cylinder. This makes the work done during the power stroke smaller.

The good news is that if you’re building a new engine, there are a-lot of tricks to control compression ratio and its effects. Designing an engine with higher compression ratios seems like a pretty straight forward process, but there are important details and innovations that we can use to optimize the compression process. There are also a few tweaks we can make to improve the efficiency of the compression process. Let’s put this knowledge to use.

The shape of the combustion chamber on the cylinder head is a key design parameter for high compression engines. As the piston moves up, you want to compress the gas with as little friction loss as possible. This is accomplished by inducing a swirl in the cylinder air as it is compressed, which reduces the friction losses. A great example is the GM Vortec cylinder head, which was designed to induce a swirl as air entered the cylinder and was compressed, reducing friction loss. A swirl will also improve air movement in the cylinder and reduce hot spots, which limits pre-ignition.

The piston head itself is also important when designing a high compression engine. Modern engines are all fuel injected, and a flat top piston doesn’t produce the right air patterns in the cylinder to ensure proper fuel mixing and combustion. Because of this, a “quench area” was created on top of the piston to allow mixing. However, this quench area also increases the vapor area at TDC, and reduces the compression ratio of the engine. Space for the valves to open and not strike the piston head is also essential. This results in a piston that can have cutouts for quench and valves, but raised areas to compensate and increase compression. A standard small block Chevy (SBC) piston is shown on the left, and a SBC piston designed for higher compression is shown on the right.

Possibly the cheapest way to improve high compression ratio operation (since you don’t actually need to buy anything) is by polishing the combustion chamber of the cylinder head. Polishing the combustion chamber reduces friction losses from airflow, but more importantly reduces the temperature of hot spots during the compression stroke. A great analogy is to hold a wire over a fire, versus a metal plate, and see which gets hotter faster. By polishing the surface, the number of metal surface particles that extend out into the hot air’s flow path are reduced, and the metal’s surface will be a more uniform temperature. This can allow higher compression ratios for a given fuel blend (octane rating). A good example of this is our ’76 Corvette which was designed for street use. It has a compression ratio of around 10.5, but by employing these and other techniques can easily run on 89 Octane gas and still use a proper amount of ignition advance. The high compression improves engine power and allows us to achieve fuel mileage in the mid-20’s.

For simplicity, most cylinder heads are either iron or aluminum. Frequently, people will discuss the generous weight benefits of aluminum vs iron, but they almost always neglect the benefits of iron vs aluminum. Iron heads have an obvious advantage when it comes to durability and strength, as well as more secure mounting threads for engine components (water pump, exhaust manifolds etc.), but what about efficiency?

We can define how efficiently a material transfers heat by calculating the “Thermal Conductivity”. Thermal Conductivity is “the quantity of heat transmitted through a unit thickness of a material – in a direction normal to a surface of unit area – due to a unit temperature gradient under steady state conditions”. FUN….we know. Thermal conductivity has the units of W/(m K) in the SI system, and Btu/(hr ft °F) in the imperial or English system.

Anyway, aluminum has a thermal conductivity of approximately 205 W/(m K), while iron has a conductivity of 80 W/(m K) and steel of ~40-15 W/(m K), depending on the composition. This means aluminum is far better at transferring heat out of the combustion chamber, and into everything else. This sounds like a good thing, but actually it’s not. Remember that this is a heat engine, and all of the work it does depends on the heat produced by the engine. If that heat is removed by the coolant system, it’s useless (and it loads up the coolant system). This applies during compression and especially combustion. Also, it means more heat will be transferred to your fuel injectors, carburetor and anything bolted to the cylinder head. This isn’t so bad for the fuel injectors, but heat transferred to the carb is a no-no, and can cause heat soak, boiling fuel, varnish to form, and more. Depending on the engine, some estimates calculate that as much as 10 horsepower is lost by using an aluminum cylinder head vs steel. But the weight savings are still significant, and may make up for this. In the end, it’s the designer’s call.

Want a shot at calculating the compression ratio of your engine? It’s easy! Just use our quick online calculator located on our Automotive Formulas page.

We hope you enjoyed Part 3 of the Teaching Engine Thermodynamics Series!

Check out the next chapter to learn about the Combustion Process and what it means for you!

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