COMPRESSION RATIO: HOW to CALCULATE, MODIFY and CHOOSE the BEST one - BOOST SCHOOL #10

Summary

This video script delves into the concept of engine compression ratio, explaining its impact on performance and efficiency. It outlines how to calculate and modify compression ratios, and the importance of choosing the right ratio for specific applications. The script also addresses the limitations of high compression ratios, such as increased risk of knock and higher emissions, and provides practical advice for balancing power output with drivability and reliability.

Takeaways

• 🔧 Compression ratio is the ratio between the largest and smallest volume of your cylinder, influencing engine performance and efficiency.
• 🛠️ Higher compression ratios compress the air-fuel mixture more, bringing molecules closer together and enhancing combustion efficiency.
• ⚙️ Spark-ignition engines benefit from higher compression ratios as it facilitates heat transfer and more effective combustion.
• 🔥 While higher compression ratios can increase power output and efficiency, they also raise combustion temperatures, which can increase nitrogen oxide emissions.
• 🚫 The main limitation of high compression ratios in spark-ignition engines is 'knock,' where the air-fuel mixture ignites prematurely, potentially damaging the engine.
• 🔩 Compression ratio is determined by factors such as engine bore, stroke, head gasket thickness, piston-to-deck clearance, and combustion chamber volume.
• 📊 Modifying compression ratios can be achieved through adjustments in bore, stroke, head gasket thickness, machining, and piston top design.
• 🔄 Increasing bore and stroke generally increases compression ratio, while thicker head gaskets or dished pistons decrease it.
• 💡 Choosing the optimal compression ratio depends on the accuracy of the engine build, the hardware and software used, and the specific goals of the build.
• 🚗 For naturally aspirated engines, higher compression ratios are generally better, whereas turbocharged engines may benefit from lower compression ratios to accommodate higher boost levels.

Q & A

What is the compression ratio of an engine?

The compression ratio of an engine is the ratio between the largest and smallest volume of the cylinder when the piston is at 'bottom dead center' and 'top dead center', respectively.

How does the compression ratio influence engine performance and efficiency?

A higher compression ratio allows for more compression of the air-fuel mixture, which can lead to more effective and rapid combustion, thus increasing both power output and efficiency of the engine.

What is the significance of the air-fuel mixture compression in spark-ignition engines?

In spark-ignition engines, a more compressed air-fuel mixture facilitates heat transfer from one layer to the next, enabling more effective and rapid combustion, which results in a more thorough burn of the mixture.

Why can't we increase the compression ratio indefinitely?

While higher compression ratios can improve engine performance, there is a sensible limit due to factors like increased combustion temperatures, which can lead to higher nitrogen oxide emissions and the risk of engine knock.

What is engine knock and why is it a concern?

Engine knock occurs when the air-fuel mixture inside the cylinder becomes so hot from compression that it ignites before the spark plug initiates the flame front, causing an uncontrolled explosion. Knock can damage the engine and should be avoided.

How can the compression ratio be calculated?

Compression ratio can be calculated using a formula or more conveniently, by using an online compression ratio calculator that takes into account factors such as engine bore, stroke, head gasket bore, and other engine dimensions.

What factors determine the compression ratio of an engine?

The compression ratio is determined by the engine bore, stroke, compressed head gasket thickness, head gasket bore, piston to block deck distance, piston dish or dome volume, and combustion chamber volume.

How can the compression ratio be modified?

Compression ratio can be modified by changing the engine's bore and stroke, using different thickness head gaskets, machining the block deck or cylinder head, grinding the combustion chambers, or modifying the piston top design.

What are the considerations for choosing the optimal compression ratio for an engine application?

The optimal compression ratio depends on the build accuracy, the hardware and software capabilities for knock control, the engine's displacement and power goals, and the specific application requirements such as street driving or racing.

How does forced induction affect the compression ratio?

Forced induction engines are more limited in compression ratio because the compressed air entering the cylinder adds heat, increasing the risk of knock. These engines typically require lower compression ratios to avoid this issue.

What is the relationship between boost and compression ratio in terms of power output?

While a higher compression ratio can increase power output, boost has a more significant effect. A single point increase in compression ratio typically results in about a 4% power increase, whereas 1 psi of boost can increase power by around 7%.