OldYat47 said:The effective compression ratio is the actual (rather than geometric) compression ratio, taking into account intake efficiency, scavenging and boosting events (where boost is used). Geometric or theoretical compression ratio is the volume of the space in the cylinder above the piston at the point where the cylinder is closed vs. the volume present when the piston is at top dead center.
What that 9.7 means is that the actual engine in operation compresses the intake charge volume at a ratio of 9.7:1.
I don't fool around with RC engines, so I can't say if 9.7:1 is "good" or "bad".
OldYat47 said:I must be clear in that it's not the geometric volume of the cylinder. It is the volume of the cylinder charge, taking into account that volume will be less than the geometric volume. You can think of it as the quantity (let's say mass) of fuel/air mixture at something less than atmospheric pressure drawn into the cylinder at intake. An effective compression ratio of 9.7 to 1 in a car engine would be admirable. As I said, I don't work with RC engines so I can't make a judgement on that.
OldYat47 said:No. What you are referring to is the geometric compression ratio. That assumes that the entire volume will fill with mixture at existing atmospheric pressure. In reality that doesn't happen because of intake inefficiencies, turbulence, etc.
The effective compression ratio is a measure of how much the fuel-air mixture is compressed inside the combustion chamber of an engine. It takes into account the volume of the combustion chamber and the volume of the fuel-air mixture before and after compression.
The effective compression ratio is calculated by dividing the total volume of the combustion chamber (including the volume of the cylinder and the head space) by the volume of the fuel-air mixture before compression. This ratio is then compared to the mechanical compression ratio, which only takes into account the volume of the cylinder.
The effective compression ratio has a direct impact on the efficiency and power output of an engine. A higher effective compression ratio can result in better fuel economy and more power, while a lower ratio can lead to decreased efficiency and power.
The design of an engine, including factors such as the shape and size of the combustion chamber, piston design, and valve timing, can all impact the effective compression ratio. A well-designed engine will have a higher effective compression ratio, resulting in better performance.
While a higher effective compression ratio can lead to improved performance, it can also increase the risk of engine knock, which can damage the engine. This is why it is important for engine designers to carefully balance the effective compression ratio with other factors to ensure optimal performance and reliability.