Evan
11-12-2002, 01:36 AM
This is from JEs website. In short it explains why protruding pistons shouldnt be used. They say the flat top pistons or pistons with less domes are better. If youve seen a 13:1 piston you know the top part stick out pretty good like a mountain peak. So I guess the highest compression used in a EX should be 12.5
The Inside Information
Pistons as the combustion chambers "floor" by Jim McFarland
There was a time, particularly in the high performance and racing community, when the shape of the piston crowns was largely focused on their influence upon mechanical compression ratio. As a rule, the higher c.r. the greater the power potential. However, the movement of air and fuel into an engine's combustion space and how it behaves upon arrival can be important to optimizing power.
More specifically, the motion of mixtures delivered into this space can have a direct bearing on combustion efficiency. In recent years, considerable study has been devoted to how piston crowns influence this "mixture motion" and the impact they have upon fuel economy, exhaust emissions or outright power. It has been through these investigations that increased fuel efficiency, the ability to raise cylinder pressure without incurring detonation and the net improvement of horsepower/cubic-inch ratios have all resulted.
The essence of "mixture motion"
Ideally, combustion begins at the spark plug and moves at a uniform rate through the combustion space. In reality, that is often not the case. If you visualize combustion flame movement much like the progression of what happens after touching a lighted piece of paper, although at much higher rates, interruptions in travel tend to alter combustion efficiency.
Fundamentally, two types of mixture motion have been explored and utilized in stock, high performance and racing engines. Greatly simplified, motion that tends to produce rotation in a horizontal plane is called "swirl" and that which revolves more in a vertical plane is called "tumble". Swirl and tumble are intended to help maintain fuel suspension (in the air) and keep fuel droplet size small for rapid combustion and a reduction in spark timing...both of which tend to increase net torque output.
Based on the design of a given combustion chamber, piston crown shape can have a favorable or detrimental effect on mixture motion. For example, protrusions from a piston crown intended to increase mechanical compression ration sometimes inadvertently cause interruptions in flame travel and reduced combustion efficiency (and power).
The role of piston crowns in riding combustion
As combustion chamber volume is decreased, the amount of piston crown material protruding into the chamber can be reduced. Correspondingly, as crown material (that may impede flame travel) is reduced, alterations to intended mixture motion (swirl and/or tumble) are less likely. Consider the following example.
Visualize two engines, both of the same mechanical compression ratio. One has a larger combustion chamber than the other, accompanied by a protruding piston crown that produces a compression ratio equal to the other engine of a smaller chamber volume and a less protruding crown.
While to both engines deliver the same numerical value for compression ratio, the one with fewer "obstructions" in the flame path (small chamber, flatter piston crown) tends to produce higher combustion efficiency...as evidenced by higher cylinder pressure, lower brake specific fuel consumption (b.s.f.c.) and increased power. In particular, the engine of small piston crown and chamber volume can provide all this utilizing the same amount of air and fuel as supplied to the engine of larger combustion chamber volume and increased piston crown material.
Of note is the importance played by the piston of smaller crown. In this case, a more uniform and rapid flame is promoted, in addition to improved air/fuel mixture quality (less air/fuel separation).
The relationship between piston crown (the combustion chamber "floor") and cylinder head (combustion chamber "roof") is important not only to establishing levels of compression ratio but also toward promoting proper flame movement during combustion. The dynamics created in this space as each of these two surfaces (crown and chamber) approach each other during the compression stroke and beginning of combustion is both vital and critical to optimizing power. JE is currently testing many different shapes to promote mixture motion that translates into horsepower.
The Inside Information
Pistons as the combustion chambers "floor" by Jim McFarland
There was a time, particularly in the high performance and racing community, when the shape of the piston crowns was largely focused on their influence upon mechanical compression ratio. As a rule, the higher c.r. the greater the power potential. However, the movement of air and fuel into an engine's combustion space and how it behaves upon arrival can be important to optimizing power.
More specifically, the motion of mixtures delivered into this space can have a direct bearing on combustion efficiency. In recent years, considerable study has been devoted to how piston crowns influence this "mixture motion" and the impact they have upon fuel economy, exhaust emissions or outright power. It has been through these investigations that increased fuel efficiency, the ability to raise cylinder pressure without incurring detonation and the net improvement of horsepower/cubic-inch ratios have all resulted.
The essence of "mixture motion"
Ideally, combustion begins at the spark plug and moves at a uniform rate through the combustion space. In reality, that is often not the case. If you visualize combustion flame movement much like the progression of what happens after touching a lighted piece of paper, although at much higher rates, interruptions in travel tend to alter combustion efficiency.
Fundamentally, two types of mixture motion have been explored and utilized in stock, high performance and racing engines. Greatly simplified, motion that tends to produce rotation in a horizontal plane is called "swirl" and that which revolves more in a vertical plane is called "tumble". Swirl and tumble are intended to help maintain fuel suspension (in the air) and keep fuel droplet size small for rapid combustion and a reduction in spark timing...both of which tend to increase net torque output.
Based on the design of a given combustion chamber, piston crown shape can have a favorable or detrimental effect on mixture motion. For example, protrusions from a piston crown intended to increase mechanical compression ration sometimes inadvertently cause interruptions in flame travel and reduced combustion efficiency (and power).
The role of piston crowns in riding combustion
As combustion chamber volume is decreased, the amount of piston crown material protruding into the chamber can be reduced. Correspondingly, as crown material (that may impede flame travel) is reduced, alterations to intended mixture motion (swirl and/or tumble) are less likely. Consider the following example.
Visualize two engines, both of the same mechanical compression ratio. One has a larger combustion chamber than the other, accompanied by a protruding piston crown that produces a compression ratio equal to the other engine of a smaller chamber volume and a less protruding crown.
While to both engines deliver the same numerical value for compression ratio, the one with fewer "obstructions" in the flame path (small chamber, flatter piston crown) tends to produce higher combustion efficiency...as evidenced by higher cylinder pressure, lower brake specific fuel consumption (b.s.f.c.) and increased power. In particular, the engine of small piston crown and chamber volume can provide all this utilizing the same amount of air and fuel as supplied to the engine of larger combustion chamber volume and increased piston crown material.
Of note is the importance played by the piston of smaller crown. In this case, a more uniform and rapid flame is promoted, in addition to improved air/fuel mixture quality (less air/fuel separation).
The relationship between piston crown (the combustion chamber "floor") and cylinder head (combustion chamber "roof") is important not only to establishing levels of compression ratio but also toward promoting proper flame movement during combustion. The dynamics created in this space as each of these two surfaces (crown and chamber) approach each other during the compression stroke and beginning of combustion is both vital and critical to optimizing power. JE is currently testing many different shapes to promote mixture motion that translates into horsepower.