Moto Man's 8 phase motor theory
08-22-2007, 10:12 PM
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Moto Man's 8 phase motor theory
This information is from Pat McGivern "Moto Man", thought it might be interesting to some. He deals with race bikes but this applies to engines in general.
TORQUE
The word "torque" is often used incorrectly to describe low RPM horsepower. In the real world, horsepower is all that matters, because torque involves no motion whatsoever. Torque is simply the static measurement of twisting force. The ultimate goal is a linear rush of acceleration from 8,000 RPM to redline. The key to achieving that goal lies in understanding the way an engine produces horsepower.
When torque (static force) is combined with RPM (motion), the result is horsepower (Work / Acceleration). Of the three factors in the equation, the only fixed constant is RPM. In other words, RPM will always increase in a perfect numerical progression. (For example: there will always be 1,000 rpms between 5,000 and 6,000 rpm) Therefore, If the engine can be tuned to produce a constant torque output over a wide range of RPM, the horsepower will "automatically" multiply in the same perfect linear progression as the RPM !
A flat torque output ! This is the heart of the tuning challenge ! Because generally when an engine is tuned to make more torque in one RPM range, there is a similar loss at all other RPM's. To see why this happens, let's take a look at the tuning compromises at different RPM's. Since each step of the process will overlap the actual strokes of the piston, it's far more accurate to think of the cycle in terms of 8 phases rather than 4 180 degree strokes.
2 EXHAUST PHASES (Exhaust Blowdown / Exhaust Return)
Exhaust Blowdown:
The exhaust must be completely cleared from the cylinder. The only way to accomplish this, is to open the exhaust valves about 30-40 degrees before the bottom of the power stroke, so that the still burning charges pressure can begin to escape out of the cylinder. If the power phase were allowed to continue to the bottom of the piston stroke, the piston would have to work hard to push against the high pressure created by the still burning (and still expanding) gasses during the upward exhaust stroke. Instead, some of it's own pressure is used to blow itself out of the cylinder while the piston is still on the down stroke.
Exhaust Return:
By the time the piston reverses direction in the exhaust return phase, the excess pressure is gone. If the exhaust silencer is positioned as shown in the Dynamic Horsepower newsletter, there will be a slight vacuum which will actually pull the piston up !!
The "textbook 4 stroke" had positive pressure during the exhaust return phase, whereas I'm saying there is vacuum !! Which one makes more sense ??
The tuning tradeoff: The best time to open the exhaust valves is a compromise between extracting the most power from the power phase at low RPM, and losing the least power from the exhaust phase at high RPM.
3 INTAKE PHASES
There are 3 distinctly different ways the intake charge enters the engine.
Intake Overlap:
The intake phase actually begins during the end of the exhaust return phase. About 15 degrees before the top of the piston stroke, the intake valves open. This is also called the camshaft overlap period because the intake and exhaust valves are both open a small amount at the same time. (the exhaust valves are closing and the intake valves are opening.)
The low pressure from the exiting exhaust creates a flow pattern across the top of the cylinder that draws fresh intake mixture into the cylinder to displace the last remaining spent gases. The truly ingenious part of this design, is that the flow of intake mixture into the cylinder has been started while the piston is still going up... against the direction of the flow it's pumping !!!
Intake Suction:
Now the piston has passed the top and now accelerating down it's stroke. At the same time, the valves are opening rapidly to allow the intake charge to enter the cylinder with minimal resistance. Since the fuel/air mixture has a certain amount of mass, it tends to lag behind the piston, and this lag time becomes more pronounced as the RPM's increase. As a result, the piston first creates a low pressure condition in the cylinder, and the mixture rushes in to fill it.
Intake Charging:
This is the time when the piston has passed the bottom of it's stroke, and begun to move up. Because of the charge momentum created by the intake suction phase, lots of fuel and air mixture is still rushing down the intake tract to fill the cylinder. This phenomenon increases with the engine speed, to the point that a progressively higher percentage of the cylinder filling occurs after the piston is no longer physically "sucking" the charge in. Because of this, it's necessary to extend the intake phase way past the physical 180 degree intake stroke. On average, the valves don’t completely close until the piston has moved up about 55 degrees past the bottom of it's 180 degree stroke !!
The tuning tradeoff: As you can see, the length of these phases has to do with the speed of the engine ! This is another compromise, because while the delayed valve closing improves high RPM cylinder filling, the charge velocity isn't high enough at lower RPM, and the piston will push some of the fuel/air mixture back into the port. This is one of the most important things to understand about the intake process !!
Also, in order to extract the most power from the intake phase, the inducted charge must burn completely. If the carburetors are jetted right, the average fuel/air mixture will be right. But, since fuel is heavier than air, it's possible for some of the fuel to separate from the mixture as it moves through the ports and into the cylinder. This causes distinct lean and rich pockets in the cylinder, which will result in poor combustion efficiency.
The fuel/air charge should remain turbulent in the cylinder to maintain a uniform mixture throughout. One popular way to do this in a two valve engine is to curve the intake port to "swirl" the mixture into the cylinder. This doesn't work with a four or five valve head though, because too much turbulence is created in the port, which disrupts the volume of flow into the cylinder.
Compression Phase
The moment the intake valves are closed during the upward compression stroke marks the end of the intake phase, and the beginning of the compression phase. Since it's the expansion of the burning charge that pushes the piston down, the more the fuel/air charge can be initially compressed, the greater the total expansion will be once it's burned.
The limit to the maximum possible compression ratio is detonation. The one factor that has the greatest effect on limiting the detonation is the combustion efficiency.
2 Burning Phases
Pre Power Burning Phase
The spark will use some time to spread into a flame all the way through the fuel. If this time is used when the piston is going down, then some of the fuel's potential power will be lost. So, the best moment to ignite the spark will be before the piston has reached the top of it's compression stroke, usually around 35-40 degrees before top dead center.
Power Production Stroke
The piston reaches the top, reverses direction, and only now is the engine finally making power! The piston is then forced down to the point of the exhaust blowdown phase, about 140 degrees down from top, and the cycle starts over.
Conclusion
As well as the four stroke engine works, it's far from perfect. In addition to the basic tuning problems, a lot of energy is used up to recharge the cylinder for the power stroke. In fact, out of the 720 degrees in a complete cycle, on average, the power phase lasts less than 140 degrees! Many improvements can be made to the remaining 580 degrees that will maximize the power phase, and just as importantly, minimize recharging losses.
The secret to producing a constant output of torque is to consistently fill the cylinders better at all RPM’s.
Modified by quicksilver1689 at 11:25 PM 8/22/2007
TORQUE
The word "torque" is often used incorrectly to describe low RPM horsepower. In the real world, horsepower is all that matters, because torque involves no motion whatsoever. Torque is simply the static measurement of twisting force. The ultimate goal is a linear rush of acceleration from 8,000 RPM to redline. The key to achieving that goal lies in understanding the way an engine produces horsepower.
When torque (static force) is combined with RPM (motion), the result is horsepower (Work / Acceleration). Of the three factors in the equation, the only fixed constant is RPM. In other words, RPM will always increase in a perfect numerical progression. (For example: there will always be 1,000 rpms between 5,000 and 6,000 rpm) Therefore, If the engine can be tuned to produce a constant torque output over a wide range of RPM, the horsepower will "automatically" multiply in the same perfect linear progression as the RPM !
A flat torque output ! This is the heart of the tuning challenge ! Because generally when an engine is tuned to make more torque in one RPM range, there is a similar loss at all other RPM's. To see why this happens, let's take a look at the tuning compromises at different RPM's. Since each step of the process will overlap the actual strokes of the piston, it's far more accurate to think of the cycle in terms of 8 phases rather than 4 180 degree strokes.
2 EXHAUST PHASES (Exhaust Blowdown / Exhaust Return)
Exhaust Blowdown:
The exhaust must be completely cleared from the cylinder. The only way to accomplish this, is to open the exhaust valves about 30-40 degrees before the bottom of the power stroke, so that the still burning charges pressure can begin to escape out of the cylinder. If the power phase were allowed to continue to the bottom of the piston stroke, the piston would have to work hard to push against the high pressure created by the still burning (and still expanding) gasses during the upward exhaust stroke. Instead, some of it's own pressure is used to blow itself out of the cylinder while the piston is still on the down stroke.
Exhaust Return:
By the time the piston reverses direction in the exhaust return phase, the excess pressure is gone. If the exhaust silencer is positioned as shown in the Dynamic Horsepower newsletter, there will be a slight vacuum which will actually pull the piston up !!
The "textbook 4 stroke" had positive pressure during the exhaust return phase, whereas I'm saying there is vacuum !! Which one makes more sense ??
The tuning tradeoff: The best time to open the exhaust valves is a compromise between extracting the most power from the power phase at low RPM, and losing the least power from the exhaust phase at high RPM.
3 INTAKE PHASES
There are 3 distinctly different ways the intake charge enters the engine.
Intake Overlap:
The intake phase actually begins during the end of the exhaust return phase. About 15 degrees before the top of the piston stroke, the intake valves open. This is also called the camshaft overlap period because the intake and exhaust valves are both open a small amount at the same time. (the exhaust valves are closing and the intake valves are opening.)
The low pressure from the exiting exhaust creates a flow pattern across the top of the cylinder that draws fresh intake mixture into the cylinder to displace the last remaining spent gases. The truly ingenious part of this design, is that the flow of intake mixture into the cylinder has been started while the piston is still going up... against the direction of the flow it's pumping !!!
Intake Suction:
Now the piston has passed the top and now accelerating down it's stroke. At the same time, the valves are opening rapidly to allow the intake charge to enter the cylinder with minimal resistance. Since the fuel/air mixture has a certain amount of mass, it tends to lag behind the piston, and this lag time becomes more pronounced as the RPM's increase. As a result, the piston first creates a low pressure condition in the cylinder, and the mixture rushes in to fill it.
Intake Charging:
This is the time when the piston has passed the bottom of it's stroke, and begun to move up. Because of the charge momentum created by the intake suction phase, lots of fuel and air mixture is still rushing down the intake tract to fill the cylinder. This phenomenon increases with the engine speed, to the point that a progressively higher percentage of the cylinder filling occurs after the piston is no longer physically "sucking" the charge in. Because of this, it's necessary to extend the intake phase way past the physical 180 degree intake stroke. On average, the valves don’t completely close until the piston has moved up about 55 degrees past the bottom of it's 180 degree stroke !!
The tuning tradeoff: As you can see, the length of these phases has to do with the speed of the engine ! This is another compromise, because while the delayed valve closing improves high RPM cylinder filling, the charge velocity isn't high enough at lower RPM, and the piston will push some of the fuel/air mixture back into the port. This is one of the most important things to understand about the intake process !!
Also, in order to extract the most power from the intake phase, the inducted charge must burn completely. If the carburetors are jetted right, the average fuel/air mixture will be right. But, since fuel is heavier than air, it's possible for some of the fuel to separate from the mixture as it moves through the ports and into the cylinder. This causes distinct lean and rich pockets in the cylinder, which will result in poor combustion efficiency.
The fuel/air charge should remain turbulent in the cylinder to maintain a uniform mixture throughout. One popular way to do this in a two valve engine is to curve the intake port to "swirl" the mixture into the cylinder. This doesn't work with a four or five valve head though, because too much turbulence is created in the port, which disrupts the volume of flow into the cylinder.
Compression Phase
The moment the intake valves are closed during the upward compression stroke marks the end of the intake phase, and the beginning of the compression phase. Since it's the expansion of the burning charge that pushes the piston down, the more the fuel/air charge can be initially compressed, the greater the total expansion will be once it's burned.
The limit to the maximum possible compression ratio is detonation. The one factor that has the greatest effect on limiting the detonation is the combustion efficiency.
2 Burning Phases
Pre Power Burning Phase
The spark will use some time to spread into a flame all the way through the fuel. If this time is used when the piston is going down, then some of the fuel's potential power will be lost. So, the best moment to ignite the spark will be before the piston has reached the top of it's compression stroke, usually around 35-40 degrees before top dead center.
Power Production Stroke
The piston reaches the top, reverses direction, and only now is the engine finally making power! The piston is then forced down to the point of the exhaust blowdown phase, about 140 degrees down from top, and the cycle starts over.
Conclusion
As well as the four stroke engine works, it's far from perfect. In addition to the basic tuning problems, a lot of energy is used up to recharge the cylinder for the power stroke. In fact, out of the 720 degrees in a complete cycle, on average, the power phase lasts less than 140 degrees! Many improvements can be made to the remaining 580 degrees that will maximize the power phase, and just as importantly, minimize recharging losses.
The secret to producing a constant output of torque is to consistently fill the cylinders better at all RPM’s.
Modified by quicksilver1689 at 11:25 PM 8/22/2007
08-22-2007, 10:26 PM
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Okay, Let's examine it a little closer !!
We were all taught to think of a 4 stroke engine by the traditional textbook explanation of Intake, Compression, Power & Exhaust.
Textbooks and magazines achieve a sort of instant credibility that comes from the association with $$.
Example:
"This source must be right ... after all, there's obviously
some big money behind it ..."
Because of this association, the vast majority of people never question the things they learn in school, or read in mainstream publications.
In the last issue of Power News you learned to think of a four stroke engine in a whole new way... 8 Phases !!
In the 8 phase engine article,
you'll remember that the intake "stroke"
actually consists of 3 phases.
( Overlap, Suction and Charging. )
,
Stock Yamaha R6 Intake Port
Uh - Oh ...
It turns out that a flowbench measures the least
important aspect of intake cycle efficiency !!
Stock Yamaha R1 Intake Port
It's true !!
When you stop to think about it, a
flowbench only measures the efficiency
of the "suction phase".
That's not the right
thing to measure.
Why ??
Because, it doesn't matter how well
the cylinder is filled at that point in
the intake cycle !
What ... that sounds crazy !!!
It's 100% true ... it's simply a matter of the sequence of events ! The success of the last event, the charging phase, determines the success of the entire intake process.
What happens when a low velocity port fills the cylinder really well, but too early ?? The result is a slower intake charge that stops flowing into the cylinder. Then some of the charge gets pushed back out of the cylinder and into the port as the piston returns up the bore during the intake charging phase. The gain in flow doesn't offset the loss in port velocity.
.
Here's an analogy:
It's just like a roadrace; you can lead for 3 laps, then run out of steam and end up in 7th place.
In racing, only the last lap counts, because the one who leads at the checkered flag wins.
In the intake cycle "race" the last lap is the charging phase,
and the checkered flag is the intake valves closing.
So in the final result, the first 2 phases don't matter if the 3rd phase is unsuccessful !! The total intake volume that will be burned is determined by the amount that remains in the cylinders after the intake valves close. That means that an early gain during the suction phase can be easily lost during the charging phase. And, if the intake charge returns back into the port during the piston's upstroke, the result is going to be a net loss !!
It gets worse !
On a carbureted bike the reversing charge will create an effect called double carburetion, which means the mixture will become even richer with fuel as it passes back over the main jet nozzle for a second time.
Have we lost enough power yet ??
Wait... it gets even worse !!
Here's a real "out of the box" idea: high flow ports also flow really well backwards !!! All that work on the flowbench comes back to haunt you when the piston now has even less resistance to pushing the intake charge back into the " high flowing port " !!!!!!! When you begin to consider the consequences of all this, the whole idea of "more flow is better" comes crashing down like a lead balloon.
What's The Secret ??
Using the race analogy, if you increase the Port Velocity, by making the port smaller, the intake cycle "race" starts out slower, gains momentum and makes a tremendous charge on the "last lap" to overtake the high flow port and win the "race". The interesting thing is, this type of port will always lose in a flowbench contest !!!
We were all taught to think of a 4 stroke engine by the traditional textbook explanation of Intake, Compression, Power & Exhaust.
Textbooks and magazines achieve a sort of instant credibility that comes from the association with $$.
Example:
"This source must be right ... after all, there's obviously
some big money behind it ..."
Because of this association, the vast majority of people never question the things they learn in school, or read in mainstream publications.
In the last issue of Power News you learned to think of a four stroke engine in a whole new way... 8 Phases !!
In the 8 phase engine article,
you'll remember that the intake "stroke"
actually consists of 3 phases.
( Overlap, Suction and Charging. )
,
Stock Yamaha R6 Intake Port
Uh - Oh ...
It turns out that a flowbench measures the least
important aspect of intake cycle efficiency !!
Stock Yamaha R1 Intake Port
It's true !!
When you stop to think about it, a
flowbench only measures the efficiency
of the "suction phase".
That's not the right
thing to measure.
Why ??
Because, it doesn't matter how well
the cylinder is filled at that point in
the intake cycle !
What ... that sounds crazy !!!
It's 100% true ... it's simply a matter of the sequence of events ! The success of the last event, the charging phase, determines the success of the entire intake process.
What happens when a low velocity port fills the cylinder really well, but too early ?? The result is a slower intake charge that stops flowing into the cylinder. Then some of the charge gets pushed back out of the cylinder and into the port as the piston returns up the bore during the intake charging phase. The gain in flow doesn't offset the loss in port velocity.
.
Here's an analogy:
It's just like a roadrace; you can lead for 3 laps, then run out of steam and end up in 7th place.
In racing, only the last lap counts, because the one who leads at the checkered flag wins.
In the intake cycle "race" the last lap is the charging phase,
and the checkered flag is the intake valves closing.
So in the final result, the first 2 phases don't matter if the 3rd phase is unsuccessful !! The total intake volume that will be burned is determined by the amount that remains in the cylinders after the intake valves close. That means that an early gain during the suction phase can be easily lost during the charging phase. And, if the intake charge returns back into the port during the piston's upstroke, the result is going to be a net loss !!
It gets worse !
On a carbureted bike the reversing charge will create an effect called double carburetion, which means the mixture will become even richer with fuel as it passes back over the main jet nozzle for a second time.
Have we lost enough power yet ??
Wait... it gets even worse !!
Here's a real "out of the box" idea: high flow ports also flow really well backwards !!! All that work on the flowbench comes back to haunt you when the piston now has even less resistance to pushing the intake charge back into the " high flowing port " !!!!!!! When you begin to consider the consequences of all this, the whole idea of "more flow is better" comes crashing down like a lead balloon.
What's The Secret ??
Using the race analogy, if you increase the Port Velocity, by making the port smaller, the intake cycle "race" starts out slower, gains momentum and makes a tremendous charge on the "last lap" to overtake the high flow port and win the "race". The interesting thing is, this type of port will always lose in a flowbench contest !!!
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08-24-2007, 07:56 AM
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Re: (2000CTR)
So basically getting your cam timing/lift events correct has more to do with power production than anything else? Makes you wonder when computer controlled valvetrains will start to appear. I wonder if that kind of "problem" is an issue with a Wankel.
08-24-2007, 02:39 PM
#9
Member
Re: (200kCivicSI)
Good artical. You need enough flow to support your BHP and RPM number. Cylinder size and RPM's dictates the flow you need, and the cam events are desigened around them. Or your flow is designed around cam and cyl. size. Pick one.
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