2003-04 Mach 1 Registry Owners Club - View Single Post

tmhutch · 05-13-2010, 06:09 AM

Intake Lobe

There are 4 major components of the intake lobe that contribute to the success of any given combination; Duration, IVC, IVO, and Lobe Centerline. Intake events are generally considered more important and less forgiving than exhaust events. Not only is the induction cycle slightly more complex but it can be dramatically affected by small changes in the engines combination. For that reason I will dedicate considerable space to the intake event.

Intake Lobe Duration

The most important place to start intake lobe design is duration. There are two main components of duration that contribute to building power (other than overlap). The first is it gives the piston access to an open valve for a longer period of time, allowing air to continue filling the cylinder efficiently at high RPM. Note that an engine will not automatically make more power when duration is added unless the engine rev’s high enough to utilize it. Putting aside inertial charging, a cylinder can only hold so much air. The volume of that cylinder when the piston is at BDC never changes and that space represents the maximum amount of air that can be drawn in. An engine makes 500 horsepower at 6500 RPM with roughly the same amount of air/fuel in the cylinder that it uses to make 50 horsepower at idle. It’s the number of revolutions per minute that produces the work known as horsepower. As engine RPM increases, the amount of time the valve remains open is reduced. Mechanically the valve is open for the same number of degrees but it is the time component that becomes limited at high RPM. At a certain point the mechanical elements are moving too fast for the air to keep up and the engine runs out of breath. Increasing the cams duration adds critical milliseconds to the amount of time the valve stays open and feeds the engine.

Adding duration also extends both the opening point and the closing point. The earlier opening places the valve at a more useful lift when the induction stroke begins, making the air/fuel mixture more responsive to the piston. Extending the closing point allows the inertia of the air/fuel to continue filling the cylinder ABDC when the piston starts moving back up the bore. This allows better cylinder fill at high RPM. As you will see later, these two power producing timing events divide the cam design industry into two different camps; those that believe the intake valve opening point is the purveyor of power production and those that think the closing point is the holy grail. I remain neutral.

Intake Valve Closing

Now we’re going to get into the nuts and bolts of things with some repetition and further explanation. The Intake Valve Closing Point is often considered the most important because of its direct effect on the engines operating range, volumetric efficiency, and dynamic compression ratio. First thing to note is that the intake valve closes after bottom dead center (ABDC). In other words, it is still open after the piston has traveled all the way to the bottom of the bore and started moving back up the cylinder.

Inertial Charging

The reason for waiting until ABDC to close the valve is that the incoming charge will keep forcing its way past the valve even after the piston has stopped drawing it in. Higher RPM means higher velocity and continued cylinder filling further into the compression stroke. On a high RPM race engine, air/fuel can continue forcing its way past the closing intake valve long after the piston has started its path up the bore because the fast moving piston has increased the speed of the air to the point that it cannot easily be stopped. To what extent the “inertial supercharging” takes place depends on many variables but the most obvious elements are velocity through the intake path and the RPM of the engine, or more specifically, piston speed. Along with pressure wave, it is inertial supercharging that is responsible for the volumetric efficiencies beyond 100% found in properly designed and built performance engines. Similar to a supercharger or turbocharger, inertia literally rams more fuel into the cylinder than would be possible using atmospheric pressure alone. More detail to come.

Dynamic Compression

If inertial supercharging is the high RPM component of Intake Valve Closing, than the Dynamic Compression Ratio (DCR) is the low RPM component. Dynamic compression is a label used to give a general idea of cylinder pressure over a limited, low RPM range of operation. DCR cannot account for non mechanical elements of the induction cycle because it is derived from a mathematical equation involving stroke, connecting rod length and intake valve closing. As a result, the actual figure never changes, even though the cylinder pressure it is supposed to represent is greatly affected by RPM. This makes it less accurate as the RPM’s increase.

To understand dynamic compression ratio it is easiest to first understand the basics of calculating static compression ratio. When an engine is rated at 10:1 compression it means that the volume of air in the cylinder when the piston is at Bottom Dead Center (swept volume) is 10 times the volume of air that the cylinder can hold when the piston is at TDC. So the piston compresses 10 parts of air into a tiny space 1/10th its original size when at TDC, or 10:1 Static Compression Ratio. That’s it, fairly straight forward.

What differentiates the Static Compression Ratio from the Dynamic Compression Ratio is that we use the pistons position in the bore at time of intake valve closing, rather than BDC. The logic behind this is that holding the intake valve open after the piston has started moving back up the bore causes some of the inducted air to be pushed back out of the cylinder and into the intake port. This means that less air is trapped and compressed. This loss of air/fuel mix into the intake port is known as intake reversion. Instead of compressing 10 parts of air into 1/10th the space (10:1 compression), we are only compressing 8 parts air into 1/8th the space, or 8:1 dynamic compression. Closing the intake valve later causes less air/fuel to be trapped, compressed and ignited which translates into a less power and torque at low to mid RPM. Conversely, an earlier closing raises DCR and increases torque/hp at low to mid RPM.

To recap, the two components critical to IVC is Dynamic Compression and Inertial Supercharging. Dynamic compression prefers an early IVC and is critical to low RPM operation while being largely irrelevant to high RPM operation. Inertial supercharging uses a late IVC and is critical to high RPM operation while being largely irrelevant at low RPM operation.

Time, Not Distance

To understand the RPM component of IVC we have to first understand that valve events are largely a function of time (milliseconds). In the case of the intake event, the air/fuel mass is supposed to fill the cylinder between the time the intake valve opens and when the piston reaches just past BDC when the valve closes. With low to mid RPM operation this is easily accomplished but as RPM increases, the air/fuel mix has less and less time to fill the cylinder before the piston reaches BDC. This is one of the major power and RPM limiting components of airflow.

Here is why: At low RPM the air is able to follow closely behind the piston on the intake stroke so that at BDC the piston has pulled in a full cylinder of air before the intake valve closes. At this point the air/fuel is trapped for compression and ignition.

As RPM increases, the intake charge or column of air/fuel mixture begins to lag behind the faster moving piston and closing intake valve. It continues to fall farther and farther behind as the engine revs higher. As a result, less of that column of air makes it past the valve before it slams shut. When this happens we end up with only a partially filled cylinder when the valve closes. This limits the RPM capability of the engine because it basically runs out of breath.

The remedy is to close the intake valve later. The high speed of the mechanical parts has two components. One is that the piston and valve outrun the intake charge and the second is positive in that the high speed of the piston also creates a more powerful pressure differential and a faster moving intake charge. At high RPM the piston outruns the intake charge at the beginning of the cycle but eventually the intake charge gets up to speed and when it does it is really moving. If we keep the intake valve open a little longer ABDC we can use the inertia of the high speed charge to make up for the slow starting air’s inability to fill the cylinder by the time the piston hits BDC. Not only that but as stated earlier, if the air/fuel is moving fast enough than the inertia can actually pack the cylinder with more air than the piston could ever pull in by itself.

One More Time

The bottom line from an IVC standpoint is that when the engines intended RPM range is low we tune around the concept of dynamic compression by closing the intake valve early to trap and compress as much air as possible. With low RPM combinations inertial charging is insignificant and the piston movement is slow enough so that the air can keep up with it. As a result the induction cycle has enough time to utilize an early IVC and capture a full cylinder of air near BDC. However, exceeding 100% cylinder fill is unlikely.

When the engines intended RPM range is high we tune around a late intake valve closing and inertial supercharging. Technically the late IVC combination has low dynamic compression but the reality is that only applies to low and mid RPM operation. As the RPM’s increase, the incoming column of air begins packing the cylinder beyond what the piston could draw in on its own. If the dynamic compression equation could account for inertial charging, the number it produced could actually be higher than the static compression. It’s not that dynamic compression is entirely irrelevant for high RPM combinations but it’s correlation to cylinder pressure becomes less accurate.

That sums up IVC so we will move on to the intake valve opening event.

05-13-2010, 06:09 AM	#3
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond Intake Lobe There are 4 major components of the intake lobe that contribute to the success of any given combination; Duration, IVC, IVO, and Lobe Centerline. Intake events are generally considered more important and less forgiving than exhaust events. Not only is the induction cycle slightly more complex but it can be dramatically affected by small changes in the engines combination. For that reason I will dedicate considerable space to the intake event. Intake Lobe Duration The most important place to start intake lobe design is duration. There are two main components of duration that contribute to building power (other than overlap). The first is it gives the piston access to an open valve for a longer period of time, allowing air to continue filling the cylinder efficiently at high RPM. Note that an engine will not automatically make more power when duration is added unless the engine rev’s high enough to utilize it. Putting aside inertial charging, a cylinder can only hold so much air. The volume of that cylinder when the piston is at BDC never changes and that space represents the maximum amount of air that can be drawn in. An engine makes 500 horsepower at 6500 RPM with roughly the same amount of air/fuel in the cylinder that it uses to make 50 horsepower at idle. It’s the number of revolutions per minute that produces the work known as horsepower. As engine RPM increases, the amount of time the valve remains open is reduced. Mechanically the valve is open for the same number of degrees but it is the time component that becomes limited at high RPM. At a certain point the mechanical elements are moving too fast for the air to keep up and the engine runs out of breath. Increasing the cams duration adds critical milliseconds to the amount of time the valve stays open and feeds the engine. Adding duration also extends both the opening point and the closing point. The earlier opening places the valve at a more useful lift when the induction stroke begins, making the air/fuel mixture more responsive to the piston. Extending the closing point allows the inertia of the air/fuel to continue filling the cylinder ABDC when the piston starts moving back up the bore. This allows better cylinder fill at high RPM. As you will see later, these two power producing timing events divide the cam design industry into two different camps; those that believe the intake valve opening point is the purveyor of power production and those that think the closing point is the holy grail. I remain neutral. Intake Valve Closing Now we’re going to get into the nuts and bolts of things with some repetition and further explanation. The Intake Valve Closing Point is often considered the most important because of its direct effect on the engines operating range, volumetric efficiency, and dynamic compression ratio. First thing to note is that the intake valve closes after bottom dead center (ABDC). In other words, it is still open after the piston has traveled all the way to the bottom of the bore and started moving back up the cylinder. Inertial Charging The reason for waiting until ABDC to close the valve is that the incoming charge will keep forcing its way past the valve even after the piston has stopped drawing it in. Higher RPM means higher velocity and continued cylinder filling further into the compression stroke. On a high RPM race engine, air/fuel can continue forcing its way past the closing intake valve long after the piston has started its path up the bore because the fast moving piston has increased the speed of the air to the point that it cannot easily be stopped. To what extent the “inertial supercharging” takes place depends on many variables but the most obvious elements are velocity through the intake path and the RPM of the engine, or more specifically, piston speed. Along with pressure wave, it is inertial supercharging that is responsible for the volumetric efficiencies beyond 100% found in properly designed and built performance engines. Similar to a supercharger or turbocharger, inertia literally rams more fuel into the cylinder than would be possible using atmospheric pressure alone. More detail to come. Dynamic Compression If inertial supercharging is the high RPM component of Intake Valve Closing, than the Dynamic Compression Ratio (DCR) is the low RPM component. Dynamic compression is a label used to give a general idea of cylinder pressure over a limited, low RPM range of operation. DCR cannot account for non mechanical elements of the induction cycle because it is derived from a mathematical equation involving stroke, connecting rod length and intake valve closing. As a result, the actual figure never changes, even though the cylinder pressure it is supposed to represent is greatly affected by RPM. This makes it less accurate as the RPM’s increase. To understand dynamic compression ratio it is easiest to first understand the basics of calculating static compression ratio. When an engine is rated at 10:1 compression it means that the volume of air in the cylinder when the piston is at Bottom Dead Center (swept volume) is 10 times the volume of air that the cylinder can hold when the piston is at TDC. So the piston compresses 10 parts of air into a tiny space 1/10th its original size when at TDC, or 10:1 Static Compression Ratio. That’s it, fairly straight forward. What differentiates the Static Compression Ratio from the Dynamic Compression Ratio is that we use the pistons position in the bore at time of intake valve closing, rather than BDC. The logic behind this is that holding the intake valve open after the piston has started moving back up the bore causes some of the inducted air to be pushed back out of the cylinder and into the intake port. This means that less air is trapped and compressed. This loss of air/fuel mix into the intake port is known as intake reversion. Instead of compressing 10 parts of air into 1/10th the space (10:1 compression), we are only compressing 8 parts air into 1/8th the space, or 8:1 dynamic compression. Closing the intake valve later causes less air/fuel to be trapped, compressed and ignited which translates into a less power and torque at low to mid RPM. Conversely, an earlier closing raises DCR and increases torque/hp at low to mid RPM. To recap, the two components critical to IVC is Dynamic Compression and Inertial Supercharging. Dynamic compression prefers an early IVC and is critical to low RPM operation while being largely irrelevant to high RPM operation. Inertial supercharging uses a late IVC and is critical to high RPM operation while being largely irrelevant at low RPM operation. Time, Not Distance To understand the RPM component of IVC we have to first understand that valve events are largely a function of time (milliseconds). In the case of the intake event, the air/fuel mass is supposed to fill the cylinder between the time the intake valve opens and when the piston reaches just past BDC when the valve closes. With low to mid RPM operation this is easily accomplished but as RPM increases, the air/fuel mix has less and less time to fill the cylinder before the piston reaches BDC. This is one of the major power and RPM limiting components of airflow. Here is why: At low RPM the air is able to follow closely behind the piston on the intake stroke so that at BDC the piston has pulled in a full cylinder of air before the intake valve closes. At this point the air/fuel is trapped for compression and ignition. As RPM increases, the intake charge or column of air/fuel mixture begins to lag behind the faster moving piston and closing intake valve. It continues to fall farther and farther behind as the engine revs higher. As a result, less of that column of air makes it past the valve before it slams shut. When this happens we end up with only a partially filled cylinder when the valve closes. This limits the RPM capability of the engine because it basically runs out of breath. The remedy is to close the intake valve later. The high speed of the mechanical parts has two components. One is that the piston and valve outrun the intake charge and the second is positive in that the high speed of the piston also creates a more powerful pressure differential and a faster moving intake charge. At high RPM the piston outruns the intake charge at the beginning of the cycle but eventually the intake charge gets up to speed and when it does it is really moving. If we keep the intake valve open a little longer ABDC we can use the inertia of the high speed charge to make up for the slow starting air’s inability to fill the cylinder by the time the piston hits BDC. Not only that but as stated earlier, if the air/fuel is moving fast enough than the inertia can actually pack the cylinder with more air than the piston could ever pull in by itself. One More Time The bottom line from an IVC standpoint is that when the engines intended RPM range is low we tune around the concept of dynamic compression by closing the intake valve early to trap and compress as much air as possible. With low RPM combinations inertial charging is insignificant and the piston movement is slow enough so that the air can keep up with it. As a result the induction cycle has enough time to utilize an early IVC and capture a full cylinder of air near BDC. However, exceeding 100% cylinder fill is unlikely. When the engines intended RPM range is high we tune around a late intake valve closing and inertial supercharging. Technically the late IVC combination has low dynamic compression but the reality is that only applies to low and mid RPM operation. As the RPM’s increase, the incoming column of air begins packing the cylinder beyond what the piston could draw in on its own. If the dynamic compression equation could account for inertial charging, the number it produced could actually be higher than the static compression. It’s not that dynamic compression is entirely irrelevant for high RPM combinations but it’s correlation to cylinder pressure becomes less accurate. That sums up IVC so we will move on to the intake valve opening event. Last edited by tmhutch; 05-15-2010 at 02:56 PM.