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

tmhutch · 05-13-2010, 06:11 AM

Exhaust Lobe

The theoretically perfect exhaust lobe wouldn’t open the valve until BDC where it would instantaneously open to full valve lift. This way every last ounce of power could be extracted from the ignited air/fuel mixture by the piston while at the same time pumping losses would be minimized. Similar to a perfect intake event, the perfect exhaust lobe would require an instantaneous full opening of the valve. This way the exhaust gasses could escape easily and efficiently without a lot of backpressure. Unfortunately we are again bound by current technology which requires a compromise in low end performance in the pursuit of high RPM horsepower. To find out more, let’s look at the individual valve events.

Exhaust Valve Closing

When we are speaking about EVC the topic will revolve around overlap, intake scavenging, exhaust scavenging, combustion chamber heat and reversion.

When closed at just the right moment the exhaust valve allows the piston to push out all the exhaust, it gives the escaping gasses time to pull exhaust from the combustion chamber where the piston cannot reach, and it delays closing just long enough so the inertia of the escaping exhaust gas can initiate the intake charge, but close soon enough to prevent it from pulling the air/fuel mix out the exhaust pipe.

Most exhaust gasses are evacuated during the blow down period at exhaust valve open but it is critical that the valve is not closed to early or we end up with a contaminated intake charge and no cross flow to the intake. This is a fairly straight forward consequence but an exhaust valve closed too late is a little more complicated. It can over scavenge the exhaust, cause reversion back into the cylinder, or prevent the piston from pulling in enough air/fuel to produce sufficient power. To what extent any of these conditions occur or do not occur has as much to do with how well the exhaust is matched to the combination as the actual specifications of the camshaft.

Ignoring the exhaust system component of this equation for a moment, we’ll address each of the potential problems of closing the exhaust valve too late. The problem with over scavenging the cylinder is that the air fuel mix will be pulled out the exhaust pipe instead of down into the cylinder, resulting in less combustible material to act against the piston and poor fuel mileage. In the case of an excessively late closing valve, not only is too much air/fuel mixture pulled out the exhaust but enough heat can be pulled from the combustion chamber to result in reduced combustion efficiency. Finally, the ultimate mismatch occurs when the valve is closed so late that the piston has traveled far enough down the bore to negatively affect the amount of air it can draw in. Remember that the piston cannot start pulling in air/fuel until the exhaust valve is completely closed. A late closing works with high RPM applications because the speed of the piston creates a more powerful pressure differential between the cylinder and the intake track so even though the piston starts to pull charge in later in the process, it is a more powerful pull that gets the job. If the exhaust valve is closed to late in relation to the engines maximum RPM capability than the piston never reaches high enough speed to create the dramatic pressure differential needed to make up for the loss of time it has to pull on the intake charge.

EVC and the Exhaust System

In addition to over scavenging, the other possible condition that can occur with a late closing exhaust valve is reversion. If the exhaust system doesn’t have either enough escape velocity or it has too much resistance, exhaust can reverse direction and come back into the cylinder, polluting the intake charge.

Back pressure usually rears its head at higher RPM when the piston cannot overcome the resistance of the restrictive exhaust, and reversion is caused by poor escape velocity in the exhaust system. Both are RPM dependent and require an exhaust system that matches the RPM range of the camshaft.

The exhaust system is a critical component to a successful exhaust lobe and has the ability to render a perfect lobe ineffective, and help a poorly spec’d lobe perform better. An engine cammed properly for high RPM with a late closing exhaust valve needs an appropriately sized exhaust to complement it. A small primary tube header combined with the late exhaust close will over scavenge the cylinder at low RPM and fail to rid exhaust gasses at high RPM.

Conversely, a primary tube that is too large for a given camshafts closing point and intended RPM range may not properly scavenge the combustion chamber until the RPM’s are beyond the combinations capability. In which case the exhaust may never be fully evacuated from the cylinder and the intake charge will experience little to know initiation during overlap.

A certain amount of reversion and inadequate scavenging is inevitable for properly designed, high RPM applications because an exhaust big enough to handle 7500 RPM will carry little to no escape velocity at very low RPM. Whether “too big” or “too small”, every possible level of mismatch can occur but there is some consolation in that it is only the extremes that amount to sizeable losses.

EVC and Overlap

When addressing the exhaust and intake scavenging component of the exhaust valve closing point, were actually talking about overlap. It is this period where both the intake and exhaust valve are open at the same time that determines how well exhaust is evacuated from the space in the combustion chamber where the piston cannot reach (clearance volume) and also how much the escaping exhaust helps initiate the intake charge.

When speaking in terms of positioning the intake and exhaust lobes to obtain proper overlap, it is the exhaust lobe that usually bears the brunt of the responsibility because in general, an intake lobe will have an appropriate lobe center for a given combination that is decided on in terms of dynamic compression, pressure differential and inertial charging near the end of the cycle. These components make the intake lobe more sensitive than the exhaust lobe to ideal placement and it is for that reason that the more forgiving exhaust lobe can often be moved around without jeopardizing its primary purpose.

Even though the exhaust lobe is typically less sensitive to movement, it is important to note that whether we are adding duration or moving the lobe, both the opening and closing events could be altered and the consequences need to be considered.

1) Adding duration to delay the closing and increase overlap will also result in an earlier opening point. Opening the exhaust valve further into the combustion period provides additional blowdown pressure for removing exhaust gasses but can have a negative effect on low to mid RPM power because it is sent out the exhaust pipe.

2) Retarding the exhaust lobe as opposed to adding duration to increase overlap will also delay the point at which the exhaust valve opens. This helps low to mid RPM power but may result in greater pumping losses and incomplete exhaust removal.

3) If we want to maintain the same opening point but alter the closing point we can add duration and retard the lobe so all of the added duration takes place on the closing side while the opening point remains the same.
Exhaust valve closing is no different from any other camshaft event. It needs to be considered from a broad perspective, spec’d properly and supported with matching components throughout the engine.

Exhaust Valve Opening

The exhaust valve opening event can make for interesting conversation because some engine combinations can make it very forgiving to improper placement while other combinations will make it a critical component to a successful build. Harold Brookshire considers it the most important valve event while others consider it the least important. Who is right? They all are, given the right context. The argument is slightly different for every engine. Let’s find out why.

As we saw earlier, the exhaust lobe closing point can be critical to the success of the intake cycle. But it is the opening event of the exhaust lobe that is most critical for expelling the burnt gasses. The end of the combustion cycle marks the beginning of the exhaust cycle. This is where the piston starts moving back up the bore shoving the exhaust out past the valve. When the piston reaches top dead center and has pushed the exhaust out, the cycle is nearly finished. This simplistic view gives a good idea of how the exhaust cycle works but as we know it isn’t quite that simple.

In order to effectively remove all the exhaust and avoid pumping losses (horsepower used by the piston to push exhaust out) the exhaust cycle actually begins around 70 degrees BBDC as the valve begins to open during the power stroke (blowdown). By cracking the valve open under high pressure, much of the exhaust can escape early in the cycle and create good velocity for scavenging later in the cycle. The downside is the energy used to rid the cylinder of exhaust will not be able to act against the piston. That energy is lost and can never be recovered, so the compromise is a balance between how much energy is lost out the exhaust pipe VS how much is used to push the piston down VS how much horsepower is used by the piston to push the remaining exhaust out (pumping losses).

Each combination has a point where opening the valve any sooner loses more horsepower out the exhaust than is gained by the pumping losses it eliminates. The opposite is also true; each combination has a point where delaying the exhaust valve opening to extract extra power from the still expanding combustion costs more power to pump the leftover exhaust gasses out than is gained by the force it exerts on the piston. In typical fashion, this is largely RPM dependent.

A low RPM combination can utilize a later closing and benefit from the extra force applied against the piston. Exhaust is still adequately eliminated with the low RPM, later closing combination because of another recurring theme in camshaft events; time. Most camshaft events are a function of time which makes them RPM centric. Exhaust opening is no different, and at low RPM there is enough time for the exhaust to escape. Additionally, the remaining exhaust gasses do not act as forcefully against a slower moving piston. A high speed piston will slam into the remaining gasses creating a strong opposing force as it tries to push the exhaust past the valve, but a slow speed piston has a much gentler encounter with the remaining exhaust, placing less demand on the valve while allowing more time to move past it with less corresponding resistance.

As RPM increases the exhaust valve needs to be opened earlier because the amount of time it has to rid the cylinder of exhaust gasses becomes shorter. Opening the valve sooner exposes the exhaust port to progressively higher cylinder pressure. As the amount of time decreases to get rid of the exhaust gas, more pressure is needed to jump start the cycle and increase exit velocity. Finding the correct pressure needed to adequately blow enough exhaust past the valve (blow down) without wasting power can be a delicate balance.

There is no hard and fast rule that defines a specific RPM but to simplify the concept I will simply note that if the exhaust valve is opened a little too early it may only require a few hundred more RPM to move the cam into its power band. From a big picture standpoint we can see that a few degrees off may not be a big deal. If we have a combination that can easily handle a few hundred more RPM in terms of adequate intake flow, intake pressure wave tuning, exhaust system capability, short block durability, valvetrain stability, etc. than it won’t be a problem. Then again if we don’t have the wiggle room, horsepower is left on the table.

When attempting to properly define the timing of the exhaust opening event, engine builders like to point out that 80% of the power is extracted from the combustion by the time the piston has reached 90 degrees past TDC. Often people will infer from this statement that the last 90 degrees is useless. For example, a popular magazine author recently wrote that; “The bottom half of the power stroke actually provides very little in terms of engine power”.
Racer Brown, a veteran cam designer states that; “Very late exhaust valve opening points, when properly done will improve engine performance through most of the engine speed range, if not all of it.”

You might think I would be more inclined to agree with a veteran cam designer rather than a writer for an automobile magazine but the fact is they are both right under the right circumstances. Racer Brown further stated the importance of considering the RPM range of the engine when determining the opening event.

There is more to the RPM element than just time. We also have to consider piston speed during the blowdown period. As the piston moves farther down the cylinder it is moving away from an ever weakening combustion charge. The piston and the combustion are moving in the same direction. As a result, force acting on the piston is going to be greater on a slower moving piston than a faster moving piston. Think about running with the wind at your back, the faster you run the less that force pushes against you. As the RPM increases, the piston simply runs away from any meaningful combustion earlier in the power stroke.

Crankshaft stroke and connecting rod length also need to be considered in terms of piston speed and position during the blowdown period but an entire chapter can be written on that subject alone so I’m going to take a mulligan on it for now.

Another related element is static and dynamic compression, or more appropriately, cylinder pressure. It affects both when the valve should be opened and the influence the combustion has on the piston throughout the power stroke. High cylinder pressure environments will exact more force against the piston further into the exhaust stroke so that a later opening event becomes more relevant in terms of the torque it provides and the ability to affectively evacuate the exhaust when the piston is further down the bore. For example, higher cylinder pressure can be just as effective at pushing exhaust out at 50* BBDC as a lower pressured cylinder at 60* BBDC. Not only that but the pressure being exerted against the piston at 50* BBDC in a high compression combination can be just as forceful as the pressure being exerted at 60* BBDC in a lower compression combination. Compression and cylinder pressure support the best of both worlds, low speed torque and high RPM horsepower.

A consideration related to cylinder pressure is volumetric efficiency. An engine operating at 75% VE will be generating less useful force against the piston at 50* before BDC than an engine running at 100%. Think of this in terms of cylinder fill or simply, more air/fuel causing a bigger explosion and more cylinder pressure. This also has an effect on when the exhaust valve should be opened.

As you can see, there are no simple valve events. Each is unique in its own way and has the potential to make or break an engine combination.

05-13-2010, 06:11 AM	#5
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond Exhaust Lobe The theoretically perfect exhaust lobe wouldn’t open the valve until BDC where it would instantaneously open to full valve lift. This way every last ounce of power could be extracted from the ignited air/fuel mixture by the piston while at the same time pumping losses would be minimized. Similar to a perfect intake event, the perfect exhaust lobe would require an instantaneous full opening of the valve. This way the exhaust gasses could escape easily and efficiently without a lot of backpressure. Unfortunately we are again bound by current technology which requires a compromise in low end performance in the pursuit of high RPM horsepower. To find out more, let’s look at the individual valve events. Exhaust Valve Closing When we are speaking about EVC the topic will revolve around overlap, intake scavenging, exhaust scavenging, combustion chamber heat and reversion. When closed at just the right moment the exhaust valve allows the piston to push out all the exhaust, it gives the escaping gasses time to pull exhaust from the combustion chamber where the piston cannot reach, and it delays closing just long enough so the inertia of the escaping exhaust gas can initiate the intake charge, but close soon enough to prevent it from pulling the air/fuel mix out the exhaust pipe. Most exhaust gasses are evacuated during the blow down period at exhaust valve open but it is critical that the valve is not closed to early or we end up with a contaminated intake charge and no cross flow to the intake. This is a fairly straight forward consequence but an exhaust valve closed too late is a little more complicated. It can over scavenge the exhaust, cause reversion back into the cylinder, or prevent the piston from pulling in enough air/fuel to produce sufficient power. To what extent any of these conditions occur or do not occur has as much to do with how well the exhaust is matched to the combination as the actual specifications of the camshaft. Ignoring the exhaust system component of this equation for a moment, we’ll address each of the potential problems of closing the exhaust valve too late. The problem with over scavenging the cylinder is that the air fuel mix will be pulled out the exhaust pipe instead of down into the cylinder, resulting in less combustible material to act against the piston and poor fuel mileage. In the case of an excessively late closing valve, not only is too much air/fuel mixture pulled out the exhaust but enough heat can be pulled from the combustion chamber to result in reduced combustion efficiency. Finally, the ultimate mismatch occurs when the valve is closed so late that the piston has traveled far enough down the bore to negatively affect the amount of air it can draw in. Remember that the piston cannot start pulling in air/fuel until the exhaust valve is completely closed. A late closing works with high RPM applications because the speed of the piston creates a more powerful pressure differential between the cylinder and the intake track so even though the piston starts to pull charge in later in the process, it is a more powerful pull that gets the job. If the exhaust valve is closed to late in relation to the engines maximum RPM capability than the piston never reaches high enough speed to create the dramatic pressure differential needed to make up for the loss of time it has to pull on the intake charge. EVC and the Exhaust System In addition to over scavenging, the other possible condition that can occur with a late closing exhaust valve is reversion. If the exhaust system doesn’t have either enough escape velocity or it has too much resistance, exhaust can reverse direction and come back into the cylinder, polluting the intake charge. Back pressure usually rears its head at higher RPM when the piston cannot overcome the resistance of the restrictive exhaust, and reversion is caused by poor escape velocity in the exhaust system. Both are RPM dependent and require an exhaust system that matches the RPM range of the camshaft. The exhaust system is a critical component to a successful exhaust lobe and has the ability to render a perfect lobe ineffective, and help a poorly spec’d lobe perform better. An engine cammed properly for high RPM with a late closing exhaust valve needs an appropriately sized exhaust to complement it. A small primary tube header combined with the late exhaust close will over scavenge the cylinder at low RPM and fail to rid exhaust gasses at high RPM. Conversely, a primary tube that is too large for a given camshafts closing point and intended RPM range may not properly scavenge the combustion chamber until the RPM’s are beyond the combinations capability. In which case the exhaust may never be fully evacuated from the cylinder and the intake charge will experience little to know initiation during overlap. A certain amount of reversion and inadequate scavenging is inevitable for properly designed, high RPM applications because an exhaust big enough to handle 7500 RPM will carry little to no escape velocity at very low RPM. Whether “too big” or “too small”, every possible level of mismatch can occur but there is some consolation in that it is only the extremes that amount to sizeable losses. EVC and Overlap When addressing the exhaust and intake scavenging component of the exhaust valve closing point, were actually talking about overlap. It is this period where both the intake and exhaust valve are open at the same time that determines how well exhaust is evacuated from the space in the combustion chamber where the piston cannot reach (clearance volume) and also how much the escaping exhaust helps initiate the intake charge. When speaking in terms of positioning the intake and exhaust lobes to obtain proper overlap, it is the exhaust lobe that usually bears the brunt of the responsibility because in general, an intake lobe will have an appropriate lobe center for a given combination that is decided on in terms of dynamic compression, pressure differential and inertial charging near the end of the cycle. These components make the intake lobe more sensitive than the exhaust lobe to ideal placement and it is for that reason that the more forgiving exhaust lobe can often be moved around without jeopardizing its primary purpose. Even though the exhaust lobe is typically less sensitive to movement, it is important to note that whether we are adding duration or moving the lobe, both the opening and closing events could be altered and the consequences need to be considered. 1) Adding duration to delay the closing and increase overlap will also result in an earlier opening point. Opening the exhaust valve further into the combustion period provides additional blowdown pressure for removing exhaust gasses but can have a negative effect on low to mid RPM power because it is sent out the exhaust pipe. 2) Retarding the exhaust lobe as opposed to adding duration to increase overlap will also delay the point at which the exhaust valve opens. This helps low to mid RPM power but may result in greater pumping losses and incomplete exhaust removal. 3) If we want to maintain the same opening point but alter the closing point we can add duration and retard the lobe so all of the added duration takes place on the closing side while the opening point remains the same. Exhaust valve closing is no different from any other camshaft event. It needs to be considered from a broad perspective, spec’d properly and supported with matching components throughout the engine. Exhaust Valve Opening The exhaust valve opening event can make for interesting conversation because some engine combinations can make it very forgiving to improper placement while other combinations will make it a critical component to a successful build. Harold Brookshire considers it the most important valve event while others consider it the least important. Who is right? They all are, given the right context. The argument is slightly different for every engine. Let’s find out why. As we saw earlier, the exhaust lobe closing point can be critical to the success of the intake cycle. But it is the opening event of the exhaust lobe that is most critical for expelling the burnt gasses. The end of the combustion cycle marks the beginning of the exhaust cycle. This is where the piston starts moving back up the bore shoving the exhaust out past the valve. When the piston reaches top dead center and has pushed the exhaust out, the cycle is nearly finished. This simplistic view gives a good idea of how the exhaust cycle works but as we know it isn’t quite that simple. In order to effectively remove all the exhaust and avoid pumping losses (horsepower used by the piston to push exhaust out) the exhaust cycle actually begins around 70 degrees BBDC as the valve begins to open during the power stroke (blowdown). By cracking the valve open under high pressure, much of the exhaust can escape early in the cycle and create good velocity for scavenging later in the cycle. The downside is the energy used to rid the cylinder of exhaust will not be able to act against the piston. That energy is lost and can never be recovered, so the compromise is a balance between how much energy is lost out the exhaust pipe VS how much is used to push the piston down VS how much horsepower is used by the piston to push the remaining exhaust out (pumping losses). Each combination has a point where opening the valve any sooner loses more horsepower out the exhaust than is gained by the pumping losses it eliminates. The opposite is also true; each combination has a point where delaying the exhaust valve opening to extract extra power from the still expanding combustion costs more power to pump the leftover exhaust gasses out than is gained by the force it exerts on the piston. In typical fashion, this is largely RPM dependent. A low RPM combination can utilize a later closing and benefit from the extra force applied against the piston. Exhaust is still adequately eliminated with the low RPM, later closing combination because of another recurring theme in camshaft events; time. Most camshaft events are a function of time which makes them RPM centric. Exhaust opening is no different, and at low RPM there is enough time for the exhaust to escape. Additionally, the remaining exhaust gasses do not act as forcefully against a slower moving piston. A high speed piston will slam into the remaining gasses creating a strong opposing force as it tries to push the exhaust past the valve, but a slow speed piston has a much gentler encounter with the remaining exhaust, placing less demand on the valve while allowing more time to move past it with less corresponding resistance. As RPM increases the exhaust valve needs to be opened earlier because the amount of time it has to rid the cylinder of exhaust gasses becomes shorter. Opening the valve sooner exposes the exhaust port to progressively higher cylinder pressure. As the amount of time decreases to get rid of the exhaust gas, more pressure is needed to jump start the cycle and increase exit velocity. Finding the correct pressure needed to adequately blow enough exhaust past the valve (blow down) without wasting power can be a delicate balance. There is no hard and fast rule that defines a specific RPM but to simplify the concept I will simply note that if the exhaust valve is opened a little too early it may only require a few hundred more RPM to move the cam into its power band. From a big picture standpoint we can see that a few degrees off may not be a big deal. If we have a combination that can easily handle a few hundred more RPM in terms of adequate intake flow, intake pressure wave tuning, exhaust system capability, short block durability, valvetrain stability, etc. than it won’t be a problem. Then again if we don’t have the wiggle room, horsepower is left on the table. When attempting to properly define the timing of the exhaust opening event, engine builders like to point out that 80% of the power is extracted from the combustion by the time the piston has reached 90 degrees past TDC. Often people will infer from this statement that the last 90 degrees is useless. For example, a popular magazine author recently wrote that; “The bottom half of the power stroke actually provides very little in terms of engine power”. Racer Brown, a veteran cam designer states that; “Very late exhaust valve opening points, when properly done will improve engine performance through most of the engine speed range, if not all of it.” You might think I would be more inclined to agree with a veteran cam designer rather than a writer for an automobile magazine but the fact is they are both right under the right circumstances. Racer Brown further stated the importance of considering the RPM range of the engine when determining the opening event. There is more to the RPM element than just time. We also have to consider piston speed during the blowdown period. As the piston moves farther down the cylinder it is moving away from an ever weakening combustion charge. The piston and the combustion are moving in the same direction. As a result, force acting on the piston is going to be greater on a slower moving piston than a faster moving piston. Think about running with the wind at your back, the faster you run the less that force pushes against you. As the RPM increases, the piston simply runs away from any meaningful combustion earlier in the power stroke. Crankshaft stroke and connecting rod length also need to be considered in terms of piston speed and position during the blowdown period but an entire chapter can be written on that subject alone so I’m going to take a mulligan on it for now. Another related element is static and dynamic compression, or more appropriately, cylinder pressure. It affects both when the valve should be opened and the influence the combustion has on the piston throughout the power stroke. High cylinder pressure environments will exact more force against the piston further into the exhaust stroke so that a later opening event becomes more relevant in terms of the torque it provides and the ability to affectively evacuate the exhaust when the piston is further down the bore. For example, higher cylinder pressure can be just as effective at pushing exhaust out at 50* BBDC as a lower pressured cylinder at 60* BBDC. Not only that but the pressure being exerted against the piston at 50* BBDC in a high compression combination can be just as forceful as the pressure being exerted at 60* BBDC in a lower compression combination. Compression and cylinder pressure support the best of both worlds, low speed torque and high RPM horsepower. A consideration related to cylinder pressure is volumetric efficiency. An engine operating at 75% VE will be generating less useful force against the piston at 50* before BDC than an engine running at 100%. Think of this in terms of cylinder fill or simply, more air/fuel causing a bigger explosion and more cylinder pressure. This also has an effect on when the exhaust valve should be opened. As you can see, there are no simple valve events. Each is unique in its own way and has the potential to make or break an engine combination.