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

tmhutch · 05-13-2010, 06:08 AM

The 4 (5) Cycles

Before going any further we need to get a solid picture in our heads of what happens in a four stroke engine as it relates to camshaft timing. The four cycles include the Intake Cycle, Compression Cycle, Combustion, and Exhaust. Refer to the chart below by as a reference while we work through a day in the life of the engines camshaft.

Here is what would be seen looking at a pushrod V-8 cam, from the front: 1) Intake lobe lift; 2) Exhaust lobe lift; 3) Intake duration; 4) Exhaust duration; 5) Overlap; 6) Lobe Centerline Angle (LCA); 7) Cam Advance (A) and Retard (R).

When looking at the engine from the front, the camshaft spins in a clockwise motion. Of the two cam lobes pictured above it is the exhaust lobe that leads the way on the right with the intake lobe following closely behind. We are going to start the explanation of the cycles with the exhaust lobe, or the one on the right in the figure above.

Visualization is the key to understanding the process so I’m going to walk through it step by step. For the sake of this conversation I will utilize the specs of Competition Cams 106400 camshafts for the 4.6 DOHC V8.

Once again, the four basic cycles include the induction cycle, compression, combustion and exhaust. The first thing we want to suppose is that the intake valve has just closed ending the induction cycle and the combustion cycle has already started. Both intake and exhaust valves are closed and the force of the ignited air/fuel mixture is shoving the piston down towards Bottom Dead Center (BDC). As it travels down, the force acting against it from the combustion is slowly dissipating. At 69.5 degrees (on Comp Cams 106400) before the piston reaches bottom dead center (BDC) the exhaust valve starts to open. There is still considerable pressure from the exploding air fuel mixture and instead of continuing to exert force against the piston it is used to help push the burnt gasses past the exhaust valve. This is called the blowdown period and its purpose is to get a jump start on eliminating all the exhaust gasses and reducing as much cylinder pressure as possible before the piston starts back up the cylinder. Any cylinder pressure or exhaust gas remaining in the cylinder acts as resistance to the rising piston. This lost power is referred to as pumping losses.

After the piston rounds BDC it starts pushing up against the remaining gasses and shoving them out the exhaust port. As the piston nears the top the exhaust valve begins to close while at the same time the intake valve begins to open. The intake opens at 22.5* Before Top Dead Center (BTDC) and the exhaust valve doesn’t close until 17.5* After Top Dead Center (ATDC). By doing a little math we can see that both the intake valve and the exhaust valve are open for 40* of crankshaft rotation. This period where both valves are open at the same time is referred to as the overlap period. In the 1950’s Ed Iskendarian discovered that overlap could be used as a powerful camshaft design element and used it with such success that it was coined the fifth cycle. He even produced a line of cams with that namesake. More on overlap later.

At 17.5* ATDC the exhaust valve is shut and the piston is drawing in the air fuel mixture from the open intake valve. As it descends down the cylinder it continues drawing in air/fuel until it reaches BDC. At this point the intake valve is still open as the piston starts to move back up the cylinder. Even though the piston is no longer pulling the intake charge in, the inertia created during overlap and the down stroke momentarily continues to ram air into the cylinder. With 106400 intake cams it’s not until the piston reaches 66.5* ABDC that the intake valve is completely shut. It is at this point on the graph above where both intake and exhaust valves remain shut as the lifters travel around the back side of the lobes. Now that both valves are closed, the piston can begin compressing the air/fuel mixture in preparation for ignition near TDC. As the piston approaches TDC the fuel mixture is highly compressed and ignited, shoving the piston back down the cylinder, putting us back where we started.

It is important to have a good visualization of the above process in order to understand what will follow. In particular, the principles of advancing and retarding both the intake and exhaust lobes.

The Fun Stuff

Alright, so you’re probably asking what all this talk is about advancing this and closing that. First thing to look at is the primary valve events. They are Intake Open (IO), Intake Close (IC), Exhaust Open (EO), and Exhaust Close (EC).

As in many things camshaft related we have to find a balance between opposing design parameters. Any time we decide we would like to change the opening or closing point of a lobe we have to remember that whether achieving this by adding duration or advancing the lobe, we affect what is happening on the other side of the lobe equally. For example we might want a later closing exhaust valve in order to help eliminate exhaust gasses. Adding duration to achieve this would both close the valve later and open it earlier, resulting in a net benefit. Retarding the lobe would close the valve later but also open it later. The later opening results in less pressure during the blowdown period which also means fewer exhaust gasses are eliminated during this period. Since blowdown is more critical to exhaust gas elimination than the exhaust valve closing point, the net result would be negative. Every adjustment has a host of other inherent changes that also occur and none of the camshaft valve events exist in a vacuum.

How do we figure out which points to design around? As you will see from the opposing opinions of the experts below, it isn’t always a clear answer:

“Now the last timing event is the most important and the most critical THE CLOSING OF THE INTAKE VALVE. The closing of the intake valve is the governor of the RPM range and the governor of the effective compression ratio!” Dimitri Elgin

“I consider the IVO and the EVO to be of equal valve, and first in importance. If anything, the EVO occurs first, and influences what the IVO sees and does.” Harold Brookshire

The closing of the intake valve, the opening of the intake valve, or the opening of the exhaust valve. It’s interesting how these two veteran cam designers with exceedingly impressive credentials have differing opinions. Dimitri Elgin is very “inertial charging” centric while Harold Brookshire focuses on the importance of airflow in the very early stages of the intake cycle. Dimitri is more road race oriented while Harold is more drag race focused. Do these differences account for the opposing philosophies? Maybe a little but the reality is they’re both pretty much talking about the same thing. I imagine if these two men were discussing cams they would agree a lot more than disagree. Often times differing design opinions are nothing more than perspective.

Since we are not actually designing camshafts we can get away with this basic outline provided by Racer Brown:

Listed in order of importance and significance to engine operation are:

1. Intake valve closing point;
2. Intake valve opening point (probable);
3. Exhaust valve closing point (probable; could be reversed with (2) under certain conditions);
4. Exhaust valve opening point;
Racer Brown

In addition to opposing design parameters we have to think in terms of levels and priorities. Adjusting one valve event can set off a chain reaction of consequences that greatly effect how the engine performs. This line of thought can be seen in Harold Brookshire’s design philosophy which basically says that if you don’t get things right at the beginning of the intake cycle, it doesn’t matter where the all important intake closing is placed.

Some design consequences are dramatic while others are hardly noticed. However, these levels of impact can all be turned upside down based on the priorities of the individual and the engines inherent strengths and weaknesses. A late EVO on a small cubic inch engine with low compression can save the day with the additional torque it provides, yet at the same time it can be disastrous if the engine is being used in a high RPM application. The negative high RPM consequences of a late EVO are not important to an individual that is prioritizing low RPM torque, yet this inconsequential element is a big priority to the high RPM enthusiast. Bottom line is that a particular strength or weakness of a given combination can sometimes magnify what would normally be an inconsequential effect of changing a valve event.

In addition to intended usage, the myriad of cylinder heads, intake manifolds, and exhaust systems, along with all the bore/stroke and connecting rod combinations can make camshaft design very technical. I will touch on some of the more obvious challenges but in general we are going to continue with the assumption that we are dealing with a reasonably well designed and applied combination that will respond accordingly to generally accepted parameters.

We will use some of Racer Browns hierarchy above to guide us as we look at each valve event and its effects on engine performance. Keep in mind that opening and closing events can be adjusted or spec’d in a couple different ways. One is by adding or subtracting duration. When we add duration to the lobe we are simultaneously opening the valve earlier and closing the valve later because the duration is added to both sides of the lobe. The other way to change opening and closing points is to simply take an existing lobe and advance or retard it. If we advance it we end up with an earlier opening point and an earlier closing point. If we add duration and move the lobe one way or the other we can maintain the original opening or closing while concentrating the additional duration on the opposite side of the lobe. Each possibility has individual consequences that are sometimes opposing. If that sounds confusing, don’t worry, I’ll repeat it a few times throughout the article.

05-13-2010, 06:08 AM	#2
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond The 4 (5) Cycles Before going any further we need to get a solid picture in our heads of what happens in a four stroke engine as it relates to camshaft timing. The four cycles include the Intake Cycle, Compression Cycle, Combustion, and Exhaust. Refer to the chart below by as a reference while we work through a day in the life of the engines camshaft. Here is what would be seen looking at a pushrod V-8 cam, from the front: 1) Intake lobe lift; 2) Exhaust lobe lift; 3) Intake duration; 4) Exhaust duration; 5) Overlap; 6) Lobe Centerline Angle (LCA); 7) Cam Advance (A) and Retard (R). When looking at the engine from the front, the camshaft spins in a clockwise motion. Of the two cam lobes pictured above it is the exhaust lobe that leads the way on the right with the intake lobe following closely behind. We are going to start the explanation of the cycles with the exhaust lobe, or the one on the right in the figure above. Visualization is the key to understanding the process so I’m going to walk through it step by step. For the sake of this conversation I will utilize the specs of Competition Cams 106400 camshafts for the 4.6 DOHC V8. Once again, the four basic cycles include the induction cycle, compression, combustion and exhaust. The first thing we want to suppose is that the intake valve has just closed ending the induction cycle and the combustion cycle has already started. Both intake and exhaust valves are closed and the force of the ignited air/fuel mixture is shoving the piston down towards Bottom Dead Center (BDC). As it travels down, the force acting against it from the combustion is slowly dissipating. At 69.5 degrees (on Comp Cams 106400) before the piston reaches bottom dead center (BDC) the exhaust valve starts to open. There is still considerable pressure from the exploding air fuel mixture and instead of continuing to exert force against the piston it is used to help push the burnt gasses past the exhaust valve. This is called the blowdown period and its purpose is to get a jump start on eliminating all the exhaust gasses and reducing as much cylinder pressure as possible before the piston starts back up the cylinder. Any cylinder pressure or exhaust gas remaining in the cylinder acts as resistance to the rising piston. This lost power is referred to as pumping losses. After the piston rounds BDC it starts pushing up against the remaining gasses and shoving them out the exhaust port. As the piston nears the top the exhaust valve begins to close while at the same time the intake valve begins to open. The intake opens at 22.5* Before Top Dead Center (BTDC) and the exhaust valve doesn’t close until 17.5* After Top Dead Center (ATDC). By doing a little math we can see that both the intake valve and the exhaust valve are open for 40* of crankshaft rotation. This period where both valves are open at the same time is referred to as the overlap period. In the 1950’s Ed Iskendarian discovered that overlap could be used as a powerful camshaft design element and used it with such success that it was coined the fifth cycle. He even produced a line of cams with that namesake. More on overlap later. At 17.5* ATDC the exhaust valve is shut and the piston is drawing in the air fuel mixture from the open intake valve. As it descends down the cylinder it continues drawing in air/fuel until it reaches BDC. At this point the intake valve is still open as the piston starts to move back up the cylinder. Even though the piston is no longer pulling the intake charge in, the inertia created during overlap and the down stroke momentarily continues to ram air into the cylinder. With 106400 intake cams it’s not until the piston reaches 66.5* ABDC that the intake valve is completely shut. It is at this point on the graph above where both intake and exhaust valves remain shut as the lifters travel around the back side of the lobes. Now that both valves are closed, the piston can begin compressing the air/fuel mixture in preparation for ignition near TDC. As the piston approaches TDC the fuel mixture is highly compressed and ignited, shoving the piston back down the cylinder, putting us back where we started. It is important to have a good visualization of the above process in order to understand what will follow. In particular, the principles of advancing and retarding both the intake and exhaust lobes. The Fun Stuff Alright, so you’re probably asking what all this talk is about advancing this and closing that. First thing to look at is the primary valve events. They are Intake Open (IO), Intake Close (IC), Exhaust Open (EO), and Exhaust Close (EC). As in many things camshaft related we have to find a balance between opposing design parameters. Any time we decide we would like to change the opening or closing point of a lobe we have to remember that whether achieving this by adding duration or advancing the lobe, we affect what is happening on the other side of the lobe equally. For example we might want a later closing exhaust valve in order to help eliminate exhaust gasses. Adding duration to achieve this would both close the valve later and open it earlier, resulting in a net benefit. Retarding the lobe would close the valve later but also open it later. The later opening results in less pressure during the blowdown period which also means fewer exhaust gasses are eliminated during this period. Since blowdown is more critical to exhaust gas elimination than the exhaust valve closing point, the net result would be negative. Every adjustment has a host of other inherent changes that also occur and none of the camshaft valve events exist in a vacuum. How do we figure out which points to design around? As you will see from the opposing opinions of the experts below, it isn’t always a clear answer: “Now the last timing event is the most important and the most critical THE CLOSING OF THE INTAKE VALVE. The closing of the intake valve is the governor of the RPM range and the governor of the effective compression ratio!” Dimitri Elgin “I consider the IVO and the EVO to be of equal valve, and first in importance. If anything, the EVO occurs first, and influences what the IVO sees and does.” Harold Brookshire The closing of the intake valve, the opening of the intake valve, or the opening of the exhaust valve. It’s interesting how these two veteran cam designers with exceedingly impressive credentials have differing opinions. Dimitri Elgin is very “inertial charging” centric while Harold Brookshire focuses on the importance of airflow in the very early stages of the intake cycle. Dimitri is more road race oriented while Harold is more drag race focused. Do these differences account for the opposing philosophies? Maybe a little but the reality is they’re both pretty much talking about the same thing. I imagine if these two men were discussing cams they would agree a lot more than disagree. Often times differing design opinions are nothing more than perspective. Since we are not actually designing camshafts we can get away with this basic outline provided by Racer Brown: Listed in order of importance and significance to engine operation are: 1. Intake valve closing point; 2. Intake valve opening point (probable); 3. Exhaust valve closing point (probable; could be reversed with (2) under certain conditions); 4. Exhaust valve opening point; Racer Brown In addition to opposing design parameters we have to think in terms of levels and priorities. Adjusting one valve event can set off a chain reaction of consequences that greatly effect how the engine performs. This line of thought can be seen in Harold Brookshire’s design philosophy which basically says that if you don’t get things right at the beginning of the intake cycle, it doesn’t matter where the all important intake closing is placed. Some design consequences are dramatic while others are hardly noticed. However, these levels of impact can all be turned upside down based on the priorities of the individual and the engines inherent strengths and weaknesses. A late EVO on a small cubic inch engine with low compression can save the day with the additional torque it provides, yet at the same time it can be disastrous if the engine is being used in a high RPM application. The negative high RPM consequences of a late EVO are not important to an individual that is prioritizing low RPM torque, yet this inconsequential element is a big priority to the high RPM enthusiast. Bottom line is that a particular strength or weakness of a given combination can sometimes magnify what would normally be an inconsequential effect of changing a valve event. In addition to intended usage, the myriad of cylinder heads, intake manifolds, and exhaust systems, along with all the bore/stroke and connecting rod combinations can make camshaft design very technical. I will touch on some of the more obvious challenges but in general we are going to continue with the assumption that we are dealing with a reasonably well designed and applied combination that will respond accordingly to generally accepted parameters. We will use some of Racer Browns hierarchy above to guide us as we look at each valve event and its effects on engine performance. Keep in mind that opening and closing events can be adjusted or spec’d in a couple different ways. One is by adding or subtracting duration. When we add duration to the lobe we are simultaneously opening the valve earlier and closing the valve later because the duration is added to both sides of the lobe. The other way to change opening and closing points is to simply take an existing lobe and advance or retard it. If we advance it we end up with an earlier opening point and an earlier closing point. If we add duration and move the lobe one way or the other we can maintain the original opening or closing while concentrating the additional duration on the opposite side of the lobe. Each possibility has individual consequences that are sometimes opposing. If that sounds confusing, don’t worry, I’ll repeat it a few times throughout the article.