arghx7 Scooby Specialist

Member#: 232940 Join Date: Dec 2009 Location: cold

Appendix A: Cams and Valve Timing





Source: original image but based on Asmus, "Valve Events and Engine Operation," 1982, SAE 820749



The chart shows on the horizontal axis a full 720 degree crank angle cycle measured in degrees ATDC firing, with top dead center (TDC) firing occurring at 0 & 720 degrees, and TDC intake occurring at 360 degrees. So 180 degrees would be the end of the expansion/beginning of exhaust stroke, and 540 degrees would be the end of the intake stroke/beginning of compression stroke. The vertical axis is valve lift. The red line shows a lift vs crank angle curve for an exhaust cam, and the blue line shows a lift vs crank angle curve for an intake cam.



The centerlines for the exhaust (ECL) cam and the intake cam (ICL) is the distance in crank angle degrees between TDC and peak valve lift. An intake cam with centerline 110 would have peak lift 110 degrees ATDC intake (or 470 ATDC Firing). An exhaust cam with centerline 110 would have peak lift 110 degrees BTDC intake (or 250 degrees ATDC Firing). Cam Phasing (AVCS) phases the whole lift profile and moves the centerline relative to the default locking position (left or right in the image). It doesnt change the peak lift, the duration, or the shape of the lift profile. Now here are some camshaft parameters to understand. These arent my own personal ideas but rather they have already been written about by acknowledged experts in valve timing.



Effective compression ratio  this is the compression ratio based on the closing timing of the intake valve. The engine may have a nominal compression ratio of say 8.0:1, but the effective compression ratio depends on the intake valve closing timing. The closer the closing timing is to BDC compression (540 degrees ATDC), the higher the effective compression ratio (up to the nominal/geometric value).





Source: original image



For purposes of this discussion, closing timing before 540 degrees (BDC compression) is early intake valve closing. Closing timing after 540 is late intake valve closing. You might hear those terms used in slightly different ways though. When we utilize very late intake valve closing for fuel economy we call it Atkinson cycle.



Effective expansion ratio  This is similar to effective compression ratio, except it relates to the expansion work in the cylinder which actually makes the torque. The earlier the exhaust valve opening timing, the lower the expansion ratio. High expansion ratio could come at the expense of poor blowdown, making it harder to evacuate gases out of the cylinder.



Blowdown volume  relates to the area under the curve before BDC expansion/exhaust. Blowdown helps evacuate gases from the cylinder and relieve backpressure at the turbine inlet. Blowdown is helpful at higher speeds and loads to evacuate gases, but it reduces expansion work.



Exhaust flow volume  relates to the area under the exhaust valve lift curve

Intake flow volume - relates to the area under the exhaust valve lift curve

Overlap volume  relates to the area under the overlap of intake and exhaust valve opening



Intake valve closing volume  relates to the area under the curve after BDC compression. Intake valve closing volume improves volumetric efficiency depending on the rpm.



Notice we use the term "volume" (like intake valve closing volume). We can take an area under the valve lift curve shown in the crank angle vs lift diagram and then do some math (which I wont get into) according to the geometry of the valves to normalize the parameter. It's a way of comparing cams between different heads. For example, a big cam should be considered in light of the number and size of the valves. Its a way to do the comparison between say aggressive cam with small valves vs mild cam with big valves, or aggressive cam with 2 valves vs mild cam with 4 valves.



Some Cam Phasing Rules of Thumb



As I advance my intake cam centerline, effective compression ratio increases and intake flow volume decreases. Overlap volume may be increasing as well depending on the exhaust valve events. As I retard my intake cam centerline, effective compression ratio decreases and intake flow volume increases. Overlap volume may be decreasing as well depending on the exhaust valve events.



For intake phasing there will always be a tradeoff between overlap, closing volume (volumetric efficiency) and effective compression ratio.



As I retard my exhaust cam centerline, effective expansion ratio increases and blowdown volume decreases. Overlap volume may be increasing as well depending on the intake valve events.



As I advance my exhaust cam centerline, effective expansion ratio decreases and blowdown volume increases. Overlap volume may be decreasing as well depending on the intake valve events.



For exhaust phasing there will always be a tradeoff between overlap, blowdown volume (evacuation of exhaust gases) and effective expansion ratio.





If my overlap volume begins before TDC intake, I get hot exhaust gases spitting back into the intake port because the piston is still rising. If my overlap volume begins after TDC intake, I get exhaust gases drawn back into the cylinder from the exhaust manifold. This behavior depends on the air pressure at the exhaust port. At low loads, I will spit hot exhaust gases back, but under boost, when intake port pressure is higher than exhaust pressure, we can scavenge residual gases out of the cylinder and also help spool the turbo.



Appendix B: Combustion Quality Metrics and Combustion Enhancement



This appendix provides context to discussion of the TGV, EGR, Atkinson Cycle, and head design. First I will show some pressure traces of a non-knocking engine at different types of combustion phasing. Since the TGV is a burn rate/combustion quality enhancement device, the next section will explain combustion speed and stability metrics by drawing an analogy. It will show another burning enhancement approach: dual spark plugs as implemented on a modern Hemi engine. Adding an additional spark plug doesnt restrict flow like a TGV or a high-tumble intake port, and it can improve combustion. Besides adding cost and complexity though it also takes up space in the head that could be used for valves, an injector, a spark plug, more casting, etc. Theres always a tradeoff.



Now, on to introducing three metrics for combustion: Combustion Phasing (and peak pressure), Combustion speed, and Combustion Stability.



Combustion Phasing



Well start with combustion phasing, which could be described as how advanced or retarded the combustion is. You might be used to thinking about advanced or retarded spark timing, but remember that spark timing only tells you when the plug was fired, not how the burning actually behaved.



We can measure combustion phasing with the location of 50% burn (called MFB50 or CA50) and the location of peak pressure (LPP). Both are in units of crank angle degrees ATDC firing. Typically your minimum spark advance for best torque (MBT) sets your CA50 to about 8 degrees, with an LPP of 11-13 degrees. This is a pretty hard and fast rule for spark ignited piston engines with conventional combustion. Take a look at the images below showing pressure traces on a crank angle basis and a Pressure-Volume diagram basis.







Source: Original Image





Source: Original Image



In an area of engine operation with a knock limitation (for that given fuel), we need to retard the combustion, and our combustion pressure looks like the retarded image above. Combustion can also be retarded to heat up the exhaust for lighting off the catalytic converter. Otherwise we want our combustion to be like the MBT pressure trace. If its advanced further than that, we get unnecessarily high combustion pressure and unnecessarily fast burn, which can stress components and cause more combustion noise.



Peak Cylinder Pressures



You can see above that advancing spark timing increases peak cylinder pressure, even if you are not knocking (like with E85). Other things that increase cylinder pressure are:



Preignition events

Engine load - airflow injested at a given rpm, as well as indicated mean effective pressure (area under the Pressure loop)

Higher engine speeds



Peak pressures fluctuate (see section on combustion stability below), and will vary over time. We can take a certain number of cycles, typically 300 cycles, and average them over time. That gives a mean (average) value for peak pressure at a given speed and load. We can also look at the max cylinder pressure cycle among those.



The image below shows the mean peak pressure curve for a Ford Ecoboost 3.5 engine and for a naturally aspirated engine. This is for full load operation for max rated torque curve.





Source: Kapp, "3.5L V6 EcoBoost: Democratization of Sustainable Engine Technology," 2008, Aachen Colloquium



You can see that the mean peak pressure increases with rpm, until it reaches 80 bar. The engine has been rated for 80 bar mean pressures, and the engine calibration keeps to that limit by not having too much spark timing/too advanced of combustion phasing. This engine will be able to withstand occasional cycles greater than 80 bar.



Also, here is peak cylinder pressure map represented as speed & brake mean effective pressure (basically, flywheel torque) on a turbo DI engine (non Subaru) with stock tune. Notice how the max is at high speed & load (peak power), even though peak torque is hit at lower speed:







and here is a chart showing the impact on cylinder pressure of switching from regular fuel (87 octane) to E85 on a naturally aspirated 6 cylinder and advancing the spark. Notice the 20% increase of cylinder pressure at 6000rpm just from advancing spark.











Combustion Speed



Combustion speed is the second metric to discuss, and it is the missing link between spark timing and combustion phasing. I may know when I fired my spark, but I have to know how fast the mixture burned to figure out my combustion phasing and whether I am at MBT or not. When you run an engine at low loads, and then start degrading the burn for fuel efficiency (Atkinson Cycle, external or internal EGR, dethrottling with low valve lifts), combustion slows down. Generally speaking (not always, but generally), fast combustion is good and slow combustion is bad. There are several metrics to measure the combustion speed, measured in crank angle degrees:





Initial combustion formation, also called burn delay or ignition delay: this is the number of crank angle degrees from 0% mass fraction burned to 10%, also called the 0-10% duration.



Bulk burn duration: this is the number of crank angle degrees from 10% mass fraction burned to 90%

You might also see other metrics such as 0-90%, 10-80%, 10-50%, but the above two are the most commonly used.



The below image demonstrates the benefit of a combustion enhancement approach on an engine. In this case it is the dual sparkplugs on the modern Hemi engines. You can see the faster burn with the dual plugs (blue line), the design that went into production, vs an experimental single plug design (red line):





Source: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815



This image shows improved fuel consumption and earlier MBT spark timing for a cruising condition:





Source: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815



High tumble flow such as that from closed TGV has a similar effect of speeding up the burn rates, which is important for offsetting slow burning from internal EGR (overlap through AVCS phasing), external EGR, or Atkinson Cycle. The improved combustion can also reduce knocking tendency, as the gases at the end of the combustion chamber have less time to auto ignite.



Combustion Stability



Combustion stability metrics are based on statistical calculations of the combustion pressure over some number of cycles. Unstable combustion can result in torque fluctuations (surging feeling in the car) or even misfire. Here are some factors that affect combustion stability:



-- Dilution of gases in the chamber (internal or external EGR, lean burn)

-- Spark plug related factors (plug gap, voltage, spark blowout etc)

-- Injection timing and rail pressure, especially on DI engines

-- Combustion phasing, especially spark retard for knock or for gearshift torque reduction (which is one of the reasons why torque curves can look noisy)

-- Motion inside the combustion chamber due to things like intake port and combustion chamber shape, piston bowl or squish clearance, injection spray pattern



Ill skip the math for calculating combustion stability metrics and give some rules of thumb along with an example of the primary burn rate enhancement for a modern Hemi engine, dual spark plugs. Youll see in the image below that at an idle condition with dual spark plugs the Hemi engine had more consistent burn with different spark timings. This makes for a smoother idle, better fuel consumption, improved emissions, etc. Here are the two main metrics:



-- Lowest Normalized Value (LNV)  0-100%, high number is better. The lower the number, the more likely the engine will experience rough running or misfire.



-- Coefficient of Variation (COV)  from less than 1% all the way up to 10+%, and lower is better. The higher the number, the more likely the engine will experience rough running or misfire.





Source: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815



The above images demonstrate ways to evaluate the quality of combustion and they show how a combustion enhancement can improve that quality. Dual spark plugs, a TGV, a restrictive high tumble port, piston crown shape, or valve timing tricks can all enhance burning quality but always with some kind of tradeoff or downside, and usually more than just cost. https://i.imgur.com/iiTnxTn.pngSource: original image but based on Asmus, "Valve Events and Engine Operation," 1982, SAE 820749The chart shows on the horizontal axis a full 720 degree crank angle cycle measured in degrees ATDC firing, with top dead center (TDC) firing occurring at 0 & 720 degrees, and TDC intake occurring at 360 degrees. So 180 degrees would be the end of the expansion/beginning of exhaust stroke, and 540 degrees would be the end of the intake stroke/beginning of compression stroke. The vertical axis is valve lift. The red line shows a lift vs crank angle curve for an exhaust cam, and the blue line shows a lift vs crank angle curve for an intake cam.The centerlines for the exhaust (ECL) cam and the intake cam (ICL) is the distance in crank angle degrees between TDC and peak valve lift. An intake cam with centerline 110 would have peak lift 110 degrees ATDC intake (or 470 ATDC Firing). An exhaust cam with centerline 110 would have peak lift 110 degrees BTDC intake (or 250 degrees ATDC Firing). Cam Phasing (AVCS) phases the whole lift profile and moves the centerline relative to the default locking position (left or right in the image). It doesnt change the peak lift, the duration, or the shape of the lift profile. Now here are some camshaft parameters to understand. These arent my own personal ideas but rather they have already been written about by acknowledged experts in valve timing.Effective compression ratio  this is the compression ratio based on the closing timing of the intake valve. The engine may have a nominal compression ratio of say 8.0:1, but the effective compression ratio depends on the intake valve closing timing. The closer the closing timing is to BDC compression (540 degrees ATDC), the higher the effective compression ratio (up to the nominal/geometric value). https://i.imgur.com/fPekmVQ.pngSource: original imageFor purposes of this discussion, closing timing before 540 degrees (BDC compression) is early intake valve closing. Closing timing after 540 is late intake valve closing. You might hear those terms used in slightly different ways though. When we utilize very late intake valve closing for fuel economy we call it Atkinson cycle.Effective expansion ratio  This is similar to effective compression ratio, except it relates to the expansion work in the cylinder which actually makes the torque. The earlier the exhaust valve opening timing, the lower the expansion ratio. High expansion ratio could come at the expense of poor blowdown, making it harder to evacuate gases out of the cylinder.Blowdown volume  relates to the area under the curve before BDC expansion/exhaust. Blowdown helps evacuate gases from the cylinder and relieve backpressure at the turbine inlet. Blowdown is helpful at higher speeds and loads to evacuate gases, but it reduces expansion work.Exhaust flow volume  relates to the area under the exhaust valve lift curveIntake flow volume - relates to the area under the exhaust valve lift curveOverlap volume  relates to the area under the overlap of intake and exhaust valve openingIntake valve closing volume  relates to the area under the curve after BDC compression. Intake valve closing volume improves volumetric efficiency depending on the rpm.Notice we use the term "volume" (like intake valve closing volume). We can take an area under the valve lift curve shown in the crank angle vs lift diagram and then do some math (which I wont get into) according to the geometry of the valves to normalize the parameter. It's a way of comparing cams between different heads. For example, a big cam should be considered in light of the number and size of the valves. Its a way to do the comparison between say aggressive cam with small valves vs mild cam with big valves, or aggressive cam with 2 valves vs mild cam with 4 valves.As I advance my intake cam centerline, effective compression ratio increases and intake flow volume decreases. Overlap volume may be increasing as well depending on the exhaust valve events. As I retard my intake cam centerline, effective compression ratio decreases and intake flow volume increases. Overlap volume may be decreasing as well depending on the exhaust valve events.For intake phasing there will always be a tradeoff between overlap, closing volume (volumetric efficiency) and effective compression ratio.As I retard my exhaust cam centerline, effective expansion ratio increases and blowdown volume decreases. Overlap volume may be increasing as well depending on the intake valve events.As I advance my exhaust cam centerline, effective expansion ratio decreases and blowdown volume increases. Overlap volume may be decreasing as well depending on the intake valve events.For exhaust phasing there will always be a tradeoff between overlap, blowdown volume (evacuation of exhaust gases) and effective expansion ratio.If my overlap volume begins before TDC intake, I get hot exhaust gases spitting back into the intake port because the piston is still rising. If my overlap volume begins after TDC intake, I get exhaust gases drawn back into the cylinder from the exhaust manifold. This behavior depends on the air pressure at the exhaust port. At low loads, I will spit hot exhaust gases back, but under boost, when intake port pressure is higher than exhaust pressure, we can scavenge residual gases out of the cylinder and also help spool the turbo.This appendix provides context to discussion of the TGV, EGR, Atkinson Cycle, and head design. First I will show some pressure traces of a non-knocking engine at different types of combustion phasing. Since the TGV is a burn rate/combustion quality enhancement device, the next section will explain combustion speed and stability metrics by drawing an analogy. It will show another burning enhancement approach: dual spark plugs as implemented on a modern Hemi engine. Adding an additional spark plug doesnt restrict flow like a TGV or a high-tumble intake port, and it can improve combustion. Besides adding cost and complexity though it also takes up space in the head that could be used for valves, an injector, a spark plug, more casting, etc. Theres always a tradeoff.Now, on to introducing three metrics for combustion: Combustion Phasing (and peak pressure), Combustion speed, and Combustion Stability.Well start with combustion phasing, which could be described as how advanced or retarded the combustion is. You might be used to thinking about advanced or retarded spark timing, butWe can measure combustion phasing with the location of 50% burn (called MFB50 or CA50) and the location of peak pressure (LPP). Both are in units of crank angle degrees ATDC firing. Typically your minimum spark advance for best torque (MBT) sets your CA50 to about 8 degrees, with an LPP of 11-13 degrees. This is a pretty hard and fast rule for spark ignited piston engines with conventional combustion. Take a look at the images below showing pressure traces on a crank angle basis and a Pressure-Volume diagram basis. https://i.imgur.com/S2yKppS.pngSource: Original Image https://i.imgur.com/r5mMQ3N.pngSource: Original ImageIn an area of engine operation with a knock limitation (for that given fuel), we need to retard the combustion, and our combustion pressure looks like the retarded image above. Combustion can also be retarded to heat up the exhaust for lighting off the catalytic converter. Otherwise we want our combustion to be like the MBT pressure trace. If its advanced further than that, we get unnecessarily high combustion pressure and unnecessarily fast burn, which can stress components and cause more combustion noise.You can see above that advancing spark timing increases peak cylinder pressure, even if you are not knocking (like with E85). Other things that increase cylinder pressure are:Preignition eventsEngine load - airflow injested at a given rpm, as well as indicated mean effective pressure (area under the Pressure loop)Higher engine speedsPeak pressures fluctuate (see section on combustion stability below), and will vary over time. We can take a certain number of cycles, typically 300 cycles, and average them over time. That gives a mean (average) value for peak pressure at a given speed and load. We can also look at the max cylinder pressure cycle among those.The image below shows the mean peak pressure curve for a Ford Ecoboost 3.5 engine and for a naturally aspirated engine. This is for full load operation for max rated torque curve. https://i.imgur.com/Bpu69jS.pngSource: Kapp, "3.5L V6 EcoBoost: Democratization of Sustainable Engine Technology," 2008, Aachen ColloquiumYou can see that the mean peak pressure increases with rpm, until it reaches 80 bar. The engine has been rated for 80 bar mean pressures, and the engine calibration keeps to that limit by not having too much spark timing/too advanced of combustion phasing. This engine will be able to withstand occasional cycles greater than 80 bar.Also, here is peak cylinder pressure map represented as speed & brake mean effective pressure (basically, flywheel torque) on a turbo DI engine (non Subaru) with stock tune. Notice how the max is at high speed & load (peak power), even though peak torque is hit at lower speed: https://i.imgur.com/ZGjhtIA.pngand here is a chart showing the impact on cylinder pressure of switching from regular fuel (87 octane) to E85 on a naturally aspirated 6 cylinder and advancing the spark. Notice the 20% increase of cylinder pressure at 6000rpm just from advancing spark. https://i.imgur.com/zzUY9nh.pngCombustion speed is the second metric to discuss, and it is the missing link between spark timing and combustion phasing. I may know when I fired my spark, but I have to know how fast the mixture burned to figure out my combustion phasing and whether I am at MBT or not. When you run an engine at low loads, and then start degrading the burn for fuel efficiency (Atkinson Cycle, external or internal EGR, dethrottling with low valve lifts), combustion slows down. Generally speaking (not always, but generally), fast combustion is good and slow combustion is bad. There are several metrics to measure the combustion speed, measured in crank angle degrees:, also called burn delay or ignition delay: this is the number of crank angle degrees from 0% mass fraction burned to 10%, also called the 0-10% duration.: this is the number of crank angle degrees from 10% mass fraction burned to 90%You might also see other metrics such as 0-90%, 10-80%, 10-50%, but the above two are the most commonly used.The below image demonstrates the benefit of a combustion enhancement approach on an engine. In this case it is the dual sparkplugs on the modern Hemi engines. You can see the faster burn with the dual plugs (blue line), the design that went into production, vs an experimental single plug design (red line): https://i.imgur.com/dwYbd5J.pngSource: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815This image shows improved fuel consumption and earlier MBT spark timing for a cruising condition: https://i.imgur.com/phZmzc5.pngSource: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815High tumble flow such as that from closed TGV has a similar effect of speeding up the burn rates, which is important for offsetting slow burning from internal EGR (overlap through AVCS phasing), external EGR, or Atkinson Cycle. The improved combustion can also reduce knocking tendency, as the gases at the end of the combustion chamber have less time to auto ignite.Combustion stability metrics are based on statistical calculations of the combustion pressure over some number of cycles. Unstable combustion can result in torque fluctuations (surging feeling in the car) or even misfire. Here are some factors that affect combustion stability:-- Dilution of gases in the chamber (internal or external EGR, lean burn)-- Spark plug related factors (plug gap, voltage, spark blowout etc)-- Injection timing and rail pressure, especially on DI engines-- Combustion phasing, especially spark retard for knock or for gearshift torque reduction (which is one of the reasons why torque curves can look noisy)-- Motion inside the combustion chamber due to things like intake port and combustion chamber shape, piston bowl or squish clearance, injection spray patternIll skip the math for calculating combustion stability metrics and give some rules of thumb along with an example of the primary burn rate enhancement for a modern Hemi engine, dual spark plugs. Youll see in the image below that at an idle condition with dual spark plugs the Hemi engine had more consistent burn with different spark timings. This makes for a smoother idle, better fuel consumption, improved emissions, etc. Here are the two main metrics:-- Lowest Normalized Value (LNV)  0-100%, high number is better. The lower the number, the more likely the engine will experience rough running or misfire.-- Coefficient of Variation (COV)  from less than 1% all the way up to 10+%, and lower is better. The higher the number, the more likely the engine will experience rough running or misfire. https://i.imgur.com/XbC82KQ.pngSource: Hartman, The New DaimlerChrysler Corporation 5.7L Hemi Engine, 2002, SAE 2002-01-2815The above images demonstrate ways to evaluate the quality of combustion and they show how a combustion enhancement can improve that quality. Dual spark plugs, a TGV, a restrictive high tumble port, piston crown shape, or valve timing tricks can all enhance burning quality but always with some kind of tradeoff or downside, and usually more than just cost. Last edited by arghx7; 07-12-2017 at 09:45 AM .