CAM PROFILE DESIGN
We will make a short analysis about some basic principles of cam profile design strategies. In the end we will explain why multiair system can affect so much the quality of the combustion. Our goal is to give an easily understandable explanation without many technical terminologies.
Lets separate the camshaft fuctions in categories :
Exhaust valve is opening after the combustion of the mixture in order to let gases escape to exhaust manifold. Combustion takes place while piston moves towards BTC. Now as piston passes BTC and starts moving towards TDC , exhaust stroke takes place. Normally exhaust valve opens immediately after the BTC for two reasons. Firstly we need to take full advantage of the thermal energy released from the combustion( which is transformed to kinetic energy of gases) as this power generated by gases pushing piston downwards, is exactly the power generated from our engine, delivered to crankshaft.
Secondly as piston moves upwards to TDC, it needs to face the smallest possible resistance. ( If exhaust valve opened after BTC, trapped gases would bring resistance against the motion of piston).
This strategy is always perfect? No. It is perfect at low rpm in order to achieve maximum torqe.
As rpm rise, piston moves faster from BTC to TDC and thus, there is not enough time for the exhaust gases to fully escape the chamber. A part of them stays in, as intake stroke begins.
The solution here is to open the exhaust valve some degrees before BTC. This would have a small negative effect in low rpm but huge gains at higher rpm.
How many deg before BTC? That one depends on the design of our head and exhaust manifold. The ability of these two to extract the gases properly will define our cam profile. Also the the application of our setup. What are we talking about? Small city car? track use? dragster? etc.
* Turbo setups
Same principles here, but there is a difference. Exhaust gases besides moving the piston , have a second role aslo. Spinning the turbocharger. Thus we need them to hit the turbine wheel as soon as possible for faster spool. But how much sooner before we loose too much thermal energy? Again depends on the factors we mentioned before. There is a critical point of course, which is different for each designer.
The basic theory here is that we don't need the exhaust vlave to close exactly at TDC for the same reason. We need to give more time to exh gases to empty the chamber. But how is this possible as piston begins to move downwards again for intake stroke? As intake valve opens at TDC ( or before that, will explain later ) there is a small period that both valves are simultaneously open . This is called OVERLAP. For these milliseconds, positive pressure from intake helps exhaust gases to perfectly empty the chamber.
Do we need overlap?
At low rpm we don't. As with overlap we cannot create vacuum just after the TDC and at low rpm there is not enough positive pressure built on intake manifold, less amount of mixture is drawn inside chamber.
At higher rpm though , we can achieve better
filling of chamber.
* Turbo setups
As on turbo applications we have positive pressure on intake manifold , 2-3 times above barometric pressure, there is a great possibility a part of mixture to escape directly to exhaust port. So generally we dont use overlap on turbo setups. It may be used on extreme applications where we consciously let some mixture to escape exhaust in order to push way more amount of it, through intake.
B ) INTAKE
Intake stroke begins just after exhaust stroke , when piston passes TDC and moves downwards to BTC. Normally intake valve opens exactly at TDC in order to take advantage of the vacuum. If we open the intake valve earlier we can take advantage of the positive boost built on intake manifold in order to push more mixture inside chamber. This again will work only at high rpm and will harm torque at low rpm. Will create also overlap if exhaust valve closes after TDC , as we explained earlier. So it is used as strategy mostly on N/A engines.
The intake valve closing point is possibly more important the the rest 3 combined.
Intake stroke ends as piston passes BTC, so compression stroke begins. Normally intake valve closes at exactly that point. What if we close intake valve , after BTC? A part of mixture returns to intake manifold and static compression drops. But if we take advantage of positive boost inside intake manifold, can we force more mixture inside chamber during compression stroke? Of course we can! The higher the rpm the longer we can keep intake valve open. So we can create more power , but sacrificing torque and drivabillity at low rpm.
* Turbo setups.
The basic principle here is that we can take advantage of the boost to force more mixture inside chamber. The higher the boost level the longer we can keep the intake valve open. The problem here is that with that strategy we loose static compression and efficiency at low rpm . Which means less exhaust gases at lower rpm. Which means that boost kicks in at higher rpm.
Generally, the higher the valve lift the less resistance for the intake mixture entering the chamber on one hand and exhaust gases emptying the chamber on the other hand. So, more flow and more power. There is a critical point that flow is no longer increased as we keep increasing valve lift. This point depends on the diameter of head ports , angle of valve to vertical axis, space between valve and cylinder walls, etc
As we see everything on cam profile design is about compromising. What we need ? At what rpm? What rpm range? Etc. The challenge of the designer is to achieve the best performance possible, minimizing drawbacks.
What if we could create a system that alters the intake valve closing , continuously? What if it could consider rpm , boost level , throttle position and calculate the best point to close the intake valve?
Wouldn't that engine be twice efficient comparing to a conventional engine?
Sure it would be.
That engine is called Multiair !
Tuning multiair engines, means altering pulse resonance on intake even by changing turbo, exhaust manifold etc. That is why, when studying and developing different stages ( designs)of camshafts, while making appropriate adjustments for multiair system calculations through ECU, brings us best of results.
MYTHS ABOUT CAMSHAFTS
Myth no1 : Reprofiling reduces lift
Via the process of reprofiling , we decrease the diameter of base circle.
Actual lift = lobe lift + x - y
At factory profiles x = y = base Circle dia/2
When reprofiling we reduse the ( y ) value
So actual lift increases
Myth no2 : Reprofiling reduces surface hardness
If reprofiling process is executed properly , surface hardness remains constant.
Factory hardening has a depth around 2 mm. That means that grinding wheel always removes material from that area of lobe. Working on a hardened surface, material temperature is rising fast because of friction. Constant coolant fluid provision and very slow process execution is demanded. After that surface hardness is tested with special tool at Rockwell C unit.
Test proves that hardness after reprofiling is same or slightly higher than factory ( pic 3 )
Μyth no3 : Total duration and lift determines profile’s characteristics
Total duration and lift values are just 2 numbers which give us a general idea about profile and nothing more. In order to fully evaluate the quality of the profile we need to know total area duration per lift , area per lift. According to each designer’s strategy ramps may open/ close faster or slower ( depending on the velocity we need for the exhaust gases or intake mixture on comparison with the time we wish the valves to stay open. Also fatigue of the components are a critical factor. That means that 2 different profiles with exactly same total lift / duration may have different total area and different area at each lift point. So the total duration ( measured at 0mm) is not representative at all . It is more representative to measure duration at 1mm. Many manufacturers announce the duration at 0.1 mm and even at 0mm , for marketing purposes. So a cam profile may look huge but actually doesn’t perform as it should. Take a look at at pic 2. The exactly same profile with and without “wide” ramps. If we measure at 0mm the differences will be around 40 deg. ( eg 260 vs 220 )
Different seems huge , but these two profiles would have really similar performance as duration at 1mm will be similar. (pic 2)
Myth no4 : Bigger is better
Bigger duration and lift ( absolute values) are not always better.
- Lift. Higher lift is not always better. That depends on the design of our head. Port diameter , valve diameter , port/valve angle etc , determines the lift that we achieve maximum flow. Above that value there is no flow increase and thus no reason to operate our valve there (pic 4) E.g if our head reaches maximum flow at 9mm , a profile which keeps the valve around 9mm ( let’s say 7-9-7) for 50 degrees will perform better than a profile that uses 10mm lift but keeps the valve at the region 7-11-7 for 40 degrees. (pic5).
- Duration. As we already mentioned if total duration is bigger , but total area is smaller or area at a specific lift where our head achieves maximum air velocity is lower , performance will be again worse instead of greater.