What makes engine components suitable for NA or FI?
Discussion
Typically, the internals must be designed to withstand MORE - more pressure, more heat, more temperature cycles / temperature variance, more loads etc for a turbocharged car. This is most true for the pistons, however conrods should also be taken into consideration. So typically, a FI engine will have stronger pistons and rods.
Camshaft design is a very tricky area - camshaft profiles, to get the optimum out of each engine, will be different. A forced induction engine will usually benefit from increased exhaust valve open duration and / or lift when compared to a NA car.
It is best to reduce compression ratio in a FI engine over a NA engine, in order to run higher boost with acceptable ignition timing to get the best power and driveability.
These are probably the main differences, although of course there are more details such as intake manifold, exhaust manifold, oil cooling etc etc.
Camshaft design is a very tricky area - camshaft profiles, to get the optimum out of each engine, will be different. A forced induction engine will usually benefit from increased exhaust valve open duration and / or lift when compared to a NA car.
It is best to reduce compression ratio in a FI engine over a NA engine, in order to run higher boost with acceptable ignition timing to get the best power and driveability.
These are probably the main differences, although of course there are more details such as intake manifold, exhaust manifold, oil cooling etc etc.
Yes, dished pistons are commonly used for precisely that reason. FI pistons also typically have a thicker crown to deal with the higer forces exerted and to give some resistance to occasional detonation which can be very destructive in an FI engine.
Cams will usually have a much lower overlap period. Since their is higer pressure in the exhaust manifold their is a tendency for the gas flow to be the wrong way during overlap on an FI engine actually shoving the mixture back the wrong way up the inlet manifold and leaving the cylinder part filled with hot exhaust gasses and a weak mixture. Some overlap is used but it very much depends on the amount of boost being used and the pressure ratio between the inlet and exhaust manifolds. The expected RPM range of the engine also has an impact on the cam profile but in virtually all case the overlap will be lower than an "equivalent" NA engine.
Jon
Cams will usually have a much lower overlap period. Since their is higer pressure in the exhaust manifold their is a tendency for the gas flow to be the wrong way during overlap on an FI engine actually shoving the mixture back the wrong way up the inlet manifold and leaving the cylinder part filled with hot exhaust gasses and a weak mixture. Some overlap is used but it very much depends on the amount of boost being used and the pressure ratio between the inlet and exhaust manifolds. The expected RPM range of the engine also has an impact on the cam profile but in virtually all case the overlap will be lower than an "equivalent" NA engine.
Jon
Z064life said:
I also read how dished pistons can help with fi applications.
Is this because dished pistons contribute to a lower cr?
Pretty much. But CR value is also dependent on how well charge is cooled, combustion chamber shape and the dynamics of the burn. That is why on modern engines you see high output turbo engines with seemingly sky high CRs, mainly because they chose a good chamber shape.Is this because dished pistons contribute to a lower cr?
It's also important to point out that the control systems are now noticeably improved over what was available in the past, so you can run components closer to their design limit, so (as an example) where you used forged pistons in the past a cast piston would now be adequate.
What that does mean is that engines are now tremendously expensive to tune if you want to increase output a lot, and the effort needed to calibrate the engine management system is going to be a lot higher.
If you limit it to specifically NA/FI issues (as opposed to the issues related to increasing BHP via either means) then it's basically just the cam and compressions ratio:
Pistons - not directly affected but can be dished to decrease compression ratio, though this isn;t the only way (machining the head/fitting thicker head gaskets/etc).
Con-rods - actually experience LESS stress in a FI engine than a NA one*.
Crank - not really affected either.
Cam - FI likes standard cams with little or no overlap, NA engines love more overlap. In the case of an NA engine you get to keep both valves open longer and have longer to get gasses in/out without too much loss. With FI the extra inlet pressure tends to push fuel in the inlet and straight out the exhaust if you have a lot of overlap.
On the compression stroke the charge in the cylinder pushes back on the piston, reducing it's pull on the con-rod. With FI you have more charge in the cylinder exerting a greater force back on the piston, further reducing the pistons pull on the con-rod.
Obviously if your going to double the BHP
Pistons - not directly affected but can be dished to decrease compression ratio, though this isn;t the only way (machining the head/fitting thicker head gaskets/etc).
Con-rods - actually experience LESS stress in a FI engine than a NA one*.
Crank - not really affected either.
Cam - FI likes standard cams with little or no overlap, NA engines love more overlap. In the case of an NA engine you get to keep both valves open longer and have longer to get gasses in/out without too much loss. With FI the extra inlet pressure tends to push fuel in the inlet and straight out the exhaust if you have a lot of overlap.
- Peak stress in the con-rod are TDC on the exhaust stroke and TDC on the compression stroke. In both cases the load is tensile - the piston wanting to keep going up out the top of the engine, pulling the con-rod on one direction and the crank starts pulling it the other.
On the compression stroke the charge in the cylinder pushes back on the piston, reducing it's pull on the con-rod. With FI you have more charge in the cylinder exerting a greater force back on the piston, further reducing the pistons pull on the con-rod.
Obviously if your going to double the BHP
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