Formula SAE Racing Vehicle Diffuser and Restrictor Design Management

Formula SAE rules says that the air passage must have a 20mm dia or less from where total air must flow at least once so rather limiting throttle bore at 20mm or making intake manifold intake at 20mm - A separate part Restrictor is used which has convergent and divergent part to boost engine performance. 

Restrictor has two child parts; 1^st is convergent nozzle and 2^nd is divergent nozzle as shown in below figure. Inlet Air pressure vary on convergent and divergent nozzle as shown in below figure consequently we must optimize it to get smooth change flow.

Child Part	Requirement	Material	Manufacturing	Cost	Weight	Life	Design Review
Restrictor	Burst Pressure 10 Bar, Vibration 30 g, Working Temperature 200 ° C,	Plastic	Moulding	C	A	B	Steel will be used and only by machining we can make final part. If steel pipe is not available then Aluminum ADC12 can be made by sand casting process and machining.
		Aluminum	Sand Casting, Machining	A	B	A
		Steel	Machining	B	B	A
Gasket	Working Temperature 200 ° C Leakage Pressure 10Bar	Paper	Die Cutting	A	A	C	Manifold and Throttle Side- Paper Gasket will be used as it is easy to design & manufactures and low cost.
		Metal	Machining	B	B	A
		Rubber	Compression Moulding	C	A	B
Fastener	30 G Vibration 100 N Load No Rust	M6 Class8.8	Casting Heat Treatment Thread Making	-	-	-	M6 For Assembly of Throttle on Restrictor. M6 For Assembly of Restrictor on In Mani.
		M6 Class10.9		-	-	-
		M8 Class8.8		-	-	-

Design Failure Mode Effect Analysis 

If you don't know DFMEA process, read throttle body DFMEA it has RPN number calculation method.

DFMEA of Restrictor:

DFMEA is unavoidable and essential step in designing process. All the possible failure modes related to performance, design, material and manufacturing have been discussed in detail. Also recommendation has been made to improve design process for each and every failure. (RPN above 100 is very dangerous) 

Function	Failure Mode	Failure Effect	SEVERITY	Potential Cause	OCCURANCE	Design Control Method		DETECTION	RPN
						Prevention	Testing
Restrictor	Flow Chocking	Uneven Torque Output Low Peak Torque , Peak Torque at inaccurate RPM	5	Wrong Nozzle Length Wrong Cross Section Steps or sharp change in Dia Flow vertex or pressure points	6	Runner Length & Cross Section Optimization. Iterations for Throttle Bore Dia and plenum inlet dia.	Sudden Acceleration Test, Up Down Test	3	5*6*3= 90
	High Pressure Waves
	Back Flow
	Low Charge Density
	Uneven Flow
Gasket	Leakage	Accident	10	Wrong thickness Wrong Mounting Hole Dia and Tolerances Wrong Material	2	Stress and Strain CFD, Thermal Stress Simulation	Engine Thermal Test	2	10*2*2=40
	Burn
Throttle  Assembly on Restrictor	Bolt Hole Mismatch	No Assembly	9	Wrong Tolerance, Wrong Material, Shrinkage due to Thermal Expansion	2	GDNT and Tolerance Stack up Analysis	Engine Durability  Test	2	9*2*2=36
	Fatigue and Crack	Accident	10		2				10*2*2=40
Diffuser Pipe Assembly on Intake	Wrong Position on Plenum	Low Volumetric Efficiency	8	Unequal Flow in different Runners	5	CFD	Engine Calibration Test	3	8*5*3=120
	Fatigue and Crack	Accident	10	Wrong Tolerance, Wrong Material, Shrinkage due to Thermal Expansion	2	GDNT and Tolerance Stack up Analysis	Engine Durability  Test	2	10*2*2=40

Based on above DFMEA, following are design recommendations 

Function	Failure Mode	Recommended Action
Restrictor	Flow Chocking	Ensure Air cooling of Restrictor. Place restrictor out let at centre of plenum. Use Resonator or Silencer. Use Smooth curved without any sharp edge in restrictor.
	High Pressure Waves
	Back Flow
	Low Charge Density
	Uneven Flow
Gasket	Leakage	High surface finish on mating surface. Have Minimum Hole Position Tolerance.
	Burn
Throttle  Assembly on Restrictor	Bolt Hole Mismatch
	Fatigue and Crack
Diffuser Pipe Assembly on Intake	Wrong Position on Plenum	Place Restrictor at centre of Plenum.
	Fatigue and Crack	Consider factor of safety in stress and select correct material.