Exhaust System Design Review and Design Failure Mode Effct Analysis

Please read previous blogs for exhaust layout, material selection, manufacturing consideration and related subjects. Here in this blog we will discuss design review and design failure mode effect analysis. To understand RPN Number and its calculation please read my blog of throttle body DFMEA or Intake Manifold DFMEA. 

Design Review of Exhaust System - 

Child Part	Requirement	Material	Manufacturing	Cost	Weight	Life	Design Review
Exhaust Manifold	Burst Pressure 40 Bar, Vibration 30 g, Working Temperature 1000 ° C, Surface Roughness Ra8	Aluminum	PDC, Machining	A	B	A	Exhaust Manifold with catalyst will be parched and welding will be done on steel pipe to regulated pipe length if required.
		Steel	Machining	B	B	A
Gasket	Working Temperature 1000 ° C,  Leakage Pressure 30Bar	Steel	Machining	A	/	C	If no change is done in exhaust pipe then same gasket from Yamaha R6 will be used. Otherwise same R6 gasket will be machined to adjust shape for exhaust.
		Alloy Steel	Machining	C	/	A
Catalyst	As per 2016 Emission Rule in UK	Currently Fitted	OEM Knowhow	A	C	B	New R6 catalyst can be used as per latest emission regulation 2016. As currently fitted catalyst is old it can cause back pressure or breach emission regulation however after performance test old part can also be used.
		New R6 2016		C	A	A
Fastener	Temp 1000 °C 30 G Vibration 100 N Load No Rust	M8 Class8.8	Casting Heat Treatment Thread Making	-	-	-	As temperature, Vibration and Chemical Reaction are high; it is suggest to use M10 Bolts.
		M8 Class10.9		-	-	-
		M10 Class8.8		-	-	-

DFMEA - Design Failure Mode Effect Analysis is mandatory and day to day work of an automotive engineer in design department. No safety related part can be and must be designed with out DFMEA. Severity is threat, Occurrence is probability of happen this failure and detection is can quality circile tools can catch it. 

For example softness of driver seat - Suppose softness of driver seat is out of specification. Severity is 1 as there is no threat to life of driver or legal issue to OEM. Occurrence is how often this happen - this data you can get from quality department or vendor development department. May be in 10000 seats, 1 seat is out of specification- so 2 or 3 rating is ok. Detection - Can quality engineer check it on manufacturing line or can final inspection engineer at OEM check it, catch it and reject it - of course yes so score will be very less.  

On the other hand  fuel tank or brake fail is life threat so severity is 10. Occurrence is high because fuel system has hundreds of parts and any part malfunction can lead to leak. Detection is difficult as many tests are destructive test and sampling is done. For example rubber hose near fuel rail. if every component is check for worst case pressure - the hose will burst - it is called burst testing so out of 1000 hose only 1 is checked. 

If you need more understanding about DFMEA, please read throttle dfmea and intake manifold dfmea. 

Function	Failure Mode	Failure Effect	SEVERITY	Potential Cause	OCCURANCE	Design Process		DETECTION	RPN
						Prevention	Testing
Exhaust Manifold	Interference in gas	Low Volumetric Efficiency High Exhaust Noise Emission Rule Violation	7	Wrong Exhaust Pipe Length, Wrong Catalyst selection	9	Pipe Length Simulation, Manifold Simulation. Burst Pressure Simulation	Emission Test, Exhaust Noise Test, Engine Durability Test	3	7*9*3= 189
	Back Flow
	Flow Chocking
	Leakage
	Catalyst Melting
Muffler Silencer	Pressure Wave	Abrupt Torque Change, High Exhaust Noise, Emission Rule Violation	8	Wrong Muffler Shape Wrong Tail Pipe Length	5	Noise Simulation, Pressure Wave Simulation	Exhaust Noise Test, Engine Durability Test	4	8*5*4 =160
	Low scavenging  Efficiency
	No Noise Reduction
	Leakage, Melting or Braking
Gasket	Leakage	Accident	10	Wrong thickness Wrong Mounting Hole Dia and Tolerances Wrong Material	2	Stress and Strain CFD, Thermal Stress Simulation	Engine Dyno Test	2	10*2*2=40
	Burn
Assembly	Air Leakage	High Noise and Emission regulation Violation	8	Wrong Tolerance, Wrong Material, Shrinkage due to Thermal Expansion	2	GDNT and Tolerance Stack up Analysis	Engine Durability  Test	2	8*2*2=32
	Bolt Hole Mismatch
	Fatigue and Crack

Suggestion based on DFMEA of Exhaust System :-  

Function	Failure Mode	Recommended Action
Exhaust Manifold	Interference in gas	421 Type Exhaust Pipe to avoid gas interference. Use new R6 Catalyst. Manifold Outlet Dia should be nearly double that single inlet pipe.
	Back Flow
	Flow Chocking
	Leakage
	Catalyst Melting
Muffler Silencer	Pressure Wave	Use simple perforated type muffler from available range from Yamaha for Bikes rather manufacturing.
	Low scavenging  Efficiency
	No Noise Reduction
	Leakage, Melting or Braking
Gasket	Leakage	Use Metal Gasket with high surface finish. Have Minimum Hole and Position Tolerance.
	Burn
Assembly	Air Leakage	Keep gasket hole bigger than hole in Manifold. Also Gasket Hole can be made Oval to move and fix it.
	Bolt Hole Mismatch
	Fatigue and Crack