VIVA Flashcards
(26 cards)
What is the difference between dBC and dBA? Why is this significant?
The are different filter weightings of sound
A is weighted for the audible range of human ear
C is weighted towards lower frequency so is applicable to automotive applications
Why was the engine tuned to 7200 RPM?
Engine speed for maximum torque, more important than power for FS events
Same speed as the intake
Same speed range as dyno data.
Does the drivetrain allow the engine to rev to 7200 in all gears?
The test bed data from the F15 car indicates it does because it revs up to 8000RPM
Why/how was the idle engine speed chosen to be 2000 RPM?
Feedback from test days - hard to get the engine to actually idle
Can be chosen for FS competition
Lower speed has lower noise output
What is impedance?
Effectively resistance.
What is the difference between a two stroke and four stroke engine. How does this effect the exhaust?
2-stroke piston engine has an intake and an exhaust stroke. All air, fuel, and combustion products must be moved in those 2 strokes.
4-stroke engine has a separate stroke for moving burned (or burning) and unburned mixtures - intake, compression, expansion, and exhaust.
Explain the combustion cycle and draw it.
- A-B: Isentropic and Adiabatic Compression (Work In)
- B-C: Isochoric (Constant Volume) heat addition (Combustion)
- C-D: Isentropic and Adiabatic Expansion – Work Out (Power Stroke)
- D-A: Isochoric Heat Rejection (Exhaust)
What is the acoustical velocity?
Speed of sound dependent of temperature and density.
Why does the amount of time that the exhaust valve is open for decrease with engine speed.
As engine speed increases then the speed that the crankshaft rotates at increases, therefore the fraction of the time that the exhaust valve is open for decreases.
What is the stroke length?
The stroke length is determined by the cranks on the crankshaft. Stroke can also refer to the distance the piston travels.
What is the cylinder bore?
Size in terms of diameter for which a piston travels
Explain the different orientations of the crankshaft cycle with regards to pumping loop graphs
In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft.
The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).
What is gas inertia?
Inertia is the resistance of any physical object to any change in its state of motion
What is a fully developed flow in an exhaust?
A fully developed fluid flow through a pipe is one in which the boundary layer extends to the centre line.
How does a perforated tube avoid turbulent flow?
-Allows fraction of the flow to pass through
-Still large volume for packing material
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Why was 3D simulation such as fluent not used?
- Needed to be able to optimise 4 different geometries
- Can’t predict volumetric efficiency in GT-POWER
- GT-POWER has the muffler pre-processor
- Validated model for GT-POWER
What sort of mounting insulates from vibration?
Isolate the resonating exhaust system from the chassis to prevent structure borne sound.
Secured using an anti-vibration mounting strap attached via stainless steel square tube welded to the chassis.
High temperature resistant silicone sponge medium
Not finalised because requires the exhaust to be tacked up first.
Why didnt i do it as torque to weight ratio?
Torque was not the main focus of the investigation
This wouldnt take into account other changes in weight across the engine package
Why was a variable geometry exhaust not investigated?
Why is it not realistically feasible?
- Increases the torque at lower RPM
- This is not necessary if the driver is competent
- Focus was on maximising noise attenuation, this would not have benefited this.
- Large increase in weight, already increased weight with the expansion chamber
- High cost
- Produces leakage and flow losses
What experimental test data should be collected if possible?
Wall Temperatures
Torque and Power Curves
- Pressure Transducers in the intake, cylinder and exhaust systems
- Pressure is sampled every crank degree
- Key to determining the accuracy of pressure pulses and tuning effects
What do flowsplits do in GT-POWER?
- Pipe junctions which allow the code to simulate 3d junctions
- Give information on the size of the junction and the losses associated with travelling through it
What is Hemoltz theory?
Tuning peak will occur when the natural Helmholtz resonance of the cylinder and header is roughly double the piston frequency.
The resonator volume is considered to be the cylinder volume with piston at mid stroke
Assumptions for Helmholtz tuning theory include:
- Heat transfer and friction are negligible
- End effects are ignored
- Gas within the header is incompressible
- Gas in the header acts as a linear spring, with the mass in the pipe oscillating on it
What are the restrictions for side routing of the exhaust?
Limited to the aero restrictions
Cant extend past the wheels
75mm clearance of front and rear tyres
Not above bar
Heat shielded for convection
How could the GT-POWER model be made more comprehensive?
Cooling
