VIVA

Question 1

What is the difference between dBC and dBA? Why is this significant?

Answer 1

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

Question 2

Why was the engine tuned to 7200 RPM?

Answer 2

Engine speed for maximum torque, more important than power for FS events

Same speed as the intake

Same speed range as dyno data.

Question 3

Does the drivetrain allow the engine to rev to 7200 in all gears?

Answer 3

The test bed data from the F15 car indicates it does because it revs up to 8000RPM

Question 4

Why/how was the idle engine speed chosen to be 2000 RPM?

Answer 4

Feedback from test days - hard to get the engine to actually idle

Can be chosen for FS competition

Lower speed has lower noise output

Question 5

What is impedance?

Answer 5

Effectively resistance.

Question 6

What is the difference between a two stroke and four stroke engine. How does this effect the exhaust?

Answer 6

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.

Question 7

Explain the combustion cycle and draw it.

Answer 7

• 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)

Question 8

What is the acoustical velocity?

Answer 8

Speed of sound dependent of temperature and density.

Question 9

Why does the amount of time that the exhaust valve is open for decrease with engine speed.

Answer 9

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.

Question 10

What is the stroke length?

Answer 10

The stroke length is determined by the cranks on the crankshaft. Stroke can also refer to the distance the piston travels.

Question 11

What is the cylinder bore?

Answer 11

Size in terms of diameter for which a piston travels

Question 12

Explain the different orientations of the crankshaft cycle with regards to pumping loop graphs

Answer 12

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).

Question 13

What is gas inertia?

Answer 13

Inertia is the resistance of any physical object to any change in its state of motion

Question 14

What is a fully developed flow in an exhaust?

Answer 14

A fully developed fluid flow through a pipe is one in which the boundary layer extends to the centre line.

Question 15

How does a perforated tube avoid turbulent flow?

Answer 15

-Allows fraction of the flow to pass through
-Still large volume for packing material
-

Question 16

Why was 3D simulation such as fluent not used?

Answer 16

• 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

Question 17

What sort of mounting insulates from vibration?

Answer 17

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.

Question 18

Why didnt i do it as torque to weight ratio?

Answer 18

Torque was not the main focus of the investigation

This wouldnt take into account other changes in weight across the engine package

Question 19

Why was a variable geometry exhaust not investigated?

Why is it not realistically feasible?

Answer 19

• 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

Question 20

What experimental test data should be collected if possible?

Answer 20

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

Question 21

What do flowsplits do in GT-POWER?

Answer 21

• 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

Question 22

What is Hemoltz theory?

Answer 22

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:

1. Heat transfer and friction are negligible
2. End effects are ignored
3. Gas within the header is incompressible
4. Gas in the header acts as a linear spring, with the mass in the pipe oscillating on it

Question 23

What are the restrictions for side routing of the exhaust?

Answer 23

Limited to the aero restrictions

Cant extend past the wheels

75mm clearance of front and rear tyres

Not above bar

Heat shielded for convection

Question 24

How could the GT-POWER model be made more comprehensive?

Answer 24

Cooling

Question 25

Why wasnt the whole exhaust made from aluminium?

Answer 25

Originally it was all stainless to facilitate easier welding

Then aluminium expansion chamber chosen because of its large size

Ferrous metals are much more resilient and can take the vibes and heat.

The aluminium parts require clamps and slip fits so that they can withstand the vibration, rather than being welded.

This leads to leakages

Question 26

How could the GT-POWER model be made more comprehensive?

Answer 26

Cooling

Feedback loops