Engineering physics Flashcards
Rotational dynamics
What effects the flywheel KE
[4]
- mass
- angular velocity
- friction
- spoked - mass spread out to the side
Rotational dynamics
three uses of flywheel
- regenerative breaking
- wind turbine
- smoothing out torque/ angular v.
how to convert any
quantity
angular -> linear
multiply by radius
Thermodynamics
What does +ve Q means
energy transfered into the system
Thermodynamics
What does +U mean
increased avg. KE of particles
Thermodynamics
What does +WD mean
work done by the particles
Thermodynamics
what is the area underneath p-v graph
Work done by gas
Thermodynamics
What is constant in isothermal process
internal energy / temp.
Thermodynamics
What is constant in adiabatic processes ?
heat energy of system
Thermodynamics
Describle the 4 steps to ideal engine cycle
Induction - constant Pa expansion
Expansion - adiabatic
Compression - adiabatic
exhaust - constant Pa compression
Thermodynamics
describle the induction step of ideal engine
- fuel mixture in
- volume increases at constant Pa
Thermodynamics
describle expansion stroke of ideal engine
- valves closed
- piston push down to compress gas
- spark generated
- heat transfered to system
- Inc. U => Inc. T and Inc. Pa
Thermodynamics
Describle power stroke
- net WD by gas to push piston
- volume increases
- Temp decreases
Thermodynamics
Differences between
real and ideal
engine cycle
- need induction and exhaust stroke
- rounded corners - valve takes time
- heating/cooling cannot occur at constant vol.
- not ideal gas
- fuel may not completely combust
- mas pressure is lower
- compression/expansion not completely adiabatic
Thermodynamics
Difference between
petrol and diesel
engine
- only draws air in induction stroke
- fuel introduced during expansion stroke
- no spark needed
Thermodynamics
Why is there less
work output
in real engine
- smaller area of loop
- area of pumping loop needed subtraction
- friction between surfaces
- energy used in pumping gas + valve op.
Thermodynamics
define 2nd law of thermodynamics
the need for engine to operate between a source and a sink
Thermodynamics
How to maximise Qh and WD
2nd law of thermodynamcis
- CHP - combine heat and power
- reduce friction
Thermodynamics
What is the purpose of refridgerator
- abstract as much heat as possible from cold place
- maximise Qc
Thermodynamic
what is the purpose of heat pump
- deliver as much as heat to hot place
- maximise Qh
Thermodynamics
Eq. of thermal efficiency
indicated / Qh
Thermodynamics
Eq. of mechanical efficiency
brake / indicated
Thermodynamics
Eq. of overall efficiency
mechanical x thermal
brake / Qh
Define input power
indicated power (WD) +
output power to sink (Qh)
Thermodynamics
How to cal. Indicated power
A(loop) * no. of piston * frq of cycle
Thermodynamics
Indicated power eq.
brake power + frictional power
how many rotations per cycle
2
Describle thermal efficiency
how well the calorific value of fuel is converted into work inside engine
Describle mechanical efficiency
work/power used to overcome friction/viscocity inside engine and operate valves, water and oil pumps.
Thermodynamics
why is it not advised to operate above optimal speed
- thermal efficiency decreases
- friction increases at higher speed
- work against greater viscosity
Thermodynamics
Why is there less WD isothermally than adiabatic
- adiabatic curve is steeper to reach the same volume
- greater area under the curve
- so more WD
