Unit 5: Applied 2 Flashcards
How do engines typically work on the most fundamental level
Converting heat energy into movement energy
What is a system in the context of thermodynamics
An object of interest whose behaviour we are monitoring in relation to its surroundings
In words, what is the first law of thermodynamics and how is that portrayed mathematically
energy cannot be created or destroyed therefore: The internal energy of a system is the sum of energy inputs and outputs. expressed mathematically as ΔU = Q – W,
Q is heat added W is work done by system
What is meant by internal energy
It is the sum of kinetic energy f the molecules and vibration of the bonds
How is the first law typically wrote in maths
ΔQ = ΔU + ΔW Q = heat entering the system, U = Internal energy, W = Work done by system
work done for a cylinder at constant pressure
ΔW=pΔV
What is an isothermal change?
Temperature stays the same
How can isothermal changes happen (very nearly)
Compressing, expanding very slowly such that heat generated can flow out very slowly
Work done by a gas is called:
expansion
Work done on a gas is called:
compression
List all isothermal processes
- pV = constant and p1V1 = p2V2;
- ΔU = 0 (becasue the internal energy is dependant on temperautre)
- ΔQ = ΔW (if a gas is to work W, Q amount should be supplied)
What is an adiabatic change
A change where there is no heat flow in/out of a system
ΔQ = 0
How can one calculate temperature change in an adiabatic change
p1V1/T1 = p2V2/T2
What is the equation for the adiabatic line?
see google
state the variables in the adiabatic line equation
- Cv = the energy needed to give unit temperature rise in 1 mole of gas where the VOLUME is kept constant
- Cp = energy needed to give unit temp rise in 1 mole of gas where pressure is kept constant
- y is a ratio which is different for certain gases
- k is constant
What is an isovolumetric process
a process occurring at a constant volume.
Key characterics and equation of isovolumetric change
- p1/T1 = p2/T2
no change in volume means no work is done therefore all heat entering becomes internal energy
What is an isobaric process
Happens at a constant pressure
Key characterics and equation of isobaric process
- v1/T1 = v2/T2
- Some heat is used to increase the internal energy and the rest to do work
what do P-V diagrams represent
A cycle of processes undergone by an ideal gas
What are the two equations for thermal efficiency
- (work done per second)/(energy supplied per second)
- (indicated power)/(input power from fuel combustion)
What is the practical efficiency
Similiar to thermal but accounts for frictional losses.
What must an engineer take into account when designing an engine to be efficient
- Considering the theory of how the gases behave as they expand and contact
- designing the engine so friction is low, valves gas tight
what are the 4 stages of the otto cycle
- Induction
- compression
- ignition
- exhaust
What is the initial step of teh otto cycle
Begins at A after the induction stroke which draws the petrol air mixture at a constant pressure.
why is in practice the thermodynamics of a petrol engine are more complex
- fuel burns during the cycle so number of moles is not constant
- cycle happens v.quick so there is swirling of gas, this Ke is not taken into account
- Ignition takes a finitie time and takes time to propagate through the fuel air mix therefore pressures will vary within gases
What happens A->B in otto cycle
Compression: The air in the cylinder is now trapped and compressed adiabatically by the incoming piston. No heat enters or leaves
What happens B->C in otto cycle
Ignition: a spark ignites the mixture raising the pressure and temp v.rapidally
What happens C->D in otto cycle
Power: High pressure in cylinder forces the piston outward. Work done adiabatically
What happens D->A in otto cycle
Cooling: the gas is cooled rapidly by contact with the internal walls of the cylinder, reducing pressure to initial pressure. assuming constant volume.
What happens just before the next cycle?
Piston moves from A to E to exhaust the gases out and then returns to draw more fuel
What is the indicated power of the engine (words and equation)
Rate of net work done by the engine
Area of indicator loop * number of cycles per second * number of cylinders
What is the input power to the engine
Calorific value of fuel*fuel flow rate
What is the calorific value
The energy released per unit mass of fuel when the fuel is burned
What are the units for calorific value
JKg^-1
What are the units for fuel flow rate
kgs^-1
Why is no spark plug needed in the diesel engine
diesel oil is prayed into the cylinder at max compression. Because air is at a high temperature after comp, the fuel ignites on contact with the air, which pushes the piston down at constant pressure
Why does fuel injection in a diesel engine happen at a constant temperature
The fuel ignites as it is injected and pushes the piston out
What are the 4 stages of the indicator diagram for a diesel engine
1 - Compression
2 - Fuel injection
3 - Power
4 - Cooling and exhaust
Why is the output power of an engine less than its indicated power
Some work done by engine is wasted due to friction between moving parts such as bearings.
What meant by brake power
Output power of an engine
What is mechanical efficiency and why is it needed
It is the fraction of power it delivers as brake power, needed to account for friction between moving parts
What is the equation for mechanical efficiency
(Brake Power)/(Indicated Power)
What is meant by friction power
Difference between the indicated power and brake power; power lost due to friction
how does one calculate brake power
(Engine torque)*(angular speed of drive shaft)
What is the overall efficiency of the engine and derive an equation from the definition.
The friction of the input power from fuel combustion that is delivered as power, therefore:
=(brake power)/(input power from fuel combustion)
Derive another equation for overall efficiency considering mechanical and thermal efficiency
Because mechanical efficiency = (Brake power)/(indicated power)
and thermal = (indicated)/(power from combustion)
overall must = mechanical*thermal
What is the Second law of thermodynamics
It is impossible for heat transfer from a high temp source to produce and equal amount of work
The reversible engine is supposed to hold what key trait
The most efficient type of engine according to the laws of thermodynamics
What is a reversible engine
One that can be reversed such that work done on the line causes max heat transfer from cold to hot as well as vice versa
State the formula showing the principle of a reversible engine and its fundamental implication
Qin = W + Qout.
This means the reversible heat engine doesn’t waste energy
In a reversible engine, what is the efficiency dependent on
The temperatures of the source temp (Th) and sink temp (Tc)
What is an equation for efficiency in a reversible engine
Work/Qin
or
(Th - Tc)/Th
What are the components of the reversible engine
- temp source
- temp sink
- Engine
Derive a formula for efficiency of reversible engine consisting of the energy transfers in out out the engine when it is in REVERSE
Since eff = W/Qin
and W = Qin - Qout
therefore eff = (Qin - Qout)/Qin
What is the relationship between the ratios of energy transfer and the heat storages
Qout/Qin = Tc/Th
Real engines operating at leess than max efficiency have what major implication
Energy becomes less and less useful every when it is transferred
What is the main reason the reversible engine cannot produce 100% efficiency
For this Tc would have to be 0, which is impossible
What are the main reasons thermal efficiency values are lower in practice (3 reasons)
- The indicator loop does not have sharp corners, therefore less work is done than if it had sharp corners
- The expansion and compression strokes are not perfectly adiabatic as assumed in calculating the theoretical thermal efficiency
- Work is done by the engine to bring about the induction and exhaust strokes
What are the reasons mechanical efficiency is less than 100%
- Friction between moving parts cannot be eliminated
- Oil used to lubricate the bearings in an engine are viscous which means they cause some resistance to motion of the moving parts
What is a heat pump
A device used to transfer energy from a cold space to a hot one
How is the coefficient of performance of a heat pump defined
(Heat transferred into hot space (Qin))/(work done to bring about the heat transfer)
What happens at stage A->B in the refrigeration process
The refrigerant is in the vapor state and is compressed adiabatically to high pressure.
What happens at stage B->C in the refrigeration process
The high pressure vapor condenses in tubes outside the refrigerator, releasing internal
energy to the surroundings.
What happens at stage C->D in the refrigeration process
The condensed liquid passes through an expansion valve, causing its pressure and
temperature to fall.
What happens at stage D->A in the refrigeration process
The liquid evaporates in tubes inside the ice box, gaining internal energy from objects
in the ice box. The low pressure vapor is then drawn into the compressor for the next cycle.
