Thermodynamics and Engines (need some editing) Flashcards
What is the first law of thermodynamics?
Q=ΔU+W
- the energy transferred to a system through heating is equal to the sum of the increase in internal energy and the work done by the system
When is Q positive?
when energy is transferred to the system by heating
when is ΔU positive?
when there is an increase in temperature
when is W positive?
when work is done by the system
What is an Open system?
-e.g?
matter can go flow in and out the system
-aerosol can
what is a closed system?
-e.g?
no matter can enter or leave the system
-air inside a balloon
what is an ideal gas?
-perfectly elastic collisions between molecules
-no intermolecular forces
-no potential energy = internal energy is equal to the sum of the kinetic energies of all its particles
what is the ideal gas equation?
pV= nRT
what is the equation between pressure, volume and temperature?
p₁V₁/T₁ = p ₂V ₂/T ₂
What is an adiabatic process?
no heat leaves or enter the system
- Q=0
- ΔU=-W
what is the equation for an adiabatic process?
pVˠ=constant
what is an isothermal process?
the temperature of the system remains constant
-ΔU=0
- Q=W
what is the equation for an isothermal change?
pV=constant
what is a constant pressure change?
(isobaric)
the pressure of the system remains constant
what is the equation for a constant pressure change?
W=pΔV
what is a constant volume change?
(Isochoric)
the volume of the system remains the same
- W=0
- Q=ΔU
what is the area beneath the pV curve?
the work done during the process
what does isothermal mean?
constant temperature
How do you find the work done per cycle on a pV graph?
the area of the loop
What are the Four steps in a four stroke petrol engine?
- Induction
- Compression
- Expansion
- Exhaust
Explain what happens during Induction?
(four stroke petrol engine cycle)
the piston moves down, gas mixture drawn in through inlet valve
-volume of gas increases
-pressure is constant
Explain what happens during Compression?
(four stroke petrol engine cycle)
inlet valve is closed and cylinder moves up, doing work on the gas
-volume of gas decreases
-pressure increases
mixture is then ignited and temperature and pressure rapidly increases at almost constant volume
Explain what happens during Expansion?
(four stroke petrol engine cycle)
gas mixture expands, doing work on the piston causing it to move down the cylinder and exhaust valve opens.
- pressure drops to slightly above atmospheric pressure
Explain what happens during Exhaust?
(four stroke petrol engine cycle)
piston moves up the cylinder, forcing the burnt gas out through the open exhaust valve
How is a diesel engine different to a petrol engine?
Diesel only intakes air, not a gas mixture, the diesel is directly inputed into the cylinder as a fine spray using a fuel injector, causing it to ignite
Why is the efficiency of a real engine far lower than in theory?
-time taken to open and close valves
-heating does not take place at constant volume
-fuel not completely burned
How are the indicator graphs for theoretical and actual engine cycles different?
-actual has rounded edges
-theoretical has higher peaks
-actual has a smaller loop area
What is indicated power?
the product of the area of the main pV loop and the number of cycles it undergoes per second and the number of cylinders
what is brake power?
-equation?
the power output of the engine
- P=T⍵
what is the input power?
the product of the calorific value of the fuel and its flow rate
what is the calorific value?
the measure of how much energy the fuel stores per unit volume or mass
what are 3 types of engine efficiencies?
-Overall efficiency
-Thermal efficiency
-Mechanical efficiency
what is the equation for Overall efficiency?
Overall efficiency = Brake power/input power
what is the equation for thermal efficiency?
thermal efficiency=indicated power/input power
what is the equation for mechanical efficiency?
mechanical efficiency =brake power/indicated power
What is the second law of thermodynamics?
a heat engine must have both a source and a sink to operate
why would an engine not abide by the second law of thermodynamics?
if the engine reached the temperature of the source (thermal equilibrium), no more heat would flow and so no work would be done. i.e not 100% efficient
what is the equation for the efficiency of an engine?
W/Q(hot)
what is regenerative braking?
allows the power caused by braking in a vehicle to be stored (in a flywheel) and be used later on to accelerate when needed
what are reverse heat engines?
engines that have work on them in order to transfer energy from a colder region to a warmer one
what are the two forms of reverse heat engines and briefly explain function?
-Refrigerators: extract energy from cold region
-Heat pumps: transfer energy to hot region
What is COP, coefficient of performance?
a measure of how effective a reverse heat engine is at transferring heat per unit of work done
what is the equation of COP(ref)?
COP(ref)= Q(cold)/W
what is the equation of COP(hp)?
Q(hot)/W
What are CHP schemes, Combined heat and power?
-problems with CHP?
use heat transferred from power plants to surroundings to heat homes and businesses close by.
-homes and businesses tend to be far away from power plants
explain, with reference to the first law of thermodynamics, why the temperature of the gas decreases as a balloon is inflated
- Q=Δu+W
- Q=0 as it’s a rapid expansion
- when balloon is inflated, work is done by the system hence W is positive.
- Δu=Q-W, therefore change in internal energy is negative so temperature decreases.
