Thermodynamics And Engines Flashcards
Give two ways of changing the internal energy
Use forces to do work or heat transfer
What is an adiabatic change?
An energy transfer where no heat transfer occurs
Give a way to cause an adiabatic change in a gas
Compress it quickly
What does γ stand for
Adiabatic constant
Compare adiabatic and isothermal curves on pv graphs.
Adiabatic curves are steeper
Describe the pv graphs for:
Change of volume at constant pressure
Change of volume and pressure at constant temp
Change of volume and pressure at change of temp
Constant pressure is horizontal line.
Constant temp is curve
Adiabatic is steeper curve
Give the first law of thermodynamics
The heat energy supplied to a system either increases its internal energy or enables it to do work or both
ΔQ=ΔU+ΔW
What does the area under a pv graph represent?
Work done
Give the equation for work done and Q in and out
Qin =W+Qout
Why do you need a low temperature sink?
To draw energy from the source
Give the second law of thermodynamics
It is impossible for heat transfer from high temp to source to produce produce an equal amount of work
Give the efficiency of an engine
Work done by engine over energy supplied to engine
Give the cycle of an engine
Do work, cooled, re heat, do work etc.
In a petrol engine what is the working substance?
Air
Give an assumption of an indicator diagram
Fixed mass of air in cylinder throughout each cycle obeying gas laws
What happens in each stage of an engine cycle?
Compression: air in cylinder trapped and compressed adiabatically by piston.
Ignition: spark ignites mixture to increase pres and temp rapidly.
Power: high pres forces piston outward. Work done.
Cooling: gas cooled rapidly reducing pressure
What does the area of the indicator loop give you?
Work done
How do you get indicated power?
=Rate of net WD by engine.
=area of loop X number of cylinders X number or cycles per second
How do you get input power?
=calorific value of fuel X fuel flow rate
Give the thermal efficiency of an engine
WD per second over energy supplied per second
= indicated power over input power from fuel combustion
Describe briefly how a Diesel engine works
No spark plug needed.
Diesel oil sprayed into cylinder at max compression.
Air at high temp immediately after compression so fuel ignites, pushing piston down at constant pressure.
What is output power referred to as?
Brake power
Give mechanical efficiency of an engine
Brake power over indicated power
What is the difference in brake power and indicated power referred to as?
Friction power
How do you measure the brake power of an engine?
Measure teh engine torque when drive shaft is turning at a constant angular speed.
What does brake power equal?
Engine torque X angular speed
What is the overall efficiency equation for an engine?
Brake power over input power from fuel combustion
=mechanical efficiency X thermal efficiency
What is a reversible engine?
One which can be operated in reverse.
Work on it causes max heat transfer from cold to hot
What does efficiency of a reversible engine depend on?
Temp of high temp source and low temp sink
What is Th over Tc equal to?
Q in over Qout
What does efficiency of a reversible engine equal?
=W over Qin
=Th-Tc over Th
What is the only way to make any engine 100% efficient?
Make low temp sink absolute zero which is impossible
What are the theoretical values of thermal efficiency in a petrol engine and a Diesel engine?
Petrol engine=58%
Diesel engine=65%
What are the actual thermal efficiency percentages for a petrol engine and a Diesel engine?
Petrol engine=30%
Diesel engine=35%
Give the four main reasons why thermal efficiency values are lower in practice.
The indicator loop area in practice doesn’t have sharp corners like in the theoretical diagram so less work is actually done.
The maximum temperature is not reached like it is in theory when perfect combustion is used.
The expansion and strokes are not perfectly adiabatic.
Work is done by engine to bring about induction and exhaust strokes.
Give the two main reasons why mechanical efficiency is less in real life than theory.
Friction between moving parts.
Oil is used in the engine to lubricate bearings. Oil is viscous so causes some resistance to motion.
What is r?
The ratio of max volume over min volume of cylinder. The compression ratio.
What is r to the negative 0.4 equal to?
Td-Ta over Tc-Tb
Give a typical r value for a petrol engine
9
Why are CHP stations better than normal power stations?
The water used to cool the engine block is heated and then used to heat local houses in CHP rather than just being wasted
What is a heat pump?
A device that transfers energy from a cold space to a hot space
What are heat pumps used for?
Heating buildings economically
What does a refrigerator do?
Transfer heat from a hot object to cool it down.
