Thermodynamics Flashcards
(61 cards)
What is the first law of thermodynamics?
Q = ΔU = W
Energy supplied by heating (J) = Increase in internal energy (J) + work done by the system (J)
What happens when work is done on the gas and by the gas?
Work is done by the gas (+ve work done) if it expands
Work is done on the gas (-ve work done) if it contracts
How do you calculate work done by a gas and what does the equation assume?
W = pΔV
Work done by the gas (J) = pressure of gas (Pa) c change in volume (m3)
This equation assumes the surrounding pressure doesn’t change as the gas expands
The pressure of the gas stays constant, the volume changes
How do you calculate work done from a p-V diagram?
Work done by the gas = area under p-V diagram
What are the 4 main thermodynamic processes?
Constant volume (w=0)
Constant pressure (Δp=0)
Isothermal (ΔT=0)
Adiabatic (ΔQ=0)
Explain a constant pressure process
A process in which no change in pressure occurs
This occurs when gases are allowed to expand and contract freely during a change in temperature
W = pΔV (equation on valid when pressure is constant)
Q = ΔU + pΔV
V1/T1 = V2/T2
Explain a constant volume process
A process where no change in volume occurs hence, no work is done
W = 0 (W=pΔV –> ΔV=0)
Q = ΔU
All energy transferred by heating is transferred to the internal energy of the gas
Vertical straight line on a p-V daogram - no area below as no work is done
P1/T1 = P2/T2
Explain an isothermal process
A process in which no change in temperature occurs
No change in temperature means no change in internal energy of the gas
ΔU = 0
Q = W
Any energy supplied to the gas is used to expand the gas
pV = constant –> p1V1 = p2V2
Explain an adiabatic process
A process where no heat is transferred into or out of the system
Q = 0 (no heat energy is supplied)
W = -ΔU
If the gas expands, the internal energy decreases
An adiabatic process will be accompanied by a change in temperature
PVγ = constant
Adiabatic vs isothermal p-V diagram
An adiabatic p-V diagram is steeper than an isothermal diagram
What is a four-stroke engine and what are the 4 stages?
A four-stroke engine is an internal combustion engine that burns fuel once every 4 stroked of the piston. Each movement of the piston up or down is called a stroke. The 4 stages are:
- Induction
- Compression
- Power
- Exhaust
Explain the induction stage of a petrol engine
The piston moves down the cylinder, increasing the volume of the petrol-air mixture which is drawn into the cylinder by the inlet valve
The pressure in the cylinder remains constant, just above atmospheric pressure
Explain the compression stage of a petrol engine
The inlet valve is closed and the piston moves back up, doing work on the gas
The volume decreases and the pressure increases. This is done adiabatically (no additionally heat energy is supplied)
Almost at the end of the cycle, the petrol-air mixture is ignited by a spark. The temperature and pressure increase rapidly, at an almost constant volume
Explain the power stage of a petrol engine
The high pressure forces the piston back down the cylinder, so work is done by expanding the gas
The exhaust valve opens when the piston is near the bottom of the strok, and the pressure reduces to almost atmospheric pressure
Explain the exhause stage of a petrol engine
The piston moves up the cylinder, forcing the burnt gases through the open exhaust valve and out of the cylinder
The pressure in the cylinder remains just above atmospheric pressure
What are the assumptions made in a theoretical indicator diagram of a 4-stroke petrol engine?
The same air/gas is constantly moving through the cycle repeatedly
The pressure and temperature can change instantaneously
The expansion and compression happens adiabatically
The engine experiences no friction
The heat source is external
Explain the stages of a theoretical indicator diagram of a 4-strok petrol engine
A-B : gas is compressed adiabatically
B : the mixture is ignited by a sparl
B-C : heat is supplied and volume is kept constant. Temperature and pressure increase rapidly
C-D : gas expands adiabatically (temperature decreases)
D-A : system is cooled at a constant volume (heat leaves the system)
Why is an actual indicator diagram different to a theoretical one?
Heating and cooling cannot occur at constant volume
Expansion and compression are not adiabatic
Combustion is not instantaneous - corners are rounded
How do you calculate work done on a petrol indicator diagram?
The work done on the gas is the area below the compression curve (A-B)
The work done by the gas is the area below the expansion curve (C-D)
The net work done is the area of the enclosed loop or work done by the gas minus work done on the gas
Explain the induction stroke of a diesel enigne
In the induction stroke, only air is drawn into the cylinder. This means tehre is no fuel in the cylinder during compression
Explain the compression stroke of a diesel engine
During the compression stage, the air is compressed at a high temperature which vaporises and ignites the diesel fuel, which is pumped directly into the cylinder through an injector. There is no spark
Explain the expansion and exhaust stages of a diesel engine compared to a petrol engine
The exhaust and expansion stages of both engines are similair
Explain the stages of a theoretical indicator diagram of a 4-stroke diesel engine
A-B : gas is compressed adiabatically
B-C : heat is supplied and pressure is kept constant (flat peak)
C-D : gas expands adiabatically
D-A : system is cooled at a constant volume and heat exits the system
Why are diesel engines more efficient than petrol engines?
Efficiency is directly proportional to the compression ratio
Diesel engines have a higher compression ratio than petrol engines and are hence more efficient
