Engines - Engineering

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  • Created by: yasminw24
  • Created on: 25-06-17 16:45
What is the equation for thermodynamics?
Q=U + W
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What is the first law of thermodynamics about?
How energy is conserved in a system through heating, cooling and doing work
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What does Q stand for?
Q = energy transferred to a system by heating. If energy is transferred away, it is -ve
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What does U stand for?
Increase in internal energy (sum of potential and kinetic energies)
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What does W stand for?
Work done BY system. It is negative if work is done ON the system
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What is the equation for a change in a closed system?
p1v1/t1 = p2v2/t2
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What is an isothermal change?
Where temperature remains constant so U=0 as internal energy ONLY depends on temperature Q=W
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What does Q=W mean?
Work a system does = amount of heat energy supplied. Work done on system = amount of heat energy lost
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Using the ideal gas law, what is the equation for an isothermal change?
p1v1=p2v2
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What is an adiabatic process?
Where no heat is lost or gained by the system so Q=0. Therefore U=-W
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What does U=-W mean?
Any change in the internal ennergy of a system is caused by work done by/on the system
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Using the ideal gas law, what is the equation for an Adiabatic process?
p1v1^=p2v2^, where ^ is the adiabatic constant
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What happens in changes at a constant pressure?
Where pressure doesn't change, work done is W=pV. For an expansion, W and V are positive, for a compression, W and V are -ve
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Using the ideal gas law, what is the equation for a chnage at constant pressure?
V1/T1=V2/T2
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What happens in changes at a constant volume?
Where volume is constant work done = zero. If W=0 then Q=U, therefore transferring heat energy to the system, only increases internal energy
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What are all the variation you can have of a system?
Constant Temp (Isotherm), No Heat loss/gain (adibabatic), constant pressure (W=pV), constant volume (no work)
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What do you use p-V diagrams for?
To represent non-flow processes, an arrow shows the direction the change is happening in, area under curve represents work dne during the process
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What are p-V curves for isothermal processes called?
Isotherms
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What does the p-V graph look like for an isothermal compression? How does the graph vary?
The position depends on temperature. Hotter the temperature, the further from the origin
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What does the p-V graph look like for an adiabatic process?
STEEPER than isothermic, LARGER area as more work is done to compress gas adiabatically
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What does a p-V graph look like at constant volume?
straight vertical line. No work is done as no line for area under
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What does a p-V graph look like at constant pressure?
Straight horizontal line, the work done is area under the graph / W=pV
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What is a cyclic process?
A system can undergo different processes one after another which form a cycle (loop). They start at a certain pressure and volume and return to them at the end of the cycle
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How do you find the net work done of a cyclic process?
Area of a loop or find difference between work done by a system and work done on a system
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What is a four-stroke petrol engine?
Petrol engines which burn fuel once every four strokes of a piston
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What is a stroke?
Each time a piston moves up and down
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What is the set up in an internal combustion engine?
Cylinders filled with air, air in cylinders is trapped by tight-fitting pistons which move up and down
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What is the name of each four stroke stage?
Induction, Compression, Expansion, Exhaust
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What happens in the Induction stage?
1. Piston starts at top and moves down, increasing volume of gas. 2. This sucks in a mixture of fuel and air through an open inlet valve. 3. Pressure remains constant, below atmospheric pressure
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What happens in the Compression stage?
1. Inlet valve closes. 2. Piston moves up and does work ON the gas, increasing pressure. 3. Just before pistons at top, the spark plug creates spark and ignites air-fuel mixture. 4. Temp and Pressure sudden increase at nearly constant volume
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What happens in the Expansion stage?
1. The hot air-fuel gas expands and does work ON the piston as it moves down 2. There is a net output of work 3. Just before piston is at bottom, exhaust valve opens and pressure reduces
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What happens in the Exhaust stage?
1. Piston moves up and burnt gas leaves through exhaust valve 2. Pressure remains almost constant just above atmospheric pressure
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What is different about a four-stroke DIESEL engine?
1.. Induction:only air pulled in, not mix. 2. Compression: air compressed till it reaches temp high enough to ignite diesel fuel. Just before end, diesel sprayed in cylinder by injector and ignites. 3. Expansion & Exhaust = same
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What is differnet about the indicator diagrams between petrol and diesel?
For Diesel - no sharp peak at start of expansion stroke. FLAT
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Why would we compare an ideal/theroetical p-V graph to one for a real engine?
To find out how much friction there is. See how much power the engine actually produces
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What is the name of the Petrol cycle?
Otto Cycle
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What do the theorectical models assume?
1. Same gas taken continuously in cycle. Gas is pure air, with adiabatic constant 1.4. 2. Pressure and temp changes are instantaneous 3. Heat source external 4. Engine frictionless
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What is the difference between the real life models and the theoretical models?
1. Corners of theore not rounded 2. heating not at constant volume 3. doesn't include small amount of -ve work 4. Engines have internal heat source 5. energy need to overcome friction (smaller loop)
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How do you calculate the enginer power form an indicator diagram?
Net work done x number of cycles per sec x number of cylinders
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Why is the indicated power not the actual output power and how would you work this out?
Power needed to overcome friction. So actual output power is less than the indicated power
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What is meant by the efficiency of an engine?
It is the measure of how much of the input power is transferred usefully
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Name the 3 kinds of efficiency?
Mechanical Efficiency, Thermal Efficiency, Overall Efficiency
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What is the equation for mechanical efficiency?
brake power / indicated power
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What is the equation for thermal efficiency?
indicated power / input power
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What is the equation for overall efficiency?
brake power / input power
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What is the 2nd law of thermodynamics?
No engine is 100% efficient
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Without the 2nd law, what would it suggest about thermodynamics?
Theoretically it suggests all of the heat energy supplied to a heat engine could be transferred into useful work
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Why would it be impossible to not have this 2nd law?
As some heat ends up increasing temperature of engine. If it reaches temperature of heat source no heat flows so no work is done
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How do you calculate the efficiency of a heat engine?
W / Qh
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Why are real heat engines' efficiencies lower than their theoretical maximum?
1. Frictional forces inside the engine 2. Fuel doesn't burn entirely 3. Engine needs to move internal components of engine
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What are CHP plants?
Combined heat and power plants try to limit energy waste by using waste heat created by engines for other purposees e.g. heating houses and businesses nearby
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How do reversed heat engines work?
They take heat energy from a cold 'space' and transfer it to a hot 'space'. Heat naturally flows from hotter to colder spaces so work must be done to transfer heat from a colder space to a hotter space
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What is an example of a reversed heat engine?
Refrigerator or Heat Pump
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How does a refrigerator work?
1. Extract heat energy from cold space 2. Keeping space as cool as possible and food fresh for longer
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How does a heat pump work?
Aims to pump as much heat as possible into a hot space. Cold space is outdoors and hot space is inside house
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What is meant by COP?
Coefficient of Performance. It measure how well work is converted into heat transfer e.g. COP of 4. Transfers 4J of energy for every 1J of work done (bigger than 1 so not efficiency)
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Card 2

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What is the first law of thermodynamics about?

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How energy is conserved in a system through heating, cooling and doing work

Card 3

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What does Q stand for?

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Card 4

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What does U stand for?

Back

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Card 5

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What does W stand for?

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