Engines

• Engines can’t work by only following first law, as heat will not flow if no work is put in when at thermal equilibrium.
• Max theoretical efficiency is for ideal gasses.
• The only way an engine can be 100% efficient is when Th is as high as possible and tc is as low as possible, absolute zero.
• The energy transferred to sink (Qc) is usually higher than the useful work output of heat engines.
• To maximise the dissipated heat energy, combined heat and power schemes are used. Power plants transfer lots of heat to surroundings which is used to heat homes and buildings.
• Can also maximise dissipated energy through regenerative breaking, flywheels, which allow energy (transferred from power) to be stored
• Reverse heat engines have work done on them to transfer energy from a colder space to warmer space
• Refrigerators try to maximise Qc to make a cold space colder.
• Heat pumps transfer as much as energy to a hot region as possible (per j) maximise QH
• Coefficient of performance is a measure of how effective a reversed heat engine is.
• COP Tc/ Th-Tc is only used when assuming it’s running at maximum theoretical efficiency. has to in kelvin.
• Theoretical indicator diagrams have the same gas throughout, p and T change instantaneously, no friction lost to heat, heat source is external.
• this is a theoretical petrol diagram

  
• This is a theoretical diesel diagram

  
• Otto / petrol cycles are ignited by a spark plug at the end of the compression stroke
• Petrol uses an air/fuel mix
• Deisel engines dont use a spark plug, but reaches a higher temperature during compression which is high enough to ignite the deal when sprayed with fuel at end of the compression stroke
• Deisel only intakes air
• For cyclic processes to be useful, energy done BY system > energy done ON system
• First step of engine cycle is induction, where the piston moves down, volume increases and the gas mixture is drawn into cylinder through inlet valve. pressure is constant.
• Second step of engine is compression, where piston moves up, work is done on the gas and volume decreases; pressure increases. At the end of stroke spark plug ignited the mixture
• When spark plug ignites the mix, temperature greatly increases and pressure also greatly increases
• The third step of engine cycle is expansion, where the gas mix expands and does work on the piston, causing piston to move down. gas is at a high temp and Work done by the gas is higher than the work done to compress it. At end of stroke exhaust valve opens, pressure decreases
• Step four is exhaust, where piston moves up, forcing burnt gas out of cylinder through exhaust valve
• Area beneath the compression curve is the work done on the gas during compression
• Area beneath expansion is the work done by the has during expansion
• Area between the induction and exhaust is the negative work done.
• Area of main loop - negative work done = true net work done
• Calorific value is the measure of how much energy fuel stores per unit volume, JM-3
• Thermal efficiency tells the engineer how much energy is used in powering the machine
• Mechanical efficiency tells an Engineer how much energy is used against friction
• actual otto cycles pV diagrams have a sharp peak, and heating isn’t at a constant volume as temp and pressure can’t increase instantaneously.
• Actual pV loops areas are much smaller than theoretical
• Efficiency is much less in real engine than theoretical
• Real engines pv loops have rounded edges as it takes time for valves to open and close
• Theoretical pV loops have higher peaks as you assume the heat source is external (can create production of higher temps)
• Real pV loops have a lower peak as not all the fuel is burnt (doesnt reach as high a temp)
• Area of real pV loop is smaller as some work is used against friction
• Mechanical = brake over indicated
• Thermal = indicated over input