A-LEVEL PHYSICS: 11.2.4: Petrol Engine Cycle (SaveMyExams) Flashcards
heat engine:
device that extracts heat energy from its environment and converts it into useful work
four-stroke engine:
commonly used in:
an internal combustion engine that burns fuel once every 4 strokes of the piston
commonly used in ordinary cars
inside a four-stroke engine:
a piston easily moves up and down in a cylinder
each movement of the piston is a ‘stroke’
steps in four-stroke petrol engine cycle:
-induction
-compression
-expansion
-exhaust
explain induction step in four-stroke petrol engine cycle: (4)
-inlet valve opens
-piston moves down the cylinder, increasing volume
-petrol-air mixture is drawn into the cylinder
-pressure inside the cylinder remains constant, just below atmospheric pressure
explain compression step in four-stroke petrol engine cycle: (5)
-inlet valve closes
-piston moves up, work done on gas by piston (-W)
-petrol-air mixture undergoes adiabatic compression: volume decreases, pressure & temperature increase
-near the top of the stroke, mixture is ignited by spark-plug
-causes rapid rise in pressure & temperature at almost constant volume
explain expansion step in four-stroke petrol engine cycle: (3)
-the high pressure forces the piston back down the cylinder, work is done on piston by gas (+W) (expansion)
-exhaust valve opens when piston is near the bottom of the stroke
-pressure drops to near atmospheric pressure
explain exhaust step in four-stroke petrol engine cycle: (2)
-piston moves up the cylinder, forcing the burnt gas through the open exhaust valve & out of the cylinder
-pressure in cylinder remains at just above atmospheric pressure
explain the four-stroke petrol engine cycle: (14)
INDUCTION
-inlet valve opens
-piston moves down the cylinder, increasing volume
-petrol-air mixture is drawn into the cylinder
-pressure inside the cylinder remains constant, just below atmospheric pressure
COMPRESSION
-inlet valve closes
-piston moves up, work done on gas by piston (-W)
-petrol-air mixture undergoes adiabatic compression: volume decreases, pressure & temperature increase
-near the top of the stroke, mixture is ignited by spark-plug
-causes rapid rise in pressure & temperature at almost constant volume
EXPANSION
-the high pressure forces the piston back down the cylinder, work is done on piston by gas (+W) (expansion)
-exhaust valve opens when piston is near the bottom of the stroke
-pressure drops to near atmospheric pressure
EXHAUST
-piston moves up the cylinder, forcing the burnt gas through the open exhaust valve & out of the cylinder
-pressure in cylinder remains at just above atmospheric pressure
assumptions of theoretical indicator diagram of a four-stroke petrol engine: (5)
-same petrol-air mixture constantly moving through cycle
-pressure & temperature can change instantaneously
-adiabatic expansion & compression
-no friction
-heat source is external
key differences between theoretical & actual indicator diagrams of a four-stroke petrol engine: (6)
in actual indicator diagrams:
-corners are rounded
-heating & cooling cannot occur at constant volume
-expansion & compression are not adiabatic
-not pure air
-fuel may not be completely burnt at the end of the cycle
-usually no induction & exhaust step
key difference between theoretical & actual indicator diagrams of a four-stroke petrol engine is that in actual, corners are rounded. explain:
because the valve takes time to open and close (combustion is not instantaneous)
key difference between theoretical & actual indicator diagrams of a four-stroke petrol engine is that in actual, heating & cooling cannot occur at constant volume.
explain:
because the temperature and pressure do not increase instantaneously
key difference between theoretical & actual indicator diagrams of a four-stroke petrol engine is that in actual, expansion & compression are not adiabatic.
explain:
because there is some heat transfer taking place to cool the gas
