AGK - Engines Flashcards
What is Boyle’s law?
Assuming constant temperature:
PV = Constant
What is Charle’s law?
Assuming constant pressure:
v ~ T
What is the combined gas law?
Combining Boyle’s (PV = Constant)
and Charle’s (V~T)
PV/T = Constant
What is it called when all the ends of an engine line up?
Top dead centre (TDC)
Bottom dead centre (BDC)
What are the four strokes?
Induction
Compression
Power
Exhaust
How is the practical 4 stroke cycle different from the theoretical one?
Practical cycle takes into account the gases having momentum.
During TDC/BDC there is little linear movement of the piston fo the same angular movement of the crank.
This is ineffective crank angle, there is little change in cyclinder pressure so gasses struggle to enter/exit.
Valve and ignition timing are adjusted in the form of valve lead/lag to account for this, this is the practical 4 stroke cycle.
What is valve lead? (Inlet)
As exhaust stroke comes to the end, but before the induction stroke, the inlet valve opens allowing the momentum of the gases leaving the exhaust to use pressure to pull more fresh mixture in.
What is valve lag? (Inlet)
The inlet valve remaining open beyond BDC after the induction stroke, allowing the intertia of the gas to pull more mixture in at this ineffective crank angle.
What is the equation for thrust?
F = M * (Vjet - Vflight)
What is 1 horsepower in watts?
1 horsepower = 740 watts
What is thermal efficiency?
Ratio between heat energy converted into power output and total heat energy within the fuel.
30% Typical value
What is mechanical efficiency?
The power output of the engine at propshaft compared to the theoretical power output.
Typically 80%
What is volumetric efficiency?
The volume of the charge compared to the avaliable volume.
What is an adiabatic process?
Change in pressure caused by change in volume with no heat energy added or removed.
What is an isochoric process?
Change in pressure with no change in volume.
What is stroke?
Distance the piston travels between the lowest and highest points (TDC & BDC)
When does ignition occur?
Between compression and power strokes
How far does the crank rotate for a stroke?
180 Degrees
What is ineffective crank angle?
Small linear movement of the piston at TDC/BDC for the same angular rotation of the crank.
What does the theoretical cycle assume about combustion time?
It’s instant
What two things do we change in the practical Otto cycle?
Valve timing
Ignition timing
When does valve lead occur on exhaust?
At the end of the power stroke the exhaust valve opens early as most of the energy has been imparted in the inertia of the gases remaining.
When does valve lag occur on exhaust?
Valve remains open during the start of the induction stroke to account for inertia of gases allowing maximum mixture to enter.
During this point we get valve overlap from the inlet valve being open at the same time as the exhaust.
What is valve overlap?
When the exhaust valve lag from the exhaust stroke and the inlet valve lead for the induction stroke, both valves are open at the same time.