Engineering - Heat Engines Flashcards
(38 cards)
Heat engine definition?
A device or system that extracts energy from its environment in the form of heat and converts it into useful work.
Working substance definition?
The substance (usually liquid) on which the thermodynamic processes are performed in the engine, by changes of temperature, pressure and volume. Enclosed in cylinder.
Egs of heat engines inc (3)
-the internal combustion engine (which uses a petrol-air mixture)
-the diesel engine (which uses a diesel-air mixture)
-the steam engine (which uses water)
The thermodynamic processes experienced by the fluid cause a piston in the cylinder to move up and down, and this motion is then converted, via a crankshaft, into…
into a rotational motion.
The working substance in each cylinder is forced through a series of thermodynamic processes, eventually returning to its starting state. The processes in each cylinder are…
staggered so that the power output is continuous.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
Idealised cycle, where working substance behaves theoretical way and main processes occur one after the other with no..
time delay.
(FIRST try drawing it. Then bring up picture of first engine cycle on ur phone!!! Don’t look at writing.)
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
A’ to A?
Inlet valve opens and exhaust valve closes. Piston moves down. Working substance drawn into each cylinder. This is the INDUCTION STROKE.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
At A?
Inlet valve closes.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
A to B?
Piston moves up, compressing the gas adiabatically. Temp rises. This is the COMPRESSION STROKE.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
B to C?
A spark plug ignites the gas mixture at B, supplying heat Qin and increasing the pressure at a constant volume. Temperature rises.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
C to D?
The increased pressure pushes the piston down as the gas expands adiabatically, decreasing both the pressure and the temperature. This is the POWER STROKE.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
D to A?
The exhaust valve opens at D and most of the burnt gas mixture is released, removing an amount of heat Qout. The pressure and temperature of the gas that remains in the cylinder decrease rapidly.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
A to A’?
A the piston moves up, the remaining gas mixture is expelled. This is the EXHAUST STROKE.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
At A’?
The exhaust valve closes and the inlet valve opens. The cycle repeats.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
Each cycle corresponds to..
Two revolutions of the engine.
The 4-Stroke Petrol Engine Cycle (Otto Cycle):
The thermal efficiency of this engine is =
the ratio of max useful work done by engine / energy supplied by fuel.
The diesel engine cycle:
Engine that compresses air, raising its temp above the — — of the fuel. No spark plug needed.
ignition point
The diesel engine cycle:
Diesel engines require —– —– and produce a higher —- than internal combustion engines.
heavier construction
torque
The diesel engine cycle:
Main difference between diesel engine cycle and 4-stroke petrol engine cycle is that, in diesel engine, …
there’s no fuel in the cylinder during compression.
(Ok now first try drawing, then get out ur diesel engine cycle pic. Again - don’t look at writing yet!)
The diesel engine cycle:
At A’?
Inlet valve opens and exhaust valve closes.
The diesel engine cycle:
A’ to A?
Air drawn into each cylinder at Patm as the piston moves down. This is the INDUCTION STROKE.
The diesel engine cycle:
At A?
Inlet valve closes.
The diesel engine cycle:
A to B?
Air in cylinder compressed adiabatically, as the piston moves up. Temp rises and becomes hot enough to ignite the fuel. This is called the COMPRESSION STROKE.
The diesel engine cycle:
B to C?
Diesel fuel is sprayed into cylinder and is ignited immediately by the hot air, supplying heat Qin. This forces the piston to move down at constant pressure and is the FIRST PART OF THE POWER STROKE.
