Piston Engine Principles

Question 1

What will the engine produce?

Answer 1

Rotation - To drive the propeller
Power - rate of work
Torque - capacity to do work (leverage)

Question 2

Piston engine power formula.

Answer 2

Power = Torque x RPM

Question 3

Piston engine torque.

Answer 3

Turning force that allows the engine to apply a mechanical force to complete work.

Question 4

The force within torque.

Answer 4

Force = pressure x Area.
Area = Surface area of a piston within the engine. Fixed volume.
Pressure = movement of throttles.

Question 5

Conversion of the 4 energies.

Answer 5

Chemical energy -> heat energy -> pressure energy -> mechanical energy

Question 6

Principle of operation.

Answer 6

Mixture drawn = charge.
Charge is ignited by spark plug.
During combustion, hot air attempts to expand but is confined by the cylinder.
Therefore, increase in pressure which pushes cylinder down.
Piston connected to a crankshaft that converts linear motion into rotary motion.
Up and down motion = reciprocal movement.

Question 7

Top dead centre.

Answer 7

Point of a piston in which the head is farthest away from the crank shaft.
Point from which ignition system measurements are made and the firing timing is determined for all the pistons.

Question 8

Bottom dead centre.

Answer 8

The lowest point the piston reaches.

Question 9

Stroke.

Answer 9

Distance between the TDC and BDC.
There is a 180 degree crankshaft rotation between TDC and BDC.

Question 10

Valve spring.

Answer 10

Ensures correct amount of spring pressure is applied to prevent valve bounces.

Question 11

Valves.

Answer 11

Control the motion of a fluid or gas along a tube.

Question 12

Inlet valve.

Answer 12

Manages the timing of the mixture entering the engine.

Question 13

Exhaust valve.

Answer 13

Transports the burned mixture out.

Question 14

Spark plug.

Answer 14

Produces a spark to ignite the mixture.

Question 15

Push rod.

Answer 15

Connects camshaft to valve. Transfer the motion from the camshaft to open/close the valves at the right time.

Question 16

Cooling fins.

Answer 16

Absorbs heat from the block/head, then dissipate that heat outside, cooling the engine.

Question 17

Camshaft.

Answer 17

Dictates timing of valve openings. As the camshaft spins, the lobes open and close the intake valve and exhaust port.

Question 18

Crankcase.

Answer 18

Holds crucial engine parts and lubricates them with oil.

Question 19

Crankshaft.

Answer 19

Converts reciprocating motion into rotational motion. Driven by pistons via the connecting rod.

Question 20

Connecting Rod.

Answer 20

Connects piston to crankshaft.

Question 21

Piston.

Answer 21

Converts the energy created through the combustion process and pass it on to the engines crankshaft.

Question 22

Cylinder.

Answer 22

Chamber where fuel is combusted and power is generated.
Image in files, 2.

Question 23

The 4 stroke (otto) cycle.

Answer 23

Induction -> Compression -> Power -> Exhaust.

Images in files: 3-4-5-6

Question 24

Induction stroke.

Answer 24

Insert valve opens. Volume increases. Pressure reduces. Temperature reduces. Piston moves down towards BDC. 180 degree crankshaft rotation between TDC and BDC.

Question 25

Compression stroke.

Answer 25

Insert valve closed. Exhaust valve closed. Volume decreasing. Pressure increasing. Temperature increasing. Piston moving up towards TDC.
180 degree crankshaft rotation from BDC to TDC.

Question 26

Powerstroke.

Answer 26

Insert valve closed. Exhaust valve closed. Volume of cavity where piston was increasing. Pressure decreases. Temperature decreases.
180 degree crankshaft rotation between TDC and BDC.

Question 27

Exhaust stroke.

Answer 27

Inlet valve closed. Exhaust valve open. Volume decreases. Pressure increases. Temperature increases.
180 degree crankshaft rotation between BDC and TDC.

Question 28

Ineffective crank angle.

Answer 28

Small linear movement. Large angular rotation of crankshaft.
Linear movement is not efficiently converted into rotational movement.

Valve timing makes use of the ineffective crank angle to maximise the time to expel exhaust gases.

Question 29

Inlet valve lag.

Answer 29

Inlet valve open as compression stroke begins.
Relatively little movement of piston.

Question 30

Inlet valve lead.

Answer 30

Inlet valve opened early.
Exhaust valve still open.
Relatively little movement of piston.

Question 31

Valve timing.

Answer 31

Both valves closed at start of power stroke.
Most of work during power stroke done before BDC
-so the exhaust valve open before the end of power stroke(exhaust valve).
Longer period available to clear cylinder of waste gas, helped by residual pressure.
Exhaust valve remains open slightly after TDC (lag).
Remaining exhaust gases exit under own momentum.

Question 32

Optimised Cycle.

Answer 32

Image in files, 22

Question 33

Ignition timing.

Answer 33

Image in files, 10

Question 34

When does inlet valve open?

Answer 34

15 degrees before TDC.

Question 35

When does exhaust valve close?

Answer 35

10 degrees after TDC.
At end of exhaust of stroke.

Question 36

When does exhaust valve open?

Answer 36

End of power stroke.

Question 37

When does intake valve close?

Answer 37

Start of compression stroke.

Question 38

When is ignition?

Answer 38

30 degrees before TDC.

Question 39

When is overlap?

Answer 39

15 degrees before TDC to 10 degrees after TDC.
Inlet valve and exhaust valve both open.

Question 40

What is swept volume?

Answer 40

Length of stroke x piston crown area.

Question 41

What is compression ratio?

Answer 41

the cylinder volume with the piston at bottom dead centre to the cylinder volume at top dead centre.

Question 42

What speed does the camshaft of a piston engine rotate at?

Answer 42

Half the speed of the camshaft.

Question 43

Specific fuel consumption .

Answer 43

mass of fuel required to produce unit power for unit time