Four stroke cycle and ignition

Question 1

what is the four-stroke cycle?

Answer 1

cycle repeated every four complete strokes of the piston. which relates to 2 revolutions of the crankshaft

Question 2

what are the four-strokes?

Answer 2

induction compression power and exhaust in that order

Question 3

what happens in the induction stroke?

Answer 3

mixture is drawn in and the piston travels downward form top dead centre to bottom dead centre (lowest and highest point of piston) with inlet valve open and outlet closed.

Question 4

is cylinder pressure initially less than atmospheric?

Answer 4

yes

Question 5

what happens in the compression stroke?

Answer 5

piston moves from bottom dead centre to top dead centre with both valves closed. as the piston moves up toward TDC fuel air mixture is compressed increasing both pressure and temperature.

just before the end, the fuel-air mixture is ignited by a spark plug which rapidly increases cylinder pressure still further.

Question 6

what happens in the power stroke?

Answer 6

increase of pressure and heat due to ignition is now utilised when the piston moves from TDC to BDC with both valves still closed.

produces maximum pressure at beginning of the stroke and is the only stroke that provides useful work to turn propellor.

Question 7

what happens in the exhaust stroke?

Answer 7

the piston moves from BDC to TDC with exhaust valve open. burnt gas is forced out cylinder through the exhaust valve by the ascending piston.

the pressure-volume line then slopes down to the left retuning to the beginning of induction stroke.

Question 8

when working under ideal conditions combustion is said to take place at what?

Answer 8

at constant volume, therefore a piston engine cycle is often referred a the constant volume cycle.

this is repeated continuously to obtain a steady power output and more cycles completed per minute the greater the power.

Question 9

the valve timing diagram is usually represented how?

Answer 9

by a spiral diagram to show one cycle. the point at which valve opens or close is referenced to crankshaft position of TDC and BDC.

Question 10

when does valve open or close?

Answer 10

never at top or bottom but a little before or after TDC or BDC.

Question 11

when valve opens or closes at crank angle before or after TDC or BDC this is known as what?

Answer 11

valve lead or valve lag

Question 12

what is the purpose of valve lag or lead?

Answer 12

to aid cylinder induction and scavenging. having inlet valve lag aids filling the cylinder with charge thus increasing volumetric efficiency.

Question 13

exhaust valve lead aids with what?

Answer 13

aids with pushing out the burnt gases from cylinder. it also benefits of removing any resistance by air pressure.

Question 14

inlet valve lead and exhaust valve lag are what?

Answer 14

coincident with one another this is called valve overlap. overlap helps both induction and scavenging

Question 15

when exhaust gases leave the cylinder what is created?

Answer 15

a depression in the combustion chamber.

this induces the new charge which in turn reduces the depression allowing more exhaust gases to exit the cylinder.

Question 16

normal ignition occurs when?

Answer 16

just before end of compression stroke usually 25 degrees before TDC.

Question 17

how is spark timing determined?

Answer 17

by the need to reach maximum combustion pressure as near to the beginning of the power stroke as possible.

Question 18

how fast do flames travel?

Answer 18

flame travel speed is approx 60ft per second when under compression therefore peak pressure isn’t reached until approx 10 degrees after TDC.

Question 19

when spark takes place before TDC the ignition is said to be what and if after?

Answer 19

advanced if it take place before and retarded if after.

Question 20

ideally, the ignition provision should be carried when?

Answer 20

with changes in RPM.

Light aircraft range is small so ignition position is usually fixed except during start when ignition is retarded to approx 15 degrees after TDC

Question 21

how do diesel engines differ to AVGAS engines?

Answer 21

they use compression ignition rather than spark.

in diesel, only air is admitted to the cylinder during induction stroke which is then compressed to a much greater compression ratio than AVGAS causing compressed air to reach as high as 550 degrees.

Question 22

just prior to TDC what is injected into diesel engines?

Answer 22

fuel is injected into either a pre-chamber attached to the cylinder or directly into the cylinder head as a fine spray.

Question 23

fuel for diesel engines is what?

Answer 23

either diesel or jet A-1 as diesel engines can burn both

Question 24

higher temperature air causes what in diesel engines?

Answer 24

atomizes and ignites fuel causing temperature and pressure in the cylinder to increase rapidly and force piston down for the power stroke.

Question 25

RPM for fixed-pitch aircraft is what?

Answer 25

the number of revolutions a crankshaft makes in one minute.

Question 26

what is valve lag?

Answer 26

late closing of the inlet valve.

Question 27

when does valve lag happen?

Answer 27

may expect it to close when the piston reaches BDC as its finished drawing fuel-air charge into cylinder but since it has mass there’ll be a slight delay in it entering the cylinder.

Question 28

When does valve lead occur?

Answer 28

the exhaust valve opens a little bit before the piston starts moving upwards.