Carburation

Question 1

2 types of carb

Answer 1

Fuel injection
Carburettor

Question 2

CCM (chemically correct mixture)

Answer 2

1:15 (fuel:oxygen)

Question 3

Fuel unit measure

Answer 3

Weight (consistent)

Question 4

Workable mixture ratios

Answer 4

1:9 x 1:18
Max power 1:12
Best economy 1:16

Question 5

Pre ignition

Answer 5

Combustion before timing due to…
Prolonged exposure to detonation
Super heated carbon deposits forming
Excessively high CHTs

Leads to…
Reduction in power or serious engine damage

Question 6

Detonation

Answer 6

After spark plug ignition
Caused by…
Auto ignition of FA mix ahead of flame wall
Usually lean (hot)
Or too high pressure

Results…
Chemical energy 5-25 times normal combustion
Violent rapidly oscillating pressure fluctuations which produce knock
Sever engine damage or engine failure

Question 7

Carburettor main components

Answer 7

Throat
Butterfly/throttle valve
Metering jet
Fuel nozzle
Float and float chamber

Question 8

Throat use

Answer 8

Venturi tube
Pressure change by Venturi controlled by butterfly valve
Pressure change main effect

Question 9

Fuel nozzle and metering jet

Answer 9

Fuel nozzle
Low pressure middle of Venturi
Pressure difference draws fuel into throat

Metering jet
Delivers correct amount of fuel (by weight) into air before intake

Question 10

Mixture control types and uses (to adjust for pressure differences with altitude)

Answer 10

Back suction
Varies pressure in float chamber
Line exposes float to Venturi equalising pressure difference
Mixture valve in line vents float chamber to atmosphere
Adjusting mixture valve adjusts pressure difference

Needle type
Jet restriction type
Position of needle controls amount of fuel into metering jet
Directly controlled by pilot

Automatic mixture control
Can be back suction or needle type
AMC controlled by expansion and contraction of aneroid capsule
In climb pressure decreases and capsule expands restricting flow
Descend capsule deflates and fuel flow increase

Question 11

Effect of over rich vs over lean

Answer 11

Over rich…
Loss of power, rough running, spark plug fowling, lead deposits being developed on valve and piston head (detonation and pre ignition)

Over lean…
Too hot (detonation)

Question 12

Carburettor icing types

Answer 12

Refrigeration…
Fuel vaporisation requires latent heat from air
Can freeze Vapor in air
Causes icing end of Venturi/ throttle valve
Can occur in 25°c when RH over 50%

Throttle ice
Pressure drops in Venturi causing temp to drop
Low power setting and high acceleration airflow more susceptible
Adds to refrigeration ice
Mixture temps between 0 and -10°c

Impact ice:
Super cooled water droplets on forward facing surfaces
Can block air intake into carb
Usually temps 0° or less flying through visible moisture

Question 13

Carb icing symptoms

Answer 13

Loss of power and poor performance
Rough running (ice enters cylinders)

Question 14

Carb icing remedy

Answer 14

Carb heat (ducting of air around engine to increase temp)

Question 15

Fuel injection

Answer 15

Direct…
Pressurised fuel to cylinder

Indirect…
Fuel to inlet manifold

Question 16

Continuous flow system (indirect)

Answer 16

Continuous injection into intake manifold

Main components…

Injection pump
Engine driven
Maintains positive pressure on fuel delivered to metering unit

FCU
filters fuels then passes it through mixture control valve to the metering valve

Manifold valve
Evenly distributes fuel to all nozzles

Injector nozzles
Inject fuel into inlet ports of each cylinder
Fuel amount linked to rpm mixture and throttle settings

Question 17

Direct injection

Answer 17

Modern
Directly injects fuel to cylinder
Controlled by throttle and mixture

Main components

FCU
Delivers correct amount of fuel based on amount of air through throttle unit

Injection pump
Divides fuel into equal proportions for cylinders
Timed to force fuel at high pressure through discharge nozzle

Injection nozzle
Spring loaded
Only opens when fuel under pressure is applied

Question 18

Fuel injection pros and cons

Answer 18

Pros
No refrigeration ice
Uniform FA delivery
Less maintenance
Better throttle response
Increased engine efficiency

Cons
Vapour lock (prevented with use of aux fuel pump purging lines)
Fuel contamination (thin fuel lines)
Carbon buildup on intake valves and no fuel flow to remove it