Piston Engine Fuel

Question 1

AVGAS burns with a flame rate of about …

Answer 1

60-80 ft per second

Question 2

What is detonation?

Answer 2

rapid and spontaneous combustion of part of the fuel mixture after the burning has begun

Question 3

What effects does detonation have on a running engine?

(3 things)

Answer 3

reduced power, increased CHT, engine runs rough

Question 4

Four main causes of detonation?

Answer 4

fuel with poor anti-knock qualities

excessively weak mixture

anything that raises temp or pressure of mixture before it is burned (low rpm with high power setting)

high engine temp

Question 5

What is pre-ignition?

what causes it? (2 primary reasons)

Answer 5

Ignition of the mixture before the spark plug has fired

caused by excessively weak mixture and local hot spots on cylinder eg carbon deposits

Question 6

What happens to pre-ignition with increased rpm?

Answer 6

gets worse

(think about hot spots getting hotter)

Question 7

What happens to detonation with increased rpm?

Answer 7

gets better

Question 8

What does the octane rating of fuel indicate?

Answer 8

resistance to knocking / detonation

Question 9

Avgas is normally made up of two octane rating numbers, what does each one indicate?

Answer 9

The anti-knock qualities at weak mixture and rich mixture respectively

Question 10

Most effective additive for controlling detonation is?

Answer 10

tetra-ethyl lead

Question 11

Ratio of air to fuel that ensures complete combustion is about?

Answer 11

15:1 by mass

Question 12

Why can’t a chemically correct mixture be used in practice?

Answer 12

increased burning temp increases risk of detonation

Question 13

A lean mixture will result in an increased CHT but cooler EGT because?

Answer 13

The burn is slower, giving more time for the heat to transfer to the engine body but the weak mixture results in cooler gas burn temp