Exercise 1

Question 1

Advantages of the 2-stroke cycle?

Answer 1

□ Smoother cycle with a power stroke from every cylinder on every revolution
□ Higher power density- more power from smaller engine
□ Can be designed with no mechanical valves- simple design for small engines
□ Can be designed to run at lower speeds- match propellor speed, no gearbox

Question 2

Advantages of the 4-stroke cycle

Answer 2

□ Can operate without intake pressure boost
□ Cleaner operation with less exhaust pollution
□ Can use crankcase for oil reservoir

Question 3

Why must a 2-stroke engine
always have an intake pressure boost?

Answer 3

Two-stroke engines have no separate intake or exhaust stroke. The exhaust gasses
must therefore be pushed out of cylinder (scavenged) by the intake air

Question 4

Why is the ideal diesel cycle always lower then the ideal otto cycle

Answer 4

• In otto cycle, heat is added at constant volume, whitch leads to greater temperature rise.
• In the diesel cycle, heat is added at a constant pressure whitch result in lower peak temperature
  -Diesel have a longer combustion duration
• As efficency is temperature dependent, the diesel cycle has a lower efficency at the same compression ratio

Question 5

Why is diesel cycle in reality more efficient then otto cycle

Answer 5

• Because diesel engines operate at higher compression ratios
• Diesel engines run at leaner air-fuel ratios, reducing heat loss at walls
• Diesel has higher fuel density

Question 7

What is volumetric efficiency?

Answer 7

the ratio of the amount of air taken into the cylinder to the total volume of the cylinder

Question 8

What are advantages of an over square engine (bore/stroke ratio is greater than 1)?

Answer 8

□ For the same cylinder displacement volume an over square engine will have
a shorter stroke length. This results in a lower average piston speed and
lower friction losses
□ Cylinder lengths will be slightly shorter
□ The over square design allows for more and larger valves in the head of
the cylinder

Question 9

What are advantages of under square engine (bore/stroke ratio is smaller than 1)?

Answer 9

□ An under square engine will have smaller diameter cylinders, resulting
in shorter flame travel distance (time)
□ Combustion chamber surface area will be smaller resulting in less heat
losses per cycle