Gasexchange

Question 1

State three requirements for fresh air supply and exhaust gas removal respectively

Answer 1

fresh charge:
- low effort
- fast and complete gas exchange
- supply of high amount of fresh gas
exhaust gas:
- low effort 
- low noise
- complete gas exchange

Question 2

State the formulae and explain three characteristic numbers for the gas exchange process

Answer 2

Gasexchange F8

• air efficiency (Liefergrad)
• volumetric efficiency (Luftaufwand)
• Trapping efficiency (Fanggrad)

Question 3

Briefly explain the gas exchange process of a two stroke engine.

Answer 3

piston moves upwards:
- exhaust gas is pushed out
- fresh charge is compressed
- in the crank case fresh charge is sucked in due to the low pressure
ignition, piston moves downwards:
- exhaust port reopens as piston moves by-> high pressure presses out some exhaust gas
- piston closes intake port and partiall compresses fresh charge in crank case
- near BDC transfer port opens-> partly compressed fresh charge transfers in comb. chamb.
- this pushes out the remaining exhaust gas (scavenging)

Question 4

Draw a p,V-diagram of the gas exchange process of an arbitrary two stroke engine

Answer 4

gasexchange F13

Question 5

Explain the term scavenging and give examples

Answer 5

fresh charge presses out remaining exhaust gas
reverse scavenging: 
- cross scavenging with deflector piston
- cross scavenging
uniflow scavenging:
- uniflow sc. with horizontal opposed pistons
- uniflow sc. with valves
gasexchange F16/17

Question 6

Name five requirements for control components in four stroke engines

Answer 6

• long open period at high valve lifts
• quick opening and closing operation
• low flow resistance
• effective sealing in closed condition
• mech. durability

Question 7

Explain why the theoretical optimum of rectangular valve lift curves is not appropriate for real applications

Answer 7

• valves need some time to open and close-> no rectangular valve lift curves

Question 8

Explain, using a p,V-diagram, how gas exchange losses affect the break power output.

Answer 8

• induction and exhaust stroke consume some amount of the break power due to gas exchange losses
  Gasexchange F31

Question 9

Explain the term valve overlap. Briefly explain its implificationon the gas exchange process

Answer 9

valve overlap
- valve timing has great influence on cylinder charge, fuel consumption, mean effective pressure(power), emissions, A/F mix-> conventional mech. valve trains are always a compromise
For good results:
- exhaust valves open BBDC and close ATDC
- inlet valves open BTDC and close ABDC
-> valve overlap
- for fast running engines valve overlap is necassary for gas exchange-> short time for g. ex. at high speeds

Question 10

State advantages and disadvantages of multi-valve technologies

Answer 10

advantages:
- lower accelerated masses-> faster valve opening possible
- greater overall crosssectional area-> lower flow losses
- spark plug in middle of comb. chamb.-> improved comb.
- lower valve temps.
disadvantages:
- complex valve train and cylinder head
- increased friction losses

Question 11

Draw a diagram of torque and power over the engine’s speed showing the influence of the use of 2 and 4 valves per cylinder respectively

Answer 11

gasexchange F41

Question 12

Name examples to optimize the gas exchange

Answer 12

Optimazation

• theory of pressure waves
• varable intake manifold unit (shrt pipe-> high speeds)
• variable valve timing