Gas exchange process in 2- and 4-stroke engines Flashcards
What is the main difference between air intake for Diesel and Otto cycle in 4-stroke engines?
Otto cycle mixes air and fuel as a direct injection while in diesel cycle you only regulate the fuel injection, never air intake
What can happen if you take in too much air compared to fuel for otto cycle
You risk the fuel not igniting
How to you regulate air intake in Otto cycle
Throttle valve- remember that this increeces pumping losses
What does the lobe do?
the lobe is mechanism used to push air vents. The higher and broader the lobe, the more air you take in.
what does the cam do?
the cam is the cylinder that rotates the loab. The cam cylinder is connected to the
What number of air intake valves in normal for each cylinder?
2 or 4
What are the advantages/disadvantages of four valve air intake
- Increased flow
-improved heat removal
-reduce mass of components (less steel) - More complex mechanisms
What is variable valve control?
Normally valve lift curves and valve timing is fixed. New technology makes room for optimized air intake by controlling/customizing intake pattern of the engines
What are the direct advantages of variable valve control?
- Imroves volumetric eff
- Improves in-cylinder flow
- Reduce work associated with gas exchange
- By variable valve control you can implement miller cycle for reduced combustion temperature (lets air intake be open longer then usual, resulting in longer compression stroke and hence increece thermal efficiency, but lower power output)
What is VVT?
??
What is Individual valve control
- Individual control of each valve for each cylinder
-Improves air flow at low speed/power (close one or two valves- improves in-cylinder air swirl)
-Sequential cylinder operation (shuts down fiering in one or more cylinders to improve valve eff of active cylinders)
Why does 2 stroke air intake from 4-stroke?
2-stroke engines need access air supplied, as they are not as effective at taking in air by them self, and removing the exhaust
What are the common methods of pressurizing air for intake in 2-stroke intake
-Crankcase compression
-Root blower
-Centrifugal compressor
What are the types of scavanging for 2-stroke engines?
-Cross flow
-Loop flow
-Uni flow- best for marine vessels
-Reverse flow
What is scavanging?
the process of clearing out exhaust gases from the combustion chamber and replacing them with fresh air-fuel mixture for the next cycle of combustion. This is crucial for maintaining the engine’s efficiency and performance.
What two types of in-cylinder flow motions do we have
swirl, squish and tumble
Describe swirl and how it is made
Swirl is the most important motion for diesel engines and is created by opening one intake-valve before another. Typical swirl number is 2 which indicated swirl rotates twice as fast as motor
What is squish?
Squish is an inward gas motion. Its created by having “sinks” on top of piston head. Squish have a minor effect compared to swirl.
What is tumble
As the piston reaches TDC, the squish motion
generates a secondary flow called tumble, where
the rotation occurs about a circumferential axis
near the outer edge of the cavity or piston bowl. The necessity of tumble motion
is to increase the turbulence
level which favours proper and
quick mixing of fresh charge
which leads to effective
combustion with reduced
emission
