Characteristics of combustion in CI-engines Flashcards
What are the advantages of CI-engines?
- Can use optimum compression ratio- the ability of diesel engines (CI engines) to operate with higher and more thermodynamically efficient compression ratios compared to spark-ignition (SI) engines like gasoline engines.
- Operates un-throttled: load is controlled by varying fuel injected per cycle
- Always operates lean: fuel/air equivalence ratio range 0.2 (idle) to 0.5 — 0.75 (full load)
- Due to points 1, 2, and 3 above, diesel engine is more energy efficient than SI engine
- Due to over-all lean combustion , diesel engine must be supercharged to obtain high
power density - Turbocharged diesel engine power 3.5 to 4.0 times that of normally aspirated engine
- No combustion limit on cylinder pressure – high thermal efficiency
- May be designed to burn Heavy Fuel Oil (HFO) and other “low” quality fuels— low fuel
cost
What is diffusion flame
Diffusion flame is the mixing of air and fuel, when it has been separated, so the opposite of pre-mixed. Guess its mentioned as it is the opposite of premixed flame.
Mention some characteristics for diffusion flames
- Yellowish light
-Soot is formed in the core of the flame - ## The equivalence ratio varies extremely depending on where you are in the flame
What can be done to fuel spray in CI engine to help air-fuel mixing when injecting?
- Move the air around
- Use high pressure fuel injection (spray break up is increased)
How do we achieve in-cylinder air motion in CI-engines?
swirl and squish
What is the main tasks of fuel injection?
-Introduce fuel into the combustion chamber
-Distribute the fuel throughout the chamber
What is the prime difference between new and old injection technology?
New motors can do multiple injections in the same compression period.
What is the requirements of fuel injections?
- Create high fuel pressure
- Precise and repeatable fuel delivery
-Possibility to vary the start of injections - Injections of small fuel quantities
What is indirect injection (IDI)?
Has a pre-chamber and swirl chamber. This allows for higher engine speed and lower injection speed and lower PM emissions, but is less efficient
What is direct injection (DI)
Fuel is injected directly in to chamber- Most used in cars
Describe the physical factors that prevent injected fuel to ignite instantaneously.
Physical factors:
- Time for the fuel spray to leave the injectors and atomize.
- Entrainment of hot air into the spray plume
- Vaporization of the fuel droplets caused by the entrained hot air
- mixing of fuel vapor with air
What are the two main components of injectos?
pump element and spray tip
what is the main difference between combustion in CI and SI engines?
In CI combustion flame occurs everywhere all at once, while for SI engines the flame starts one place and moves in a flame front.
What is the definition of ignition delay?
Ignition delay is defined as the time (or crank angle interval) from when the fuel injection
starts to the onset of combustion.
How to achieve high fuel spray atomization?
high fuel pressure, small injector hole diameter, optimum fuel
viscosity, high cylinder pressure (large divergence angle)
What is cetane number?
Defines the ignition quality of a fuel by a number ranging from 1 to 100 where cetane is 100. Low cetane number means high ignition delay.
what is autoignition?
The spontaneous ignition of a fuel and oxidizer mixture in the absence of any external
ignition source
what is fuel dissociation?
a general process in which fuel molecules separate or split into other compounds. Good word for describing what happends when fuel during compression and combustion.
What is a cooperative fuel research engine?
Standardized motor to check combustion delay. Used to find cetane number
what machine is used to measure auto-ignition?
Rapid compression machine
What are the four stages of combustion?
ignition delay period, premix combustion phase (the spice before TDC that occurs becaure the outer layer of well mixed fuel and air from the injection catch fire), mixing-controlled phase, late combustion phase
What are the three basic stages of combustion?
- Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the
compression stroke. - The liquid fuel atomizes into small drops and penetrates into the combustion chamber.
- The fuel vaporizes and mixes with the high-temperature high-pressure air
