Combustion Flashcards
State three primary objectives of the combustion process
primary objectives:
- complete combustion
- ideal combustion process (efficiency)
- Avoidance of to high cylinder pressure peaks (mech. stress)
- Avoidance of to loud combustion noise
State four properties of SI-and CI-engines respectively
Spark ignition SI:
- premixed flame
- stoichiometric (lambda=1), homogenous mixture
- Flame propagation starts at single point (spark plug)
Compression ignition:
- diffiusion flame
- flame colour shiny yellow
- inhomogenous mixture(heterogenous)
- stochastic start of combastion, not locally defined
Name and explain the five sub-processes of the conventional SI-engine
5 sub-processes of conventional SI-engines
- Time of ignition
- Initialisation of flame front
- Transition from laminar to fully turbulent flame propagation
- main combustion with turbulent flame prapagation
- extinguish phase (flame termination)
Briefly explain the influence of the AFR on the behavior of the combustion process of an SI-engine
Fuels F25!
Briefly explain the influence of the charge motion on the behavior of the combustion process of an SI-engine
charge motion has a big influence on the flame speed
Wf=Wlb+Wt
laminar burning rate-> depends on fuel
transport velocity->inflamable by charge motion
Briefly explain the influence of the positioning of the spark plug on the behavior of the combustion process of an SI-engine
spark plug in the middle:
- short distance->shorter combustion time
- combustion of fuel faster-> retarding ignition point possible
- steep rate of heat release, high efficiency
Briefly explain the influence of the ignition timing on the behavior of the combustion process of an SI-engine
iginition timing:
- ignition delay-> cycle by cycle variations (15 bar)
- by advancing the TOI the peak of the pressure curve is adjusted near to TDC
- it can be found, that the pressure peak should be located at approx. 8° ca
Explain the term cycle-to-cycle variations
cycle by cycle variations:
- even though compress. ratio, ignition point and equivalence ratio are kept constant, the pressure peak varies between cycles about 15 bar
- Combustion starts local at spark plug
- Influence factors on combustion: temp., pressure, equivalence ratio, charge motion
- > cyclic variations in the combustion process, especially at part load
- variation of cylinder pressure patterns around TDC
- small volume change
- > limited influence on delivered work
Explain the effects of abnormal combustion (Pre-ignition and Knocking)
Pre-ignition(combustion F33f):
- air/fuel mix ignites before spark plug fires
- initiated by an ignition source, like hot spots, to hot spark plug, carbonaceous deposits
Engine knocking(F35f):
- consequence of an uncontrolled combustion of the air/fuel mix
- local pocket ignites before flame zone reaches them-> collision of flame fronts
Explain the different operation modes of an SI-engine with direct injection (DI)
operating modes SI with DI
- homogenous operation: air fills the chamber, fuel injection during intake stroke
- stratified charge operation: fuel injection during compression stroke
What are the requirements for self-ignition in a CI-engine?
requirements for self-ignition (CI):
- air in comb. cham. has at least be heated up to ignition temp. of diesel fuel
- ignition temp. of diesel fuel about 220°
- for fast ignition and good combust., air is heated up over 500°
- > compression ratio of CI engines 18-20
Briefly explain the processes occurring in the injection beam in a CI-engine
primary spray break up: - growth of surface waves due to airodynamic forces - turbulance - relaxation of the velocity profile - cavitation air diffuses into fuel jet
What is the influence of the charge-motion in a CI-engine?
- swirl transports flames through the comb. cham.
- is the swirl to strong-> overlapping with fuel jet-> incomplete combustion
Distinguish indirect and direct injection in a CI-engine
DI - short ignition lag - slow rise in pressure - low formation of particular matter - well suited for high engine speeds - 10% higher fuel consump.( heat losses , overflow losses) DI - longer igniton lag - fast rise in pressure - more formation of part. matter - engine speed limited by combustion time - higher NOx emissions - lower fuel consump. -> disadvanteges of DI can be reduces by modern multiple ijection processes
What is the aim of new combustion processes? Name and explain one alternative process
Aim:
combining advantages of CI-engines (high efficiency at part load) and SI-engines (high power density, smooth runnung) to one combustion process
HCCI homogenous charge compression ignition
- compression of A/F mix-> simultanous self-ignition of whole cylinder charge
- fast combustion process
