CI Engines Flashcards
What is cetane number
A measure of ignition delay when diesel is compressed
inverse of octane number
CN = 100 for normal cetane, short ignition delay
CI injection
direct injection into the cylinder
spray atmoisation, vaporisation, and mixing
autoignition during compression
mixing controlled combustion
Name some spray parameters
injection pressure (high for good penetration and atomisation as high velo)
injection profile
spray patterns (geometry of the injector)
droplet size and distribution (smaller is good)
evaporation time
what is Common rail injection
all injectors supplied by a common rail
fuel injectors electornically activated
ECU executes precise timing and amounts
What is result of increased injectino pressure
higher velocity and penetration
high penetration = better mixing and air utilisation
reduces emissions with higher performance due to complete combustion
decreases spray angle
Injection rate shaping types
Square = high pressure and large penetration, high heat relase
Ramp - med penetration, heat release and NOx levels
Boot - low penetration, heat relase and NOx levels
Split and multipoint injection
Pilot injection -> at lower temp and pressure to help reduce engine noise and NOx
Main injection -> square or boot shape rate helps to reduce NOx
Post injection -> coupled post injection at high pressure help to reduce soot formation, aim to reduce exhaust gas temp
Piston bowl designs
Want the piston to aid in mixing, want homogenous mixing for complete combustion.. induce swirl and turbulence to acheive good mixing
Hemispeherical
Torodial -> best reduction in emissions and highest brake thermal eff
Shallow
Main design issues
Compression ratio needs to be high to ensure high cylinder pressure to enable unaided cold start… cold start depends on
- compression ratio (high is good)
- ambient temp (high is good)
- fuel injection timing and amount
- fuel air mixing (homogenous good)
- heat transfer loses (minimise)
Good mixing results in homogenous mixture leading to better air utilisation and sfc
optimal swirl intensity tends to decrease.. high injection pressure favoured at low to medium speed
low surface to volume ratio preffered to minimise heat loss
metal temp of combustion bowl needs to be optimised for best airflow turbulence and thermal fatigue life
combustion noise can be reduced by tubrocharging
What are the types of CI systems
Direct injections -> single chamber direct injection
Indirect injection (IDI) -> chamber divede into 2 regions
- fuel injected into prechamber and mixed (swirl/turbulence)
- connected to main chamber via a nozzle
- rich airfuel mixture in prechamber but overal lean in main chamber
Draw valve timing diagram
what is SOI, SOC and ignition delay
SOI = start of ignition
SOC = start of combustion
ingiton delay = time between the 2
What does ignition delay depend on
atomisation of the fuel -> high pressure low viscosity is good
vaporisation of fuel droplets -> size and distribution of particles (smaller are better)
mixing of fuel vapour with air -> high swirl and tubrulence to create a homogenous mixture
chemical process
Draw combustion diagram and name the phases
1 - igntion delay to auto-ignition temp of fuel used
2 - premixed burning due to the formation of premixed fuel in the ignition delay phase (high heat released)
3 - mixed control combustion -> controlled by rate at which mixture becomes available
4 - late burning -> soot and emission form at lower temp
Name some CI abnormal combustion processes
diesel knock -> due to late autoigntion and rapid combustion due to lots of premixed airfuel
misfiring at cold start -> charge is unable to ignite due to low temp and pressure
incomplete combustion -> results from poor mixing or if flame is quenched due to low temp in cylinder
CI combustion control
air available for all loads is the same
use EGR to reduce emissions
use turbo/supercharger to increase volumetric eff
direct injection used
complex control system compared to SI
main objective is minimise fuel consumption and ocmply with exhaust emissions and noise regulations
