SI Engines Flashcards
What is octane number
octane number equal to the volumetric percentage of iso-octane in the fuel mixture… indicates a fuels resistance to knock
High octane = 100% -> high antiknock performance
Low octane = 0% -> very poor antiknock
Outline the fuel injector control for an SI engine
Throttle valve linked to accelerate regulates amount of air in
A sensor sends a signal to the ECU
ECU sends signal to electronic fuel injector
An electromagnet moves the plunger to open the valve within the injector, pressurized fuel forced through a nozzle creating a mist
Injector located in the manifold to create premixed airfuel mixture
Port Injection (PI)
Supply a proper ratio of fuel and air to the cylinder
1 injector per cylinder
Injector placed on the side of intake manifold near valve
Mixture passes through intake valve into cylinder
Multipoint fuel injection -> injector for each cylinder
Gasoline Direct Injection (GDI)
and the 2 types
A future tech for SI engines
Better control over injection, can optimise
Move towards lean comnbustion
Liquid fuel injection
- high pressure injection
- well atomised fuel
- higher emissions due to incomplete combustion
Direct injection of premixed airfuel
- lower pressure injection
- mixture formed outside cylinder by inject fuel into the air
- short duration needed for complete mixture formation
- low emission due to complete combustion
GDI Operating modes
Stratified mode
- overall lean eq ratio (better sfc)
- fuel rich near the spark but lean combustion elsewhere
- late injection used
- non-homogeneous fuel air mixture (lower power)
- injector located within the cylinder
Homogeneous stoich mode
- fuel injected early into suction stroke
- draws in air and fuel to mix
- used for med higher loads
- charge cooling from cylinder
Mode switching
- ideally want both modes
- complex control system required
GDI advantages
Improved sfc
- less pump loses
- high compression ratio due to charge cooling
- increased volumetric eff due to charge cooling
- better cooling due to fuel evaporation
No manifold film required
Precise airfuel ratio control
Reduced cold start issues
GDI disadv
Complex control system required
high rate of formation of injector deposit (hot spots)
increased electrical power needed
difficult to control stratified mode over operating range
three way catalyst cannot be utilised to full advantage
Ignition
Lower compression ratio = self-igntion temp is higher, so we want high rc (also means more power)
spark plug used to ignite fuelair mixture just before TDC
Valve timing diagram
check book
Combustion (SI) and phases
spark towards end of compression stroke (just before TDC)
flame propagates through premixed fuel
- forms smooth sphere
- turbulence and mixing cause it to wrinkle and grow
1 - combustion and propagation
2 - wrinkle formation due to moderate tubulence
3 - high turbulence phase
What is flame propagation controlled by
What are the flame speed factors
reaction rate -> pure chemical reaction
transposition rate -> physical movement of flame front
flame speed
- turbulence and mixing (high turbulence = faster)
- fuel air ratio (more fuel = faster)
- T, P and pressure ratio
- engine speed and size (flame speed scales with rpm)
Draw crank angle diagram and lable
A - spark and lag (about 20 CA deg before TDC)
B - flame propagation
C - combustion (max temp and pressure just after TDC)
Knock
auto ignition of the gas ahead of the main flame front
or surface ignition due to hot spots
- overheated valves, spark plug, hot deposits on piston head or cylinder wall
leads to
- general engine damge
- piston head corrosion
- gasket cracking
- loss in power and eff
Factor effecting knock
- compression ratio -> lower to reduce
- inlet temperature -> lower to reduce
- AFR -> make very rich to reduce
- Engine speed -> increase to reduce
ECU main tasks
Saftey
Fuel management strategy
provide torque according to driver input
engine/exhaust emission control (AFR/stoich)
SI engine fuel control loops
fuel injection forward loop
airfuel ratio feed back loop
control realises a suitable injection timing based on property of intake air, intake mass flow, manifold pressure or throttle plate angle/engine speed
the airfuel feedback control system anaylsises the content of the exhaust gas looking for unburnt fuel which indicates rich combustion… system compensates for this by making the injection less rich until stoich conditions are met
SI ignition control loops
Ingition angle control loops
knock feedback loop
controller computes a nominal ignition timing based on engine speed and load, aims to achieve max brake torque while avoiding knock and excessive pollution
feedback control system uses the knock detection system to adapt the ignition timing to a safe level while maintaining fuel efficiency
Methods to improve fuel economy
Variable valve actuation (VVA) - traditionally valves control by camshaft, this is slow and not optimal, electronic components can operate faster and reduce pumping loses
Variable Valve timing (VVT) - phase conversion of valve timing, optimum timing and better control, improves sfc and emissions
variable compression ratio (VCR) - control rods can change to change the compression ratio
- high rc at low speed to improve efficiency
- low rc as high speed to improve power
Miller atkinson alternative ICE - compression start centre slightly above BDC, inlet valve open for longer, essentially reduces the compression ratio
