SI Engines

Question 1

What is octane number

Answer 1

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

Question 2

Outline the fuel injector control for an SI engine

Answer 2

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

Question 3

Port Injection (PI)

Answer 3

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

Question 4

Gasoline Direct Injection (GDI)

and the 2 types

Answer 4

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

Question 5

GDI Operating modes

Answer 5

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

Question 6

GDI advantages

Answer 6

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

Question 7

GDI disadv

Answer 7

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

Question 8

Ignition

Answer 8

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

Question 9

Valve timing diagram

Answer 9

check book

Question 10

Combustion (SI) and phases

Answer 10

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

Question 11

What is flame propagation controlled by

What are the flame speed factors

Answer 11

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)

Question 12

Draw crank angle diagram and lable

Answer 12

A - spark and lag (about 20 CA deg before TDC)

B - flame propagation

C - combustion (max temp and pressure just after TDC)

Question 13

Knock

Answer 13

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

Question 14

ECU main tasks

Answer 14

Saftey

Fuel management strategy

provide torque according to driver input

engine/exhaust emission control (AFR/stoich)

Question 15

SI engine fuel control loops

Answer 15

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

Question 16

SI ignition control loops

Answer 16

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

Question 17

Methods to improve fuel economy

Answer 17

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