IC Engines Flashcards
TDC
Top Dead Centre. Highest Point of engine cylinder
BDC
Bottom Dead Centre. Point at the bottom of the cylinder motion where it stops and changes direction
Four stroke Cycle stages
- Inlet 2. Compression 3. Expansion (Power) 4. Exhaust
Engine Torque Delivery
Highly irregular. Maximum Torque in the Power (Expansion) Stroke. ICE use multiple cylinders to smooth the engine torques.
Flywheel use
Large disk at the end of the crankshaft to add inertia to the shaft to smooth the torque
Combustion Equation
CxHy + O2 + (79/21) N2 > CO2 + H2O + N2 in a perfect combustion - in reality also get Soot (Carbon particles); NOx (Nitrous oxides; pollutant and greenhouse gas); HC (Unburnt fuel) and; CO (Carbon Monoxide)
Pre-mixing the air with the fuel
Flame burns blue, no orange or soot formation
Air/Fuel Ratio (AFR) Equation
ma / mf = .ma / .mf ( dot indicates mass flow
Stoichiometric
Correct AFR to perfectly convert fuel to CO2
lambda equation
lambda = AFRactual / AFRstoich = 1/phi
SI (petrol) engines lambda value
Usually run at lambda = 1 (Stoichiometric) at all loads
AFR = 14.5 - 14.7
CI (diesel) engines AFR
Usually run lean on fuel but move towards lambda = 1 at full load
AFR = 15-70
AFR for best power output
approx 12-13. fuel rich
AFR for emmisions targets
14.5-14.7. SI runs here as 3 way catalyst can brake up all of the emmisions
Peak NOx AFR
Little over 16. This is due peak temperatures at slightly lean fuel mix
SI combustion
Fireball grows from spark ignition. The outer surface forms some NOx but less soot
CI combustion
Fuel auto ignites after injection and sufficient time to mix with air. Local ignition occurs at the ends of each injected spray. Irregular fuel mixing leads to high NOx and Soot. Less unburned HC and CO
HCCI combustion
Fuel is premixed with hotter gases. Multiple ignition sites after compression but prior burned gases lead to a cooler combustion so zero NOx and zero soot. Similar HC and CO exhaust to gasoline
SI
Spark Ignition - Petrol
CI
Compression Ignition - Diesel
HCCI
Homogeneous Charge Compression Ignition - Petrol
Benefits of Diesel
Much greater Fuel Economy and reduced CO2 emissions. As well as reduced HC and CO emissions
Downsides of Diesel
High Soot and particulates - increased cost. Increased NOx emmisions
HCCI benefits
Much lower NOx emissions and reduced CO2 and improved fuel economy
HCCI downsides
Greater cost and reduced combustion control
Diesel Fuel economy reasoning
Higher compression ratio and therefore greater expansion ration which means more work is obtained during the power stroke
Do not use a throttle so there is no pumping losses under partial load conditions
Use high amounts of EGR
EGR
Exhaust Gas Recirculation. Is used to reduce peak combustion temperatures and reduce NOx emissions
Road transport CO2 emmisions by transport type
32% petrol cars
30% diesel cars
18% HGV
16% vans
3% buses
1% other
22% of UK total CO2 emissions
Crude Oil
Main products of Crude oil distillation are gasoline and diesel.
Diesel market share in the EU
Between 2005 and 2015 Diesel cars made up over 50% of new passenger cars in the EU. Now on the decline
Diesel Popularity
Excelent Fuel Economy - linked to tax incentives
Fun to drive due to low speed torque
Diesel Combustion Dirtiness
Main combustion occurs near the TDC and is contained within the piston bowl
Fuel injector typically has 6 or more injector orifices so multiple flame zones
Diesel after emissions treatment
- Oxidation catalyst
- Passive Particulate Filter to oxidise particulates
- NO2 decomposition catalyst
NOx trap has been replaced with a selective catalytic reduction due to VW scandal in 2015
2017 Ban
New sales of conventional ICE vehicles banned from 2040. Hybrids currently not included
