Post Midterm Flashcards
The Kp of a reaction depends on “_____” only?
Temperature
The “______” the Kp, the more complete the reaction
Larger
- As Kp approaches 0 - no reaction
- As Kp approaches ∞ - complete reaction
Affects of pressure on Kp?
Pressure does not affect Kp, however it does affect the equilibrium composition.
If P increases the ratio of partial pressures changes:
- Increases then Δv > 0
- Decreases then Δv < 0
- ΔP = 0 then Δv = 0
Effects of inert gases on Kp?
Kp is not affected by presence of inert gases. However, it affects the equilibirum composition
What happens to Kp when the stoichiometric coefficients are doubled?
Value of Kp is squared
When determining Kp, free electrons in the equilibrium constant can be treated as an ?
Ideal Gas
Exothermic or Endothermic?
- Breaking C-C
- Breaking C-H
- Isomerization
- Polymerization
- Breaking C-C (Endothermic)
- Breaking C-H (Endothermic)
- Isomerization (Exothermic)
- Polymerization (Exothermic)
The 3 Cracking Types
- Thermal Cracking
- Catalytic Cracking
- Hydro Cracking
Properties of Thermal Cracking (Temp, Pressure, O.N)
- 450-650°C
- 80-100atm
- O.N is approximately 85
How does temperature affect thermal cracking?
Effect of temperature is to break the bonds near the ends of the HC
How does pressure affect thermal cracking?
Effect of pressure is to break the bonds near the middle of the HC
How does time affect thermal cracking?
Time increases the amount of breakage
What is Catalytic Cracking and its approximate O.N?
Catalytic Cracking: Process is in the presence of various catalysts and is typically a high quality gasoline with an O.N of approximately 90
What is Hydro Cracking and its properties (pressure, O.N)?
Hydro Cracking: Process is in the presence of a catalyst and a high pressure hydrogen (30-40 atm) with an O.N of approximately 92
What is Reforming and its typical feed?
Otherwise known as Catalytic Reforming, is a process in the presence of a catalyst and hydrogen. The feed is low quality gasoline and some light kerosene.
What is the process called if the catalyst in reforming is Platinum? Approximate O.N?
Platforming and an O.N of approximately 95-98
What are the 12 fuel properties?
- Calorific Value
- Volatility
- Viscosity
- Specific Gravity
- Fire Point
- Flash Point
- Spontaneous Ignition Temp (S.I.T)
- Flame Velocity
- dp (delay period) or Ignition Delay
- Oiliness
- Impurities
- Octane Number
L.H.V of gasoline and diesel fuel?
- LHV)gas = 42 MJ/kg
- LHV)diesel = 42.5 MJ/kg
Affects on the mixture as volatility increases?
Better evaporation and mixing will be obtained
What is Vapour Lock?
When there is only vapour in the pump which results in no flow. This is especially disadvantagous to high volatile fuels.
What is Carborator Icing?
Due to a high rate of evaporation, a layer of ice will cover the device and result in a loss of engine control.
Issues with low volatility?
- Poor Mixing
- Cannot Cold Start
Issues with high volatility?
- Vapour Lock
- Carborator Icing
- Not good for hot starts
2 methods in measuring volatility?
- ASTM Method
- Vapour Pressure (Reid Method)
On an ASTM cruve, what do Ta and Tb indicate?
- Ta = Initial Temp or Bubble Temp
- Tb = End Temp or Dewpoint (First few droplets of condensation when cooling)
On an ASTM curve, what does it mean if Fuel 1’s curve is higher than Fuel 2’s?
- Fuel 1 has more volatile fraction than Fuel 2
- Fuel 1 has more heavy fraction than Fuel 2
Do we need a more volatile or less volatile gasoline in the winter compared to the summer?
More volatile
What happens to the volatility as you move down on a Temperature vs Volume % graph?
Moving down → more volatile
In terms of volatility, what are the 5 steps in running an engine on an ASTM curve?
- Cold Starting/Starting
- Warming Up
- Acceleration/Cruising
- Cruising
- Diluting Lube/Oil and deposits on the surface
What is Cruising?
Running at 80% of max power
Why can’t we use diesel fuel in a petrol engine?
We need a volatile fuel in a gasoline engine. Diesel is less volatile and is not a suitable mixture for combustion.
What does NPSH stand for?
Net Positive Suction Head
What is the Reid Test or otherwise known as RVP (Reid Vapour Pressure)?
- Common measure in the volatility of gasoline, oil, petroleum products except for LPG
- Absolute vapor pressure exerted by a liquid at 100°F (37.8 °C)
- A pressure gauge is used to determine the volatility of the fluid
- The dry air is 5-8 times larger than the fuel
Four methods in measuring viscosity?
- Rotary Viscometer
- Discharge Viscometer
- Bubble Movement
- Falling Ball Viscometer
What is Saybolt Universal Seconds (SUS)?
A measure of kinematic viscosity using time. It is the time that 60 cc of oil takes to flow (discharge method) through a calibrated tube at a controlled temperature, 38°C.
Three types of discharge viscometers and their differences?
- Saybolts (or Furol)
- Redwood
- Engler
Principle remains the same, however it differs by orifice dimensions and discharge volume.
What is the difference between Saybolts and Furol?
SUS is the unit for low viscosity fluids while FUS is for high viscosity
What is a Hydrometer?
Instrument that measures the specific gravity (relative density) of liquids
Higher API gravity indicates?
Lighter fuel
Define Fire Point
Temperature at which the fuel has evaporated enough to burn (by a spark) for at least 5 seconds
Define Flash Point
Lowest temperature at which vapours of a volatile material will ignite, when given an ignition source. The mixture continues to burn.
A low flash point indicates?
Highly flammable (hazardous)
What are the flash points for petrol and diesel fuel?
- Petrol
- T_flash ≃ 60°C (flammable)
- Dependent on composition
- Diesel
- T_flash > 60°C (inflammable)
What is Spontaneous Ignition Temperature (SIT)?
Otherwise known as the autoignition temperature, it is the lowest temperature at which combustion starts without an external source of ignition (spark, flame, etc)
What are the two types of flame velocity?
- Laminar Flame Velocity (S_L)
- Turbulent Flame Velocity (S_T)
5 factors that affect flame velocity?
- Temperature
- Pressure (density)
- Turbulence
- Chemical Formula of the Fuel
- F/A ratio
Impact of temperature and pressure on flame velocity?
Increase in temperature or pressure, increases flame velocity
What is a delay period?
Interval between the spark and the start of combustion
“dp” depends on what 3 factors?
- Temperature
- Pressure
- F/A Ratio
How do temperature and pressure impact dp?
Increase in P or T, lowers dp
At what fuel air ratio is dp lowest?
FA stoic
What is Oiliness?
A measure of tendency to adhere to the surface and causes better lubrication
True or False, is there less oiliness for long chain HCs?
False, for a long chain HC there is more oiliness
Does gasoline or diesel fuel have more oiliness?
Diesel has oiliness while gasoline is considered dry.
Why can’t we use petrol in a diesel engine?
We cannot use petrol in a diesel engine as it will not have enough lubrication and causes seizure
Examples of impurities in fuel
- Sulfur
- Gum
- Ash
What is knock?
Knock is abnormal combustion in IC engines or early combustion of the end charge
What is the mechanism of knock?
High compression as a result of expanding gases due to the flame. This leads to a higher temp causing an explosion before the flame hits.
What are 5 issues as a result of knock?
- Rough running of the engine
- High rate of heat release (hot spots)
- High mechanical and thermal stresses (failure)
- Fouling the surfaces by residues
- Drop in engine efficiency
How is the early spark and late spark figure illustrated in a graph from tdc to bdc?
Refer to Emma’s Notes Combined (page 60/85). The weird sound noises we always do with our hands.
What occurs at optimum sparktiming?
At optimum sparktiming the work loss will be minimum and the torque output will be maximum. Recall what it looks like on the graph.
7 factors affecting knock
FTPFARMEC
- Fuel Type
- Temperature
- Pressure
- Fuel Air ratio
- Mixture Turbulence
- Engine Size
- Compression Ratio
What are the two factors associated with temperature that affect knock?
- Higher flame velocity
- Shorter dp
- When temp increases, the dp is shorter
- Higher T increases knock
How does pressure affect knock?
Increase in P, increases knock