A large We number indicates that the deforming aerodynamic forces are much larger than the reforming surface tension forces.
What do the terms dng and sg represent in the log-normal distribution?
dng is the number geometric mean dropsize and sg is the geometric standard deviation.
dng is the mean diameter of a droplet and sg is the standard deviation.
The terms have the same significance as the mean and standard deviation of a normal distribution function.
dng is the mean number of droplets in a sample and sg is the standard deviation.
What is the acronym SMD stand for?
Sauter Mean Diameter
Specimen Mean Diameter
Sample Mean Diameter
SMD describes the "average" diameter of an ensemble of droplets by weighting the average by both the surface area and the volume.
Why is SMD used to characterize droplet size?
Evaporation is a surface behavior.
The local AFR is volume dependent.
It explains the tendency of a spray to evaporate and form a particular AFR.
Reflects the spread of fuel through mass weighting.
It helps describe what happens to the droplets in theory.
It helps describes what happens to the droplets in reality.
What is the We number?
It is a dimensionless number describing the ratio of deforming aerodynamic forces to the reforming surface tension forces.
It is a dimensionless number describing the ratio of activation energy to thermal energy.
It is a dimensionless number describing the ratio of inertial fluid forces to viscous fluid forces.
The log normal distribution for droplet size is given by this equation.
What is one key advantage of the log-normal distribution?
It can be written such that there is a simple relationship between the surface and volume distributions.
It can be written such that there is a simple relationship between the weight and area distributions.
It can be written such that there is a simple relationship between the volume and diameter distributions.
What are some dropsizing techniques?
Malvern Particle Sizer (MPS)
Phase Doppler Velocimetry
Laser Sheet Dropsizing
Dynamic Light Scattering
Dilute Solution Viscometry
The log normal distribution is used because it has been found experimentally to be a good fit for sprays.
What are some of the disadvantages of using PDV as a dropsizing technique?
Line of sight
Low spatial resolution
Requires a specific off axis forward scatter geometry
I want to measure a dense spray. Which drop-sizing technique should I use?
A "hollow core" or "air core" is formed by the swirl of the liquid.The tangential velocity of the outgoing liquid is given by two equations, relating the mass of liquid, the port area, and the chamber radius. At a radial position of r=0, the tangential velocity is equal to infinity. As this is impossible, a low pressure or vapor core is observed.
The process that govern CO formation are .....
.....controlled by the mass of fuel burnt.
..... controlled through AFR.
In the premixed flame, the CO concentration.....
...increases rapidly in the flame zone to a super equilibrium maximum.
....decreases slowly in the flame zone to a near zero state.
The Zeldovich mechanism is the mechanism that describes the formation of NO by breaking down O2 to O atoms and N2 to N atoms. The subsequent reactions produce NO.
What are some mechanisms of HC formation?
Incomplete combustion or a misfire in a fraction of the engine cycles.
Lean AFR resulting in knocking.
Trapping of unburnt mixture in crevices formed by the pistons and ring packs.
Burning a nitrogen rich fuel.
Flame quenching at the combustion wall.
Combustion behavior is substantially controlled by the AFR. What is this?
It is the mass ratio of air to fuel present in a combustion process.
It is the ratio of actual air to the theoretical air required for the complete combustion of fuel.
It is the ratio of the number of moles of fuel and air to the number of moles of fuel and air in a stoichiometric mixture.
How is No emissions mainly formed?
It is formed by the reaction of nitrogen and oxygen present in the combustion air.
It is formed by the reaction of nitrogen and fuel present during the combustion.
What are the pollutant emissions produced by a conventional gasoline engine?
Carbon Dioxide (CO2)
Carbon Monoxide (CO)
Nitrogen Oxide (NOx)
Sulphur Dioxide (SO2)
The reactions to produce NO are very sensitive to temperature because the production of O atoms is essentially exponential with temperature.
How can the production of NOx be controlled or reduced?
Lean burn operation for AFR 22:1
Excess air and exhaust gas recirculation
Increasing or decreasing temperature of combustion
What are some of the requirements for using catalytic abatement to reduce NOx?
It needs an AFR of 14.5:1.
It needs a specially designed engine.
It needs a high temperature to function.
In order to reduce the production of NO, a lean burn operation is used. This requires a specifically designed engine where a port fuel injector is made through an open inlet valve.
Increasing the AFR can significantly reduce the formation of CO and HC.
Ignition delay period (or time) is the delay time between the start of injection and the start of combustion in a Diesel engine. What determines the delay time?
Fuel air mixing
Calorific value of fuel (ie how much energy the fuel gives off when it burns)
What happens when there is a long ignition delay period?
It risks a large amount of mixture since the in-cylinder temperature will be low
The larger part of premixed combustion may take place
It leads to an increase in compression work (and thus reduces engine power output)
It leads to an increase of soot formation because a proportion of fuel may remain unburnt
It results in high engine temperatures and "knocking"
The three distinct phases of combustion for a diesel engine are premixed combustion, mixing controlled combustion, and end-phase combustion.
Which phase of the combustion process in a diesel engine produces the most NO?
End phase combustion
During which phase of combustion in a diesel engine is most of the soot formed?
What is the break up time of spray?
Defined from the experimental results, it is the rate at which the spray tip penetration changes.
Defined from the theory of sprays, it is the rate at which the spray tip penetration changes.
It is the rate at which a jet spray changes to a fully atomized spray.
Advanced diesel combustion combines split injection strategies with catalytic abatement.
What is the prime parameter in determining the quantities of NOx formed in a traditional gasoline engine?
Mass of fuel burnt
Incomplete combustion or misfires.
A "split injection" system allows the rich zone of the fuel to be significantly reduced whilst maintaining flexibility with engine load (eg the amount of fuel injected) .
How does a split injection system (single pulse) help reduce smoke production?
It minimizes rich and hot zones of combustion
It reduces the combustion efficiency and lowers the amount of soot and smoke produced
It maximizes the rich and hot zones of combustion
It increases the combustion efficiency and lowers the amount of soot and smoke produced
How does using EGR help with diesel combustion?
It increases the heat capacity which reduces NOx
It dilutes the combustion which reduces NOx
It increases the overall efficiency
It reduces the amount of smoke production
What are some benefits of using high pressure injection in a DI Diesel Engine?
Higher pressures improve fuel customization and enhance fuel-air mixing
A smaller SMD (Resulting from high pressure) enhances the formation of fuel-air mixture in diffusion combustion phase
It reduces both soot and NOx production.
A DIG achieves a lean burn by manipulating a rich region to the spark plug by controlling the timing and form of injection in conjunction with the in-cylinder air flow.
How does lean burn increase bfsc?
It decreases pumping losses
It allows for more air to be drawn into the cylinder for a given amount of fuel
There is less throttling
It improves the overall efficiency
It restricts the amount of air drawn into the cylinder for a given amount of fuel
There is more throttling
The cone angle is defined as the angle between two straight lines originating from the orifice exit of the nozzle and being tangent to the spray outline. This angle
usually ranges from 5 to 30 degrees.
Fuel spray tip penetration is defined as the maximal distance measured from the injector to the spray tip. It represents the maximum penetration length
achieved by the droplets in the center of the spray.
How is the cone angle mainly affected?
Geometric characteristics of the nozzle
The fuel and air density
Reynolds number of the fuel
Velocity and Pressure of the fluid
Change of crank angle