FINAL TEST
1. What is the Compression ratio for the engine?
We often talk about the power output when it comes to internal combustion engines, from over a century various research has been conducted to study and modify the factors affecting the power output of an IC engine. so let's understand the concept of compression ratio.
The compression ratio of an IC Engine is the ratio of the total volume of the combustion chamber to the volume left after complete compression i.e clearance volume. In simple words, it is the ratio between the total volume of the combustion chamber which is left when the piston is at its bottom dead center, and the volume left inside the combustion chamber when the piston moves to its top dead center.
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`CR = (V_s + V_c) / V_c`
Where,
`Vs` = swept volume. This is the volume inside the cylinder displaced by the piston from the beginning of the compression stroke to the end of the stroke.
`V_c` = clearance volume. This is the volume of the space in the cylinder left at the end of the compression stroke.
`V_s`can be estimated by the cylinder volume formula
`n_v = pi/4*D^2*L`
Where,
D = cylinder bore (diameter)
L = piston stroke length
For example - Let's consider an engine having 1000cc total volume out of which 900cc is the swept volume i.e the volume covered by the piston when it moves from BDC to TDC, and having 100cc clearance volume i.e volume left inside the cylinder when the piston reached to its TDC. So the compression ratio of the engine will be 1000:100 or 10:1
It is found that the greater the compression ratio more will be the power output of the engine.
The compression ratio of diesel engines is much higher than that of petrol engines i.e for petrol engines the CR varies from 10:1 to 14:1 and for diesel engines, the CR varies from 18:1 to 23:1.
Changing the compression ratio without changing geometrical parameters is possible only by changing clearance or swept volume.
2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters?
As we know that to modify the compression ratio of the engine without changing its geometrical parameters, either the clearance volume (or) swept volume should be changed.
Originally,
`V_max implies V_c + V_s = 0.0005742 m^3`
`V_min = V_c = 5.704*10^-5 m^3`
`V_s = V_max - V_c implies 0.0005742 - 5.704*10^-5 implies 0.0005168 m^3`
`V_s = 0.0005168 m^3`
CASE 1: Swept volume is constant and clearance volume is varied.
`CR = (V_s + V_c) / V_c`
`10.3 = (0.0005168 + V_c) / V_c`
`V_c = 5.5569*10^-5 m^3`
Thus, the required compression ratio of 10.3 can be achieved by changing the clearance volume to `5.5569*10^-5 m^3` while keeping the swept volume constant.
CASE 2: Clearance volume is constant and swept volume is varied.
`CR = (V_s + V_c) / V_c`
`10.3 = ((V_s + 5.5569*10^-5 )) /(5.5569*10^-5)`
`V_s = 0.0005168 m^3`
Hence to get the required compression ratio of 10.3, the swept volume is changed to 0.0005168 m^3 by changing the bore (or) stroke length of the engine by keeping the clearance volume constant.
3. Calculate the Volumetric Efficiency of the engine?
When it comes to the performance parameters of an internal combustion engine, one of the most important factors to consider is Volumetric efficiency. It is the ratio of the volume of the air or charge that is drawn into the cylinder during the suction stroke to the total displacement of all the cylinder or cylinders during suction stroke to the total displacement of all the cylinders when they are operating at atmospheric pressure. To put it another way, it refers to the capacity of the engine to carry out the work in an effective manner. However, this is dependent on the speed of the engine as well as the load. It also depends heavily on the configuration of the inlet and exhaust systems, as well as the dimensions of the valve ports and the timing of the valve.
The formula of Volumetric Efficiency
The mathematical expression for the volumetric efficiency is as follows :
`eta_v` = Actual volume/Swept volume`
`eta_v = V_a/V_s`
It can also be written as,
`eta_v = (pi/4) D^2L`
Where D equals the diameter of the bore
L = the length of the stroke of the piston
Assume the air used is ideal in nature, therefore using the ideal gas equation,
`P*V_a = m*R*T`
`V_a = (m*R*T)/P`
where,
P - Pressure of air at atmospheric condition (101325 N/m^2)
m - mass of air
R - Universal gas constant (287 J/kg-k)
T - Temperature of air at atmospheric condition (293k)
`V_a = [(0.00053486*287*293)]/101325`
`V_a = 0.0004438 m^3`
So. `eta_v` = `V_a`/`V_s`
Volumetric Efficiency (`eta_v`) = (0.0004438) / (0.0005168) *100`
(`eta_v`) = 85.87%
Q 4. Measure the air mass flow rate for the engine as kg/s.
The mass of the air(m) is taken as 0.00053486kg
Speed of the engine (N) = 3000 rpm = 3000/60 = 50 revolution/sec
Mass flow rate (`dot m`) = (`m_a`* N)
= 0.00053486*50 = 0.026743 kg/ s
Q5. Why is the cell count varying during the simulation?
The cell count is varying during the simulation because of the Adaptive Mesh Refinement(AMR) and Fixed embedding used to capture the physics accurately in critical
areas.
The velocity and temperature AMR are used in the PFI engine.
For instance, in velocity AMR if we set SGS as 1m/s then the cell will become finer by a factor of 2n, where n is the embedded level. If there are 2 mesh become finer by 2^2 4 times.
Q6. In a real engine, valves are in contact with the cylinder head but while running a simulation, there is a small gap.
- Why?
- What happened if the gap is big?
Why?
In real engines, the valves are in contact with the cylinder head to avoid escaping of fuels and heat from the combustion chamber. The movement of valves is controlled by using cam and follower mechanisms.
In our PFI simulation, we have not designed any mechanism to drive the valves. So there is a small gap between the cylinder head and the valve. During the simulation, converge automatically creates disconnect triangles between the gap.
What happens if the gap is big?
When the gap is big, heat dissipation between the cylinder head and cylinder wall cannot take place. This increases the temperature in the cylinder which can lead to problems like detonation(or) Knocking and it can also increase emissions like NOx, unwanted HC, and soot which decrease the efficiency and life of the engine.
7. How many seconds does combustion last in this case?
As we can see from the above plot,
The duration of combustion is 240.199 degrees
Engine speed(N) = 3000rpm => 50rps
Revolution in degree = 360*50 = 18000 degree/sec
Combustion time = 240.199 degree
Combustion time in seconds = 240.199/18000 = 0.0133 seconds
Questions based on DIESEL ENGINE:
1. What is an advantage of SECTOR Simulation?
Sector simulation is a process of using only the sector of the model for CFD simulation. Since the size of the engine is reduced the time taken for the simulation will also reduce such that it saves computational time. As this is a closed-cycle analysis the boundary conditions and parameters required to define a case are also minimum. The main benefit of using this method is dramatically reducing the computational time.
2. When will the sector approach not work?
The sector modeling does not work in the case of open-cycle engine analysis. In open cycle analysis, the position of walls varies with time which cannot be captured when we use sector modeling. So, sector modeling cannot be applied for open cycle analysis it can be used for closed cycle analysis where the valves are not considered.
3. How to choose Start Crank Angle(SCA) and End Crank Angle (ECA) for a sector simulation?
The sector simulation is good in the case of closed-cycle analysis. So, the start crank angle can be taken at the beginning of the compression stroke and the end crank angle can be taken at the end of the expansion stroke.
For example, if we assume the fuel to be injected at 0 degrees then 180 degrees can be taken as the start crank angle (SCA) and 180+360 = 540 degrees can be taken as the end crank angle (ECA) for the diesel engine.
4. Can we convert the PFI case into a SECTOR case? Explain your answer.
PFI case is an open cycle analysis type. As I said before SECTOR modeling is suitable only for closed cycle analysis where a closed part of a 360-degree chamber can be simulated for saving computational time.
As in the case of PFI the valves are considered which makes it an open-cycle analysis. So the PFI case cannot be converted into a sector case.