FINAL TEST
FINAL CHALLENGE
Questions based on PFI Engine:
Q1. What is the Compression ratio for the engine?
The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.
CR = (Vd + Vc) / Vc
CR – Compression ratio
Vd – Displacement volume
Vc – Clearance volume
The petrol engine compresses the air & fuel with a ratio between 10:1 to 14:1. The petrol engine mixes petrol with air & compresses this mixture in the combustion chamber. Diesel engines normally have compression ratios between 14:1 to 25:1. Higher compression ratios mean a higher thermal efficiency, which means that theoretically maximum efficiency can be achieved at an infinite compression ratio.
A fundamental specification for such engines, it is measured two ways: the static compression ratio, calculated based on the relative volumes of the combustion chamber and the cylinder when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. The dynamic compression ratio is a more advanced calculation which also takes into account gasses entering and exiting the cylinder during the compression phase.
Supercharged engines usually have a compression ratio between 8:1 and 8.5:1. However, when it comes to diesel engines, the absence of spark plugs requires a higher compression ratio from about 14:1 to as much as 22:1. They use hot air to vaporize and then ignite the fuel.
Lower compression ratios also allow for engine downsizing, which tend to operate at higher specific engine loads in order to achieve lower fuel consumption. In addition to decreasing CO2 emissions, a fuel consumption reduction of 9% is possible with a displacement reduction from 2.0 to 1.6.
Q2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters.
Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase fuel efficiency while under varying loads. Variable compression engines allow the volume above the piston at top dead center to be changed.
Higher loads require lower ratios to increase power, while lower loads need higher ratios to increase efficiency, i.e. to lower fuel consumption. For automotive use this needs to be done as the engine is running in response to the load and driving demands.
To increase the compression ratio without changing the geometry is not possible, but without disassembling can increasing the compression ratio. (i) Fix the displacement volume with varying the clearance volume or (ii) fix the clearance volume with increase the displacement volume.
To fix the compression ratio = 10.3
(Vc + Vd) / Vc = 10.3
Vd = Max volume – Min Volume = 0.0005738 – 0.000057032 = 0.0005167 m3.
Vc = 0.000057032 m3.
(Vd + Vc)/Vc = 10.3
Vd + Vc = 10.3 Vc
Vd = 10.3 Vc – Vc = 9.3 Vc
New Vc = Vd / 9.3 = 0.0005167 / 9.3 = 0.0000555591 m3.
Here to change the Clearance volume as 0.0000555591 m3 without changing the displacement volume to reach the compression ratio is 10.3.
Vd = 10.3 Vc = 10.3 * 0.000057032 = 0.0005874 m3.
Here to change the displacement volume as 0.0005874 m3 without changing the clearance volume to reach the compression ratio is 10.3.
Q3. Calculate Volumetric Efficiency for this engine
Volumetric efficiency (VE) in internal combustion engine is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure (during the intake stroke) to the mass density of the same volume of air in the intake manifold.
Volumetric efficiency = Va / Vd
Vd = V max – V min = 5.742 * 10-4 – 5.702 10-5 = 0.00051718 m3.
Ideal gas Equation P V = m R T
Here P = 101325 Pa T = 300 K R = 287 J/ Kg – K.
Mass from the graph m = 0.0005342 kg
V = m R T / P
V = 0.0005342 * 287 * 300 / 101325
= 0.00045392 m3.
Volumetric efficiency = Va / Vd = 0.00045392 / 0.00051718 = 0.8776 = 87.76 %
Q4. Measure the air mass flow rate for this engine in kg/s
Mass flow, also known as “mass transfer” and “bulk flow”, is the movement of fluids down a pressure or temperature gradient, particularly in the life sciences. volumetric flow at standard conditions translates to a specific mass flow rate.
Max mass flow rate from the graph = 0.0005342 kg
Speed of the engine is 2400 RPM
Number of revolutions per cycle = 2
Number of cycles per second = 1200 / 60 = 20
Mass flow rate = 0.0005342 * 20
= 0.010684 kg /s.
Q5. Why is the cell count varying during the simulation?
In simulation process, in the case set option grid selection has fix the cell count of the simulation, depending upon the geometry the cell count has been fixed. In converge simulation the adaptive mesh refinement and also fixed embedding option are used to varying the cell count during the simulation. In adaptive mesh refinement, fix the cell count for various regions separately, fix with the conditions like velocity, Temperature, Pressure, Density, Species and also controlling the mesh refinement with timing control (sequential or periodic or permanent). To fix the embedded type in this option of sub grid selection and also change the orientation level of the sub-gird selection. For the fixed embedding option also vary the cell count, in this option select the entity type separately and fixed the start time and end time of the embedding. Also fix the mode of the embedding system in the fixed embedding option.
Q6. In a real engine, valves are in contact with the cylinder head but while running simulation, there is a small gap WHY?
The valve which allows mixture into the cylinder is the inlet valve; the one through which the spent gases escape is the exhaust valve. They are designed to open and close at precise moments, to allow the engine to run efficiently at all speeds.
The operation is controlled by pear-shaped lobes, called cams, on a rotating shaft, the camshaft, driven by a chain, a belt, or a set of gears from the crankshaft.
Where the camshaft is mounted in the engine block, small metal cylinders tappets sit in channels above each cam, and from the tappets metal pushrod extend up into the cylinder head. The top of each pushrod meets a rocker arm which bears against the stem of a valve, which is held in a raised (closed) position by a strong coiled spring the valve spring.
As the pushrod rises on the cam it pivots the rocker arm, which pushes the valve down (open) against the pressure of its spring. As the cam lobe rotates further, the valve spring acts to close the valve. This is called an overhead-valve (OHV) system.
Some engines have no pushrods; the valves are operated more directly by single or double camshafts in the cylinder head itself the overhead-cam system.
As there are fewer moving parts between the camshaft and the valve, the overhead-cam (OHC) method is more efficient and produces more power for a given engine capacity than an engine with pushrods, because it can operate at higher speeds. With either system, there must be some free play in the operating gear, so that the valve can still close completely when parts have expanded through heat.
A pre-set gap tappet clearance is essential between the valve stem and the rocker arm or cam, to allow for expansion. Tappet clearances vary widely on different cars, and faulty adjustment can have serious effects.
What happens if this gap is big?
If the gap is too large, the valves open late and close early, reducing power and increasing engine noise. Too small a clearance prevents the valves from closing properly, with a consequent loss of compression. Some engines have self-adjusting tappets, which are hydraulically operated by the engine oil pressure.
Q7. How many seconds does combustion last in this case?
The duration of combustion in one cycle is 240 deg from the engine performance calculator,
The speed of the engine is 3000 RPM,
In one revolution, the engine rotates per minute is 3000 * 360 = 1080000 deg
Degrees per rotation per sec = 1080000 / 60 = 18000 degree
In the last combustion takes place = duration of combustion in one cycle / degrees per rotation per sec
= 240 / 18000 = 0.0133 sec
Questions based on DIESEL Engine:
Q1. What is an advantage of SECTOR Simulation?
One of the primary advantages of simulators is that they are able to provide users with practical feedback when designing real world systems. This allows the designer to determine the correctness and efficiency of a design before the system is actually constructed. Sector simulation is very low cost compared to full working set up simulation and also get the accurate result in the specified region to compared with the complete set up. Also the sector simulation reduce the simulation time.
Q2. When will the sector approach not work?
The full geometry must be centered at z=0 and any cut plane through the geometry must form a closed loop, implying you would need to remove the valves and ports from your geometry and close the holes. The sector approach not work the geometry is not symmetric and not suitable for which geometry has an inlet and outlet.
Q3. How to choose Start Crank Angle (SCA) and End Crank Angle (ECA) for a sector simulation?
In sector simulation the crank angle calculated for the compression and expansion stroke. So, that the start crank angle is defined in the start of compression stroke and the end crank angle is calculated end of expansion stroke. In start crank angle start of compression stroke at 180 degrees and the end crank angle in the end of expansion stroke is 540-degree angle.
Q4. Can we convert the PFI case into a SECTOR case? Explain your answer?
In the port fuel injection system is not possible to set the sector geometry, in that port fuel injection, it is set to be in the air in line flow region, so in that region has inlet and outlet, if possible we fix the cylinder region we don’t have the inlet and outlet region, so it does not affect the sector case and also in that cylinder geometry has the axisymmetric object. In the port fuel injection system, the low cetane number fuel only gives a high amount of efficiency like gasoline, diethyl alcohol are the low cetane number fuels gives the high efficiency and low emissions.
Full hydrodynamic simulation is needed for the Port fuel injection system to attain the best results in the combustion modeling for efficiency and also the emission modeling for reducing the emission. In a simplified simulation, fluid would move through solid structures without imposing any deformation of the topology. Realistically, however, the fluid pressure and shear stress calculated in the fluid simulation cause the solids to deform. Thus, it is important to capture the forces and interactions between the fluid and solid for realistic simulation results. The nature of a PFI simulation is that the forces imposed by the fluid may deform the object in ways that you cannot anticipate. In CONVERGE, Adaptive Mesh Refinement increases mesh resolution in the areas that require higher resolution as the FSI object deforms. This allows for the optimal use of computational cells by placing cells where they are needed, when they are needed.