FINAL TEST

FINAL TEST

PFI QUESTIONS

Q1. What is the Compression ratio for the engine?

The compression ratio (CR) is defined as the ratio of the volume of the cylinder and its head space (including the pre-combustion chamber, if present) when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel (‘top dead centre’, tdc). Typically, petrol engines have a CR of 8–10, while diesel engines have a CR of 15–20. The CR of petrol engines is limited by the requirement that the fuel burns uniformly in the cylinder and does not ignite thermally prior to the spark (so-called ‘engine knocking’).

i.e. CR=`V_(BDC)/V_(TDC)`=`(V_1+V_2)/V_2`

where `V_(BDC)`-> Volume of the gas when cylinder is at the bottom most position.

`V_(TDC)`->Volume of the gas when the cylinder is at the top most position.

BDC->Bottom Dead Centre

TDC->Top Dead Centre

For internal combustion engine(Diesel Engine), a high compression ratio is favorable because of higher thermal effciency of the system. As a result, the internal combustion engine can reach high combustion temperature which provides more work output for the same amount of fuel. This is because the IC engine is conceptually based on the heat engine and as a result, longer expansion cycle provide more work output and less heat wasted to the surrroundings in the form of exhaust.

Q2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters.

Geometrical parameters of an engine are crank rod length, bore,stroke,etc. These parameters are constant for a particular engine. The only way that we can change the compression ratio of this engine to 10.3 is by varying

`V_1 and V_2`, where they are swept volume and clearance volume respectively.

From the PFI engine data, we ge that the maximum and minimum volumes respectively are as follows:

`V_max=5.74**10^-4 m^3;V_min=5.7**10^-5 m^3`

Note that since maximum volume occurs when the piston is at BDC and minimum volume occurs when piston is at TDC, therefore

`V_2=V_min;V_1=V_max-V_2`

`impliesV_2=5.7**10^-5 m^3` and `V_1=5.17**10^-4 m^3`

Substituting in the CR equation and evaluating for `V_2`, we get

10.3=`(5.17**10^-4 +V_2)/V_2`

`V_2=5.56**10^-5 m^3`

Now following the same procedure and evaluating for `V_1`, we get

`V_1=5.301**10^-4 m^3`

Q3. Calculate Volumetric Efficiency for this engine

The easier the engine breathes, the greater its volumetric efficiency, and the more power it produces.

The volumetric efficiency of an engine can be reduced by restricting the airflow in and out of the engine.

Volumetric efficiency is how an engine's pumping capacity is defined.

i.e.

`eta_v=V_(air)/V_1`

From our plot, we get mass flow of air as `5**10^-4 kg`

`implies PV=mRT`and using this relation, we get `V_(air)=4.42**10^-4 m^3 implieseta_v=0.85`

Our engine is 85% efficient in converting fuel to mechanical power.

Q4. Measure the air mass flow rate for this engine in kg/s

mass flow rate is defined as the amount of mass flow that is crossing the intake port per unit time.

From our data, the average mass flow that entered into the cylinder is `5**10^-4 m^3`. But the engine RPM is 3000

`implies rps=3000/60=50rps`.

Therefore, mass flow rate for this engine in kg/s is `2.5**10^-2 (kg)/s`

Q5.Why is the cell count varying during the simulation?

The cell count is subjected to variation(note that we have reached million cells) because of the usage of AMR(adaptive mesh refinement) and SGS(sub grid scaling). Note that by not using these two, we get the total number of cells close to 10000. But inorder to capture the physics behind the flow such as turbulence and mixing efficiently, we use AMR and SGS which will follow Converge's state of the art built in model which subdivides the mesh automatically depending upon the need such as at the nozzle and at the walls. Hence the cell count is subjected to variation.

Q6. In a real engine, valves are in contact with the cylinder head but while running simulation, there is a small gap.WHY?What happens if this gap is big?

In reality, the engine ports and valve leave a small gap in between them. This is done so as to protect the cylinders from mechanical ear and tear. If there werent any gas, the valve motion over the period of time could damage the liner walls. To provide this detail to the Converge software, we use something called as disconnect triangles(virtual triangles) by setting up events. The main usage of events is that the user can specify the times at which the valve motion should occur. Because during one stroke, two valves are opened and during the other stroke, the other two valves are opened. So to make sure that converge understands this, we setup events.

If we setup the gap between the port and valve larger, then leakage of fuel and air mixture might happen into the cylinder, which could cause incomplete combustion giving unburnt products of combustion and thereby in the process, producing more pollutants. Another effect is that the larger gap can give rise to unreliable simulation data. Hence the close proximity need to be kept.

Q7. How many seconds does combustion last in this case?

From the engine performance calculator, we get

If we see the figure below, we can observe that combustion starts from -120.09 degrees to 120.11 degrees. The total duration is roughly 240 degrees.

WKT engine rpm-3000

Therefore, rps-50

dps(degrees per second)=360*50=18000

time taken=`240/18000=0.11331s`

DIESEL ENGINE QUESTIONS

Q1. What is an advantage of SECTOR Simulation?

TIME & RESOURCE! Using the sector simulation helps us to save a lot of time in CFD calculations, which reduce the overutilisation of computer resources. As discussed, Diesel engine has an almost symmetric geometry, which can be exploited because math becomes easy once symmetry is found. The results that we get for sector simulation is applicable for full engine as well. Note that this method is advantageous only in close-cycle systems.

Q2. When will the sector approach not work?

Sector methodical approach will definitely fail when we introduce fuel injection systems,exhaust and intake systems, turbocharger systems and so on. Moreover, upon introducing these elements, we have to stick with open cycle analysis rather than close cycle analysis. If we use sector modeling, we cant simulate fuel injection process and motion of valves and turbocharger actions which form part and parcel of the bigger picture.

Q3. How to choose Start Crank Angle (SCA) and End Crank Angle (ECA) for a sector simulation?

As we know, that the sector simulation is based on the close cycle assumptions, so the analysis is only valid at the beginning of the compression stroke and ends at the completion of the expansion stroke. For instance if the fuel inkection takes place at 0 CAD then the compression stroke starts from 180 CAD and is valid till the end of the expansion stroke at 540 CAD.

Q4. Can we convert the PFI case into a SECTOR case? Explain your answer?

A big NO!

A port fuel injection cannot be converted into a sector model. The sector model is symmetryc because we ignore more crucial aspects like fuel injection etc. If we do the same in a PFI engine, then we will get inaccurate mixing and hence, wrong physics! Also, the chief difference between PFI and diesel engine is the presence of a spark plug whereas in diesel engine, there isnt any. Modeling of spark plug will become a tedious job once we include symmetricity. Also, we have ports that get divided into two near the combustion chamber, so applying symmetry conditions there seems obsolete. Hence in PFI engine, we cant use Make engine sector tool.