Week 11: Project 2 - Emission characterization on a CAT3410 engine

 

Compression Ignition (CI) Engine

The compression ignition engine is an internal combustion engine that uses temperature increase during the compression stroke as a source to ignite the fuelcharge (fuel-air mixture). This is also called auto-ignition. These engines are always fuel injected.Air is drawn into the cylinder through the intake manifold and compressed by the piston.

The mostimportant function of the CI engine combustion chamber is to provide proper mixing of fuel and airin a short time to lessen theignition lag phase. In order to achieve this, an organized air movement called air swirl is provided to produce high relative velocity between fuel droplets and air. When the liquid fuel is injected into thecombustion chamber, the spray cone gets disturbed due to the air motion and turbulence inside.

The onset of combustion will cause an added turbulence that can be guided by the shape of the combustion chamber.

The combustion and emission formation processes in diesel engines have a close relationship with the piston bowl geometry which can strongly affect the air fuel mixing before the combustion starts.

Importance of Combustion Chamber

The performance and emission characteristics of CI engines mainly depend upon the combustion process. Combustion of the fuel inside the cylinder, in turn depends on various factors like, fuel injection timing, fuel injection pressure,engine design such as shape of combustion chamber and positionof injector, fuel properties, number and size of injection nozzle hole, fuel spray pattern, air swirl, fuel quantity injected, etc.

Most important function of CI engine combustion chamber is to provide proper mixing of fuel and air in short possible time. In CI Engine, only air enters inside the combustion chamber during suction stroke. The air gets compressed inside the combustion chamber during compression stroke, near the end of compression stroke fuel enters the combustion chamber by means of injector so there is heterogeneous mixture in CI engine.

After injection of fuel inside the combustion chamber, fuel and air mixed with each other but it take certain time for mixing so to increase the rate of mixing turbulence is required in CI Engine. If turbulence is applied inside the combustion chamber the fuel mixes with the air completely and it passes throughout the chamber.

The temperature and pressure of the air inside the chamber is high so the injected fuel reaches to its self-ignition temperature and start burning. Due to turbulence the flame propagate throughout the combustion chamber and because of this complete combustion of mixture may occur inside the chamber.

Piston Geometry

The shape of the combustion chamber and the fluid dynamics inside the chamber are important in diesel combustion. As the piston moves upward, the gas is pushed into the piston bowl. The geometry of the piston bowl can be designed to produce a squish and swirling action which can improve the fuel/air mixture formation before ignition takes place.

The main goals desired from the design of chamber geometry are to optimize the mixing of the fuel and air, before and during ignition, and to improve the flow of the exhaust products once combustion is complete.

Case Setup

Engine Geometrical Parameters

Bore = 0.13716 m.

Stroke = 0.1651 m.

Connecting Rod Length = 0.263 m.

Speed = 1600 Rpm.

 Simulation Parameters

Simulation Type = Gas Simulation

Material = Diesel 2

Simulation Start Time = -147 Crank Angle Degrees

Simulation End Time = 135 Crank Angle Degrees

Iniital and Boundary Conditions

•  Physical Modelling

• Spray Modelling: Spray modelling is used to configure the nozzles, injectors , injection rate-shapes during the simulation.In this simulation we have a single injector which consists of 4 nozzles. Sprays are divided into small droplets and itroduced into the system as lagrengian parcels which are clustered near the cone center.The O'Rourke turbulent dispersion  model and frossling evaporation model is used in this simulation.
• Combustion Modelling: SAGE chemistry solver along with Zeldovich NOx and Hiroyasu soot models are used.

• Meshing 

Base grid of 1.4 mm is used in the simulation.AMR and fixed embedding has been used in certain regions in order to refine the mesh near the cylinder, exhaust, intake boundaries.AMR is trigerred using velocity and temperature, both having 3 embedding levels.

The above image shows the various processess that takes place during the simulation in terms of crank angle.

Results 

• Mesh Generation

 video link:

https://photos.google.com/u/0/photo/AF1QipPSbUtPrdbibc5kumYlL9kMaQP-Kxe_n6DmvR0u

 

 

• Mean Temperature

 video link:

 https://photos.google.com/u/0/photo/AF1QipMKlFRN1xIsxwg7Fb7FMj8-0qXLK4WbcFNd-3qK

 

 

The peak temperature in Omega Piston is much higher than the Open W piston.Hence, complete combustion takes place in the Omega Piston.

• Pressure

 video link:

https://photos.google.com/u/0/photo/AF1QipPNI95QkUa-wkijw-a7B-gtuBU9HqI3HoZQMS-i

 

The peak pressure in both the pistons is approximately same.

• Heat Release

Omega Piston has a high heat release rate than the Open W piston. Hence, we can say that omega piston has a high combustion efficiency.

• Pressure - Volume Plot

Since the area under the PV plot for the omega piston is greater than the open-w piston.The pdV work of omega piston is higher.

Calculation

 Time per degree= $\frac{60}{360\cdot RPM}=\frac{60}{360\cdot 1600}=0.000104\frac{\sec }{deg}$

For 270.306 degrees,

Time, T = $270.306\cdot 0.000104=0.0281\sec$

• For Omega Piston

Power, $P=\frac{W}{T}=\frac{3409.28}{0.0281}=121.33KW$

Torque, $T=\frac{60\cdot P}{2\cdot \pi \cdot RPM}=\frac{60\cdot 121326.69}{2\cdot \pi \cdot 1600}=724.11Nm.$

 Power and Torque for Omega Piston are 121.33KW and 724.11 Nm respectively.

• For Open W-Piston

Power,$P=\frac{W}{T}=\frac{3028.53}{0.0281}=107.776KW$

Torque,$T=\frac{60\cdot P}{2\cdot \pi \cdot RPM}=\frac{60\cdot 107776.86}{2\cdot \pi \cdot 1600}=643.245Nm.$

 Power and Torque for Open W-Piston are 107.776KW and 643.245 Nm respectively.

 Omega Piston is more efficient than the Open W-piston and been proved both graphically and analytically.

 

Emissions

• NOx 

The amount of produced NO_x is a function of the maximum temperature in the cylinder, oxygen concentrations, and residence time. Most of the emitted NO_x is formed early in the combustion process, when the piston is still near the top of its stroke. This is when the flame temperature is the highest. Increasing the temperature of combustion increases the amount of NO_x by as much as threefold for every 100 °C increase.

Since Omega piston has high peak temperature, the NOx formation in the engine is also high when compared to Open W piston.

• Soot

Diesel engines offer the possibility of combining very high thermal efficiencies with very low emissions, and their good fuel efficiency results in low carbon dioxide emissions. The main problem areas for diesel engines are emissions of nitrogen oxides (NOx) and particulates, and these two pollutants are traded against each other in many aspects of engine design. Very high temperatures in the combustion chamber help reduce the emission of soot but produce higher levels of nitric oxide (NO). Lowering the peak temperatures in the combustion chamber reduces the amount of NO produced but increases the likelihood of soot formation. Better mixing of the air and fuel is the key to lower emissions. The NO produced rapidly oxidises to NO₂ (collectively called NOx). NOx combines with hydrocarbons or volatile organic compounds in the presence of sunlight to form low level ozone. This leads to smog formation.

• Carbon Monoxide (CO)

Carbon monoxide results from the incomplete combustion where the oxidation process does not occur completely. This concentration is largely dependent on air/fuel mixture and it is highest where the excess-air factor (λ) is less than 1.0 that is classified as rich mixture. It can be caused especially at the time of starting and instantaneous acceleration of engine where the rich mixtures are required. In the rich mixtures, due to air deficiency and reactant concentration, all the carbon cannot convert to CO₂ and be formed CO concentration. Although CO is produced during operation in rich mixtures, a small portion of CO is also emitted under lean conditions because of chemical kinetic effects.

From the above graph is evident that omega piston has a high emission of Carbon Monoxide than Open-W piston.