Page 269 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

2 Engine: The NOx reduction process starts with an efficient CRD engine design CRD engine design that burns
clean ultra low sulfur diesel (ULSD) and produces inherently lower exhaust emissions- exhaust that is already
much cleaner due to leaner and more complete combustion.
3 Diesel exhaust fluid (def) tank and pump: Under the direction of the vehicle’s onboard computer, Def is
delivered in precisely metered spray patterns into the exhaust stream just ahead of the SCR converter. DEF is
a urea based solution, Composition – 67.5% de-ionized water – 32.5% urea. Urea- Under heat, decomposes
to ammonia (NH3) and Carbon dioxide (CO2) Ammonia (NH3) reacts with NOx in the presence of a Catalyst
DEF is required for the selective catalytic reduction (SCR) system to function.
4 SCR Catalytic converter: This is where the conversion happens. Exhaust gases and an atomized mist of
DEF enter the converter simultaneously. Together with the catalyst inside the converter, the mixture undergoes
a chemical that produces nitrogen gas and water vapor.
5 Control device: Exhaust gases are monitored via a sensor as they leave the SCR catalyst. Feedback is
supplied to the main computer to alter the DEF flow if NOx levels fluctuate beyond acceptable parameters.
Positive Crankcase Ventilation
Purpose of crankcase ventilation: The first controlled emission was crankcase vapors. While the engine is
running during combustion some unburned fuel and other products of combustion leak between the piston rings
and the cylinder walls, down into the crankcase. This leakage is called blow-by. Blow by gases are largely HC
gases
Unburned fuel, and water from condensation, also find their way into the crankcase, and sump. When the engine
reaches its full operating temperature, the water and fuel evaporate. To prevent pressure build - up, the crankcase
must be ventilated.
In earlier vehicles, crankcase vapors were vented directly to the atmosphere through a breather tube, or road
draught tube. It was shaped to help draw the vapors from the vapors from the crankcase, as the vehicle was being
driven.
Modern vehicles are required to direct crankcase breather gases and vapors back into the inlet system to be
burned.
A general method of doing this is called positive crankcase ventilation, or PCV.
PCV working principle: The PxCV vacuum circuit works as follows (Fig 1). Air for the system enters the air
cleaner area. The air then goes through the air filter, through a tube, and through the closed oil filler cap.
The intake manifold vacuum the draws the crankcase vapors and gases back to the PCV valve. From the PCV
valve, the vapors and gases are drawn into the intake of the engine to be burned by combustion.

If too many vapors and gases get into the intake manifold, it may upset the air-fuel ratio. The PVC valve helps to
control the amount of vapors and gases going back into the intake manifold.
As shown in the diagram (Fig 2), the PCV valve consists of a tapered plunger and two springs, and limits the air
flow based on intake manifold vacuum.
During idle and deceleration when blow-by gases are minimal, the low pressure (or “high” vacuum) in the intake
manifold pulls the plunger against the springs and restricts the airflow through the valve.

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CITS : Automotive - Mechanic Diesel - Lesson 80 - 83