CH33 emission control systems STUDENT VERSION rev

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Chapter 33

Emission Control Systems

• Emissions of most concern are HC, CO,

NOx, CO2, and O2

• O2 content indicates combustion efficiency

• The allowable amounts of emissions are

regulated by the government

• All vehicles for the last 40 years have been equipped with devices to reduce emissions

Hydrocarbons (HC)

• Evaporative fuel emissions also a source

• Other causes of HC emissions

– Unburned fuel in sheltered areas of combustion chamber

– Fuel is absorbed into oil on cylinder walls

– Fuel is not adequately mixed with air

– Misfires

– Very rich or lean mixtures

Carbon Monoxide (CO)

• Formed when there is not enough oxygen to combine with carbon during combustion

• Caused by a lack of air or too much fuel

• CO will not occur without combustion

• As the mixture gets richer, CO increases

Nitrogen Oxides (NOx)

• As combustion reaches more than 2300ºF (1261ºC) N and O2 combine

• NOx formation controlled by controlling combustion temperature

• A rich mixture reduces NOx but increases

CO and HC

Carbon Dioxide (CO2)

• Byproduct of ideal combustion

• CO2 production directly related to the amount of fuel consumed

• Levels are highest with a slightly lean mixture

• CO2 also produced by manufacturing processes

Oxygen (O2)

• Too much O2 in exhaust indicates an

improper air/fuel ratio

• Very low O2 readings indicate a rich

mixture

• High O2 readings indicate a lean mixture

• O2 levels are not affected by the catalytic converter

Diesel Emissions

• Diesels are most efficient of internal

combustion engines

• Typical diesel emissions include:

– Carbon (soot) and various carbon compounds – NOx Sulphur dioxide

– Water Various HC compounds – CO2

Inspection and Maintenance

• If the emissions are lower, the vehicle is

certified for use

I/M Programs (cont’d)

• After the 1990 Clean Air Act, more precise testing started, called the I/M240 test

• A vehicle is operated on a dyno and tested for 240 seconds under differing load

conditions

• Also tests the EVAP system

Vehicle Emission Control

Evaporative Emissions System

Enhanced EVAP System

• Similar to previous EVAP systems but can detect small (0.020” or 0.5mm) leaks and monitors canister purge flow

• Use a fuel tank pressure sensor and EVAP canister vent solenoid

• Improved cooling systems

• Spark control systems

PCV Systems

• Reroute blowby gases, mixed with outside air, back into the intake manifold.

• Blowby can cause sludge, oil dilution, and

increased crankcase pressures.

– PCV spring moves the valve to increase the opening.

• Wide Open Throttle

– Valve is open and allows more blowby gasses to flow.

Fixed Orifice Tube PCV Systems

• Some engines use a fixed orifice instead of

a PCV valve

• The orifice limits the amount of blowby

flow into the intake

• The air/fuel system is calibrated for this air leak

Heated PCV Systems

• Crankcase vapors contain moisture

• To prevent freezing of the moisture, some engines use heated PCV systems

• Can be coolant or electrically heated

• The PCM uses IAT input to determine if PCV heating is needed

EGR Systems

• Recirculates an amount of exhaust gas which dilutes the air/fuel mixture

• This lowers combustion temperatures

• EGR amount is proportional to throttle opening

• Many modern engines do not use EGR

Common Types of EGR Valves

Other Converter Designs

• Two-way or oxidizing converters had no

Air Injection Systems

• Pumps fresh air into the exhaust

• Caused combustion of exhaust gases

• Older systems pumped air into exhaust manifolds

• Later systems pumped air into catalytic converters

Pulse (Non-Pump) Systems

• Instead of using

an air pump, some

systems used the

natural exhaust

pressure pulses to

pull air from the

air cleaner

Diesel Emission Controls

• Modern diesels have emission levels similar

to gasoline engines

• Particulate filters and new fuel injection

systems reduce soot emissions

• Diesels emit less HC, CO, and CO2

Low-Sulfur Fuel

• Eighty percent of all diesel fuel sold must

be low-sulfur

• Reduced sulfur from 500 PPM to 15 PPM

• Allows use of particulate filters and NOx catalysts

Diesel Fuel Injection

• When injection begins at precisely the right time, emissions are minimized

• Many diesels use common rail injection:

– Uses high-pressure to provide better

atomization of the fuel

– Engine runs cleaner and quieter

– Many systems fire the injectors several times

Glow Plugs

• Diesels can be hart to start when cold

• Most diesels use small electric heaters to heat the intake air

• Driver is alerted to wait when starting to allow warm up

PCV System

• Diesels produce very little vacuum so

traditional PCV systems do not work

• A multi-stage filter may be used to collect, coalesce, and return crankcase oils to sump

• Other systems work with the turbocharger

to draw crankcase gases out and back into the intake

Crankcase Depression Regulator

(CDR)

• Similar to a PCV valve

• Designed to work with very low vacuum

• Minimizes crankcase pressures

• Used on turbo and non-turbo engines

• Must be calibrated to the engine

EGR Systems

• Very similar to those on gas engines

• Most diesel EGR applications cool the

incoming gases before the cylinders

• Most EGR coolers use engine coolant that passes through a separate circuit to cool the gases

• Also reduces soot emissions.

• May have a NOx adsorber built in or

separate

Particulate Matter (PM) Filter

• Placed after the catalytic converter

• Early filters needed regular cleaning

• Newer designs periodically burn the

collected PM

• Use of regular diesel fuel in place of sulfur fuel will destroy the PM filter

low-Selective Catalyst Reduction

(SCR) Systems

• Used to reduce NOx emissions

• A reductant is injected into the exhaust stream and then absorbed onto a catalyst

• The reductant removes oxygen from a

substance and combines another substance

with oxygen to form another compound

• Ammonia and urea water are commonly used

SCR Systems (cont’d)

• Work well only if exhaust is within a

specific temperature range

• The PCM controls exhaust temperature and reductant injection

• Reductant supply tanks must be refilled

• Organic compound made of carbon,

oxygen, and hydrogen

• Helps eliminate over 90% of nitrogen

oxides in exhaust gases

• Urea systems are low cost and do not affect engine performance