Exhaust, Emissions, and Aftertreatment Systems

Chapter Introduction

This chapter covers all systems related to exhaust gas evacuation, emissions control, and aftertreatment devices for heavy-duty vehicles (Class 3 to 8). For the Red Seal exam, you must master not only the operating principles, but also diagnostic procedures, reference values, applicable Canadian standards, and the interactions between these systems and the engine.

Modern aftertreatment systems have become an essential component of the powertrain. A competent technician must understand how these systems influence performance, fuel consumption, and engine durability.

Fundamental Principles of Exhaust Gases

Exhaust Gas Composition

Exhaust gases from a diesel engine contain several regulated pollutants:

Carbon monoxide (CO): produced by incomplete combustion

Unburned hydrocarbons (HC): unburned fuel

Nitrogen oxides (NOx): formed at high temperatures (combination of N₂ + O₂)

Particulate matter (PM): soot and ash

Sulfur dioxide (SO₂): originating from sulfur in the fuel

NOx formation increases with combustion temperature. Conversely, particulates increase when combustion is incomplete (lack of oxygen). This trade-off is known as the NOx-PM trade-off curve.

Temperatures and Pressures

Exhaust gas temperature varies depending on the measurement point:

Measurement Point	Typical Temperature (Diesel)
Turbocharger outlet	250–450 °C
DPF inlet	200–350 °C (normal operation)
DPF regeneration	550–650 °C
DPF outlet	150–250 °C
Final exhaust outlet	100–200 °C

Exhaust back pressure is a critical parameter. An excessive value (generally > 10 kPa at idle or > 25 kPa under load) indicates a clogged DPF or a catalyst problem.

Exhaust System Components

Exhaust Manifold

The manifold collects gases from each cylinder and directs them to the turbocharger turbine. Modern manifolds are often made of cast iron or stainless steel. Manifold leaks can cause:

Loss of performance

Abnormal noise

False readings from oxygen sensors (where applicable)

Engine compartment contamination

Turbocharger

The turbocharger uses exhaust gas energy to compress intake air. Key points:

Pressure ratio: typically 2.5:1 to 4.5:1 for modern engines

Rotation speed: can reach 150,000 to 200,000 RPM

Axial clearance: 0.025–0.125 mm (depending on manufacturer)

Radial clearance: 0.30–0.60 mm

Excessive clearance indicates bearing wear. Clearance checks must be performed cold, using a dial indicator.

Exhaust Brake System (Retarder)

The exhaust retarder (e.g., Jacobs Engine Brake, PacBrake) works by creating back pressure in the exhaust system. Principle:

34.A butterfly valve partially closes in the exhaust pipe

35.Pressure builds up in the manifold

36.The engine must work against this pressure during the exhaust stroke

37.The energy is dissipated as heat

Braking power can reach 60–80% of the engine's driving power.

Diesel Aftertreatment Systems

Technology Overview

Modern diesel engines use a combination of technologies to meet EPA 2010 / Euro VI / Canadian standards:

Technology	Abbreviation	Primary Function
Diesel oxidation catalyst	DOC	Oxidation of CO and HC
Diesel particulate filter	DPF	Capture of particulate matter
Selective catalytic reduction	SCR	NOx reduction
Ammonia oxidation catalyst	AMOX	Removal of residual ammonia

Diesel Oxidation Catalyst (DOC)

The DOC is a ceramic or metal monolith coated with precious metals (platinum, palladium). It oxidizes:

CO into CO₂

HC into CO₂ and H₂O

A portion of NO into NO₂ (important for passive DPF regeneration)

The optimal operating temperature is 250–450 °C. Below 200 °C, the DOC is ineffective (light-off temperature).

Diesel Particulate Filter (DPF)

The DPF captures soot particles through physical filtration. Silicon carbide (SiC) or cordierite substrates offer filtration efficiency greater than 95%.

Regeneration mechanisms:

52.Passive regeneration: occurs at high temperature (> 350 °C) in the presence of NO₂. Carbon reacts with NO₂ to form CO₂ and NO.

53.Active regeneration: fuel injection into the cylinder during the exhaust stroke (post-injection) or direct injection into the exhaust to raise the temperature to 550–650 °C.

Regeneration parameters:

Parameter	Typical Value
Soot threshold for regeneration	40–60% load
Target temperature	550–650 °C
Typical duration	20–45 minutes
Interval between regenerations	300–800 km (city) / 1500–3000 km (highway)

Soot load calculation:

Soot load is estimated by the engine control module (ECM) based on:

The pressure difference between the DPF inlet and outlet (ΔP)

Exhaust gas flow rate

Gas temperature

Driving history

The simplified formula: ΔP = k × Q² × (soot load)

Where k is a system constant and Q is the volumetric flow rate.

Selective Catalytic Reduction (SCR)

The SCR system injects urea (DEF - Diesel Exhaust Fluid) into the exhaust stream upstream of the SCR catalyst. The urea decomposes into ammonia (NH₃), which reacts with NOx to form N₂ and H₂O.

Main chemical reactions:

Urea hydrolysis: (NH₂)₂CO + H₂O → 2NH₃ + CO₂

NOx reduction: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

NO₂ reduction: 6NO₂ + 8NH₃ → 7N₂ + 12H₂O

DEF composition:

32.5% high-purity urea

67.5% demineralized water

Freezing point: -11 °C

Density: 1.09 g/cm³ at 20 °C

Dosing requirements:

The typical dosing ratio is 2 to 4% of fuel consumption. For an engine consuming 40 L/100 km, DEF consumption will be approximately 1 to 1.6 L/100 km.

DEF quality:

The ISO 22241 standard defines quality requirements. DEF must be stored between 0 °C and 25 °C, away from direct light. Contamination by metals or impurities can damage the SCR catalyst.

AMOX Catalyst

The AMOX is placed downstream of the SCR to oxidize excess ammonia (NH₃) into N₂ and H₂O. It prevents ammonia from being released into the atmosphere (a phenomenon called ammonia "slip").

Sensors and Actuators

Differential Pressure Sensor (DPF)

The sensor measures the pressure difference between the DPF inlet and outlet. Two stainless steel tubes connect the sensor to the exhaust system. Clogged tubes can cause erroneous readings.

Reference values:

Condition	Typical ΔP
Empty (clean) DPF	1–3 kPa
DPF at 50% load	5–10 kPa
Clogged DPF	> 15 kPa

Temperature Sensor

Type K thermocouples are commonly used. They produce a voltage proportional to temperature (approximately 41 µV/°C). Exhaust temperature sensors must be checked with a multimeter and compared against a reference source.

NOx Sensor

The NOx sensor is typically mounted downstream of the SCR. It uses electrochemical cell technology to measure NOx concentration in the gas. A high reading downstream of the SCR may indicate:

Insufficient DEF dosing

A degraded SCR catalyst

A faulty sensor

An air leak in the exhaust system

DEF Quality Sensor

The quality sensor measures the urea concentration in the DEF tank using an ultrasonic transducer. It also detects temperature and level. An out-of-specification concentration (less than 30% or more than 35%) triggers vehicle performance derating.

Canadian Standards and Regulations

Emissions Standards

In Canada, emissions standards for heavy-duty vehicles are aligned with US EPA (Environmental Protection Agency) standards. The main requirements:

Tier 4 standards for off-road engines (equivalent to EPA 2010 standards)

EPA 2010 standards for on-road engines: NOx ≤ 0.20 g/bhp-hr, PM ≤ 0.01 g/bhp-hr

Euro VI standards (reference for some manufacturers): NOx ≤ 0.40 g/kWh, PM ≤ 0.01 g/kWh

Canadian Electrical Code

The Canadian Electrical Code, Part I (CSA C22.1-21) applies to vehicle electrical installations. Rule 8-200 concerns grounding conductors and bonding. For exhaust systems, this rule is relevant for:

Grounding of sensors and actuators

Protection against electrostatic discharge

Isolation of control circuits

CSA B149.1

The CSA B149.1 standard (Natural Gas and Propane Installation Code) applies to vehicles operating on compressed natural gas (CNG) or propane. The exhaust systems of these vehicles must comply with the requirements of this standard regarding:

Combustion gas evacuation

Engine compartment ventilation

Gas leak detection

Canadian Standards Association (CSA) Standards

The CSA B620 standard covers fuel tanks and fuel systems for highway vehicles. Although primarily focused on fuel, it has implications for exhaust systems regarding sealing and leak prevention.

Diagnostic Procedures

Exhaust Back Pressure Test

Required equipment:

Pressure gauge (0–100 kPa)

Adapter for pressure tap

Infrared thermometer

Procedure:

121.Install the pressure gauge on the pressure tap before the DPF

122.Start the engine and bring it to operating temperature

123.Record the pressure at idle (should be < 5 kPa)

124.Accelerate to 2000 RPM and record the pressure (should be < 15 kPa)

125.Compare with manufacturer specifications

Interpretation:

Symptom	Probable Cause
High pressure at idle	Clogged DPF, obstructed exhaust
High pressure under load	Clogged DPF, faulty turbocharger
Fluctuating pressure	Leak in the system, faulty sensor

SCR System Verification

DEF dosing test:

130.Check the DEF level and quality

131.Inspect the integrity of the tank and lines

132.Verify the operation of the dosing pump

133.Measure the DEF flow rate with a flow meter (if available)

134.Compare with the expected value: DEF flow rate = fuel consumption × 0.03 to 0.04

DEF circuit leak test:

Pressurize the circuit to 100 kPa with compressed air

Hold the pressure for 5 minutes

A pressure drop > 10 kPa indicates a leak

Sensor Diagnostics

Temperature sensor:

141.Disconnect the connector

142.Measure the sensor resistance (thermocouple: very low resistance)

143.Heat the sensor with a heat gun and check the voltage variation

144.Compare with manufacturer values

Differential pressure sensor:

146.Disconnect both tubes from the sensor

147.Apply a known pressure with a manual vacuum pump

148.Check the output voltage (typically 0.5–4.5 V for 0–20 kPa)

149.Check the integrity of the tubes (no leaks, no blockages)

Preventive Maintenance

Visual Inspection

Check the condition of exhaust hangers and isolators

Look for signs of leaks (black deposits, soot traces)

Check the condition of flexible couplings and gaskets

Verify the tightness of clamps and flanges

DPF Replacement

The DPF must be replaced when:

Regeneration can no longer reduce the soot load

The substrate is cracked or melted

The pressure difference remains high after cleaning

The DPF has exceeded its useful service life (generally 400,000–800,000 km)

DPF Cleaning

Cleaning can be performed by:

Thermal cleaning: oven at 600 °C to burn off soot

Hydraulic cleaning: high-pressure washing with specific solutions

Combined cleaning: thermal followed by hydraulic

Common Pitfalls to Avoid

168.Confusing the functions of the DOC and the DPF: the DOC oxidizes gases, the DPF filters particulates. They are not interchangeable.

169.Neglecting the differential pressure sensor tubes: clogged or blocked tubes give false readings and can trigger unnecessary regenerations.

170.Using poor-quality DEF: contamination can destroy the SCR catalyst. Always use DEF that complies with ISO 22241.

171.Forgetting to check the DEF level during diagnosis: an empty tank is the most frequent cause of SCR system failure.

172.Confusing active and passive regeneration: passive regeneration requires high temperature and NO₂; active regeneration uses fuel post-injection.

173.Ignoring performance derate codes: modern vehicles progressively reduce power when the aftertreatment system is faulty. Do not confuse this with an engine problem.

174.Measuring back pressure at the wrong location: the measurement must be taken before the DPF, not after.

175.Not respecting torque specifications: exhaust connections must be tightened to manufacturer specifications. Over-tightening can deform flanges.

176.Forgetting to check for software updates: manufacturers regularly publish ECM software updates to improve aftertreatment management.

177.Confusing emissions standards: Tier 4 standards apply to off-road engines, EPA 2010 standards to on-road engines. The limit values are different.

Summary

The modern exhaust system includes the manifold, turbocharger, DOC, DPF, SCR, and AMOX.

The DOC oxidizes CO and HC; the DPF filters particulates; the SCR reduces NOx with the help of DEF.

DPF regeneration can be passive (high temperature + NO₂) or active (fuel post-injection).

DEF is a 32.5% urea solution that decomposes into ammonia to reduce NOx.

Key sensors are: differential pressure, temperature, NOx, DEF quality.

Canadian standards are aligned with US EPA standards for heavy-duty vehicles.

The Canadian Electrical Code, Part I (Rule 8-200) applies to vehicle electrical circuits.

Exhaust back pressure is a key indicator of DPF condition.

Diagnosis should begin with checking fault codes, then sensors, then actuators.

Preventive maintenance includes visual inspection, sensor verification, and periodic DPF cleaning.

Self-Assessment Questions

190.What is the typical active regeneration temperature of a DPF?

a) 250 °C

b) 350 °C

c) 550–650 °C

d) 800 °C

195.What is the percentage of urea in DEF?

a) 20%

b) 32.5%

c) 50%

d) 67.5%

200.Which sensor is used to estimate the soot load of the DPF?

a) Temperature sensor

b) Differential pressure sensor

c) NOx sensor

d) Mass air flow sensor

205.Which Canadian standard applies to vehicle electrical installations?

a) CSA B149.1

b) CSA B620

c) Canadian Electrical Code, Part I

d) ISO 22241

210.What is the typical DEF-to-fuel consumption ratio?

a) 0.5–1%

b) 2–4%

c) 5–8%

d) 10–15%

Answers: 1-c, 2-b, 3-b, 4-c, 5-b

Exhaust, Emissions, and Aftertreatment Systems