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Study notes

Qwestrum Engineering360 · Automotive & Manufacturing · Internal Combustion Engines

Combustion and Emissions

Combustion quality controls power, while temperature and oxygen availability control regulated emissions.

Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

HC unburned fuel; CO incomplete combustion
EGR reduces NOx by lowering peak T
Particulate filter DPF for diesel PM

Topic details

Introduction

Heywood and Bosch both emphasize that emissions are not independent outputs; they are direct consequences of in-cylinder combustion phasing and mixture formation. Indian university questions typically connect source formation and after-treatment conversion in one integrated answer.

Key relations & formulas

Formulas (Indian textbook notation)

$CO_{2} from carbon mass balance in fuel$

Formulas (Indian textbook notation)

$NOx forms at high flame temperature$

Formulas (Indian textbook notation)

$three-way catalyst: CO + O_{2} → CO_{2}; NOx + CO → N_{2} + CO_{2}$

Notation and sign conventions

Relation 1 —

CO_{2} from carbon mass balance in fuel

Formulas (Indian textbook notation)

$CO_{2} from carbon mass balance in fuel$

Write this relation with symbols exactly as in Internal Combustion Engines — V. Ganesan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

NOx forms at high flame temperature

Formulas (Indian textbook notation)

$NOx forms at high flame temperature$

Write this relation with symbols exactly as in Internal Combustion Engines — V. Ganesan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

three-way catalyst: CO + O_{2} → CO_{2}; NOx + CO → N_{2} + CO_{2}

Formulas (Indian textbook notation)

$three-way catalyst: CO + O_{2} → CO_{2}; NOx + CO → N_{2} + CO_{2}$

Write this relation with symbols exactly as in Internal Combustion Engines — V. Ganesan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

NOx rises with high local temperature and oxygen residence time, so EGR and injection timing are primary in-cylinder controls. CO and HC increase when oxidation is incomplete, while particulate emissions are dominant in diffusion-controlled diesel zones; after-treatment elements then trade conversion efficiency against backpressure and thermal window.

Assumptions and validity limits

State assumptions explicitly before using any relation for combustion and emissions — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In IC Engines (Automotive) viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in IC Engines (Automotive) papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to combustion and emissions.
4. Use equation 1:

CO_{2} from carbon mass balance in fuel

.
5. Use equation 2:

NOx forms at high flame temperature

.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Combustion and Emissions appears in OEM powertrain development. In Indian automotive curricula this topic is tested because it connects theory to engine cycles and performance.

GATE and semester exams often combine combustion and emissions with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use combustion and emissions?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

Students frequently claim EGR reduces all pollutants, which is incorrect because PM and HC may worsen if calibration is poor. Another mistake is writing catalyst reactions without stating lambda window and light-off temperature conditions.

Quick revision checklist

Before attempting combustion and emissions problems, confirm you can:

1. HC unburned fuel; CO incomplete combustion
2. EGR reduces NOx by lowering peak T
3. Particulate filter DPF for diesel PM

Revise the solved examples in Internal Combustion Engines — V. Ganesan and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Simple carbon-to-CO2 estimate

Problem

If fuel contains 0.86 kg carbon per kg fuel and combustion is complete, estimate kg of CO2 produced per kg fuel.

Solution

C to CO2 mass ratio is 44/12. CO2 formed = 0.86 × (44/12) ≈ 3.15 kg CO2 per kg fuel.

Conceptual check — Combustion and Emissions

Problem

In a IC Engines (Automotive) semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of combustion and emissions." What should a complete answer include?

📖 Standard books (India)

Internal Combustion Engines — V. Ganesan
Read: Syllabus unit
Standard IC engine text in Indian universities

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