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Combustion in SI & CI Engine
Combustion in SI And CI Engine
Combustion is the chemical reaction between a hydrocarbon fuel and oxygen that liberates energy in the form of heat and light. For hydrocarbon fuels used in spark ignition (SI) engines (petrol), the chemically correct (stoichiometric) air-fuel ratio is approximately 1 : 15 by mass. The practical ignition limits for petrol are typically around minimum fuel-air ratio = 1 : 30 (very lean) and maximum fuel-air ratio = 1 : 7 (very rich).
Stages of Combustion in SI Engine
- Stage I - Ignition lag (or delay): The time interval between the spark discharge and the beginning of appreciable combustion (rise of pressure) is called the ignition lag. It includes the time required for the formation of a flammable mixture in the spark region and the initial chemical reactions that lead to a self-sustaining flame kernel.
- Stage II - Flame propagation: After formation of the flame kernel, a flame front propagates through the unburnt mixture. Turbulence, mixture composition and pressure-temperature conditions determine the flame speed. The start of flame propagation is seen as the sudden rise of pressure in the cylinder.
- Stage III - After burning: This final stage is the burning of residual or less-volatile portions of the charge and slow completion of chemical reactions. It contributes to tailing of the pressure rise.
Effect of engine variables on ignition lag
| Parameter increased | Effect on ignition lag |
|---|---|
| Self‐ignition temperature (of fuel) | Increases ignition lag |
| Initial temperature and pressure | Decreases ignition lag |
| Turbulence | Decreases ignition lag (faster flame kernel formation) |
| Compression ratio | Decreases ignition lag (higher temperature and pressure at TDC) |
| Engine speed | Ignition lag measured in time (ms) decreases; measured in crank angle may increase |
| Advancing spark timing | Reduces required ignition lead (effective ignition lag in crank angle decreases) |
Effect of engine variables on flame propagation (flame speed)
| Parameter increased | Effect on flame propagation (flame speed) |
|---|---|
| Compression ratio | Increases flame speed |
| Initial temperature and pressure | Increases flame speed |
| Engine load | Increases flame speed (richer mixtures up to optimum) |
| Turbulence (moderate) | Increases flame speed |
| Engine speed | Generally increases turbulence and hence flame propagation, though detailed effect depends on geometry |
Illustration: flame propagation before self‐ignition and after self‐ignition (schematic behaviour of flame front).
Combustion with and without knocking
- Detonation or knocking: If a portion of the unburnt charge auto‐ignites (without the flame front reaching it) because of high local temperature or pressure, a pressure wave is generated and intense metallic‐pinging vibrations occur. This phenomenon is called knocking or detonation. It is caused by uncontrolled auto‐ignition of parts of the mixture ahead of the propagating flame front.
Effect of engine variables on knocking tendency (SI engine)
| Parameter increased | Tendency to knock |
|---|---|
| Compression ratio | Increases |
| Supercharging (intake pressure) | Increases |
| Initial air temperature and pressure | Increases |
| Engine load | Increases |
| Advancing spark timing | Increases |
| Turbulence | Decreases (turbulence promotes smooth burning and reduces unburnt hot spots) |
| Engine speed | Generally decreases tendency to knock |
| Octane number | Decreases (higher octane reduces knocking) |
| Length of hydrocarbon chain | Increases (straight long‐chain paraffins tend to knock more) |
- Maximum knocking in SI engines often occurs for a mixture slightly richer than stoichiometric (approximately 10% richer than stoichiometric in many practical cases). Making the mixture significantly leaner or richer than this tends to reduce knocking tendency.
Other ignition phenomena in SI engines
- Surface ignition: Ignition initiated by a hot surface (hot spot) such as hot deposits on spark plug, valve heads or cylinder walls. Surface ignition is independent of the spark.
- Pre‐ignition: If a hot surface ignites the charge before the spark event, it is called pre‐ignition. Pre‐ignition can severely damage the engine because combustion begins too early.
- Squish: Rapid expulsion (squishing) of gas from the piston-crown periphery into the combustion chamber recess as piston approaches TDC. Squish produces strong turbulence that shortens flame travel and reduces knocking tendency.
Combustion chamber design principles (SI engines)
- Use a large inlet valve area to improve volumetric efficiency.
- Maintain an appropriate surface‐to‐volume ratio: lower surface area relative to volume reduces heat loss but careful shaping reduces flame‐travel time.
- Position the spark plug centrally to shorten flame travel and reduce tendency for surface ignition; place the exhaust valve away from hot walls to avoid hot spots.
Common types of combustion chambers
- T‐head combustion chamber: Long passageways cause longer flame travel and higher knocking tendency; used historically in early engines.
- L‐head combustion chamber (side‐valve): Valves on the side; used for low compression ratio engines; lower manufacturing cost but poorer breathing and flame travel.
- I‐head combustion chamber (overhead valve / OHV): Valves in the head; suitable for higher compression ratios; generally lower surface‐to‐volume ratio, higher volumetric efficiency and shorter flame travel.
- F‐head combustion chamber: A mixed layout with some advantages of both side‐valve and overhead arrangements; designed to balance volumetric efficiency and combustion quality.
CI Engines (Compression Ignition) - Principles and Stages of Combustion
In a CI (diesel) engine air alone is compressed to a high compression ratio (typically 12 : 1 to 22 : 1), which raises the air temperature above the ignition temperature of diesel fuel. Fuel is injected into this hot, compressed air where it atomises, vaporises and mixes with air; ignition occurs by self‐ignition of the fuel-air mix.
Stages of combustion in CI engines
- I. Delay period (ignition delay): The interval between the start of injection and the start of combustion. It is commonly subdivided into:
- Physical delay: Time for atomisation, vaporisation and mixing of fuel with air.
- Chemical delay: Time required for the chemical reactions to proceed to ignition once the local conditions for ignition are reached.
- II. Rapid or uncontrolled combustion: Fuel accumulated during the delay period burns rapidly when ignition begins, producing a steep initial pressure rise; this phase is responsible for most of the indicated mean effective pressure if delay is long.
- III. Controlled combustion (diffusion burning): Combustion continues as more fuel is injected and burns in a diffusion‐controlled manner at near‐constant pressure for direct‐injection diesel engines (rate depends on atomisation and mixing).
- IV. After burning: Combustion of less volatile fuel fractions and any remaining spray droplets; completes the combustion process and reduces smoke.
Effect of variables on the delay period in CI engines
| S.No | Increase in variable | Effect on delay period | Reason |
|---|---|---|---|
| 1 | Cetane number of fuel | Reduces | Higher cetane number fuels ignite more readily (lower chemical delay) |
| 2 | Injection pressure | Reduces | Finer atomisation increases surface‐to‐volume ratio and speeds vaporisation |
| 3 | Injection advance (earlier injection) | Increases delay period (measured in crank angle) | Lower temperatures and pressures at the time of injection start increase the time to ignition |
| 4 | Compression ratio | Reduces | Higher compression raises air temperature and pressure, reducing auto‐ignition time |
| 5 | Intake temperature | Reduces | Hotter intake air reduces fuel vaporisation time and chemical delay |
| 6 | Jacket water temperature | Reduces | Warmer cylinder walls raise ambient air temperature and reduce delay |
| 7 | Fuel temperature | Reduces | Warmer fuel vaporises more readily and mixes faster with air |
| 8 | Intake pressure (supercharging) | Reduces | Higher density increases local pressures and temperatures and shortens delay |
| 9 | Engine speed | Delay reduces in milliseconds but increases when expressed in crank angle | Less time per cycle (ms) but for given crank angle combustion conditions change |
| 10 | Load (fuel-air ratio) | Generally reduces | Higher load increases cylinder pressure and temperature |
| 11 | Engine size | Little change in ms; crank‐angle effect depends on rpm | Geometric and thermal inertia differences |
| 12 | Type of combustion chamber | Varies; pre‐combustion chamber often shows lower delay | Chamber geometry affects mixing and turbulence |
Knocking in CI engines
- If the ignition delay is long, a larger quantity of fuel accumulates in the combustion chamber before ignition. The sudden auto‐ignition of this accumulated fuel produces a high rate of pressure rise and may cause knocking in diesel engines.
- All factors that increase the delay period tend to increase the knocking tendency in CI engines.
Comparison of knocking phenomenon in SI and CI engines
| SI engine | CI engine |
|---|---|
| Knocking occurs towards the end of combustion | Knocking occurs at the beginning of combustion (if delay is long) |
| Rate of pressure rise is very high due to homogeneous mixture | Rate of pressure rise is usually lower because the mixture is heterogeneous |
| Knocking often associated with short ignition delay (rapid auto‐ignition of end gas) | Knocking associated with long delay period (large fuel mass accumulated) |
| Knocking easily audible and detectable | Knocking not always easily detectable by ear |
The factors that reduce knocking tendency in SI engines often have the opposite effect in CI engines and vice versa. Engine calibration and fuel selection should therefore be matched to the type of engine.
| Factors | SI engine (effect) | CI engine (effect) |
|---|---|---|
| Self‐ignition temperature | High (helps avoid auto‐ignition) | High (longer delay - undesirable) |
| Delay period | Short | Long |
| Compression ratio | Lower to avoid knock | High for ignition and efficiency |
| Intake temperature | Lower to reduce knock | Higher to reduce delay |
| Engine speed | High speed reduces knocking tendency | Effect varies; transient and thermal effects are important |
| Octane number / Cetane number | High octane reduces SI knock | High cetane reduces CI delay and knock |
Turbulence, Swirl and Squish - definitions and role
- Turbulence: Disordered, random fluid motion that increases mixing rates, accelerates combustion and helps break flame nuclei so as to induce flame propagation.
- Swirl: An organised rotational motion of the charge about the cylinder axis. Swirl helps supply fresh air to burning droplets and assists evacuation of combustion products from the spray region, promoting better mixing and cleaner combustion.
- Squish: Secondary radial inward movement of air produced when the piston crown approaches the cylinder head and squeezes the gas into a combustion recess. Squish produces strong turbulence and reduces flame travel distances.
CI engine combustion chamber types and swirl generation
- Open combustion chamber with induction swirl: Swirl is produced by directing the incoming air through ports or passages during the intake stroke. Useful for large, low‐speed engines and for cold starting. Less suited to wide speed ranges.
- Divided combustion chamber with compression swirl: Air is forced into a separate swirl chamber through tangential passages during compression; a single‐pintle injector injects into the swirl chamber. Good for variable speed operation and higher volumetric efficiency, but cold starting can be more difficult and manufacturing cost is higher.
- Pre‐combustion chamber: A small portion of the fuel is ignited in a pre‐chamber producing high turbulence jets that ignite the main chamber. Pre‐chambers improve ignition and multi‐fuel capability but suffer higher heat loss.
Auxiliary systems
- Glow plug: An electric heater used to raise the temperature of the combustion chamber or intake port to assist cold starting of diesel engines. It is typically used for a short time before and just after cranking in cold conditions.
The document Combustion in SI & CI Engine is a part of the Mechanical Engineering Course Mechanical Engineering SSC JE (Technical).
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FAQs on Combustion in SI & CI Engine
| 1. What is the difference between combustion in SI and CI engines? |  |
Ans. Combustion in SI engines (spark ignition) is initiated by a spark plug, while combustion in CI engines (compression ignition) is initiated by the high temperature of compressed air.
| 2. What is knocking in combustion and how does it affect engine performance? | |
Ans. Knocking in combustion refers to the abnormal combustion of fuel in the engine, causing a knocking sound. It can lead to engine damage, loss of power, and decreased fuel efficiency.
| 3. What are the stages of combustion in an engine? | |
Ans. The stages of combustion in an engine are intake, compression, power, and exhaust. During these stages, fuel is mixed with air, compressed, ignited, and the resulting energy is converted into mechanical work.
| 4. How does combustion differ when knocking occurs in a CI engine? | |
Ans. When knocking occurs in a CI engine, the combustion process becomes erratic and uncontrolled, leading to increased pressure and temperature inside the cylinder. This can cause engine damage and reduce efficiency.
| 5. How can knocking be prevented in combustion engines? | |
Ans. Knocking can be prevented in combustion engines by using higher octane fuel, optimizing the air-fuel mixture, adjusting ignition timing, and ensuring proper engine maintenance.
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