MECH9761 Chap.5 Spark Ignition Combustion and Air Management
Spark Ignition Combustion and Air Management
This chapter follows the causal chain of spark-ignition combustion: mixture preparation, ignition and turbulent flame propagation. It compares port and direct injection, explains how boosting and valve timing manage trapped air, and links tumble breakdown, spark timing and chamber geometry to burn rate, torque, efficiency and emissions. Every technology is evaluated at an operating condition rather than as a universal winner.
The chapter therefore treats hardware as a set of interacting controls whose benefit, penalty and calibration region must be stated together.
What this chapter covers
- 01
Port injection compared with direct injection
- 02
Charge cooling, atomisation, wall wetting and particulates
- 03
Turbochargers, superchargers and volumetric efficiency
- 04
Variable intake-valve timing and trapped charge
- 05
Tumble breakdown into turbulence near ignition
- 06
Spark timing, flame propagation and best torque
Worked example · free
Select an injection mode for two conditions
- +1At low load, port injection provides longer mixing time and lower particulate tendency.
- +1It also supports low noise under the course operating-map description.
- +1At maximum power, combined port and direct injection increases total fuel-delivery capacity.
- +1The direct portion adds charge cooling, while calibration must control locally rich zones and wall wetting.
Key terms
- Port Fuel Injection
- Low-pressure fuel delivery into the intake path before charge induction.
- Direct Fuel Injection
- High-pressure fuel delivery directly into the cylinder.
- Charge Cooling
- Temperature reduction caused by fuel vaporisation within the inducted charge.
- Tumble Vortex
- Large-scale in-cylinder charge motion that can break down into turbulence near ignition.
- Best Torque Timing
- The spark timing that places combustion to maximise torque under the stated operating condition.
- Flame Quenching
- Suppression of flame reactions near a relatively cool combustion-chamber surface.
- Turbocharger
- An exhaust-driven compressor system used to raise intake charge density.
- Supercharger
- A mechanically driven compressor that raises intake pressure while consuming shaft work.
- Volumetric Efficiency
- A measure of inducted fresh charge relative to a displacement-based reference amount.
- Spark Advance
- Ignition timing placed earlier in crank angle relative to top dead centre.
- Pool Fire
- Particulate-producing combustion of liquid fuel accumulated on a chamber surface.
Spark Ignition Combustion and Air Management FAQ
Why can direct injection increase particulate emissions?
Direct delivery shortens the available mixing time and can create locally rich pockets. Spray impingement may wet the piston or liner and form a liquid pool that burns incompletely. Higher injection pressure improves atomisation but does not remove the need for careful targeting, timing and wall-temperature control.
How does tumble improve combustion?
Tumble is organised large-scale motion during intake and compression. Near ignition it should break down into small-scale turbulence, which wrinkles the flame surface and accelerates propagation. Its benefit depends on both intensity and timing, because turbulence that decays before the spark cannot support the intended burn rate.
Why is more ignition advance not always better?
Advancing from a retarded condition can place pressure rise more usefully during expansion and raise work. Excessive advance creates high pressure too early, increasing negative compression work and knock or component constraints. Best timing is therefore an operating-point result rather than the earliest possible spark.
When can port and direct injection be combined?
Port delivery can provide long mixing time at lower demand, while direct delivery adds charge cooling and fuel-flow capacity at high load. A combined strategy can use both benefits across an operating map. The split still depends on particulate control, wall wetting, targeting, pressure and available preparation time.
How do turbochargers and superchargers differ?
A turbocharger recovers exhaust-flow energy to drive its compressor, while a supercharger takes mechanical drive from the engine. Turbo systems must manage response and back pressure; superchargers impose direct shaft demand. Both can increase trapped air, but their costs and low-speed behaviour differ.
What can intake-valve timing change?
Changing intake opening or closing alters gas exchange and the amount of fresh charge retained for compression. The useful setting depends on speed and load because flow inertia and available filling time change. An answer should name the event, trapped-charge consequence and pumping effect rather than claim a universal efficiency gain.
Why does chamber shape affect flame travel?
Chamber geometry sets the distance from spark location to the far regions, influences tumble and turbulence, and determines wall area near the reacting charge. A central spark and compact path can shorten propagation, while cool walls can quench reactions. The outcome still depends on mixture quality and timing.
Exam move
Create three linked maps. The first runs from injector location through evaporation, mixture uniformity and particulate risk. The second runs from exhaust energy or mechanical drive through boost, trapped charge, pumping and torque. The third runs from tumble through turbulence, spark, flame travel, pressure phasing and work.
For each technology, practise one benefit, one penalty and the speed or load region where the comparison is valid. Redraw each map from its final outcome back to the controlling hardware so causal gaps become obvious. Compare port, direct and combined injection at low and high load. Then explain a spark-timing sweep using start of combustion, peak pressure, CA50 and integrated work in that order.
Finish every response with the operating condition that limits the recommendation.
