7TOPICSpark Ignition Engines

What do we need from engines?High power

Low fuel consumption

Low emissions

All these depend on COMBUSTION

In turn, combustion process is a function of:Air/fuel ratioIgnition timingCombustion chamber designEngine speedEngine load

Let us learn step-by-step starting from

Part IMixture preparation

Sections 7.1 to 7.3

Power of SI Engine is controlled by amount of mixture inducted

Mixture requirements are different for full-load (wide-open throttle) and for part-load operation

At WOT we are interested in maximum power; all inducted air must be utilized

At part-load operation fuel must be utilized efficiently

At very light load mixture must be fuel-rich for stable combustionSI Engine mixture requirements

Response of specific fuel consumption and power output to changes in air/fuel ratio at WOT and constant speed

Specific fuel consumption vs. power output for varying air/fuel ratio at different throttle settings(fish-hook curves)

SI Engine mixture requirementsAir-fuel mixture must be:

slightly rich for maximum power output

slightly lean for best efficiency at partial loads

rich enough for idling operation

satisfy emissions required by regulations

Elementary carburetor, how does it work?Inlet sectionVenturi throatFloat chamberPressure equalizing passageCalibrated orificeFuel discharge tubeThrottle plateDoes it meet engine requirements?

Performance of elementary carburetorPerformance of elementary carburetorAt low loads mixture becomes leaner; engine requires enriched mixture

At partial loads equivalence ratio increases slightly as air flow increases; engine requires almost constant equivalence ratio

As air flow approaches the maximum WOT value, equivalence ratio remains essentially constant. Engine requires enriched mixture for maximum power

Elementary carburetor cannot compensate for transient phenomena in the intake manifold. Nor can it enrich the mixture during engine starting and warm-up

Elementary carburetor cannot adjust to changes in ambient air density due to changes in altitude

Changes required in elementary carburetorThe main metering system must be compensated to provide essentially constant lean or stoichiometric mixtures over 20 to 80 percent air flow rateIdle system must be added to meter fuel flow at idle and light loadsAn enrichment system must be added so the engine can provide its maximum power at WOTAn accelerator pump which injects additional fuel when throttle is opened rapidly is requiredA choke must be added to enrich mixture during engine starting to ensure a combustible mixtureAltitude compensation is required to adjust the fuel flow to changes in air density

Modern carburetor designMain venturiBoost venturiMain metering spray tube or nozzleAir-bleed orificeEmulsion tube or wellMain fuel-metering orificeFloat chamberThrottle plateIdle air-bleed orificeIdle fuel orificeIdle mixture orificeTransition orificeIdle mixture adjusting screwIdle throttle setting adjusting screw

Fuel injection systemsFuel injection systems for conventional spark-ignition engines inject fuel into intake system

With multipoint systems fuel is injected into intake port of each cylinder

In single-point systems fuel is injected into air flow directly above the throttle body

In modern SI engines with advanced combustion systems fuel is injected directly into the cylinder

Multipoint vs. single-pointIncreased power and torque through improved volumetric efficiency and more uniform fuel distribution

More rapid engine response to changes in throttle position

More precise control of the equivalence ratio during cold-start and engine warm-up

Multipoint port injectionSpeed-density electronic multipoint port fuel-injection system: Bosch D-Jetronic

Multipoint port injectionElectronic multipoint port fuel-injection system with air-flow meter: Bosch L-Jetronic

Advantages of direct air-flow measurementsAutomatic compensation for tolerances, combustion chamber deposit buildupDependence of volumetric efficiency on speed and exhaust backpressure is automatically accounted forLess acceleration enrichment is required because the air-flow signal precedes the filling of the cylindersImproved idling stabilityLack of sensitivity of the system to EGR since only the fresh air flow is measured

Multipoint port injectionMechanical multipoint port fuel-injection system: Bosch K-Jetronic

Single-point throttle-body injectionTwo-injector throttle-body electronic fuel-injection systemADVANTAGELow costDISADVANTAGESlower transport of the fuel than the air from upstream of the throttle plate to the cylinder

Engine performance depend on air/fuel ratio. Mixture must byslightly rich for maximum power outputslightly lean for best efficiency at partial loadrich enough for idling operationElementary carburetor does not provide required air-fuel ratioSome supplementary systems are added to correct the characteristics of an elementary carburetorFuel injection systems are used to substitute carburetorsSummary of Mixture preparation

Fuel injector

Exhaust oxygen sensor

Air mass flow rate through the carburetor Venturi

or in terms of the pressure drop pa = po pT

whereFlow through the Venturi

Mass flow rate of the fuel

To prevent fuel spillage when the engine is inclined, the fuel level in the float chamber is held below the fuel discharge nozzle. Thus

where h is typically of order 10 mmFlow through the fuel orifice

Carburetor performanceAir/fuel ratio delivered by carburetor

and equivalence ratioback

Carburetor double-venturi systemback

Main metering system with air-bleed compensationback

Metering characteristics of system with air-bleed compensation