Fuel Metering

SI Engine Fuel Metering

• The SIE meters fuel into the air stream of amount

dictated by the speed and by the load

– This is usually done by either carburettor or by fuel injector

– It must satisfy the requirements of the engine over its

entire operating regime

– The optimum air-fuel ratio for a SIE is that which gives

required power output with the lowest fuel consumption,

consistent with smooth and reliable operation

– The constraints of emissions control may dictate a different

air-fuel ratio, and may also require the recycling of a

fraction of the exhaust gases (EGR) into the intake system


• The ratios of air and fuel are required for various

conditions of speed and load, which are:

– Idling and low-load range (throttle almost closed)

– Economy (cruise) or medium-load range (throttle partially

open)

– Power of full-load range (throttle wide open)


• Fuel system includes - fuel tank, fuel-level indicator, fuel

lines, fuel pump, fuel filter, air cleaner, throttle body, and

intake manifold

• Fuel pumps may be mechanical or electric (in-line or in-

tank)

– They deliver more fuel than required

– Fuel is pressurised - prevents vapour lock

• There are two main types of carburettor:

– Fixed jet (or fixed venturi)

– Variable jet (or variable venturi)

• Fuel injectors are also of two types:

– Single-point (or throttle-body (TBI)) injector

– Multi-point (or port-fuel (PFI)) injector


• The fuel metering system and manifold have to perform

satisfactorily in both steady-state and transient

conditions

– Fuel will be present in the intake manifold as vapour, liquid

droplets, and liquid film

• There are three basic parts of a carburettor – which are,

air horn, float bowl, and the throttle body

• Internal parts of a carburettor includes

– Float system

– Idle system and low speed system

– Main-metering system

– Accelerator pump system

– Choke system

Carbuettor Float System

Carbuettor Idling and Low-Speed System

Carbuettor Main Metering System

Carbuettor Accelerator Pump System

Carbuettor Choke System

Fixed Jet Carburettor

• Simple fixed jet or fixed venturi carburettor can only

sense air flow rate without distinguishing between fully

open throttle at a slow engine speed or partially closed

throttle at a higher engine speed

• The fuel outlet is at the smallest cross-sectional area so

that the maximum velocity promotes break-up of the

liquid jet and mixing with air; the minimum pressure also

promotes fuel evaporation

– The pressure drop at the maximum air flow rate is about

0.05 bar

– The maximum air flow is when the velocity at the venturi is

supersonic

Fixed Jet Carburettor

• To make allowance for the mixture becoming richer at

larger flow rates a secondary flow of fuel, which reduces

as the air flow rate increases, should be added

– A method of achieving this is the compensating jet and

emulsion tube

• When the choke system is in operation, fuel is drawn

from the float chamber directly, and the manifold acts as

a surface carburettor

Variable Venturi Carburettor

• Variable venturi carburettor is fitted with a piston

• When the throttle is opened, the air flow through the

venturi increases that decreases the pressure

downstream of the venturi and causes the piston to rise

until the pressure on the piston is balanced by its weight

and the force from a light spring

• A tapered needle attached to the piston moves in or out

in the main jet or orifice depending on the piston position

• For starting, extra fuel is provided by a lever that lowers

the jet

Multiple Barrel Carburettor

• Added barrel or barrels improve engine breathing or

volumetric efficiency, especially at high speed

– Two-barrel carburettor

• In one system, each barrel supplies AF mixture for half of the

engine cylinders

• In a staged carburettor, the primary barrel supplies all

cylinders until its throttle valve opens more than 45 degrees -

improves medium to high speed operation

– Four-barrel carburettor – Basically two two-barrel staged

carburettors in a single assembly - one pair of barrels

makes up the primary side, the other two barrels make up

the secondary side

– Multiple carburettors are used in some high-performance

engines; some engines use separate carburettors for each

cylinder

Multiple Barrel Carburettor

• Electronically controlled carburettor

– Operation carburetted electronic engine control system is

very similar to EFI system

– The major difference is that the ECM sends pulses to a

carburettor mixture control solenoid instead of to a fuel

injector

– mixture control solenoid controls mixture richness

– solenoid replaces the vacuum- or mechanically-operated

power system - a plunger extends from the solenoid, the

tipped end acts like the metering rod

– the ECM turns the solenoid on and off very rapidly by

grounding and ungrounding the solenoid coil

EFI System in SI Engines

• Carburettors atomize the fuel by processes relying on

the air speed being greater than the fuel speed at the

fuel nozzle; they also meter the fuel using the air flow as

the independent variable

• Fuel injection differs in both respects

– The fuel speed is greater than the air speed to atomize the

fuel and the fuel is metered proportionally to air flow but

not by air flow itself – a pump is used to generate the

pressure difference necessary to flow the fuel

• Fuel injection system can be classified according to the

following characteristics:

– Single point – one fuel injector serves more than one

cylinder


– Multipoint – one fuel injector per cylinder

– Electronic – the metering of the fuel by solenoid action

– Mechanical – the metering of the fuel by cam actuation

– Port – fuel is sprayed into the air stream at the inlet port

– In-cylinder – fuel is sprayed directly into the cylinder

– Low pressure – fuel is injected into gas at a pressure level

on the order of the intake pressure

– High pressure – fuel is injected near the end of

compression into gas at a pressure level on the order of

the compression pressure

– Timed or pulse – each injection has a finite duration,

injection is timed to begin and end at specific times in the

cycle


– Continuous or steady – fuel flows through the injector

through all times in the engine cycle; fuel is metered by

controlling the pressure upstream of the fuel injectors

• Some advantages of fuel injection system over

carburetted system are:

– Increased volumetric efficiency and therefore increased

power and torque

– Better thermal efficiency because of improvements

realized in mixture control

– Lowest exhaust emissions because atomization does not

depend on cranking speed, and also fuel supply may be

cut off during deceleration

– More fuel tolerant because distribution can be independent

of vaporization and therefore of the fuel volatility


• With improved mixture distribution, the probability that one

cylinder is more prone to knock than the others is reduced so

a lower octane fuel or higher compression ratio can be used

– Charge stratification – with timed fuel injection late in the

intake process or during the compression stroke, it is

possible to stratify the charge

• A rich mixture near the spark plug ensures easy ignition, a

lean end gas reduces the tendency to knock, and a lean

overall charge has favourable emission characteristics

– Elimination of short circuited fuel in turbo or supercharged

four-stroke engines and in all two-stroke engines since the

injection can be timed to begin once the exhaust valve or

port is closed


• Normally, electronic Fuel Injection (EFI) system uses

electronic engine control (EEC), sometimes called the

electronic control module (ECM)

• Injectors are fitted with solenoid operated valves

• PFI can deliver fuel more accurately - improves fuel

economy and engine performance

• EFI system contains, among others

– Fuel injector

– Different sensors – throttle position sensor, oxygen sensor,

intake-air temperature sensor, manifold absolute pressure

(MAP) sensor, etc.

– Relay controller

– Fuel pump inertia switch


Single-point,

continuous

fuel injection

system


Mechanical

fuel injection

system


• Signals from various electronic sensors come into the

ECM - fuel injection is metered accordingly

EFI System in SI Engines – Injector

CI Engine Injection System

• Diesel fuel injection systems are high pressure for two

reasons:

– The fuel pressure must be greater than the compression

pressure in order to inject the fuel into the cylinder at the

time combustion is to commence

– To impart a high fuel velocity relative to the air so that the

maximum stable size of the droplets will be small enough

that the fuel can evaporate quickly so as not to imepede

the ignition delay

• CI engines use a common rail system (CRS) for fuel

injection

– One pump delivers the fuel to each injector from a fuel

distributing manifold


When the cam allows the injector

plunger to rise, fuel enters the

orifice (A)

Some of the fuel enters the cavity

ahead of the plunger through the

metering orifice (B) – the greater

the fuel pressure, the greater is

the mass of fuel that enters the

cavity

Most of the fuel (80% or so)

passes through the drain and

return to tank (C)

The bypass flow serves to cool

the injector


• The only high pressure in the system is in the injector;

metering and distribution takes place at a low pressure in

the common rail

• In the high-pressure system, the plunger, called a needle

valve, is lifted rather than forced down which opens a

flow path through the nozzle


• In an electronically controlled fuel

injector, metering is initiated by

actuation of a solenoid operated

three-way valve

– The rail pressure is then applied to

the low pressure piston which is

directly connected to the high

pressure plunger

– The cross-sectional area of the low

pressure piston is 15 times that of the

high pressure piston – thus the high

pressure plunger will have a pressure

of 15 times that of common rail

pressure – therefore, called intensifier


• Another CI engine injection system employs a positive

displacement pump so that the mass injected is

independent and the fuel pressure adjusts itself

accordingly – known as the jerk-pump system

– The mass of fuel injected is controlled by varying the

displacement of the pumping plunger

Fuel Metering

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