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Fuel Systems

Fuel Systems1. Carburetors

2. TBI

3. PFI

4. CPFI

5. DFI

6. Diesels

7. Forced Induction

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Carburetors

Mechanical devices that use pressure differential to meter and vaporize fuel for engine demands

Contains 6 circuits designed to deliver air/fuel mixture for different operating modes

Float/choke/idle/acceleration/main/power

Mounted on top of intake manifold

Receives fuel from mechanical pump driven by camshaft

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Carburetors

Low fuel pressure: 3-5 psi

Fuel is vaporized inside venturis and travels through intake manifold to cylinders

4 cyls: 1-2 venturis (barrels)

6 & 8 cyls: 2-4 venturis

80’s & 90’s:

Higher fuel costs

Increasingly tight emission laws

Onboard computers

4

TBI (Central Fuel Injection)

1 or 2 injectors mounted inside throttle body on top of intake manifold

Injector is a simple on/off valve controlled (pulsed) electronically by onboard computer (ECM)

Fuel is sprayed into intake stream above throttle plate- vaporization takes place inside throttle body

Uses tank mounted electric fuel pump

Low pressure: 10-15 psi

Eliminates possibility of vapor lock

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TBI

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TBI

Advantages:

Simplicity

Low manufacturing cost

Low cost of maintenance

Disadvantages

Fuel traveling through intake manifold

Difficult to meter mixtures to individual cylinders

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PFI

One injector per cylinder

Injector is mounted in intake port in manifold or cylinder head

Injectors are connected by fuel rail

Injectors can be controlled as groups or individually

Higher pressure: 30-50 psi

Necessary to break up fuel particles

Vaporization takes place in front of intake valve

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PFI

PFI

Electric fuel pump mounted inside fuel tank

Fuel pressure in fuel rail is regulated and excess is returned to fuel tank

Fuel filter is usually mounted under body of car

Throttle body controls air only

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PFI

Advantages:

Precise control of fuel mixtures to cyls

No fuel traveling through intake manifold

Disadvantages:

Higher cost of manufacturing: complexity

Higher cost of maintenance

Injector location causes deposits

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CPFI

Designed as a “hybrid” system

Allows for precise fuel control of PFI

Simplicity of TBI

Uses central injector mounted inside intake manifold

Injector connects to nozzles that feed individual cyls

Injector is electronically actuated

Nozzles are activated by pressure

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CPFI

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CPFI

Advantages

Simplicity allows for low production cost

Low cost of maintenance

While unit is located inside intake manifold, fuel is injected at intake port

Disadvantages

Extremely sensitive to pressure changes

Plastic lines that connect injector with nozzles are prone to breakage

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DFI

Injector sprays fuel directly into combustion chamber

Uses much higher pressure: 2000-3000 psi

Fuel must overcome compression pressures

Vaporization must take place during injection

Uses electric feed pump inside fuel tank

Uses mechanical high pressure pump to increase fuel pressures

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DFI

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DFI

Advantages:

Allows for complete air fuel mixture control

Better atomization from higher pressures

Vaporization process cools combustion chambers allowing higher compression ratios

Disadvantages

Complexity

High cost of manufacture and maintenance

High pressure fuel pump

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Diesel Systems

Diesel fuel is heavier and requires much higher temperatures to ignite

Engine is designed to create high heat with compression

Fuel is injected once air is heated from compression

Ignition occurs once fuel is injected into chamber

“Compression ignition engine”

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Diesels

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Diesels

Fuel is injected directly into combustion chamber with precise timing

Uses very high pressures to overcome compression pressures and to provide atomization of fuel

Most systems use electric in-tank pumps to supply high pressure injection pumps

Injection pressures can be created by pumps or injectors themselves, both driven by engine camshaft

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Diesels

Pressures:

Supply pressures: 5 to 75 psi

Injection pressures: 3000-40000 psi

Injection timing is critical for:

Power

Noise

Smoke

Air is unthrottled

Engine speed is controlled by fuel delivery alone

Engine acceleration is slower than spark ignition engine

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Diesels

Advantages

Extremely high engine output (torque)

Higher energy content of fuel allows for better economy

Disadvantages

Higher emissions caused by high combustion chamber temperatures

High compression ratios require expensive internal engine components

Fuel systems components are precision and expensive

Different operating characteristics: slower acceleration

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Diesels

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Diesels

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Superchargers

Uses engine power to pressurize intake manifold

Boost pressures on stock engines: 7-10 psi

Forces more air into engine creating more power

Usually found on V-type engines

Simplified plumbing

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Superchargers

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Turbochargers

Exhaust driven turbine forces air into intake manifold

Same operating principles as supercharging

Usually found on inline engines

Plumbing complexity

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Turbochargers

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Intercoolers

Cools compressed air going into intake manifold

Cooler air= more dense charge

More air= more power

Cooler is mounted in front of radiator and transfers heat to ambient air

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Intercoolers

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