This document summarizes fuel systems for spark ignition (SI) engines. It discusses the requirements of a good fuel system, including proper air-fuel ratios for different engine operating conditions. It then describes the basic components and functioning of carburetor systems, including single jet and complete carburetors. It notes the limitations of simple carburetors and discusses additional systems in complete carburetors to overcome these limitations, such as compensating jets, emulsion tubes, back suction controls, auxiliary valves/ports, idling systems, power enrichment systems, and chokes. It briefly introduces throttle body fuel injection and multi-point fuel injection as alternatives to carburetion before petrol/electronic injection systems.

1. FUEL SYSTEM For SI ENGIES
Prof. N.R. Kumbhar

2. Introduction
 The task of the engine induction and fuel systems is to
prepare an air-fuel mixture that satisfies the requirement of
the engine.
 This preparation is to be carried out over entire engine
operating system.
 In principle, the optimum air-fuel ratio for an engine is that
which give the required power output with the lowest fuel
consumption.
 It should also ensure smooth and reliable operation.
 The fuel Induction systems for SI engine are classified as:
 Carburetors.
 Throttle body Fuel Injection Systems.
 Multi Point Fuel Injection Systems.

3. Fuel System Layout

4. Requirements of Good Fuels
 Viscosity
Low & Constant over wide range of temp
 Pour point
Higher than operating conditions
 Sulphur Contain
No
 Volatility
Moderate so to mix with air
 Heat of Combustion
High
 Product of Combustion
No or Very Less
 Cost
Low

5. Air –Fuel Ratio
The air-fuel ratio is the proportions of air and gasoline mixed by
the carburettor as required for combustion by the engine.
This ratio is extremely important for an engine because there are
limits to how rich (with more fuel) or how lean (with less fuel) it
can be, and still remain fully combustible for efficient firing.
Engine can operate range from 8:1 to 18.5:1

6. Stoichiometric Ratio 14.7:1
The best fuel economy at 15:1 to 16:1
maximum power output at 12.5:1 to 13.5:1
A:F Power Economy Co/HC Nox
Lean Low High Low High
Rich High Low High Low

7. Mixture Requirements of
Engine
 Operating conditions:
 Idling
 Cursing
 High Power
Operation Governing Factor Mix Requirement
Idling Dilution of Mixture Rich
Cursing / Normal Economy Slight Lean
High Power/ Acceleration Full Air Utilization Rich
Cold Starting & Warmup Engine Temperature Rich
Accelerating Liquid film / non-
vaporized droplet
Rich

8. Carburetor & Carburetion
 Carburetor is device that atomizes and vaporizes the
fuel and mixes with air in varying proportion to suit SI
engine operating range.
 Carburetion is process of braking up and mixing the
fuel with air .
 Factors affecting carburetion
 Engine Speed
 Vaporization Characteristic of Fuel
 Temperature of Incoming Air
 Design of Carburetor

9. Simple Carburetor working
Principle

10. Types of Simple Carburetor
Downdraft Up-Draft Horizontal
Draft

11. Requirements of Good
Carburetor
 Easy Starting or Cold Starting
 Must atomize fuel and air mix homogeneously
 Provide correct air fuel ratio
 Must run engine smoothly without wastage of fuel
 Must provide rich mixture during idling and starting and
sudden acceleration
 It should function correct during changing climatic
condition

12. Limitations of Simple Carburetor
(Single Jet Carburetor)
 Can’t provide Rich Mixture during Starting or No / low
Load
 Can’t provide Rich Mixture during sudden acceleration
 Cold starting problem
 Can’t provide correct A:F during operating conditions
like Idling, Normal Running and Accelerating.
 Can’t adjust mixture quantity for Change in altitude

13. Complete Carburetor
 Main metering system :
(To supply correct A:F ratio during required operating conditions)
 Idling system
 Power enrichment system
 Acceleration system
 Choke

14. Main Metering System
 Compensating Jet Device
 Emulsion tube or Air Bleeding System
 Back Suction Control or Pressure Reduction
 Auxiliary Valve and Auxiliary Port

15. Compensating Jet

16. Emulsion Tube / Air Bleeding
System

17. Back Suction Control or
Pressure Reduction System

18. Auxiliary Valve & Auxiliary
Port

19. Idling System

20. Power Enrichment or
Economizer System

21. Acceleration Pump

22. Choke (With Spring Loaded by Pass)

23. Solex Carburetor

24. Cold Starting

25. Idling Condition

26. Normal Running

27. Accelearation

28. Petrol Injection System

29. Electronic Injection System