ENGINE PERFORMANCE II

LESSON 2

Engine Fuel Requirements

 

Gasoline - Hydrocarbons - molecule of Hydrogen and Carbon

Advantages:   Easily vaporizes.

                       Burns quickly, but under control.

                       High heat value to produce large amount heat.

                       Easy to store, handle, transport.

Disadvantages: Air pollution   

                          1.  Hydrocarbons  (HC)

                          2.  Carbon monoxide  (CO)

                          3.  Nitrogen Oxides  (NOX)

                          4.  Carbon Dioxide  (CO2)

Air Fuel Ratio - ratio of air to fuel based on weight.

An  gasoline engine can run on an A/F ratio from 8 to 1(rich)

to 18.5 to 1(lean).

At 15 to 1:        15 pounds of air for 1 pound of gasoline

9000 gal. of air for 1 gal. of gasoline

Stoichiometric ratio = best mixture when fuel and air are both

                                                  completely burned.   14.7 to 1.

A/F requirements for different conditions:

   Economy: 15:1 to 16:1 (Extra air, all the fuel is used)

   Power:       12.5 :1 to 13.5 to 1    10% rich is optimum torque A/F ratio

(All the air is used, maximum fuel is used, and fuel is wasted)

Rich load & high speed.

Leaner at cruise and light load.

Conditions that effect A/F requirements:

    1.  Engine speed

    2.  Temperature

    3.  Load

    4.  Atmospheric pressure

Engine operating factors affecting A/F Ratio (Actually in cylinder)

   1.  Exhaust in mixture at idle or low speeds,

add fuel to make up for the fuel that combines with exhaust.

   2.  Improper vaporization,

cold engines require more raw fuel to create enough vapors

 

Fuel Composition - hydrocarbon(s) blended together

Blending Considerations:

 Volatility - blended for temperature and season

                 blended for altitude

                        Reid vapor pressure

                 blended to burn cleaner, oxygenated. 

Impurities

   1.  Sulfur > contributes to sulfuric acid in crankcase.

   2.  Gum > contributes to plugged injectors,

                                        sticky valves and rings.

 

Additives

1.  Anti-icers - alcohols which act as anti-freeze.

2.  Anti-oxidant inhibitors - prevent gum formation.

3.  Metal deactivators - prevent chemical reactions with metal.

4.  Detergents - keep carbs and injectors clean.  Top Tier fuel

5. TEL (tetraethyl lead) - octane booster, valve seat lubricant.   Before 1974

6.  MTBE - adds oxygen burns cleaner (oxygenated fuel). Before 2007

7.  Ethanol - adds oxygen burns cleaner (oxygenated fuel).

Gasohol - 90% gasoline; 10% ethanol, ethyl alcohol

                    Comes from corn or sugarcane

Ethanol is an octane booster and adds oxygen,

         but it has less heat value.

Problems caused by too much alcohol:

1.  Fuel system corrosion.

2.  Breakdown of plastic fuel tank liners.

3.  Breakdown of rubber or plastic fuel system components.

4.  Driveability problems.

5.  Water absorbed by alcohol collects in bottom of tank.

Stoichiometric A/F ratio for alcohol is 9 :1.

So when used in a mixture of greater than a10%, a lean A/F mixture results.

 

Octane Rating > antiknock value, which indicates the gasoline’s

                                                   resistance to detonation.

   Higher-octane fuel burns slower.

                           Octane up > Detonation down

 

Detonation > uncontrolled burn > explosion

                       spark knock or ping

Causes loss of power and combustion chamber overheating.

 

Forced Air Induction

The purpose of forced air induction is to boost intake manifold pressure and increase the air intake charge. An increase in manifold pressure increases the volumetric efficiency of the engine. Additional fuel can be added when the intake charge is increased which in turn increases power output from the engine. Engines with forced air induction have lower compression ratios to help prevent detonation. An inter-cooler may be added to cool the compressed air charge and increase its density.

Types of forced air induction:

Turbocharger – Waste energy from the exhaust gas is used to spin a turbine on a shaft. A centrifugal compressor is mounted on the opposite end of the shaft to increase the pressure of the intake charge.  The shaft may spin up to 150,000rpm. Turbos begin to work when the engine is under load and the exhaust gas is high in kinetic energy. Turbos use a pressure activated waste gate to limit boost pressure.

 

Supercharger – A belt from the crankshaft turns a supercharger.  

        Varieties of Superchargers;

                Roots Blower

                Centrifugal

                Twin Screw

Superchargers have oil reservoirs that should be checked and filled periodically. Turbochargers are lubricated and cooled by the engine’s lube system. Turbos require a cool down period before engine shutdown.