Chapter 39 - Automotive Fuels, Combustion Efficiency

Octane number requirement tends to go down when:

1. Car is operated at higher altitudes (lower barometric pressure). 2. Fuel-air ratio is richer or leaner than that producing maximum knock. 3. Spark plug location in combustion chamber provides shortest path of flame travel. 4. Combustion chamber design gives maximum turbulence of fuel-air charge. 5. Compression ratio is lowered. 6. Exhaust gas recycle system operates at part-throttle. 7. Ignition timing retard devices are used. 8. Humidity of the air increases. 9. Ignition timing is retarded. 10. Inlet air temperature is decreased. 11. Reduced engine loads are employed.

Cetane Vs. Octane

A cetane number, in some ways. is the opposite of a gasoline octane number. A high cetane number means the fuel will ignite easily from heat and pressure. In a diesel. the fuel must ignite and burn as soon as it touches the hot air in the combustion chamber.

Diesel Fuel Cetane Rating

A cetane rating Indicates the cold starting ability of diesel fuel. The higher the cetane number, the easier the engine will start and run in cold weather. Most auto-makers recommend a cetane rating of about 45. This is the average cetane value of No. 2 diesel fuel.

Diesel Fuel Injection/Combustion

A diesel engine is a compression-ignition engine. High pressure heats air in the cylinder.When fuelis sprayed into the hot air, combustion begins Since diesel fuel is thicker and has different burn-ing characteristics than gasoline, a high-pressure injection system must be used to spray the fuel directly into the combustion chambers. A low-pressure injection system or carburetor would not meter the thick diesel fuel properly. Diesel fuel will not vaporize (change from a liquid to a gas) as easily as gasoline. If diesel fuel were to enter the intake manifold of an engine, it would collect on the inner walls of the manifold. This would upset engine operation. Combustion requires fuel to be in a vapor state. Diesel engines inject the diesel fuel directly into the combustion chamber. The compressed. hot air vaporizes and burns the fuel.

Diesel Combustion

A diesel engine is a compression-ignition engine. This engine compresses air until it is hot enough to ignite the fuel. A spark plug would not ignite diesel fuel properly. If gasoline were used in a diesel engine. it would detonate on the compression stroke and not produce useful energy. Diesel fuel is thick and hard to ignite. A high-pressure mechanical pump and nozzle force the fuel into the engine combustion chamber. An extremely high compression ratio heats the air in the cylinder. When fuel is sprayed into the hot air. it begins to burn. Note that fuel is injected into the engine for several degrees of crankshaft rotation. This causes a smooth, steady buildup of pressure for quiet diesel engine operation.

Lean Air-Fuel Mixture

A lean air-fuel mixture contains a large amount of air compared to fuel. For gasoline, 20:1 is a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel is burned. Too lean a mixture, however, can cause poor engine performance (lack of power, missing, and even engine damage).

Rich Air-Fuel Mixture

A rich air-fuel mixture is the opposite of a lean mixture; more fuel is mixed with a smaller portion of air. For gasoline, 8:1 (8 parts air to one part fuel) is a very rich fuel mixture. A slightly rich mixture tends to increase engine power. However, it also increases fuel consumption and exhaust emissions. An over-rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust).

Normal Gasoline Combustion Operation

A spark at the spark plug starts the fuel burning. A small ball of flame forms around the tip of the plug. The piston is moving up in the cylinder, compressing the fuel mixture. The flame spreads faster and moves about halfway through the mixture. Generally, the flame is moving evenly through the fuel mixture. The piston is nearingTDC, causing increased pressure. The piston reaches TDC. The frame picks up more speed. The flame shoots out to consume the rest of the fuel in the chamber. Combustion is complete with the piston only a short distance down in the cylinder

LPG Fuel System

A special fuel system is needed to meter the gaseous LPG into the engine. An LPG fuel system uses a high-pressure storage tank. A fuel strainer-fuel lock cleans fuel and prevents leakage when the engine is not running. A converter uses heat from the engine coolant to change the liquid LPG into a gas. A special carburetor meters LPG into the engine.

Alcohol

Alcohol has the potential to be an excellent alternative fuel for automotive engines. The two types of alcohol used in automobiles are ethyl alcohol and methyl alcohol. Alcohol is a clean-burning fuel for automobiles. It is not commonly used because it is expensive to use and produce. Also, the vehicle's fuel system requires modification before it can burn straight alcohol. Almost twice as much alcohol must be burned. compared to gasoline. This reduces fuel economy by 50%.

Automotive fuel

Any substance (liquid, gas, or solid) that can be easily burned to produce heat.

Petroleum (Crude Oil - Chemical)

Chemically, crude oil consists of highly flammable hydrocarbons. Hydrocarbons are chemical mixtures of about12%hydrogen (flammable gas vapor) and82%carbon (heavy, black solid). Crude oil also contains sulfur, nitrogen, metals, and saltwater, which must be removed.

Processing Crude Oil

Crude oil deposits are contained inside the earth. Oil companies perform exploration tests (seismic studies, surface mapping, and test drilling) to find oil. After determining where oil might be located, a drill crew bores a hole thousands of feet into the ground. A huge steel derrick is used for the deep earth drilling operation. The derrick has a cutting bit capable of passing through dirt, sand, and rock. Once the oil deposit has been reached, the oil is pumped to the surface and sent to a refinery. The refinery converts the crude oil into more useful substances, such as motor oil, gasoline, and diesel fuel.

Petroleum (Crude Oil)

Crude oil, or petroleum, is oil taken directly out of the ground. It is used to make gasoline, diesel fuel, liquefied petroleum gas, and many nonfuel materials (asphalt, motor oil,etc.).

DEF Composition/Usage

DEF is mixture of urea and de-ionized water. It is stored in a separate tank that typically has a blue filler cap and is located near the fuel tank. The rate of diesel exhaust fluid (DEF) consumption is termed the 'dosing rate." Most diesel vehicles that use DEF have a dosing rate of 2% per gallon of diesel fuel burned. For every 100 gallons of fuel burned. 2 gallons of DEF would be injected into the exhaust system in front of the catalytic converter.

Diesel fuel should not be confused so with fuel oil or home heating oil. Why?

Diesel fuel contains fewer impurities than fuel oil. Fuel oil should never be used in a diesel engine or damage will result.

Diesel Fuel Grades

Diesel fuel grades ensure that diesel fuel sold all over the country has uniform standards. Diesel engine makers are then able to select a diesel fuel grade that meets the needs of their engines. There are three diesel fuel grades: No. 1 diesel. No.2 diesel. and No. 4 diesel.

Diesel Fuel

Diesel fuel is the second most popular type of automotive fuel. A gallon of diesel fuel contains more heat energy than a gallon of gasoline. It is a thicker fraction (part) of crude oil. Diesel fuel can produce more cylinder pressure and vehicle movement than an equal amount of gasoline.

Dieseling

Dieseling also called after-running or run-on, is a problem in which the engine keeps running after the key is turned off. A knocking. coughing. or fluttering noise is heard as the fuel ignites and the crankshaft spins uncontrollably. When dieseling. the gasoline engine ignites the fuel from heat and pressure. some-what like a diesel engine. With the ignition key off. the engine runs without voltage to the spark plugs.

Crude Oil Distillation

Distillation is the first conversion process. During distillation, a fractionating tower breaks the crude oil down into different parts, or fractions (LPG, gasoline, kerosene, fuel oil, and lubricating oils). A fractionating tower allows crude oil vapors to condense and separate into trays. Each weight of vapor flows off its tray and into storage containers After distillation, other processes purify and alter the content of each crude oil fraction.

Ethyl Alcohol

Ethyl alcohol. also called grain alcohol or ethanol, is made from farm crops. Grain, wheat, sugarcane. potatoes, fruit, oats. soybeans, and other crops rich in carbohydrates can be made into ethyl alcohol. This type of alcohol is a colorless. harsh tasting. toxic. and highly flammable liquid.

Gasoline Additives

Extra chemicals, called additives (detergents, antioxidants, etc.), are mixed into gasoline to improve its operating characteristics. Anti knock additives slow down the burning of gasoline. This helps prevent engine ping, or knock (knocking sound produced by abnormal, rapid combustion).

Flex Fuel

Flex-fuel vehicles are designed to operate on a mix of up to 85% ethanol (alcohol) and only 15% gasoline. Their fuel system can sense the alcohol/gasoline percentages and adjust fuel injection as needed to maintain the correct air-fuel ratio.

Gasoline Combustion

For gasoline (or any other fuel) to burn properly, it must be mixed with the right amount of air. The volatile (flammable) air-fuel mixture must then be compressed, pressurized, and ignited. The resulting combustion, or rapid oxidation, produces heat, expansion of gases, and engine cylinder pressure. The cylinder and combustion chamber pressure pushes down on the pistons to rotate the crankshaft.

Air-Fuel Mixture

For proper combustion and engine performance, the correct amounts of air and fuel must be mixed. If too much fuel or too much air is used, engine power, fuel economy, and efficiency will suffer. A stoichiometric fuel mixture is a chemically correct air-fuel mixture. For gasoline, it is a mixture ratio of 14.7:1 (14.7 parts air to 1 part fuel, by weight). Under constant engine conditions, this ratio helps ensure that all the fuel is burned as efficiently as pos-sible during combustion.

Gasohol

Gasohol, as the name implies. is a mixture of gasoline (usually 87 octane gasoline) and alcohol (usually grain alcohol). The mixture can range from 2-20% alcohol. In most cases, gasohol is a blend of 10% alcohol and 90% gasoline. Gasohol's commonly used as an alternative fuel in motor vehicles because fuel system and engine modifications are not needed.

Gasoline

Gasoline is the most common type of automotive fuel. It is an abundant and highly flammable part of crude oil.

Hydrogen

Hydrogen is a highly flammable gas that is a promising alternative fuel of the future. Hydrogen is one of the most abundant elements on our planet. It can be produced through the electrolysis of water (sending electric current through saltwater). Hydrogen is an ideal fuel. Hydrogen burns almost perfectly. leaving only water and harmless carbon dioxide as by-products. Many manufacturers are now working to develop hydrogen-powered vehicles. Presently. hydrogen is expensive to make and store. However. as we use up our supply of crude oil. we may someday make hydro-gen a major source of automotive fuel.

Octane number requirement tends to go up when...

Ignition timing is advanced. Air density rises due to supercharging, a larger throttle opening, or higher barometric pressures. Humidity or moisture content of air decreases. Inlet air temperature goes up. Lean fuel-air ratios. Compression ratio is increased. Coolant temperature is raised. Antifreeze (glycol) engine coolant is used. Combustion chamber design provides little or no quench area. Vehicle weight is increased. Engine loading is increased, such as when climbing a grade, pulling a trailer, or increasing wind grade, or increasing wind resistance with a car-top carrier.

Alternative fuels

Includes any fuel other than gasoline and diesel fuel. Liquefied petroleum gas alcohol. meth-ane gas. and hydrogen are examples of alternative fuels.

LPG Applications

LPG is commonly used in industrial equipment. such as forklifts. It is also used in some fleet vehicles that always refuel at the same location. Being a gas, LPG burns cleanly, producing few exhaust emissions.

Gasohol Availability

Many gas stations sell gasohol as a 141-octane fuel. The alcohol tends to reduce the knocking tendencies of the gasoline. It acts as an antiknock additive. For example. 10% alcohol can increase 87 octane gasoline to 91 octane. Gasohol can be burned in a high-compression, high-horsepower engine without detonating and knocking.

Methanol

Methanol is commonly used as a racing fuel. It bums cooler than gasoline and does not produce a visible flame. This can be very dangerous because you cannot see the flames if there is a fire. For increased safety, racing organizations have required the use of additives in methanol racing fuel to make its flame more visible.

Methyl Alcohol

Methyl alcohol, also termed "wood alcohol' or methanol. can be made out of wood chips. petroleum. garbage. and animal manure. It has a strong odor, is colorless. poisonous. and very flammable.

Engine Combustion

Most engines burn liquid gasoline, while a few consume diesel fuel, gasohol, alcohol, and alternative fuels. Combustion is the chemical reaction that occurs when a fuel is burned with an oxidant (oxygen). Normal combustion is a primary factor for good fuel economy, max engine power, low exhaust emissions, and long engine service life. Abnormal combustion is when combustion happens too quickly (explosion) or too slowly (incomplete burn). Combustion efficiency is a measure of how well combustion consumes all the fuel's heat energy for maximum cylinder pressure (propulsion) and minimum exhaust by-products (pollution).

Petroleum (Crude Oil - Natural)

Natural crude oil is a mixture of semi-solids (neither solid nor liquid), liquids, and gases.

No. 1 Diesel

No. 1 diesel is thinner than No. 2 diesel and is sometimes recommended as a winter fuel. Low temperatures tend to thicken diesel fuel, causing performance problems (hard starting. poor fuel delivery). In some engines. the thin No. 1 diesel fuel will not provide adequate lubrication. Metal-to-metal contact may occur. causing serious engine damage.

No. 2 Diesel

No. 2 diesel is normally recommended for use in diesel automotive engines. It is also the only grade of fuel available at many service stations. No. 2 diesel has a medium viscosity (thickness or weight). which provides proper operating traits for the widest range of conditions. Some auto manufacturers allow the use of only No.2 diesel fuel with special cold weather additives being used in the winter.

Normal Gasoline Combustion

Normal gasoline combustion occurs when the spark plug ignites the fuel and burning progresses quickly and smoothly through the air-fuel mixture. Combustion flame produces heat. Heat causes gases to expand. Expansion causes pressure. Pressure pushes the piston down on power stroke Maximum cylinder pressure should be produced a few degrees of crank rotation after piston TDC on the Power stroke. Normal combustion takes only about 3/1000 of a second. This is much slower than an explosion. Dynamite explodes in about 1/50,000 of a second. Under some undesirable conditions, however, gasoline can burn too quickly, making part of combustion like an explosion.

Octane Enhancers (Oxygenates)

Octane enhancers, or oxygenates, are now used as blending components in gasoline to increase octane levels and reduce engine knock. Octane enhancers are alcohols made up of hydrogen, carbon, and oxygen. Examples of octane enhancers include toluene, ethanol, and MTBE.

Diesel Fuel Cloud Point

One of the substances in diesel fuel is paraffin (wax). At very cold temperatures. this wax can separate from the other parts of the diesel fuel. When this happens. the fuel will turn cloudy or milky. Cloud point is the temperature at which paraffin separates out of the fuel. At cloud point. the paraffin can clog the fuel filters and prevent diesel engine operation.

Preignition stages

Preignilion is caused by early ignition of the fuel mixture. Abnormal and normal flames collide. producing a pinging noise. Hot carbon deposit ignites fuel mixture. Spark plug ides and two llama fronts form. Both flame fronts shoot toward each other at high speed. Two names collide causing pressure 'spike' and a knock.

Preignition

Preignition. or surface ignition. results when an overheated surface in the combustion chamber ignites the air-fuel mixture. A "hot spot" (overheated bit of carbon. sharp metal edge. hot exhaust valve) causes the mixture to burn prematurely. A ping or a mild knock is a light tapping noise that can be heard during preignition. It is not as loud or as harmful as a detonation knock. Preignition is similar to detonation. but the actions are reversed. Detonation begins after the start of normal combustion. Preignition begins before the start of normal combustion. Prolonged preignition can produce harmful detonation. If an engine pings or knocks excessively, serious engine damage can result. Correct the problem right away.

Liquefied petroleum gas (LPG)

Sometimes used as a fuel for automobiles and trucks. It is one of the lightest fractions of crude oil. Chemically. LPG is similar to gasoline. However. at room temperature and pressure. LPG is a vapor. not a liquid.

Spark Knock

Spark knock is an engine combustion problem caused by the spark plug firing too soon in relation to the position of the piston. Spark timing that is advanced too far causes combustion pressure to slam into the upward-moving piston. This causes maximum cylinder pressure before TDC. not just atter TDC as it should. Spark knock and preignition produce about the same symptoms—pinging under load. To find the cause of pinging, first check the ignition timing. If timing is correct, check other possible causes.

Spark Knock Stages

Spark knock occurs because of excessive pressure in cylinder. Spark knock is a ping or knock caused by an ignition timing problem. Spark plug "fires" too soon Piston moves toward flame font. Pressure builds as piston slams into combustion flame. Spark knock occurs because of excessive pressure in cylinder

The four phases of detonation.

Spark occurs, combustion is slow but normal. Normal combustion spreads very slowly. End gas auto-ignites and two flame fronts spread rapidly. Flames collide with pressure "spike" and knock.

Synthetic Fuels

Synthetic fuels are fuels made from coal, shale oil rock (rock filled with petroleum), and tar sand (sand-filled with petroleum). Shale rock can be converted into oil, coal can produce about two and one-hall barrels of oil per ton, four and one-half tons of tar can be changed into about one barrel of oil. Synthetic fuels are synthesized (changed) from a said hydrocarbon state to a liquid or gaseous state. Synthetic fuels are being experimented with as a means of supplementing crude oil. As crude oil-based fuels become more expensive. synthetic fuels will become more practical.

Selective Catalytic Reduction

Systems Selective Catalytic Reduction (SCR) systems can be found on some late-model diesel-powered vehicles. These systems automatically spray diesel exhaust fluid (DEF) into the vehicle's exhaust stream. The DEF tank generally has a blue cap for easy identification. A catalytic converter located downstream of the DEF spray nozzle breaks down toxic NO emissions into harmless water and nitrogen.

Lead Dangers

Tetraethyl lead, or lead, is a naturally occurring heavy metal. For many years, lead was added to gasoline as an octane booster to prevent detonation and engine wear. After scientists learned that lead was a common cause of birth defects and other serious illness, all fuel and oil additives containing lead were banned from the market.

DEF Regulations

The EPA requires vehicle manufacturers to install safeguards that help prevent diesel engines from being operated without DEF. When DEF is low. a warning Indicator on the instrument panel will illuminate. If the driver ignores the warning indicator and operates the diesel engine without DEF. the engine's ECU will greatly reduce engine power and limit driving speeds until the DEF container is refilled.

Gasoline Grade

The gasoline grade indicates the antiknock value of gasoline blend. Octane ratings are given as numbers on the gas station pump. Automakers recommend octane ratings of fuel for their engines. The owner's manual will give the octane rating recommended for the car's engine. Use a fuel with an octane rating as high as the automaker's recommendations.

Normal diesel combustion stages

The piston moves up to compress and heat the air in the cylinder. Note that this is different than a gasoline engine. which compresses both fuel and air. Diesel fuel is injected directly into the combustion chanter. The hot air makes the fuel begin to burn and expand. More fuel is sprayed into the chamber. More pressure is developed and the piston begins to move down in the cylinder. The rest of the fuel is injected into the chamber. Pressure continues to form, pushing the piston down on the power stroke.

Diesel Fuel Water Contamination

Water contamination is a common problem with diesel engines. When mixed with diesel fuel. water can clog filters and corrode components. The parts in diesel injection pumps and nozzles are very precise. They can be easily damaged by water. Many late-model diesel injection systems have water separators to prevent water damage.

Causes of Dieseling

a high idle speed. leaking injectors, carbon deposits in the combustion chambers. low-octane fuel. an overheated engine. or spark plugs that have too high a heat range.

Racing Fuels

have additives to greatly increase their resistance to preignition and detonation. Racing fuels come in many blends, with ratings from 100 to 120 octane and higher.

Racing Fuel Example

if a small-displacement turbocharged engine is developing over 1000 horsepower at 50 pounds of boost pressure, the car better be running 118 octane or higher racing fuel. If a lower octane rating fuel is used, a split second of detonation will usually result in instant engine destruction. At these boost levels, even a split second of detonation can cause burned pistons, broken piston rings, badly bent connecting rods, and engine block damage.

Abnormal combustion

occurs when the flame does not spread evenly and smoothly through the combustion chamber. The lean air-fuel mixtures. high operating temperatures. and low-octane fuels of today make abnormal combustion problems more common.

Detonation

results when part of the unburned air-fuel mixture explodes violently. This is the most severe and engine-damaging type of abnormal combustion. Engine knock is a symptom of detonation. The combustion chamber pressure rises so quickly that parts of the engine vibrate. Detonation sounds like a hammer hitting the side of the engine.

Why should you avoid using high octane fuel in an engine designed to burn low octane fuel?

There is no benefit to engine power or fuel economy by purchasing the higher-priced fuel. Contrary to what many people believe, a high-octane fuel actually is less volatile and burns more slowly than low-grade fuel. High octane fuel may not burn properly in low compression, naturally-aspirated engine. Its improper use can actually decrease engine power and gas mileage. Use the cheaper, low-octane fuel in engines with lower compression ratios.

Why should you never recommend a racing fuel to customers driving their vehicles on city streets?

This is against federal law and can result in expensive fines to you or your repair facility. Improper fuel can also void a vehicle's warranty and result in catalytic converter and engine damage. Always recommend fuel with an octane rating specified by the vehicle manufacturer. A fuel sticker displaying recommended octane rating for a specific engine is installed on the car. This sticker is usually located over the fuel filler opening. Recommended octane ratings will also be listed in the owner's manual.