Chap 5 Fuels and Systems

8/2/2019 Chap 5 Fuels and Systems

1/25

35

Fuels and fuel systems

(based on H.Heywood Internal combustion engine fundamentals

and DieselNet website)

5.1 Fuels for combustion engines

Combustion engines need fuel in order to transform chemical energy into useful work.

Different engine fuels are used for different types of engine. The three main classes of fuel

are:

gasoline (petrol)

diesel

alternative fuels.

5.1.1 Gasoline

Gasoline (petrol) is a complex blend of carbon and hydrogen compounds. Additives are then

added to improve performance. All gasoline is basically the same, but no two blends are

identical. The two most important features of gasoline are its volatility and its resistance to

knock (octane). Volatility is a measurement of how easily the fuel vaporizes. If the gasoline

does not vaporize completely, it will not burn properly (liquid fuel will not burn). If the

gasoline vaporizes too easily the mixture will be too lean to burn properly. Since high

temperatures increases volatility, it is desirable to have a low volatility fuel for warm

temperatures and a high volatility fuel for cold weather. The blends will be different for

summer and winter fuels. Vapour lock, which was a persistent problem years ago, exists very

rarely today. In today's cars the fuel is constantly circulating from the tank, through thesystem and back to the tank. The fuel does not stay still long enough to get so hot that it

begins to vaporize. Resistance to knock or octane is simply the temperature the gas will burn

at. Higher octane fuel requires a higher temperature to burn. As compression ratio or

pressure increases, so does the need for higher octane fuel. Most engines today are low

compression engines and therefore require a lower octane fuel (87). Using a higher octane

than necessary is just wasting money. Other factors that affect the octane requirements of

the engine are: air/fuel ratio, ignition timing, engine temperature, and carbon build up in the

cylinder. Many automobile manufacturers have installed exhaust gas re-circulation systems to

reduce cylinder chamber temperature. If these systems are not working properly, the car will

5


2/25

Internal combustion engines

36

have a tendency to knock. Before switching to a higher octane fuel to reduce knock, make

sure to have these other causes checked.

5.1.2 Diesel fuel

Diesel fuel, like gasoline, is a complex blend of carbon and hydrogen compounds, and it, too,

requires additives for maximum performance. Two grades of diesel fuel are used in

automobiles today: 1-D and 2-D. Number 2 diesel fuel has a lower volatility and is blended for

higher loads and steady speeds. It therefore works best in large truck applications. Because

number 2 diesel fuel is less volatile, it tends to be harder to start in cold weather. On the

other hand, number 1 diesel is more volatile, and therefore more suitable for use in an

automobile where there are constant changes in load and speed. Since diesel fuel vaporizes

at a much higher temperature than gasoline, there is no need for a fuel evaporation control

system, as with gasoline.

Diesel fuels are rated with a cetane number rather than with an octane number. While a

higher octane number of gasoline indicates resistance to ignition, a higher cetane rating of

diesel fuel indicates the ease at which the fuel will ignite. Most number 1 diesel fuels have a

cetane rating of 50, while number 2 diesel fuel has a rating of 45. Diesel fuel emissions are

higher in sulphur, and lower in carbon monoxide and hydrocarbons than gasoline and are

subject to different emission testing standards.

5.1.3 Alternative fuels

There are many different kinds of alternative fuels. The world energy authorities recognize

the following alternative fuels:

- Alcohols - ethanol and methanol.- Biodiesel similar to diesel fuel, but made from plant oil or animal fat.- Liquefied Petroleum Gas (LPG) - hydrocarbon gases under low pressure.- Compressed Natural Gas (CNG) - natural gas under high pressure.- Liquefied Natural Gas (LNG) - natural gas at very low temperature-

Hydrogen- Liquids made from coal - gasoline and diesel fuel not based on petroleum.

The Alcohols - Ethanol and Methanol

Alcohols have been popular alternative fuels for many years. In fact, Henry Ford's first car

was fuelled with alcohol. Both ethanol and methanol are now used as transportation fuels and

will likely play an increasingly important role in the future.

Ethanol. Ethanol (sometimes called grain alcohol), is generally made from corn (a grain). It

can also be made from biomass (organic materials), which includes agricultural crops and


3/25

Chapter 5. Fuels and fuel systems

37

waste (like rice straw), plant material left from logging and trash including cellulose (paper).

Brazil, which is by far the largest producer in the world, makes ethanol from sugar cane.

The alcohol found in alcoholic beverages is ethanol. However, the ethanol used for motor

fuel is denatured, which means poison has been added so people can't drink it.

Methanol. Unlike gasoline, which contains many different chemicals and can vary greatly

from one batch to another, methanol is made of a single chemical and is therefore much

purer. It is also very poisonous and very harmful if swallowed.

Methanol can be made from various biomass resources, such as wood (it is sometimes

called wood alcohol) as well as from coal. However, today nearly all methanol is made from

natural gas, because it is cheaper.

Methanol is safer in case of accidental fire than gasoline, because it burns cooler. One

problem, however, is that the flame from a methanol fire is difficult to see in bright sunlight,

and accidental fires may not be detected immediately, because the fire is hard to see.

Methanol contains about half the energy of gasoline per volume. Lower energy means less

distance can be travelled for the same amount of fuel, but not less power.

Since the 1960s, methanol has been the required fuel for the Indianapolis 500 and other

types of racing. There are also other types of vehicles fuelled by methanol which are

generally called Flexible Fuel Vehicles (FFVs). Flexible fuel vehicles are specially designed

vehicles that can operate on alcohol, gasoline or any combination of the two. FFVs have

become quite popular in certain parts of the world (e.g. in California).

Although some vehicles run on pure alcohol, most FFVs operate on alcohol blends for two

main reasons. Firstly, adding a small amount of gasoline improves engine starting in cold

weather. Secondly, it improves flame visibility as pure alcohols burn with a nearly invisible

flame in daylight. By adding gasoline, the flame is easier to see and therefore safer. The

alcohols used in FFVs are E85 (85 % ethanol with 15 % gasoline) or M85 (85 % methanol and 15

% gasoline). FFVs are specially designed to tolerate the corrosive nature of alcohols.

Biodiesel is a lot like diesel fuel, but made from vegetable oil or animal fat.

Biodiesel is not regular vegetable oil and is not safe to swallow. Biodiesel is biodegradable

though, so it is much less harmful to the environment if spilled. Biodiesel is made througha process called transesterification. Transesterification turns vegetable oil and animal fat into

esterified oil, which can be used as diesel fuel, or mixed with regular diesel fuel. RME (Rape

Methyl Ester) or FAME (Fat Acid Methyl Ester) are examples of esterified oils.

Ordinary diesel engines can run on biodiesel. Practically any type of vegetable oil or

animal fat can be used to make biodiesel. But the most popular types of vegetable oils are

soybean and rapeseed oil. Soybeans are used to make tofu and soy sauce. Soybean and

rapeseed oil have been tried as biodiesel because they are less expensive than most other

types of vegetable oil. Although soybean and rapeseed oil are more expensive than regular

diesel fuel, most other types of vegetable oils are too expensive to even be considered for

use as diesel fuel. Animal fat also is too expensive for this use, but used oil from restaurants


4/25


38

has been tried for biodiesel. Biodiesel has been shown to produce lower tailpipe emissions

than regular diesel fuel. The best thing about biodiesel is that it is made from plants and

animals, which are renewable resources.

Liquefied Petroleum Gas (LPG) is a natural hydrocarbon fuel made of a mixture of propane

and other similar types of hydrocarbon gases (e.g. butane). The common name for LPG is

"propane", because this is its main constituent.

LPG has the special property of becoming liquid when under pressure, and reverting to gas

at atmospheric pressure. This means it can be easily and conveniently stored as a liquid in

tanks under a pressure of usually about 1,4 MPa (200 lbs psi). This gives it a big advantage

over natural gas, which will only turn to liquid at extremely low temperatures. Whats more,

LPG is 250 times denser as a liquid than as a gas. So, a lot of fuel can be stored in a relatively

small space, for use almost anywhere.

About 60 % of the world supply of LPG comes from the separation of natural gas products,

and 40 % is a by-product from the refining of crude oil. In the past, LPG was considered as

waste and flared off; now it is recognised as a major energy source. It is produced in vast

quantities (the UK produced 6.8 million tonnes in 2003) and exported over 3 million tonnes. It

is particularly abundant in the North Seas wet crude oil and could provide a secure supply

of fuel for many years.

In some countries where there are (or were) no natural gas pipelines, LPG is used for

domestic applications such as cooking, heating, hot water or gas barbecues. The LPG used in

the home is the same as that used for vehicles, and in some countries LPG is a relatively

popular vehicle fuel. In the Netherlands, for example, over 10 % of the motor fuel used is

LPG.

LPG offers several advantages over traditional fuels; not only does it usually cost less than

gasoline for the same amount of energy, but LPG-fuelled engines pollute less than gasoline or

diesel engines. Moreover, compared with petrol, vehicles running on LPG emit about 20 % less

CO2, while with respect to NOX emissions, 1 diesel car emits the equivalent of 20 LPG cars.

For particulates, the ratio is 1 diesel car to 120 LPG vehicles. Diesel engines also emit

substantially more fine particles than LPG engines, and LPG engines are quieter than diesel

engines. Finally, LPG will quickly evaporate in the event of a fuel spill and, unlike with petroland diesel, there is no risk of ground or water contamination.

Compressed Natural Gas (CNG) is a natural gas under high pressure. Like oil (petroleum),

natural gas comes from underground. However, natural gas, as the name implies, is a gas

much like air, rather than a liquid like petroleum. It has been found to be one of the most

environmentally friendly fuels, and its popularity is growing.

Natural gas is mostly methane (about 95 %). The other 5 % is made up of various gases

along with small amounts of water vapour. These other gases include butane, propane,

ethane and other trace gases.


5/25


39

Methane is a hydrocarbon, meaning its molecules are made up of hydrogen and carbon

atoms. Its simple, one carbon, molecular structure (CH4) makes possible its nearly complete

combustion.

Because of its clean burning nature and the fact that it is not made from petroleum, as

gasoline and diesel are, many automakers around the world are developing vehicles to run on

natural gas. Cars, vans, buses and small trucks generally use natural gas that has been

compressed (called compressed natural gas or CNG) and stored in high-pressure cylinders (at

around 20 MPa).

Several vehicles are available today (such as the Honda Civic CGX or the Ford Crown

Victoria) that operate on compressed natural gas. Some run on natural gas only and others

can run on natural gas or gasoline (called bi-fuel vehicles).

Liquefied Natural Gas (LNG) is a natural gas at very low temperature. Natural gas can be

made into three forms. One, is the low-pressure form for domestic use (cooking or heating,

for example. Another form is compressed natural gas (CNG), as mentioned above. CNG is

dispensed from special CNG fuel stations. The third form is liquefied natural gas (LNG). LNG is

made by refrigerating natural gas to condense it into a liquid. The liquid form is much more

dense than natural gas or CNG. It has much more energy for the amount of space it takes up.

So, much more energy can be stored in the same amount of space on a car or truck. That

means LNG is good for large trucks that need to go a long distance before they stop for more

fuel.

Liquefied natural gas is made by refrigerating natural gas to minus 160 Celsius to

condense it into a liquid. This is called liquefaction. The liquefaction process removes most of

the water vapour, butane, propane, and other trace gases, that are usually included in

ordinary natural gas. The resulting LNG is usually more than 98 % pure methane. Several

engine manufacturers (e.g., Caterpillar, Cummins, Detroit Diesel, Mack, Navistar) sell heavy-

duty natural gas engines that can operate on LNG.

Hydrogen is regarded by many scientists as the fuel of the future. While at the time of writing

it is mainly experimental vehicles which are operating on this fuel, the potential for this

unique energy source is excellent. Anyone who has taken a chemistry class knows thathydrogen is number one on the periodic chart of elements and the lightest of all elements. It

is easy to produce through electrolysis; simply splitting water (H20) into oxygen and hydrogen

by using electricity. Nowadays, however, nearly all hydrogen is made from natural gas.

Because hydrogen burns nearly pollution-free, it has been thought of as the ultimate clean

fuel. When burned, it turns into heat and water vapour. When burned in an internal

combustion engine, the combustion also produces small amounts of other gases. These other

gases are mostly oxides of nitrogen, because the hydrogen is being burned with air, which is

about two-thirds nitrogen. Being a non-carbon fuel, the exhaust is free of carbon dioxide

(which is one of the major contributors to global warning).


6/25


40

Hydrogen is normally in a gaseous state and can be compressed and stored in cylinders.

Compressed hydrogen contains less energy per volume compared to liquid fuels like gasoline

or ethanol, and the bulkiness of the cylinders (or fuel tanks) is one of the main problems with

storing hydrogen. Hydrogen can also be cooled to produce liquid hydrogen, but it is a costly

process.

Hydrogen's clean burning characteristics may, one day, make it a popular transportation

fuel. For now, the problem of how to store enough hydrogen on a vehicle for a reasonable

range, and its high cost, compared to gasoline, are critical barriers to widespread commercial

use. Nearly all hydrogen currently is made from natural gas. For that reason, hydrogen usually

costs more than natural gas.

There are still only a relatively small number of hydrogen-powered vehicles. Most of these

have been experimental prototypes made by car manufacturers, and nearly all of them were

equipped with internal combustion engines, similar to ones that run on gasoline.

Liquids made from coal are gasoline and diesel fuels that don't come from petroleum. Like oil

and natural gas, coal is a non-renewable, fossil fuel formed in the earth from what was once

living plants. Being a solid, coal is not easy to use for most transportation fuel needs.

However, there are ways to make gasoline, diesel fuel, methanol, and other chemicals from

coal. These processes have been used for many years in certain countries (such as South

Africa), to produce gasoline and diesel fuel from coal.

The methods to produce gasoline and diesel from coal were used by the Germans during

World War II. At that time, Germany was rich in coal deposits but had no oil. Nor could it be

brought in from abroad due to war operations, so synthetic fuels (oil, gasoline and diesel)

were made from coal. These processes can be used today, but the process is expensive. It is

cheaper to use crude oil pumped from below the ground.

5.1.4 Safety first with motor fuels

Remember:

1. Never swallow any type of motor fuel!

Gasoline and diesel fuels are poisonous to swallow and also are flammable. Great care mustbe used to avoid accidental fire. All types of motor fuel can kill you or make you sick if you

swallow even small amounts.

2. Ethanol fuel is "denatured." That means poison is added to stop people from drinking it.

Ethanol is flammable and care must be used to avoid accidental fire.

3. Even a small amount of methanol can make you very sick or even kill you. Methanol is

flammable and care must be used to avoid accidental fire.


7/25


41

4. Biodiesel made from vegetable oil is not safe to drink. Biodiesel is also flammable and care

must be used to avoid accidental fire.

5. Liquefied petroleum gas (LPG) or propane is kept under low pressure. Accidentally

releasing that pressure can be dangerous. LPG is flammable and care must be used to avoid

accidental fire.

6. Compressed natural gas (CNG) is kept under high pressure. Accidentally releasing that

pressure can be dangerous. Natural gas is flammable and care must be used to avoid

accidental fire.

7. Liquefied natural gas (LNG) is very cold and can injure by freezing. Natural gas is

flammable and care must be used to avoid accidental fire.

8. Compressed hydrogen is under high pressure. As with compressed natural gas, care must be

taken to avoid accidental release. Liquefied hydrogen is very cold and can injure by freezing.

Hydrogen is flammable and care must be used to avoid accidental fire.

5.2 Fuel supply systems

5.2.1 Introduction

A combustion engine needs a fuel system to store and supply fuel to the cylinder combustion

chamber where it can be mixed with air, vaporized, and burned to produce energy. In

general, a fuel system consists of a fuel tank, where the fuel is stored, a fuel pump which

draws the fuel from the tank through the fuel lines (pipes) and delivers it through a fuel filter

to either a carburettor or a fuel injector. It is then delivered to the cylinder chamber for

combustion.

Fuel tank location and design are always a compromise with available space. Mostautomobiles have a single tank located in the rear of the vehicle, and nowadays tanks are

fitted with internal baffles to prevent the fuel from sloshing back and forth. Noises from the

rear on acceleration and deceleration could indicate that the baffles are broken. All tanks

have a fuel filler pipe, a fuel outlet line to the engine and a vent system. Cars fitted with a

catalytic converter are also equipped with a filler pipe restrictor so that leaded fuel, which is

dispensed from a thicker nozzle, cannot be introduced into the fuel system.

The law requires that all fuel tanks be vented. Before 1970, fuel tanks were vented to the

atmosphere, emitting hydrocarbon emissions. Since 1970 all tanks are vented through a

charcoal canister, into the engine to be burned before being released to the atmosphere. This


8/25


42

is called evaporative emission control. Since 1976 all new cars have vehicle rollover

protection devices to prevent fuel spills.

Steel lines and flexible hoses carry the fuel from the tank to the engine. Special rubber is

used for the hoses. Ordinary rubber, such as used in vacuum or water hose, will soften and

deteriorate.

Fuel pumps Two types of fuel pump are used in automobiles; mechanical and electric. All fuel

injected cars today use electric fuel pumps, while most carburetted cars use mechanical fuel

pumps. Mechanical fuel pumps are diaphragm pumps, mounted on the engine and operated by

an eccentric cam usually on the camshaft. A rocker arm attached to the eccentric moves up

and down flexing the diaphragm and pumping the fuel to the engine. Because electric pumps

do not depend on an eccentric for operation, they can be located anywhere on the vehicle. In

fact they work best when located near the fuel tank. In many cars today, the fuel pump is

located inside the fuel tank.

While mechanical pumps operate on pressures of 0,04 MPa, electric pumps can operate on

pressures of 0,3 MPa. Current is supplied to the pump immediately the key is turned, which

allows for constant pressure on the system for immediate starting.

The fuel filter is the key to a properly functioning fuel delivery system. This is especially true

for fuel injection more than carburettors, because fuel injectors are more susceptible to

damage from dirt because of their close tolerances. A further reason is because fuel injected

cars use electric fuel pumps. When the filter clogs, the electric fuel pump works so hard to

push fuel past the filter, that it burns itself up. Most cars use two filters. One inside the gas

tank and one in a line to the fuel injectors or carburettor. Unless some severe and unusual

condition occurs to cause a large amount of dirt to enter the gas tank, it is usually only

necessary to replace the filter in the line.

5.2.2 Gasoline engine carburettor

In the C20

th

, the most popular fuel supply systems in gasoline internal combustion engineswere systems with carburettors. The carburettor is a device which mixes air and fuel for an

engine, and was invented by the Hungarian engineer Donat Banki in 1893.

Frederick William Lanchester of Birmingham, England, experimented early on with the

wick carburettor in cars. In 1896, Frederick and his brother built the first petrol driven car in

England, a single cylinder 5hp (4kW) internal combustion engine with chain drive. Unhappy

with the performance and power, they re-built the engine the following year into a two

cylinder horizontally opposed version using the new wick carburettor design. This version

completed a tour of almost 1500 km in 1900, successfully incorporating the carburettor as an

important step forward in automotive engineering. Carburettors are still found in small

engines and in older or specialized automobiles.


9/25


43

Most carburetted (as opposed to fuel-injected) engines have a single carburettor, though

some, primarily with greater than 4 cylinders or high-performance engines, use multiple

carburettors. Most automotive carburettors are either downdraft (the flow of air is

downwards) or side-draft (the flow of air is sideways). Small propeller -driven flat airplane

engines have the carburettor below the engine

The fundamental function of a carburettor is fairly simple, but the implementation is fairly

complex. A carburettor must provide the proper fuel/air mixture under a wide variety of

different performance conditions such as:

- cold start,- idling or slow-running,- acceleration,- high speed / high power at full throttle,- cruising at part throttle (light load)

Most carburettors contain equipment to support several different operating modes, called

circuits.

Fig. 5.1. Diagrammatic view of a carburettor.

A carburettor (see Figure 5.1) basically consists of an open pipe, the carburettor's "throat"

or "barrel", through which the air passes into the inlet manifold of the engine. The pipe is in

the form of a Venturi - it narrows in section and then widens again. Just after the narrowest

point is a butterfly valve or throttle - a rotating disc that can be turned end-on to the airflow,

so as to hardly restrict the flow at all, or be rotated so that it (almost) completely blocks the

flow of air. This valve controls the flow of air through the carburettor throat and thus the

quantity of air/fuel mixture the system will deliver. This in turn affects the engine power and

speed. The throttle was connected, usually through a cable or a mechanical linkage of rods

and joints (by pneumatic link or drive-x-wire nowadays), to the accelerator pedal on a car or

the equivalent control on other vehicles or equipment. Fuel is introduced into the air through

fine calibrated holes, referred to asjets.

Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too"lean". The "mixture" is normally controlled by adjustable screws on an automotive


10/25


44

carburettor, or a pilot-operated lever on a propeller aircraft (since mixture is air density

(altitude) dependent). The correct air to petrol ratio is 14.6:1, meaning that for each weight

unit of petrol, 14.6 units of air will be burned. For SI engines this is the stoichiometric ratio

(that is, the ideal or most efficient air/fuel mixture), but for more power a richer mixture of

around 11:1 is used and for fuel economy an 18:1 mix. In order to ensure the carburettor is

delivering the correct mixture, the carbon monoxide (CO) and oxygen content of the exhaust

fumes are measured and the carburettor adjusted accordingly. A more sophisticated way to

determine the correct mixture, as used in modern fuel injected engines, is by using a lambda

sensor in the exhaust system. Some famous manufacturers of carburettors are the following:

Amal, Autolite, Carter, Holley, Pierburg, Rochester, Solex, SU, Weber, Weber/Magneti-

Marelli, Zenith, Villiers.

5.2.3 Gasoline engine fuel injection systems

The automotive industry is faced with the task of having to work towards a further reduction

of exhaust emissions while at the same time further improving fuel economy. To solve these

problems some manufactures, but especially Bosch, offer various gasoline-injection systems.

So far, Bosch has successfully introduced the K-Jetronic, KE-Jetronic, L-Jetronic, LH-Jetronic,

Mono-Jetronic and Motornic system to world markets.

Fuel injection systems supply the precisely metered amount of fuel required for

dependable combustion. They are designed to react instantly to sudden changes in the

engine's operating conditions, and help development engineers produce engines which are

more economical and more compatible with the environment. Today - depending upon engine

design, driving conditions such as traffic and driving habits, an injection system can reduce

fuel consumption by between 5 % and 15 %. Bosch is a pioneer in the field of emission control

by catalyst converter. In 1976 the first vehicle in the world with Lambda closed-loop control,

three-way catalyst converter and a Bosch gasoline-injection system went into production.

Today every gasoline-injection system can be supplied with Lambda closed-loop control.

Automobile manufacturers throughout the world are taking increasing advantage of the ideas

developed by Bosch, and at the time of writing more than 13 million vehicles worldwide have

been equipped with Bosch gasoline-injection systems (not including Bosch licenses). Some 320different vehicle models from numerous automobile manufacturers in Europe, the United

States and Japan use Bosch gasoline-injection technology. The fuel injection systems are

described below.


11/25


45

Fig. 5.2. A gasoline injection system in action.

The K-Jetronic is an air-flow sensing system, and needs no mechanical drive. In the fuel

distributor (see Figures 5.3 and 5.4), the fuel is continuously metered by means of control

slits and downstream differential valves, and is supplied to the individual cylinders via

injection valves. The air flow is measured by means of a sensor plate, which is deflected

against a hydraulic force by the intake air flow. The fuel distributor control plunger is

actuated by means of a lever. Through appropriate design of the taper of the air-flow sensor,

the mixture ratio can be adapted precisely to the requirements of every engine. A hydraulic

counterforce, which acts on the control plunger of the fuel distributor, permits mixture

corrections during warm-up and, if necessary, under full load. The system can be

supplemented with lambda closed-loop control. Through a timing valve, the electronic

control unit influences the differential pressure across the control slits and thus the quantity

of fuel injected.

Fig. 5.3. The K-Jetronic scheme (Bosch Corporation)


12/25


46

Fig. 5.4. The K-Jetronics components (Bosch Corporation)

The KE-Jetronic (Figures 5.5 and 5.6) was developed to improve mixture adaption particularly

during warm-up and for changes of load. The basic principle behind the K-Jetronic was

retained, but in the KE-Jetronic, warm-up control and additional control functions are

performed by an electro-hydraulic pressure actuator which replaces the warm-up regulator of

K-Jetronic and which is mounted directly on the fuel distributor. This varies the differential

pressure across the control slits, thus influencing the quantity of fuel injected. The system

features an electronic control unit which processes information on engine temperature,

engine speed, air flow and throttle position as well as air-fuel ratio and atmospheric pressure.

The "limp-home" properties of the K-Jetronic also apply to the KE-Jetronic. The pressure

actuator of KE-Jetronic also has the capability to perform additional functions, such as

acceleration enrichment, overrun cut-off, lambda closed-loop control and altitude

compensation. The electronic control unit can also perform idle-speed control as well as

further functions. The control unit employs analogue or digital technology with a micro-

computer, depending on the scope of functions.


13/25


47

Fig. 5.5. The KE-Jetronic scheme (Bosch Corporation)

Fig. 5.6. The KE-Jetronics components (Bosch Corporation)

The L-Jetronic gasoline-injection system (see Figures 5.7 and 5.8) also operates in accordance

with the air flow sensing principle. The fuel is distributed by means of solenoid-operated

injection valves. The pressure drop at the metering point in the injection valve is kept at a

constant level by means of a pressure regulator, thereby making the injected volume

dependent solely on the opening period of the injection valves. The electronic control unit

receives signals from various sensors which characterize the operating condition of the

engine. The control unit employs analogue and digital technology depending on the scope of

functions. An air-flow meter, located in the engine's intake air flow, provides a signal as a

function of the intake air volume. The injection valves are actuated twice per camshaft


14/25


48

revolution. Corrections to the air-fuel ratio are carried out by changing the opening period of

the injection valves according to the dynamic engine requirements and additional control

functions such as cold-start control, warm-up enrichment, acceleration enrichment, idle

correction, overrun cut-off, engine-speed limitation and lambda closed-loop control.

Fig. 5.7. The L-Jetronic scheme (Bosch Corporation)

Fig. 5.8. The L-Jetronics components (Bosch Corporation)

The LH-Jetronic (Figures 5.9 and 5.10) is a further development of the L-Jetronic and it

operates on the same basic principle. Instead of the flat-type air-flow meter, a hot-wire mass

air-flow meter is used to measure the intake air. This measuring device makes it possible for

the first time to measure the inducted air mass directly, irrespective of density and


15/25


49

temperature. The control unit employs digital technology. A microcomputer controls adaption

to the engine characteristics. In addition to the functions found in the L-Jetronic, the LH-

Jetronic in its most basic form is equipped with idle-speed control

Fig. 5.9. The LH-Jetronic scheme (Bosch Corporation)

Fig. 5.10. The LH-Jetronics components (Bosch Corporation)

The Mono-Jetronic is an compact single-point injection system (see Figures 5.11 and 5.12). In

the Mono-Jetronic the fuel is metered at a centralized point by means of only one solenoid-

operated injection valve which is positioned directly above the throttle plate. This ensures

that the fuel is injected in the area of maximum air speed, resulting in optimum mixture


16/25


50

preparation. Apart from the throttle plate and the injection valve, the central injection unit

also contains the pressure regulator, the throttle-position potentiometer and the idle-air

thermo-actuator. This low-profile and compact design is easily mounted on the intake

manifold. The main control variables of the system are the position of the throttle valve and

the engine speed. The control unit contains a microcomputer and is provided with self-

adapting functions. Corrected values are stored and constantly updated in a non-volatile

memory. The Mono-Jetronic is a low-cost gasoline-injection system which is used in small and

medium-sized vehicles in order to comply with stringent exhaust-emission legislation.

Fig. 5.11. The Mono-Jetronic scheme (Bosch Corporation)

Fig. 5.12. The Mono-Jetronics components (Bosch Corporation)


17/25


51

The Motronic is a digital engine control system which combines injection and ignition (see

Figures 5.13 and 5.14). The heart of the Motronic is its microcomputer which can process

engine specific data on spark advance and injected fuel quantity. The data is stored in maps

after it has been established by the test engineer on the test bench. Sensors inform the

computer of the inducted air quantity, engine speed, crankshaft position, engine temperature

and air temperature. The computer then calculates the most favourable ignition point and

the optimum quantity of fuel to be injected. The microcomputer controls the quantity of fuel

injected and ignition timing precisely to the various operating conditions such as idle, part

load, full load, warm-up, overrun and load change. The result is a possible reduction in

gasoline consumption by 5-20 % depending on peripheral conditions, driving cycle and

reference basis.

Fig. 5.13. The Motronic scheme (Bosch Corporation)


18/25


52

Fig. 5.14. The Motronics components (Bosch Corporation)

Gasoline direct injection(GDI and HPI)is a fuel system with fuel split directly to cylinder.

Pioneer engine was developed by Mitsubishi Company in 1996 (see Figures 5.15).

Fig. 5.15. Scheme of GDI system

The main advantage of using of the GDI system is creation of mixture during as well suction as

compression strokes by double fuel injection (see Fig. 5.16). The results of it is lower fuel

consumption and emission (up to 20%) and increasing power about 10% in comparison to other

fuel injection systems.

Technique of direct injection realizes conception of lean mixture operation engine. Engine

like those, based on precision injection of very small doses of fuel and almost complete

combustion, can consume small amount of fuel.

for conventional gasoline engine Air/Fuel (A/F) ratio is equals 14,7:1

for GDI engine: A/F = 13:1 24:1 (dynamic operation) and A/F = 30:1 40:1 (50:1 for

Toyota prototype - economic run).


19/25


53

Fig. 5.16. Double fuel injection in GDI system conception

Combustion of so lean charge is possible because of stratification of air-fuel mixture in the

cylinder. That stratification is organised by swirl (see Fig. 5.17) of the air first and then

mixture according to shape of piston crown (see Fig. 5.18).

Fig. 5.17. Swirl in chamber of GDI engine Fig. 5.18.Piston for GDI system

Increasing of power in GDI conception is reach among other thing by increasing compression

ration without negative result of knocking (see Fig. 5.19).

Fig. 5.19. Volumetric efficiency for Multipoint Injection and GDI vs. compression ratios


20/25


54

Disadvantage of GDI application is producing high amount of NOx - much more than for

conventional fuel systems, so the engine using GDI has to be equipped with catalytic

reduction converter and exhaust gas recylculation device.

5.2.4 Diesel engine fuel systems

The performance of diesel engines is heavily influenced by their injection system design. In

fact, the most notable advances achieved in diesel engines resulted directly from superior

fuel-injection system designs. While the main purpose of the system is to deliver fuel to the

cylinders of a diesel engine, it is how that fuel is delivered that makes the difference in

engine performance, emissions, and noise characteristics.

Unlike its spark-ignited engine counterpart, the diesel injection system delivers fuel under

extremely high injection pressures. This aspect implies that the system component designs

and materials should be selected to withstand higher stresses, while still performing for

extended durations matching the engines durability targets. Greater manufacturing precision

and tight tolerances are also required for the systems efficient functioning. In addition to

expensive materials and manufacturing costs, diesel injection systems are characterized by

more intricate control requirements. All these features add up to a system whose cost may

represent as much as 30 % of the total cost of the engine.

Diesel fuel injection systems can be classified into three categories, as follows:

Pump-Line-Nozzle

Unit Injector and Unit Pump

Common Rail

Pump-line-nozzle(P-L-N) is a fuel system using central injection pump driven off the engine

geartrain (see Figure5.15 below). The injection pump feeds separate injection nozzles

located in the cylinder head above each cylinder. Lines which must be of exactly equal

length link the pump with the nozzles. In the case of the in-line pump, the central pump

incorporates a number of separate plunger/barrel pumping elements, each serving oneinjector. Each nozzle incorporates a needle valve and the orifices which provide fuel

atomisation.

The P-L-N fuel system used to be the most common type of diesel injection, dominating

most diesel engine applications. In addition to the in-line pump design, where each injector is

fed by a separate pumping element, there are several other configurations that have been

developed, including the distributor/rotary pump.


21/25


55

Fig. 5.15. Pump-Line-Nozzle System Principle (DieselNet)

Unit injector (UI) is a system that incorporates the high pressure pumping element and the

injector as one device, thus eliminating the need for high pressure fuel lines connecting the

pump with injectors (see Figure 5.16 below). Elimination of the injection lines decreases the

possibility of wave superposition, which may lead to after-injections and contribute to

injection delays. Traditionally, unit injector systems have been able to deliver extremely high

injection pressures. One of the main reasons for this unique capability is the absence of the

high pressure fuel line.

The related unit pump (UP) system also uses an individual, camshaft-driven injection pump

for each cylinder, but the pump and the injection nozzle body are separate and connected by

a short fuel line. Therefore, the UP system may be also classified as a variation of the P-L-N

fuel system.

It is a characteristic feature of the P-L-N and unit injector systems that their fuel pressure

depends on engine speed. It follows that when the engine is running at lower speeds, fuel

pressure will be rather low. Conversely, the higher the engine speed, the higher the injection

pressure. Such dependency on the engine speed for a very important injection system

function is regarded by many engine and fuel system designers as too binding. Moreover,

tough new restrictions on emissions and customer demands for performance as well as good

fuel economy require as much control flexibility as possible.


22/25


56

Fig. 5.16. Cross Section of Electronic Unit Injector (DieselNet)

In the course of developing the modern diesel engine, engineers have discovered the need

for, and benefits of, the following fuel injection equipment features:

- high injection pressure that can be controlled without dependence on engine speed,- high injection pressure independent of fuel quantity,- precision in fuel metering or injected quantity,- flexibility in injection timing or injection timing that could be varied according to

parameters decided by the designers,

- ability to perform multiple injections in one combustion cycle,- ability to meter extremely small fuel quantities,- ability to maintain minimum variation in injected quantity from one cylinder to

another in multi-cylinder engines,

- ability to minimize cycle-to-cycle variability in injected quantity,- ability to minimize cylinder-to-cylinder injection timing variation,- ability to minimize cycle-to-cycle injection timing variability,- maintenance of accurate fuel metering and injection timing over the expected life of

the engine,

- ability to interact with other subsystems to maximize performance and fuel economyand minimize exhaust emissions.

The last of the above functions involves the preparation of the combustible mixture. For

instance, the injector nozzle design must atomise the fuel to enhance its evaporation and

interaction with the available air in the combustion chamber. It must also produce the tiniest

of fuel particles with the capability of penetrating deep into the combustion chamber volume

and seek more air/oxygen to enhance combustion. This is a long list of desirable features that

comes with a corresponding list of engineering challenges.


23/25


57

The common rail injection system seems to meet many of the features desired by design

engineers. It is a simple concept of a diesel injection system employing a common pressure

accumulator, called the rail, which is mounted along the engine block (see Figure 5.17

below). The rail is fed by a high pressure fuel pump that can be driven at crank speed (engine

speed o twice the camshaft speed). High pressure injection lines connect the common rail to

the engine injectors that (in the mechanical incarnation of common rail) are actuated via

overhead cams or rocker arm mechanisms. In modern, electronically controlled systems, the

injectors are actuated by solenoid valves.

Fig. 5.17.Bosch Common Rail Diesel Fuel Injection System (DieselNet)

The total common rail system includes a low pressure supply pump that draws fuel from

the tank and feeds it to the high pressure pump. High pressure fuel delivered into the rail

may bring along pressure pulsations, therefore the volume of the common rail should be

designed to dampen those pulsations. These pulsations result from the delivery

characteristics of the multi-plungers radial fuel pump. Driving the pump at engine speed

increases fuel pumping capacity and improves the potential for high injection pressure.

Control of fuel metering and injection timing is similar to that in the in-line pump and unit

injector systems. In spite of the availability of these desirable features in common rail

systems, it needs to be emphasized that such systems could not attain their potential without

the help from electronic controls. In fact, electronics were introduced in all types of fuelinjection systems to expand their capabilities and improve their performance.

Each of the above injection system categories (P-L-N, UI and Common Rail) has branched

out into many unique and distinct designs, and each has a number of sub-categories. For

instance, pump-line-nozzle systems include in-line, distributor (rotary), as well as unit

pumps. Not only could we differentiate them as such, but we can also separate between them

on the basis of whether they are mechanically- or electronically-controlled. Similarly, unit

injectors can be mechanically- or electronically-controlled. Unit injector systems can also be

designed to deliver extremely high injection pressures, in which case they may feature

mechanical intensifiers in the form of plungers having two different diameters. They may also

be actuated by brute force imparted to the top of the injector by a large cam such as the


24/25


58

Cummins pressure-time controlled (PT) system. The common rail category enjoys a similar

variety of design details.

5.2.5 The search for increased injection pressures

The evolution of the diesel injection systems from the old mechanically-controlled P-L-N

designs to the modern electronically-controlled unit injector and accumulator designs, are

driven, in general, by the need to achieve tighter injection control and lower emissions, and

follow one distinct trend: increased injection pressures. In fact, this trend began in and

continued through the last century. For heavy-duty applications, it accelerated in the 1970s

to meet the demand for cleaner, as well as more fuel-efficient engines. Eventually the high

speed, passenger car market adopted the same philosophy, and even though it started a

decade after the heavy-duty applications, both light-duty and heavy-duty applications have

now similar injection pressures. The historical evolution in peak injection pressure is

illustrated in Figure 5.18 below.

Fig. 5.18. Peak Injection Pressure Versus Model Year (DieselNet)

The highest injection pressures have been realized in the unit injector system. Elimination

of high pressure fuel lines in the UI system allowed considerably higher injection pressures

(up to 200 MPa) compared with P-L-N systems. Injector pressure capabilities of various

systems are illustrated in Figure 5.19.

Fig. 5.19. Injector Pressure Capability of Various Injection Systems

UI - unit injector; UP - unit pump; CR - common rail; RP - rail pressureElectronic Control in Fuel Injection (DieselNet)


25/25


Review questions

What kind of fuels do you know?

Why does carburettor stop use?

Describe components of fuel system for SI engines.

What advantages does the common rail system has got?

Documents

Chap 5 Fuels and Systems