Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a
process that converts chemical potential energy contained in a carrier fuel into kinetic
energy or work. Overall fuel efficiency may vary per device,
which in turn may vary per application fuel efficiency, especially fossil fuel power plants or industries
dealing with combustion, such as ammonia production during the Haber
process.
In the context of transport,
fuel economy is the energy efficiency of a particular vehicle, is given as a ratio of distance
travelled per unit of fuel consumed. Fuel economy is expressed in miles per gallon (mpg) in the
USA and usually also in the UK (imperial gallon);there is sometimes confusion as the
imperial gallon is 20% larger than the US gallon so that mpg values are not
directly comparable. In countries using the metric
system fuel economy is stated in kilometres per litre (km/L) in the Netherlands,
Denmark and in
several Latin American or Asian countries such as India, Japan, South Korea,
or as the reciprocal ratio, "fuel consumption" in liters per 100 kilometers
(L/100 km) in much of Europe, Canada, New Zealand and Australia. Litres
per mil are used in Norway and Sweden.
Fuel consumption is a more accurate measure of a vehicle’s
performance because it is a linear relationship while fuel
economy leads to distortions in efficiency improvements.
Weight-specific efficiency (efficiency per unit weight) may
be stated for freight,
and passenger-specific efficiency (vehicle efficiency per passenger).
Vehicle design
Fuel efficiency is dependent on many parameters of a
vehicle, including its engine parameters, aerodynamic
drag, weight, and rolling resistance. There have been advances in
all areas of vehicle design in recent decades.
Hybrid vehicles use two or more power sources for
propulsion. In many designs, a small combustion engine is combined with
electric motors. Kinetic energy which would otherwise be lost to heat during
braking is recaptured as electrical power to improve fuel efficiency. Engines
automatically shut off when vehicles come to a stop and start again when the
accelerator is pressed preventing wasted energy from idling.
Fleet efficiency
Fleet efficiency describes the average efficiency of a
population of vehicles. Technological advances in efficiency may be offset by a
change in buying habits with a propensity to heavier vehicles, which are less
efficient, all else being equal.
Energy efficiency terminology
Energy efficiency is similar to fuel efficiency but the
input is usually in units of energy such as British thermal units (BTU),
megajoules (MJ), gigajoules (GJ), kilocalories (kcal), or kilowatt-hours (kW·h).
The inverse of "energy efficiency" is "energy intensity",
or the amount of input energy required for a unit of output such as
MJ/passenger-km (of passenger transport), BTU/ton-mile (of freight transport,
for long/short/metric tons), GJ/t (for steel production), BTU/(kW·h) (for
electricity generation), or litres/100 km (of vehicle travel). Litres per
100 km is also a measure of "energy intensity" where the input
is measured by the amount of fuel and the output is measured by the distance
travelled. For example: Fuel economy in automobiles.
Given a heat value of a fuel, it would be trivial to convert
from fuel units (such as litres of gasoline) to energy units (such as MJ) and
conversely. But there are two problems with comparisons made using energy
units:
- There are two different heat values for any hydrogen-containing fuel which can differ by several percent (see below).
- When comparing transportation energy costs, it must be remembered that a kilowatt hour of electric energy may require an amount of fuel with heating value of 2 or 3 kilowatt hours to produce it.
Energy content of fuel
The specific energy content of a fuel is the heat energy
obtained when a certain quantity is burned (such as a gallon, litre, kilogram).
It is sometimes called the heat of combustion. There exists two different
values of specific heat energy for the same batch of fuel. One is the high (or
gross) heat of combustion and the other is the low (or net) heat of combustion.
The high value is obtained when, after the combustion, the water in the exhaust
is in liquid form. For the low value, the exhaust has all the water in vapor
form (steam). Since water vapor gives up heat energy when it changes from vapor
to liquid, the liquid water value is larger since it includes the latent heat
of vaporization of water. The difference between the high and low values is
significant, about 8 or 9%. This accounts for most of the apparent discrepancy
in the heat value of gasoline. In the U.S. (and the table below) the high heat
values have traditionally been used, but in many other countries, the low heat
values are commonly used.
Neither the gross heat of combustion nor the net heat of
combustion gives the theoretical amount of mechanical energy (work) that can be
obtained from the reaction. (This is given by the change in Gibbs
free energy, and is around 45.7 MJ/kg for gasoline.) The actual amount
of mechanical work obtained from fuel (the inverse of the specific fuel consumption) depends
on the engine. A figure of 17.6 MJ/kg is possible with a gasoline engine,
and 19.1 MJ/kg for a diesel engine. See Brake specific fuel consumption for
more information.
Fuel efficiency of vehicles
The fuel efficiency of vehicles can be expressed in more
ways:
- Fuel consumption is the amount of fuel used per unit distance; for example, litres per 100 kilometres (L/100 km). In this case, the lower the value, the more economic a vehicle is (the less fuel it needs to travel a certain distance); this is the measure generally used across Europe (except the UK, Denmark and The Netherlands - see below), New Zealand, Australia and Canada. Also in Uruguay, Paraguay, Guatemala, Colombia, China, and Madagascar., as also in post-Soviet space.
- Fuel economy is the distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or miles per gallon (MPG), where 1 MPG (imperial) ≈ 0.354006 km/L. In this case, the higher the value, the more economic a vehicle is (the more distance it can travel with a certain volume of fuel). This measure is popular in the USA and the UK (mpg), but in Europe, India, Japan, South Korea and Latin America the metric unit km/L is used instead.
Converting from mpg or to L/100 km (or vice versa)
involves the use of the reciprocal function, which is not distributive.
Therefore, the average of two fuel economy numbers gives different values if
those units are used, because one of the functions is reciprocal, thus not
linear. If two people calculate the fuel economy average of two groups of cars
with different units, the group with better fuel economy may be one or the
other. However, from the point of energy used as a shared method of measure,
the result shall be the same in both the cases.
The formula for converting to miles per US gallon (exactly
3.785411784 L) from L/100 km is
, where
is value of L/100 km. For miles per Imperial gallon
(exactly 4.54609 L) the formula is
.
In parts of Europe, the two standard measuring cycles for
"litre/100 km" value are "urban" traffic with speeds up to
50 km/h from a cold start, and then "extra urban" travel at
various speeds up to 120 km/h which follows the urban test. A combined
figure is also quoted showing the total fuel consumed in divided by the total
distance traveled in both tests. A reasonably modern European supermini
and many mid-size cars, including station wagons, may manage motorway travel
at 5 L/100 km (47 mpg US/56 mpg imp) or 6.5 L/100 km in city traffic
(36 mpg US/43 mpg imp), with carbon
dioxide emissions of around 140 g/km.
An average North
American mid-size car travels 21 mpg (US) (11 L/100 km)
city, 27 mpg (US) (9 L/100 km) highway; a full-size
SUV usually travels 13
mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup
trucks vary considerably; whereas a 4 cylinder-engined light pickup can
achieve 28 mpg (8 L/100 km), a V8
full-size pickup with extended cabin only travels 13 mpg (US) (18
L/100 km) city and 15 mpg (US) (15 L/100 km) highway.
The average fuel economy is higher in Europe due to the
higher cost of fuel. In the UK, a gallon of gas without tax would cost US$1.97,
but with taxes cost US$6.06 in 2005. The average cost in the United States was
US$2.61. Consumers prefer "muscle cars" but choose more fuel
efficient ones when gas prices increase.
European-built cars are generally more fuel-efficient than
US vehicles. While Europe has many higher efficiency diesel cars, European
gasoline vehicles are on average also more efficient than gasoline-powered vehicles
in the USA. Most European vehicles cited in the CSI study run on diesel
engines, which tend to achieve greater fuel efficiency than gas engines.
Selling those cars in the United States is difficult because of emission
standards, notes Walter McManus, a fuel economy expert at the University of
Michigan Transportation Research Institute. "For the most part, European
diesels don’t meet U.S. emission standards", McManus said in 2007. Another
reason why many European models are not marketed in the United States is that
labor unions object to having the big 3 import any new foreign built models
regardless of fuel economy while laying off workers at home.
An example of European cars' capabilities of fuel economy is
the microcar
Smart
Fortwo cdi, which can achieve up to 3.4 l/100 km (69.2 mpg US)
using a turbocharged
three-cylinder 41 bhp (30 kW) Diesel engine. The Fortwo is produced
by Daimler
AG and is currently only sold by one company in the United States.
Furthermore, the current (and to date already 10 year old) world record in fuel
economy of production cars is held by the Volkswagen
Group, with special production models (labeled "3L") of the Volkswagen Lupo and the Audi A2,
consuming as little as 3 L/100 km (94 mpg-imp;
78 mpg-US).
Diesel engines generally achieve greater fuel
efficiency than petrol (gasoline) engines. Passenger car diesel engines have energy efficiency of up to 41% but
more typically 30%, and petrol engines of up to 37.3%, but more typically 20%.
That is one of the reasons why diesels have better fuel efficiency than
equivalent petrol cars. A common margin is 25% more miles per gallon for an
efficient turbodiesel.
For example, the current model Skoda Octavia, using
Volkswagen engines, has a combined European fuel efficiency of 41.3 mpg for the
105 bhp (78 kW) petrol engine and 52.3 mpg for the 105 bhp
(78 kW) — and heavier — diesel engine. The higher compression ratio is
helpful in raising the energy efficiency, but diesel fuel also contains
approximately 10% more energy per unit volume than gasoline which contributes
to the reduced fuel consumption for a given power output.
In 2002, the United States had 85,174,776 trucks, and
averaged 13.5 miles per US gallon (17.4 L/100 km; 16.2 mpg-imp).
Large trucks, over 33,000 pounds (15,000 kg), averaged 5.7 miles per US
gallon (41 L/100 km; 6.8 mpg-imp).
The average economy of automobiles in the United States in
2002 was 22.0 miles per US gallon (10.7 L/100 km; 26.4 mpg-imp).
By 2010 this had increased to 23.0 miles per US gallon
(10.2 L/100 km; 27.6 mpg-imp). Average fuel economy
in the United States gradually declined until 1973, when it reached a low of
13.4 miles per US gallon (17.6 L/100 km; 16.1 mpg-imp)
and gradually has increased since, as a result of higher fuel cost. A study
indicates that a 10% increase in gas prices will eventually produce a 2.04%
increase in fuel economy.
Fuel efficiency in microgravity
How fuel combusts affects how much energy is produced. The National Aeronautics and
Space Administration (NASA) has investigated fuel consumption in microgravity.
The common distribution of a flame under normal gravity
conditions depends on convection, because soot tends to rise to the top of a
flame, such as in a candle, making the flame yellow. In microgravity or zero
gravity, such as an environment in outer space,
convection no longer occurs, and the flame becomes spherical, with a
tendency to become more blue and more efficient. There are several possible
explanations for this difference, of which the most likely one given is the hypothesis
that the temperature is evenly distributed enough that soot is not formed and
complete combustion occurs., National Aeronautics and Space Administration,
April 2005. Experiments by NASA in microgravity reveal that diffusion
flames in microgravity allow more soot to be completely oxidised after they
are produced than diffusion flames on Earth, because of a series of mechanisms
that behaved differently in microgravity when compared to normal gravity
conditions.LSP-1
experiment results, National Aeronautics and Space Administration, April
2005. Premixed flames in microgravity burn at a much
slower rate and more efficiently than even a candle on Earth, and last much
longer.
Transportation
Fuel efficiency in transportation
Vehicle efficiency and transportation pollution
Fuel efficiency directly affects emissions causing pollution
by affecting the amount of fuel used. However, it also depends on the fuel
source used to drive the vehicle concerned. Cars for example, can run on a
number of fuel types other than gasoline, such as natural gas, LPG or biofuel or
electricity which creates various quantities of atmospheric pollution.
A kilogram of carbon, whether contained in petrol, diesel,
kerosene, or any other hydrocarbon fuel in a vehicle, leads to approximately
3.6 kg of CO2
emissions. Due to the carbon content of gasoline, its combustion emits
2.3 kg/l (19.4 lb/US gal) of CO2; since
diesel fuel is more energy dense per unit volume, diesel emits 2.6 kg/l
(22.2 lb/US gal). This figure is only the CO2 emissions of the
final fuel product and does not include additional CO2 emissions
created during the drilling, pumping, transportation and refining steps
required to produce the fuel. Additional measures to reduce overall emission includes
improvements to the efficiency of air
conditioners, lights and tires.
Driving technique
There is a growing community of enthusiasts known as hypermilers
who develop and practice driving techniques to increase fuel efficiency and
reduce consumption. Hypermilers have broken records of fuel efficiency, for
example, achieving 109 miles per gallon in a Prius. In non-hybrid
vehicles these techniques are also beneficial. Hypermiler Wayne
Gerdes can get 59 MPG in a Honda
Accord and 30 MPG in an Acura MDX.
All drivers have the potential to improve their fuel
efficiency. Applying these five
basic fuel-efficient driving techniques can potentially reduce your fuel
consumption by up to 25% and save you hundreds of dollars a year in fuel.
Fuel-efficient driving is easy. In fact, it’s one of the simplest and most
effective ways any driver can cut their fuel costs, shrink their environmental
footprints and ward off climate change.
Advanced Technology improvements to improve fuel
efficiency
The most efficient machines for converting energy to rotary
motion are electric motors, as used in electric
vehicles. However, electricity is not a primary energy source so the
efficiency of the electricity production has also to be taken into account.
Currently railway
trains can be powered using electricity, delivered through an additional
running rail, overhead catenary system or by on-board generators used in diesel-electric locomotives as common
on the UK rail network. Pollution produced from centralised generation of
electricity is emitted at a distant power station, rather than "on
site". Some railways, such as the French SNCF and Swiss federal railways
derive most, if not 100% of their power, from hydroelectric or nuclear power
stations, therefore atmospheric pollution from their rail networks is very low.
This was reflected in a study by AEA Technology between a Eurostar train
and airline journeys between London and Paris, which showed the trains on
average emitting 10 times less CO2, per passenger, than planes,
helped in part by French nuclear generation. This can be changed using more renewable
sources for electric generation.
In the future, hydrogen
cars may be commercially available. Powered either through chemical
reactions in a fuel cell that create electricity to drive very efficient
electrical motors or by directly burning hydrogen in a combustion engine (near
identically to a natural gas vehicle, and similarly compatible
with both natural gas and gasoline); these vehicles promise to have near zero
pollution from the tailpipe (exhaust pipe). Potentially the atmospheric
pollution could be minimal, provided the hydrogen is made by electrolysis
using electricity from non-polluting sources such as solar, wind or hydroelectricity
or thermochemically by the use of the Thorium fuel cycle in a molten salt reactor.
In any process, it is vitally important to account for all
of the energy used throughout the process. Thus, in addition to the energy cost
of the electricity or hydrogen production, we must also account for
transmission and/or storage losses to support large-scale use of such vehicles.
For this reason the use of the idea "zero pollution" should be
avoided.
In 2004, a consortium of major automakers — BMW, General
Motors, Honda,
Toyota and Volkswagen/Audi — came up with "Top
Tier Detergent Gasoline Standard" to gasoline brands
in the U.S. and Canada that meet their minimum standards for detergent
content and do not contain metallic additives. Top Tier gasoline contains
higher levels of detergent additives in order to prevent the build-up of
deposits (typically, on fuel injector and intake
valve) known to reduce fuel economy and engine performance.
SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment