Now that we know how a diesel engine works, it is time to discuss advantages
and disadvantages. Diesel engines are more efficient than gasoline engines of
the same power rating, resulting in lower fuel consumption. A common margin is
40 percent more miles per gallon for an efficient turbo diesel. For example,
the current model Škoda Octavia, using Volkswagen Group engines, has a combined
Euro rating of 6.2 L/100 km (46 mpg-imp; 38 mpg-US)
for the 102 bhp (76 kW) gasoline engine and 4.4 L/100 km
(64 mpg-imp; 53 mpg-US) for the 105 bhp (78 kW)
diesel engine.
However, such a comparison does not take into
account that diesel fuel is denser and contains about 15 percent more energy by
volume. Although the calorific value of the fuel is slightly lower at
45.3 MJ/kg (megajoules per kilogram) than gasoline at 45.8 MJ/kg,
liquid diesel fuel is significantly denser than liquid gasoline. This is
significant because volume of fuel, in addition to mass, is an important
consideration in mobile applications. No vehicle has an unlimited volume
available for fuel storage.
Adjusting the numbers to account for the energy
density of diesel fuel, the overall energy efficiency is still about 20 percent
greater for the diesel version.
While a higher compression ratio is helpful in
raising efficiency, diesel engines are much more efficient than gasoline
(petrol) engines when at low power and at engine idle. Unlike the gasoline
engine, diesels lack a butterfly valve (throttle) in the inlet system, which
closes at idle. This creates parasitic loss and destruction of availability of
the incoming air, reducing the efficiency of gasoline engines at idle. In many
applications, such as marine, agriculture, and railways, diesels are left
idling and unattended for many hours, sometimes even days. These advantages are
especially attractive in locomotives (see dieselisation).
Even though diesel engines have a theoretical fuel
efficiency of 75 percent, in practice it is lower. Engines in large diesel
trucks, buses, and newer diesel cars can achieve peak efficiencies around 45
percent, and could reach 55 percent efficiency in the near future. However,
average efficiency over a driving cycle is lower than peak efficiency. For
example, it might be 37 percent for an engine with a peak efficiency of 44
percent.
Another inherent tendency
of diesel engines is the ability to produce greater torque or work than a
gasoline engine. For commercial uses requiring towing, load carrying and other
tractive tasks, diesel engines tend to have better torque characteristics than
gasoline engines. Diesel engines tend to have their torque peak quite low in
their speed range (usually between 1600 and 2000 rpm for a small-capacity
unit, lower for a larger engine used in a truck). This provides smoother
control over heavy loads when starting from rest, and, crucially, allows the
diesel engine to be given higher loads at lower speeds than a gasoline engine,
making them much more economical for these applications. This characteristic is
not so desirable in private cars, so most modern diesels used in such vehicles
use electronic control, variable geometry turbochargers and shorter piston
strokes to achieve a wider spread of torque over the engine's speed range,
typically peaking at around 2500–3000 rpm.
While diesel engines tend to have more torque at
lower engine speeds than gasoline engines, diesel engines tend to have a
narrower power band than gasoline engines. Naturally aspirated diesels tend to
lack power and torque at the top of their speed range. This narrow band is a
reason why a vehicle such as a truck may have a gearbox with as many as 18 or
more gears, to allow the engine's power to be used effectively at all speeds.
Turbochargers tend to improve power and torque at high engine speeds;
superchargers improve power at lower speeds; and variable geometry
turbochargers improve the engine's performance equally by flattening the torque
curve. The Paxman Hi-Dyne engine was a 1950s attempt to widen the power band of
a diesel engine.
In addition to torque, diesel engines generally
create more power than comparable gas engines. In diesel engines, conditions in
the engine differ from the spark-ignition engine, since controlling the air
supply directly controls by the fuel supply, rather than power.
The average diesel engine has a poorer
power-to-weight ratio than the gasoline engine. This is because the diesel must
operate at lower engine speeds and because it needs heavier, stronger parts to
resist the operating pressure caused by the high compression ratio of the
engine and the large amounts of torque generated to the crankshaft. In
addition, diesels are often built with stronger parts to give them longer lives
and better reliability, important considerations in industrial applications.
Diesel engines usually have longer stroke lengths
chiefly to facilitate achieving the necessary compression ratios, but also to
reduce the optimal operating speed (rpm). As a result piston and connecting
rods are heavier and more force must be transmitted through the connecting rods
and crankshaft to change the momentum of the piston. This is another reason
that a diesel engine must be stronger for the same power output as a gasoline
engine.
Yet it is this characteristic that has allowed some
enthusiasts to acquire significant power increases with turbocharged engines by
making fairly simple and inexpensive modifications. A gasoline engine of
similar size cannot put out a comparable power increase without extensive
alterations because the stock components cannot withstand the higher stresses
placed upon them. Since a diesel engine is already built to withstand higher
levels of stress, it makes an ideal candidate for performance tuning at little
expense. However, it should be said that any modification that raises the
amount of fuel and air put through a diesel engine would increase its operating
temperature, which will reduce its life and increase service requirements.
These are issues with newer, lighter, high-performance diesel engines,
which are not “overbuilt” to the degree of older engines, and they are being
pushed to provide greater power in smaller engines.
The addition of a turbocharger or supercharger to
the engine greatly assists in increasing fuel economy and power output,
mitigating the fuel-air intake speed limit mentioned above for a given engine
displacement. Boost pressures can be higher on diesels than on gasoline
engines, due to the latter's susceptibility to knock, and the higher compression
ratio allows a diesel engine to be more efficient than a comparable spark
ignition engine. Because the burned gases are expanded further in a diesel
engine cylinder, the exhaust gas is cooler, meaning turbochargers require less
cooling, and can be more reliable, than with spark-ignition engines.
Without the risk of knocking, boost pressure in a
diesel engine can be much higher; it is possible to run as much boost, as the
engine will physically stand before breaking apart.
A combination of improved mechanical technology
(such as multi-stage injectors which fire a short "pilot charge" of
fuel into the cylinder to warm the combustion chamber before delivering the
main fuel charge), higher injection pressures that have improved the
atomization of fuel into smaller droplets, and electronic control (which can
adjust the timing and length of the injection process to optimize it for all
speeds and temperatures) have mitigated most of these problems in the latest
generation of common-rail designs, while greatly improving engine efficiency.
The poor power and narrow torque bands have been addressed by superchargers,
turbochargers, (especially variable geometry turbochargers), intercoolers, and
a large efficiency increase from about 35 percent for IDI to 45 percent for the
latest engines in the last 15 years.
The increased fuel economy of the diesel engine
over the gasoline engine means that the diesel produces less carbon dioxide (CO2)
per unit distance. Recent advances in production and changes in the political
climate have increased the availability and awareness of biodiesel, an
alternative to petroleum-derived diesel fuel with a much lower net-sum emission
of CO2, due to the absorption of CO2 by plants used to
produce the fuel. Although concerns are now being raised as to the negative
effect this is having on the world food supply, as the growing of crops
specifically for biofuels takes up land that could be used for food crops and
uses water that could be used by both humans and animals. However, the use of
waste vegetable oil, sawmill waste from managed forests in Finland, and
advances in the production of vegetable oil from algae demonstrate great
promise in providing feed stocks for sustainable biodiesel that are not in
competition with food production.
When a diesel engine runs at low power, there is
enough oxygen present to burn the fuel- diesel engines only make significant
amounts of carbon monoxide when running under a load.
Diesel fuel is injected just before the power
stroke. As a result, the fuel cannot burn completely unless it has a sufficient
amount of oxygen. This can result in incomplete combustion and black smoke in
the exhaust if more fuel is injected than there is air available for the
combustion process. Modern engines with electronic fuel delivery can adjust the
timing and amount of fuel delivery (by changing the duration of the injection
pulse), and so operate with less waste of fuel. In a mechanical system, the injection
timing and duration must be set to be efficient at the anticipated operating
rpm and load, and so the settings are less than ideal when the engine is
running at any other RPM than what it is timed for. The electronic injection
can "sense" engine revs, load, even boost and temperature, and
continuously alter the timing to match the given situation. In the gasoline
engine, air and fuel are mixed for the entire compression stroke, ensuring
complete mixing even at higher engine speeds.
Diesel exhaust is well known for its characteristic
smell; but this smell in recent years has become much less because the sulfur
is now removed from the fuel in the oil refinery. Diesel exhaust has been found to contain a long
list of toxic air contaminants. Among these pollutants, fine particle pollution
is perhaps the most important as a cause of diesel's harmful health effects.
A combination of improved
mechanical technology such as multi-stage injectors which fire a short
"pilot charge" of fuel into the cylinder to initiate combustion
before delivering the main fuel charge, higher injection pressures that have
improved the atomization of fuel into smaller droplets, and electronic control
(which can adjust the timing and length of the injection process to optimize it
for all speeds and temperatures), have partially mitigated these problems in
the latest generation of common-rail designs, while improving engine
efficiency. For most industrial or
nautical applications, reliability is considered more important than lightweight
and high power.
The lack of an electrical ignition system greatly
improves the reliability. The high durability of a diesel engine is also due to
its overbuilt nature (see above), a benefit that is magnified by the lower
rotating speeds in diesels. Diesel fuel is a better lubricant than gasoline and
thus, it is less harmful to the oil film on piston rings and cylinder bores; it
is routine for diesel engines to cover 400,000 km (250,000 mi) or
more without a rebuild.
Due to the greater compression ratio and the
increased weight of the stronger components, starting a diesel engine is harder
than starting a gasoline engine of similar design and displacement. More torque
is required to push the engine through compression.
Either an electrical starter or an air-start system
is used to start the engine turning. On large engines, pre-lubrication and slow
turning of an engine, as well as heating, is required to minimize the amount of
engine damage during initial start-up and running. Some smaller military diesels
can be started with an explosive cartridge, called a Coffman starter, which
provides the extra power required to get the machine turning. In the past, Caterpillar
and John Deere used a small gasoline pony engine in their tractors to
start the primary diesel engine. The pony engine heated the diesel to aid in
ignition and used a small clutch and transmission to spin up the diesel engine.
Even more unusual was an International Harvester design in which the diesel
engine had its own carburetor and ignition system, and started on gasoline.
Once warmed up, the operator moved two levers to switch the engine to diesel
operation, and work could begin. These engines had very complex cylinder heads,
with their own gasoline combustion chambers, and were vulnerable to expensive
damage if special care was not taken (especially in letting the engine cool
before turning it off).
Petrol/gasoline engines are limited in the variety
and quality of the fuels they can burn. Older gasoline engines fitted with a carburetor
required a volatile fuel that would vaporize easily to create the necessary air-fuel
ratio for combustion. Because both air and fuel are admitted to the cylinder,
if the compression ratio of the engine is too high or the fuel too volatile
(with too low an octane rating), the fuel will ignite under compression, as in
a diesel engine, before the piston reaches the top of its stroke. This
pre-ignition causes a power loss and over time major damage to the piston and
cylinder. The need for a fuel that is volatile enough to vaporize but not too
volatile (to avoid pre-ignition) means that gasoline engines will only run on a
narrow range of fuels. There has been some success at dual-fuel engines that
use gasoline and ethanol, petrol and propane, and gasoline and methane.
In diesel engines, a mechanical injector system
vaporizes the fuel directly into the combustion chamber or a pre-combustion
chamber (as opposed to a Venturi jet in a carburetor, or a fuel injector in a
fuel injection system vaporizing fuel into the intake manifold or intake
runners as in a petrol engine). This forced vaporization means that
less-volatile fuels can be used. More crucially, because only air is inducted
into the cylinder in a diesel engine, the compression ratio can be much higher
as there is no risk of pre-ignition provided the injection process is
accurately timed. This means that cylinder temperatures are much higher in a
diesel engine than a gasoline engine, allowing less volatile fuels to be used.
Diesel fuel is a form of light fuel oil, very
similar to kerosene (paraffin), but diesel engines, especially older or simple
designs that lack precision electronic injection systems, can run on a wide
variety of other fuels. Some of the most common alternatives are Jet A-1 type jet
fuel or vegetable oil from a very wide variety of plants. Some engines can be
run on vegetable oil without modification, and most others require fairly basic
alterations. Biodiesel is a pure diesel-like fuel refined from vegetable oil
and can be used in nearly all diesel engines. Requirements for fuels to be used
in diesel engines are the ability of the fuel to flow along the fuel lines, the
ability of the fuel to lubricate the injector pump and injectors adequately,
and its ignition qualities (ignition delay, cetane number). Inline mechanical
injector pumps generally tolerate poor-quality or bio-fuels better than
distributor-type pumps. Also, indirect injection engines generally run more
satisfactorily on bio-fuels than direct injection engines. This is partly
because an indirect injection engine has a much greater 'swirl' effect,
improving vaporization and combustion of fuel, and because (in the case of
vegetable oil-type fuels) lipid depositions can condense on the cylinder walls
of a direct-injection engine if combustion temperatures are too low (such as
starting the engine from cold).
It is often reported that Diesel designed his
engine to run on peanut oil, but this is false. Patent number 608845 describes
his engine as being designed to run on pulverulent solid fuel (coal dust).
Diesel stated in his published papers, "at the Paris Exhibition in 1900 (Exposition
Universelle) there was shown by the Otto Company a small diesel engine,
which, at the request of the French Government ran on Arachide (earth-nut or
peanut) oil (see biodiesel), and worked so smoothly that only a few people were
aware of it. The engine was constructed for using mineral oil, and was then
worked on vegetable oil without any alterations being made. The French
Government at the time thought of testing the applicability to power production
of the Arachide, or earth-nut, which grows in considerable quantities in their
African colonies, and can easily be cultivated there." Diesel himself
later conducted related tests and appeared supportive of the idea.
Diesel engines can operate on a variety of
different fuels, depending on configuration, though the eponymous diesel fuel
derived from crude oil is most common. The engines can work with the full
spectrum of crude oil distillates, from natural gas, alcohols, gasoline, wood
gas to the fuel oils from diesel oil to residual fuels. Many automotive diesel
engines would run on 100 percent biodiesel without any modifications. This
would be such a potential advantage since biodiesel can be made so much more
cheaply than it takes to have traditional diesel fuel from your fuel station's
pump.
The
type of fuel used is selected to meet a combination of service requirements,
and fuel costs. Good-quality diesel fuel can be synthesized from vegetable oil
and alcohol. Diesel fuel can be made from coal or other carbon base using the Fischer-Tropsch
process. Biodiesel is growing in popularity since it can frequently be used in
unmodified engines, though production remains limited. Recently, biodiesel from
coconut, which can produce a very promising coco methyl ester (CME), has
characteristics which enhance lubricity and combustion giving a regular diesel
engine without any modification more power, less particulate matter or black
smoke, and smoother engine performance. The Philippines pioneers in the
research on Coconut based CME with the help of German and American scientists.
Petroleum-derived diesel is often called petrodiesel if there is need to
distinguish the source of the fuel.
Pure plant oils are increasingly being used as a
fuel for cars, trucks and remote combined heat and power generation especially
in Germany where hundreds of decentralized small- and medium-sized oil presses
cold press oilseed, mainly rapeseed, for fuel. There is a Deutsches Institut
für Normung fuel standard for rapeseed oil fuel. Normal diesel fuel is more difficult to
ignite and slower in developing fire than petrol because of its higher flash
point, but once burning, a diesel fire can be fierce.
Fuel contaminants such as dirt and water are often
more problematic in diesel engines than in gasoline engines. Water can cause
serious damage, due to corrosion, to the injection pump and injectors; and
dirt, even very fine particulate matter, can damage the injection pumps due to
the close tolerances that the pumps are machined to. All diesel engines will
have a fuel filter (usually much finer than a filter on a petrol engine), and a
water trap. The water trap (which is sometimes part of the fuel filter) often
has a float connected to a warning light, which warns when there is too much
water in the trap, and must be drained before damage to the engine can result.
The fuel filter must be replaced much more often on a diesel engine than on a gasoline
engine, changing the fuel filter every two to four oil changes is not uncommon
for some vehicles.
Diesels are considered much safer in general than gasoline-powered
vehicles. Diesel fuel has low flammability, leading to a low risk of fire
caused by fuel in a vehicle equipped with a diesel engine. The United States Army and NATO use only
diesel engines and turbines because of fire hazard. Although neither gasoline
nor diesel is explosive in liquid form, both can create an explosive air/vapor
mix under the right conditions. However, diesel fuel is less prone due to its
lower vapor pressure, which is an indication of evaporation rate. The Material
Safety Data Sheet for
ultra-low sulfur diesel fuel indicates a vapor explosion hazard for diesel
indoors, outdoors, or in sewers.
Fuel injection introduces potential hazards in
engine maintenance due to the high fuel pressures used. Residual pressure can
remain in the fuel lines long after an injection-equipped engine has been shut
down. This residual pressure must be relieved, and if it is done so by external
bleed-off, the fuel must be safely contained. If a high-pressure diesel fuel
injector is removed from its seat and operated in open air, there is a risk to
the operator of injury by hypodermic jet-injection, even with only 100 pounds
per square inch (690 kPa) pressure. The first known such injury occurred
in 1937 during a diesel engine maintenance operation.
Diesel exhaust has been classified as an IARC Group
1 carcinogen. It is a cause of lung cancer and is associated with an increased
risk for bladder cancer. Many
advancements in diesel technology have taken place in the last few years due to
increasingly stringent emissions regulations both here and in Europe.
High-speed (approximately 1,000 rpm and greater) engines are used to power
trucks (lorries), buses, tractors, cars, yachts, compressors, pumps and small electrical
generators. As of 2008, most high-speed engines have direct injection. Many
modern engines, particularly in on-highway applications, have common rail direct
injection, which is cleaner burning.
As of 2008, many common rail and unit injection
systems already employ new injectors using stacked piezoelectric wafers in lieu
of a solenoid, giving finer control of the injection event. Variable geometry turbochargers have flexible
vanes, which move and let more air into the engine depending on load. This
technology increases both performance and fuel economy. Boost lag is reduced as turbo impeller inertia
is compensated for.
Accelerometer pilot control (APC) uses an accelerometer
to provide feedback on the engine's level of noise and vibration and thus
instruct the ECU to inject the minimum amount of fuel that will produce quiet
combustion and still provide the required power (especially while idling).
The next
generation of common rail diesels is expected to use variable injection
geometry, which allows the amount of injected fuel to be varied over a wider
range, and variable valve timing (see Mitsubishi's 4N13 diesel engine) similar
to that on gasoline engines. Particularly in the United States, coming tougher
emissions regulations present a considerable challenge to diesel engine
manufacturers. Ford's HyTrans Project has developed a system, which starts the
ignition in 400 ms, saving a significant amount of fuel on city routes, and
there are other methods to achieve even more efficient combustion, such as homogeneous
charge compression ignition, being studied.
Japanese and
Swedish vehicle manufacturers are also developing diesel engines that run on dimethyl
ether (DME). Some recent diesel engine
models utilize a copper alloy heat exchanger technology (CuproBraze) to take
advantage of benefits in terms of thermal performance, heat transfer
efficiency, strength/durability, corrosion resistance, and reduced emissions
from higher operating temperatures.
Some research for this article is from
Wikipedia.org
There are no
new recalls this week.
