The first subject of this week’s article is how a diesel engine operates. The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed hot air to ignite the fuel rather than using a spark plug (compression ignition rather than spark ignition).
In the true diesel engine, only air is initially
introduced into the combustion chamber. The air is then compressed with a
compression ratio typically between 15:1 and 22:1 resulting in 40-bar
(4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to
1.4 MPa; 120 to 200 psi) in the petrol engine. This high compression
heats the air to 550 °C (1,022 °F). At about the top of the compression
stroke, fuel is injected directly into the compressed air in the combustion
chamber. This may be into a (typically toroidal) void in the top of the piston
or a pre-chamber depending upon the design of the engine. The fuel
injector ensures that the fuel is broken down into small droplets, and that the
fuel is distributed evenly. The heat of the compressed air vaporizes fuel from
the surface of the droplets. The heat from the compressed air in the combustion
chamber then ignites the vapor; the droplets continue to vaporize from their surfaces
and burn, getting smaller, until all the fuel in the droplets has been burnt.
The start of vaporization causes a delay period during ignition and the
characteristic diesel knocking sound as the vapor reaches ignition temperature
and causes an abrupt increase in pressure above the piston. The rapid expansion
of combustion gases then drives the piston downward, supplying power to the
crankshaft.
As well as the high level of compression allowing
combustion to take place without a separate ignition system, a high compression
ratio greatly increases the engine's efficiency. Increasing the compression
ratio in a spark-ignition engine where fuel and air are mixed before entry to
the cylinder is limited by the need to prevent damaging pre-ignition. Since
only air is compressed in a diesel engine, and fuel is not introduced into the
cylinder until shortly before top dead centre (TDC), premature detonation is
not an issue and compression ratios are much higher.
Diesel's original engine injected fuel with the assistance of compressed air, which atomized the fuel and forced it into the engine through a nozzle (a similar principle to an aerosol spray). The nozzle opening was closed by a pin valve lifted by the camshaft to initiate the fuel injection before top dead centre (TDC). This is called an air-blast injection. Driving the three-stage compressor used some power but the efficiency and net power output was more than any other combustion engine at that time.
Diesel engines in service today raise the fuel to
extreme pressures by mechanical pumps and deliver it to the combustion chamber
by pressure-activated injectors without compressed air. With direct injected
diesels, injectors spray fuel through 4 to 12 small orifices in its nozzle. The
early air injection diesels always had a superior combustion without the sharp
increase in pressure during combustion. Research is now being performed and
patents are being taken out to again use some form of air injection to reduce
the nitrogen oxides and pollution, reverting to Diesel's original
implementation with its superior combustion and possibly quieter operation. In
all major aspects, the modern diesel engine holds true to Rudolf Diesel's
original design, that of igniting fuel by compression at an extremely high
pressure within the cylinder. With much higher pressures and high technology
injectors, present-day diesel engines use the so-called solid injection system
applied by Herbert Akroyd Stuart for his hot bulb engine. The indirect
injection engine could be considered the latest development of these low speed hot
bulb ignition engines.
A vital component of all diesel engines is a
mechanical or electronic governor which regulates the idling speed and maximum speed
of the engine by controlling the rate of fuel delivery. Unlike Otto-cycle
engines, incoming air is not throttled and a diesel engine without a governor
cannot have a stable idling speed and can easily overspeed, resulting in its
destruction. Mechanically governed fuel injection systems are driven by the
engine's gear train.
These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern electronically controlled diesel engines control fuel delivery by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal, as well as other operating parameters such as intake manifold pressure and fuel temperature, from a sensor and controls the amount of fuel and start of injection timing through actuators to maximize power and efficiency and minimize emissions. Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is measured in degrees of crank angle of the piston before top dead centre. For example, if the ECM/ECU initiates fuel injection when the piston is 10° before TDC, the start of injection, or timing, is said to be 10° BTDC. Optimal timing will depend on the engine design as well as its speed and load.
These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern electronically controlled diesel engines control fuel delivery by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal, as well as other operating parameters such as intake manifold pressure and fuel temperature, from a sensor and controls the amount of fuel and start of injection timing through actuators to maximize power and efficiency and minimize emissions. Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is measured in degrees of crank angle of the piston before top dead centre. For example, if the ECM/ECU initiates fuel injection when the piston is 10° before TDC, the start of injection, or timing, is said to be 10° BTDC. Optimal timing will depend on the engine design as well as its speed and load.
Advancing the start of injection (injecting before
the piston reaches to its SOI-TDC) results in higher in-cylinder pressure and
temperature, and higher efficiency, but also results in increased engine noise
due to faster cylinder pressure rise and increased oxides of nitrogen (NOx)
formation due to higher combustion temperatures. Delaying start of injection
causes incomplete combustion; reduced fuel efficiency and an increase in
exhaust smoke, containing a considerable amount of particulate matter and
unburned hydrocarbons.
Diesel engines have several advantages over other internal combustion engines:
- They burn less fuel than a petrol engine performing the same work, due to the engine's higher temperature of combustion and greater expansion ratio. Gasoline engines are typically 30 percent efficient while diesel engines can convert over 45 percent of the fuel energy into mechanical.
- They have no high voltage electrical ignition system, resulting in high reliability and easy adaptation to damp environments. The absence of coils, spark plug wires, etc., also eliminates a source of radio frequency emissions, which can interfere with navigation and communication equipment, which is especially important in marine and aircraft applications.
- The life of a diesel engine is generally about twice as long as that of a petrol engine due to the increased strength of parts used. Diesel fuel has better lubrication properties than petrol as well.
- Diesel fuel is distilled directly from petroleum. Distillation yields some gasoline, but the yield would be inadequate without catalytic reforming, which is a more costly process.
- Diesel fuel is considered safer than petrol in many applications. Although diesel fuel will burn in open air using a wick, it will not explode and does not release a large amount of flammable vapor. The low vapor pressure of diesel is especially advantageous in marine applications, where the accumulation of explosive fuel-air mixtures is a particular hazard. For the same reason, diesel engines are immune to vapor lock.
- For any given partial load the fuel efficiency (mass burned per energy produced) of a diesel engine remains nearly constant, as opposed to petrol and turbine engines, which use proportionally more fuel with partial power outputs. They generate less waste heat in cooling and exhaust.
- Diesel engines can accept super- or turbo-charging pressure without any natural limit, constrained only by the strength of engine components. This is unlike petrol engines, which inevitably suffer detonation at higher pressure.
- The carbon monoxide content of the exhaust is minimal; therefore diesel engines are used in underground mines.
- Biodiesel is an easily
synthesized, non-petroleum-based fuel (through transesterification) which
can run directly in many diesel engines, while gasoline engines either
need adaptation to run synthetic fuels or else use them as an additive to
gasoline (e.g., ethanol added to gasohol).
Many configurations of fuel injection have been used over the course of the twentieth century. Most present-day diesel engines use a mechanical single plunger high-pressure fuel pump driven by the engine crankshaft. For each engine cylinder, the corresponding plunger in the fuel pump measures out the correct amount of fuel and determines the timing of each injection. These engines use injectors that are very precise spring-loaded valves that open and close at a specific fuel pressure. Separate high-pressure fuel lines connect the fuel pump with each cylinder. Fuel volume for each single combustion is controlled by a slanted groove in the plunger, which rotates only a few degrees releasing the pressure, and is controlled by a mechanical governor, consisting of weights rotating at engine speed constrained by springs and a lever. The injectors are held open by the fuel pressure. On high-speed engines the plunger pumps are together in one unit. The length of fuel lines from the pump to each injector is normally the same for each cylinder in order to obtain the same pressure delay.
A cheaper configuration on high-speed engines with fewer than six cylinders is to use an axial-piston distributor pump, consisting of one rotating pump plunger delivering fuel to a valve and line for each cylinder (functionally analogous to points and distributor cap on an Otto engine).
Many modern systems have a single fuel pump which supplies fuel constantly at high pressure with a common rail (single fuel line common) to each injector. Each injector has a solenoid operated by an electronic control unit, resulting in more accurate control of injector opening times that depend on other control conditions, such as engine speed and loading, and providing better engine performance and fuel economy.
Both mechanical and electronic injection systems can be used in either direct or indirect injection configurations. Two-stroke diesel engines with mechanical injection pumps can be inadvertently run in reverse, albeit in a very inefficient manner, possibly damaging the engine. Large ship two-stroke diesels are designed to run in either direction, obviating the need for a gearbox.
An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a pre-chamber or ante-chamber, where combustion begins and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. This system allows for a smoother, quieter running engine, and because combustion is assisted by turbulence, injector pressures can be lower, about 100 bar (10 MPa; 1,500 psi), using a single orifice tapered jet injector. Mechanical injection systems allowed high-speed running suitable for road vehicles (typically up to speeds of around 4000 rpm. The pre-chamber had the disadvantage of increasing heat loss to the engine's cooling system, and restricting the combustion burn, which reduced the efficiency by five to ten percent. Indirect injection engines are cheaper to build and it is easier to produce smooth, quiet-running vehicles with a simple mechanical system. In road-going vehicles most prefer the greater efficiency and better-controlled emission levels of direct injection. Indirect injection diesels can still be found in the many ATV diesel applications.
Direct injection diesel engines have injectors mounted at the top of the combustion chamber. The injectors are activated using one of two methods - hydraulic pressure from the fuel pump, or an electronic signal from an engine controller.
Hydraulic pressure activated injectors can produce harsh engine noise. Fuel consumption is about 15 to 20 percent lower than indirect injection diesels. The extra noise is generally not a problem for industrial uses of the engine, but for automotive usage, buyers have to decide whether or not the increased fuel efficiency would compensate for the extra noise.
Electronic control of the fuel injection transformed the direct injection engine by allowing much greater control over the combustion.
Unit direct injection also injects fuel directly into the cylinder of the engine. In this system the injector and the pump are combined into one unit positioned over each cylinder controlled by the camshaft. Each cylinder has its own unit eliminating the high-pressure fuel lines, achieving a more consistent injection. Volkswagen AG uses this type of injection system, also developed by Bosch, in cars (where it is called a Pumpe-Düse-System—literally pump-nozzle system) and by Mercedes Benz ("PLD") and most major diesel engine manufacturers in large commercial engines (CAT, Cummins, Detroit Diesel, Electro-Motive Diesel, Volvo). With recent advancements, the pump pressure has been raised to 2,400 bars (240 MPa; 35,000 psi), allowing injection parameters similar to common rail systems.
In common rail systems, the separate pulsing high-pressure fuel line to each cylinder's injector is also eliminated. Instead, a high-pressure pump pressurizes fuel at up to 2,500 bar (250 MPa; 36,000 psi), in a "common rail". The common rail is a tube that supplies each computer-controlled injector containing a precision-machined nozzle and a plunger driven by a solenoid or piezoelectric actuator.
In cold weather, high-speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition due to the higher surface-to-volume ratio. Pre-chambered engines make use of small electric heaters inside the pre-chambers called glowplugs, while the direct-injected engines have these glowplugs in the combustion chamber.
Many engines use resistive heaters in the intake manifold to warm the inlet air for starting, or until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are used in cold climates when an engine is turned off for extended periods (more than an hour), to reduce startup time and engine wear. Block heaters are also used for emergency power standby Diesel-powered generators, which must rapidly pick up load on a power failure. In the past, a wider variety of cold-start methods were used. Some engines, such as Detroit Diesel engines used a system to introduce small amounts of ether into the inlet manifold to start combustion. Others used a mixed system, with a resistive heater-burning methanol. An impromptu method, particularly on out-of-tune engines, is to manually spray an aerosol can of ether-based engine starter fluid into the intake air stream (usually through the intake air filter assembly).
Diesel fuel is also prone to waxing or gelling in cold weather; both are terms for the solidification of diesel oil into a partially crystalline state. The crystals build up in the fuel line (especially in fuel filters), eventually starving the engine of fuel and causing it to stop running. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a spill return system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank.
Due to improvements in fuel technology with additives, waxing rarely occurs in all but the coldest weather when a mix of diesel and kerosene may be used to run a vehicle. Gas stations in regions with a cold climate are required to offer winterized diesel in the cold seasons that allow operation below a specific Cold Filter Plugging Point. In Europe these diesel characteristics are described in the EN 590 standard.
Most diesels are now turbocharged and some are both turbocharged and supercharged. Because diesels do not have fuel in the cylinder before combustion is initiated, more than one bar (100 kPa) of air can be loaded in the cylinder without pre-ignition. A turbocharged engine can produce significantly more power than a naturally aspirated engine of the same configuration, as having more air in the cylinders allows more fuel to be burned and thus more power to be produced. A supercharger is powered mechanically by the engine's crankshaft, while the engine exhaust, not requiring any mechanical power, powers a turbocharger. Turbocharging can improve the fuel economy of diesel engines by recovering waste heat from the exhaust, increasing the excess air factor, and increasing the ratio of engine output to friction losses.
A two-stroke engine does not have a discrete exhaust and intake stroke and thus is incapable of self-aspiration. Therefore all two-stroke engines must be fitted with a blower to charge the cylinders with air and assist in dispersing exhaust gases, a process referred to as scavenging. In some cases, the engine may also be fitted with a turbocharger, whose output is directed into the blower inlet. A few designs employ a hybrid turbocharger for scavenging and charging the cylinders, which device is mechanically driven at cranking and low speeds to act as a blower.
As turbocharged or supercharged engines produce more power for a given engine size as compared to naturally aspirated engines, attention must be paid to the mechanical design of components, lubrication, and cooling to handle the power. Pistons are usually cooled with lubrication oil sprayed on the bottom of the piston. Large engines may use water, seawater, or oil supplied through telescoping pipes attached to the crosshead.
As with petrol engines, there are two classes of diesel engines in current use: two-stroke and four-stroke. The four-stroke type is the "classic" version, tracing its lineage back to Rudolf Diesel's prototype. It is also the most commonly used form, being the preferred power source for many motor vehicles, especially buses and trucks. Much larger engines, such as used for railroad locomotion and marine propulsion, are often two-stroke units, offering a more favorable power-to-weight ratio, as well as better fuel economy
Two-stroke diesel engine operation is similar to that of petrol counterparts, except that fuel is not mixed with air before induction, and the crankcase does not take an active role in the cycle. The traditional two-stroke design relies upon a mechanically driven positive displacement blower to charge the cylinders with air before compression and ignition. The charging process also assists in expelling (scavenging) combustion gases remaining from the previous power stroke.
The archetype of the modern form of the two-stroke diesel is the (high-speed) Detroit Diesel Series 71 engine, designed by Charles F. "Boss" Kettering and his colleagues at General Motors Corporation in 1938, in which the blower pressurizes a chamber in the engine block that is often referred to as the "air box". The (very much larger medium-speed) Electro-Motive Diesel engine is used as the prime mover in EMD diesel-electric locomotive, marine and stationary applications, and was designed by the same team, and is built to the same principle. However, a significant improvement built into later EMD engines is the mechanically assisted turbo-compressor, which provides charge air using mechanical assistance during starting (thereby obviating the necessity for Roots-blown scavenging), and provides charge air using an exhaust gas-driven turbine during normal operations—thereby providing true turbocharging and additionally increasing the engine's power output by at least fifty percent.
In a two-stroke diesel engine, as the cylinder's piston approaches the bottom dead centre exhaust ports or valves are opened relieving most of the excess pressure after which a passage between the air box and the cylinder is opened, permitting air flow into the cylinder. The airflow blows the remaining combustion gases from the cylinder—this is the scavenging process. As the piston passes through bottom center and starts upward, the passage is closed and compression commences, culminating in fuel injection and ignition. Refer to two-stroke diesel engines for more detailed coverage of aspiration types and supercharging of two-stroke diesel engines.
Normally, the number of cylinders is used in multiples of two, although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced to prevent excessive vibration. The inline-six-cylinder design is the most prolific in light- to medium-duty engines, though small V8 and larger inline-four displacement engines are also common. Small-capacity engines (generally considered to be those below five liters in capacity) are generally four- or six-cylinder types, with the four-cylinder being the most common type found in automotive uses. Five-cylinder diesel engines have also been produced, being a compromise between the smooth running of the six-cylinder and the space-efficient dimensions of the four-cylinder. Diesel engines for smaller plant machinery; boats, tractors, generators and pumps may be four-, three- or two-cylinder types, with the single-cylinder diesel engine remaining for light stationary work. Direct reversible two-stroke marine diesels need at least three cylinders for reliable restarting forwards and reverse, while four-stroke diesels need at least six cylinders.
The desire to improve the diesel engine's power-to-weight ratio produced several novel cylinder arrangements to extract more power from a given capacity. The uniflow opposed-piston engine uses two pistons in one cylinder with the combustion cavity in the middle and gas in- and outlets at the ends. This makes a comparatively light, powerful, swiftly running and economic engine suitable for use in aviation. An example is the Junkers Jumo 204/205. The Napier Deltic engine, with three cylinders arranged in a triangular formation, each containing two opposed pistons, the whole engine having three crankshafts, is one of the better known.
Some information for this article obtained from Wikipedia.org.
Recent Recalls:
27,933 Ford 2012-2013 Edge vehicles equipped with 2.0L engines.
The fuel line pulse damper metal housing may crack as a result of an improper manufacturing process. A cracked fuel line pulse damper housing may result in a combination of fuel odor, weepage, or a continuous leak while the fuel system is pressurized. A fuel leak in the presence of an ignition source may result in a fire.
300 Ford 2011-2012 Explorers certain replacement steering gears installed as service parts.
The affected gears may lock, preventing the driver from being able to steer the vehicle. The inability to steer the vehicle increases the risk of a crash.
Contact your local dealership for more information and how to proceed.
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