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Diesel Engines

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Title: Diesel Engines


1
Diesel Engines
2
  • A diesel engine (also known as a
    compression-ignition engine) is an internal
    combustion engine that uses the heat of
    compression to initiate ignition to burn the
    fuel, which is injected into the combustion
    chamber. This is in contrast to spark-ignition
    engines such as a petrol engine (gasoline engine)
    or gas engine (using a gaseous fuel as opposed to
    gasoline), which uses a spark plug to ignite an
    air-fuel mixture. The engine was developed by
    Rudolf Diesel in 1893.
  • The diesel engine has the highest thermal
    efficiency of any regular internal or external
    combustion engine due to its very high
    compression ratio. Low-speed Diesel engines (as
    used in ships and other applications where
    overall engine weight is relatively unimportant)
    often have a thermal efficiency which exceeds 50
    percent.
  • Diesel engines are manufactured in two stroke and
    four stroke versions. They were originally used
    as a more efficient replacement for stationary
    steam engines. Since the 1910s they have been
    used in submarines and ships. Use in locomotives,
    trucks, heavy equipment and electric generating
    plants followed later. In the 1930s, they slowly
    began to be used in a few automobiles. Since the
    1970s, the use of diesel engines in larger
    on-road and off-road vehicles in the USA
    increased. As of 2007, about 50 percent of all
    new car sales in Europe are diesel.

3
Largest Diesel Engine in the World The The
Wartsila-Sulzer RTA96-C Turbocharged Two-Stroke
Diesel Engine
  • Total engine weight 2300 tons  (The crankshaft
    alone weighs 300 tons.)
  • Maximum power 108,920 hp at 102 rpm 

4
  • 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 151 and 221 resulting in
    40-bar (4.0 MPa) pressure compared to 8 to 14
    bars (0.80 to 1.4 MPa) (about 200 psi) in the
    petrol engine. This high compression heats the
    air to 550 C . 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 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 vapour is then
    ignited by the heat from the compressed air in
    the combustion chamber, the droplets continue to
    vaporise from their surfaces and burn, getting
    smaller, until all the fuel in the droplets has
    been burnt. The start of vaporisation causes a
    delay period during ignition and the
    characteristic diesel knocking sound as the
    vapour 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. Engines for scale-model aeroplanes
    use a variant of the Diesel principle but premix
    fuel and air via a carburation system external to
    the combustion chambers.

5
  • 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-tension electrical ignition
    system to attend to, 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.

6
  • 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.
  • 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).

7
The Ideal Air Standard Diesel Cycle
8
  • The diesel cycle analysis shows that the
    efficiency increases as the compression ratio is
    increased, but reduces slightly as the load ratio
    is increased. However, increasing the compression
    ratio has a diminishing effect on increasing the
    ideal cycle efficiency, and that for a practical
    engine, there are no benefits in increasing the
    compression ratio above approximately 20l. This
    is true because increases in frictional losses
    (associated with the higher pressures) and
    increases in heat transfer (because of the higher
    temperatures and pressures, and the adverse
    surface-area-to-volume ratio) outweigh any gains
    in the ideal cycle efficiency. Combustion is
    initiated by self-ignition of the fuel (hence the
    term compression ignition engines), and the main
    requirement of the fuel is that it is sensible to
    self-ignition.

9
  • Cold weather performance
  • 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. These engines also
    generally have a higher compression ratio of 191
    to 211. Low-speed and compressed-air-started
    larger and intermediate-speed diesels do not have
    glowplugs and compression ratios are around 161.

10
  • The compression ratio is likely to be selected on
    the basis of the lowest compression ratio that
    will provide satisfactory cold-starting
    performance, and even this compression ratio can
    be higher than that for optimum efficiency. For
    direct injection diesel engines with a
    displacement of 0.5 L cylinder, the compression
    ratio is approximately 18 1, whereas for larger
    engines (say, 1 L cylinder and greater), the
    compression ratio will be approximately 15 1 or
    slightly lower. The larger displacement engines
    have a better volume-to-surface-area ratio thus,
    less heat transfer occurs, and satisfactory
    cold-starting can be achieved with a lower
    compression ratio.

11
  • The performance of a diesel engine is critically
    dependent on its combustion system, and this
    means both the combustion chamber and the fuel
    injection system. There are two types of diesel
    combustion systems direct injection, and
    indirect injection. In diesel engines the
    air-fuel ratio is always weaker than
    stoichiometric (to avoid hydrocarbon and
    particulate emissions). In consequence, the
    specific output of a naturally aspirated diesel
    engine is much lower than that of a spark
    ignition engine. Fortunately, turbocharging can
    increase both the efficiency and output of diesel
    engines

12
Direct and Indirect Injection Combustion Chambers
  • Two types of combustion chambers are used in
    diesel engines. These are direct injection (DI)
    and indirect injection (IDI). The IDI engine
    offers faster combustion and thus the potential
    for higher engine speeds.
  • The maximum mean piston speed is a result of
    mechanical considerations and the flow through
    the inlet valve, and is limited to approximately
    12 m/s. For an engine speed of 3000 rpm, this
    corresponds to a stroke of 0.12 m, or a swept
    volume of approximately 1 L/cylinder. Thus, if
    the combustion speed limits direct injection
    engines to 3000 rpm, then for engines of less
    than 1 L/cylinder swept volume, higher speeds are
    obtainable only by using the faster indirect
    injection combustion system. With careful
    development of the combustion system, direct
    injection engines can now operate at speeds of up
    to 5000 rpm.

13
  • The pre-chamber of the IDI engine is
    approximately half of the clearance volume, and
    the lower part of the swirl chamber is an insert
    made from a heat-resisting material that is
    thermally isolated from the cylinder head. The
    insert heats when the engine has started, thereby
    reducing heat transfer losses, and its high
    temperature helps fuel evaporation, ignition, and
    combustion. Indirect injection engines invariably
    have a heater plug projecting into the
    pre-chamber. This is switched on prior to
    cranking the engine, so that when the fuel is
    injected, its ignition is provided. Small DI
    engines (say, 500 cm3/cylinder and smaller) also
    can use heater plugs, and these are sited so that
    their tips are close to the injector. Sometimes
    the heater plugs remain switched on (possibly at
    a reduced power setting) so that cold running is
    quieter.

14
  • Direct injection engines demand perfect matching
    of the fuel spray and air motion. Initially, this
    was achieved in high-speed DI engines by having a
    reasonable injection pressure (say, 600 bar) and
    swirl (a rotating flow with its axis parallel to
    the cylinder axis). However, the kinetic energy
    associated with swirl comes from the pressure
    drop in the induction process thus, there is a
    tradeoff between swirl and volumetric efficiency
    (and power output). In addition to promoting good
    mixing of the fuel and air, swirl increases heat
    transfer, which, of course, lowers the engine
    efficiency. Therefore, the current trend is
    toward lower levels of swirl, in which case
    four-valves-per-cylinder layouts can be used,
    with benefits for the volumetric efficiency and
    power output. The good air-fuel mixing now comes
    from more advanced fuel injection equipment.

15
Fuel Injection Equipment
  • To obtain small droplets that will evaporate
    quickly, the fuel injector nozzle holes can be as
    small as 0.15 mm. To inject sufficient fuel in
    the short time available, injection pressures
    must be 1500 bar or higher. These high pressures
    also ensure that the fuel jet disperses well
    within the combustion chamber. Likewise, these
    high pressures have led to the use of electronic
    unit injectors (EUI) and common rail (CR)
    injection systems in preference to the
    traditional pumpline-injector (PLI) systems.

16
  • Unit injectors have the pumping element and
    injector packaged together, with the pumping
    element operated from a camshaft in the cylinder
    head. This eliminates the high-pressure fuel line
    and its associated pressure propagation delays
    and elasticity. Common rail fuel injection
    systems have a high-pressure fuel pump that
    produces a controlled and steady pressure, and
    the injector controls the start and end of
    injection.

17
Pump-Line-Injector (PLI) Systems
  • In-line fuel-injection pumps have one pump
    element for each engine cylinder. These are
    arranged in a row. The camshaft of the in-line
    fuel-injection pump is driven by the gear wheels
    or chains of the combustion engine. The in-line
    fuel-injection pump runs at half the speed of the
    engine and always synchronously to the piston
    movements of the diesel engine. The fuel reaches
    the nozzle-holder assemblies with the injection
    nozzles via high-pressure lines.

18
  • The most important part of the fuel injector is
    the nozzle. The nozzle has a needle that closes
    under a spring load when it is not spraying.
    Although less prone to blockage, open
    (needleless) nozzles are not used because they
    dribble. When an injector dribbles, combustion
    deposits build up on the injector, and the engine
    exhaust is likely to become smoky. The
    needle-opening and needle-closing pressures are
    determined by the spring load and the projected
    area of the needle. The pressure to open the
    needle is greater than that required to maintain
    it in an open position, because in the closed
    position, the projected area of the needle is
    reduced by the seat contact area. A high
    needle-closing pressure is desirable because it
    maintains a high seat pressure, thereby giving a
    better seal. This also is desirable because it
    keeps the nozzle holes free from blockages caused
    by decomposition of leaked fuel.

19
  • The injector shown in Fig. 4.5 has a two-stage
    spring arrangement that provides pilot injection.
    Pilot injection, in which a small amount of fuel
    is injected during the ignition delay period, is
    a means of reducing diesel knock, by limiting the
    amount of mixture that is burned rapidly. The
    weak spring (I), acting through the central
    pressure pin, allows the nozzle to open a small
    amount (H1, usually less than 0.1 mm ).As the
    pressure rises further, the nozzle needle and
    strong spring lift further (H1 H2) for the main
    part of the injection. The pre-loads from the two
    springs are controlled by shims.

20
Electronic Unit Injectors (EUI)
  • In the Delphi Diesel Systems electronic unit
    injector (EUI) (Fig. 4.6), both the quantity and
    the timing of injection are controlled
    electronically through a Colenoid actuator. The
    Colenoid is a solenoid of patented construction
    that can respond very quickly (injection periods
    are of the order 1 ms), to control very high
    injection pressures (up to 1600 bar or so). The
    Colenoid controls a spill valve, which in turn
    controls the injection process. The pumping
    element is operated directly from a camshaft, and
    the whole assembly is contained within the
    cylinder head.

21
Common Rail (CR) Fuel Injection Systems
22
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23
Diesel Emissions
Unburned Hydrocarbons (HC), and Particulates in
Compression Ignition Engines
  • The three sources of hydrocarbon (HC) emissions
    in diesel engines are as follows
  • a. Fuel that is introduced too late into the
    reaction zone, such as from the tip of the
    injector nozzle, or fuel that impinged on the
    combustion chamber walls
  • b. Over-diluted mixture that occurs at the
    extremities of the fuel spray
  • c. Fuel that does not burn fully in the rich
    mixture zones
  • With diesel engine combustion, it is essential to
    remember that a wide range of air fuel ratios are
    present, and these extend beyond the weak and
    rich mixture flammability limits. The HC
    emissions will be present in the gaseous phase
    and as part of the smoke that is a major
    component of the particulates.
  • Particulates are any substance apart from water
    that can be collected by filtering diluted
    exhaust at a temperature of 325 K. Particulates
    include sulfates and fuel that has been partially
    pyrolyzed, as well as high molar mass
    hydrocarbons that have been condensed. The black
    smoke associated with a poorly regulated diesel
    engine consists of carbon particles produced by
    the thermal decomposition (pyrolysis) of
    hydrocarbons within the rich part of the air-fuel
    mixture during the diffusion-controlled
    combustion stage. The carbon reunites into
    particles that are visible as smoke in the
    exhaust.

24
  • Diesel exhaust particulates will comprise carbon
    (20-50), sulfates (5- 5), unburned fuel
    (10-30), unburned lubricant (1 0-20), and
    unknown (- 10). The composition will depend on
    the engine, its operating point, and the fuel
    being used (sulfur and other inorganic content).

25
  • The NOx emissions increase with load because of
    the increase in combustion temperature and this
    increase in combustion temperature is why the
    hydrocarbon emissions fall. Similarly, retarding
    the injection timing in all cases leads to lower
    combustion temperatures. This lowers the NOx
    emissions but increases the hydrocarbon
    emissions. The DI diesel has lower NOx emissions
    than the IDI engine because the lower compression
    ratio gives lower in-cylinder temperatures. The
    hydrocarbon emissions are higher from the Dl
    engine because there is a longer ignition delay
    period, resulting in more over-dilution at the
    fringes of the spray. The ignition delay period
    increases with a reduced load in the Dl engine
    because the combustion chamber surface
    temperatures are lower, and there will be more
    cooling of the charge during compression. In
    contrast, the pre-chamber insert in the IDI
    engine is always hot enough to ensure a short
    ignition delay period.

26
Diesel Engine Emissions Control
  • Exhaust Gas Recirculation (EGR)

27
Turbocharging
  • A turbocharger, or turbo, is a centrifugal
    compressor powered by a turbine that is driven by
    an engine's exhaust gases. Its benefit lies with
    the compressor increasing the mass of air
    entering the engine (forced induction), thereby
    resulting in greater performance (for either, or
    both, power and efficiency). They are popularly
    used with internal combustion engines (e.g.,
    four-stroke engines like Otto cycles and Diesel
    cycles).

28
Turbocharging versus supercharging
  • In contrast to turbochargers, superchargers are
    not powered by exhaust gases but are connected
    directly or indirectly to an engine. Belts,
    chains, shafts, and gears are only a few of the
    ways this is performed. Most automotive
    superchargers are positive-displacement pumps,
    such as the Roots supercharger. Some
    superchargers are compressors such as World War
    II piston aircraft engines, to be specific the
    Rolls-Royce Merlin and the Daimler-Benz DB 601,
    which utilized single-speed or multi-speed
    centrifugal superchargers.
  • A supercharger uses mechanical energy from the
    engine to drive the supercharger. For example, on
    the single-stage single-speed supercharged Rolls
    Royce Merlin engine, the supercharger uses up
    about 150 horsepower (110 kW). Yet the benefits
    outweigh the costs For that 150 hp (110 kW), the
    engine generates an additional 400 horsepower and
    delivers 1,000 hp (750 kW) when it would
    otherwise deliver 750 hp (560 kW), a net gain of
    250 hp (190 kW). This is where the principal
    disadvantage of a supercharger becomes apparent
    The internal hardware of the engine must
    withstand generating 1150 horsepower.
  • In comparison, a turbocharger does not place a
    direct mechanical load on the engine. It is more
    efficient because it converts the waste heat of
    the exhaust gas into horsepower used to drive the
    compressor. In contrast to supercharging, the
    principal disadvantages of turbocharging are the
    back-pressuring (exhaust throttling) of the
    engine and the inefficiencies of the turbine
    versus direct-drive.
  • A combination of an exhaust-driven turbocharger
    and an engine-driven supercharger can mitigate
    the weaknesses of the other. This technique is
    called twincharging.
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