Diesels have been revolutionised in the last decade
– just look at how many diesel passenger cars are now being sold around the
world. So what’s transformed them from being smelly, noisy and dirty machines
into their current refined form? In most cases, it’s their common rail direct
injection systems. Let’s take a detailed technical look.
Diesel Engines
Although the basic designs of petrol
and diesel engines are similar (both are two or four stroke designs which use
reciprocating pistons driving a crankshaft), a diesel engine does not compress
its fuel/air charge and then initiate combustion by the use of a spark plug.
Instead, in a diesel engine just air is compressed. When the piston is near Top
Dead Centre, the fuel is sprayed by an injector into the combustion chamber,
whereupon it mixes with the hot compressed air and self-ignites.
In order that the air within the
diesel combustion chamber reaches an adequate temperature for self-ignition to
occur, the compression ratio needs to be much higher than found in a spark
ignition engine. Compression ratios in the range of 16:1 to 24:1 are commonly
used, giving forced aspirated diesel engines a compression pressure of up to 150
Bar. This generates temperatures of up to 900 degrees C. Since the ignition
temperature of the most easily combustible components of diesel fuel is only 250
degrees C, it is easy to see why the fuel burns when it is injected after the
piston has risen on the compression stroke.
Diesel engines are designed to
develop high torque at low engine speeds, resulting in better fuel economy. In
recent years, the use of turbochargers and common rail direct injection have
dramatically improved the specific torque output of diesel car engines. This
diagram shows that specific torque has risen from about 70 Nm/litre to more than
182 Nm/litre over the last 20 years. At the same time, specific fuel consumption
has fallen by over 60 per cent!
Compared with petrol-powered engines
that most often run with stoichiometric mixtures (that is, the theoretically
correct air/fuel ratio for complete combustion, which is about 14.7:1), diesels
use very lean air/fuel ratios. The air/fuel ratios for diesel engines under full
load are between 17:1 and 29:1, while when idling or under no load, this ratio
can exceed 145:1. However, within the combustion chamber, localised air/fuel
ratios vary – it is not possible to achieve a homogenous mixing of the fuel with
the air within the combustion chamber. To reduce these in-chamber air/fuel ratio
variations, large numbers of very small droplets of fuel are injected. Higher
fuel pressure results in better fuel atomisation, so explaining the increase in
injection pressures now being seen.
Injection
Diesel engines are not throttled.
Instead, the combustion behaviour is affected by these variables:
Since the use of electronically
controlled common rail injection allows these variables to be individually
controlled, we’ll briefly look at each.
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Timing of Start of
Injection
The timing of the injection of fuel
has a major affect on emission levels, fuel consumption and combustion noise.
The optimal timing of the start of injection varies with engine load. In car
engines, optimal injection at no load is within the window of 2 crankshaft
degrees Before Top Dead Centre (BTDC) to 4 degrees After Top Dead Centre (ATDC).
At part load this alters to 6 degrees BTDC to 4 degrees ATDC, while at full load
the start of injection should occur from 6 – 15 degrees BTDC. The duration of
combustion at full load is 40 – 60 degrees of crankshaft rotation.
Too early an injection initiates
combustion when the piston is still rising, reducing efficiency and so
increasing fuel consumption. The sharp rise in cylinder pressure also increases
noise. Too late an injection reduces torque and can result in incomplete
combustion, increasing the emissions of unburned hydrocarbons.
Unlike a conventional port fuel
injected petrol engine, where the amount of fuel injected can be considered to
be directly proportional to the injector opening time, a diesel injector will
vary in mass flow depending on the difference between the injection and
combustion chamber pressures, the density of the fuel (which is temperature
dependent), and the dynamic compressibility of the fuel. The specified injector
duration must therefore take these factors into account.
Diesel fuel injectors do not add the
fuel for a combustion cycle in one event, instead they operate in up to four
different modes. The first is pre-injection, a short duration pulse which
reduces combustion noise and Oxides of Nitrogen (NOx) emissions. The bulk of the
fuel is then added in the main injection phase, before the injector is turned
off momentarily before then adding a post-injection amount of fuel. This
post-injection reduces soot emissions. Finally, at up to 180 crankshaft degrees
later, a retarded post-injection can occur. The latter acts as a reducing agent
for an NOx accumulator-type catalytic converter and/or raises the exhaust gas
temperature for the regeneration of a particulate filter.
The injection amounts vary between 1
cubic millimetre for pre-injection to 50 cubic millimetres for full-load
delivery. The injection duration is 1-2 milliseconds.
Common Rail System Overview
Unlike previous diesel fuel injection systems -
even those electronically controlled – common rail systems use, as the name
suggests, a common fuel pressure rail that feeds all injectors. (In this
respect, common rail diesel systems are like traditional electronic fuel
injected petrol engines.) By separating the functions of fuel pressure
generation and fuel injection, a common rail system is able to supply fuel over
a broader range of injection timing and pressure than previous systems.
This diagram shows a simple common rail fuel
injection system. A high pressure mechanical pump (1) pressurises the fuel which
flows to the common rail (3). A fuel rail control valve (4) allows the fuel
pressure to be maintained at a level set by the Electronic Control Unit (8). The
common rail feeds the injectors (5). Sensor inputs to the ECU comprise fuel
pressure (2), engine speed (9), camshaft position (10), accelerator pedal travel
(11), boost pressure (12), intake air temperature (13) and engine coolant
temperature (14). (6) and (7) are the fuel filter and fuel tank, respectively.
More complex common rail systems use these
additional sensors:
Not shown on these diagrams are the glow plugs.
Common rail diesels still use glow plugs, however their use is not normally
required except for starting in ambient temperatures below 0 degrees C.
Extra ECU outputs can include control of
turbocharger boost pressure, exhaust gas recirculation and intake port tumble
flaps.
Common Rail System Components
Fuel pressures of up to 1600 Bar are generated by
the high pressure pump. This pump, which is driven from the crankshaft, normally
comprises a radial piston design of the type shown here. The pump is lubricated
by the fuel and can absorb up to 3.8kW. So that pump flow can be varied with
engine load, individual pistons of the pump are able to be shut down. This is
achieved by using a solenoid to hold the intake valve of that piston open.
However, when a piston is deactivated, the fuel delivery pressure fluctuates to
a greater extent than when all three pistons are in operation.
The fuel pressure control valve comprises a
fuel-cooled solenoid valve. The valve opening is varied by its solenoid coil
being pulse width modulated at a frequency of 1 KHz. When the pressure control
valve is not activated, its internal spring maintains a fuel pressure of about
100 Bar. When the valve is activated, the force of the electromagnet aids the
spring, reducing the opening of the valve and so increasing fuel pressure. The
fuel pressure control valve also acts as a mechanical pressure damper, smoothing
the high frequency pressure pulses emanating from the radial piston pump when
less than three pistons are activated.
The fuel rail feeds each injector. It is made
sufficiently large that the internal pressure is relatively unaffected by fuel
being released from the injectors. As indicated earlier, the rail is fitted with
a fuel pressure sensor. To guard against dangerously high fuel pressure, a fuel
pressure relief valve is also fitted.
The fuel injectors superficially look like the
injectors used in conventional petrol injection systems but in fact differ
significantly. This diagram shows a common rail injector. Because of the very
high fuel rail pressure, the injectors use a hydraulic servo system to operate.
In this design, the solenoid armature controls not the pintle but instead the
movement of a small ball which regulates the flow of fuel from a valve control
chamber within the injector.
The life of a common rail diesel fuel injector is
certainly a hard one. Bosch estimates a commercial vehicle injector will open
and close more than a billion times in its service life.
Emissions
Five major approaches are taken to
reducing diesel exhaust emissions.
Within the engine itself, the design
of the combustion chamber, the placement of the injection nozzle and the use of
small droplets all help reduce the production of emissions at their source.
Accurate control of engine speed, injection mass, injection timing, pressures,
temperatures and the air/fuel ratio are used to decrease emissions of oxides of
nitrogen, particulates, hydrocarbons and carbon monoxide.
-
Exhaust Gas
Recirculation
Exhaust gas recirculation, where a
proportion of the exhaust gas is mixed with the intake charge, is also used to
reduce oxides of nitrogen emissions. It does this by reducing the oxygen
concentration in the combustion chamber, the amount of exhaust gas passing into
the atmosphere, and the exhaust gas temperature. Recirculation rates can as high
as 50 per cent.
Diesel oxidation-type catalytic converters can be
used to reduce hydrocarbon and carbon monoxide emissions, converting these to
water and carbon dioxide. So they rapidly reach their operating temperature,
this type of catalytic converter is fitted close to the engine.
NOx accumulator-type catalytic converters are also
used. This type of design breaks down the NOx by storing it over periods from 30
seconds to several minutes. The nitrogen oxides combine with metal oxides on the
surface of the NOx accumulator to form nitrates, with this process occurring
when the air/fuel ratio is lean (ie there is excess oxygen). However, the
storage can only be short-term and when the ability to bind nitrogen oxides
decreases, the catalytic converter needs to be regenerated by having the stored
NOx released and converted into nitrogen. In order that this takes place, the
engine is briefly run at a rich mixture (eg an air/fuel ratio of 13.8:1)
Detecting when regeneration needs to occur, and
then when it has been fully completed, is complex. The need for regeneration can
be assessed by the use of a model that calculates the quantity of stored
nitrogen oxides on the basis of catalytic converter temperature. Alternatively,
a specific NOx sensor can be located downstream of the accumulator catalytic
converter to detect when the efficiency of the device is decreasing. Assessing
when regeneration is complete is done by either a model-based approach or an
oxygen sensor located downstream of the cat; a change in signal from high oxygen
to low oxygen indicates the end of the regeneration phase.
In order that the NOx storage cat works
effectively from cold, an electric exhaust gas heater can be employed.
-
Selective Catalytic Reduction
One of the most interesting approaches to diesel
exhaust treatment is Selective Catalytic Reduction. In this approach, a reducing
agent such as dilute urea solution is added to the exhaust in minutely measured
quantities. A hydrolysing catalytic converter then converts the urea to ammonia,
which reacts with NOx to form nitrogen and water. This system is so effective at
reducing NOx emissions that leaner than normal air/fuel ratios can be used,
resulting in improved fuel economy. The urea tank is filled at each service.
Exhaust particulate filters are made from porous
ceramic materials. When they become full, they can be regenerated by being
heated to above 600 degrees C. This is a higher exhaust gas temperature than is
normally experienced in diesels and to achieve this, retarded injection and
intake flow restriction can be used to increase the temperature of the exhaust
gas.
Conclusion
As can be seen, dramatic changes in both the fuel
injection system and exhaust aftertreatment have occurred in diesel technology.
Next week, we’ll look at how the electronic control system makes it all
function.
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