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In a direct injection engine, the
injection nozzle is located inside the combustion chamber, rather than
in the induction pipe as in multi-port or throttle-body fuel injection.
Like the spray from an atomizer bottle one might use to keep cool in
the summer, the fine mist generated by each solenoid-controlled
injector’s tiny outlet holes creates a well atomized air/fuel mixture.
Each
bank of cylinders has a high-pressure fuel rail that feeds the
individual injectors and a fuel rail pressure sensor on each rail that
helps the vehicle powertrain control module precisely control the fuel
pressure. Fuel injectors use internal solenoids to switch on and off
the flow of fuel extremely precisely. Fuel flows through six tiny
pinholes in each injector.
Injectors are positioned to the side
of each cylinder, aiming the fuel directly into the cylinder adjacent
to the spark plug and alongside the intake and exhaust valves. Fuel is
sprayed into the cylinders at pressures of up to 2,150 psi, about 35
times more intense than port fuel injection.
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| To
control the injection valves, the new Common-Rail injectors use a
rapid-action actuator made of piezo crystals to control the injection
valve. The movement of the piezo package is transmitted non
mechanically and entirely without friction to the rapidly switching
nozzle needle. This doubles the injector's switching speed, allowing a
more precise measurement of the amount of fuel injected and thus
leading to a reduction in harmful combustion products. |
The
spark plug is surrounded by a relatively small, precisely shaped volume
of ignitable air/fuel mixture that results when fuel is sprayed toward
the spark plug just before ignition. Only the area directly around the
spark plug at the top of the cylinder contains air/fuel mixture. Other
areas inside the combustion chamber merely contain air or recirculated
exhaust gas. This stratification of the charge allows the engine to
burn mixtures with a much higher rate of air than conventional engines.
Air/fuel ratios can
increase to 60 parts of air (instead of 14.7) for every part of fuel.
As
fuel is injected into the cylinder, the shaped piston crown guides the
air/fuel mix to the spark plug. As the spark plug fires, igniting the
mixture, surrounding areas contain only air or recirculated gases,
forming an insulating cushion at the cylinder walls and cylinder head.
The cushion of non-combustible gas around the combustion chamber also
means that less combustion heat has to be evacuated. This improves the
thermal efficiency of the engine, improving fuel economy.
Another
factor contributing to improved fuel economy is the ability to increase
the compression ratio to nearly 12:1 without the need for premium fuel,
because direct injection reduces the tendency of engine knock. The
higher compression ratio alone increases efficiency by about two
percent.
However, the major fuel reduction potential is realized
because of the way we drive. Direct injection charge stratification
works best at low and medium loads in the lower half of the engine
speed range, where traditional gasoline engines are least efficient.
Because most engines operate under these driving conditions, the direct
injection engine operates in a stratified-lean mode most of the time,
thus increasing fuel economy by nearly 21 percent.
Evolution
The future of direct injection involves coupling the system with other
technologies, such as turbocharging and Start/Stop. By playing off the
efficiencies of multiple systems, it enables automakers to develop
smaller, more fuel-efficient engines while improving torque and
performance.
Turbocharging direct injection engines is the most
promising fuel economy technology for U.S., according to Paul Whitaker,
chief technologist – gasoline engines for AVL Powertrain Engineering
Inc., the world’s largest independent, privately owned company for the
development of gasoline, diesel and alternative fuel powertrain systems.
“By
turbocharging a direct injection engine, it combines existing and
proven technologies in a synergistic manner and offers double digit
fuel economy benefits with a much lower cost than diesel or hybrid
technology,” he says.
At the 2009 SAE World Congress, there was a sense of
urgency in the air as automotive engineers attempt to tackle the U.S.
government’s 35-mpg Corporate Average Fuel Economy (CAFE) standard that
goes into effect in 2020.
The new standard increases fuel economy
by 30 percent from today’s standard. While most manufacturers are
taking a multi-faceted approach — which includes diesel, hybrid, fuel
cell and electric power sources — one transitional technology is
helping to extend the life of the gasoline engine and push fuel mileage
and performance forward by leaps and bounds: gasoline direct injection.
 |
| Like
the spray from an atomizer bottle one might use to keep cool in the
summer, the fine mist generated by each solenoid-controlled injector's
tiny outlet holes to create a well atomized air/fuel mixture. |
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It
also allows manufacturers to meet future emissions standards using
typical catalytic converters and can be applied across a manufacturer’s
entire engine portfolio, including Flex Fuel applications.
Ford
is working on changes in the coolant system to improve fuel economy for
a direct injection equipped vehicles. A typical feature of direct
injection engine thermodynamics is the difference in thermal losses,
depending on whether the engine is operated in the economy or full-load
mode. In the economy mode, an insulating blanket of air and
recirculated exhaust gas helps keep heat away from the cylinder walls
and head. In the high-powered mode, more heat is released.
A new
control system for the coolant circuit is being designed to shut off
the fan motor over a longer period of time or reduce the operating
speed of the water pump, during economy mode operation, thus reducing
operating drag on the engine and improving fuel economy.
GM
utilizes Dual Cam Phasing on the camshafts of its Ecotec 2.0-liter
Turbo Engine. The phasers continuously vary the intake and exhaust
valve timing and use cam position sensors so the engine control module
can control the timing accurately. The crankshaft and camshaft position
sensors are digital. A new engine controller, specific to the engine,
is used to sense and control the engine’s performance parameters.
Variable
intake and exhaust timing works synergistically with both the gasoline
direct injection and turbocharging systems. The variable engine timing
enabled by cam phasing allows the combustion process to be optimized.
Also, valve overlap at low rpm can be adjusted by the controller to
increase the response of the turbocharger, lessening the feeling of
turbo lag.
Servicing
With almost every
manufacturer (including Ferrari) having at least one direct injection
engine available, technicians should be seeing these vehicles in their
bays for service.
“The biggest item to consider when servicing
(direct injection) systems is the high voltage and fuel pressures the
systems generate,” says Al Krenz, director of service for Bosch North
America. A direct injection system typically will operate between 725
psi up to 2050 psi, so bleeding down the fuel system properly is
important.
 |
| Each
bank of cylinders has a high-pressure fuel rail that feeds the
individual injectors and a high pressure pump with rail pressure sensor
that helps the vehicle powertrain control module precisely control the
fuel pressure. |
“Always follow the
manufactures procedure to bleed the high pressure system down before
performing any repairs to the system,” Krenz recommends.
Caution
also should be used when diagnosing the voltage signals of the
injectors. The high-pressure injectors typically actuate at
approximately 70 volts and 10 amps, with the capability to rise over
120 volts.
As with diesel direct injectors, carbon can build up
on the tip of the injector and interfere with the distribution and
atomization of the fuel. Even the slightest loss of the fuel delivery
will have an adverse effect on the engines drivability, power output,
fuel economy and exhaust emissions.
Injectors can have varying
types of spray patterns, depending on the engine requirements. While
typical port injectors produce a fuel droplet of approximately 165
micron, direct injectors atomize a much smaller fuel droplet size of
only 65 micron.
The use of direct injection and other
complementary systems creates a win-win-win situation for the
environmentalists, government and consumers. Environmentalists get a
reduction in smog forming emissions, consumers get the high performance
they desire from a smaller engine while saving money at the pump and
manufacturers are able to achieve better fuel economy, assisting them
in reaching the 35.5 mpg national standard. It seems that performance,
economy and ecology can peacefully co-exist.
