What is a “modern” diesel,
and is it really that much different?
By
C.J. Baker
There’s
a lot of talk these days about “modern” diesels,
and how today’s diesels have changed. The modern
diesel is described as powerful, clean, quiet, responsive,
economical, and smoke-free, but what actually constitutes
a modern diesel? Generally speaking, a diesel must
incorporate five basic features to be considered modern.
Those five features are: (1), four valves per cylinder;
(2), direct fuel injection; (3), computer controlled
fuel management; (4), common-rail fuel injection; and
(5), a “pilot injection” noise suppression
cycle. You may notice that a turbocharger wasn’t
included. That is because virtually all automotive
diesels manufactured in America since ‘94 are
turbocharged, so a turbocharger is assumed to be part
of the basic diesel design. Let’s look at each
of these defining characteristics one at a time.
First,
by using a cylinder head design with four valves per
cylinder (two intake valves and two exhaust valves)
maximum airflow can be obtained for a given bore diameter
with less shrouding (flow restriction) by the cylinder
wall. The cylinder head ports can also be aligned to
promote maximum swirl within the cylinder for high
combustion efficiency. Most importantly, a four-valve
configuration allows the fuel injection nozzle to be
positioned in the center of the combustion chamber
for a symmetric fuel injection pattern directly down
the centerline of the cylinder. This arrangement provides
the most uniform fuel dispersion for combustion efficiency
with a minimum of exhaust pollutants. Earlier designs
with only two valves per cylinder did not allow such
advantageous injector placement.
Second,
in recent past history, many automotive diesel engines
were designed with indirect injection (IDI). IDI used
a precombustion chamber. Fuel was injected into the
prechamber where compression ignition occurred. The
burning mixture then expanded through a passage or
throat into the cylinder. This arrangement could be
designed to produce high swirl in the cylinder, but
the main purpose was to reduce engine noise. The clatter
of pre-modern direct injection (DI) diesels is the
result of a rapid pressure spike when the main injection
pulse occurs. The resultant combustion produces the
pressure spike that generates the noise. By initiating
combustion in a much smaller precombustion chamber,
the clatter wasn’t nearly as loud. Unfortunately,
IDI diesels are not nearly as efficient or as clean
as DI diesels, so all modern diesels are direct injection.
The noise problem was solved with “pilot injection”,
which we’ll cover later.
Direct
injection diesels are as much as 15 percent more efficient
than IDI diesels in terms of thermal efficiency. Power
and torque output is up to 40 percent better with DI.
Additionally, fuel consumption is up to 30 percent
less, and that means carbon dioxide and hydrocarbon
emissions are reduced by a similar amount. The transition
of the burning gases from the precombustion chamber
to the cylinder in IDI diesels results in significant
heat losses into the cylinder head and cylinder walls,
which reduces efficiency.
The
third item that distinguishes a modern diesel is computer
controlled fuel management. What this really means
is computerized electronic control of the fuel injection
pulses in response to fuel throttle position, engine
speed and load, and feedback from numerous engine sensors.
A computer can control the fuel injectors, pump pressure,
and duration of the fuel injection pulse (called pulse
width) with the speed necessary to permit real time
combustion control. It also permits multiple injection
events for each cylinder firing. Computer control provides
the precision to properly time these events to meet
both today’s and tomorrow’s emissions and
fuel economy requirements. For example, some diesel
fuel injection manufacturers are talking about the
necessity to have as many as four or five separate
injection events per combustion cycle. If you do the
math for such a system on a V8 running 4000 RPM, that
would be 80,000 injection events per minute! Only a
computer is capable of controlling such a system with
accurate timing. Of course, the injection system must
also be up to the task.
The
fourth requirement of a modern diesel is common-rail
fuel injection. In order to allow the computer to control
the timing, pulse width, and fuel pressure of the injection
system, the injection system must be able to maintain
pressure independently from engine speed, and to achieve
injection pressures adequate for efficient direct injection.
Currently, common-rail injection systems provide this
needed flexibility. Operating independently of engine
speed, the common-rail system can supply fuel pressure
from 2000 to 24,000+ PSI to a common fuel rail that
supplies each bank of injectors. The high pressure
allows the use of injectors with multiple small holes
for efficient distribution and atomization of the fuel
pulse into the cylinder. Ultra high pressure creates
small fuel droplet size and high injection velocity
to promote complete burning of the fuel. This provides
maximum power and minimal pollution. The high fuel
pressure also allows the delivery of a programmed amount
of fuel in a very short time. This is especially important
to accommodate multiple injection events during each
combustion cycle.
The
fifth requirement of a modern diesel is pilot injection.
Pilot injection is used on direct injection diesels
to dramatically reduce noise, especially at low engine
speeds and idle. Pilot injection is featured on ’01-03
Chevy/GM 6.6L DuraMax diesels, on ‘03 Cummins
5.9L diesels for automotive applications, and on ’03
Ford 6.0L Power Stroke diesels. Pilot injection is
the introduction of a small amount of fuel that starts
combustion immediately prior to the main, power-producing
fuel pulse. This smoothes the start of combustion,
eliminating the pressure spikes that produce the clatter
common to previous diesels, especially at idle. Done
properly, a diesel with pilot injection will idle as
quietly as a gasoline engine. Pilot injection also
improves the cold start capability of diesels to equal
that of gasoline engines at temperatures as low as
-40º F. Additionally, pilot injection can help reduce
the oxides of nitrogen from a diesel by lowering peak
combustion temperatures.
Timing
of the pilot injection pulse and the amount of fuel
injected is critical to noise reduction. The time between
the pilot pulse and the main injection pulse needs
to be extremely short. Some injection systems are now
capable of reducing this interval between pulses to
as little as .0007-second. The result is combustion
starts on a small scale that doesn’t generate
noticeable clatter, and then builds gently, but quickly.
This reduces noise, produces less vibration, and quieter
cold start warm-ups.
Modern
diesels have come a long way in just the last few years.
They are no longer noisy and smoky, emissions have
been significantly reduced, power and economy are up,
and performance is astounding. However, the evolution
of the diesel is far from over. Many other technologies
are being tried and tested, such as cooled exhaust
gas recirculation, cold-burn post injection, closed-loop
ion-sensing for real time combustion control, and piezoelectric
injection. Most of these developments are aimed at
further reducing diesel emissions, and most have a
positive effect on power and economy. Low sulfur fuel
has been mandated by 2006, which will eliminate any
remaining diesel odor and permit the use of catalytic
converters to reduce oxides of nitrogen. Self-cleaning
particulate traps are coming to remove the last vestiges
of smoke from diesel exhaust too. We’re also likely
to see a reduction in diesel engine weight and increases
in maximum engine speed. The modern diesel is already
incredible. One can only wonder what future development
will bring. One thing is for sure, you’ll be seeing
a lot more exciting diesel-powered vehicles in the
future. One of them might even be yours.
|
