Francisco Espinosa-Loza and Daniel Flowers work on the stationary
homogeneous charge compression ignition engine.
TRANSPORTATION accounts for 65 percent
of U.S. oil consumption. More efficient vehicle engine designs
would reduce U.S. dependence on foreign oil and help to mitigate
global climate change caused by carbon dioxide emissions. Cleaner
engines would contribute to reducing toxic pollutants in the atmosphere,
such as nitrogen oxide (NOx), hydrocarbon (HC), and carbon monoxide
(CO). One promising technology is called the homogeneous charge
compression ignition (HCCI) engine. Livermore engineers Salvador
Aceves and Daniel Flowers are leading a team that is applying modeling
processes originally developed by nuclear weapons researchers to
study the HCCI engine’s potential.
HCCI technology could be scaled to virtually every size and class
of transportation engines, from small motorcycles to large ships.
Stationary HCCI engines could replace the spark-ignited and diesel
engines currently used for backup-power generation and in businesses
such as hotels where recovered exhaust heat is used for swimming
pools and other facilities. Caterpillar Inc. has donated a six-cylinder
stationary engine for the Livermore team to verify modeling results
In the HCCI engine, fuel is homogeneously premixed with air, as
in a spark-ignited engine, but with a high proportion of air to
fuel. When the piston reaches its highest point, this lean fuel–air
mixture autoignites (spontaneously combusts) from compression heating,
as in a diesel engine. A feature of the HCCI engine is that it
burns cooler than spark-ignited and diesel engines. Lower temperature
combustion considerably reduces the emissions of nitrogen oxide.
In addition, premixed combustion in HCCI engines reduces particulate
matter emissions to very low levels. An HCCI engine can operate
using any fuel, so long as the fuel can be vaporized and mixed
with air before ignition.
Although the advantages of the HCCI engine are clear, significant
challenges must be overcome before the engine is commercially viable.
Once engineers resolve these issues, HCCI engines could achieve
approximately 40 percent peak efficiency versus 30 percent for
spark-ignited engines. Diesel engines can achieve high efficiency
similar to HCCI engines, but diesel engines are major sources of
NOx and particulate matter emissions. The Department of Energy’s
Office of Energy Efficiency and Renewable Energy and the California
Energy Commission are funding the Livermore research. Professors
Karl Hedrick, Robert Dibble, and J. Y. Chen at the University of
California at Berkeley are collaborating with Livermore researchers
to address some of the engine combustion control challenges.
KIVA, a three-dimensional code, is
used to simulate gaseous flow in a model of the HCCI engine
divided into spatial zones. Together with the HCT code,
researchers determine where and when in the combustion
chamber the hydrocarbons and carbon monoxide are formed.
greatest challenge is controlling the HCCI engine’s ability
to operate under a wide range of speeds and loads. The HCCI engine
does not have a combustion trigger such as a spark plug or fuel
injector found in conventional engines. Instead, combustion is
achieved by controlling the temperature, pressure, and composition
of the fuel–air mixture so that it spontaneously ignites
at the proper time. The required control system is fundamentally
more challenging than conventional engines because the ignition
is sensitive to very small changes in temperature. When a load
is suddenly added, as when a vehicle goes from idle to cruising
speed, the control system must adjust the temperature, pressure,
and composition rapidly enough to maintain stable combustion.
HCCI engines lack traditional combustion-control systems, understanding
the chemical kinetics of combustion is key to addressing
its challenges,” says Aceves. Livermore’s chemical
kinetics code, known as HCT, and Los Alamos National Laboratory’s
fluid mechanics code KIVA, used in combination, have allowed Laboratory
researchers to make critical contributions to HCCI technology.
calculates problems involving gas hydrodynamics, transport, and
chemical kinetics (how molecules react). The code has been used
to study the combustion properties of many compounds. The Livermore team has modified HCT by incorporating
models for heat transfer, a turbocharger, and exhaust-gas recirculation, which
are all required for engine analysis.
KIVA is a three-dimensional
fluid mechanics code that simulates liquid and gaseous flow under steady-state
and transient conditions. Livermore researchers use KIVA
to simulate flow in a model of the HCCI cylinder divided into spatial zones.
This multizone approach allows temperature and charge velocity distributions
to be calculated separately from the much more computationally intensive chemical
kinetics processes, which are solved using only a few zones. These codes allow
engineers to visualize the inner workings of the combustion process.
in HCCI engines is mainly controlled by chemical kinetics and minimally by turbulence
effects, the Livermore team developed a two-step
process to analyze combustion. The effect of turbulence is first considered by
running KIVA to obtain a temperature distribution within the cylinder. The results
from KIVA are then used in HCT, which calculates the combustion parameters related
to HCCI. This two-step process makes it possible to obtain accurate predictions
for the turbulent combustion inside an HCCI engine, within a reasonable computational
time for the Laboratory’s computers. The significance of this computer-based
analysis is that for the first time, researchers can accurately predict combustion
rates, emissions, and performance of HCCI engines.
Intake temperatures necessary for
various fuels to operate a heavy-duty engine running at
maximum power. The intake temperature required for proper
ignition is shown as a function of the compression ratio
(the ratio of the volume of the combustion space in the
cylinder at the bottom of the piston stroke to the volume
at the top of the stroke).
Finding the Optimal Fuel
2003, Livermore researchers used the HCT and KIVA codes to evaluate
fuels and additives that might improve HCCI engine performance.
several fuels, including propane, methane, natural gas, ethanol, iso-octane,
and a variety of additives.
Modeling and experiments
have shown that operating conditions for satisfactory HCCI combustion are, in
part, dictated by the characteristics of the fuel. Fuels
with a relatively low octane number, such as n-heptane or diesel fuel, require
lower intake temperatures, and fuels with a high octane number, such as methane
and natural gas, require higher intake temperatures. Understanding the limitations
of each fuel leads to specific engine design options. Flowers notes, “Certain
engine features will work better depending on the type of fuel. We can make any
given fuel work with HCCI, but we’re still defining the characteristics
that work best.”
were conducted using small amounts (10 parts per million) of 913 additives to
determine if one additive might better control combustion.
Some of the peroxides showed the most promise, considerably advancing ignition
timing. Although this finding is significant, the benefit of using additives
to control combustion must be compared to other potential control mechanisms,
such as adjusting the intake temperature to obtain satisfactory ignition timing
and combustion. Using an additive may increase the engine’s complexity
because additional systems would be needed to store and deliver the additive
to the combustion chamber.
combustion also affects its emissions. Although
the engine produces lower NOx and particulate emissions, it releases higher amounts
of hydrocarbon and carbon monoxide than spark-ignited or diesel engines. At very
low loads, CO levels can reach as high as 60 percent of the total fuel carbon
because of incomplete combustion. Fortunately, CO and unburned HC are easily
controlled with commercially available oxidation catalysts.
used the two-step KIVA–HCT method to investigate
when and where in the cylinder the HC and CO are formed. The simulations showed
that CO reaches its maximum level immediately following the main heat release
during combustion. CO then decreases and slowly rises again during the expansion
stroke of the piston. This increase happens because fuel is not completely consumed
in the lowest temperature regions of the combustion chamber. Aceves says, “There
is enough time during the expansion stroke for unburned fuel to migrate from
the crevices and boundary layer into the hot core of the cylinder, where it can
react and produce additional CO and CO2.”
Because HCCI is a
thermal autoignition process, temperature sensitivity is an important issue.
Controlling cylinder temperatures will help not only to address
the emissions issue but also to control combustion itself. “A few degrees
in temperature,” says Aceves, “can make the difference between combustion
and no combustion. HCCI needs a control mechanism that detects ambient temperature
and adjusts the mixture of air to fuel in the cylinders to obtain ignition at
the right time, and the controller has to be fast enough to handle the adjustments.” One
possibility is for gases coming into the cylinder to be warmed by heat recovered
from exhaust gas.
Controlling the temperature
is also an important factor for enabling multiple cylinders to work together
efficiently. Flowers explains, “Because HCCI
is very sensitive to temperature changes, the interactions and differences between
cylinders have a greater effect in an HCCI engine than in a spark-ignited or
The Livermore team
projects that a stationary HCCI engine will be commercially available in the
next five years. Nissan currently sells a hybrid HCCI–diesel
automobile in Japan and in Europe. Laboratory engineers are hopeful that in a
decade or so, HCCI engines will be powering the transportation industry in the
Key Words: combustion; homogeneous charge compression ignition
(HCCI) engine; hydrodynamics, chemistry, and transport (HCT) code;
For further information contact Salvador Aceves
(925) 422-0864 (email@example.com)
or Daniel Flowers
(925) 422-0529 (firstname.lastname@example.org).
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