26-02-2013, 12:25 PM
Combustion Phasing Model for Control of a Gasoline-Ethanol Fueled
SI Engine with Variable Valve Timing
Combustion Phasing.pdf (Size: 333.4 KB / Downloads: 157)
Abstract
Concern over the availability of fossil fuels and
energy usage have produced an interest in both alternative
fuels and new engine technologies such as variable valve timing
to improve engine efficiency. Fuel-flexible engines permit the
increased use of ethanol-gasoline blends. Ethanol is a renewable
fuel which has the added advantage of improving performance
in typically knock-limited operating regions due to the higher
octane rating of the fuel. Furthermore, many modern engines
are also being equipped with variable valve timing (VVT), a
technology which allows increased control of the quantity of
burned gas in-cylinder and can increase engine efficiency by
reducing the need for throttling. The burned gas fraction as well
as the blend ratio of ethanol impact the combustion timing and
capturing these effects is essential if the combustion phasing is
to be properly controlled.
INTRODUCTION
Fuel-flexible SI engines incorporating variable valve timing
enable the efficient utilization of a nearly CO2 neutral,
domestically available renewable fuel. While ethanol is an attractive
option for offsetting dependence on petroleum based
gasoline, the differences in fuel properties between gasoline
and ethanol (as summarized in Table I) can significantly
alter engine performance [8], [24], [7]. Due to the different
molecular structure of ethanol, the stoichiometric air-fuel
ratio of ethanol is substantially different than gasoline and
the air and fuel controllers must target different values for
ethanol blends. Ethanol also has a different laminar flame
speed than gasoline; therefore, the rate at which flames
propagate during combustion will differ depending on the
fuel ethanol content and combustion phasing will be directly
affected. In addition, as a result of its higher octane rating,
ethanol has a higher resistance to knock than gasoline.
Gas Exchange Modeling
In order to capture the dynamics of combustion phasing,
it is crucial to have an accurate estimate or measurement of
the contents of the cylinder at intake valve closing (IVC).
The mixture in the cylinder will be made up of fresh air,
residual and reinducted exhaust gases, and fuel. The mass of
fuel entering into the cylinder is controlled by the existing
engine fueling control and thus, is known. However, the mass
of fresh and burned gases in the cylinder must be calculated
from appropriate physically-based models. The fuel-flexible
engine considered in this work is also equipped with variable
valve timing (VVT). With a VVT system, it is possible to
alter the intake and exhaust valve opening and closing such
that both the intake and exhaust valves are open for a period
of time. This valve overlap can significantly influence the
amount of fresh air and burned gases present in the enginecylinder
at IVC.
CONCLUSIONS AND FUTURE WORK
In this work, a physically-based, generalizable combustion
phasing model for fuel-flexible SI engines with VVT
has been developed and demonstrated to accurately capture
changes in CA50 due to variation in thermodynamic
conditions, valve overlap, spark advance, and ethanol blend
fraction at over 500 points across the engine operating range
of a 4-cylinder fuel flexible SI engine utilizing VVT. The
model captures the rate of fuel burn from SIT to CA50
within 10% of the actual experimental values at a majority
(over 90%) of these points. Furthermore, its computational
simplicity and the fact that it uses only available engine
sensor measurements make in extremely valuable for efforts
to control combustion phasing.