Troels Dyhr Pedersen.indd - Solid Mechanics

DESCRIPTION OF THE EXPERIMENT
OBJECTIVE
The goal of this study was to determine the
influence of compression ratio (CR) and engine
speed on HCCI combustion of DME at selected
excess air ratios.
MOTIVATION
The primary motive for investigating HCCI
combustion of DME is that of avoiding highpressure
pumps and injectors. As DME has a very
low lubricity, pumps and injectors are susceptible to
rapid wear. If DME can be introduced with a low
pressure nozzle instead, these problems are
avoided.
Prior experiments have shown that the combustion
of pure DME in the HCCI process necessitates a
careful choice of CR to achieve the best possible
result, e.g. a high IMEP and indicated efficiency. It
is also of interest to investigate the influence of
engine speed on the combustion process. Higher
engine speeds may improve the characteristics of
the combustion, which may be of interest in smaller
engines.
EXPERIMENTAL OUTLINE
Excess air ratios
From earlier experiments it was found that the
richest mixture that could be used without
excessive knock would be an excess air ratio
(lambda) of 2.5. It was also found that compression
ignition of mixtures leaner than the excess air ratio
of 4 required a high compression ratio of about 12-
14, and that such lean mixtures cannot produce
sufficient torque to keep an engine running except
for idle conditions.
Three different excess air ratios were chosen to
represent the rich limit (lambda 2.5), the lean limit
(lambda 4) and a value in between (lambda 3). As
keeping an equivalence ratio constant can be a
challenge when air flow changes, three constant
values of fuel mass per cycle were used instead.
The three values are however referred to as
lambda 2.5, 3 or 4 in the text and figures.
Variation of engine speed
The HCCI combustion is a premixed and
homogeneous combustion which is mainly
governed by chemical kinetics. The reactions
therefore occur very fast compared to DI CI
engines and SI engines, but may still be affected by
engine speed.
It is well known that higher engine speeds lead to a
more adiabatic compression of the mixture, but also
increases the heat flux from the combustion
chamber and therefore results in higher in-cylinder
temperatures.
To investigate the possible effects of engine speed
on the combustion event and engine performance,
an interval of engine speed comparable to that of a
medium duty engine has been chosen. To avoid
excessive amounts of data, the fixed engine
speeds used were 1000, 2000 and 3000 RPM.
Variation of compression ratio
The CR has been varied from 8.8 to 12.8 in steps
of 0.2. The purpose was to accurately determine
the range of CR necessary to achieve satisfactory
engine performance within the desired range of
excess air ratios and engine speeds.
Determining the usable interval of compression
ratio at different engine speeds and equivalence
ratios
In this study, the lowest acceptable compression
ratio is found at the point where the engine is
capable of self-sustaining combustion. It was
necessary to preheat the inlet air at the lowest
compression ratios to initiate HCCI combustion, but
if combustion did not proceed after preheating was
removed, the operating point was not considered
valid.
With lambda 4 it was however difficult to determine
whether HCCI combustion was completed while the
CR was in the lower end. To determine if
combustion was satisfactory, the amounts of CO2
and CO were monitored to determine if the
combustion efficiency was satisfactory. It turned out
that the combustion resulted in higher amounts of
CO than CO2 until the CR was raised enough to
complete the final CO oxidation. A figure displaying
emissions of CO and CO2 is found in the results
section.
At the higher engine speeds it was more difficult to
initiate HCCI combustion of the leaner mixtures
lambda 3 and 4. As preheating the air could not
effectively solve this problem, preheating was done
by running the engine with the rich mixture for a
short time before shifting to the leaner mixture. If
the compression ratio was adequate the
combustion would continue and stabilize; otherwise
it would only continue for a short while before
entering a mode of partial combustion.
The highest acceptable compression ratio is found
at the point at which no further improvement in
performance is achieved. While it is difficult to
determine this limit during the experiment, it is clear
from the results that IMEP as well as indicated
efficiency peaks at the optimum compression ratio
and then decreases as compression is further
increased. This happens fast with lambda 2.5, while
lambda 4 benefits from a higher compression ratio
than the lowest acceptable.
While the intensity of knock or pressure rise rate is
sometimes used as a subjective limit of operation,
this limit is not used in this study as it is desired to
show the amplitude of engine knock when
operating limits are exceeded.