Troels Dyhr Pedersen.indd - Solid Mechanics

Engine modification
To obtain a lower CR than the original, the piston
top was lowered to give a geometric CR of 14.3.
This CR is empirically high enough to ensure
ignition and complete combustion of very lean
charges with DME. To enable further adjustment of
the CR, a cylinder with an internal diameter of 25
mm and an external diameter of 30 mm was
inserted through the cylinder head to connect with
the compression volume. The volume of the
regulating cylinder could then be used to adjust the
compression volume by the adjustment of a piston.
To allow space for this arrangement, the existing
passage for the fuel injection valve was expanded
and used for the regulating cylinder. Figure 2
illustrates the arrangement.
With the regulating cylinder and piston arrangement,
the compression volume is adjustable from 14.3 to
8.8 by extracting the piston up to 50 mm out of the
regulating cylinder.
As both cylinder liner and regulating cylinder are
subjected to the engine cooling water it is assumed
that thermal conditions are approximately equal in
both volumes. Combustion should therefore occur
simultaneously in both volumes.
Fuel injection system
The DME was injected in pulses into the intake
manifold of the modified cylinder by means of a
standard gasoline injector with a free flow capacity
of 190 g/min. The arrangement is seen in figure 1.
The fuel supply tank was pressurized at 8 bars in
order to avoid formation of vapor bubbles in the fuel
supply line and near the injector. The amount of
fuel per injection was calibrated using a precision
scale with a resolution of 0.1 gram intervals to
determine fuel consumption in 500 injections.
Injection timing and duration was controlled using a
programmable microcontroller. The signal from a
quadratic encoder on the engine shaft was used as
reference for the injection. In this study, the amount
of fuel is kept constant for all speeds for each of the
three target values of lambda. Volumetric efficiency
is however not constant. For the three target values
of lambda, the actual values are therefore slightly
different, as shown in table 3.
Table 3: Target and actual excess air ratios
RPM 1000 2000 & 3000
Target 2.50 3.0 4.0 2.5 3.0 4.0
Fuel [mg] 25 20 15 25 20 15
Inject. [ms] 4.7 3.6 2.5 4.7 3.6 2.5
Vol. eff. 0.96 0.88
Actual 2.45 3.06 4.08 2.25 2.81 3.74
The variation in actual lambda is considered to be
of less importance, as the concentration of fuel is
limiting the rate in lean combustion and not the
concentration of oxygen, which is abundant.
Cylinder pressure data acquisition
The cylinder pressure was measured using a Kistler
6052A pressure transducer. A Kistler 5001 charge
amplifier was used for amplification of the signal
from the transducer. The acquisition of the pressure
was made using a computer with a data acquisition
card. The clock and trigger signals for the
acquisition was supplied by an encoder with 3600
pulses per revolution. TDC alignment was made by
software correction.
The setup used for acquiring the cylinder pressure
was designed to provide a high resolution of the
cylinder pressure, as the combustion durations
were expected to be from 5 - 10 CAD. Thus, using
a lower resolution such as 1 sample per CAD would
be insufficient, as samples would not provide much
information about the event. In addition, a sampling
frequency at least 2.5 times higher than the highest
frequency expected should be used to avoid
aliasing.
Emissions
It is a common observation that the HCCI process
with gaseous fuels does not produce soot, and that
the emission of NO is very low due to the low peak
temperatures of HCCI combustion. Those
emissions are thus not the primary concern with
this combustion principle and have therefore not
been measured either.
The emissions of CO2, CO and THC were
measured with a Bosch ETT 8.55 portable exhaust
analyzer. The emissions were monitored with the
purpose of identifying the quality of the combustion,
which was used to determine when satisfactory
combustion occurred, and also when combustion
was stabilized. It is easily seen from the values of
hydrocarbons, CO and CO2 if the combustion
process is at steady state or in transition.
DATA TREATMENT
Cylinder pressure data treatment
Due to the nature of the HCCI combustion, one or
more shockwaves of some amplitude always
follows the combustion. The shockwave causes a
resonating wave in the combustion chamber which
is similar to that caused by knocking combustion.
This phenomenon has been studied in detail by
Vressner et al [14]. Apart from the noise emitted
from the engine, knocking combustion is believed
to enhance the heat transfer to the combustion
chamber walls [15]. This is another good reason
why engine knock must be limited.
The frequency of the shockwaves observed is in
the region of 4500 – 6000 Hz, with higher engine
speeds and in-cylinder temperatures leading to
higher frequencies. Wavelengths were determined
by close examination of the oscillating parts of the
pressure curves. The heat release analysis is
heavily biased by these pressure pulsations, as
these are interpreted as heat release as well as the