Troels Dyhr Pedersen.indd - Solid Mechanics

Emissions
The emissions were used during the experiment to
judge if combustion had stabilized after changing
the operating point. With a stable and effective
combustion, only small levels of CO were
measured, indicating high combustion efficiency.
The emission levels can also be used to judge the
quality of the experiment. A few outliers of higher
CO are found in figure 35 at the lower compression
ratios with lambda 2.5 and 3, which together with
corresponding lower values of CO2 in figure 34
indicate that fuel supply has been below target.
Values of CO2 are altogether not as constant as
desired, indicating that fuel supply has not been
absolutely constant.
8
7
6
5
4
3
2
Concentration of CO2 [% vol]
CO2 emissions
2.5, 1000
2.5, 2000
2.5, 3000
3, 1000
3, 2000
3, 3000
4, 1000
4, 2000
1
4, 3000
0
8.4 9.0 9.6 10.2 10.8 11.4 12.0 12.6 CR
Figure 34: Measured emissions of CO2
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Concentration of CO [% vol]
CO emissions
2.5, 1000
2.5, 2000
2.5, 3000
3, 1000
3, 2000
3, 3000
4, 1000
4, 2000
0.1
4, 3000
0.0
8.4 9.0 9.6 10.2 10.8 11.4 12.0 12.6 CR
Figure 35: Measured emissions of CO
1200
1050
900
750
600
450
300
Concentration of THC [ppm]
THC emissions
2.5, 1000
2.5, 2000
2.5, 3000
3, 1000
3, 2000
3, 3000
4, 1000
4, 2000
150
4, 3000
0
8.4 9.0 9.6 10.2 10.8 11.4 12.0 12.6 CR
Figure 36: Measured emissions of THC
The values of CO at lambda 4 indicate that
combustion efficiency is not as high as for lambda
2.5 and 3. Values are somewhat scattered, but it
seems that combustion efficiency improves at the
higher compression ratios, thereby resulting in less
CO.
Emissions of THC are generally at levels that may
be expected to be caused by the crevice volume
between the cylinder liner and piston [16]. Levels
are decreased with engine speed, presumably due
to higher in-cylinder temperatures and more postoxidation.
Emission of unburned DME does not pose a health
risk with at concentrations, as DME is non-toxic at
low concentrations. DME is also short-lived in the
atmosphere where it is degraded in a
photochemical process to produce CO2 and water.
Therefore emission unburned of DME is not a
concern. It is however unknown to which extend
DME is partially combusted, and therefore the
composition of exhaust gases is unknown. But as
cheap catalytic converters can be used to reduce
hydrocarbons in the exhaust, emissions of
hydrocarbons are not really an issue with these
levels.
Comments on the uncertainties of the experiment
The compression ratio and engine speed are only
some of the parameters that influence the
combustion process. Other parameters are
important too, but they may be more difficult to
control. Some of those are:
• Temperature of cylinder wall, piston and valves
• Temperature of cooling water
• Initial cylinder pressure and temperature
• Amount of residual gas
• Volumetric efficiency
• Engine geometry
The influence of these variables means that it is
difficult to isolate the effect of changing just the
compression ratio or engine speed, as conditions in
the cylinder changes as well. It also makes it
difficult to make consistent results without careful
consideration to these issues.
In a parameter study on HCCI [17] focused on
temperature influence it was found that the limits of
operation were highly dependant on coolant
temperature. It was also experienced in this study
that cooling water temperature could have
significant influence on the combustion timing.
Therefore cooling water temperature was
monitored and cooling water flow carefully adjusted
to maintain a constant temperature of the coolant at
the outlet.
The temperature of piston and cylinder liner was
also noticed to be of great importance. When
changing from lambda 2.5 or 3 to lambda 4, it was
observed that combustion was slowly retarded as
piston and liner cooled under the reduced thermal
load.