Troels Dyhr Pedersen.indd - Solid Mechanics

close to the lean limit. HC is composed of both unburned fuel from the crevice volumes and partially burnt fuel
from wall quenching. HC avoids post oxidation due to the lower temperatures in the cylinder and exhaust
system. HC and CO emissions are however efficiently reduced in a DOC with surplus amounts of oxygen.
The lowest equivalence ratio that may be applied in HCCI combustion is determined by the minimum
temperature, which must be reached to ensure rapid oxidization of CO to CO2. This temperature is
approximately 1500 K, which means that for the typical range of compression ratios in internal combustion
engines the equivalence ratio cannot be less than 0.15 to 0.2. This corresponds approximately to idle condition.
The highest applicable equivalence ratio is usually limited by engine knock which may not surpass a specified
tolerance limit. An equivalence ratio of 0.3 to 0.4 is commonly the limit with DME as fuel, depending on the
magnitude of knock and pressure rise rates that are tolerated.
An important feature of HCCI combustion is that it relies on auto-ignition. This means that combustion timing
depends on both fuel and engine properties, and a proper timing requires that fuel and engine is matched
properly. In reality, changing operating conditions and demands for a large operating range mean that either
engine or combustion chemistry must be manipulated to maintain an optimal operation at all times [1]. For
engines, this usually means changing the effective compression ratio through variable valve timing or similar,
whereas combustion chemistry may be changed by mixing fuels with different properties.
DME is an interesting fuel due to its auto ignition characteristics. It has a low temperature of auto ignition and a
short ignition delay. The cetane ranking of DME is 55-60, which makes the fuel an excellent alternative to
standard diesel fuel which has a cetane ranking of 45-50. The disadvantage of DME is that it has a very low
lubricity. This requires that the fuel system components are designed to handle a fuel with no lubrication. In the
current study a test engine with a common rail system specifically made for DME is used [18], and the lubricity
is increased with Lubrizol.
DME is also suitable for HCCI operation in engines with a compression ratio is around 10. If a higher
compression ratio is used, some means of delaying the combustion onset must be used to ensure optimal
position of the heat release.
In this paper, methanol addition has been used to control the combustion timing in HCCI combustion of DME.
The use of methanol in combination with DME has previously been demonstrated by other authors [2, 3, 4].
The common observation is that up to 30 % of methanol by mass is possible before combustion becomes
retarded to the point where it fails to complete. The exact amount does however depend on the compression
ratio of the engine.
Exhaust gas recirculation can also be used to delay the combustion timing. Some authors [5, 6] have looked into
the subject. The use of EGR is however a challenge, since it requires a high ratio of EGR to inlet air to achieve
the desired effect, as demonstrated in this paper. EGR works mainly by increasing the concentration of CO2 and
H2O, which increases the specific heat capacity and decreases the specific heat ratio. This reduces the
temperature increase during compression and combustion. Secondly, it reduces the amount of oxygen which is
normally not desired in diesel engines, meaning they are usually not capable of operating with more than 30 %
EGR. In HCCI combustion however, it is possible to operate with much lower oxygen concentrations. It is only
required that the concentration of oxygen is sufficient to complete the combustion.
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