Troels Dyhr Pedersen.indd - Solid Mechanics

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fuel concentrations lead to faster reactions and therefore higher differences in pressure,
the acoustic noise is increased with the engine load.
Controlling the rate of combustion is thus a challenge. It is possible to control the low
temperature heat release by constraining the development of radicals in the initial phases
of the combustion with additives or dual fuel combinations that consume radicals through
chain terminating reactions. There is however currently no methods that allow control of
the heat release rate in high temperature premixed combustion, which is fully governed
by reaction kinetics.
6.3 Combustion phasing parameters
6.3.1 Compression ratio
The most important factor to combustion phasing is the compression ratio. Fuels vary
greatly in their auto ignition characteristics. Their individual reaction chemistry gives
large differences in ignition delays. Some fuels, such as gasoline and methanol which
have high octane ratings, require a high compression ratio and hence temperature to
obtain auto ignition. Other fuels, typically those with high cetane rankings such as diesel
and DME in particular, have low auto ignition temperatures and low ignition delays. Such
fuels require only modest compression ratios to ignite.
6.3.2 Equivalence ratio
The equivalence ratio is also important to combustion phasing. If the equivalence ratio is
increased during operation of the engine, the rate of reaction is increased as well due to a
higher fuel concentration and a higher temperature increment during combustion. This
results in an instantaneous advancement of the combustion phasing. As the temperature
of the cylinder liner and piston increase slowly hereafter, the charge reacts even earlier
due to increased heat addition. Both the instantaneous and the slower advance of the
phasing pose a challenge in terms of maintaining the combustion at an optimum point in
the cycle.
6.3.3 Inlet temperature
The timing of the combustion is sensitive to inlet temperature, since the rate of formation
of the radicals needed to initiate the combustion is a function of the temperature. The
position of the initial part of the heat release may therefore be moved by controlling the
inlet temperature. A common approach is to preheat the inlet air. By using a regulated
mixture of hot and cold air, the temperature can be regulated on a cycle-to-cycle basis.
This approach has been used in a number of studies [8, 9 and 10].
6.3.4 Exhaust gas recirculation
The use of exhaust gas recirculation (EGR) is a well known method for reducing the
nitric oxides in engines in part load operation. The exhaust gas returned to the inlet
decreases the specific heat ratio, which means that combustion temperatures will be
lowered as well. In diesel engines the usual limit is approx. 30 % EGR, as the oxygen
concentration eventually decreases to a level which reduces the combustion efficiency.
This is because the diffusion flame is incapable of consuming all the available oxygen in