As the load continues to increase, maximum MSP is reduced in
order to avoid roar combustion or combustion noise. Along the roar
combustion bound, maximum MSP is limited by its PPRR of
approximately 1.5 MPa/CA. Fig. 5 illustrates the cylinder pressure
and AHRR under a roar combustion condition. First it can be seen,
as expected, that the compression pressure of DMDF mode
decreases significantly compared with the neat diesel combustion,
which is mainly due to the higher heat of vaporization of methanol
fuel. The results also show that at DMDF mode almost all the diesel
fuel and the entrained air–methanol mixture are burned in premixed
stage with a sharp peak on the AHRR trend. As the injection
timing of pilot diesel, as is shown in Fig. 5, is much more advanced
compared with that of neat diesel mode, start of ignition under
DMDF mode is much more advanced. Increasing the methanol fuel
at constant speed and load results in an increase in the mass of
premixed fuel admitted to the engine and a decrease of the pilot
diesel fuel mass injected. This increase in the mass of methanol
then causes an increase in the ignition delay period of pilot diesel
which then auto-ignites and starts burning the premixed methanol
fuel at a higher rate of pressure rise. This is also shown by Nielsen
et al. [23] on dual fuel engine where natural gas is admitted in the
inlet air manifold. Furthermore, compared with misfire bound, the
shorter diesel injection event is able to provide enough energy to
lead multi-site combustion. Meanwhile, compared with lower
load, the amount of methanol is large enough to form a richer
homogeneous mixture. In addition, because of the enhanced
charge temperature, the combustion is phased to near thermodynamically
ideal conditions. The rapid burn of DMDF mode also
shortens the combustion duration and has a positive effect on BTE.
As the load continues to increase, maximum MSP is reduced inorder to avoid roar combustion or combustion noise. Along the roarcombustion bound, maximum MSP is limited by its PPRR ofapproximately 1.5 MPa/CA. Fig. 5 illustrates the cylinder pressureand AHRR under a roar combustion condition. First it can be seen,as expected, that the compression pressure of DMDF modedecreases significantly compared with the neat diesel combustion,which is mainly due to the higher heat of vaporization of methanolfuel. The results also show that at DMDF mode almost all the dieselfuel and the entrained air–methanol mixture are burned in premixedstage with a sharp peak on the AHRR trend. As the injectiontiming of pilot diesel, as is shown in Fig. 5, is much more advancedcompared with that of neat diesel mode, start of ignition underDMDF mode is much more advanced. Increasing the methanol fuelat constant speed and load results in an increase in the mass ofpremixed fuel admitted to the engine and a decrease of the pilotdiesel fuel mass injected. This increase in the mass of methanolthen causes an increase in the ignition delay period of pilot dieselwhich then auto-ignites and starts burning the premixed methanolfuel at a higher rate of pressure rise. This is also shown by Nielsenet al. [23] on dual fuel engine where natural gas is admitted in theinlet air manifold. Furthermore, compared with misfire bound, theshorter diesel injection event is able to provide enough energy tolead multi-site combustion. Meanwhile, compared with lowerload, the amount of methanol is large enough to form a richerhomogeneous mixture. In addition, because of the enhancedcharge temperature, the combustion is phased to near thermodynamicallyideal conditions. The rapid burn of DMDF mode alsoshortens the combustion duration and has a positive effect on BTE.
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