Optimisation of the Dual Fuel combustion

The project partners (OEMs, engineering partners and fuel suppliers) discussed about possible sustainable fuels and combustion systems which are promising with regards to high thermal efficiency, low engine-out emissions and suitability for current and modified fuel infrastructures. The results have been presented in deliverable D2.1.
Methanol was one of the fuels selected being the best compromise in the tradeoff between fuel production cost and transportability. In contrast to typical spark-ignited methanol combustion, a new combustion concept was selected where methanol is burned diffusively in a compression ignition (CI) engine. To enable CI of methanol, a diesel pilot is used to preheat the combustion chamber (called Dual Direct Injection Compression Ignition, DDI CI). The resulting process allows to operate on a high compression ratio thereby achieving high thermal efficiency.
In addition to engine testing, the diffusive combustion of methanol was investigated via Computational Fluid Dynamics (CFD), enabling to vary a broader range of parameters in a less cost-intensive manner. The main simulation objective is the optimization of pilot injector spray target and strategy. The pilot fuel share, pilot injection timing and the rail pressure for pilot as well as for main injection are the calibration parameters for the desired combustion system. Methanol injection timing will be adjusted to maintain the center of combustion.

The tests on FEVs Single Cylinder Engine (SCE) showed for best efficiency point that the diffusive dual fuel combustion concept is promising regarding efficiency (~50 %) and especially NOX emissions (e.g. 6 g/kWh requires almost no EGR). Moreover, combustion is soot free and unburned fuel and CO emissions remain on a low level typical for diffusive combustion. Those thermodynamic tendencies could be found also at different engine operation points. Potential to further improve the combustion concept lies especially in the pilot injection timing and quantity. The potential was numerically evaluated by IFPEN.
The CFD simulations performed by IFPEN reproduced the experimental results and lead to the following key conclusions: The minimum pilot fuel quantity, while maintaining a minimum flow rate for injector cooling, should be injected. It reduces NOX production and increases efficiency. SOI pilot should be as close to SOI main as possible, maintaining an optimum CA50. As EGR increases, overall NOX production decreases. NOX coming from pilot injection combustion is almost eliminated at higher EGR rate (>20 %) and NOX is mainly produced during main combustion. The pilot injection can be further optimized by reducing the hydraulic flow rate to allow small quantity of diesel injection and by targeting optimization (reducing the cone angle). As the results were very promising overall, the partners suggest investigating the DDI CI concept in more detail in future projects.

Please also refer to our publications list where you will soon find “An experimental and simulation study on the potentials of Methanol Dual Direct Injection Compression Ignition for heavy-duty long-haul applications” by IFPEN and FEV.