Understanding Alcohol Autoignition to Enable Diesel Engine Decarbonization

Abstract

In order to decarbonize heavy-duty applications, new sustainable fuels must be introduced for diesel engines. Many promising sustainable fuels are alcohols, which have ignition processes that are distinct from those of conventional diesel fuel. Understanding these processes is paramount to making their use practical. To support efforts to decarbonize diesel engines, this dissertation presents experimental and modeling studies of ignition process for alcohol fuels. These studies enhance our understanding of combustion processes for sustainable fuels, and demonstrate how they can best be used in diesel engines with limited, well-defined modifications to current designs.

Alcohol fuels studied include ethanol, 1-octanol, and a novel mixture of alcohols produced via catalytic upgrade of ethanol. Conventional diesel fuel is used as a basis for comparison in many experiments. These fuels cover a wide range of properties, and span degrees of current market readiness and compatibility with unmodified diesel engines.

Autoignition of ethanol pilot injections was studied in a diesel engine with an electrically heated intake manifold. Results demonstrate that the pilot injection strategy which maximizes heat release varies as a function of intake manifold temperature. Earlier pilot timings were generally preferred as intake manifold temperature dropped. Limited dependence of pilot ignition timing on injection timing is observed, in contrast with conventional diesel fuel.

To explain these experimental results, ethanol pilot injections were simulated in a computational fluid dynamics code. By tracking the reactivity of the injected fuel as the mixture develops inside cylinder, it was shown that the change in the pilot injection timing which maximizes heat release as intake manifold temperature changes can be attributed to the evolution of temperature and equivalence ratio distributions inside the cylinder. It was also shown that fuel-air mixing is the primary mechanism by which the injected fuel approaches the temperature necessary for ignition.

Combustion of 1-octanol was studied in a constant volume combustion chamber (CVCC) and modified Cooperative Fuels Research (CFR) engine. It was found that hydrotreated vegetable oil ignited most rapidly in both devices, as expected in according to its high cetane number. It was also shown that 1-octanol ignited at a lower compression ratio than diesel fuel in the CFR engine, despite having a lower cetane number and longer ignition delay in the CVCC. This result illustrates the potential for saturated, straight chain compounds to reduce hydrocarbon emissions from premixed combustion, even as the hydroxyl moiety in alcohols tends to suppress ignition. These results also show that, as a function of their chemical structure, alcohols ignite more readily than their cetane number might predict under lean premixed conditions.

Finally, combustion of a novel, n-butanol based sustainable fuel was demonstrated in a diesel engine, utilizing exhaust rebreathing to promote autoignition. Whereas idling typically presents the greatest challenge for fuels with low cetane numbers, this study achieved idle with GrenOl with comparable performance to diesel fuel. With modified valve timings still in place, peak torque was achieved, demonstrating that exhaust rebreathing did not derate the engine at high load. Thermal efficiency and NOx emission levels remained at diesel-like levels, and particulate emissions were reduced relative to diesel levels at all conditions. These results indicate that near-market sustainable alcohols with carbon intensities near half that of diesel fuel can be used as neat fuels for diesel engines with well-defined changes to engine operation.