Mixing-Controlled Combustion of Low-Cetane Fuels

Viswanathan, A. B. Mixing-Controlled Combustion of Low-Cetane Fuels. University of Wisconsin-Madison, 2023.

Interest in utilizing low-cetane fuels in heavy-duty compression ignition (CI) engines is on the rise to take advantage of fuel availability trends and improve urban air quality. This comes from the need to continue taking advantage of the superior efficiency of the mixing-controlled combustion mode used in CI engines while transitioning away from their current fuel of choice, diesel, toward cleaner low-cetane fuels. A thorough understanding of the challenges that could arise during mixing-controlled combustion of low-cetane fuels is needed to successfully navigate this transition. This research surveys the existing literature, enumerates the chief challenges to such a transition, proposed a novel approach with the potential to mitigate those challenges and conducts studies to study the viability of the technology and its potential to improve products on the field.

One of the major challenges that is expected to arise during mixing-controlled combustion of low-cetane fuels is the difference in autoignition propensities between diesel (very high) and low-cetane fuels (typically low). Analysis of the literature pertaining to the use of gasoline in CI engines (referred to as GCI) is expected to offer extensive guidance to the present study of low-cetane fuels and identify areas that need addressing. Thus, a detailed survey of the current state of GCI research is presented in this report to enumerate the most significant challenges that are faced during mixing-controlled combustion of low-cetane fuels. The survey leads to the conclusion that stable combustion at low loads and the ability to operate the engine within emission limits under cold engine conditions will pose the most significant hurdles to a transition away from diesel.

Reduced cylinder operation is identified as a technology with the potential to mitigate these challenges while also being a sufficiently developed technology to offer a realistic pathway to OEM implementation. Since RCO has never been studied as a combustion stabilization technology, experiments were carried out to assess the viability of cylinder deactivation in this novel role, by comparing the combustion stabilities of GCI operation when different sets of cylinders were deactivated. These experiments demonstrate that even at no-load conditions, cylinder deactivation enables stable GCI combustion while offering significant thermal and fuel efficiency benefits. Building on these results, experiments are conducted to compare RCO against intake pressurization, the chief alternative being proposed to stabilize GCI combustion. The intake pressurization is achieved by using another highly developed technology: 48V mild hybridization. Two electrically assisted air handling topologies are considered and compared against RCO from stabilization as well as performance standpoints. The results show RCO to have a clear advantage when considering aftertreatment performance, although the mild-hybrid components provide more robust stabilization. Combined optimization shows that these advantages can be combined with careful optimization.

RCO is shown to alter several boundary conditions simultaneously and a 3-D CFD study is carried out to isolate the respective roles of each in combustion stabilization. In addition to significant roles for the pressure and temperature at intake valve closing, the combustion dwell (between the end of the injection event and the start of combustion) is seen to have an unexpectedly significant role in the combustion efficiency and stability.

A subset of simple RCO strategies is then implemented on a heavy-duty engine and steady-state and transient experiments are conducted. These experiments show substantial benefits to implementing even simple RCO strategies on a heavy-duty engine, including faster aftertreatment warmup and slower aftertreatment cooldown as well as stability and fuel consumption benefits. Subsequently, simulations are conducted to assess the value of the remaining RCO strategies to GCI and even more substantial fuel consumption, aftertreatment thermal management and emissions benefits are predicted.