Recent achievements in peak indicated thermal efficiency in diesel engines with advanced LTC modes. The leftmost bar shows conventional diesel combustion. The bars to the right depict observations for various forms of LTC, including partially premixed compression ignition (PPCI) and dual fuel PPCI. All efficiencies depicted are based on first law analysis. Loss terms indicated at the top of the bar graphs represent incomplete combustion of fuel. Presented by Dave Foster, U. Wisc., at USCAR Colloquium 2010. Click to enlarge.

There was general consensus that the practical limit for peak BTE for slider-crank engines is significantly less than 60% when additional factors (particularly cost) are considered. The participants agreed that achieving BTEs of more than 60% would require radical changes to present engines, including cycle compounding, new engine architectures, and more constrained combustion reactions. Such radical changes require long-term R&D, but work in this direction needs to begin now, they said.

Currently, the highest peak brake thermal efficiency (BTE) of current passenger vehicle engines is slightly above 40% (i.e., more than 40% of the energy released by the fuel is converted into crankshaft work under ideal conditions). Existing engines lose 20–25% of the fuel exergy due to the irreversibility of unrestrained (non-equilibrium) combustion. This destroyed exergy appears as heat that cannot be transformed into useful work.

In the nearer term, it appears that additional gains in peak efficiency are still possible for slider-crank combustion engines. Many of these gains are likely to come from reduced heat loss achieved by lowering combustion temperatures through various forms of lean, premixed, or partially premixed combustion...The lower reaction temperatures in LTC are also useful for reducing engine-out nitrogen oxide (NO_x) emissions, thereby reducing the need to consume additional fuel for exhaust aftertreatment. Thus, at least for NO_x, low emission combustion does not necessarily have a negative impact on efficiency.

...Various versions of LTC have been intensely investigated for the past several years because of their potential for generating reduced combustion temperatures and NO_x emissions. Lower engine-out NO_x is beneficial for diesel and lean gasoline engines because it reduces the fuel required to reduce the NO_x with post–engine aftertreatment for emissions control. The benefits from LTC also extend directly to engine efficiency, primarily because of reduced cylinder heat loss (due to the lower combustion temperature) and the potential for very dilute combustion (due to different reaction kinetics). By reducing cylinder heat loss and changing the molecular properties of the expanding combustion gases, LTC allows more of the energy released by combustion to be extracted in the expansion stroke...

This is beneficial even though the combustion reactions are not moved significantly closer to equilibrium and thus the overall combustion thermodynamics are still irreversible. The combustion irreversibility even increases further with dilute combustion due to the added entropy generation from inert gas mixing. Yet in spite of these irreversibility penalties, the reduced heat loss and better gas properties in LTC are enough to improve net work output. Thus it is now understood that it is possible to actually increase combustion irreversibility (at least a modest amount) and still improve overall engine efficiency.

—Report on the Transportation Combustion Engine Efficiency Colloquium Held at USCAR

Effective LTC needs to avoid an uptick in CO and UHC emissions. Source: Gurpreet Singh, SAE High Efficiency IC Engines Symposium. Click to enlarge.

In his overview of the US Department of Energy’s (DOE) Advanced Combustion Engine R&D presented at the Merit Review, Gurpreet Singh, Team Leader for the Advanced Combustion Engine R&D Subprogram, outlined the focus on LTC, which is used generically to represent many processes such as Homogeneous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI), and a large number of variants.

Singh noted that challenges to achieving LTC include combustion phasing; load range (e.g., stability and control at high loads where engine efficiencies are high); heat release rate; transient control; HC and CO emissions; and fuel characteristics.

In one of his Merit Review presentations of work being done at Oak Ridge National Laboratory (ORNL) on high efficiency clean combustion in multi-cylinder light-duty engines, Tom Briggs noted that advanced combustion strategies are increasingly using engine hardware that looks similar, and that these strategies are also blurring the lines between fuel selection and combustion propagation mechanisms.

I think this has come out a number of times in the conference in the comments from the reviewers, but I would say that we are moving to something that you could call a fuel-neutral combustion world. If you look at all these advanced combustion approaches, they start looking very similar in terms of the hardware they require, high EGR rates, in-cylinder injection, and they kind of blur the lines between the fuel you select and the combustion propagation mechanisms. [The different mechanisms] start looking very similar in the way combustion tends to form, propagate, even some of these HCCI approaches that John Dec was talking about. Looking at these across the spectrum you can see a lot of the advanced work converging on very similar approaches, just with a different fuel and perhaps a slightly different fuel delivery system, but a lot of the challenges...are the same.

—Tom Briggs

Briggs’s spectrum of combustion strategies. Source: Briggs, ACE016. Click to enlarge.

Next up: a report on RCCI (Reactivity Controlled Compression Ignition, earlier post) activities and results.

Resources

• Gurpreet Singh, Overview of the DOE Advanced Combustion Engine R&D (2011 Merit Review)

• Tom Briggs, High Efficiency Clean Combustion in Multi-Cylinder Light-Duty Engines (2011 Merit Review ACE016)

• Report on the Transportation Combustion Engine Efficiency Colloquium Held at USCAR, March 3–4, 2010