Christopher F. Edwards, Associate Professor, Mechanical Engineering; Patrick Caton, Shannon Miller, Kwee-Yan Teh, Graduate Researchers, Stanford University

This project aims to improve the efficiency of reactive engines by implementing the concept of low-irreversibility combustion.

Combustion engines release chemical energy contained in an air-fuel mixture by burning it. The resulting hot products serve as the working fluid whose sensible energy is converted to useful mechanical work. Once initiated, combustion in a conventional engine is rapid and unconstrained. The chemical reaction is confined to a very thin zone – the flame front – that propagates until all reactants are consumed. The exothermic process is rate-limited by local diffusive and convective transport of energy and species at the flame front. The efficiency of energy conversion by such a process can be quantified using thermodynamics principles. The concept of low-irreversibility combustion stems from the realization that, from the second law standpoint, there is opportunity, currently unexploited, to extract additional useful work during the combustion process. Instead of allowing the reactant mixture to combust rapidly without constraint, a low-irreversibility engine would harness useful work from chemical energy released as combustion is occurring.

Background

Engines that execute the energy conversion process continuously are referred to as "steady-flow" engines. For example, gas turbine engines are included in this category. In contrast, piston engines are "batch-flow" engines that process the charge in a sequence of discrete events. In each type of engine, atmospheric air is compressed to high pressure and mixed with fuel. The reactant mixture reacts, and the resulting combustion products (at high temperature) are expanded to develop work.

Figure 1 overleaf shows the result of an ideal Brayton cycle analysis of a steady-flow engine on a Mollier (enthalpy h versus entropy s) diagram. The net work developed by the engine is given by the enthalpy difference between the initial reactant and final product mixtures, both at atmospheric pressure. A theoretical "isentropic" chemical reaction (1 – 4s) would yield the maximum work available, w_MAX. State-of-the-art gas turbine combustors, on the other hand, operate adiabatically with minimal pressure loss. The rapid combustion process (2 – 3) leads to maximum rise in temperature (up to the adiabatic flame temperature) and high entropy production. As a result, the net work, w_NET actually developed by the engine is less than w_MAX; the difference is the irreversibility (or lost work), i due to combustion. The ratio of w_NET tow_MAX, a form of second law efficiency, is therefore a sensible measure of how well the engine utilizes its fuel.

The piston engine can be similarly modeled and analyzed. The Otto cycle is a simplified model of piston engine operation: The reactant gas mixture is compressed and then ignited while the piston is at top dead center (TDC). The adiabatic combustion products are expanded back to the original volume and thereby produce mechanical work. The total internal energy u versus entropy s diagram (Figure 2) shows the result of an ideal Otto cycle analysis. w_MAX is given by a theoretical "isentropic" chemical reaction from reactants to products at the same specific volume (1 – 4s). Again, adiabatic, constant-volume combustion at TDC generates entropy and irreversibility, so only a fraction of the maximum available energy is converted to useful work.

This project is studying possible processes (e.g., from 2 to 4* in Figure 2) that can achieve such entropy and irreversibility reductions. Implementation of this combustion concept requires the chemical reaction be controllable. Additionally, careful design of the work extraction process, aided by intelligent control strategies, will likely be necessary to drive the combustion process to completion and avoid quenching. Figure 2 illustrates the potential efficiency improvement based on this concept. A 20% reduction in entropy generated during combustion near TDC (due to work extraction), for instance (2 – 4*), would increase w_NET by 30%.

Approach

In a batch-flow engine, the mode of combustion has been altered from transport limited (and therefore uncontrollable) to chemical-kinetics rate-limited (and therefore controllable). This has been accomplished via dilution of the air-fuel mixture with re-inducted exhaust. The hot exhaust raises the sensible energy content (and thus temperature) of the mixture to sustain the reaction and avoid quenching. At the same time, the temperature rise during combustion is moderated due to the overall lower chemical energy content of the dilute reactant mixture. Control over the re-induction process is accomplished by a special variable valve actuation system in which arbitrary control of valve motion can be implemented.

Preliminary design work has begun on a steady-flow version of a low-irreversibility engine. The basic processes are the same as for the batch version: dilution, energy addition, autoignition, and energy extraction during combustion. Dilution and preheating are again accomplished using hot combustion products. An adjustable supersonic nozzle may be used to provide a proof of concept demonstration by extracting kinetic energy during the final combustion process.

By kinetic coupling of work extraction to chemical energy conversion, both the batch-flow and steady-flow low-irreversibility engines stand to significantly improve the efficiency with which we consume conventional fuels. Theoretical studies via chemical kinetics are also being pursued in parallel with the experimental work.