|The effect of increasing levels of fuel stratification on observed pressure. Increased stratification leads to longer combusstion duration, no stratification results in the highest PRR. Krisman et al. Click to enlarge.|
Researchers from the University of New South Wales (Australia) and Sandia National Laboratories in the US report that ethanol fuel thermal stratification has the potential to reduce significantly the pressure-rise rate (PRR) and so to extend the high-load limit of homogeneous-charge compression-ignition (HCCI) engines.
In a simulation study, published in the journal Fuel, the team developed and applied a multi-zone model of thermal stratification to assess the potential of ethanol fuel stratification to reduce PRR in HCCI engines and to understand the possible trade-off with increased NOx formation.
As discussed elsewhere, the operating principle of HCCI engines combines characteristics of SI [spark ignition] and diesel engines. As with diesel engines, the lack of throttling losses and (potentially) high compression ratios results in high thermal efficiencies. As with SI engines, the fuel-air mixture is premixed, additionally, it is usually locally lean (or diluted) throughout the cylinder. These features lead to a clean burn that all but eliminates particulate emissions, such as soot. Furthermore, the lean or diluted mixture burns at a lower temperature, dramatically reducing NOxemissions.
Despite the benefits of HCCI engines, a number of significant barriers exist to the deployment of this technology. One of the key issues is that at high loads, excessive pressure-rise rates (PRRs) occur, leading to knocking/ringing that increases noise and can potentially damage the engine. This is a direct result of the charge homogeneity: ignition happens everywhere in the cylinder almost simultaneously, unlike in an SI engine where it is limited by flame propagation rates or in diesel engines where it is limited by mixing and injection rates. The deliberate introduction of fuel stratification has been proposed as a technique to address this issue.—Krisman et al.
Approaches to achieving this fuel stratification strategy—which the researchers call Stratified Charged Compression Ignition (SCCI)—include later injection of a nearly homogeneous charge with kinetically controlled ignition, and two-stage ignition fuels, where induction time is chemically sensitive to the fuel concentration, and therefore that the fuel stratification can be used to reduce the PRR.
This paper explores an alternative strategy: thermal stratification via the evaporative cooling of a stratified fuel-air charge. When a liquid-phase fuel is directly injected, the local cooling induced by vaporization introduces a thermal stratification. Since the induction time of single-stage ignition fuels is highly sensitive to temperature, this can potentially lead to a staged ignition process and reduction of the pressure-rise, if the fuel heat of vaporization is sufficiently high. Note: this strategy also requires that the induction time is not chemically sensitive to fuel concentration, since this would produce an opposite stratification effect to that of evaporative cooling.—Krisman et al.
There are trade-offs to this approach, they note, including the potential for increased NOx formation—i.e., increased NOxversus decreased PRR.
Ethanol has already been shown to be an excellent HCCI fuel; since water has a very high heat of vaporization, they also considered mixtures of ethanol and water. (As a further benefit, hydrated ethanol reduces fuel cost and the energy intensity of the fuel life-cycle.)
They developed a multi-zone model of the engine and validated it against an experimental data-set for natural stratification in HCCI engines. They then extended the model to allow for fuel stratification, in addition to an underlying thermal stratification. Stratified fuel evaporation is accounted for in the model and leads to enhanced thermal stratification in the cylinder. Results of the study included:
Their model was able to predict correctly the temperature dependence of ethanol ignition timing under the conditions considered. It also predicted the resulting PRR of the thermally stratified charge.
No stratification resulted in the higher PRR; Increasing stratification led to longer combustion duration up until the point where partial misfire began to occur. The increased combustion duration is due to the enhanced thermal stratification, caused by different amounts of evaporative cooling. The increased thermal stratification leads to a grater range of induction times, increasing combustion duration,
Unlike stratification for two-stage ignition fuels, leaner ethanol mixtures ignite first and richer mixtures ignite last, which is potentially beneficial for reduced NOx formation.
Downsides of stratification include increased NO formation due to locally high equivalence ratios and combustion product temperatures, as well as reduced combustion efficiency and IMPE due to very rich zones not igniting because they are too cold and retarded.
Water addition results in reduced NO formation and provides additional potential for PRR reductions. The effect is more pronounced for higher levels of stratification.
Alex Krisman, Evatt R. Hawkes, Sanghoon Kook, Magnus Sjöberg, John E. Dec (2012) On the potential of ethanol fuel stratification to extend the high load limit in stratified-charge compression-ignition engines, Fuel, Volume 99 pp. 45-54, doi: 10.1016/j.fuel.2012.04.001