ExxonMobil Research files patent application on methods for expanding HCCI and other advanced combustion modes load range with fuel-alcohol blends
ExxonMobil Research and Engineering Company has filed a US patent application (#20100326387, published 30 December 2010) on methods using a wide range of fuel-alcohol blends to expand the operating envelope of engines operating in advanced combustion modes. Advanced combustion modes covered include homogeneous charge compression ignition (HCCI); premixed charged compression ignition (PCCI); low-temperature combustion (LTC) mode; or other non-traditional highly-mixed combustion modes.
As an example of the benefits, the developers say that the fuel-alcohol blend methods can expand the HCCI operating load range by about 10% to about 30% than the loads achieved when using a reference gasoline without encountering unacceptable engine noise, metallurgical stress, or elevated NOx emission levels.
The patent application is wide-reaching, with alcohols defined as including one or more compounds selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol, and fuels defined as including (but not limited to) a gasoline, a diesel fuel, kerosene, a jet fuel, a biofuel blend (e.g. biodiesel), a renewable diesel, a Fischer-Tropsch-derived fuel, a gasoline-diesel blend, a naphtha, other fuels derived from petroleum or non-petroleum feed stocks, and any combination or blend.
The patent application covers blends of between 5% and 95% alcohol. Under the ExxonMobil application, the alcohol and fuel can be blended before introduction into the engine cylinder, or injected separately into the cylinder to form a blend containing the requisite amounts of alcohol and fuel.
In methods of the invention described in the patent application, the use of the fuel-alcohol blends to expand the load range reduces or eliminates the need for exhaust gas recirculation (EGR), variable valve timing (VVT), negative valve overlap (NVO), rebreathing, multiple fuel injection, and inlet cylinder pressure boosting. According to the filing and the testing referenced within, the approach shows:
- significantly reduced peak NOx emission levels;
- prolonged ignition delay;
- delayed and broadened HTHR (high-temperature heat release); and
- reduced maximum rates of pressure increase during HTHR.
HCCI is of great interest because it can combine the low-NOx exhaust emissions of gasoline engines with three-way catalysts with the high thermal efficiency associated with diesel engines.
HCCI combustion results from the spontaneous auto-ignition at multiple points of thoroughly mixed fuel-air charge throughout the volume of the charge gas. HCCI combustion typically occurs in two stages: a low-temperature heat release (LTHR) occurs first, followed by a high-temperature heat release (HTHR). Broadening LTHR and HTHR, and reducing the maximum rate of pressure increase during LTHR and HTHR, increases the operating range of a HCCI engine.
Despite that being well known, it is still difficult to operate in HCCI mode over a wide range of loads:
- combustion phasing (the timing of auto-ignition) is inherently difficult to control;
- the rapid rate of heat release by a HCCI engine as its load increases can lead to mechanical and noise problems;
- with rapid combustion, the maximum rate of pressure rise limits the ability of HCCI engines to achieve medium and high loads;
- HCCI is sensitive to fuel composition; and
- fuels often do not auto-ignite at low loads.
Although EGR and VVT help to control the combustion heat release, rate of pressure rise, and NOx emissions of HCCI and other IC engines, each of these design options has its detriments, the ExxonMobil researchers noted. External EGR leads to a slow response rate since EGR gases must flow through the exhaust and EGR system. External EGR also requires substantial heat dissipation; EGR must often be cooled prior to introduction into the engine.
Further, they said, to achieve high load performance with EGR, a larger engine size is needed (due to the displacement of air by EGR), which leads to a loss of efficiency and power. While internal EGR strategies using VVT have faster response rates, these valve strategies contend with delayed intake valve closure time, which also decreases power and efficiency.
In a set of combustion experiments referenced in the application, the researchers demonstrated that including more than 10% by volume of ethanol in gasoline significantly improved the combustion performance of the fuels studied.
The data shown...demonstrate that there is a longer time interval between the low and high temperature heat release points for the fuels containing more than 10% ethanol when compared to a fuel without ethanol that has about the same (RON+MON)/2 value. This longer time interval probably provides an opportunity for some hot reactive gases to mix uniformly in the cylinder before the main combustion event and the presence of such reactive gases could assist with combustion phasing or auto-ignition in challenging environments. Thus, the unusual combustion characteristics of the processes of the invention could also enable engine designers to better optimize injection timing.
The researchers also found that the use of 15% or more ethanol reduced cycle average peak NOx levels by about 86-89% at 87 (RON+MON)/2 and 14% at 90 (RON+MON)/2.
While both sets of fuels at lower and higher octane levels saw significant NOx reduction, the result for the 87 (RON+MON)/2 fuel is an impressively large reduction. It is theorized that at the lower octane number, use of the non-ethanol fuel results in a not completely homogeneous fuel/air mixture. This can lead to less efficient combustion, increased peak combustion temperatures, and locally hot zones. NOx formation is very temperature-dependent, and tends to increase significantly with increasing peak combustion temperatures. It is believed that ethanol delays combustion long enough to significantly improve the homogeneity of the fuel mixture, thereby reducing both combustion temperatures and NOx levels. The fact that the NOx level decrease is smaller with the higher octane fuels supports this conclusion. With the higher octane base fuel (Base 90), the HTHR50 is already noticeably delayed at 26.1 msec. A further delay of HTHR50 to 35.5 msec in Fuel 90-20 does not impact the homogeneity nearly as much as with the lower octane fuel set, where the Base 87A HTHR50 is only 16.2 msec.
Yao et al. (2008) An investigation on the effects of fuel chemistry and engine operating conditions on HCCI engine (SAE 2008-01-1660)
Lü et al. (2007) Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors, Combustion and Flame 149 261-270 doi: 10.1016/j.combustflame.2007.01.002
Shibata, et al. (2005) Correlation of Low Temperature Heat Release with Fuel Composition and HCCI Engine Combustion (SAE 2005-01-0138)