Oak Ridge Lab study finds E30 blend and EGR can deliver significant efficiency improvements in optimized SI engines
Researchers at Oak Ridge National Laboratory’s National Transportation Research Center (NTRC) report that an E30 (30% ethanol) mid-level ethanol blend shows promise as a means for significant improvement in vehicle efficiency in optimized spark-ignited (SI) engines. Results of the study by Derek Splitter and Jim Szybist suggest that it could be possible to implement a 40% downsize + downspeed configuration (1.2 L engine) into a representative midsize sedan using this combination of optimized engine and mid-level alcohol blend.
As an example, for a midsize sedan at a 65 mph (105 km/h) cruise, estimated fuel efficiency of 43.9 mpg (5.4 l/100 km) with engine-out CO2 of 102 g/km could be achieved with similar reserve power to a 2.0 L engine fueled with regular gasoline (38.6 mpg/6.1 l/100km, engine out CO2 of 135 g/km). The data suggest that, with midlevel alcohol–gasoline blends, engine and vehicle optimization can offset the reduced fuel energy content of alcohol–gasoline blends and likely reduce vehicle fuel consumption and tailpipe CO2 emissions.
In a set of two papers published in the ACS journal Energy & Fuels, Splitter and Szybist show that midlevel ethanol blends offer, in addition to their high octane and heat of vaporization values, improved combustion and knock mitigation phenomena. They also found that E30 is more tolerant to EGR (exhaust gas recirculation) than other fuels, with short combustion duration maintained under knock-limited conditions. These results demonstrate that the use of mid-level blends can enable additional engine downsizing with the potential for λ = 1 operation, they said.
In the study, Splitter and Szybist investigate spark-ignited combustion with 87 AKI E0 gasoline in its neat form and in midlevel alcohol−gasoline blends with 24% vol/vol isobutanol−gasoline (IB24) and 30% vol/vol ethanol−gasoline (E30). They conducted experiments with all fuels to full-load conditions with λ = 1, using both 0% and 15% external cooled EGR.
|“Improvements to engine efficiency made possible with ethanol fuels may be a synergistic approach to simultaneous compliance with CAFE and RFS II. This presents a unique and infrequent opportunity to dramatically alter internal combustion engine operation by improving fuel properties.”|
—Splitter and Szybist (2014b)
The first paper investigates the impacts of high-octane biofuels and EGR on engine load range and downsizing and downspeeding. In it, Splitter and Szybist explore engine efficiency, stoichiometric torque capability at high compression ratio, and downsizing + downspeeding potential of each fuel-EGR combination relative to one another, with E30+15% EGR also compared with high-efficiency conventional diesel engine data.
The second investigates fuel and EGR effects on knock-limited load and speed. This paper takes a more detailed look at the combustion, thermal efficiency, and knock phenomena of each fuel−EGR combination.
Background. The efficiency of spark-ignited (SI) engines is fundamentally hindered by the throttling of air, and its compression ratio is limited by combustion knock. As a result, SI engines offer lower thermal efficiency relative to compression-ignited engines (i.e., diesel engines) or lean-burn SI engines.
Although downsizing and turbocharging with direct injection offer increased engine power/torque density—with equal or similar performance when downsizing and downspeeding of engines—the opportunity for downsizing and downspeeding becomes limited by combustion knock from the octane number and physical−chemical proper-ties of current market-available fuels, thereby limiting thermodynamic efficiency.
Alcohol fuels tend to have a high octane number and lower carbon intensity, giving these fuels a 2-fold reduction potential in tailpipe CO2 through molecular advantage and the ability to tolerate higher engine compression ratios. Additionally, alcohol fuels exhibit two other properties that can be favorable for increasing engine efficiency, Splitter and Szybist note:
First, the high latent heat of vaporization (HoV), used in conjunction with direct injection (DI) fueling, can increase the incoming charge density caused by a reduction in charge temperature. When exploited properly, the high HoV improves engine breathing.
Second, the amount of thermodynamic work that can be extracted from ethanol on a second law basis is higher than is suggested by its lower heating value (LHV) alone. This is attributed to a high yield of molar products for alcohols on both a stoichiometric and energy basis relative to petroleum distillates, increasing expansion pressure.
These ... results demonstrate that reductions in CO2 emissions without a decrease in MPG could be possible with intermediate ethanol−gasoline blends. A major reason for this prediction is the ability of ethanol addition to reduce combustion knock and enable an increased compression ratio. Interestingly, work by Szybist and West demonstrates that blending ethanol, even with very low-octane gasoline blendstocks, offers significant antiknock resistance and that a high-octane fuel can be produced through blending intermediate levels of ethanol with straight-run-gasoline. This is because of the highly nonlinear response of octane number blending with ethanol on a volumetric basis, as previously explained in detail by Anderson et al. and more recently by Foong et al. These cited studies show that intermediate-level ethanol blends might be promising for the next generation of SI engine fuels.
The noted inherent benefits provide the potential to increase the power output and efficiency of the engine through fuel-based knock mitigation coupled with engine optimization. However, as pointed out in the 2013 SAE International High Octane Fuel Symposium [earlier post], multiple levels of cooperation from the fuel industry, legislation and regulatory bodies, distribution systems, and point of sale vendors are required if fuel octane number is to be increased.—Splitter and Szybist (2014a)
External cooled exhaust gas recirculation (EGR) provides another approach to increase SI engine efficiency; external EGR is a proven method to reduce the knocking tendency for a given fuel. Although external-cooled EGR is widely used in diesels—with recent interest coming from the SI side—the constituents of EGR in SI engines differ from those in their diesel counterparts, Splitter and Szybist note.
Specifically, λ = 1 SI EGR is oxygen deficient, meaning that SI EGR offers the potential to increase charge mass without changing the oxygen content. The lack of oxygen in SI EGR is important when considering both catalyst and throttling requirements of SI engines (i.e., λ = 1). In addition to reducing throttling losses, the introduction of EGR into SI engines improves the thermodynamic properties of the working fluid (i.e., the ratio of specific heats [γ]), reducing in-cylinder temperatures and improving knock resistance.—Splitter and Szybist (2014a)
However, EGR also slows the flame kernel growth because of slowed reaction rates. Other work has suggested that higher EGR levels in SI engines might require the incorporation of different higher turbulence combustion chamber flows to increase EGR tolerance; or the use of high-energy long-spark systems.
Splitter and Szybist set out to explore a number of questions stemming from the properties of alcohol fuels and EGR:
Can knock resistance of conventional distillate fuels be sufficiently improved to the levels of midlevel alcohol blends through the addition of EGR?
What, if any, role does EGR have on engine efficiency in midlevel alcohol blends?
What are the combustion-specific differences between intermediate alcohol−gasoline blends and neat gasoline?
What, if any, potential performance and fuel economy incentives do midlevel alcohol−gasoline fuel blends offer, both with and without external-cooled EGR?
Can intermediate alcohol−gasoline fuel blends enable new powertrain possibilities?
Experimental setup. The ORNL team examined SI engine operation at five engine speeds (1200, 1600, 2000, 2500, and 3000 r/min) and two different EGR rates (0% and 15%), each with three different fuels (87AKI E0 “regular” gasoline, 30% by vol ethanol−gasoline, and 24% by vol isobutanol−gasoline). Gross load increments (IMEPg, indicating mean effective pressure gross) were 50 ± 5 kPa.
The rationale for the selected blending ratios was:
24% isobutanol has near identical oxygen content as E15, which the EPA has approved for use in 2001 and newer light duty vehicles.
E30 was selected because, as the EPA recently stated, there is no foreseeable issue with higher ethanol−gasoline blends, citing blends as high as E30 as likely to be permissible.
A highly modified 2.0 L GM Ecotec SI engine with stock side-mounted direct fuel injector was the experimental engine; three cylinders of the production engine were disabled to allow single-cylinder operation with an installed custom-domed piston, which increases the compression ratio to 11.85:1 (stock 9.2:1).
The increase in compression ratio changes many attributes of the combustion chamber geometry, therefore complicating a direct comparison of the higher compression ratio data to the stock compression ratio.
|“If further pursued, extreme downsizing might require significant changes to the engine beyond the parameters investigated in the present study. For example, air-handling system(s) for downsized systems demand higher per unit displacement mass flow rates, requiring base engine and air-handling optimization. Likewise, heat transfer effects may be of greater importance in downsized platforms, as surface-to-volume relations scale as the square and cubic of bore and radius, respectively.|
The present study ignores these dimensional effects that could be present in application; however, the present findings demonstrate the general trends that could be expected with thorough and proper engineering of the base engine and supporting systems.”
—Splitter and Szybist (2014a)
The experimental engine operated with a laboratory air-handling system. The engine is equipped with separate electromechanical valves for backpressure and external EGR, enabling the capability for independent control of intake manifold pressure, exhaust manifold pressure, and EGR.
Cooled EGR mixes with fresh air upstream of an air heater, followed by the intake plenum and then the intake manifold.
The researchers equipped the engine with a hydraulic valve actuation (HVA) system to enable fully variable valve actuation. To accommodate the small research module HVA system from Sturman Industries, they machined the cylinder head, disabling the functionality of the production cam and fuel pump systems.
Operating conditions—e.g., valve open, close, lift—were constant for all fuels, engine speeds, loads, and EGR combinations.
All fuels and EGR rates were tested at maximum brake torque (MBT) timing until combustion knock was encountered. Once knock-limited, the combustion was phased through spark timing to maintain a constant level of knock through visual inspection of the indicated pressure trace and by maintaining constant AVL combustion noise.
Results. Broadly, the results demonstrated that E30 offers the highest stoichiometric torque capability at high compression ratio compared to the other tested fuels, and that in combination with 15% EGR can offer similar or higher power/torque density as a Euro IV diesel engine.
Among the more specific findings:
The engine’s stoichiometric torque capability at high compression ratio can be doubled with E30 fueling as compared to 87AKI gasoline.
Regardless of the fuel type, 15% EGR increased BTE (brake thermal efficiency) and NTE (net thermal efficiency) through reductions in throttling losses (PMEP, pumping mean effective pressure) and increased GTE (gross thermal efficiency).
GTE increased with 15% EGR for each fuel type. However, the specific operating limits and GTE benefits were observed to be fuel-dependent, with 87AKI and IB24 receiving a higher relative improvement in GTE with 15% EGR. This is primarily because 87AKI and IB24 receive higher antiknock improvements with 15% EGR than E30, Splitter and Szybist said.
15% EGR offered less of an advantage at mitigating knock with E30 than the other tested fuels. Furthermore, E30 is observed to have little speed dependency and fast combustion durations in the knock-limited regime. These properties enable an expansion of the knock-limited regime to a wider load and speed range, with a smaller efficiency penalty in the knock-limited regime.
The increased knock suppression with 15% EGR in 87AKI gasoline enabled a similar stoichiometric torque capability at high compression ratio to IB24 without EGR but with a 1−2 absolute % increase in BTE. However, the overall GTE benefits of E30 were seen to be the highest in addition to expanding the maximum load at a given speed.
E30 and IB24 offered faster spark−50% MFB (mass fraction burned) times, whereas 50−90% MFB times are dominated by knock and load.
The combined findings demonstrate that midlevel ethanol blends—such as E30—open the potential for engine compression ratios and expanded downsize + downspeed powertrain approaches, providing clear pathways to improved vehicle fuel economy using existing engine technologies. The unique properties of midlevel alcohol−gasoline blends were shown to be the enabling technology toward higher engine efficiency, leading to feasible near-term increases in vehicle efficiency and reductions in CO2. The present study focused only on the engine efficiencies, downsize, and downspeed possibilities with two intermediate alcohol−gasoline fuels. The study has not focused on the fact that IB24 and E30 are not currently market-available fuels or that to design mass production engines for them requires their market presence to be significantly increased. Regardless, the present findings demonstrate that, if adopted, intermediate alcohol−gasoline fuels, in particular E30, show promise as a means to increase vehicle efficiency in optimized SI engines.—Splitter and Szybist
Derek A. Splitter and James P. Szybist (2014a) “Experimental Investigation of Spark-Ignited Combustion with High-Octane Biofuels and EGR. 1. Engine Load Range and Downsize Downspeed Opportunity” Energy & Fuels doi: 10.1021/ef401574p
Derek A. Splitter and James P. Szybist (2014b) “Experimental Investigation of Spark-Ignited Combustion with High-Octane Biofuels and EGR. 2. Fuel and EGR Effects on Knock-Limited Load and Speed” Energy & Fuels doi: 10.1021/ef401575e