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MIT study: higher octane standard fuel in US could lower fleet fuel consumption & GHG an extra 4.5-6% by 2040

24 April 2014

Offering a higher-octane gasoline to the consumer market in the US as the standard grade could deliver an incremental 4.5% to 6% reduction in fleet fuel consumption and greenhouse gas emissions by 2040, on top of a projected 26.8% reduction by then in the baseline case (i.e., without higher octane fuel, but with other projected vehicle and powertrain technology improvements), according to a new analysis by a team at MIT. For their paper, the team proposed a 98 RON gasoline—currently the US premium grade—as the new standard fuel. In other words, they proposed making the current premium fuel the new standard grade.

The analysis by Eric Chow, John Heywood and Raymond Speth, presented as a paper at the recent SAE 2014 World Congress, seeks to quantify the reductions in consumption and GHG if new vehicles designed to use the higher-octane fuel were deployed. Raising octane reduces engine knock constraints, enabling the design of new spark-ignition engines with higher compression ratios and boost levels. This leads to improved engine efficiencies and the sought reductions. (Earlier post.)

Octane numbers
The octane number (ON) reflects the resistance to auto-ignition and knock of given fuel. Two standardized methods are used for determining the octane rating of a fuel: the research method and the motor method. (Thereby leading to the Research Octane Number (RON) and the Motor Octane Number (MON)). Gasoline at US pumps is labeled with AKI (anti-knock index), the average of RON and MON.
To determine the ON, the results of testing—done in a single-cylinder, variable compression ratio engine—are compared to primary reference fuels; the fuel under test is assigned the ON of the most closely matching reference fuel.
However, notes the MIT team, the engine and measurement conditions of the RON and MON tests do not represent real world operating conditions. Results of a number of studies suggest that the anti-knock performance of modern engines correlates better with RON than MON, and may benefit from lower MON values.

The MIT team used GT-Power simulations and a literature review to determine the relative brake efficiency gain that is possible as compression ratio is increased. Due to a spread in the resulting data, the team used an average value of 2.35% relative efficiency gain.

They then performed engine-in-vehicle drive-cycle simulations are then performed in Autonomie (a simulation program developed at Argonne National Laboratory) to determine an effective, on-the-road vehicle efficiency gain. For modeling, they used a pre-loaded model for a mid-size, 2-wheel drive vehicle with automatic transmission, with chassis attributes modified to reflect a 2013 Toyota Camry.

Running cases for both naturally aspirated and turbocharged engines, they found a reduction in fuel consumption ranging from 3.01% to 4.45% for the former, and 3.01% to 7.34% for the latter, all with the 98 RON fuel.

With the possible efficiency gains determined at an individual vehicle level, the MIT team then used a fleet model to calculate the aggregate benefit for the LDV fleet. (Production of vehicles not designed to use the higher octane gasoline is assumed to stop after 2030.) Results of the simulations indicated that:

  • By 2040, hybrids (HEVs), plug-in hybrids (PHEVs); and turbocharged SI (spark ignited) vehicles make up more than 50% of the in-use fleet.

  • Higher-octane vehicles represent about 28% of all LDVs on the road by 2030, and about 69% by 2040

  • Fuel consumption for all NA-SI vehicles still accounts for almost 50% of total fleet fuel consumption in 2040 even though they represent only about 38% of the fleet.

  • Diesel remains a small fraction.

  • 98 RON gasoline accounts for roughly 39% of total gasoline demand by 2030 and almost 80% by 2040, due to the large share of redesigned, higher-octane vehicles. (Currently, 8.8% of gasoline consumed in 2012 was 98 RON premium.)

  • The baseline projected decrease in fuel consumption from 2012 levels is 10.6% by 2030 and of 26.8% by 2040. Introducing higher octane (98 RON) vehicles adds an additional relative decrease over the baseline of 1.9% by 2030 and 4.5% by 2040.

  • A 100 RON fuel could bump that incremental decrease to 6.0%

Chow-baseline
Chow-ho
Baseline vehicle sales mix for fleet model. Data from Chow et al. Click to enlarge.   Vehicle sales mix for fleet model including high-octane (HO) vehicles. Data from Chow et al. Click to enlarge.

These savings would need to be compared to any penalties on the refinery side—i.e., possible higher energy consumption and greenhouse gas emissions due to an increased refining severity.

However, the authors noted, a parallel companion study, submitted to the journal Environmental Science & Technology, indicates that due to declining gasoline demand over time leading to excess RON producing capacity and other factors, there are likely to be reductions in refinery CO2 emissions and costs, at least up to RON levels of about 98. In other words, the production energy and GHG impacts of increasing RON could be positive, rather than negative.

The MIT authors ventured a potential pathway for the possible implementation of a new higher-octane standard:

  • Change current US octane standard from AKI (87 regular / 93 premium) to RON (92 regular / 98 premium). The anti-knock performance of modern engines correlates better with RON than MON, and recent studies have shown that the effective octane index of an engine increases as RON and fuel sensitivity increase.

    Relaxing the MON requirement gives refineries more processing flexibility, allowing them to produce more high-octane fuel. Most other countries—with the exception of the US, Canada, Brazil and a few others—already use RON, the MIT team point out.

  • Shift production towards higher volumes of premium (98 RON) while decreasing volumes of regular (92 RON).

This seems to be the most pragmatic approach based on logistical consideration. By simply shifting the production volumes towards more premium gasoline, no additional grades of gasoline have to be sold at the pump. Consequently, retail gas stations avoid the need to invest money to build new infrastructure. Furthermore, confusion among consumers can be mitigated since they are offered essentially the same grades as current gasoline.

Meanwhile, the continued availability of current regular gasoline allows older, legacy vehicles (not designed for higher octane) to not “waste the extra octane of the new fuel.”

Ultimately, multiple complementary pathways focused on reducing US LDV flet fuel consumption will be needed to meet the increasingly more stringent fuel economy regulations. Approaches and initiatives designed to improve engine powertrain technology, promote adoption of more fuel efficient vehicles, explore alternative fuels, and later driver behavior will be paramount.

—Chow et al.

Resources

  • Chow, E., Heywood, J., and Speth, R. (2014) “Benefits of a Higher Octane Standard Gasoline for the US Light-Duty Vehicle Fleet,” SAE Technical Paper 2014-01-1961 doi: 10.4271/2014-01-1961

April 24, 2014 in Engines, Forecasts, Fuel Efficiency, Fuels, High Octane Fuels | Permalink | Comments (4) | TrackBack (0)

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Comments

My Gen II Prius shows several lower MPG running 89 Octane fuel than 87 Octane fuel. This illustrates the fact that at lower loads, lower octane fuel will increase efficiency for a given compression ratio (CR).

Of course, if engines have significantly increase CR as the result of higher octane fuel, even efficiency at lower loads will still increase when using high-octane fuel. However, using lower-octane fuel at low loads at high CR will give the best increase in efficiency.

Therefore, the best gain in efficiency is to have two separated fuel tanks: One for high-octane fuel for use at high loads while a larger tank using cheaper low-octane fuel for use at typical cruise loads. To prevent confusion at the pump, the filling nozzles will have different shapes and won't fit if the wrong fuel type is to be filled.

Indeed, the "octane on demand" scheme (going all the way back to the Ford/MIT ethanol-injection scheme of 2006) was the first thing I thought of when I read this.  The engine doesn't need the 98 RON fuel all the time, and would benefit if it could get 100+ RON fuel when it needed it.  Whether you add the high-octane fuel separately or generate it by separation from the generic fuel supply, it should work the same.

That said, 6% improvement by 2040 is pathetic.  I have to wonder how much you could get out of e.g. Transonic Combustion's fuel system, given that it would not only run at much higher CRs but would also recycle some exhaust heat back to the combustion chamger.

Gee, that sounds like the ICE (Internal Combustion Engine) in my Chevy Volt. It uses 92 Octane and has a higher compression ratio. Wait, don't tell me, those midwest engineers with the cheap wristwatches and bad haircuts have already figured all of this out?

Of course who uses the ICE in their Volt very often anyway?

At Engineer-Poet and Roger Pham: I was thinking the exact same thing.

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