Fueling the Future of the SI Engine

Faced with significant CO2 emissions reduction, auto makers are turning to downsizing and turbocharging as the most promising technologies for the development of high efficiency gasoline engines. However, the combustion conditions of these engines continue to move away from traditional naturally aspirated technologies, and especially from the Cooperative Fuels Research standardized single-cylinder engine, used to define the octane rating scales.

With engineers increasing Break Mean Effective Pressures and maximum mean pressures, knock resistance is proving to be a major issue limiting the performance of turbocharged, downsized spark ignition engines.

Image credit: www.blog.nulon.com.au

Octane ratings have formed the basis of fuel quality evaluation for decades, but modern engine development has somewhat unsettled previous absolute theories on the effect of these on real world engine performance.

High octane, fast burning fuel can optimise modern engine efficiency

In 2009 Milpied, Jeuland et al conducted a study into the effects of fuel quality on the performance of a downsized turbocharged Direct Injection SI engine. Using two adapted fuel matrixes they separated and evaluated the impact of three major fuel properties on engine performance: Research Octane Number (RON), Motor Octane Number (MON) and Latent Heat of Vaporization (LHV).

The engine tests were performed using a single-cylinder engine under steady state operating conditions. The GDI SI engine with a displacement of 300 cm3 and an Indicated Mean Effective Pressure (IMEP) of up to 30 bar was used to evaluate each fuel for knock limitations at various engine speeds. The effects of RON, MON and LHV on knock resistance were isolated, evaluated and compared. The study confirmed the important role of high RON values as well as the positive influence of the "cooling effect" linked to high LHV values. However, no clear impact of the MON was noticed. Interestingly the "cooling effect" of ethanol in a 20%v ethanol blend increased knock resistance from 30 to 60% (depending on engine speed) when compared to the gasoline base fuel.

The recently completed ‘ULTRABOOST’ collaborative research project confirmed the positive effects of increasing the RON ratings. A downsized, highly boosted, 2.0L in-line 4 cylinder prototype engine was used in an attempt to achieve a 35% CO2 emissions reduction whilst still delivering the performance of the normally aspirated 5.0L V8 base-engine used for comparison.

To probe engine response to fuel, a matrix of 14 formulations was tested under wide ranging engine conditions.
To compare the performance of various octane ratings the knock limited spark advance was determined for a series of fuels with RON varying from 95 to 112: The effect of octane was shown to provide a 5 and 10° crank angle advance at 2000 and 3000 rpm respectively.

Although octane rating, in particular RON, is important in optimizing the new generation of downsized turbo charged engines the laminar burning velocity is a fundamental combustion property of any fuel/air mixture.
Several research projects have shown a clear correlation between engine performance and fuel burning velocity. In principle a gasoline engine should be calibrated to give optimal ignition timing (also known as MBT - minimum spark advance for best torque) without engine knock.

However, modern downsized/boosted engines frequently tend to be limited by knock, resulting in retarded ignition timing in respect to the optimum. Under these conditions, faster burning fuels provide better combustion phasing, resulting in a more efficient energy transfer and better performance. Tests carried out using fuel blends with different burn-velocity enhancing components on a single cylinder engine with retarded spark timing, showed significant benefits (e.g. 1.5% performance benefit using a gasoline blend with 20% aromatic added).

Although not directly comparable to modern road car requirements the F1 engines of the 1980’s also faced output constraints because of knocking; the solution was a specialized fuel using the aromatic hydrocarbon, methyl benzene, also known as toluene. C7H8, to give it its chemical formula can be found in small quantities in normal service station pump gasoline although limited to amounts of less than 1% by European legislation.

Image credit: High Power Media

At the time the FIA regulations covering Formula One specified that the race cars use 'pump' fuel with a maximum RON rating of 102. Any fuel which satisfied these requirements could be used, and blending 84% toluene, with a RON of 124 and MON of 112, together with 16% n-heptane produced a fuel that fell just below the 102RON limit. This fuel allowed boost pressure to exceed 4 Bar!

Notwithstanding the importance of fuel quality on engine performance, the composition of fuel also plays a significant role in engine wear.

Increased olefins reduce lubricant oxidation

A SAE Technical Paper, compiled by Cracknell and Head, examined the effect of gasoline on the oxidative stability of engine lubricants. For the research, lubricant samples were aged on a SI bench engine that was run using ten different gasoline fuels. For each gasoline tested, the oxidative stability of the lubricant and the extent of engine wear were assessed, according to a number of different parameters.

Surprisingly, it was found that fuels containing higher levels of olefin (whether C8 olefin, or a C5/C6 olefin blend, or a catalytically cracked refinery stream) performed proportionally better than the reference gasoline with low levels of aromatics and olefins. Fuels with a higher final boiling point and higher aromatic content, appeared to be associated with elevated levels of sludge formation when compared to the reference gasoline, but did not give rise to accelerated engine wear.

Whilst there has never been a time in automotive history where the ICE has seen such dramatic increases in specific power output, there are many improvements that could be made to fuel composition which would allow for even better optimization. The higher mechanical and thermal demands of the new generation of engines will no doubt see new fuels developed to meet the needs of this technology.

It’s clear that fuel quality is vitally important for the performance of emerging downsized engine technologies. Furthermore, the trend for continued engine downsizing will increase the potential performance benefit associated with knock resistant fuels. However car manufacturers need to be mindful of the fact that fuel composition is not homogenous across all markets: By way of example RON values range from 102 for Aral Ultimate 102 in Germany down to 88 in countries such as Indonesia. This could prove to be technically challenging when vehicles are developed in first world countries using high spec reference fuel, and then operated in emerging markets.

Sources:

• Exploring a Gasoline Compression Ignition (GCI) Engine Concept – SAE international technical paper (Roger F. Cracknell et al).
• Alternative fuels – High Power media.
• FAQ: Automotive Gasoline - Bruce Hamilton.

Peter Els is a technical writer for Automotive IQ