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UTS study finds against petrol and ethanol blending
Sydney research team backs theory that ethanol and petrol work best injected separatelyHow do you get the most out of an ethanol/petrol blend? Don’t blend them – keep them separated until they arrive in the combustion chamber. It’s an idea first mooted by a team from Massachusetts Institute of Technology (MIT) in 2006, now given new impetus by a team from the University of Technology, Sydney (UTS).
The essence of the thesis is that introducing them into the combustion chamber pre-blended compromises the performance of both. Ethanol’s higher octane number, burn rate and latent evaporation temperature mean it needs higher compression ratios to achieve optimal thermal efficiency.
The answer lies in delivering the petrol to the chamber by port injection and ethanol by high-pressure direct injection, cutting petrol consumption without compromising engine performance. The MIT team showed that this method boosts the engine’s overall thermal efficiency, with independently variable inputs allowing the ethanol component to make the most of every drop of petrol.
It's a result that goes against accepted thinking in the automotive industry, where ethanol blended with petrol – in what is commercially described as 'E85' or 'flex fuel' – is an increasingly common option available at service stations. Even in a small market such as Australia, one mainstream manufacturer – Holden – offers an E85-compatible car. With E85 denoting the 85 per cent ethanol content, it's a fuel that has the potential to boost energy security by reducing the country's dependency on fossil fuel supplied from abroad. And Ethanol is considered a carbon-neutral fuel, since it is refined from plant crops, which have absorbed CO2 from the atmosphere while growing in the ground. The UTS team's research points to efficiency gains to be made by keeping the petrol and the ethanol separate, which could be achieved with relative ease in the case of the E85 Commodore, since the direct-injection SIDI V6 is based on the previous HFV6 that was port injected.
For the study, the UTS team, Yuan Zhuang and Guang Hong, used a modified Yamaha YBR250 single-cylinder engine, equipping it with a direct injection system for ethanol and a port injection system for petrol, with an ECU reconfigured for the purpose.
This allowed the engine to operate at considerably different intake pressures for each fuel. The port injection petrol system functioned at a fixed 250kPa, while the common-rail DI system for the ethanol was adjustable to run at fixed values from 3-13MPa. The team used 95 RON petrol.
Varying the ethanol/petrol ratio from zero (petrol only) to 60.1 per cent ethanol, they ran the engine on light and medium loads at speeds from 3500 to 5000rpm, going up in 500-rpm increments. When the ethanol component reached 48.3 per cent, they boosted its injection pressure from 4 to 6Mpa. Timing was set to ensure a homogeneous mixture for combustion throughout the experiment.
This is complex stuff that’s hard to put in lay language, but in essence they found that boosting ethanol input increased torque and the volumetric efficiency for each of the fuel types. It increased in-cylinder pressure, leading the team to conclude they’d boosted thermal efficiency.
Among other factors, the team attributes engine performance improvements to the cooling effect of ethanol directly injected into the combustion chamber.
But this had less desirable consequences in emissions. While nitrous oxide (NO) levels dropped with the increase in ethanol, carbon monoxide (CO) and other hydrocarbon (HC) emissions rose. The team tentatively attributed the NO drop to in-cylinder temperature reductions caused by the difference in injection pressures, and the higher CO and HC emissions to incomplete combustion caused by relatively low temperature.
What has yet to be established is whether such parallel dual delivery systems can ever find the efficiency they would require to justify the spend on their development and the inconvenience of filling separate tanks.
– with staff
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Written byJeremy Bass
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