Butanol vs. ethanol as a fuel component

April 5, 2017 |

Is your business model based on government regulations or fuel science?

 By Bill Brandon, Special to The Digest

In light of the recent article on Butamax, I feel a response from the perspective of automakers is appropriate.  One of the earlier articles referenced asked “ where’s butanol, or other substitutes for gasoline besides ethanol?” and went on to ask why the auto industry is not co-operating by switching to E85 or E100.  The simple answer is that E85 or E100 are not very good fuels for everyday use.

So what makes a ‘good fuel’?  I will limit my comments to spark ignition engines, because high compression diesel type engines are not compatible with aluminum blocks and heads that are necessary for vehicle ‘light weighting’ (a diesel type aluminum engine can be blown apart).  Unless you are an airline, energy density has little or nothing to do with what a good fuel is except as it impacts on cost.  Auto manufacturers have been asking for a 100 RON fuel for several years now to advance the efficiency potential of downsized, direct injection engines with turbochargers.  This is the way auto manufacturers would like to go rather that the expensive, though theoretically possible, mechanical improvements that EPA is advancing.

So how do we get a 100 RON fuel?  First, we must recognize that the ‘blending octane number’ is generally lower than the RON number; Butanol is about 93 and ethanol is about 117.  Of the potential ethanol components, only methanol and ethanol have blending octane numbers above 100.  Aromatics have high blending octane numbers, but have significant health risks and many in the health arena advocate for total elimination of aromatics from our fuel.

Now, just like the info-mercials on TV. WAIT THERE’S MORE!  Auto manufacturers know the value of the Heat of Vaporization – higher levels cool the engine.  In direct injection engines, this means a flatter torque curve and the initial compression level can be tweeked up a bit.  Torque means performance, but also efficiency without the need for nine speed transmissions and a whole lot of other mechanical approaches that drive auto prices up just to account for poor torque from engines due to poor quality fuel.

The final quality of a good fuel is volatility.  E85 generally only has about 80% or less of ethanol because it needs more volatility.  This volatility comes from short chain hydrocarbons, preferably paraffinic hydrocarbons.  Some components of a good fuel should have a boiling point of 100F or less.  Butanol cannot add sufficient octane, sufficient heat of vaporization or sufficient volatility.  So what is it good for?

As Butamax originally said on their website, Butanol’s value is as a co-blend with ethanol.  About 100 years ago, Charles Kettering and his Dayton Engineering Laboratory Co. (DELCO) knew the value of butanol to stabilize the octane additive, ethanol, when blended with a hydrocarbon.  The same is true today.  A modern fuel specification would be one with a 100 RON rating, a heat of vaporization of about 490 – 500 kj/ kg and a T10 of about 100F – 150 F, depending on the time of year, no other  specific distillation curve other than a maximum boiling temperature of about 400F, and no aromatics.  The percentage of butanol would probably be a minimum of  5%.  This specification would account for about 55 – 60% of the fuel with the remaining portion made up of whatever is a cheap source of BTUs.

More volatile hydrocarbons would have an average heat of vaporization of about 310 kj/kg, other hydrocarbons about 275 kj/ kg, butanol 430 kj/kg and ethanol 920 kj/kg.  RON values of different blends depend on empirical testing.  I know of no blending values of a combination of ethanol and butanol.  It may be that a large amount of butanol can be used with a more limited amount of ethanol.  In any event we should be looking at the fuel specification rather than government regulations when moving toward a more modern transportation fuel.

Category: Thought Leadership

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