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Pure Energy

All BTUs Are Not Created Equally

I sometimes have to pause and remind people that I am not anti-ethanol. As I have said numerous times, I am opposed to recycling fossil fuel into ethanol, and paying massive subsidies to do it. This is what we do with corn ethanol. That is a false solution to our fossil fuel dependence. If we could produce corn ethanol as we can sugarcane ethanol – with minimal fossil fuel inputs – that would address the vast majority of my ethanol objections. I think I made that clear over two years ago with my support for E3 Biofuels attempt to produce corn ethanol in a more sustainable fashion.

But ethanol has one particularly compelling argument: Ethanol has a high octane rating (103), which means it does not easily pre-ignite. This means higher engine efficiencies could be obtained than can be achieved with gasoline.

It is known that ethanol added to gasoline normally causes the fuel efficiency to drop. Ethanol contains about 2/3rds of the BTUs (heating value) as the same volume of gasoline, and gasoline/ethanol blends normally shows the drop in fuel efficiency one would expect. However, because of ethanol’s resistance to preignition, it should be theoretically possible to design an engine with a much higher compression ratio, which could then extract more useful work from the ethanol. Diesel engines are designed with high compression ratios, which is the key to their engine efficiencies of around 45%, versus 25-30% for a gasoline engine.

Let’s take a simple example, to show how ethanol’s BTU deficit could be made up with an increase in engine efficiency. Gasoline contains about 115,000 BTUs/gallon. If the engine efficiency is 25%, then 28,750 BTUs/gallon ultimately power the vehicle. The rest are expelled as heat. Ethanol contains about 75,000 BTUs/gallon. One could in theory achieve the same fuel efficiency with ethanol as with gasoline if an engine was designed with an efficiency that resulted in the same 28,750 BTUs/gallon powering the vehicle (assuming same weight, frictional losses, etc.) That means that if the efficiency of the ethanol-powered car was 28,750/75,000 – or 38.33%, then 1 gallon of ethanol could provide the same power to the vehicle as 1 gallon of gasoline. And of course if the efficiency of the ethanol vehicle could be increased further, it is possible to use 1 gallon of ethanol to travel farther than one could travel on 1 gallon of gasoline – despite the BTU deficit.

This has been true in theory, and some small scale engines have been created. The Saab Biopower, which debuted a couple of years ago, showed that the BTU-deficit could be partially compensated for. The Saab engine was designed with a higher compression ratio, so that on E-85 it showed a 12.5% drop in fuel efficiency instead of the typical 20-30% drop that one typically sees on E-85. The Saab also achieved a reported 20% extra power and 15% extra torque from this engine.

But I was recently made aware that Swedish automaker Scania has been producing ultra-high compression ratio engines designed for ethanol usage, and they reach engine efficiencies as high as 43%:

Scania’s Ethanol Diesel-Engine, Runs On Biodiesel Too

That means that if all else was equal (no significant weight penalty from the high-compression engine), a gallon of ethanol could enable a vehicle to travel farther than it could on a gallon of gasoline.

In reality, the comparison is not quite apples and oranges, as these Scania engines are used in heavy, commercial applications. I wrote to the company a couple of months ago and asked them some questions about any possible plans to produce a smaller engine for passenger vehicles, but they never responded.

But the point of the essay was to show that all BTUs aren’t the same for liquid fuels, and that a modified compression ratio has the potential to give the counter-intuitive result that a fuel with few BTUs per gallon can actually provide better fuel efficiency in some cases.

January 20, 2009 Posted by | compression ratio, diesel engine, ethanol, fuel efficiency, octane, Scania | 96 Comments