Diesels 10X More Efficient than Gasers

When it comes to sustainable living, efficiency is the ruler.  Diesel engines are more efficient than their gasoline counterparts.  But any engine is so ill equipped to make use of the fuel it burns, it needs a fancy radiator to make sure the “waste heat” is cast off “efficiently” – LOL.  To make matters worse, almost 2 gallons of gasoline are used to pump a gallon of gasoline into your tank!

The title of this page is a conclusion that comes from combining the efficiency of the engine (what we commonly call the “gas-mileage”) with the efficiency of the fuel itself (not something that directly effects the price therefore something that we commonly do not think about).  The efficiency of a fuel itself is the energy it contains relative to the energy required to produce it.  The industry characterizes this with a metric called the Energy Balance, Energy Ratio, Energy Return on Energy Investment, or Fossil Energy Ratio (FER).  It is a measure of how much energy the substance contains relative to how much un-sustainable or non-renewable energy it takes to generate it.  A recent study indicates the FER of biodiesel is about 5.5 (see below), while gasoline is about 0.6.

The thermal effeciency of diesel engines is about 20%, and that of gasoline engines is about 15%.  Here I am comparing 5.5X20%  vs 0.6X15%.  A physicist would say, “an engine running biodiesel provides about the same amount of usable work that goes into it, while running an engine on gasoline provides about a tenth of the work that went into it.”  Using B20 (a common blend in North America of 20% biodiesel 80% petro-diesel) gets about a third of the energy back than was used to create it.

This study, published in June 2011 by leading experts at the University of Idaho, derived this estimate using Life Cycle Analysis (LCA) that sums up the contributions to a Life Cycle Inventory (LCI) of what energy is needed to extract, process, and distribute the fuel. As this is a measure of sustainability, only non-renewable inputs are used in the calculation.  So the sunlight used in photosynthesis is not counted, and of the electricity that is used, only 70% counts as the rest is from sources that replenish themselves (more on this later).

I will paraphrase the LCI by breaking it into three stages. Here they are with the energy required for each:

1) farming and crushing: 2.3 MJ/L of Biodiesel
2) transporting the oil and processing: 3.3 MJ/L of Biodiesel
3) distribution: 0.3 MJ/L of Biodiesel.

As the oil used to cook food in my neighborhood comes from 1) and 3),  I can conservatively estimate the energy it takes to generate the locally harvested waste vegetable oil I run in in my diesel Mercedes-Benz by simply not including 2) in a LCI.

The analysis has a muddy patch regarding the soy meal and glycol byproducts as their energy content is not the reason they are used.  There is not an ideal basis to estimate the energy based credit to be used to offset their generation.  The authors of the paper follow in the footsteps of the first soybean based biodiesel LCA in 1998 which simply does a mass allocation.  In this assumption the credit offset about 4/5 of the fossil energy in the inventory as not much oil is produced from each soybean. In 2005 Pimente, an ecologist from Cornell, and Patzek, an environmental engineer from Berkley estimated the credit to be more like 1/4.  Their goal was to make a new LCI the resulted in showing the lowest efficency/FER of renewable fuels.  This of course illicited an immediate response from the UIdaho group.  Then in 2008 Argonne National Lab published this massive report in which they came up with a weighted “Hybrid” estimate of the credit based on the byproducts’ market value, energy content, protien content, etc. and concluded the credit should be about 2/3.  Using a credit of 2/3 for the farming stage, the June 2011 LCA would give a soybean based biodiesel FER of 4.0, still the highest FER of any fuel.  Pure Ethanol is around 1.5 depending on who you ask.

Whether estimating the FER  or the efficiency of a fuel, the LCA is based on the “boundaries of the system” (often human labor is not taken into consideration or the energy to fix equipment, or even the energy needed to build the plants in the first place).  But I beleive that some back of the envelope estimations have been done to ensure in the life of an operation these costs are some small percentage of the energy input.

To compare with the efficency of petroleum fuel, all we have to do is put the renewable energy part back into the UIdaho study which is the extra 0.7 MJ/L biodiesel that came from electricity for the the crusher (86%) and the processing plant (14%).  After adjusted for the byproduct credits, the efficency comes out to about 5.4. (3.8 when using the hybrid credit).  The efficency of petroleum diesel is about 0.83 and gasoline is even lower.  How can the fuel have an “efficiency” greater than 1?  Because we do not consider the energy that made the oil in the first place – petro or vegetable (photosynthesis etc).

Although these numbers are with respect to a liter of biodiesel, they can be used when discussing straight vegetable oil as a fuel because the energy content of them are very similar. In reports from the Institute for Energy and Environmental Research in Heidelberg, German you can see soybean and rapeseed oil have Low Heating Values within 2% of their biodiesel counterparts. So using the parameter in the latest study from UIdaho, my Waste Vegetable Oil (WVO) requires 2.6 MJ/L WVO to generate, then it is used to cook food for a few days in which maybe a third is consumed by humans, it is poured into a barrel that I pick up with my truck, and drive a few miles home. In the summer I simply let it settle for a couple weeks, and pump it into my tank all for insignificant energy cost resulting in a FER of about 33/2.6=13. Using the hybrid credit for the soy meal reduces it to 6.9. The efficiency of the fuel is 12 (6.7 when using the hybrid credit).  So as long as the thermal efficiency of my car is more than 15%, I am getting more usable work out of the fuel than went into it.

And what of emissions?  When considering greenhouse gasses veggie oil is carbon neutral.   All the CO2 coming out of the tailpipe was originally absorbed from the atmosphere by the canola flowers.  Particulate emissions are not an issue when considering the sustainability of the atmosphere because they are only around for a matter of weeks before being “destroyed during the natural cleaning of the atmosphere” (Greasecar – Vegetable Fuel Systems).  In any event, as pointed out in that article, there is a reduction in particulate matter emissions running on vegetable oil than diesel fuel.

The skinny: gas powered vehicles utilize about one tenth the energy stored in the dead dinosaurs, diesel powered vehicles utilize about all of the energy stored in the flowers.