Foreword: Fuel economy has a lot of focus today with the higher cost of energy. There is a lot of information out there of which much of it is not accurate. I tried to get to the bottom of what is going on and understand the processes. If possible, I wanted to seek a way to improve the fuel or blend for real fuel economy. In this blog post I explore the science, blending, and politcs of gasoline.
Gasoline: Nemesis or Necessity
Every gas tank is tracked for fuel economy as fuel economy can mean major improvements in cost. Over the years, I noted that the gas mileage was erratic. At times, the miles per gallon were very high and a short time later it would be the worse ever. Only to return to a very high value sometime later. There was little discernible pattern at first glance. I used the same gas station for years and only bought premium unleaded. The vehicle was in excellent running condition and the highest grade of oil filtration and oil was used in the engine. So for all intents and purposes there should have been some stability in the fuel economy but there was not. Something was going on but what?
I read innumerable articles and began to observe that there was circular reporting as well as severe group think on this topic of fuel economy. Everyone was copying everyone else to the point that they were recycling their own works yet citing other people who copied something of theirs. Every article read that engine knock was a outcome of low Research Octane Number (RON) or simply the fuel's Octane rating. Step up the Octane number and rid yourself of engine knock. Otherwise, there was no other purpose to Octanes. This did not sound accurate to me as purpose of Octane is not simply to rid engines of knock and pings which was actually a symptom of something else. Octane ratings are designed as a measure of fuel performance in order to optimize the application of thrust out of an engine. Engine knock is a symptom of poor fuel performance.
The Science and Engineering
The reciprocating engine involves usually four strokes; a draw stroke, compression stroke, power stroke, and expunge stroke. See Figure 1. On the draw stroke air and fuel is drawn into the cylinder past the open valve. The compression moves upward increasing pressure on the fuel-air mixture. The spark plug ignites the fuel-air mixture and consumes all the oxygen. The nitrogen remains and expands driving the piston downward in a power stroke. The piston then travels upward as the valves open to expunge spent fuel, gases, and smoke.
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| Figure 1: The 4-Stroke Engine Cycle |
The concept behind the Octane rating affects the power stroke. The Octane numbers relate to the rate of burn of the fuel. At higher Octane numbers the fuel burns more slowly applying constant pressure to the piston head for the entire power stroke as opposed to a sudden flash at the top of the stroke that could be at the incorrect time due to temperature and pressures in the cyclinder causing the knock sound. Ideally, the burn rate of the fuel should perfectly match the entire power stroke. This results in a smooth transfer of the fuel's undirected explosive energy into directed mechanical energy that moves the vehicle. Thus, the Octane rating is a measure of an efficient process in which the fuel is completely consumed over the power stroke. At lower Octane ratings a portion of the fuel is not spent. This residual fuel cokes (a hard black soot) the values, springs, and rocker arms in the head. Technically, at too high of an Octane rating the fuel-air mix could still be burning during the expunge stoke which is not good either. The design challenge is to match the power stroke travel to the burn rate of the fuel in order to optimize energy transfer and minimize waste. In doing so, the engine runs very smoothly. Thus, the octane rating for a vehicle is a fixed design feature of the engine. There is a little flexibility and with the age of the engine higher octanes improve performance.
Now that we understand the process and Octane rating efficiency of the fuel, we need to turn our attention to the fuel blending and composition. Once again I was awestruck by the degree of misinformation in the articles I read. Numerous articles cycled on the internet discussing how to improve the grade or blend of gasoline using off-the-shelf products such as Methyl Ethyl Keytone, Benzene, Toluene, or Xylene. Wild claims were made that the Octane numbers could be boosted to over 100 at lower costs. Many articles claimed that up to 50% of gasoline was Xylene. Therefore, I looked up the chemical formulas for gasoline as well as other chemicals involved and the US standards for the various blends.
First, let us look at the refining process. The older method is called fractional distillation. The crude oil is a mixture of several products, each having identifiable characteristics such a boiling points and weights. The crude oil is heated to the boiling point causing vapors to form which rise in distillation tower then settles based on their weight and Archimedes Law of Buoyancy. Collection plates are placed in the tower at specific locations that cause the vapors of a specific compound to condense on the plate before being drawn away for further processing. Methyl Ethyl Keytone, Benzene, Toluene, and Xylene are called feed stock commanding higher prices than if sold in the blend of gasoline. For example, Xylene costs about $18.00 per gallon. Gasoline is roughly $3.00 per gallon and has less than 1% Xylene. Other chemical processing methods of separation exist such as cracking, unification, and alteration. The main point is that many compounds are removed and not included in the final fuel mixture due to component economic value.
In the end, the gasoline that results from the continuous manufacturing process is further treated to remove impurities, contamination, and water. Pure gasoline is a blend of hydrogen and carbon chains. Specifically, the chains from C7H16 through C11H24 are blended together to form gasoline. The combustion process for a Hydrogen-Carbon chain that is 2 parts C8H18 combined with 25 parts Oxygen results in 16 parts CO2 and 18 parts water, Figure 2. The 25O2 in this formulation is drawn in via the air intake on a carbuerated engine.
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| Figure 2: The Chemical Formula for Combustion of Gasoline |
As the fuel makes its way to the pump, additives are included such as upper cylinder lubricants, dyes, and ethanol. Some of the additives improve the fuels performance and others such as ethanol degrade the fuels performance. In brief, ethanol burns hotter and quicker than gasoline which degrades the fuels performance. At 10% ethanol the fuel efficiency is usually degraded by 4 miles per gallon. This is a good segway into the politics of fuel.
Politics
The politics of energy covers a huge spectrum of issues mostly in the environmental realm. However, energy is also a huge revenue boon for nation-states through taxation. Essentially, the government through the mechanisms of taxation and process obfuscation take just rewards (consumer hard earn money) and redistribute them based on the whims of the aristocracy or the seat of power. This is also known as institutional theft or more commonly called social justice. A good example of process obfuscation is the formulation and blending of gasoline. Governments through their environmental agencies enforce blends of gasoline at different seasons and additives such as ethanol. Changing the formulation of the gasoline blend and ethanol mix can reduce the fuel economy resulting in more fuel to go the same distance. The taxes on gasoline are flat based on a gallon of gasoline. If traveling a fixed distance takes more fuel than before due to lower fuel economy then the tax revenue increases in relation to the fuel economy reduction. This can actually cause a reduction of disposable income if the reformulated fuel economy loss is greater than the lower prices. See Figure 3.
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| Figure 3: Relationships Between Fuel Economy and Tax Revenues. |
If the fuel economy is 20 miles per gallon when pure and at 16 miles per gallon with 10% ethanol then the differential on a 15 gallon tank is 3.75 gallons more to go the same distance when using 10% ethanol. That is a considerable increase in cost to the consumer and a huge increase in tax revenues for the government. For the consumer, there is little that can be done to improve the fuel efficiency that is also cost effective from the pump. Setting up a mini-fractional distillation plant to remove the ethanol does not justify the cost as at 10% there is 1 gallon out of 10 gallons that is ethanol. The consumer still pays for the ethanol. Simply purchase fuel without ethanol added.
Returning to the original issue of erratic fuel efficiency, the issue is mostly political as the government seeks to optimize tax revenues by managing fuel economy. The erratic fuel economy is due to seasonal fuel adjustments and the political climate seated at the time.
Achieving Lower Fuel Costs
Of course, electing public officials who do the right thing is ideal. However, the current political climate in which corruption can be obfuscated in complex or layered processes is too tempting for self-serving public servants. Demanding transparency is an option but even that is difficult to achieve as the seated power base simply stalls and debates the issue rather than takes action. Consumers are relegated to purchasing more fuel efficient vehicles and seeking alternative fuels or transportation solutions in order to save costs. A time may be coming when single passenger vehicles for the work commute become more popular. One example is the ELIO.
