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Human Fuel Consumption

How many miles to the gallon does a person get? How does this compare to a car or truck?

OK, it’s a silly question, but let’s take a first order look at the energy involved (because you can’t, after all, drink a gallon of gasoline then see how far you can walk!)

Seriously, don’t try to drink gasoline! Ever! Don’t even think about it. It’s horrible, and potentially fatal in anything other than small quantities (There’s technically no such thing as a poison, only a poisonous concentration of any substance "Sola dosis facit venenum").

PSA – If you accidentally swallow a small amount on gasoline (for instance swallowing some whilst trying to start a syphon), don’t panic. Drink water, and don’t try to make yourself vomit. You might feel sick (and burp gas fumes), but it’s best to keep the gasoline in your alimentary canal and out the regular way. A bigger danger is getting gasoline into your lungs (and if you induce vomiting, it could cause the gas to be aspirated into the lungs); gas in the lungs spreads out, coating the alveoli, and interferes with the body getting oxygen. Here are some more details on what to do if you swallow gasoline.

What about diesel?

Diesel fuel is, arguably, less toxic than gasoline, but don’t try drinking diesel either!

There is a type of diesel, however, called biodiesel, that is sourced entirely from biomass.


Image: EcoWatch

As the name implies, biodiesel refers to fuel that is based on vegetable oils and animal fats; it is not petroleum based.

Biodiesel can be made from animal fats, vegetable oils, and even recycled restaurant grease. Experiments are already on the way to manufacture it from algae.

What would happen if you drank a gallon of vegetable oil?

We'll pick this up a little later, but first a diversion to investigate diesel engines.

What would happen if you drank a gallon of vegetable oil?

Diesel Engines

Let’s take a quick look at the technology behind diesel engines and why they are so versatile.

Diesel engines are incredibly unopinionated on the heritage of their fuel, which is why they can run on biodisel. As long as their fuel contains long-chain carbon molecules, most oils can be used without any engine modifications (if the oils are blended with regular diesel), or neat with minor changes to make sure the viscosity is appropriate (they get too thick at cold temperatures).

In fact, when Rudolf Diesel, the father of the engine that bears his name, demonstrated his engine at the 1900 World Exhibition in Paris, he was running it on Peanut oil (and before that, he experimented using a slurry of fine coal dust in water!)

Technically speaking, there is a difference between biodiesel fuel and raw vegetable oil fuel, even though a diesel engine will run on both. Biodiesel has gone through a process call transesterification, which separates the glycerin from the oil, leaving behind methyl esters. The glycerin byproduct is valuable for use in other products, such as soaps.

There are many advantages of diesel engines over gasoline engines.

Diesel engines can be agnostic on their fuel source because of the high temperatures at which they operate; this enables them to burn fuels with higher energy content than regular gasoline. Diesel engines are efficient.

Diesel engines (sometimes called compression-ignition engines) don’t have spark-plugs, the way that petrol (gasoline) powered engines have, to ignite their fuel. Instead, they simply inject their fuel directly into the cylinder that contains hot enough air to spontaneously combust.

The piston in a diesel engine compresses a cylinder of air, adiabatically, to about a twentieth of its initial volume, and this heats it up to many hundreds of degrees. Atomized fuel is injected into this air, which is far hotter than the flash-point of the fuel, causing a miniature explosion and pushing the cylinder back down.

The energy extracted from a well configured diesel engine can exceed 50% of the chemical energy stored in the input fuel. In comparison, a conventional petrol engine can only achieve 30%. This efficiency has additional, subtle, benefits in that, since they ‘waste’ less energy, less heat is generated for the same amount of power. They run ‘cooler’ and require smaller radiators to remove the excess heat.

Without the need for spark plugs, and their high-tension ignition systems, diesel engines are less complex, more reliable, and produce less electrical noise. They rev at lower speeds, and though they tend to be heavier (they are built stronger to help support their higher compression ratios), if well maintained, it’s not unheard of for diesel cars and trucks to exceed half a million miles (meaning the engine lasts longer than the rest of the vehicle!).

Diesel fuel is more stable than gasoline. Its vapor does not ignite if sparked, and it’s unlikely to explode in an accident. Diesel is, therefore, preferred for inboard engines of boats where there is enclosed space. If a boat has a gasoline engine-room it needs venting fans to make sure any leaking or evaporated fuel does not linger in the air to form a dangerous stoichiometric mixture that could explode at the simplest of a spark.

The high temperature of combustion means that very low amounts of carbon monoxide are present in exhaust gases.

Diesel engines are designed to withstand high cylinder pressures (because of their high compression ratio), so this means that very little needs to be done turbo charge them, if desired or needed. They produce a lot more torque at lower revs than petrol engines and thus are very useful in machinery that just needs to chug along all day long at constant speed (boats, trucks, generators, and agricultural machinery).

A Gallon of Oil

A cup of cooking oil contains approx. 1,950 calories.

There are 16 cups in a gallon.

A gallon of cooking oil contains 31,200 calories.


For a 180lb person, approximately 100 calories are burned per mile at a slow walking pace.

With simple division the equates to 312 mpg.

A human can walk approximately 300 miles for every gallon of oil!

This is a good order of magnitude higher than a light truck which typically gets 20-25 mpg.


Cycling is more efficient the walking. Cycling gently, the same 180lb person would consume approx 30 calories for each mile traveled. This triples the efficiency.

A person can cycle approx 1,000 miles on a gallon of oil.

Fuel efficiency and fuel consumption

In a previous post, I wrote about how it is sometimes easier to think about things in terms of fuel consumption "gallons per mile", instead of fuel efficiency "miles per gallon".

The oil consumed for every mile walked is 1/312th of a gallon. This is about 2½ teaspoons.

Every mile takes two and a half teaspoons of oil.