Published on July 24th, 2008 | by Rod Adams43
On Board Energy Storage – Reason Automobile Engineers Chose (Choose) Fossil Fuel
July 24th, 2008 by Rod Adams
I thought it might be worth taking a few minutes to remember that people who developed internal combustion engines were not people focused on selling fossil fuels, they were people interested in solving a very real challenge – energy storage and delivery on a moving vehicle. When all factors are taken into account, fossil fuels provide a compact, lightweight form of energy that can be readily converted to power in device that is moving – sometimes very rapidly and without any connection to the earth.
There are certainly times in all of our lives when we feel like the big oil companies have us over a barrel, but their dominance came as a result of the high performance that their product gave to automobiles, trains, trucks, ships and aircraft. By many measures, their product remains the best technical choice available.
The high performance results from chemistry and thermodynamics. Both carbon and hydrogen – the two main components of the hydrocarbon fossil fuels – will combine with oxygen in an exothermic – heat generating – reaction. In an environment with an excess of oxygen the reaction will result in a rather benign exhaust of carbon dioxide and water vapor plus a good quantity of heat.
When you look at the products of the reaction in a balanced equation – where all of the input elements are accounted for in the outputs – you will discover that the products weigh about 4.5 – 5 times as much as the hydrogen and carbon input.
The rest of the weight comes from oxygen. Here is the chemical equation often used to describe gasoline combustion (gasoline is actually a complex combination of various hydrocarbons each with different numbers of carbon and hydrogen atoms, but C8H18 is representative of them all.)
C8H18 + 12.5 O2 –> 8 CO2 + 9 H2O
By mass, only 114 units out of 514 units are in the gasoline, while the rest is in the oxygen. This is important for vehicles because oxygen does not need to be carried – it can be sucked in as needed. There is also no technical requirement – in the absence of new regulations – to capture and store the waste products and carry them around.
The people who developed the internal combustion engines were seeking a way to eliminate the weight of the water, piping and pressure vessels that limited the portability of steam engines. They figured out that they could use the hot products from combustion to directly move pistons and turbines as long as the input fuel did not have too many contaminants that could damage the engine parts. Coal and wood contain a lot of contaminants and both of those solid materials cannot be moved with pumps.
Batteries have to contain all of the chemicals on both sides of their energy releasing equation. The very best batteries available today can store about 0.4 MJ/kg (0.05 kw-hr/lb) including the cases and safety systems. In contrast, gasoline carries about 46 MJ/kg (5.7 kw-hrs/lb).
Even with a 20% efficient IC engine, a gasoline tank stores 20 times as much energy as a battery of equal weight. As the vehicle is moving it gets rid of some of that weight. Battery powered vehicles must carry the full weight of their energy source.
The energy density difference also plays a key role in the time that it takes to put more energy back on the vehicle once a fuel load is consumed. A two minute fill-up of a 12 gallon tank puts the equivalent of 87 kilowatt-hours into the vehicle, again, taking into account the 20% thermal efficiency.
87 kilowatt-hours in 2 minutes works out to 2.6 MegaWatts. Even with a 220 volt connection, that would require about 11,800 amperes of current. Just imagine the size of the electric cables for that current.
There are certainly places and applications where electric vehicles have a role, but it is worth remembering that at least five or six generations of engineers have looked very hard at trying to meet transportation needs and they keep coming back to the same fact – when you want to move a vehicle, you need power, (energy per unit time).
The more energy you can store on board the vehicle, the longer the power will be available. The more weight per unit energy that you need to add, the quicker you get to the diminishing returns where more and more of the energy is being used to drag the energy itself around. The quicker you can refill the tank, the more use you can get out of the vehicle and the more freedom and independence you can have.
After all, those are the truly valuable service provided by vehicles of all kinds – freedom and independence of movement.
Photo credit – Adams Family car circa 1986 – taken by the mom who is not in the photo.