The internet, as well as the local coffee shop, is full of claims of super secret gadgets that will make your car get 200mpg, or 300. The story is always that the only reason we don’t have these fantastic high mileage cars is that “Big Oil” has somehow managed to suppress every single company who has ever tried to make one, or has “bought out” every device that could magically do this to your car. Well, All of them except for ONE - the person peddling it.

Well, As I have said many times on this blog (and in the coffee shop) for any particular vehicle it takes a fixed, quantifiable amount of energy to push it around. This is based on the most basic laws of physics, the same laws that give us airplanes, microwave ovens, and, yes, the automobile. So, I’m gonna do it again.

Yes, once more, we will look at how much energy it takes to move your vehicle. Our purpose will be to define the maximum MPG.

A vehicle will achieve the best MPG or energy efficiency traveling at a steady speed on level ground. In that condition, the only energy it needs is what is required to overcome aerodynamic drag and rolling resistance (friction).

Our test subject for this week will be My Late Model Chevy Malibu, a mid-sized four passenger sedan. It weighs 3,460 pounds, has a frontal area of 24.1 square feet, and a Coefficient of Drag of .37, Totally Mainstream. To simplify the math, I didn’t do any. I used this calculator: http://ecomodder.com/forum/tool-aero-rolling-resistance.php.

Here is a summary of the results. Remember this is for a steady speed!

Speed | Horsepower | Watts | BTU/min |
---|---|---|---|

35 mph | 6.15 | 4586 | 261 |

40 mph | 7.94 | 5920 | 337 |

50 mph | 12.64 | 9425 | 536 |

55 mph | 15.66 | 11677 | 664 |

60 mph | 19.17 | 14295 | 813 |

65 mph | 23.23 | 17322 | 985 |

70 mph | 27.87 | 20782 | 1182 |

80 mph | 39.12 | 29171 | 1660 |

Before we continue, look at that chart closely. Note how the power required goes up rapidly? Aerodynamic drag is the largest force opposing your movement at any reasonable speed. That drag increases with the square of the speed. Doubling the speed creates four times as much drag. But, interestingly, power requirements increase at the cube of the speed. So that doubled speed will take eight times as much power.

You will also note, I have not mentioned MPG in that chart. It is irrelevant so far. This chart is the amount of power the vehicle needs. It does not matter whether that power comes from a Gasoline or Diesel engine, an electric motor, compressed air, rubber bands or a hamster wheel. The amount of force it needs to keep moving is the same.

Also, before we look at MPG, which implies liquid fuels, lets look at the chart and apply it to an electric car. Now it turns out an electric motor is very efficient, turning about 95% of the electricity fed into it into mechanical power. If you look at the 60 MPH row, you will find maintaining that speed requires 14,295 watts of power. To quantify that as energy consumed, or work, we have to add a time element. So, at 60 MPH over the course of an hour, we will go 60 Miles – duh. In that hour we will consume 14,295 Watt Hours of electricity, or 14.2 Kilowatt-hours (kWh). That works out to .236 kWh per Mile. In an amazing coincidence, this is the same as is claimed for the Chevy Volt in electric mode. The Chevy Volt, is indeed, nearly Identical in size to the Malibu. The Tesla Roadster with a smaller frontal area, and slight better Cd, claims .217 kWh per mile in mixed driving where it’s lower weight is also a factor.

OK, Finally, lets talk about MPG. The calculator I used will give you a MPG figure for each speed based upon the efficiency of the engine and drivetrain. Putting 100% efficiency in it will yield you your 200mpg at 45 mph. (Try it!). At 60 mph, you will get 140 mpg. How did we do that? Well, there are 114,000 BTU’s in a gallon of gasoline, and we are using 813 of them per mile (minute). That is 140mpg with a perfect engine at 60 mph. Right here, we know that a 200 mpg car is impossible at any speed over 45 mph. Even if it were perfect.

Well, sad to say, nothing is perfect. Certainly not your car’s engine.

The Second law of Thermodynamics puts an upper limit on the efficiency of a heat engine. This is known as the Carnot efficiency. A modern fuel injected, steel, Otto cycle, internal combustion engine – the one in your car, can achieve a range of efficiencies from about 15% at idle, to 35% at it’s torque peak and with a wide open throttle. A diesel will achieve the upper end of that range most of the time. What that means, is in the best case, only 35% of the BTU’s in that gallon of gas are being converted to mechanical energy. The rest is wasted as heat out your radiator and exhaust pipe.

The maximum possible Highway MPG my Chevy Malibu can achieve without violating the laws of thermodynamics is thus about 49 MPG at 60 MPH. The only way to improve this number, is to improve the aerodynamics, reduce the frontal area, reduce the rolling resistance, go much slower, or only drive downhill. The Toyota Prius is currently the highest mileage car in the US with a Highway MPG of 48. It achieves this primarily by having a Coefficient of drag of .25, and an estimated engine efficiency of 30% at 65 mph. Try putting these numbers in the calculator and then changing them. Getting the picture?

But, wait, it’s a hybrid. Isn’t that why?

Well, I’m glad you asked. Remember we are talking steady speed. The Hybrid function recovers energy lost through braking. That energy recovered was only the energy we used to accelerate the vehicle. At a steady speed, the hybrid has no advantage over a non-hybrid. In fact, you will notice the city mileage (stop and go) is actually higher at 51mpg. That is where the Hybrid does it’s work recovering energy.

And, that brings us to our last topic for now – acceleration. My Malibu weighs 3,460 pounds Every time we accelerate that mass to 65 mph it uses approximately 650 BTU’s of energy to do so. Per the laws of physics, that energy is then stored in the mass of the car as kinetic energy until we decelerate (slow down). When we apply the brakes, we convert that kinetic energy to heat which is then lost to the air. This is why stop and go driving normally has much lower MPG numbers.

My Malibu achieves a real world highway mileage of 26 MPG. That means we use a total of 4,384 BTU’s for each mile driven (Remember 75% - 3288 BTUs - are wasted). If we accelerate twice each mile to 65 (using an additional 1300 BTUs), then stop, that consumption will rise to 5,684 BTU’s per mile. Doing the math again puts our resulting mileage at 20 MPG.

I hope I have shed some light on this topic, and on the impossibility of some of the snake oil. While there are various other factors that will change these results in the real world – air temperature, hills, tires, road surface, and specific engine configuration to name just a few, they will not vary by much. And the results for the “perfect” engine will still set a maximum limit on what is achievable.

Ain’t science wonderful!

## Our Natural Gas supply – The numbers game.

It does not take long to realize that there are more varied predictions on how much Natural Gas this country has, than there are flavors in your local store’s Ice Cream freezer. And, unlike that Ice-Cream, many of the natural gas numbers leave a sour taste in the mouth. Which numbers are correct, which are biased towards an agenda, and which are just plain old pie in the sky – (a-la-mode)?

Now, I am not a geologist so I cannot offer my own independent conclusions, although that does not seem to inhibit many others. What I can do, is to look at those numbers, compare them, and attempt to derive some sensible compromises based on reality, technical awareness, and just plain common sense.

First, lets look at the numbers reported by what are considered by most to be credible and reliable sources. Unlike some, these sources also have facts and figures to back up their analysis.

The United States Geological Survey in their latest report ( Dec 2008) says we have 742 Trillion cubic feet of proved conventional reserves, 378 Trillion Cubic feet of Unproved reserves, and 743 Trillion Cubic Feet of technically recoverable Unconventional resources (Shale and tight gas).

Ref: http://certmapper.cr.usgs.gov/data/noga00/natl/tabular/2008/summary_08.pdf

The Energy Information Administration in Jan 2007 puts US total recoverable conventional reserves at 211 proved and 373 unproved, and technically recoverable unconventional reserves (Proved and unproved) at 1,366 Trillion Cubic feet. Ref: http://www.eia.doe.gov/oiaf/aeo/assumption/pdf/oil_gas.pdf

Already we have some serious disagreement, although both sources agree on Unproved Conventional gas. The difference in the unconventional reserves likely centers around the definition of “Technically recoverable”.

The big difference comes in the proven reserves. For some reason the EIA comes up with a very low figure for this, (I did the math 3 times). Yet, both sources add up to roughly the same total of 1,800 Trillion Cubic feet of reserves.Now, lets turn to the Natural Gas Association. First of all, they agree with me that there is a wide disparity in these assessments. I feel so good! They base their own assessment on EIA data as above with total proven reserves at 211 Trillion Cubic Feet, and a total of 1,536 Trillion Cubic feet of unproven reserves.

http://www.naturalgas.org/overview/resources.asp (Very useful page!).

OK. The above references, ignoring what they consider to be technically recoverable, all predicate that we have a total of about 1,800 Trillion Cubic Feet of Natural gas reserves in the US. Now, I could fill a few pages with references to other reports that “grow” this estimate in leaps and bounds. It all culminated for me in a purported report from JP Morgan Chase that our reserves are NOW 8,000 Trillion Cubic Feet. I have not been able to find the actual report, only references to it. It does strike me that many of these inflated numbers are coming, not from geologists, or even energy companies, but from institutions with a financial stake in the trading of Natural Gas. Hmmmm.

I would like to point out also, that there is wide disparity on how much of that gas can be recovered. Certainly not all. According to the Natural Gas association, about 10% of the unconventional. Many sources put it closer to 30 percent. So far shale gas production in the Barnett shale has not lived up to expectations, and they are recovering about maybe 35% of what they thought they would. http://www.aspousa.org/index.php/2009/08/lessons-from-the-barnett-shale-suggest-caution-in-other-shale-plays/

One final issue I would like to address. A common talking point is how many years this supply of Natural Gas will last us. First some baseline numbers. We used 22 Trillion Cubic Feet of Natural Gas last year. That is 1.8 TCF per month, or 60 Billion Cubic Feet a Day! In the same month we will use about 11 Billion gallons of gasoline, 5 billion gallons of diesel fuel and 342 Million-Megawatt hours of electricity.

Well, if we change nothing, and manage to recover all 1,800 TCF of our natural gas, we are good for 81 years. If we manage to only recover 35%, then about 28 years worth.

But, what if we try to replace our gasoline use with natural gas? That 11 Billion Gallons of gasoline a month is about 1,364 trillion BTUs. Equivalent to 1.3 trillion Cubic Feet of Natural Gas, increasing our consumption by 72%. If we add in the diesel, that would equal .65 Trillion more Cubic feet. So replacing our transportation fuel would more than double our consumption of Natural Gas, and reduce our remaining supply to 14 or 40 years, depending on your optimism level. http://www.theoildrum.com/node/5615.

Let’s replace electricity from coal instead. Coal supplies 48% of our electricity, or 164 Million-Megawatt hours a month. Each one has 3.414 BTUs of energy. So, that is 560 Trillion BTU’s. (I checked the decimal point). It would require .6 trillion Cubic Feet of Natural gas to replace our coal. About a 33% increase, assuming the power plant efficiency is the same – it is close. That would make our natural gas last anywhere from 19 years to 54 years, again depending on how much natural gas we can actually recover.

Finally, I would recommend reading this report on The Oil Drum. It addresses some of these points, and also makes the point that the whole natural gas reserves picture is steeped in unfounded numbers and hype.

http://www.theoildrum.com/node/5676

As for me, I think I will have some Ice Cream.

Posted at 09:47 PM in Commentary, Natural Gas, Technicalities, Transportation | Permalink | Comments (18)

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