Under a Gibbous Moon

A new article from Wired showcases the 2011 Ford Mustang which comes standard with a 305 horsepower V6 and has an EPA rated highway mileage of 31.

One of the first comments struck me as interesting.

Posted by: fletc3her | 03/4/10 | 4:50 pm |

I think it’s great that the auto industry is starting to improve mileage even in sports cars and muscle cars. But, I do wonder what the actual day to day mileage of one these cars will be. If you push the car to get the advertised 0-60 speed then you are not going to get the advertised mileage. Less powerful cars force the driver to drive efficiently simply by virtue of not having enough horsepower to waste a lot of gas.

He says that less powerful cars force the driver to be more efficient because it simply does not have the power. Apparently he believes that an increase in horsepower necessitates a drop in fuel economy. It is true that they tend to be highly correlated but vehicle performance is more complicated than that.

For a brief primer, horsepower is a measurement  of work with work being force expended over a distance. The next big engine measurement is torque. Torque is a measure of instantaneous force and typically refers to a twisting action, in the case of an internal combustion engine, the crankshaft acting upon the input shaft of the transmission.

Torque is then multiplied through the gears in the transmission (or divided in the case of an overdriven gear). It is then multiplied one last time in the rear differential before being divided by the wheels (think of them as really big gears).

The power output of the engine varies with engine speed, torque reaching its peak before horsepower. When these peaks are reached varies from engine to engine but it is the gear multiplication that picks up the slack at low engine speeds.

The reason that this is important is that the engine is most efficient when it is producing the most power. This is why mileage in the “city” is always worse than “highway”, you are constantly running the vehicle through the lower and less efficient RPMs. An excellent example is watching the black smoke on a diesel from a stop. Diesels are very inefficient at low RPMs and the momentary black smoke is cause by a lot of unburnt fuel. Physics also comes into play, with regards friction and momentum (it takes a lot of energy to move a vehicle from a stop than it is to just keep going).

This brings me back to my earlier point about “forcing efficiency”. Smaller displacement engines (four cylinder and even smaller six cylinder) have very high power peaks four, five, and even six thousand RPMs. The little power that the engine does produce does not actually come into play for a long time. As stated earlier, this is overcome through the use of aggressive gearing. This has the dual purpose of multiplying torque and spinning up the engine faster.

The interesting thing about this, is that the smaller engine (in a similar weight and shaped vehicle) is working harder than a larger engine would. It uses less fuel simply by virtue of having a smaller displacement.

This comes back the Mustang mentioned in the beginning of this post. It (and the new Camero) are part of a new breed of small displacement, high power engines. While this is not a new trend (think of when the Mustang dropped the 5.0 for the 4.7). it has been accelerating due to rising fuel costs.

This extra power, gives you the ability to motivate the vehicle with less effort (fuel) which is why you are now seeing amazing performance in vehicles with mileage ratings once associated with compact cars.

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Here are the photos I took of my Jeep:

From the front

Passenger side

Driver's side

The flat tire

Widshield. Note the wire in the lower left, that was what my cell phone was attatched to.

From the inside

Yes, the windshield does look broken in. That is because my backpack from the passenger seat was embedded in it. When I removed it, it bent the glass back inwards.

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Yesterday, while on my way to work I was in a car accident on the highway. I think the front driver’s side tire blew and, anyway, my jeep flipped over and rolled about three times. Lucky for me, it landed on its wheels. I got hurt, but nothing was broken. Blogging will slow down for a while though.

When I go to get my personal effects from the vehicle, I will make sure that I take some pictures.

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WARNING – THE FOLLOWING POST CONTAINS MATH AND BIG NUMBERS – YOU’VE BEEN WARNED.

Recently, upon reading several articles dealing with the upcoming release of a variety of electric vehicles (EVs) onto the market I noticed that nobody was discussing a critical point: Where was all this electricity going to come from. Electrical power is, after all, a finite resource. With all the gasoline that is used in the United States, I wondered if there was enough production capacity to meet the demand of a large scale deployment of EVs.

I had no idea of the answer to this when I first started. Indeed, I initially assumed that the reason I saw no discussion on this is that there wasn’t a problem to begin with, but I wanted to see the numbers for myself. So, without further ado, here is what I found. All statistical data is based upon currently available figures (2007).

First, I needed to find out how much energy is consumed by gasoline powered vehicles in the United States. I found that approximately 390 million gallons per day¹. Gasoline has 34.8 megajoules (10⁶) (MJ) per liter or about 131.7 MJ per gallon. So with this, 390 million gallons of gasoline has about 51,363,000,000 MJ or 51.363 petajoules (10¹⁵) (PJ) of energy. To put that into terms of power production, I found that 3.6 MJ is equal to one Kilowatt-hour (KWh) of energy². Therefore, 51.363 PJ equals 14,267,500,000 KWh or 14.268 Terawatt-Hours (10¹²)(TWh).

The next step is to compensate for lost energy due to idling. It is estimated that 3.8 million gallons of gasoline is wasted in the United States by an idle engine⁵. This is approximately 1% of daily gasoline usage. Taking 1% from 14.268 TWh leaves us with 14.125 TWh.

Also, gasoline engines do not convert 100% of the available energy into useful work, they are about 30% efficient in converting energy into work³. Therfore, out of 14.125 TWh we actually will use 4.234 TWh.

Now, for the United States electrical generating capacity. The United States generated 4,157,000,000 MWh in 2007⁴. For comparison purposes, I shall move the decimal point so it reads 4,157 TWh. Dividing that by 365, I came to a daily average of 11.389 TWh.

That puts the replacement energy needs at approximately 35% over the current energy output. From 2006 – 2007, United states electrical output increased by 2.3%⁴. In the highly unlikely circumstance of no other growth in demand for electrical energy, it would take over 15 years to completely replace our gasoline fleet with and EV one and most likely it would take decades longer than this.

This leaves us with an undesirable time table for either removing dependence on foreign oil or reducing greenhouse emissions (assuming the ability for a rapid migration to renewable electrical production).

With this data, the conclusion that I have come to is that EVs are not a viable large scale alternative to gasoline powered vehicles. Due to limitations in electrical production they will only be able to fill a niche role for many more decades.

References

1. Petroleum Basic Statistics http://www.eia.doe.gov/basics/quickoil.html

2. Kilowatt Hour http://en.wikipedia.org/wiki/Watt-hour

3. Engine Efficiency http://en.wikipedia.org/wiki/Engine_efficiency

4. Electric Power Annual http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html

5. Anti-Idling Primer http://www.thehcf.org/antiidlingprimer.html

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