I haven't posted here in a while because it seems like nobody is paying attention. I checked the counter today and noticed this blog is starting to show up on Google search in the top 10 of 37,000 for several searches and it is starting to get some good web traffic. Not bad!
I always chart my fuel economy in my car. For every tank of gas I record the amount of fuel used, cost of the fuel, approximate outdoor temperature, the miles driven, miles with the air conditioner on, highway miles, interstate miles, city miles, the weight I am carrying, and any other significant notes. I have a big Excel sheet with all of this information and I do many calculations with the data.
Last weekend I drove about 420 miles round trip to visit family and achieved my highest fuel economy ever: 46 mpg! The car is a Toyota Corolla S with manual transmission. It is rated with the old mpg rating system to have 32 city/41 highway mpg. The awesome thing about the high fuel economy is about 40% of the miles were on the interstate at higher speeds, and 70% of the miles required the air conditioner.
My lifetime average fuel economy with the car is about 36 mpg. Most of the miles I have driven between 3 and 5 mph over the speed limit. This past weekend was different, I never exceeded the speed limit on highways or the interstate.
The benefits of going slow was high fuel economy. The downside was mostly psychological. Driving 55 seemed like I was going very slow and I could be going faster. In reality, the difference was minimal. Driving just a few mph faster does very little for short trips, but for long trips a few mph can easily add up. I actually found the extra time required was negligible. It took me 3h 45min to make the drive one way at the posted speed. My previous trip was 5 mph over the speed limit for most of the time and it took me 3h 35min. I saved 10 minutes, but my fuel economy was about 8 mpg less.
In addition to driving slow, I do use mild hypermiling techniques that I have used regularly over the past several years. The main thing to do is just pay attention to driving, which you should be doing anyway. If you see a stop sign ahead, don't hold the gas pedal down until the last moment and then brake hard. You would be surprised how far your car can coast. For one highway stop sign on the route, I put my car in neutral 0.6 miles away from the sign and coasted in. When I hit my brakes as I neared the stop sign, my car was still doing 45 mph and had only dropped 10 mph from my previous speed. If there is a slight downhill grade, I can put the car in neutral even sooner. The extra time this added to my trip was very small, probably less than 5 seconds, but I was able to coast for 0.6 miles at probably 200 mpg or higher.
A similar technique can be used for stoplights. If one turns red, brake early to slow down, and then coast in. If you time it right, you will still be rolling and nearing the car in front of you as the light turns green. Slowing early will actually help you maintain a higher speed when the light turns green.
Two other things I do are slow acceleration and using my cruise control. These are simple things that will save you several mpg. I would not recommend drafting other vehicles as some others suggest. Drafting is unsafe and illegal.
The biggest time lost in the trip is not from the highway driving speed; city driving is the worst. As I travel through small towns, it is only 4% of the total trip miles but it takes 10% of the trip time. Doing the math, it only saves 12 minutes over the whole trip to drive 4 mph over the speed limit, which agrees with my observed results.
Changing interstate speeds back to 55 mph might not be such a bad idea. Even if people still go over the speed limit, the net effect will be lower overall speed and people using less fuel. For my 210 mile trip, the difference in time between current speed limits and if all roads were 55 mph is about 25 minutes. The benefits of increased safety and reduced foreign dependence on fuel are probably worth the extra time.
Showing posts with label Fuel Economy. Show all posts
Showing posts with label Fuel Economy. Show all posts
Wednesday, July 2, 2008
Wednesday, January 23, 2008
A number check on ethanol
I haven't posted on here for a while. I've been very busy over the past six weeks.
The grad school work has kept me busy. Recently I have been looking at biomass, which led me to do some interesting calculations.
Here is the question (when you get into engineering, all questions have multiple parts): If you have an acre of corn grown and converted to ethanol, how much energy do you get? How does it compare to the solar energy striking the ground? How does it compare to a single wind turbine?
From various sources, the amount of ethanol per acre comes out to about 450 gallons. The energy content is equivalent to about 40 GJ (gigajoules, it didn't take long for me to run away form the inferior US units). To put 40 GJ in perspective, that is equivalent to about 11,000 kilowatt hours. That is a deceiving number, because the energy is not available except as heat in that quantity. If the ethanol is used to drive an engine to power a car or produce electricity, the actual amount of energy available will be about 3,500 kWh.
Solar energy strikes the land at a year round average in Iowa of about 160 Watts per square meter. Working out the numbers, 5,650,000 kWh of solar energy strikes an acre annually. Once again, not all can be harvested. Lets figure a photovoltaic panel is used with 10% conversion efficiency, and they are placed to collect about half of the energy available. Even with these low numbers for efficiency, 282,500 kWh of electrical energy is produced. This is about 80 times the amount of energy gained from ethanol.
Wind energy varies, but typical wind turbines output between 4,500,000 and 6,000,000 kWh a year. Each one takes up 1/4 acre of land. They must be spaced apart or else the turbines will interfere with each other and disrupt the wind. Working out the numbers, it takes about 37 acres for a wind turbine. From a one acre plot of land, a wind turbine will output about 120,000 kWh of energy, or about 34 times the output of growing corn for ethanol.
To put the numbers in perspective, if fields were not cultivated at all and turned into a natural prairie with wind turbines covering the landscape, about the same amount of energy would be produced. If we covered 600 square feet of a house roof in solar panels, we would output the equivalent energy of an acre of corn (43,560 square feet).
None of this analysis takes into account the energy input for ethanol. A wind turbine must be constructed, and it typically pays back the amount of energy for construction in a few years. Photovoltaics are the same way, with payback of a few years. Very little fuel consumption goes into the annual upkeep of the wind and solar technologies, compared to the huge amount of fuel that goes into an acre of corn for ethanol.
The problem with any biocrop is the efficiency of photosynthesis. The efficiency is less than 0.5% at best. Corn is only growing for a few months out of the year, and the rest of the year the field is bare and not collecting energy, further reducing the amount of solar energy collected.
Does it require more fuel to make ethanol than we get from the fuel? I am not totally sure. A lot of the pro-ethanol studies are done by pro-ethanol groups. I did a calculation the other day showing that it takes about 1,000 kWh of energy to make the ammonia to feed an acre of corn. Considering we only get 3,500 kWh out of the crop, and I didn't account for transportation of ammonia, corn, ethanol, or the huge amount of natural gas used at the ethanol plant, the net energy gain is going to be very very small.
Keep in mind that the energy food energy consumption of a human is about 2.5 kWh per day. We can feed thousands of meals off of that acre of land if used for food. We can fuel an SUV for less than half a year on the corn energy from an acre of land. Which one should be our priority? Which method of land use (solar, wind, corn ethanol) is the most efficient and best use of our resources?
The grad school work has kept me busy. Recently I have been looking at biomass, which led me to do some interesting calculations.
Here is the question (when you get into engineering, all questions have multiple parts): If you have an acre of corn grown and converted to ethanol, how much energy do you get? How does it compare to the solar energy striking the ground? How does it compare to a single wind turbine?
From various sources, the amount of ethanol per acre comes out to about 450 gallons. The energy content is equivalent to about 40 GJ (gigajoules, it didn't take long for me to run away form the inferior US units). To put 40 GJ in perspective, that is equivalent to about 11,000 kilowatt hours. That is a deceiving number, because the energy is not available except as heat in that quantity. If the ethanol is used to drive an engine to power a car or produce electricity, the actual amount of energy available will be about 3,500 kWh.
Solar energy strikes the land at a year round average in Iowa of about 160 Watts per square meter. Working out the numbers, 5,650,000 kWh of solar energy strikes an acre annually. Once again, not all can be harvested. Lets figure a photovoltaic panel is used with 10% conversion efficiency, and they are placed to collect about half of the energy available. Even with these low numbers for efficiency, 282,500 kWh of electrical energy is produced. This is about 80 times the amount of energy gained from ethanol.
Wind energy varies, but typical wind turbines output between 4,500,000 and 6,000,000 kWh a year. Each one takes up 1/4 acre of land. They must be spaced apart or else the turbines will interfere with each other and disrupt the wind. Working out the numbers, it takes about 37 acres for a wind turbine. From a one acre plot of land, a wind turbine will output about 120,000 kWh of energy, or about 34 times the output of growing corn for ethanol.
To put the numbers in perspective, if fields were not cultivated at all and turned into a natural prairie with wind turbines covering the landscape, about the same amount of energy would be produced. If we covered 600 square feet of a house roof in solar panels, we would output the equivalent energy of an acre of corn (43,560 square feet).
None of this analysis takes into account the energy input for ethanol. A wind turbine must be constructed, and it typically pays back the amount of energy for construction in a few years. Photovoltaics are the same way, with payback of a few years. Very little fuel consumption goes into the annual upkeep of the wind and solar technologies, compared to the huge amount of fuel that goes into an acre of corn for ethanol.
The problem with any biocrop is the efficiency of photosynthesis. The efficiency is less than 0.5% at best. Corn is only growing for a few months out of the year, and the rest of the year the field is bare and not collecting energy, further reducing the amount of solar energy collected.
Does it require more fuel to make ethanol than we get from the fuel? I am not totally sure. A lot of the pro-ethanol studies are done by pro-ethanol groups. I did a calculation the other day showing that it takes about 1,000 kWh of energy to make the ammonia to feed an acre of corn. Considering we only get 3,500 kWh out of the crop, and I didn't account for transportation of ammonia, corn, ethanol, or the huge amount of natural gas used at the ethanol plant, the net energy gain is going to be very very small.
Keep in mind that the energy food energy consumption of a human is about 2.5 kWh per day. We can feed thousands of meals off of that acre of land if used for food. We can fuel an SUV for less than half a year on the corn energy from an acre of land. Which one should be our priority? Which method of land use (solar, wind, corn ethanol) is the most efficient and best use of our resources?
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