How to save energy running your central A/C

Isn't central A/C wonderful? Its so cool and comfortable but on the other hand, so darn expensive.

My last house had an evaporative cooler on the roof. It's basically a system that draws hot, dry air through water soaked pads. The heat in the air is absorbed when the water evaporates.  Hot, dry air goes in and cool, more humid air comes out. When the humidity is low, it works beautifully. When it's muggy outside, well, that's where the evaporative cooler gets it's “swamp” nickname. In a dry climate like Utah, the swamp cooler is extremely efficient, somewhere on the order of 48 SEER. A typical central A/C is only 10-18 SEER.

Psychometric chart

A properly tuned and maintained swamp cooler will produce air that is a couple degrees above the wet-bulb temperature.  So if it's 110 °F outside with only 10% humidity, the wet-bulb temperature will be 68 °F and the air coming out of the swamp cooler will be about 71°F. Not a bad system for a really dry climate. 

A few weeks ago, I was pondering the idea of combining a central A/C with a swamp cooler. They do it all the time in the chillers on large commercial air conditioning systems. Why don't A/C manufactures offer it in their products to homeowners?

After a few hours reading on the home HVAC installer forums, I learned that people who ask this question, “Why not wet down a central A/C?” are flogged and burned at the stake so to speak.

Evaporating lots of water on thin metal cooling fins is the quickest way to rust and corrode them. A modest savings of power up front will end up costing the homeowner a new A/C unit in a few years.

Being an exceedingly curious man, I just can't put the issue to bed that easily. Yes, I can see how wetting down cooling fins with hard water is just asking for trouble, not to mention increased maintenance and the potential for flooding problems. But I believe that under the stretched out tent walls of a generalized bad idea lie stringent ruled, brilliant ideas.

To protect the uneducated public from injury and damage, an idea that is dangerous some of the time is stereotyped as a bad idea all the time.

For example, “Never put metal in the microwave”. Now this might seem like good advice to follow because we have all been raised to believe it is true in all cases; that it is dangerous to put metal in the microwave.

But you forget that microwave ovens are made of metal.  Some microwave ovens even come with metal shelves and metal temperature probes. “No metal in the microwave oven” is a contradiction. The more correct statement should be, “Never put metal in the microwave oven when the dielectric properties and electrical dimensions of the metal and food pieces will cause excessive heating and current flow across the metal.”  To simplify our lives by not having to crunch a bunch of energy density functions and calculations each time you need to re-heat something, it is generally easier just to not put any metal in the microwave oven.

There will always be exceptions to rules and contradictory exceptions to those exceptions. 

FYI#1:You can defrost an open can of juice concentrate in the microwave oven, even though the bottom of the can is made of metal.

FYI#2: Re-heating in a microwave oven an Arby's roast beef sandwich shrouded in its metal lined paper wrapping will cause it to catch fire.  Right Marv? 

Just as with exceptions to the "no metal in the microwave" rule, I believe that under the right circumstances, cooling the A/C coils with water (softened water) is a good idea and will save energy without significantly shortening the life of the A/C system itself.  If I am wrong, I get to buy a more energy efficient A/C coil to replace the one I currently have. 

It is my goal to confirm the following statements: 

• Removing direct sunlight by shading the outside A/C coil (without limiting airflow) will lower the power consumption of the compressor during those hours of direct sun-light. 
• Lowering the temperature of the air conditioner coils (outside unit) will lower coolant pressure, lower compressor current draw and make the A/C run more efficiently.
• Adding powered attic ventilation in a properly air sealed home will make the central air conditioner run less and reduce overall power consumption.

I know that doing the above steps could potentially save energy.  How much?  Well, that's where experimenting comes in.  

My house is already pretty well insulated.  I also air sealed major air penetration areas of the home, (recessed lights, plug sockets, windows) and added more passive attic ventilation. These low lying, energy saving fruits are already harvested.

Central Air Conditioner Coil Shade: 

Using my TED house power monitoring system, I noticed that between the hours of 5-8PM, the duty cycle of the air conditioner goes way up. I also noticed that it was during those same hours that the setting sun is beating down on the air conditioner unit outside.

I decided to build a shade to see if that would improve anything.  

To my surprise, with the tarp shade, the power consumption of the air conditioner while it was running did not change much but the duty cycle of the air conditioner's "on" cycle, shortened.

That worked so well that I spent $100 on a couple sheets of grey PVC lattis and channel pieces.  I built a more permanent trellis shade that looks nice and still shortens the A/C duty cycle during the evening sun hours.

This shade results in a 1.9KWH/day air conditioner energy savings with the added benefit of looking less like a crime scene than the tarp. 

Central Air Conditioner Misting System:  
To cool the outside air temperature, I originally thought of building a swamp cooler around the central A/C unit outside. That cost estimate quickly rocketed up into the $400 range. I'm all for advancing science and mankind's well-being but I don't want to blow that much money this early in the game.

I ended up going with a simple misting design (with 15-20 misting heads) that would allow for more airflow to the A/C coils. There are products on the web that do just that for about $100, but they are for smaller A/C units and adding enough misters for my size A/C unit add up to over $200. Plus I didn't like the toilet flapper float design they used for turning on and off the water supply line. 

For about $45 I built my own misting system that uses an electronic sprinkler valve controlled by the existing HVAC thermostat.

 

The water feeds through from an unused hose bib near by into the sprinkler valve. 

 

How cool and and refreshing is that?  

The misting system reduces the outside air temperature, in turn reducing the pressure of the whole system, in turn reducing the air conditioner's power consumption 850 watts (or 21%) while it is running its cooling cycle. This results in a 4.8KWH/day air conditioner energy savings.

I still need to run a soft water line out to the misting system, before the A/C coil is sentenced to a corroded, rusty fate.

Attic Fans:

A couple of years ago, I installed a thermostat controlled attic fan.  I had read that venting out the heat in the attic will reduce cooling costs, (makes sense). But it also takes energy to spin a large fan for 10 hours every day.

If the house isn't properly air sealed, cool conditioned air from the house will be sucked into the attic space and vented outside. In this case, the central air conditioner will use more energy with an attic fan running.

If the attic is far under ventilated, the attic fan will not be able to draw out enough hot air to make a difference but still draw a lot of power. 

If the attic has superior ventilation (most homes, including mine don't), an attic fan will not do any good either because the attic vents are already doing the an attic fan's job only passively. 

Ever since I air-sealed my home, I have been wanting to quantify the attic fan's performance and usefulness.  I got the chance this weekend when my entire family was away and left me home alone. No stoves cooking, no kids leaving outside doors open, no TV's being left on. Just a relatively constant, measurable whole house power consumption. To help in comparing apples to apples, the weather forecast all weekend was sunny and in the low 90's .

I ran one day with the attic fan on and one day with it off.

Notice how the air conditioner has shorter duty cycles on the day the attic fan is running. 

All things being equal and even with R-38 insulation in the attic, the attic fan lowered the central air conditioner's power consumption by 20%.
Even though my attic fan draws 190 watts when it is running (190x10hrs/day = 1.9KWH), it actually reduced the central A/C usage by 6.6KWH for a net gain of 4.7KWH.
After operating for 2 summer seasons and at an electrical savings of $70/ season, my attic fan just paid for itself last year.

Between the A/C trellis shade (1.9KWH savings), the A/C misting system (4.8KWH savings) and the attic fan (4.7KWH savings), my whole house net energy savings is 11.4KWH/day or a 22% reduction. COOL! 

Shoot, with the energy saved by not using an extra 11.4KWH/day all summer long, I could drive an electric vehicle 3420 miles, or commute in it to work for 5 months, FOR FREE!

Update 7/27/2011:
I have since plumbed my hose bib to the soft water line.  Now the misting system will not corrode the coil nearly as fast; maybe not at all.  

Water Conservation:

I live in the desert landscape of the Wasatch front in northern Utah. I say desert because that is what it used to be when the Mormon pioneers settled here almost 164 years ago. When they arrived, the Prophet and leader Brigham Young proclaimed, “This is the place!” I wonder if this was actually a question and not a statement? Perhaps he really meant, “This is the place?”
Either way, those early Mormon pioneers took a dry piece of desert land and transform it into a fruitful, lush, green oasis. Today over 2 million people live here and it is a beautiful, green, mountainous landscape.
We all have to start being more conservative if we expect to sustain our American lifestyle on such a finite water resource. It is possible for all of us and our children to have luxuries and amenities. We just have to be smart with how we use our resources.

Use low-flow shower heads:
This is good practice but you can even take it a step further. Hop in the shower, get wet, rinse off, then turn off water. At this point, take your time lathering up and scrubbing yourself clean. When done, turn the water back on and rinse off. You will only use 3-8 gallons for your shower.  Without the water washing the soap away before it has had a chance to do its job, you will get cleaner and use less soap in the process.

Drain Water Heat Recovery:  
It takes a lot of energy to heat water. It's a shame that all that energy ends up going down the drain.
By using a Heat-recovery drain pipe, one can recover 80-90% of that wasted energy. When water runs down a vertical pipe, rather than falling down the center very quickly, it glides down the inside wall of the pipe. In a heat recovery drain, a section of vertical sewer pipe is replaced with a thermally conductive copper one, a smaller copper culinary water pipe is wrapped around the copper sewer pipe. In this manner, heat from warm waste water (and nothing else) is transferred to the cold water on its way to the water heater. A typical system costs about $500-$800 and the pay off ranges from 2-7 years depending on how much you use it.  Heat from shower water is easily reclaimed because there is a continual flow of warm waste water preheating the continual flow of cold water entering the water heater.  I want to install one of these someday.

Use an efficient washing machine:  
Spend a little extra up front and save a bunch throughout the life of the appliance.  Energy Efficient washing machines will save you thousands of gallons of water each year and reduce the energy consumed drying clothes because they spin clothes faster making them drier. 
I recently replaced my old washing machine and electric clothes drier with an energy efficient, front loading washing machine and a gas drier. My electric bill went down 120KWH/month by drying 8 loads/week with gas instead of electricity. But to my surprise, my gas bill also went down. Even though my clothes drier now runs on natural gas, the washing machine uses so little water now (2.2 gallons/load) that the water heater doesn't have to heat up nearly as much water to feed the washing machine.  The result is a net reduction in electricity and natural gas consumption. 

Fix drippy faucets and running toilets: 
A drippy faucet can annoy the crap out of you and also wastes 20-100 gallons of water in a day.  Leaky hot water wastes energy too because your water heater has to heat up all that water before it gets lost. 

Saving Water Around the Yard:
I am not very stringent in this area. I own a large ½ acre lot, that is mostly lawn. Kentucky Bluegrass is not native to arid Utah. It takes a lot of water to keep it green and happy. But that is what the cavalier American dream demands, right?  A lush, manicured, green lawn?  You have to admit though, it does look pretty good. 

It also feels good walking barefoot or laying down on a healthy green lawn. The kids love to play on it too. Even for an energy sipping tree-hugger like myself, I justified putting it in.  My wife may have twisted my arm a little bit. 
I read on the Internet that an acre of grass sequesters 3 tons of CO2 each year.  There is a lot of research ongoing regarding this topic.  The sequestration numbers vary from several tons of CO2 per acre per year to lawns actually being a net carbon source. 

Even with my property having a large irrigation water share, I still hate to see any of it going to waste. 
If you have a choice, buy a house that has secondary water for watering the yard.  Living in Utah, I am baffled why most homes here only have a culinary water supply.  It's expensive to water a large lawn with culinary water, not to mention a terrible waste of a limited resource. 

Regardless of the water source, water the lawn in the early morning to limit evaporation. Make sure sprinkler heads are in good repair and automatic sprinkler valves turn on and off correctly.  In my last house, I only had culinary water.  I had a sprinkler zone dedicated to watering several strawberry plants.  One day, the valve got stuck in the on position.  I didn't notice it until after returning from a vacation.  Needless to say, the strawberries were amazing, but not as amazing as my water bill. 

Only water the lawn from a couple times a week to every other day. When you do water, give the lawn a good soaking, then allow the ground to dry out for a few days. This promotes a healthier lawn by sending the roots deep in search of water. Deep roots enable the plant to go longer between watering intervals.

Collect rain water for use later in the year:  
Why pay for water when mother nature already gives you the water for free?  Acquire a large cistern that can collect snow melt and rain water from the roof. During the summer months when it is dry, this water can be conservatively issued to the plants in the yard.

Xeriscape yard:  
Only plant native plants in the yard that thrive solely off of the natural water cycle.  No irrigation is required.
Check out your local water conservation district. They have examples and ideas of yard landscapes that will work in your local geography.

Drink Water:  
Anyone else thirsty? Don't buy a Coke! Drink water!! Fill up a bottle from home and drink up. It's free!
Milk is for Graham Crackers. Juice is for special occasions. Water is great anytime.

Solar Panels 6 Months Later

It has been over 6 months since I installed solar panels and started making my own electricity.

To recap, December was not a good month for solar energy. My panels only made 160kwh all month and I used 310kwh more than I made. Just by looking at daily solar panel production for December, you can extrapolate when the winter solstice took place (TED started collecting data on December 4th). 

Even though solar panels are more efficient at making power when they are cold, having less hours of the day really hurts production.

January more than doubled December's production with 380kwh. February almost doubled it again and nearly zeroed out the net meter. March was the first month where production equaled consumption. Even with all the record rain fall and cloudy weather in Northern Utah, the months of April and May produced enough power to run my house for 3 months. I can't wait to see production on months where it doesn't rain every day. This unseasonably cold weather and cloudy skies hampered available solar energy but at the same time, I hardly had to turn on the air conditioner.

I am currently producing twice the energy of what I consume. On a sunny day, I make over 3X. 
My utility company allows me to build up a one-for-one credit with them so long as I use up that credit within 12 months of accruing it.  Under this system, the grid is effectively my battery. A non-volatile, unlimited capacity, low cost, maintenance free battery. All I pay is the $5/month fee, well worth the price in my opinion.  The electric company lets me do this because I am helping them reduce their peak demand.  Solar panels are the perfect solution to preventing rolling blackouts.

As of today, I have built up nearly 2 months worth of energy credit.

In hindsight, a 6.2Kilowatt solar system is a little too large for powering just my house. But when I first looked into getting solar panels, my energy consumption was much higher than it is now. An un-calculated side effect of having solar power (and a TED whole house monitoring system) is a lifestyle change.  Having a device that points out to you that you are being extremely wasteful will change the way you act and think forever. 

This increased awareness has indirectly reduced my energy consumption by 1/3^rd.  All the while, my standard of living has remained the same or even increased. 

Now that I am making more energy than I will use in a year, I need to put it to good use before I have to give it away for free, which isn't exactly the end of the world either -- actually I would be saving the world then wouldn't I.  Here are a few ideas. 

Make some hydrogen

Using electricity, one can split water molecules into oxygen and hydrogen.  Storing hydrogen is tricky because it takes a lot of space and doesn't have a very high energy density.  But once you have it stored in tanks, you could then use it to power a fuel cell and generate electricity during a power outage. Or you could burn it directly for cooking or heating. 

Heat the home in the winter months

A deca-therm of natural gas is equivalent to 293KWH.  Heating my home for the winter (using an electric furnace instead of natural gas) would require 16,500KWH.  I would need a solar system nearly twice as large as what I have now just to run the furnace for the winter.  This would be a stupidly inefficient way to heat a house.  Still, if I have energy to burn, why not?  Nah. 

Charge an electric car

Charging an electric Chevy S-10 pickup truck after driving it 40 miles will require somewhere in the neighborhood of 9KWH of electricity.  I drive my current commuter car about 8000 miles a year.  That works out to only needing 1800KWH/year.  That would be perfectly doable with my estimated surplus and save me about $820 in gas each year.  This would also knock the return on investment for the solar system down from 6 years to just over 3 years.  Hmmmm. 

Power a Time Machine
Too cliche.  By the way, a few years from now, I actually invented a time machine and went back in time to a few hours from now.  Apparently, it didn't go very well and an even older version of myself had to go back in time to a few minutes ago and stop the whole event from taking place.  Time travel is tricky like that, not to mention it takes a ton of power and the stupid machine kept blowing the main breaker. 
Time travel, Shmime travel, I just want a microwave oven clock that doesn't have to be reset each time the power goes out. 

Nissan Leaf vs. Chevy S10 pickup

A few years ago, my sister was getting rid of her Geo Metro so I bought it off of her for $1.   

I had great plans to convert it to an all electric car. I only needed a 30 mile max range for my commute to work and back. The Geo was the ideal car for converting to EV. It was light weight and had good handling.  People have used them for years as a donor car for electric cars.

I stripped out the engine and acquired most the parts I needed,  One day, we decided to move to a larger house. Sadly I had to admit defeat when the house we moved in to was out of the range of the electric car's design. I chocked it up to a learning experience and sold the parts, liquefying my asset.

To this day, I still consider this poor excuse for an excuse my greatest failure. I vow that my next vehicle will be an all electric car.

I have been thinking about two options: The Nissan Leaf or converting a Chevy S10 pickup truck to an electric vehicle.  

Let's list the pros and cons: 

• The Nissan Leaf is very expensive but super cool. The Chevy S10 EV will be about 1/3^rd the cost and also super cool.
• The Nissan Leaf is a new car filled with the latest technological gadgets.
• The Chevy S10 pickup EV will be very basic and plain. I've never had such fancy stuff in a car so why start now?
• The Leaf has a 100 mile range. The S10 EV will be lucky to get 60 miles. My commute is only 20 miles each way.
• The S10 being a pickup truck will be able to haul lumber, sheet-rock, dirt and bags of concrete mix.
• My current vehicle (1992 Honda Accord) has been my reliable “work truck” and has served me well in hauling everything except for Sheetrock. The Leaf would be too expensive to use in this manner.
• The S10 would be a total blast to strip down and convert over to an electric car.   I need me a new hobby!  The Leaf, well....  Not much to do on it.  Drive it home and then what? 
• The Leaf will cost me almost $31,000 where as the S10 will set me back $11,600. I could buy a lot of whatever with the $18,900 saved in rolling my own instead of buying a bran new car.  

Nissan Leaf (100 miles)         Chevy S-10 (60 mile range) 
Car                      $33,720.00   Minor repair work                 $250.00

Charging station    $2,200.00   Donor vehicle '94-98 S10   $1,750.00

Rebate                 -$7,500.00   A/C Electric Kit                  $8,115.00

Acquisition Fee        $595.00   x18 T-145 6V batteries       $3,222.00

Taxes                    $1,958.51   Miscellaneous                        $500.00

Sell Honda              -$500.00   Sell Honda                            -$500.00

                                              Tax Credit                          -$1,333.70

                                              Sell old truck parts                -$400.00

    

Net Cost Leaf    $30,473.51  Net Cost Chevy S10 EV   $11,603.30

Savings of converting vs. buying new  $18,870.21

• Which begs the question, why not drop an extra $4000 on Lithium batteries instead of lead acid ones.
• Pound for pound, lithium batteries have 3 times the capacity of lead acid. Not only would the S10 have extended range but it would also have increased weight carrying capacity. Since the batteries are the last thing I would buy when converting to an electric car, I have time to give this more thought.

The Results Are In! 
I'm going to try my hand at converting another electric car. Now I just need to find a cheap S10 with a zillion miles but a good frame, body and interior.

Lawn Mowers

I prefer electric lawn mowers to gas ones because they are super quiet, they don't pollute and I don't have to buy gas.   The only problem is corded electric models are a hassle and cordless mowers, due to limited battery capacity are only suited for small lawns.  For my ½ acre lot, neither option was acceptable. 

John's Rule #106:  If you can't find what you need, invent it!  

My lawn mower uses a 115 Amp-Hour, 12V deep cycle battery to power a 750 watt AC inverter that in turn powers a 157 VDC motor that has been connected to a bridge rectifier so it can run off of normal household AC power. Granted, it’s kind of stupid to convert DC to AC and back again to DC but that’s exactly when I have done.  The down side to my Frankenstein mower is that it's not self propelled and the deep cycle battery on board weighs a lot.
I'm still looking for the parts so I can connect a 12 volt motor to the back wheels.  In the mean time, mowing the lawn is good exercise. 
When my mower is drawing the maximum amount of power, the 12 volt battery  has to provide 62.5Amps continuously.  That gives me 1 hour 50 minutes before the battery goes totally dead.  This isn't a problem because my lawn only takes about an hour to mow and the mower doesn't always draw maximum power anyway. 

750 watts is only 1 horsepower but an electric motor is way more efficient than a gas engine. My lawn mower has the equivalent grass cutting abilities of a 3.5HP gas mower. 

This season, I have been baffled as to why it has been taking me 2 deep cycle batteries to mow my large lawn instead of only one.  Last season, I would boast that I could mow the entire lawn and only use ½ the capacity of my battery. 
Having my lawn recently aerated left thousands of little grass poops all over the lawn.  You should have seen my 2-year-old daughter, (thinking they were dog poops), try to walk across the lawn with ever-so-much care and caution. She had such a cute, disgusted and worried look on her face.

Mowing the lawn after aeration, the occasional lawn poop would hit the blade with a “twang” before being obliterated. It was fun to do but it wrecked havoc on the blade.

I sharpened my spoon sharp lawnmower blade to that of a spinning-death steak-knife.  The revived blade helped a moderate amount but my mower would still bog down in long, thick grass.  What is going on here?  
Then it occurred to me that the first time I mowed the lawn this season, I lowered the blade down to a 2" cutting height and I neglected to raise it back up to the original height of 2 ¾”. After raising the blade back up, I was able to mow the entire lawn using only one battery. 

After measuring my lawn mower's performance (with a Kill-A-Watt meter) and applying some crude calculations, I arrived at these surprising statistics: 

• A sharp lawnmower blade will use 33% less energy than a dull blade. 
• Raising up the deck only ¾", extends the range of the lawn mower by 40%. 

This is true for gas or electric lawn mowers. 

Unless you have a putting green, there is no practical reason to cut the lawn short.  I found that cutting the grass a little bit longer will in return yield several benefits:

1. A healthier lawn. 
2. The lawn remains greener with less water.   
3. Takes less time to cut. 
4. The lawn mower does a better job of cutting/mulching the grass.  
5. And amazingly, it uses less energy. 

After collecting data from a few of my neighbors about their lawn mowers, I found that a ½ acre lot can be mowed using 1 tank of gas (0.4 gallons) at a fuel cost of $1.50/week, spanning a 21 week grass mowing season.  Using these figures, my $84 battery needs to last at least 2.4 years to pay for itself.  
Well, it has already done that since I am into my 3rd year of using this battery. 

After mowing, the battery is only 45% discharged.  Conveniently, this also extends the overall life of the battery. 

The x-axis is the discharge percentage of the battery.  The y-axis is the number of charge cycles. 

Amazingly, only discharging 45% will allow a battery to last 1200 charge cycles, (1200/21 = 57 years).  Due to the cheapness of my battery, I'm conservatively estimating mine will only last for 250 charge cycles or 12 years.  We'll see. 

My ½ acre lot with ~¼ acre of lawn requires ~0.7KWH of energy from the battery and 1KWH of electricity to recharge the battery. That’s about 10 cents per week to mow my lawn.   Since solar panels provide all my electricity, my lawn mower is effectively solar powered.

Nerdy Fun Facts:

• There are 125,000 BTU in a gallon of gasoline and 3412 BTU in a KWH.  An equivalent gallon of electricity would be 36.6KWH. 
• Gasoline has an amazingly high energy density.  Storing the same amount of electrical energy (as a gallon of gas) into cheap deep cycle lead acid batteries would require 2036 lbs of batteries.  Since you can never retrieve 100% of the energy stored in a battery, you would actually need more than this. 
• A typical push mower has a 0.4 gallon gas tank.  My electric lawn mower uses 15 times less energy to mow a lawn than a gas mower. 
• The deep cycle battery on my lawn mower weighs 78 lbs but only releases the (usable) equivalent energy contained in 2¼ ounces of gas. 
• The ideal grass cutting height (in my opinion) is: 2 ¾“.  This saves the most energy but still keeps the grass relatively short and manicured. 
• You can measure your own lawn mower's cutting height by placing the lawn mower on flat cement.  Measure from the ground to the bottom of the lawn mower deck.  The cutting height will be 0" -¼" higher than this. 

Update: 
The weak link in the run-time for my lawn mower is the inverter's low voltage cutoff circuit.  Once the voltage at the inverter drops below 10.7 volts, it's game over.  After drawing 60 amps from a 12 volt battery for 50 minutes continuously, the battery voltage (while drawing a heavy load) is about 10.9 volts.  The voltage drop across the 18" cables running from the battery to the inverter is about 0.2 volts with a resistance of 0.004 ohms.  After beefing up the cables with much larger gauge wire, the voltage drop is only 0.03volts and only 0.0007 ohms. 

The small red and black cables were what was used before.  I added the larger red and blue cables.

I can always tell when my lawn mower battery is around 55% because (while pulling 60 amps), that is when my inverter starts squawking about low voltage.  Before beefing up the cables, I was never able to discharge the batteries below 50% before the inverter's low voltage circuit pulled the plug on my mowing "fun".  After I reduced the voltage drop across my cables, I can complete the entire lawn without the inverter voicing a single complaint.  This effectively bought me an extra 25 minutes of run-time.

I am toying with the idea of swapping out my 78 lb lead acid battery for a 28 lb, 100AHR LiFe battery.  But at over $500, can I justify the cost?  Dang!  Science can be expensive. 

Increase Your Gas Mileage by 41%, No! Wait!! By Over 63%

Last month I wrote an article about how you can increase your gas mileage 41%.  I'm here to report that this figure was grossly inaccurate.  The truth is, you can actually increase your gas mileage by over 63%.  I know this because I have done it, reliably.  As I learn more techniques and tips, my gas mileage just keeps getting better and better.  Achieving such dramatic efficiencies in a normal internal combustion engine does not require doing anything drastic, dangerous or illegal.  There are some people that use more"shady" techniques like drafting off of semi trucks.  While this may increase their fuel economy, it is illegal, dangerous and doesn't reduce the environmental impact of burning fossil fuels.  It merely passes the costs of fuel from the drafter to the truck driver. 
I was able to increase my performance an additional 22% (above the 41% I was already achieving) for a total of a 63% increase.  This latest 22% requires installing a tool that provides instant feedback about how driving style is effecting the fuel economy and changing your driving style to attain maximum fuel economy.  Instant feedback is the key to such dramatic fuel savings. 
One such tool is the vacuum gauge.  It will work on any vehicle, costs you less than $30 yet it will save you hundreds at the pump. 

The Vacuum Gauge that I installed in my 1992 Honda Accord displays a vacuum pressure ranging from 0 to 30 Hg (inches of Mercury).

The gauge's vacuum hose connects to the extra port on my intake manifold. How convenient that there was an extra port. No cutting or splicing involved.

Vacuum pressure at the intake manifold is loosely correlated to fuel efficiency.  The harder you push on the gas pedal, the lower the vacuum pressure and the more gas your vehicle will consume. For my car, the gauge displays 0Hg at full throttle and as high as 24Hg when it is coasting in gear.  My car gets much, much higher gas mileage when I drive in a manner that keeps the vacuum pressure as high as possible.  Typically this is in the 12-18Hg range. 
Cruising on the freeway I can maintain a vacuum pressure of 15Hg.  The vacuum gauge also doubles as an inclinometer.  Driving up a hill (like an overpass) while maintaining a constant speed, the vacuum gauge will drop down toward 10Hg.  Driving down the other side, the gauge jumps up toward 20Hg. 
By driving so that the vacuum gauge remains at a constant vacuum pressure, your speed may fluctuate, your acceleration will be smaller but your fuel efficiency will be much, much higher. 

From last week's blog post, we learned that when a vehicle is driven at a low engine RPM, fuel economy goes way up.  But pushing too hard on the gas pedal while in an upper gear will bog the engine down, potentially negating any fuel savings (automatic transmissions don't suffer from this but they don't get as good of fuel economy either). 
Fortunately, the vacuum gauge will indicate if you are bogging the engine or not.  If you are cruising along a 30mph street in 4th or 5th gear, with very little throttle, the vacuum gauge will still display a nominal value.  But even an unnoticeable increase in throttle position will bog the engine down and kill the vacuum pressure at the intake manifold, increasing fuel consumption.  Having this feedback is key to driving at the lowest possible RPM while still preventing bogging and preserving fuel efficiency. 

For vehicles newer than 1996, the superior version of the vacuum gauge is the Scan Gauge II. 
Not only does the Scan Gauge calculate and display dozens of parameters like miles per gallon, gallons per hour, total trip cost, closed loop status and engine temperatures, it can also be programed with vehicle specific gauges for your particular vehicle.  For example, for my wife's Chevy Venture, it will display horse power, engine torque, air/fuel ratio, transmission temperature and even an obscure parameter called “knock retard”, (how many degrees of ignition time advance must be removed to compensate for low octane fuel, thus preventing engine knocking).  For hybrid vehicles, the Scan Gauge will display "state of charge" and the regenerative charge rate of the battery system. 

Efficient driving habits will save you money at the pump in exchange for a slight increase in commute time. 

For me, slower acceleration and reduced cruising speed adds 3 additional minutes to my 20 mile commute.  In exchange, I only have to pay to fill up for gas tank every 4 weeks instead of every two and a half.  Not a bad compromise in my opinion.

Questions/Myths about Vehicles answered by John

Lindsey from Texas:  What is the difference between driving with low air pressure in the tires and normal pressure?  What is the ideal tire pressure? 
I ran several tests using a 2002 Chevy Venture mini-van and a Scan Gauge tool.  I lowered the tire pressure in each tire to 25psi and took several samples while driving on a flat road at 30mph.  I then pumped up each tire to 32psi and took several more measurements.  Finally, I pumped all the tires up to 42psi and drove around for several more measurements.  Here are my results: 

While driving at 30mph on a flat road. 
Tire Pressure    GPH (avg)    MPG (avg)
25psi                  1.55gph          20.0mpg
32psi                  1.23gph          23.85mpg
42psi                  0.92gph          33.08mpg

Having low tire pressure will reduce your fuel efficiency over 40%.  Keeping your tires at close to the maximum pressure written on the sidewall will give you the most fuel efficiency. 

Niko from California:  Does driving with the windows open hurt fuel efficiency? 
To my suprise, and amazingly, no!   I had to run this test over and over just to make sure.  A few of my results even show an improvement in fuel efficiency when rolling down the windows.  My "baffle you" theory is modern vehicles have a more aerodynamic shape.  This creates a bubble of air around the vehicle as it drives.  Rolling down the window does not slow the car down because the bubble preserves the aerodynamics. 

Speed       Windows up    Windows down
75mph       29.5mpg           29.8mpg
70mph       25.5mpg           24.0mpg
65mph       26.1mpg           28.1mpg
60mph       29.0mpg           32.5mpg
55mph       33.7mpg           31.5mpg

Karen from Oregon:  Is it bad to start the car with the A/C on?  Also will it hurt the car to keep the A/C on when driving up a big hill? 
Starting the car with the A/C turned on doesn't hurt it at all.  If you leave the A/C button on all the time, when you start your engine, the vehicle computer delays the A/C from turning on.  My van takes about ¼ second after the engine is turned on before it automatically engages the A/C compressor.  Cars are pretty smart.  On steep hills, when the engine is really working hard, most cars automatically turn off the A/C to save performance.  Next time you are driving up a steep hill this summer, leave the A/C on.  You may notice the air coming out of your vents won't be as cool as it should be.  This is an indication that your vehicle is switching off the A/C so all available power can be used for getting up the hill.  Or maybe it's about to blow up (j/k).  By the way, it is normal for the air conditioner compressor turns itself on and all the time.  But during the times when it is on, it only uses 3 horsepower.  On average it will draw even less power. 
A/C On/off       idle/parked    30mph          55mph
no A/C              4.9hp              18.7hp          27.0hp     
max A/C           7.1hp               22.8hp         30.0hp
delta                 2.2hp                4.1hp           3.0hp

What is the ideal, most fuel efficient cruising RPM?  What is the best gear to be in?
For manual transmissions, lower is better as long as you don’t bog the engine down.  This will depend on the type of vehicle you drive and its transmission gear ratio.  For an automatic, a lighter push on the gas pedal will send it into a higher gear
Look at how the fuel efficiency is effected by what gear you are in.  These measurements were taken while driving on a flat road at a constant 55mph and manually shifting into that particular gear. 

Gear               rpm             mpg
Overdrive       1680rpm      35mpg
3rd Gear         2380rpm      30mph
2nd Gear        3700rpm      18mpg

Angie from Utah:  Does it really save fuel to drive the speed limit?  Driving faster will get you there quicker, but is the extra burn worth it?  5 mph over?  10 mph over? 
Not only will driving the speed limit save you tons on gas, it will save you speeding tickets and in turn, give you lower insurance premiums.  Plus, its safer. 
MPG of 2002 Chevy Venture
mph             mpg
55mph          35mpg
65mph          30mpg
75mph          22mpg

For a 15 mile commute, driving 75mph takes 12 minutes.  Driving 65mph takes 13:50 minutes.  Driving 55mph takes 16:21. 
Assuming you drive to work and back 5 days a week for a year, you will drive 7800 miles a year just driving back and forth to work.  Using the figures above and assuming $4/gallon gas:
Driving 75 will require 354 gallons/year and cost $1418. 
Driving 65 will require 260 gallons/year and cost $1040. 
Driving 55 will require 222 gallons/year and cost $891. 
At the cost of less than 4 ½ minutes each way, you will save $527 in gas each year by driving 55mph instead of 75mph.  That's $44/month. 
All those extra minutes spent and dollars saved, add up to a wage of $13.97/hour, tax free. 

For a short daily commute, a few minutes really doesn't matter much.  But what if you are going on a 600 mile road-trip?  In this case, the time really adds up.  To drive 55mph for 600 miles would take 10.9 hours.  Driving 75mph would only take 8 hours.  At a savings of $40 in gas, your trip will cost you nearly 3 extra hours.  To me, a vehicle full of sticky, tired, crying kids easily justifies driving as fast as possible. 

What is the most fuel efficient speed to travel at? 
That depends slightly on the type of vehicle you drive.  For highway driving, going the speed limit or even slightly under will always achieve the maximum fuel savings. 
Here is a plot of the fuel efficiency vs. speed for my 2002 Chevy Venture. 
mph         mpg
40            32
45            35
50            36
55            35
60            31
65            30
70            26
75            22

While driving 50 is the most fuel efficient speed, it's also painfully slow for a long road-trip.  Driving 55mph during a short commute doesn't use much more fuel and will still get you there in a reasonable time frame.

Ben from Utah: When parked, when is the break-even point of fuel consumption between idling and turning off the engine then restarting it? 
Using a scan gauge tool in my 2002 Chevy Venture van, I measured how much fuel it takes to idle for about 5 minutes.  Then I measured the amount of fuel it takes to stop/start the engine 10 times.  

Idling in park burns 0.3 gallons of gas every hour or about 1 penny of gas every 30 seconds. 
Stopping/starting the engine10 times consumes about 5 cents of gas.  That averages out to 1/2 cent of gas per stop/start.  The break even point is 15 seconds. 

How much fuel is consumed leaving the car in drive vs. putting it in neutral while waiting?
A warmed up engine with the transmission in drive (but not moving anywhere) will consume 0.31 gallons per hour. In neutral will consume 0.29 gph. 
A cold start engine burns 0.8 gph at idle while in park but it steadily drops as the engine warms up.

How much gas does it take to run the A/C or heater on full blast?
When idling, running max A/C uses 59% more gas than just idling alone. Running the heater on max uses 6% more gas. Running the heater and defrost uses 56% more gas, (the defrost also uses the A/C compressor). The rear defrost (which is just a resistive heater in the back windshield) used 3-6% more fuel.

Engine mode                         gph
Idling with max heater             0.34gph
Idling with max A/C                0.51gph
Idling with no heat or A/C       0.32gph

Cruising on the freeway, my Chevy Venture only uses 27HP. With max A/C, it uses 30HP. This is only about 10-12% increase. Well worth it on a hot summer day in my opinion. 

How much horse power does a vehicle use to drive 55mph?  65mph?  75mph?
Speed      horsepower      mpg
55            26.1                   34.3
65            35.1                   30.3
75            47.0                   25.0

Gavin, New Zealand:  Does the octane in fuel affect your gas mileage?  Which one works out to be cheaper overall?  A higher octane fuel will not increase your fuel efficiency one bit.  Don't waste your money.  If you car doesn't care, neither should you. 

Octane          Speed         Throttle position     Ignition timing       gph
85                  30 mph       5                                44                           1.06
92                  30 mph       5                                40                           1.00
85                  in park        0                                18                           0.30
92                  in park        0                                18                           0.31

How does extra weight affect the fuel efficiency of a vehicle? 
I measured the fuel efficiency of my Chevy Venture and then I removed all the seats, weighed them and measured the fuel efficiency again. 
The curb weight of a 2002 Chevy Venture is 3699 lbs.  The weight of 4 removable seats and other misc stuff I had in the van is 288 lbs.  Removing all that stuff, I reduced the van's weight about 8% and at the same time, increased its fuel efficiency by 10%. 

Weight                                   gph           mph
Van with seats = 3699lbs       0.92          33.08
Van no seats   =  3410lbs       0.83         36.54

Does cruise control help fuel efficiency?  It will help your vehicle achieve better fuel efficiency if you tend to drive too fast.  Otherwise, you can do better just by paying attention to the terrain of the road and allowing the vehicle to slightly speed up/ slow down accordingly.  Cruise control tries to maintain a constant speed no matter what the terrain.  This is a very in-efficient way to drive.  Slowing down while climbing hills (even an overpass) and speeding up on the downhills will save more fuel. 

A more fuel efficient cruise control would be a throttle position sensor control, or an intake manifold vacuum pressure control or a fuel gallons per hour control.