Measuring fuel economy

The EPA measures the fuel economy of most cars on the road in the US at fueleconomy.gov These estimates are computed using a fairly detailed procedure:

• Drive the vehicle on a dynamometer, whose resistance is adjusted to account for air resistance and vehicle weight.
• For city estimates: drive the vehicle for about 30 minutes at an average speed of 21 mph, covering a distance of 11 miles.
• For highway estimates: drive the vehicle for about 10 minutes, at an average speed of 48 mpg, covering a distance of 10 miles.
• Additional specialized tests are done for high-speed driving, air conditioning, and so on.

For the city estimate, note that the 21 mph average speed can be considerably higher than the speed you might experience in heavy stop-and-go traffic. If you live in an area where there are persistent traffic delays, you can easily fall far short of this 21 mph average, so your gas mileage will likely be worse than the EPA's estimate.

Let's look at a concrete example: the 2013 VW Golf, 2.5 L gasoline engine:

The EPA estimates that this car will achieve 24 MPG in the city, 31 MPG on the highway, and a "blended" rate of 26 MPG. This blended rate depends on your personal mix of city/highway driving: the EPA default is 45% highway, 55% city, but you can personalize it to reflect your circumstances.

Note that, although the default city/highway mix is pretty close to 50%, the blended MPG is a lot closer to the city estimate than the highway estimate. I discuss this in a separate blog post, but the basic idea is that, for any car, the city MPG matters a lot more than the highway MPG towards the final blended MPG.

Are these EPA estimates "accurate", that is will they match what you, as the driver of the car, will actually see in terms of how much fuel you are using? The EPA website allows users to report their own MPG numbers, using the "Your MPG" feature. For the car above, you can see that 1 driver of this particular car reports an actual MPG of 27, which is not far from the official EPA estimate.

Using data from only 1 driver is not very accurate, however, so there are websites which aggregate far more data, from many more drivers, to provide more accurate fuel consumption estimates:

• fuelly (World-wide, although seems to be heavily skewed towards the US)
• honestjohn (UK): uses imperial gallon in its MPG definition, which is different from a US gallon
• spritmonitor (Germany): uses liters/100km for fuel efficiency

In the case of the above vehicle, fuelly reports a comparable average of 26.2 MPG, from 11 different cars:

When evaluating a new car, be sure to look at both the EPA estimates as well as user-reported estimates, to get a more accurate picture of the vehicle's fuel efficiency. Better yet, report your own fuel efficiency estimates at the appropriate site above, so that everyone else can benefit from your information.

Diesel and CO2

Diesel engines are more fuel efficient than gasoline equivalents: it takes less Diesel fuel to drive any distance vs. a a comparable gasoline engine. However, Diesel engines produce more CO2 from burning Diesel fuel vs. a comparable gasoline engine. If you simply compare fuel consumption, Diesel engines appear to be much better than gasoline equivalents. However, if you compare CO2 emissions, Diesel engines, although still better than gasoline equivalents, are a lot closer.

All numbers in this post were obtained from fueleconomy.gov I try to compare equivalent engines as much as possible (i.e. similar engine volume and horse-power, on the same model car).

• 2014 Audi Q5
• 3.0 TDI: 27 MPG, 385 CO2 grams/mile
• 3.0 Gasoline: 21 MPG, 429 CO2 grams/mile
• If you drive 10000 miles in one year:
• Diesel will burn 22% (=21/27) fewer gallons of gas overall
• But Diesel will only produce 11% (=385/429) less CO2 overall
• 2014 VW Golf
• 2.0 TDI: 34 MPG, 297 CO2 grams/mile
• 2.5 Gasoline: 26 MPG, 342 CO2 grams/mile
• Diesel will burn 23% fewer gallons of gas, but produce 13% less CO2 overall
• 2014 VW Jetta SportWagen
• 2.0 TDI: 33 MPG, 310 CO2 grams/mile
• 2.5 Gasoline: 26 MPG, 342 CO2 grams/mile
• Diesel will burn 21% fewer gallons of gas, but produce 9% less CO2 overall
• 2014 VW Passat
• 2.0 TDI: 34 MPG, 290 CO2 grams/mile
• 2.5 Gasoline: 25 MPG, 350 CO2 grams/mile
• Diesel will burn 26% fewer gallons of gas, but produce 17% less CO2 overall
• 2014 Chevrolet Cruze
• 2.0 Diesel = 33 MPG, 307 CO2 grams/mile
• 1.8 Gasoline = 27 MPG, 333 CO2 grams/mile
• Diesel will burn 18% fewer gallons of gas, but produce 8% less CO2 overall

Diesel engines do reduce overall CO2 emissions. However, the gains are more modest that it appears from fuel consumption alone. Additional gains can be obtained from reducing the size of the engine, and the weight of the car.

Beware of MPG bringing gifts

Background

The standard measure for a vehicle's fuel efficiency in the US is MPG (Miles per Gallon). It represents how many miles you can drive this vehicle on one gallon of gas. Vehicles have two important MPG numbers: one for city driving and another for highway driving. It is also possible to compute a blended MPG number, as a function of how much you drive in the city vs. the highway. Here's an example from fueleconomy.gov for the 2013 Honda Civic:

In contrast, the standard measure for fuel efficiency in many European countries is "liters per 100 kilometers". It represents how many liters of fuel you need to drive this vehicle for 100km. For example, the 2013 VW Golf has an advertised fuel efficiency of 3.2 liters (of Diesel fuel) per 100 kilometers. 

MPG and liters/100km are interchangeable. Knowing that:

• 1 mile = 1.6 kilometers
• 1 gallon = 3.78 liters

The 2013 Honda Civic has a fuel efficiency of:

• 32 MPG = (32 * 1.6) / 3.78 = 13.54 km / liter
• To drive 100 km, you need 100 / 13.54 = 7.38 liters / 100 km

Conversely, the 2013 VW Golf has a fuel efficiency of:

• 3.2 liters / 100 km = (3.2 / 3.78) / (100 / 1.6) = 0.84 gallons / 62.5 miles
• This car will drive 74.4 MPG on one gallon of gas

Is there a reason to prefer MPG vs. liters/100km?

Suppose you want to upgrade your car to a newer model that burns less fuel. Which upgrade will result in greater fuel savings:

• (A) Going from a car with 18 MPG to a car with 20 MPG?
• (B) Going from a car with 30 MPG to a car with 32 MPG?

Choice (A) results in more than twice as much fuel saved than choice (B), even though the absolute MPG difference is the same (2 MPG), because the MPG varies inversely with gallons (and therefore cost).

 

Say you drive 10,000 miles in one year, and gas costs $4/gallon.

• Choice (A)
• At 18 MPG you spend: 10000 / 18 * 4 = $2222 on gas
• At 20 MPG you spend: 10000 / 20 * 4 = $2000 on gas
• Net savings: $222 (= 55 fewer gallons of fuel)
• Choice (B)
• At 30 MPG you spend: 10000 / 30 * 4 = $1333 on gas
• At 32 MPG you spend: 10000 / 32 * 4 = $1250 on gas
• Net savings: $83 (= 20 fewer gallons of fuel)

You save more than twice as much fuel by going from 18 to 20 MPG than by going from 30 to 32 MPG.

Now let's look at the same calculation in liters/100km:

• Choice (A)
• 18 MPG = 13.06 liters/100km
• 20 MPG = 11.76 liters/100km
• Net savings: 1.3 fewer liters of fuel/100km
• Choice (B)
• 30 MPG = 7.84 liters/100km
• 32 MPG = 7.35 liters/100km
• Net savings: 0.5 fewer liters of fuel/100km

In liters/100km, Choice (A) saves more than twice as much fuel than Choice (B), because liters/100km varies proportionally with gallons (and therefore cost).

Conclusion

Equal improvements in MPG do not translate to equal savings in fuel. Upgrading an 18 MPG car to a 20 MPG car makes a lot more sense than upgrading a 30 MPG car to a 32 MPG car, even though the difference is 2 MPG in both cases. The same goes for upgrading an 18 MPG car to a 19.8 MPG car vs. a 30 MPG car to a 33 MPG car, even though the difference is 10% in both cases.

Liters/100km is unambiguous about fuel savings. Upgrading a car from 8 to 7 liters/100km will result in the same fuel savings as upgrading a car from 15 to 14 liters/100km.

By the same reasoning, the city MPG number for a car matters more than the highway MPG number. A 2013 Honda Civic driven for 5000 miles in the city (at 28 MPG) and 5000 miles on the highway (at 39 MPG) will have an overall fuel economy of 32, which is a lot closer to the city number than to the highway number.

If your goal is to reduce fuel consumption by purchasing a more efficient car, be sure to convert from MPG to liters/100km in order to get a more accurate picture!

No Impact Man

We watched "No Impact Man" a few days ago. I was really looking forward to the movie, as I've often thought about the same exact themes in my own life.

I found the movie to be informative, entertaining, but I also fell that it fell short in a number of important ways.

The basic idea of the movie is to ask if it's possible to live in such a way that you produce as little impact as possible on the environment around you. Impact is defined in a number of ways:

• Trash
• Personal transportation (= direct pollution)
• Food transportation, electricity generation (= indirect pollution)
• Buying stuff (= consumer culture, which also leads to direct and indirect pollution)

The movie explores how much we need in order to live a happy life vs. how much we want for the sake of convenience, or because modern society has conditioned us for to want it. The protagonist and his family take some of the following steps to reduce their impact:

• Don't buy new things as much as possible.
• Instead, buy old things that someone else no longer wants.
• For instance, no new clothes, buy all clothes used.
• This reduces direct impact (no packaging trash) and indirect impact (no resources consumed to produce new items).
• This is a reaction to modern consumer culture.
• Reuse things as much as possible.
• For instance, no kleenex (use a handkerchief), no toilet paper (use textile rags that can be washed and reused).
• This is a reaction to the culture of using something once and throwing it away.
• Buy food locally.
• Locally here is defined as a 250 mile radius around NY.
• This is a reaction to the fact that modern agriculture is very oil-intensive: food is produced using fertilizer (generally, oil-derived) and transported from far away (also using oil).
• Stop using electricity.
• Live by sun-light alone, use candles at night.
• Electricity generation is very dirty, more than 50% of electricity in the world today comes from coal.
• Don't drive anywhere.
• Bike or walk.

Overall, the family manages to pull through this year long experiment and find that their life, while radically changed in many ways, was still largely happy and enjoyable. For instance, they traded TV for more quality time with friends and family; they lost weight and got into much better physical shape from eating less sugar-rich highly-processed foods and biking/walking everywhere; and so on.

What the movie did not address, unfortunately, is that such a life-style, while possible, depends on a number of unstated assumptions:

1. Time. You need lots more time to walk everywhere, cook meals from raw materials (as opposed to buying them pre-processed), and so on. In my own life, time is a scarce commodity, even though I'm keenly aware of it and try to budget it carefully.
2. Money. You have to pay the rent, pretty much no matter where you live. The movie hardly explored the fact that the wife had a high-paying job that covered their bills, and allowed the husband to basically not work for a year and stay home to conduct this experiment (with all that entails).
3. Distance. To make such a lifestyle possible, you have to be able to walk or bike reasonable distances to get food, or to go to work, etc. This is possible in NY, since it's one of the densest cities in the world. This may not be possible in a more rural, or even less dense city somewhere else.
4. Luck. Trading the fridge turned out to be very difficult because their food spoiled fast. In my opinion, the family was lucky that they didn't get sick during the second half of the movie. They probably mitigated this by buying their food daily or every other day and not storing it over any length of time. This is possible, but requires even more time investment.

Some of these issues could be addressed by living on a self-sufficient farm -- a mostly closed-loop system that provides for most of your needs, without needing to go outside it for other stuff. It's much less clear to me if an impact-free life is possible in a modern urban environment, especially one that depends on fossil fuel for energy. After all, your food must come from outside the city, and for that you basically need oil for transportation.

Even with these shortcomings, the movie was still entertaining and informative. I liked the fact that the movie took a very optimistic tone and genuinely tried to look at these problems and see what solutions might exist.

The movie also highlighted the fact that one person's actions do matter. Many people get discouraged by the fact that they might be alone in a sea of other people who don't care or are unwilling to change, so why bother? The protagonist answers, and I agree: "Being optimistic [...] is the most radical political act there is."

In terms of our own life, it prompted me to think harder about what other changes could we make to reduce our impact:

• Could we reduce single-use items (like Kleenex, shaving cream cans) in favor of multiple-use items (like handkerchiefs, shaving soap)?
• Could we go to the farmers market down the street every week instead of buying so much packaged food at grocery stores?
• Could we reduce TV/Internet use in favor of other activities?
• Could we buy more stuff used (craigslist, antique stores, etc.) instead of new?

Given where we live and where my job is located, it is unlikely that I will be able to reduce the impact of transportation, at least for the time being. But I remain optimistic.

Vampire energy

Standby power (a.k.a. "vampire energy") is the power consumed by devices that are simply plugged in but not really turned on. For example, a device with a remote control, uses a trickle of power to listen for the signal from the remote control in order to fully turn on. The amount of standby power is typically small. For example, a modern TV will consume 0.5W in standby, but can consume over 200W when fully turned on.


So why care about vampire energy? The reason is that when you add it all up over all electrical devices in your household, it turns out to be a really big number. In the UK, for example, in 2006, estimates are that 8% of all electrical energy consumed goes to standby power. In the US, this would amount to almost 20 average-size power-plants!

I was curious to see how much standby power we use at home. The public utility recently upgraded our power-meter to a digital smart-meter, so it's easy to just read out the wattage consumed at any point in time. I waited until everyone was asleep and everything in the house was turned off (but still in standby). I also made sure that the fridge did not have its compressor running at the time.

The number? 85W.

Wow. This is surprisingly high! At 24h/day, 30days/month, this works out to 61kWh (around $8/month at today's energy prices). Given that we consume 300-400kWh average total per month, this is between 15-20% of our total consumption! For an average US household, which consumes closer to 1000kWh/month, this would be around 5%, which is about what the British study revealed.

How could we possibly consume 85W in standby mode? I broke it down by circuit, by switching the circuit breakers on and off and studying the devices connected to each circuit:

• 29W for the DSL modem + wireless router.
• 11W for the gas heater
• 8W for various stuff in the bedrooms (alarm clock, cell phone chargers, night-light, etc.)
• 5W for the garage door remote
• 4W for the PC
• 4W for the microwave oven
• 3W for the washing machine
• 3W for the electric oven
  
• 18W for other stuff I couldn't track down precisely

The modem + wireless router are an interesting case: while I could turn them off at night (from, say, midnight to 6am), they do need to be on during the day given the level of internet use at our place. So the 29W really needs to be pro-rated down to 7W to indicate it's standby power for only about 6 hours. Another option is to get a combination router + modem that consumes less power by itself, but that's harder since I'm picky about the routers I like. :)

The astonishing one is the gas heater, at 11W. I suspect this is because the heater uses an electric element to fire up the gas burner, and this electric element has to be always on. There isn't much I can do about that, and I don't really feel like messing with the heater since it a large, expensive, and scary device.

The remaining ones are relatively small. For things like the microwave oven and washer, I could get power strip with an external switch that fully turns off the appliance, but the cost of the power strip will easily outweigh the savings from the electricity for a few years at least.

I should also probably track down the remaining 18W and see where it's wasted, but I have a feeling it's going to be small amounts here and there, not one significant consumer.

The conclusion? The best way to eliminate standby power is to do it at the source (that is design the device to not consume standby energy as much as possible). The second best way is to use timers or switched power strips and force devices off. This only makes sense if the device is of a certain kind (indoor device, that doesn't suffer problems when turned off, like the gas heater). For the remaining devices, the ideal situation is to combine them all on one (or a few) power strips and force them off with a switch. If the devices are spread throughout the house (like the microwave oven in the kitchen and the washing machine in the garage), this is not really possible.

In the end there's little I can do about this. How frustrating. :(

Here comes the Sun

Renewable energy is becoming increasingly more visible in our society. The recent oil and food price spikes, the impending opening of the Northwest Passage, the coral bleaching in the ocean all point to the fact that we consume fossil fuels at unsustainable rates, and are changing our environment for the worse. Changing to renewable energy makes both economic and moral sense.

Of the many ways to produce renewable energy, solar is a big focus these days. In the US, the federal government has a generous subsidy, which looks to be extended in the following years. In California, there is an important state subsidy, and a generous San Francisco subsidy. (Lest you wonder, even foggy San Francisco gets plenty of sun.) In California, grid electricity is produced by PG&E, mostly using natural gas. The solar incentives aim to encourage private individuals and businesses to install solar panels and feed electricity back into the grid, thereby offsetting some of their consumption. If the solar installation produces more than the individual consumes, their PG&E bill can be negative (they get a check each month). In most cases, the solar panels would offset some fraction of the consumption, typically the expensive kWh's, more on this below. Here's what this looks like (video credit Solar City): One natural question at this point is: why feed the electricity back into the grid, instead of running your house directly on it? For one, the solar panels only work during the day. To have electricity at night, you would need to install a fairly large set of batteries to store excess energy. Batteries are very costly and often an environmental nightmare (containing acid or rare metals that are expensive to synthesize or extract). Second, solar panels energy output varies considerably between seasons (in the northern hemisphere, the sun's efficiency is very different in the winter vs. the summer), or even between days (on a cold, stormy day with cloudy skies, the output is quite different than on a warm, sunny day). Third, most electrical appliances expect a steady electrical output (110v, with small error margins), which are difficult to maintain even from a good battery bank. The goal of solar is not to necessarily replace the grid entirely, but rather to offset enough to substantially reduce our pollution and dependence on fossil fuels. Solar cells convert sunlight into electrical current. The conversion is pretty inefficient, around 20% of sunlight gets transformed to electricity. However, given that direct sunlight on average produces 120 W/square meter, on an 8 hour sunny summer day we can recover almost 200 kWh of electricity using a modest 1 square meter solar cell array. Solar panels produce DC current, but the grid operates on AC, so the output from solar panels has to be converted to AC using an inverter. This exacts another small efficiency penalty (around 20%), and has to be tuned to the size of the solar array. Solar panels are expensive (largely because they're not yet mass produced, so they can't leverage economies of scale). Absent generous subsidies, in order for them to make financial sense, they have to be sized as a function of household consumption. As of the time of this posting, PG&E uses a tiered price structure for electricity: the first 256 kWh are the cheapest, at 11c. If you consume more than 256 kWh, the price increases quickly, up to more than triple:

• 11c/kWh - 0 -100% baseline (256 kWh)
• 13c/kWh - 100-130% baseline
• 22c/kWh - 130-200% baseline
• 31c/kWh - 200-300% baseline
• 35c/kWh - over 300% baseline

For a residence, it makes sense to look at a year's worth of electricity bills and figure out what is the consumption pattern. In a warm area like California, odds are you'll use lots of electricity in the summer (A/C) and less in the winter when it's cooler, but not cold enough to require heating. One solar strategy is to get a solar array big enough to offset only the expensive kWh in the summer (those at 30c or more). There is no magic formula here, each house is different, although in very broad terms a 2.5-3.5 kW solar array should do the trick for a lot of average-size homes. Before embarking on a solar project, it makes sense to first optimize your consumption using the cheapest tools: replace all incandescent bulbs with CFLs, configure computers and TVs to go into standby when not used, increase the temperature of the fridge and freezer, insulate the attic to keep cold air in, and so on. This can have a dramatic effect on your electrical consumption, as much as 30% reduction! At this point, take stock of your usage and size the solar array as a function of the new energy consumption numbers. Go solar!

Gas vs. Electric

The two major sources of energy used in California homes are gas and electricity. In our home, for example, the stove uses gas, the water heater uses gas, the washer/dryer use electricity to spin and gas to heat, and the house heater uses an electric motor to push air over a metal tube heated with gas. It's no accident that the California's major utility company is called PG&E: Pacific Gas & Electric. I recently stumbled upon an interesting article about energy efficiency in home appliances. Among others, the article recommends using an electric room heater instead of running the home gas heater. I was generally under the impression that "gas is better" because it's cheaper and pollutes less (gas burns cleaner, whereas electricity is generally produced in coal burning power-plants that are far dirtier). So I decided to do some some research into the matter. For the baseline, I looked at the January bill from 2008 and 2007 (January is the coldest month around here, when one would expect the bill to be the highest, and this past January was especially cold):

• January 2007
• Gas: 53 therms @ $1.13
• Electric: 133 KWh @ $0.11
• Total: $74
• January 2008
• Gas: 49 therms @ $1.14
• Electric: 136 KWh @ $0.11
• Total: $71

Based on our usage patterns, I would estimate that roughly half the gas we consume is for heating the air in the home. So what would it look like if we used an electric Vornado heater instead? Based on the Vornado's specifications, it uses between 750 and 1500 Watts, depending on the temperature setting. I measured the wattage, and we're between 600 and 1200 Watts, since we never set it at the max (it gets too hot). For the purposes of this simple calculation, I'll assume the average consumption is 1000 Watts = 1 KW. We use the heater for a maximum of 5 hours per night (5pm - 10pm), so in one month, that's 30 * 5h * 1KW = 150 KWh. With this in mind, our January 2008 bill would have looked like:

• Gas: 25 therms @ $1.14
• Electric: 286 KWh @ $0.11
• Total: $56

That's a 22% reduction in cost! The heater would pay for itself in 3 months. In terms of quality of life, we've started spending more time in one room, with the door closed, in order to keep the heat inside. The Vornado works best in such a closed environment, and it often heats up the room far more than the gas heater. It takes a bit longer to get the room warm, but once it's warm, it consumes very little electricity to keep it that. I spoke to some colleagues at work about this, and the general consensus is that if you can thermally insulate individual rooms in the house, it makes sense to individually heat them using electricity, otherwise a gas heater is more efficient and economical for the entire house. What about the environmental impact? It turns out that in California, most electricity is also produced using natural gas, which is a reasonably clean way to do it. Some electric energy is lost in transmission, but it appears to be reasonably small (average 10%). The big upside, however, is that California is aggressively pursuing electricity generation using renewable energy: solar, wind, and so on, in which case electricity is definitely the way to go. You also have the option to offset the carbon used by your consumption, which is a nice bonus. In typical maverick fashion, San Francisco wants to become fully energy independent, and there are projects underway for that. In my case, it seems that electricity makes more sense than gas. At the end of the day, however, the most important thing is to be aware, measure the impact, and think about what makes sense. More on that, however, in another post.