A more nuanced view of BEVs

To date I haven’t been kind to BEVs.  With the appearance three weeks ago of the Chevy Bolt on dealer floors I’ve come to realize that my objections to BEVs were based on the fact that every production model BEV was either too expensive or had too short a range.

The point of electric vehicles is that they don’t emit CO2, a growing hazard for climate today.  Some approaches to climate mitigation have been denounced by their opponents as too painful to consider, much as one would not go to a medieval dentist for fear that the cure might be more painful than the disease.

The climate counterpart of modern dentistry is painless mitigation. At least in first world countries austerity will be no more effective for global warming than diet has proved for obesity.

Painless climate mitigation is coming slowly, give it time. As an early adopter I’ve put 7000 miles since May on my Toyota Mirai.  I have found it a pure joy to drive. There are just enough hydrogen stations in California for me to be able go pretty much wherever I want within the triangle bounded by Ukiah, Reno, and upper Baja.  A fill-up takes only five minutes every EPA 312 miles (best I’ve managed is 370 miles). California has 25 hydrogen stations now, almost all commissioned within the last 18 months. In 2013 California Assembly Bill 8 authorized the California Energy Commission to build a hundred at an expenditure rate of $20M a year, so there are 75 to go. The expectation is that private investment will step in once this bootstrapping phase has created a market.

Toyota and Honda have committed to ramping up delivery to California of respectively the Mirai and the Clarity. In their price bracket and range the only BEV competitor today is the Chevy Bolt which I test drove just two days ago. The past three weeks Peninsula Chevrolet Cadillac has been selling them like hot cakes a mere two miles from where I’m typing this at my beach house. (GM picked them over their nearby Salinas counterpart because last year they sold ten times as many Volts.) My wife’s 1987 MBZ 300TD needs to find a new home (even though it should still be good for another 300,000 miles) because she’d like something with more modern safety features. Interesting options for us are the Honda Clarity FCV and the GM Bolt BEV, with respective EPA ranges of 366 and 238 miles, both leasing for around $370/mo (the Mirai is $349/mo but we already have one).

While there’s no shortage of sour grapes about these cars online, most of the complaints concern mere teething problems. I’m convinced these EVs are the cars of the future, with batteries for commuting and short trips and fuel cells for that plus longer trips, infrastructure permitting. (BEVs are ok for long trips provided you set aside time for refueling, e.g. at meal stops.) Germany and Japan are way ahead of the US with FCV infrastructure and other European countries, in particular Nordic ones. are starting to put their toe in the water.

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  1.  Green hydrogen production via low-temperature alkane cracking

    (The following distinguishes renewable and green energy.  Renewable energy does not entail consumption of fossil fuels, for example wind, solar, hydro, biofuels, etc. In contrast green energy does not entail emission of CO2, for example wind, solar, hydro, and carbon-based fuels with carbon capture and sequestration (CCS).  In particular biofuels are renewable but without CCS are not green as understood here.)

    Hydrogen can be captured as an otherwise unwanted byproduct of various industrial processes.  Even if those processes are not themselves green, using such hydrogen instead of discarding it could arguably be considered green.

    Hydrogen can also be manufactured intentionally by steam reforming of methane, with oxides of carbon (CO and CO2) as byproducts.  Typically these are emitted to the atmosphere, making this method not green.

    Another commonly considered method is electrolysis of water producing hydrogen and oxygen.  This method is thermodynamically unfavorable, particularly at room temperature but less so at higher temperatures.  When the electricity comes directly from a solar panel the resulting hydrogen is clearly green.  However the 22% efficiency of solar panels in combination with the thermodynamic inefficiency of electrolysis makes this method expensive relative to the other uses to which the electricity could be put.

    Although fossil fuels are not renewable, they are still plentiful today and are projected to remain so for many decades to come, making them of considerable interest for the foreseeable future.  Natural gas (largely methane, CH4), propane (largely C3H8), gasoline (largely iso-octane, C8H18), and other alkanes (CnH{2n+2}) are refined from fossil fuels and and their principal constituent elements are carbon and hydrogen.  Hydrogen produced by removing and sequestering the carbon is green hydrogen as understood here.  Understood as a chemical reaction, this process can be expressed as the reversible reaction CnH{2n+2} <=> nC + (n+1)H2, for example CH4 <=> C + 2H2 in the case of methane.

    A straightforward implementation of this process is thermolysis or heat cracking carried out in the absence of other chemicals, especially avoiding oxygen!   In the case of methane, at 500 C and atmospheric pressure an equilibrium is reached in which half the methane has separated into hydrogen and carbon.  By 1000 C essentially all of the CH4 has been “cracked”.

    A problem with this method of producing hydrogen is that at these high temperatures the carbon tends to form carbon nanotubes that stick to the furnace orifices, eventually clogging them.  Carbon nanotubes are notoriously strong, greatly complicating cleaning them out.  Recently teams at IASS in Germany and KIT in Sweden have bubbled methane through molten tin at temperatures of 1000 C and above, with the carbon nanotubes collecting at the surface as an easily removed scum.  While this method seems very promising technically, its economics are unlikely to become clear for several years.

    We are therefore interested in alternative approaches to removing the carbon from alkanes that does not involve molten metals.  We are currently exploring methods that avoid production of nanotubes, in particular at low temperatures and low pressures.

  2. Home hydrogen

    Currently there are no practical methods of producing hydrogen at home.  As one application of the foregoing project, we would like to make a prototype of a 34″ tall device that sits on the ground directly beside the Toyota Mirai’s fuel receptacle and pumps purified hydrogen into it.

    Since natural gas is more prevalent than either propane or gasoline in typical suburban residences, and moreover is 25% hydrogen by weight, the feedstock for this device would be natural gas.   The electricity needed for the prototype can come from the house mains initially.  A more elaborate model would have a small rechargeable battery and a fuel cell powered by the generated hydrogen; alternatively it could draw its power from the Mirai’s 12 volt DC 10A power outlet.

    The pumping pressure could be anywhere from 20 to 70 MPa.   20 MPa would suffice for a range of 90 miles, 35 MPa for 155 miles, and 70 MPa for the Mirai’s EPA-rated 312 miles.

  3. Fast US crossing in an Electric Vehicle

    The record for an electric vehicle crossing the US from Los Angeles to New York, 3011 miles, is 58 hours and 55 minutes.  This included 12 hours and 48 minutes of charging time.   A fuel cell vehicle could easily shave ten hours off the recharging time.  The obstacle is the lack of infrastructure.

    Continuous production of hydrogen from a readily obtainable alkane would permit much faster refueling at either fast-fill CNG stations, purchase of bottled propane, or refilling a gasoline tank.  The tradeoffs between these alkanes need to be explored.

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Internal combustion engine vehicles, ICEVs, are powered by burning hydrocarbons like gasoline, diesel, ethanol, compressed natural gas, propane, and mixtures thereof. They pollute by emitting byproducts of the combustion, mainly CO2 as the principal cause of long-term global warming but also with traces of much more toxic emissions that can impact the immediate neighborhood more short-term [1].

Moreover ICEVs are inefficient. This is because they exploit the thermodynamics of a heat engine that converts a temperature difference into mechanical energy. Nicolas Carnot’s theorem, which he proved in 1824, states that the efficiency of any such heat engine is at most the ratio of that difference to the higher temperature. In order to have anywhere near 100% efficiency the lower temperature would need to be near absolute zero, -273 C. This might be feasible on frigid Pluto far from the Sun’s warmth, but not on Earth, and especially not in an ICE whose lower temperature as a heat engine is that of its exhaust as it leaves the cylinder! Car engines are typically well below even the already low Carnot limit, being about 20-30% efficient.

Thermodynamics is the last place efficiency experts should be looking.


Electric vehicles, EVs, are powered more indirectly, namely by converting the chemical energy of an on-board fuel of one kind or another into electrical energy. In turn an electric motor converts this electrical energy (very efficiently with the present state of the art) into mechanical energy.

An important aspect of this chemical-to-electrical conversion is that it is slow, limited by the impracticalities of driving at over 150 mph on most roads or doing twenty consecutive 0-60 mph timing runs. It therefore does not do the sort of damage to the chemicals that faster conversion can cause. This is relevant to the first essential difference below between BEVs and FCVs.

By avoiding combustion EVs are far less polluting than ICEVs, and by avoiding thermodynamics they are also much more efficient.

All this however is merely background for the immediate question of the form the chemical energy of an EV should best take.


There are two main choices: battery chemistry and fuel cell chemistry.

Critics of FCVs like Joe Romm, Tony Seba, and the Wikipedia article on fuel cell vehicles have long lists of faults that would seem to suggest many differences from BEVs. By contrast the Wikipedia article on battery electric vehicles was unable to find a single fault with BEVs.

But some of those differences are not essential. Yes there are many more Tesla supercharger stations than hydrogen stations, but that’s more an accident of timing and deployment than an essential difference. Likewise there are many more BEVs than FCVs, but that too is timing and the ratio has been shrinking rapidly over the past 12 months.


I see only two properties of BEVs and FCVs that distinguish them in anything like an essential way. In decreasing order of obviousness:

1. Refueling a BEV entails transforming electrical energy to chemical.

2. The FCV’s independent fuel cell and tank decouples its available power from its energy capacity.

The first distinction is significant because such conversion in either direction without permanent damage is slow, as noted above in the case of conversion from chemical to electrical. Charging a battery without seriously degrading its life it can take an hour or more.

In contrast the energy in an FCV’s fuel is chemical to begin with. It can therefore be pumped into an FCV’s tank much faster than a BEV’s battery can be recharged, in minutes rather than hours (but not seconds because compressing requires energy and is also thermodynamically inefficient).

The second distinction is significant because the available power from a battery is in proportion to its energy capacity. Hence if you want a BEV with a high capacity battery for longer range, you must employ a battery capable of delivering higher power, whether you want that power or not!

The Tesla S P100D’s Mirai-beating EPA range of 315 miles [2] is achieved with a battery capable of delivering 500 kW (670 HP) [3], good for a 0-60 mph time of 2.5 seconds. Tesla makes this almost record-breaking power a feature of its long-range vehicles, as well it should.

However the price for this mind-boggling power that comes with merely respectable range is a 1200 lb battery and an MSRP of $134,000 [4].   On the one hand, among cars that quick the Tesla may well be the cheapest.  On the other, if all you want is its 315-mile range you can’t get it without paying for luxury-level performance even though you may never use it.

In contrast the Mirai achieves its EPA range of 312 miles with a 204 lb tank (including 11 lb of hydrogen). Independently it achieves its 114 kW (153 HP) of power with a 124 lb fuel cell, good for a 0-60 mph time of 9.0 seconds that would have been considered very sporty a mere two decades ago [5]. The drive train weight therefore totals 328 lbs (408 with the traction battery) vs. the Tesla’s 1200 lb battery pack. And the 2017 Mirai leases for a quite modest $349/month, likely about a quarter of what the P100D would lease for were Tesla to offer a lease option.


There is a third difference, involving efficiency, but it remains to be seen whether this is an essential one. The lithium-ion batteries typically used in BEVs have an efficiency about 1.5 times that of a fuel cell. This is easily deduced from the fact that one gasoline gallon equivalent (GGE) of electrical energy (33.7 kWh) can take a Leaf or Tesla about 1.5 times as far as one GGE of hydrogen energy (about 1 kg).

So what? Both types of EV are far more efficient than ICEVs. What does that extra 1.5 factor do for the BEV owner? Obviously it doesn’t buy more range: Tesla had to go way out on a limb to match the Mirai’s range.

Which leaves cost.

So what’s the cost of hydrogen energy vs. electrical energy?

Solar energy is currently wholesaling at about 3 cents/kWh in several parts of the world. 1 kg of hydrogen has 33.7 kWh. Hence in order to match electrical power, taking into account the 1.5 efficiency factor, hydrogen would need to wholesale at $0.03*33.7/1.5 = $0.67/kg.

Is this possible? Anaerobe Systems’ Mike Cox believes his biofuel approach can achieve $0.50/kg. The jury is out, probably for quite a while.

But this overlooks the capital cost of a 300-mile range BEV. While its electrical fuel may well be cheaper than the FCV’s hydrogen fuel, mile for mile, how many years before that savings makes up for the BEV’s insanely higher price?

5? 10? The car’s lifetime? Your lifetime?

You do the math.


Meanwhile there is a positive side to inefficiency that ICEVs get in spades during winter: vast amounts of heat. While FCVs don’t get that much heat, they get more than enough to keep both cabin and equipment toasty warm. This is why FCVs have much noisier radiator fans than BEVs.

There are two advantages of this inefficiency warmth. First, there is no need to steal power from the fuel cell’s electricity to heat the cabin. (The Mirai’s electrically heated seats are a different matter, though once the cabin has warmed up they shouldn’t be needed.) Second, just as a warm battery runs more efficiently than a cold one, so does a warm fuel cell run more efficiently.  The fuel cell’s advantage in the cold is that its inefficiency provides it with free heat that can keep it operating at peak efficiency, whereas any heat used to keep a battery warm cannot be had for free but as with heating the cabin must be stolen from the battery’s electrical power.

Winter is as cruel to BEVs as it is harmless to FCVs, which start easily and run efficiently at twenty below. Winter is also tough on ICEVs, which rely on batteries to start: the cold sucks the life out of the 12 volt battery needed to start the car, and ignition is harder in a cold cylinder.

The bottom line for efficiency then is that while the jury is out on the cost benefit of the BEV’s higher efficiency, there is no doubt that the FCV’s inefficiency is a great benefit in cold climates.


For short range EVs, batteries may be fine. However for long range EVs FCVs have two clear advantages over BEVs: faster refuelling, and an independence of range and performance that greatly reduces both weight and cost. Whether the greater efficiency of BEVs is of any value remains to be seen.


[1] http://www.ehhi.org/exhaust06.pdf

[2] Page 32 of http://www.fueleconomy.gov/feg/pdfs/guides/FEG2016.pdf

[3] https://electrek.co/…/tesla-model-s-p90d-ludicrous-power-o…/

[4] https://evannex.com/…/how-elon-musk-just-built-the-quickest…

[5] (a) Road & Track credits the Leaf with 9.4 seconds. (b) One of the eight Mirais that drove from Rocklin to Truckee for the opening ceremony on August 27 was Brad’s from Santa Rosa (didn’t catch his last name but he’s in real estate). He’d bought his Mirai just weeks earlier after learning that a BEV with the same range would cost twice as much. When I told him the Mirai’s 0-60 mph time was 9 seconds he was adamant that his Mirai was much quicker than that. I didn’t want to contradict him because that’s how the Mirai feels to me too, even if it isn’t true.

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If a Mirai ran on liquid hydrogen, how many miles per gallon would it get?

The shocking answer is 18.

But wait. The EPA says the Mirai has a fuel economy of 67 MPGe. How could being liquid make that much of a difference. Is it because it is so cold it doesn’t have much energy?

No, the difference is between MPG and MPGe. Whereas a gallon is a unit of volume, a Gasoline Gallon Equivalent, 1 GGE, is a unit of energy defined as 33.7 kWh. 1 GGE of liquid hydrogen has a volume of 3.7 gallons. So one gallon of it can only take you 67/3.7 = 18 miles.

This comes about because the density of liquid hydrogen is 0.07085 kg/L (kilograms per liter). Water is more than 14 times as dense. A gallon of liquid hydrogen therefore weighs 0.07085*3.785 = 0.268 kg and can only get you 67*0.268 = 18 miles.

But then what’s the point of liquid hydrogen if isn’t dense?

Well, the combined capacity of the Mirai’s two tanks is 122.4 liters. That volume of hydrogen at room temperature and 70 MPa pressure weighs 5 kg, a density of 5/122.4 = 0.04085 kg/L. Liquid hydrogen is therefore 0.07085/0.04085 = 1.73 times as dense as H70.

So, light as it is, liquid hydrogen requires only 58% of the volume of H70. It does this by reducing the distance between hydrogen molecules by a factor of the cube root of 1.73, namely 1.2.

So then why have these two big tanks taking up space under and behind the rear seats when liquid hydrogen can shave 42% off its volume?

Well, the trouble with liquid hydrogen is that at atmospheric pressure it must be cooled to 20 K (-253 C) to liquefy. A temperature so close to absolute zero might be ok for a huge truck but in an ordinary passenger car it would greatly increase both its complexity and cost.

If compressed, would less cooling suffice? No, compressing it to 13 atmospheres (1.3 MPa) only raises that temperature by 13 degrees.

That temperature and pressure, 33 K and 13 atmospheres, is called the critical point of hydrogen. Beyond that point there is no such thing as liquid hydrogen because there is no sharp demarcation between liquid and gas, just a compressible “supercritical” (not to be confused with political campaigns) fluid that can be shrunk by cooling, compressing, or both. (But with enough of that shrinking hydrogen solidifies, i.e. freezes.)

To save on the cost and complexity of liquid hydrogen the Mirai relies on pressure alone to save space.

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Home Hydrogen

I believe the technology for home hydrogen fueling that will win out in the end will be a 1-5 cubic foot unit operating overnight, taking as inputs natural gas and electricity, and outputting hydrogen at a rate of 1-5 kg per 10 hours depending on the size (and therefore cost) of the unit. Every 36 seconds 1-5 grams of H2 at room temperature are pumped into the car at a pressure proportional to how much is already in the tank, and 3-15 grams of dry soot (broadly construed to include carbon powder, graphite, etc.) are added to a container at the bottom. Every week your waste management company collects the accumulated 25 lbs of soot per FCV separately from your garbage and recycling.

No tanks external to the car, no emissions of any gas. Assuming no subsidies, operating costs per kg should come to whatever 100 cf of gas costs, around $1.50 today, plus $0.90 for the electricity ($2.50 if you fill up during peak hours in the afternoon).  $2.40 for 60 miles worth of H2 comes to 4 cents a mile. With gasoline at $2.40 a gallon you’d need a 60 mpg ICEV to match that, and you’d also miss out on the convenience of home refueling while getting frowns from your neighbors for continuing to emit 20 lbs of CO2 for every gallon you use.

Although no such unit is available today, progress towards it has been made at IASS and KIT.   I see no obvious technical or economic obstacle to refining their current design to meet the above specifications, perhaps based on a different approach to methane cracking.

The 5 cf unit will fit comfortably in the Mirai’s 20 cf trunk, allowing you to take it on trips. A network of 1500 overnight filling stations at motels, hotels, and campsites every 20 miles along the major interstates would permit travel throughout the country at 300 miles a day, and further when FCVs with larger tanks come on the market.  Each station would consist of one or more parking spots each with a hookup providing gas and electricity and a bin in which to deposit the night’s accumulation of 33 lbs of soot in the morning, emptied by the hotel’s waste management service.  Stations could start with a single spot booked like a hotel room.  With the current small FCV population, two cars are unlikely to want the same spot and when they do the second can just book a nearby spot.  This very simple infrastructure could be built up gradually nationwide with only a few million dollars a year total, comparable to the cost of installing just a few $2M hydrogen stations around the country. Both will happen, but the former could easily happen faster.

Eventually FCVs will incorporate the unit the same way BEVs incorporate their charger today. Since the construction and operating costs of a regular hydrogen station will be at least a hundred times that of an overnight gas-and-electric parking spot, it is reasonable to expect the nation’s infrastructure to grow at the rate of a hundred such spots for every regular station, with the latter spaced apart ten times the spacing of the spots.

PG&E currently offers special electricity rates to BEV owners charging at home, so if they did the same for home hydrogen your cost could be as low as 2 cents a mile. This could be reduced to zero with a federal carbon tax of $0.001 on gasoline, rising to $0.002 when the milestone of a million FCVs in the US is reached. A portion of this tax would be paid to the utilities to compensate them for giving you free gas and electricity for your FCV. (Maybe this is already happening for their BEV subsidies.)

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The Mirai’s air conditioning and fuel consumption

1. 𝐔𝐬𝐞𝐬

As with any car with air conditioning, the Mirai’s ac needs fuel to operate. How much fuel does it consume? This question is of interest to fuel economy: what hit does the MPGe take with the ac on? In the Mirai’s case it is also of interest when keeping the car cool using the ac while you’re away: what does it cost per hour, and will sufficient range remain when you return?

If the latter application of the fuel cell seems like a wasteful perversion, using it to power your house would have to seem even more so. You won’t think so if you aim the leftmost vent at the steering wheel before you leave: on your return you’ll be able to hold the left side of the wheel without burning your fingers—no such luck however with the right side that I can see, sadly.

The ac might even save a life if you accidentally forget and lock your kid or pet in a hot car for even a fraction of an hour, see the gruesome statistics for kids at http://noheatstroke.org/ . But only if accidentally: don’t rely on the ac to save a life, as occasionally ac’s fail, sometimes with fatal consequences.

2. 𝐀𝐧 𝐞𝐱𝐩𝐞𝐫𝐢𝐦𝐞𝐧𝐭

To answer these questions I parked in the sun at 2 pm yesterday with the windows up. The outside air temperature (measured in the shade as usual) was 80 F, a typical mid-July afternoon in Palo Alto. (Temperature of anything exposed to direct sunlight is not well defined as it depends very heavily on the nature of the surface so exposed.)

Now pick whichever one of the following two paragraphs is more to your taste and skip the other. 🙂

1. The car got hot inside. The car is 16′ long: 5′ for the white hood and trunk, 5′ for the black roof, and 6′ for the transparent windshield and rear window. Therefore around 20% more solar heat enters the car through those two windows than heats the black roof, and then there’s also the side windows. The white hood and trunk contribute next to nothing to cabin temperature.

2. With the sun’s rays beating down through the glass onto the dash, rear parcel shelf, steering wheel, seats, and other upholstery, all black, these interior surfaces became painfully hot to the touch. And although the windows are transparent to short wave radiation (SWR) from the 5700 K sun and are therefore not heated by it, they are opaque, i.e. black, to the 20x longer wavelength radiation (LWR) from the car’s black interior surfaces at 300-340 K. In this way the solar radiation that doesn’t heat the car’s exterior by virtue of passing through the windows, instead heats all surfaces in the interior including the inside surface of the windows. The radiation trapped in this way accumulates to make the car literally a hothouse (aka greenhouse).

Opening the windows a crack goes some way to allowing that accumulated heat to escape, namely via convection.

Running the ac does a far better job than cracking the windows. (To lock the doors while the ac is on, use the mechanical key, which as a side effect renders the remote inoperative for all but the trunk until you unlock the doors again with the mechanical key.)

But at what cost?

3. 𝐓𝐡𝐞 𝐤𝐞𝐲 𝐦𝐞𝐚𝐬𝐮𝐫𝐞𝐦𝐞𝐧𝐭

To get some idea of this I set the climate control (both driver and passenger) to 75 F, and ran the ac for two hours with the car stationary, with ECO ac off during 2-3 pm and on during 3-4.

The ac consumed a total of 0.1 kg, evenly split between the two hours, hence 0.05 kg/hour. I found no detectable difference with ECO on, either in consumption or passenger comfort, but perhaps it might make a difference on a more cloudy day.

Using 33.7 kWh = 115,000 BTU (in agreement with each other to within a surprising 0.01%) for the energy of 1 kg of hydrogen, that comes to 0.05*33.7 = 1.685 kWh or 5750 BTU. Room air conditioners need about 30 BTU per square foot so the Mirai’s ac would be adequate for a 200 s.f. bedroom. This might seem like overkill for the Mirai’s cabin area of only 50 s.f. but bear in mind that the average bedroom does not sit out baking in the hot sun on a hot road or parking lot trapping LWR: I know of no http://noheatstroke.org/ for kids’ bedrooms.

4. 𝐂𝐨𝐧𝐜𝐥𝐮𝐬𝐢𝐨𝐧𝐬

𝐶𝑜𝑛𝑐𝑙𝑢𝑠𝑖𝑜𝑛 1. If at 65 mph you get 65 MPGe (miles per kg) without the ac on, that means the car uses 1 kg/h. Turning on the ac therefore raises that to 1.05 kg/h, i.e. 5% more fuel consumed.

My 50 mph (indicated) run that I reported here the other day over 43 miles (therefore taking time 43/50 = 0.86 hours) got 100 MPGe with the ac on. That’s 50/100 = 0.5 kg/h, hence a consumption of 0.5*.86 = 0.43 kg. But in 0.86 hours the ac uses 0.05*.86 = 0.043 kg, so 10% more fuel consumed with the ac on than with it off! (This morning I repeated that run at an indicated 57 mph with the ac off and got 82 MPGe, a big drop from 100 MPGe and showing that turning off the ac doesn’t come near compensating for going 7 mph faster.)

So as a percentage the ac hurts more when you’re trying to be economical by driving more slowly. That’s obvious when you consider that the ac burns fuel at a steady rate whereas slow driving reduces overall fuel consumption while also taking longer to reach your destination and so requiring more ac.

𝐶𝑜𝑛𝑐𝑙𝑢𝑠𝑖𝑜𝑛 2. If you use the ac to keep your car cool while parked, 1 kg of hydrogen will last 1/.05 = 20 hours. A full tank can therefore keep it cool for 5*20 = 100 hours. A quick way to see if you have plenty of hydrogen is to look at the gas gauge at upper left of the dash, which seems to be marked off in 8 divisions of about 0.6 kg each with perhaps a reserve in the neighborhood of 1/3 kg, just guessing. So each of those 8 divisions should give 0.6*20 = 12 hours of cooling. But don’t forget to allow enough extra in order to remain well in range of a hydrogen station.

5. 𝐌𝐞𝐭𝐡𝐨𝐝𝐨𝐥𝐨𝐠𝐲

Burning question: where did I get 0.05 kg/h for ac consumption? Certainly not from watching the range decrease. The way the Mirai calculates range is far too inscrutable to infer any reliable kg numbers!

What I did was to scroll right on the multi-function display to (i) (Information), then scroll down to the Drive Monitor and watch the MPGe going down for one or both of TRIP A and TRIP B (selected on the left of the steering wheel). This works provided there are already some miles traveled on that meter (shown at the lower far left), otherwise MPGe will remain at zero. The more miles the better the precision; fortunately A had 145.6 miles though B only had 28.5. Neither changed during the experiment because I didn’t go anywhere, but their MPGe’s in the Drive Monitor kept decreasing during the 2 hours of ac operation.

Here’s the key bit of magic for obtaining the figure of 0.05 kg/h.

𝐌𝐢𝐥𝐞𝐬 𝐝𝐢𝐯𝐢𝐝𝐞𝐝 𝐛𝐲 𝐌𝐏𝐆𝐞 𝐠𝐢𝐯𝐞𝐬 𝐤𝐠 𝐨𝐟 𝐇2 𝐜𝐨𝐧𝐬𝐮𝐦𝐞𝐝 𝐬𝐢𝐧𝐜𝐞 𝐫𝐞𝐬𝐞𝐭𝐭𝐢𝐧𝐠 𝐭𝐡𝐚𝐭 𝐦𝐞𝐭𝐞𝐫

So as MPGe declines, that quotient steadily rises, reflecting the ongoing consumption of hydrogen. The amount it rises in one hour is the amount of hydrogen consumed per hour. Need I say more? (No, but that never stopped me before. 😉 )

6. 𝐔𝐧𝐜𝐞𝐫𝐭𝐚𝐢𝐧𝐭𝐲 𝐚𝐧𝐚𝐥𝐲𝐬𝐢𝐬

As MPGe is only given to 3 digits of significance you get very little accuracy after just 10 minutes. An hour of ac however is enough to tell that consumption per hour is in the range 0.045 to 0.055 kg/h and that 0.05 kg/h should therefore be good to about 10%. (One way to improve accuracy is to wait until each change of the MPGe and immediately record the time on a stopwatch to the nearest second, but I didn’t think of that until later.)

Following my usual practice I had reset TRIP A after the last fill-up. I noticed that when 2.5 kg had been consumed the gas gauge at upper left showed slightly less than half empty. This suggested that “half-empty” on the gas gauge was not really exactly 2.5 kg down but perhaps only 2.4 kg down. Since the gauge is divided into 8 parts, that suggests 0.6 kg per division rather than 5/8 = 0.625.

7. 𝐂𝐚𝐯𝐞𝐚𝐭𝐬

All the above numbers are only preliminary – my estimates may be inaccurate, and your mileage may vary. More data from other Mirai owners might improve these numbers, or for that matter from Toyota who may know them by now to three decimal places.

Also I’ve assumed a perfect odometer but from what I’ve seen so far the odometer is not calibrated to be spot on until the tires are almost bald, at which time one wheel revolution will take the car forward about 78.5″. It therefore must be reading about 3% low when the tires are new, taking the car forward 81″. (Some day someone will invent a more accurate odometer that’s independent of tire wear; in the meantime I rely on GPS for accuracy.)

And lastly many manufacturers add 2 mph to their speedometer reading.

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The Mirai’s MPGe at 50 mph

The US Environmental Protection Agency (EPA) has estimated the combined city/highway miles-per-gallon-equivalent (MPGe) of the Toyota Mirai at 67 MPGe, and the range of its 5 kg tank as 312 miles.  Since neither of these numbers are terribly useful without knowing the speed and traffic conditions, I decided yesterday to get a better idea of how the MPGe depended on speed.

I’d already observed that the EPA’s 67 MPGe number was almost exactly reached on a trip on June 25 from the Saratoga station down US-101 to the Santa Barbara station, and that on average over the 1700 miles I’d driven the car so far I was getting 57 MPGe, due mainly to highway driving at the prevailing traffic speed of 70-80 mph on US-101 between Palo Alto and Pacific Grove.  What I didn’t know however was whether a higher MPGe was possible by driving at a steady 50 mph on country-road conditions unimpeded by stalled traffic and traffic lights.

So on the afternoon of Yellow Pig Day 2016 (July 17) I spent a couple of hours on various roads within 30 miles of Palo Alto with the cruise control set at 50 mph.   Being a hot day (85 F) I set the A/C to ECO and the climate control to 77 F, which was much more comfortable than with the A/C off.

The Mirai provides a number of gauges for assessing fuel economy, selectable using some of the buttons on the right of the steering wheel.   The one I found most useful (I’ll call it the TSF gauge) shows the Elapsed Time (HH:MM), Average Speed (MPH), and Average Fuel Economy (MPGe) for the selected trip meter.  There are four trip meters, A, B, P (my name for it), and ODO.  These can be selected by pressing the TRIP button on the steering wheel, and the corresponding miles-traveled is shown underneath the speedometer, independently of whether the TSF gauge is selected.  A and B can each be reset to zero by holding down the TRIP button on the steering wheel for two seconds.   P is reset to zero at Power-on, and ODO is the odometer.  For any of the four trip meters the product of miles-traveled and Average Fuel Economy on the TSF gauge gives the kilograms of hydrogen consumed since that meter was reset.

(It would have been more useful to have P reset automatically at fill-up than at Power-on, though resetting A manually at fill-up is almost as good provided you remember each time and don’t mind using up one of the two trip meters in this way.  With that trip meter, subtracting kilograms-consumed (as calculated above) from the tank’s capacity of 5 kg gives the remaining hydrogen.  I find remaining-hydrogen far more useful than the Mirai’s estimate of remaining range because the latter is based on the last eight fill-ups and is therefore not very useful when you can predict better than the car can what your likely driving pattern will be for the remaining tank contents.

After experimenting inconclusively on I-280 (too hilly to be informative), I settled on Foothill Expressway for my first experiment.  I started from Page Mill Expressway and ended a mile or so after passing under CA-85 where the 45 mph speed limit dropped to 40 mph.   After the first couple of miles the TSF gauge showed around 100 MPGe but then I started encountering red lights which soon knocked it down to the 70s.  Since the goal was to measure economy at a steady 50 mph I discarded that and decided that 50 mph on CA-85 would be easier to maintain.  So I went back to 85, and since the Saratoga hydrogen station was only six miles away I filled up there so as to start the measurement with a full tank, with the plan being to refill afterwards and compare the pump’s and car’s estimates of how much hydrogen had been used.

As usual I reset Trip Meter A at the station.  However to get a better idea of the MPGe at a sustained 50 mph I waited until I was on 85 and going at 50 mph before resetting Trip Meter B.

My plan was to drive to Gilroy and back, a round trip of 70 miles.  However at Cochrane Road, 21.8 miles, I could see stalled traffic ahead so I cut the trip short, took the Cochrane Road exit, and returned to the Saratoga station.

At Cochrane Road the MPGe was 99.6, which blew me away.  I figured the trip must have been downhill some of the way for the MPGe to be so high, and therefore expected the return to be lower.   So I was enormously surprised to see it actually increase on the way back, reaching 100.9 MPGe as I pulled into the Saratoga station 0.7 miles from the exit off CA-85.

My Mirai’s odometer is consistently showing 3% less distance traveled than what Microsoft Streets and Trips (one of the better navigators for PCs) claims. So going by S&T my real MPGe should be 104 MPGe. Not that there’s a big difference there when compared with the EPA’s city/highway figure of 67 MPGe, nor with my own result of 56.6 MPGe averaged over 1800 miles of lead-footed keeping-up-with-traffic driving since acquisition.

When I filled up on the return to Saratoga the pump showed 0.446 kg. The car judged the round trip at 46.0 miles (the 45.0 miles on the gauge plus 1.0 miles from the station to where I reset Trip Meter B on reaching 50 mph on CA-85), so the 100.9 MPGe figure means that the car thought it had consumed 46.0*100.9 = 0.456 kg, a difference of 0.01 kg or 1/3 oz (and .01/.446 = 2.2%) more than the pump’s reading. This difference is well within the pump’s claimed accuracy of 5%, but it’s only 0.2% of the tank’s capacity and therefore insignificant. I’ll start keeping an eye on the car’s MPGe number in future to get a better idea of car-pump agreement on longer trips.

On the social-interaction aspect of driving at 50 mph in the rightmost lane of a 65 mph freeway, I have only two observations. In the three hours I spent on these experiments, only one motorist expressed any arguably overt hostility and that was a taxi driver who briefly honked at me as he passed. (And for all I know he was just appreciating my PROTON POWERED license plate. Honking doesn’t offer quite the same explanatory bandwidth as Tolstoy’s War and Peace.)

What I did find however was a number of leeches that attached themselves almost to my rear bumper, each for several miles. My first thought was that this was how they complained, and my blood pressure went up a bit. But then it occurred to me that maybe some motorists are happy not only to drive slowly for better mileage but in the draft of another slow driver for even better mileage. This thought was reinforced by the observation that sometimes I had a caravan of several cars tightly anchored to me, none seizing any of the frequent opportunities to merge left into a faster lane.

So what does this mean for the Mirai’s so-called range? Well, if you believe Streets and Trips over the Mirai’s odometer, and you can avoid traffic jams and traffic lights, and that all 5 kg of the Mirai’s two tanks is available, and you aren’t experiencing major altitude changes, then by setting your cruise control to 50 mph and setting your A/C to a tolerable level and driving off into the sunset you should be able to cruise into the next hydrogen station after driving 520 miles. Or if you drive out and back from say Truckee or San Juan Capistrano with your cruise control at 50 mph you should be able to go as far as 260 miles (252 by the Mirai’s odometer until further confirmation) before turning around.

Suggestion to Toyota: a trip meter that resets at each fill-up. This could replace the one that resets each time you start the engine, which doesn’t seem terribly useful. Currently I use Trip Meter A for that but this entails resetting it at the station.

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