Guest Post: Efficiency or Range? You Can’t Have Both.

The i3 is the most efficient production car available today

Every now and then I have a reader send me an article they wrote and ask if I'd like to post it here. Usually it's not exactly what I'm looking for and politely explain why I won't be posting it and thank them for sending it nonetheless.  Occasionally I'll get something interesting though, like the post below which was sent to me by Robert Kasper. I think it's particularly timely since just last week I posted the Tesla/BMW comparison piece and I think this is an interesting follow up to it.  I hope you enjoy:                    

  Efficiency or Range?  You Can’t Have Both.

…But Advanced Technology Can Help.

By Rob Kasper

In the world of electric vehicles, whether Battery Electric Vehicles (BEVs) or Plug-in Hybrid Electric Vehicles (PHEVs), there is a clear trade off between range and efficiency.  For a given technology, efficiency suffers as range increases due to the weight of not only additional battery capacity, but the increased structure and volume to haul that capacity around.  Now that there are a significant number of plug-in vehicles being manufactured, and a recognized standard to test them, we can identify trends.  Consider Table 1 and Figure 1, a plot of efficiency (as measured in EPA MPGe) vs. range in miles for 2014 plug-in electric vehicles measured by the EPA.  They are grouped into Battery Electric Vehicles (BEVs) and Plug-in Hybrid Electric Vehicles, and further identified as either conventional or advanced technology design and construction.  Conventional technology is generally characterized by a manufacturer’s use of an existing gasoline powered platform modified for battery electric drive, steel frame construction and cladding, and standard battery technology.  Advanced technology is generally characterized by a clean sheet, purpose built EV design, extensive use of aluminum or aluminum plus Carbon Fiber Reinforced Plastic (CFRP) for weight savings, higher energy density lithium ion battery packs, with the bonus of performance equivalent to or exceeding the best of conventional technology plug-in vehicles.

Figure 1: Efficiency vs. Range

Table 1: EPA Electric Range and MPGe

Beyond the obvious observation that the price of greater range is lower efficiency within a given technology, it is important to note the significance of advancing technology.  The ground-up EV design, significantly lighter weight construction, and advanced battery technology of the BMW i3 and Tesla Model S push the blue trend line significantly up and to the right of conventional BEVs’ green trend line.  As significant is the single data point (in purple) representing the only advanced technology PHEV currently available – The BMW i3 REx.  Not only is it capable of greater efficiency and far more range than any conventional PHEV (the red trend line), it is more efficient than all but two conventional BEVs, with only slightly less range than all but the most inefficient conventional BEVs.

It is this outlier of a data point, the BMW i3 REx that might best help illustrate why a smart means of increasing the range of an EV may not necessarily be to add more battery capacity.   Battery energy is clean and well suited for powering vehicles for relatively short-range transportation but due to its weight and lengthy charge times, inefficient and inconvenient for long distances.   On the other hand, the benefits of energy density and convenience make gasoline/diesel energy better suited for longer range transportation with the trade-off being greater well to wheel emissions in many parts of the world.  In the case of the BMW i3 REx, each mile of range requires either 0.15 pounds of gasoline, or 5.7 pounds of battery capacity.  At 37 times the mass specific energy density of battery power, very little gasoline is required to extend range for a given tank size, and that tank can be replenished in minutes nearly anywhere in the well developed fossil fuel infrastructure that currently exists worldwide.  This capability requires a 265 pound increase in the weight3 of the vehicle for the REx engine and associated systems, which imposes a 6% decrease in efficiency, but once set, that efficiency does not appreciably decrease as more energy in the form of gasoline is added to increase range.  Increasing battery capacity cannot increase range as efficiently, as not only must the weight of the battery increase by 37 times the weight of gasoline per mile in the first increment, but by the weight of increased structure and volume, as well as even greater battery capacity to offset the reduction in efficiency resulting from the weight increase.  There comes a point where the sacrifice in efficiency may no longer be worth the additional range to be gained.
See figure 2:

Figure 2: EV Energy Storage (and Generation) Weight vs Range for Advanced Technology EVs

1- EPA testing protocol does not account for approximately 4 miles of range remaining after REx fuel exhaustion when publishing a 72 mile battery powered electric range before REx activation, but does account for it in the total range calculation of 150 miles:  72 electric miles + 1.9 gal x 39 mpg + 4 electric miles = 150 EPA range (76 electric + 74 gasoline).  76 miles of range is also the result of dividing the EPA measured total i3 wall to wheel consumption of 22.0 kWh by the i3 REx EPA measured consumption rate of 0.288 kWh/mile.  This value is further corroborated by the CARB BEVx designation awarded to the i3 REx which requires the electric range not only be at least 75 miles, but that it must exceed the gasoline range, neither of which would be possible without accounting for the ~4 miles of range remaining after REx fuel exhaustion.

2- The EPA’s 95 MPGe rating of the Toyota Prius Plug-In Hybrid includes 0.2 gallons of gasoline operation plus 29 kWh of electric operation per 100 miles.  Subtracting the 10 mile of gasoline operation contribution to the total (0.2 gal X 50 mpg) yields 29 kWh per 90 miles, or 32.2 kWh per 100 miles, which results in 105 MPGe for electric only operation. (MPGe = 33,705 divided by watt hours per mile.)

3- While EPA rated at 87 miles of range in its base form, purchasers of the Mercedes-Benz B-Class can choose to pay an additional $600 for the Range Package, which makes an additional 17 miles of range available.  There is no difference in total battery capacity between the two configurations, only the percentage of SOC made available to the driver.

4- The 8 BMW battery pack modules weigh 55 lbs. each, for a total of 440 lbs.  Reference page 17 of the BMW i3 Service Managers Workshop Participant Guide at http://darrenortiz.com/website_pdfs/BMWi3PG.pdf.

5- 265 lbs for the REx engine and all associated equipment is the difference in weight between the i3 BEV and i3 REx as published on BMW’s spec pages:  http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3/Features_and_Specs/BMWi3Specifications.aspx
http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3RangeExtender/Features_and_Specs/BMWi3RangeExtenderSpecifications.aspx.  Adding the 440 lb. battery weight makes the total energy production and storage weight at 76 mile of range 705 lbs.  This increases by 11.4 lbs. of gasoline for every 74 miles driven beyond 76.

6- Widely quoted in other sources, Car and Driver claims the Telsa Model S 85 kWh battery pack weighs 1323 lbs: http://www.caranddriver.com/reviews/2013-tesla-model-s-test-review.  This is exactly 600 kg, making it appear to be an estimate, but it is the only number we have to work with, as Tesla does not publish the spec.

7- Weight of the 60 kWh Tesla Model S battery pack is estimated from the 85 kWh figure to be 60/85 X 1323 lbs. = 934 lbs.

The Silence of The Cars

Well Clarice, can you still hear the sounds of the engine screaming in your dreams?

There has been a lot of discussion about adding artificial noise to electric cars to serve as pedestrian alerts. I've been driving electric for over four years now and have driven over 120,000 miles, and I say they are simply not necessary. However as much as I wish it were true, I know the discussion doesn't begin and end with my opinion on this. Personally I'm all in favor for a backup beeper, which I actually think should be on all cars regardless of the type of fuel they use. But artificial noise when the vehicle is moving forward is silly and simply unnecessary. The quietness of the EV is something to celebrate, not shun.

Electric cars are quieter than gas cars, but only while driving at very low speeds (like under 10 mph) is there much of a difference. At speeds higher than that you can hear the tires on the pavement and the whine of the electric motor. Automakers are spending tons of money to make their gas cars quieter, and most cars - especially premium brands are extraordinarily quiet unless they are under heavy acceleration. In situations where pedestrians are most likely to have an issue, say crossing the street, cars are usually rolling along at lower speeds, coasting much of the time and much of the noise you hear anyway is the tire noise on the pavement. Plus, I think a bigger issue here is simply getting people to pay attention. Get off the cell phone, pull the Earbuds out and pay attention when you are crossing the street! And drivers need to stop texting and just drive the car. Look it's your responsibility not to run people over, it's not their responsibility to hear you coming and jump out of the way just so you can send "lol" to your bff! This is a sore subject for me because I was hit while crossing the street by a person texting three years ago and even needed surgery. I heard the car coming (a gas car!) just before it hit me and couldn't manage to get out of the way in time anyway.

Unfortunately our opinions may not eventually matter because our government may very well mandate a solution to a problem we don't have. The National Highway Traffic Safety Administration has proposed a new rule requiring hybrid and electric cars to make sounds to alert pedestrians. Even though our friends across the pond have actually done studies and come to the conclusion that these artificial noises are not necessary. Still, We'll likely rush to legislate something we don't have actual data on. In the UK, a study commissioned by the Department for Transport came up with these findings: "At low speeds of 7-8 kmh (5mph) electric cars were just one decibel quieter than petrol cars. When speeds were increased to more than 20 kmh (12mph), the noise levels were "similar", with tyre noise dominating. "There does not appear to be any significant difference in the acoustic nature of [internal combustion engine] vehicles and [electric and hybrid] vehicles, and as such nothing suggests a pedestrian would clearly be able to differentiate between vehicle types,"

Anyway as I said I'm just one opinion. I want to know your opinion on this issue. Automakers are struggling with this. They are conflicted because they aren't clear exactly what the consumer wants and they are also not sure themselves if it is actually needed. I know because I've talked with people from various OEM's about this very subject and they have asked me what I think about it. Some believe that while hardened EV supporters like me don't think it's necessary, that people new to electric vehicles may believe it is. That may be true in fact. So I'm putting up a poll here and asking people to vote how they would like to see their EV deal with this issue. Please vote, I can assure you this blog is monitored by every major OEM, and nearly on a daily basis. You never know who's listening. The poll is at the top of the right sidebar. Let your opinion be heard! By the way, all cars already have a pedestrian alert, it's called a horn!

How CARB May Make The i3's Range Extender Less Attractive

The range extender engine for the i3 is seen here to the left of the electric motor

According to the UK price list for the i3 the range extender will automatically come on when the state of charge falls to 18%. I've been guessing that will happen at about 20% so I was pretty close. It will then attempt to maintain the battery SOC at 18%, while allowing the car to continue to drive along relatively uninhibited. The only time there will be a problem is if your driving is demanding a high level of energy output for a prolonged period of time.

For instance, driving along at 60 mph on a flat surface you may only need 10 or 11 kW's to sustain the charge because that's about all you'll be consuming. That's no problem for the REx because it can provide up to 25kW's of constant supply. However if you need to drive up a steep grade at highway speeds for 10 continuous miles or so you may have a problem because the car will likely draw more than 25kW's under these strenuous conditions. The 18% buffer combined with the REx pumping out it's maximum output will allow the drive to continue for quite some time, but after a while of using more energy then it is capable of replacing, it will eventually need to reduce power output. What happens then is unclear but I would imagine the car would slow down to a speed it can maintain power for. Again, this should not happen on flat land, as the energy consumption should be able to be replace by the REx. It will also have plenty of power for most hills and bursts of speed when needed. I'm talking about long, extended drives up steep inclines that happen at the end of your journey after you've already depleated the battery and the range extender has come on. Personally I have a situation where this could come into play myself. My in laws live in Vermont and the last 10 to 15 miles to their house is mostly uphill. I'd already have the range extender on by the time I get to this final leg of the journey so I'm curious if I'll have a problem making it. I could stop along the way and charge for a while if necessary but I'd prefer just driving nonstop. After all, that's why I'd get the range extender; so I don't have to stop to charge along the way of a trip.

So what can be done to alleviate this? The Chevy Volt has a "Hold Mode" that the driver can initiate at any time. This manually turns on the range extender without waiting for it to automatically turn on when the battery is depleted and holds the battery state of charge at the level it was when you turned it on. Sounds like a great idea, so is BMW going to do the same thing? Yes, and maybe no. If you look at page 8 of the UK price list that I provided the link to above, you'll see it says: "Manually activated when the vehicle is below 80%". Brilliant! So if you buy an i3 in the UK, you can turn on the range extender once the state of charge drops below 80%. Therefore if you know you'll be driving up a long, steep hill or mountain at the end of your journey, you can turn on the range extender and "hold" the charge so when you arrive at the mountain you'll have plenty of charge to complete the journey. Perfect, so US customers will get this feature also, right? Unfortunately, maybe not.

CARB's restrictions may hinder the REx

California is the #1 market for electric vehicles in the US and one of the reasons they sell so well there is zero emission vehicles are allowed carpool lane access regardless of the amount of passengers. This is a highly sought after perk in California and cars that qualify for it usually sell very well. The all electric i3 will definitely qualify, but the under the new more strict rules for PHEV's, an EV with a range extender will only qualify for the valuable HOV access sticker if it operates this way: "engine operation cannot occur until the battery charge has been depleted to the charge-sustaining lower limit". So that means the range extender cannot be manually turned on at 80% and still qualify for HOV access in California.

It's clear to me BMW will make the i3 conform with CARB's rules so it will have HOV access is California, but will they do this to all the US cars, or just for the ones shipped to California? I don't have the answer. I tried to get clarity on this at the i3 Premier but nobody wanted to confirm it one way or the other. Hopefully that means a decision hasn't been made on this yet and there is hope for the rest of us. If so and the powers to be at BMW find their way to read this blog post, please consider offering the same ability to manually turn on the REx for US customers outside of California. The vast majority of customers in the US don't need carpool access, why should they have their range extender neutered so people in California can have it? This is a simple software change. It's available in the UK and probably for the rest of Europe so it's not like it will cost BMW anything to develop. Let's hope BMW does the right thing and makes this feature available to US customers outside of California. It really makes the range extender a more useful asset, this shouldn't be a hard decision to make.