Mazda Skyactiv-X: Compression Ignition and Superchargers

[CodeRedR51]

Mazda's game plan: Compression ignition, superchargers, EVs — and still fun to drive

Highlights:

• First to use compression ignition (at low RPM - plugs still there for high RPM)
  
• Superchargers used for 10 to 30% increase in torque
• Electric vehicles arriving starting in 2019

Today, Mazda announced it's new Sustainable Zoom-Zoom 2030 initiative. Dumb marketingspeak name aside, this is Mazda's general plan to make its cars more efficient while still keeping Mazda's fun-to-drive character. The most notable part of this announcement is Mazda's new Skyactiv-X engines and the addition of EVs and electrified powertrains. As we reported back in January, this new supercharged Skyactiv-X engine family will be the first to use compression ignition rather than traditional spark plugs.

Each year, fuel economy and CO2 emissions standards grow ever tighter. Each and every automaker around is looking for ways to improve both factors. Initially, most have chosen to downsize engines and add turbochargers. That's why Mazda's new Skyactiv-X engines are going to be so special. They essentially work like a diesel engine, using extremely high compression to ignite the fuel rather than the flame from a spark plug. Adding a supercharger rather than a turbo retains a smooth and immediate engine response. Look for a 10 to 30 percent increase in torque.

Mazda's engine already have some of the highest compression ratios around, but these new engines will push well into diesel territory. The biggest issue so far with gasoline compression ignition is controlling when the gasoline ignites. Mazda claims to have solved this issue, seamlessly moving from spark to compression ignition.

This new plan comes 10 years after the first Sustainable Zoom-Zoom initiative was announced. With Sustainable Zoom-Zoom 2030, Mazda wants to focus on the Earth, society, and people. Together with the new engines, Mazda hopes to create a sustainable future that still involves engaging and fun-to-drive automobiles.

First, Mazda wants to consider emissions over the entire life of a car. That means reducing emissions generated while building the car or fueling and charging a car in addition to those released while it's on the road. The goal is to reduce emissions to 50 percent of 2010 levels by 2030, and a 90 percent reduction by 2050. New electrified models will debut in 2019 in markets that have a high ratio of clean energy for power generation.

Mazda's focus on society aims to improve safety with the Mazda Proactive Safety philosophy. Like with Skyactiv, this ethos goes far beyond simply improving an engine or adding new active safety technology. Mazda believes a better driving position with a standard steering wheel and pedal placement will improve safety. Mazda will start testing autonomous technology, looking for a full rollout sometime in 2025.

Finally, Mazda's focus on people is intended to improve driver engagement and pleasure. That includes the satisfaction of driving a green vehicle in addition to one that's fun to toss into a corner. Expanding and updating the current Mazda design language is also part of the focus on people.

 

[FT-1]

Good on Mazda for trying something different. Hope this tech turns out to be a success and we have more superchagred cars on the market.👍

 

[Eunos_Cosmo]

I continued to be enthusiastic about the innovation happening within Mazda. They are crushing it this decade, even if their sales don't really reflect that. They are on a level like Honda was in the 1990s or Toyota in the 1980s, perhaps even Citroen in the 1950s.

Mazda's corporate culture seems to be one in which the entire company is structured in total support of it's engineers & designers. This is in contrast of course to GM where the company is structured in total support of its accountants and lawyers.

It's pretty stunning to look at where Mazda was in 2007 and compare it to now, all without enormous capital or outside investment. Every car they make is great.

Sidenote: The choice of a supercharger rather than a turbocharger has me thinking this could be a miller cycle engine of some sort. Mazda definitely has experience with Miller-cycle engines, and I can conceptually see how it could work with HCCI.

 

[Eunos_Cosmo]

Sidenote: The choice of a supercharger rather than a turbocharger has me thinking this could be a miller cycle engine of some sort. Mazda definitely has experience with Miller-cycle engines, and I can conceptually see how it could work with HCCI.

I was not exactly correct.

What they've done is even more fascinating. See this excellent Jalop article for full text.

Basically they are creating the additional compression (durring HCCI operation) not by mechanical means like I speculated, but by creating a preliminary fireball/explosion at the top of the combustion chamber which creates a pressure wave that compression the main air-fuel charge to an eye-watering 16:1 ratio which then spontaneously and homogeneously combusts. On top of that the air-fuel mixture is a ludicrous 36.8:1! A good, if perhaps not so timely analogy I saw was that of a Hydrogen bomb. A first-stage fission bomb compresses a hydrogen fuel source to the point of spontaneous fusion, creating an even larger explosion. It's a stunningly elegant solution that gives even more credibility to the excellence of engineering at Mazda.

The supercharger is still an essential part of this whole system as a method of control.

The more interesting part of this is that the same concept could be applied to a rotary engine, perhaps with even better results due to the discreet 'regional' temperatures of a rotary engine layout.

 

[Sage Ages]

Anyone have an idea why they're using a supercharger in particular instead of turbocharging? This isn't really something I know too much about.

 

[Omnis]

Anyone have an idea why they're using a supercharger in particular instead of turbocharging? This isn't really something I know too much about.

Because a turbo doesn't spool at 1500 RPM. And the supercharger is almost guaranteed to be hooked up to mild hybrid that can drive the blower as opposed to having an alternator.

Remember, the purpose of forced induction in this application is not to pressurize the system per se, but to act as an "air injector" to control for optimum HCCI conditions. If they were trying to make big power, they'd slap on a turbo for big boost at low compression on spark ignition. They're actually trying to achieve the opposite. Rather than maximizing power per cylinder volume, the goal is maximizing power per fuel volume.

 

[Vegard]

Like @Omnis said, it's probably related to the more linear boost characteristics of a supercharger being better suited to deliver the ideal conditions for HCCI over a wide RPM range.

 

[Sage Ages]

Because a turbo doesn't spool at 1500 RPM.

Like @Omnis said, it's probably related to the more linear boost characteristics of a supercharger being better suited to deliver the ideal conditions for HCCI over a wide RPM range.

Alright. I had a feeling that response time would be an issue but I didn't know for sure.

And the supercharger is almost guaranteed to be hooked up to mild hybrid that can drive the blower as opposed to having an alternator.

I didn't even think of that.

 

[Omnis]

If you have an hour to kill, I'd suggest watching the complete presentation:



I put it on when going to bed, so that I have have Skyactiv-Zzz's

 

[88GTA]

^ Added to watch later. I'll be interested in seeing the development in this because the lease on my '17 6 ends in December '19.

I'm happy with my decision 9 months in. The only thing I don't like about the car is the auto transmission but coming from a 6 speed manual, any auto would suck. It has a pretty slow downshift on acceleration but one of the reasons I got the 6 was because it was still a traditional auto and not a CVT.

 

[Obelisk]

^ Added to watch later. I'll be interested in seeing the development in this because the lease on my '17 6 ends in December '19.

I'm happy with my decision 9 months in. The only thing I don't like about the car is the auto transmission but coming from a 6 speed manual, any auto would suck. It has a pretty slow downshift on acceleration but one of the reasons I got the 6 was because it was still a traditional auto and not a CVT.

Oh, how's your 6 holding up? You liking the experience?

--

This all looks really interesting, but does it also boost engine output? I know the OP said up to 30% more torque, but I was wondering if it bumped the car's base horsepower. Having a 180 hp 2.0 engine would be quite fun on top of the added fuel efficiency - which, to be honest, the 2.0 could benefit from. It's a tad sluggish at low revs, based on my year and 2 months with my 3.

 

[88GTA]

So far so good with the 6. Wishing it had a 3rd pedal but it wasn't in the cards. I'm averaging about 29 MPG which is nice.

 

[CodeRedR51]

 

[Eunos_Cosmo]

Littlest supercharger ever?

 

[Keef]

Littlest supercharger ever?

No, actually that is a "highly responsive air supply unit". Do you watch technical videos or do you just skim through them?

EDIT: I just had a genius idea.

I want Mazda to partner with Tesla.

Mazda needs to work on hybridization and electrification, stat. Tesla already has it. They could supply Mazda with everything they need.

But Tesla has this new electric semi truck thing with a range of only 300 miles. That won't work cross-country. What's the perfect compromise? A tiny range-extending diesel engine, Mazda's Skyactiv-D. The engine would take up very little room in Tesla's truck, probably mounting it behind that forward-mounted cabin, and obviously could be fueled off our existing diesel infrastructure. Current long-haul trucks carry over two days worth of diesel fuel, 300 gallons at 5.5 mpg from a 12 liter diesel engine. Imagine the fuel consumption of Mazda 2.2 Skyactiv-D running as a battery range-extending generator fueled by 50-100 gallons of diesel. Holy crap. That thing could run for a week without needing refueled, and continually keep the battery at best capacity. And because that battery will run all the systems while the driver is sleeping, even more diesel is saved by not running an engine overnight (many truckers do this because diesel engines run extremely efficiently at idle). The driver gets up after his rest, starts driving, and the Skyactiv-D regenerates the battery, and then maintains it as needed. Simple. Diesel-electric hybrid eliminates any charging delay, allows the battery to have an immense range, and makes use of a fully-fledged diesel infrastructure, allowing the electric charging infrastructure to be built at a less demanding pace.

I know Tesla is supposed to be an all-electric company, but semi trucks have needed diesel-electric options for decades now and nobody as delivered. This seems like the perfect opportunity to make it happen. Both companies need each others' technology and combined assets. Both companies are awesome at what they're doing.

 

[Eunos_Cosmo]

No, actually that is a "highly responsive air supply unit". Do you watch technical videos or do you just skim through them?

EDIT: I just had a genius idea.

I want Mazda to partner with Tesla.

Mazda needs to work on hybridization and electrification, stat. Tesla already has it. They could supply Mazda with everything they need.

But Tesla has this new electric semi truck thing with a range of only 300 miles. That won't work cross-country. What's the perfect compromise? A tiny range-extending diesel engine, Mazda's Skyactiv-D. The engine would take up very little room in Tesla's truck, probably mounting it behind that forward-mounted cabin, and obviously could be fueled off our existing diesel infrastructure. Current long-haul trucks carry over two days worth of diesel fuel, 300 gallons at 5.5 mpg from a 12 liter diesel engine. Imagine the fuel consumption of Mazda 2.2 Skyactiv-D running as a battery range-extending generator fueled by 50-100 gallons of diesel. Holy crap. That thing could run for a week without needing refueled, and continually keep the battery at best capacity. And because that battery will run all the systems while the driver is sleeping, even more diesel is saved by not running an engine overnight (many truckers do this because diesel engines run extremely efficiently at idle). The driver gets up after his rest, starts driving, and the Skyactiv-D regenerates the battery, and then maintains it as needed. Simple. Diesel-electric hybrid eliminates any charging delay, allows the battery to have an immense range, and makes use of a fully-fledged diesel infrastructure, allowing the electric charging infrastructure to be built at a less demanding pace.

I know Tesla is supposed to be an all-electric company, but semi trucks have needed diesel-electric options for decades now and nobody as delivered. This seems like the perfect opportunity to make it happen. Both companies need each others' technology and combined assets. Both companies are awesome at what they're doing.

I sincerely doubt a 2.2 liter engine could supply the power required for a 30,000lbs+ semi truck. Power is power whether there is an intermediary (electric drive) or not, you would still be effectively trying to move freight with a <200hp engine.

Unless you mean the diesel is only generating electricity when the truck is stopped for rest?

 

[Vegard]

I sincerely doubt a 2.2 liter engine could supply the power required for a 30,000lbs+ semi truck. Power is power whether there is an intermediary (electric drive) or not, you would still be effectively trying to move freight with a <200hp engine.

Unless you mean the diesel is only generating electricity when the truck is stopped for rest?

That's literally what he suggested:

Mazda 2.2 Skyactiv-D running as a battery range-extending generator

 

[Eunos_Cosmo]

That's literally what he suggested:

"The driver gets up after his rest, starts driving, and the Skyactiv-D regenerates the battery, and then maintains it as needed. "

The prime mover (electric motor) will drain the battery way faster than the small diesel engine can replenish it. Maybe you get 100 more miles range? It would probably be more useful to just run accessories with the diesel engine.

There is a reason the diesel engines in locomotives (also series hybrids as described) have 6,000hp

 

[homeforsummer]

There is a reason the diesel engines in locomotives (also series hybrids as described) have 6,000hp

Hmm. While I can see what you're getting at, it's a bit of an apples/oranges comparison.

The locomotive on its own weighs 200-odd tons (metric - I'm sticking with that for all the numbers below), and in the record-setting example given it was capable of pulling 109,000 tons for 171 miles.

The US limit for commercial vehicles is 36 tons - 8 tons less than it is even in Europe and give or take 3000 times less than a 6000-horsepower locomotive capable of covering 171 miles.

Now obviously there are lots of different factors at play here (railroads have less elevation than roads, tracks have less friction etc), but I think @Keef probably isn't as far from the mark as you're making out.

The average diesel in a semi truck is operating somewhere around the 40% thermal efficiency mark at the moment (how much work you actually get compared to the energy you put in - the rest of the energy being lost as heat, noise etc), not including further efficiency losses as a result of using a transmission. Electric motors tend to be somewhere in the 90% range and reduction gear transmissions are pretty efficient too.

So call that putting half (being generous to combustion power) the energy into an electric drivetrain to get an equivalent output to the diesel one - as well as favourable characteristics such as improved low-down torque useful for getting up to speed (the most energy-intensive thing any vehicle has to do - cruising requires relatively little power).

It'll take someone far more intelligent with numbers than me to work out whether a 2.2 Skyactiv-D really would be enough to keep a semi going for a length of time (assuming it's also charged at each stop, since keeping the battery topped up in the first place would reduce demand on the range-extender) but it's certainly a feasible idea.

Current semis do maybe 5-6mpg on the highway. Which is pretty good for something weighing up to 36 tons, but also pretty low-hanging fruit as far as efficiency improvements are concerned. If you could turn 5mpg into even 10mpg you'd save billions of barrels of oil a year in freight transport alone, and transport companies would save billions of dollars on costs.

There are other questions here too though, of course. Like why nobody has seriously explored making semis much more aerodynamically efficient, which would also yield improvements.

 

[Eunos_Cosmo]

I mean look at the Fisker Karma. Per reviews of the Karma, 260hp/260lbs-ft from the Ecotec (nominally similar torque to the Skyactiv-D but considerably more power) was adequate for the 5300lbs Karma, but the results were not exactly spectacular. Putting a similar generator set in a vehicle 10-18x heavier? I'm not sure that would be a great result...maybe on very long stretches of flat road.

 

[homeforsummer]

The Karma also had a battery half the size of a current Nissan Leaf. It wasn't exactly a high water-mark for engineering either!

The idea with a range-extended truck would still be that the electric component - including a large battery pack - would do most of the work, and an efficient engine would effectively reduce the speed at which the batteries discharged and in some cases help maintain the charge.

And, in the case of @Keef's particular example, also act as a night heater/power supply rather than having to use a 13-litre (or other vast capacities) engine to do the same job.

The less realistic bit of Keef's suggestion is that it would "eliminate charging delay". Other than in fairly light load cycles it would probably struggle to maintain charge while in motion (though at night it would certainly be enough to recharge it if the driver couldn't find somewhere to plug in) and most of the efficiency benefit of this system would come from the batteries rather than the engine itself, but I'd be surprised if it didn't have a measurable effect on extending the usable range.

Even if it does as you mention - adds say, 100 miles to the 300 miles of a Tesla semi - that's a pretty worthwhile improvement. Makes it more usable for haulage firms, while being vastly cleaner and giving an additional option for charging should the driver stop anywhere without a plug.

 

[Eunos_Cosmo]

The Karma also had a battery half the size of a current Nissan Leaf. It wasn't exactly a high water-mark for engineering either!

The idea with a range-extended truck would still be that the electric component - including a large battery pack - would do most of the work, and an efficient engine would effectively reduce the speed at which the batteries discharged and in some cases help maintain the charge.

And, in the case of @Keef's particular example, also act as a night heater/power supply rather than having to use a 13-litre (or other vast capacities) engine to do the same job.

The less realistic bit of Keef's suggestion is that it would "eliminate charging delay". Other than in fairly light load cycles it would probably struggle to maintain charge while in motion (though at night it would certainly be enough to recharge it if the driver couldn't find somewhere to plug in) and most of the efficiency benefit of this system would come from the batteries rather than the engine itself, but I'd be surprised if it didn't have a measurable effect on extending the usable range.

Even if it does as you mention - adds say, 100 miles to the 300 miles of a Tesla semi - that's a pretty worthwhile improvement. Makes it more usable for haulage firms, while being vastly cleaner and giving an additional option for charging should the driver stop anywhere without a plug.

Well it would certainly be better than nothing.

My interpretation of Keef's post would be that a 175hp Skyactiv-D generator set would get 1500 miles of continuous range (diesel truck standardish) or so in a hybrid configuration, basically adding 1200 miles of range to Tesla's 300 mile range.

 

[Keef]

Well it would certainly be better than nothing.

My interpretation of Keef's post would be that a 175hp Skyactiv-D generator set would get 1500 miles of continuous range (diesel truck standardish) or so in a hybrid configuration, basically adding 1200 miles of range to Tesla's 300 mile range.

How much power does it take to charge a Model S's batteries? Does anybody know that? How much power does it output while cruising the highway?

Because this here Kohler generator runs a 2.5L 4-cylinder turbodiesel and outputs 28 sustained kW (whatever that means) at 100% power while burning 9.1 gph.

EDIT: The P100D has a 100 kWh battery, right? It's maximum output in an hour is 100 kW. I have to assume this generator's output is 28 kWh, but it doesn't say kWh anywhere on its spec sheet. Idk how electromagic works. Anyway, this generator would not be able to charge a Model S while he car is at full throttle, but obviously its highway cruise figure would be much smaller than 100 kWh.

Surely there's somebody here smart enough to answer our questions lol. It's just math.

 

[Obelisk]

28 kilowatts is about 1/2 of a Tesla S 60's power output.

I don't know much past that.

 

[Eunos_Cosmo]

I did some maths based on max loading (obviously not entirely useful, but something). It's also probably wrong. lol

1kw/hr = 3.6 MJ

In 1-hr, at max load, a 130kw Skyactiv-D will produce 468MJ of energy while consuming something like 32 liters of diesel worth about 1.3GJ of potential chemical energy.

In 1-hr, at max load, an (ideal) 400kw (sufficient for a semi truck I think) electric motor will consume 1440MJ of energy from batteries

If you have a 100kw/hr battery pack, you have 360MJ/hr energy reserve.

Therefore, at maximum load, a 400kw electric motor will deplete a 100kw/hr battery in about 15 mins.
If the diesel generator is running at full load, I calculate you'll get about another 7 minutes of run time.

I have no idea how you'd calculate partial load.

As an aside, 300 gallons of diesel fuel has a potential energy of 43,500MJ. Quite a bit more than a 100kw/hr battery back

Edit: Working backwards...

If the Skyactiv-D is at full load, again, it's producing 468MJ of energy per hour. If you have a 400kw electric motor, you can use up to 32% of it's power while not depleting the batteries. So at a steady state cruise (maybe doable with 130kw?) and the generator running at full load, you'll be using approximately 32 liters/9 gallons of fuel per hour (based common diesel engine fuel consumption for this power rating). If you're cruising at 60mph and using 9 gallons of fuel that's about 6mpg. Factor in a 300 gallon tank and you are looking at around 1800 mile range, assuming a perfectly flat, straight road with no acceleration and provided that 130kw is sufficient to maintain that speed.

Edit 2: Per this janky website, and assuming a 60,000lbs load, 120sf frontal area, and 0.96 drag coefficient (all reasonable for a semi-truck) you need about 290hp to maintain a 60mph cruising speed. So the Skyactiv-d wouldn't really be enough to keep up with the electric motor(s). If you had a 500kw/h battery in the same condition, you could go about 150 miles on electricity, and after that, with the Skyactiv-D working it's heart out I figure your speed would be reduced to 45mph max. At 45mph with the same fuel consumption, you're down to 5mpg and therefore 1500 mile range. Add the 150 mile electric range and you have something like 1650 mile range total.

^^All assuming magical 100% efficient electric motors^^

 

[homeforsummer]

The math looks good, but the figures still seem pessimistic for a diesel like that in the Mazda. At top speed on the autobahn in a diesel Mazda 6 I'd be very surprised if you'd get as low as 6mpg, and that's with frictional drivetrain losses, rolling resistance and aerodynamic resistance to work against. Spinning a generator seems like it'd have lower load than the combined efforts of all that - and whether it'd be necessary for the engine to spin at its maximum RPM to generate the necessary power is also unclear.

Using very, very rough math though I'd assume benefits to be proportional to the 15 minutes/7 minutes figures you quote. While those numbers are obviously very low, I think it's notable that the generator would do enough to provide more than 50% extra range in that particular example.

Running at significantly less than maximum load (and all other factors being equal) it's not unreasonable to expect the generator to add more than 50% extra range, if that math is correct.

 

[Eunos_Cosmo]

The math looks good, but the figures still seem pessimistic for a diesel like that in the Mazda. At top speed on the autobahn in a diesel Mazda 6 I'd be very surprised if you'd get as low as 6mpg, and that's with frictional drivetrain losses, rolling resistance and aerodynamic resistance to work against. Spinning a generator seems like it'd have lower load than the combined efforts of all that - and whether it'd be necessary for the engine to spin at its maximum RPM to generate the necessary power is also unclear.

Using very, very rough math though I'd assume benefits to be proportional to the 15 minutes/7 minutes figures you quote. While those numbers are obviously very low, I think it's notable that the generator would do enough to provide more than 50% extra range in that particular example.

Running at significantly less than maximum load (and all other factors being equal) it's not unreasonable to expect the generator to add more than 50% extra range, if that math is correct.

I don't disagree that you would add a lot of range, but I don't think the engine itself would be really adequate (extremely slow acceleration and low maximum speed) unless you vastly improved the aerodynamics of the truck.

Also, it's worth mentioning (I think) that an 85kwh battery pack in the P85D weighs 1200lbs. If the output scales linearly with weight (reasonable to assume because you are adding cells), that means a 500kwh battery pack (enough to push a fully loaded semi 150 miles per some assumptions) would weigh a staggering 7,000lbs. That's close to 3x the weight of a normal diesel truck engine. Combine that with about 2,000lbs of fuel and you have around 9,000lbs (4100kg) of energy storage for this hypothetical machine. I feel like this is where the electric-truck equation starts to fall apart with current battery tech. But my have we gotten off topic....apologies.

 

[homeforsummer]

I don't disagree that you would add a lot of range, but I don't think the engine itself would be really adequate (extremely slow acceleration and low maximum speed) unless you vastly improved the aerodynamics of the truck.

Does that actually matter when the engine isn't directly powering the truck? The electric motor handles the torque and power demands of the driving - the diesel generator just sits there whirring away at a constant rpm topping up the batteries. At no point is it connected to the driven wheels.

You could have a 50cc moped motor in there to do the same job in theory - it'd be useless at keeping the batteries topped up but it wouldn't make a blind bit of difference to the truck's performance.

 

[Eunos_Cosmo]

Does that actually matter when the engine isn't directly powering the truck? The electric motor handles the torque and power demands of the driving - the diesel generator just sits there whirring away at a constant rpm topping up the batteries. At no point is it connected to the driven wheels.

You could have a 50cc moped motor in there to do the same job in theory - it'd be useless at keeping the batteries topped up but it wouldn't make a blind bit of difference to the truck's performance.

The point I was trying to make that the power demand to keep a loaded semi truck going at 60mph/100kmh is higher than the maximum power output of the ICE engine. This means that the electric motors will deplete the batteries at a faster rate than the ICE engine can replenish them. When the speed reduces to about 45mph (by my calculations) the ICE will then be able to provide enough output to maintain the batteries at the same rate the electric motors are consuming electricity, the equilibrium point. You would need to find an engine that makes around 300hp at peak efficiency. Maybe one of the larger VAG V6 turbo diesels or a Powerstroke or something.

Cars like the Fisker Karma work because they are very aerodynamic and steady state cruising speed (60mph) only requires something like 20hp or less, which can easily be provided by a small ICE engine at relatively high efficiency.

The acceleration would only be impacted when the batteries are depleted, I agree on that point.

 

[homeforsummer]

The point I was trying to make that the power demand to keep a loaded semi truck going at 60mph/100kmh is higher than the maximum power output of the ICE engine. This means that the electric motors will deplete the batteries at a faster rate than the ICE engine can replenish them. When the speed reduces to about 45mph (by my calculations) the ICE will then be able to provide enough output to maintain the batteries at the same rate the electric motors are consuming electricity, the equilibrium point. You would need to find an engine that makes around 300hp at peak efficiency. Maybe one of the larger VAG V6 turbo diesels or a Powerstroke or something.

I don't disagree with any of that, but that isn't what you said in the post I quoted

Anyhoo, we're going in circles here. I think it can all be summed up as "a Skyactiv-D wouldn't be enough to maintain battery charge at cruising speed, but would be enough to extend the range of an electric truck by a useful amount".