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Transportation’s Future Is Hybrid – in More Ways Than One

by Alec Shkolnik, co-founder & CEO of LiquidPiston

Months before stepping down as CEO of Toyota, Akio Toyoda zigged at a time when the rest of the automotive industry zagged: he argued for an electrified future powered by both EVs and hybrid electric vehicles (HEVs).

I couldn’t agree more.

New hybrid innovation is just as important as innovation in fully electric vehicles. And if our goal is to reduce carbon footprint or increase energy efficiency, we must view it as a crucial optimization.

We can achieve it by adopting a multi-pronged, multi-dimensional approach that considers factors like propulsion technology, energy sources, energy conversion, and storage options under a hybrid framework.

HEVs Are Vital to Electrification Efforts

I want to start by noting that EVs are a marvel of engineering, resulting from decades of improvements in battery, motor, and control technology. They offer a range of benefits, from emission mitigation (or at least moving emissions away from urban populated areas – more on that later), to improved efficiency via regenerative braking. But there are significant hurdles to widespread EV adoption.

EVs still cost more than conventional ICE vehicles. Despite five decades of steady improvement to EV batteries, liquid fuels still offer more than 30x greater energy density (an EV must carry around 1,200 pounds of battery to replicate the energy of 36 pounds of fuel).

This is, in a word, expensive. And despite increasing economies of scale, the price of batteries actually increased in 2022 by seven percent. We’re also struggling to build enough chargers (as is Europe). To reach the US federal government’s goal of 50 percent zero-emission vehicle sales by 2030, the US needs 1.2 million public and 28 million private EV chargers. Today, there are only around 130,000 public chargers.

And this doesn’t even consider the future demands that will be placed on energy generation. Transportation alone requires almost as much energy (27 quadrillion BTU or “quads,” as of 2021 in the United States) as the entire electric grid can supply (37 quads).

And maybe most strikingly: it takes more than six times the raw materials and minerals to produce an EV compared with a conventional car. This puts an incredible strain on supply, requiring significant energy (50 kWh of energy are required to produce one kWh of battery) and water (500,000 gallons of water to produce each ton of lithium). That comes at a cost—financially and environmentally.

According to a recent Toyota memo, the raw materials in one long-range EV could instead be used to produce six plug-in hybrid vehicles or 90 HEVs. The lifetime carbon reduction of those 90 HEVs is 37 times greater than that of a single EV.

This is why we need HEVs (something Renault and Geely recognize, per the recent launch of Horse, an $8.8-billion joint venture which aims to develop low-emission ICEs and hybrid powertrains for the automotive market). HEVs combine the strengths of EVs (like regenerative braking) and those of conventional ICE vehicles. HEVs help us optimize our carbon emission reductions with the infrastructure and technology we have today.

But to properly harness the power of HEVs…

We Can’t Overlook Innovation for the Internal Combustion Engine

The battery component of hybrid vehicles is getting its fair share of attention. But if we believe in hybrids (and we should), we need to be as focused on the engine as we are on the battery.

If we reduce the weight and volume of the engines in HEVs, that, in turn, reduces the:

  • Vehicle’s weight.

  • Manufacturing costs.

  • Emissions produced.

  • The amount of energy required to propel the vehicle.

For example, my startup LiquidPiston has developed a rotary engine that is five times smaller and lighter than a traditional piston engine of similar power. For a hybrid application, decreasing the engine size and weight increases the under-the-hood “budget” for a battery.

But shrinking engines isn’t the only way to rethink them to improve an HEV’s efficiency. We are also building a rotary ICE that burns hydrogen and other low-carbon fuels. As sustainable fuels scale, this type of ICE development will be just as important as ongoing battery innovation to power our net-zero transportation future.

There’s No One-Size-Fits-All – We Need Application-Specific Hybrid Vehicles

Embracing hybrid vehicles as part of decarbonizing transportation means embracing an incredible array of options: series and parallel configurations, engines and batteries of various horsepowers, different battery and fuel types, and so on.

No one option is objectively best for all use cases; instead, the ideal combination is always application-dependent. A vehicle driven mostly on highways (like long-haul trucks) would likely perform best with a parallel hybrid configuration. Passenger cars meant to be driven in cities may do better as a series hybrid.

But the optimal hybrid configuration should always achieve these two objectives:

  1. Operate the engine at the peak of its efficiency curve during its most common driving conditions.

  2. Operate as efficiently as possible during edge-case driving.

That optimization will be complex, of course: cost, government policy, fuel type, and drive cycles will all impact what constitutes “optimal.” But starting to think about engine innovation together with battery innovation will help us ultimately accelerate the decarbonization of transportation.

To Decarbonize Transportation Quickly, We Must Embrace Hybrid Approaches

I’m not just talking about the engine-battery combinations people may see under the hoods of their Toyota Highlander Hybrids. I’m talking about the energy we draw from, how we convert that energy, how we scale our grid’s capacity and resiliency, and how we make all of this affordable. Maybe that comes from a mix of biofuels and fossil fuels paired with more charging infrastructure and diversified vehicle types and business models (higher tax credits, government incentives, etc.).

But the takeaway here is clear. An ICE that’s designed with a focus on hybrid power system compatibility, optimized thermodynamics for increased fuel efficiency, and multi-fuel capability can play a pivotal role in powering and accelerating our hybrid transportation future.

Alec Shkolnik is co-founder & CEO of LiquidPiston, which is a leading developer of combustion engines and hybrid power solutions that are scalable, compact, and capable of utilizing efficient fossil or renewable fuel. Alec has a PhD from MIT’s Computer Science and Artificial Intelligence Laboratory, where he was an NSF Graduate Research Fellow and a postdoctoral researcher.



I would agree generally with what he says - batteries for pure BEVs have become way too large, heavy and expensive.
Hence we need some kind of PHEV with a range extender (or full hybrid) source.
The problem is that you can end up with 2 complete engines (ICE and EV) which is a lot of complexity and cost, especially with every more stringent pollution controls.
Nonetheless, it seems like a good idea to have a 12 kWh battery and a range extender than a 50-80 kWh battery which is only fully utilized a few times per year.
A range extender need only run at one speed and one power setting, so it can presumable be made cheaper and more efficient than a variable speed one.
There are various tasks that are not suited to PHEVs, like use in hilly areas and towing boats, etc, so just don't use them for these tasks.

Albert E Short

Happy owner of a 2018 Honda Clarity PHEV here, but I think the ship sailed on this one a few years ago, at least for the vast bulk of cars. The "36 lbs of fuel = 1200 lbs of battery" is a bit specious. It takes a 800+ lbs of engine(explosive reactor), transmission, radiator, and exhaust to get the energy at 40% efficiency for the good ones. 200 lbs of electric motor gets you the same or more hp with many, many fewer moving parts. With batteries doubling in density with safer, more heat tolerant materials, it looks like the Europeans bet right going all in on electric.

Roger Pham

Your concern is valid for the exist engines that are very heavy and less efficient. However, Liquid Piston company have come up with an engine that is much lighter and more efficient than existing engines to address this issue.
In fact, a PHEV should have a much smaller and lighter engine than an ICEV because the battery and the e-motor can made up for the power lacking in the downsized engine, and NO need for a gear-change transmission because the e-motor can act as an e-CVT transmission.
The problem with BEV is that it is totally dependent on the grid, and sometimes, the grid is so stress-out with excessive demand of longer and longer extreme heat waves, while PHEVs can add power to the grid at these moments to avoid grid burn out.
Natural disasters like forest fires, hurricanes are becoming more common, and evacuation with a 600-mi range PHEV that can be filled up in 3 minutes would be much more re-assuring than using a 300-mi range BEV that takes a long time to charge...and during those peak travel times...the long lines at fast-charging stations will be UNBEARABLE...with on-coming fires and smokes, or storm...


Generally speaking, I agree with him; batteries for pure BEVs have become excessively huge, heavy, and expensive.Therefore, a PHEV that can use a range extender (or full hybrid) is required.The issue is that you may end up with two full engines (ICE and EV), which adds a lot of complexity and cost, particularly if pollution regulations become worse.But if the battery is only going to be completely used a few of times a year, it seems more practical to have a 12 kWh battery with a range extender.Since a range extender just needs to operate at a single speed and power level, it should be possible to make it at a lower cost and higher efficiency than a variable speed version. You shouldn't use a plug-in hybrid redactle unlimited electric vehicle (PHEV) for hauling a boat or driving up and down steep hills, among other things.


As others commenting here note, the hassle with things like liquid piston is that you still have essentially two systems, one combustion, the other electric.

Fuel cells don't have that problem, as they are also wholly electric.

The present low temperature PEM versions in cars have other issues though, like requiring very pure (expensive) hydrogen, and the need to balance moisture.

HT PEMS should hopefully solve those issues, but we have not yet got them in volume commercial production, with aircraft looking the first candidates.

But I am no friend of sweeping theoretical justifications, rather of fitting presently do-able technology to the application, and I certainly think that advanced combustion engines have a role.

That is one of my gripes about the move to batteries in everything, it is far too broad, sweeps difficulties under the carpet, and as the the article argues you end up with solutions which are massively sub-optimal in many respects, including resource use as highlighted.

IOW, iit is Toyota's broad front approach which is optimal, not Tesla and their psycopath in chief.


is liquid piston's engine really more efficient? I only see one page claiming 45% thermal efficiency. I am not sure this much better than existing engines using the Atkinson cycle or good old diesel.


@ Davemart:
You're right about some of those points that you mentioned. What you forgot to mention, that FCs - when fed with green H2 - have an absolutely rotten overall efficiency.
Whilst talking about psycopaths, when was the last time that you took a long enduring look in the mirror?



There is a difference between making deploring public figures, in this case one who to my personal knowledge enabled and applauded the doxxing of a critic, not me, so as to put pressure on those who disagree with him - free speach advocate - so long as it is in agreeement with his bullying and overbearing agenda,and simply indulging in ill mannered abuse of private citizens, as you chose to do.

You are not only foollsh, but ill mannered.

As for the efficiencies of hydrogen, it all depends on the total system.

Home fuel cells in Japan currently using natural gas, but hydrogen would to fine, hit over 90% electrical plus thermal efficiency.

Also many things are inefficient but still desirable.

Solar is only 20% or so efficient, so would you advocate stopping using it?

But whatever, bye, I am not going to waste more time on someone who is as ignorant and prejudiced as he is ill-mannered.


This article/infomercial unfortunately repeats a lot of talking points that don't pass the sniff test.
For instance, batteries may be heavier than gasoline, but EVs are 4x more energy efficient, so they only need to store a fraction of the energy. Also, as others have pointed-out, you need much more than a fuel tank to make an ICE car.
We don't need as many fast chargers as claimed, especially in the US, because people have electricity and parking at home. Fast chargers are only needed for long interstate trips. Also, most US homes already handle heavy loads: HVAC, cooking, hot water, etc. Home charging mostly happens during off-hours when those loads are minimal.
There's also the rhetorical device of projecting yesterday's grid on tomorrow's demand. Peak coal was 100 years ago (1920s), so let's stop using that to forecast tomorrow's grid mix.
I don't doubt that gasoline will be a large part of the US's energy mix for a long while, but I think it will be mostly in less-populated areas that have antiquated grids.


This Post states:
“ liquid fuels still offer more than 30x greater energy density (an EV must carry around 1,200 pounds of battery to replicate the energy of 36 pounds of fuel).”
Only if the rest of the Powertrain has zero weight and is 90% efficient. No one should ever use those numbers period.

PHEV batteries are not lightweight either and use significant quantities of Nickel and Cobalt. The Honda Insight, an excellent PHEV has a battery that weighs over 300 kg, add another 150 kg for the ICE and the Powertrain weighs 500 kg (the Honda does not have a transmission). Similar excellent PHEV like the Chevy Volt and Toyota RAV4 Prime have 300+ kg batteries. Also, they have only 14 kWh of Usable Capacity (that is the important metric) and a good all electric range.

The Tesla Model 3 SR+ and the Toyota BZ4X (in China with the CATL battery) have battery weights of less than 500 kg and are LFP which means No Cobalt or Nickel.
The 2024 Tesla will probably have the new CATL Shenxing battery which will reduce charging times and will have a long life even with fast DC charging. BTW the Mercedes eActros 600 kWh battery Class 8 truck also uses these batteries.

So PHEV for everyone is no advantage. However, there are some applications where a lightweight. Range Extender ICE like the Shkolnik Liquid Piston engine will be useful. If your requirements are that you are pulling a trailer and travel more than 450 miles in one day or live in remote areas without an access to EV chargers, then the Shkolnik engine looks good. Particularly if you use renewable diesel or Ethanol.
Maybe put that Range Extender on the trailer so you only use it when necessary.


IMO, the range extender does not have to be super efficient, as it will not be used that much. It does have to be small and light and not too expensive, and as it should have a reduced operating envelope, should be able to be made reasonably efficient at reasonable cost.
Not good if you want to pull boats a lot, but great for long haul motorway cruising.

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