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Motiv Engines introduces 2nd-generation split-cycle concept; MkII Clarke-Brayton heavy-duty engine being designed for LNG

8 May 2014

Mkii-section
Section of the MkII Clarke-Brayton split-cycle engine. Note the three different cylinder sizes. Air moves sequentially from induction (top cylinder) to combustion (middle cylinder) to expansion (bottom cylinder). Click to enlarge.

Motiv Engines, LLC introduced the second-generation of its engine concept dubbed the MkII Clarke-Brayton Engine, which it intends to develop into a heavy-duty on-highway engine fueled by liquid natural gas (LNG). The prototype is fueled by diesel, a first step in proving the technology before developing a new LNG fuel system.

The MkII Clarke-Brayton Engine is a boxer-configuration split-cycle engine implementing what Motiv calls the Clarke-Brayton cycle. The thermodynamics of the engine are virtually identical to the company’s previous CCI (Compact Compression Ignition) design, as described in a 2013 SAE paper, but are implemented in a much more conventional way.

Motiv suggests that the MkII engine will be up to twice as efficient as a conventional automobile engine and up to 20% more efficient than modern diesel engines. It achieves this by splitting the processes of an engine into three separate optimized spaces—i.e., across three optimized cylinders—rather than performing all the operations in a single “compromised” space. This allows it to implement a fundamentally more efficient cycle, achieve an extremely high 56:1 compression ratio leading to 30 MPa peak pressure, and lose much less of its energy to heat than a conventional engine due to the very small bore of the combustion chamber (the middle set of cylinders).

(The very high compression ratio also enables the engine to burn natural gas as a compression ignition fuel, without any ignition assistance.)

As illustrated in the animation above, air moves sequentially through the 3 cylinders from top to bottom. The small combustion piston area leads to lower forces on the crank than a conventional engine would have if it were able to reach similar pressures, reducing the friction compared to conventional architectures. It expands all the way to ambient pressure before the exhaust stroke.

Gas transfer from one cylinder to the next is begun at equal pressures on either side of the valve, keeping velocities low and minimizing pumping losses. The power is produced in almost a 50-50 split between the combustion (central) and exhaust (largest) pistons.

All valves are actuated by overhead cams. Piston ring sealing is completely conventional, eliminating the dynamic effects of the first-generation design. The reciprocating mass is greatly reduced.

We have taken what we learned from our first engine and applied it to the new design. This is a major step toward our goal of building the most efficient engine in the world for trucks, automobiles and power generation. The improvement in efficiency and the use of natural gas makes this engine a natural fit to meet the upcoming greenhouse gas emissions regulations while reducing costs for operators.

—Ed O’Malley, Motiv CEO

Motiv said that the major components of the MkII Clarke-Brayton Engine have been released for casting and everything else should be released for fabrication within a couple of weeks. The company will test the engine this summer.

Background. The technology for the MkII Clarke-Brayton Engine originated at Caterpillar, Inc. where it was invented by John Clarke, now the Chief Scientist at Motiv Engines. Motiv has advanced the technology from basic concept through the construction and testing of the first prototype to the MkII.

The Clarke-Brayton Engine uses a modified Brayton Cycle—the same cycle used by gas turbine engines. At equal pressure ratios, the Brayton Cycle is inherently more efficient, but even more so at the extremely high 56:1 or greater compression ratio of the Clarke-Brayton engine.

At the Engine Stretch Efficiency Colloquium in 2010, Clarke noted that the classic piston and crank mechanism constrains thermodynamic efficiency, while separating the cylinders allows them to be individually optimized.

There are a number of advantages to this split-cycle concept:

  • The expansion cylinder can be larger than the compression cylinder, thereby realizing the expansion advantage of the Atkinson or Brayton cycles over the conventional Otto and Diesel cycles.

  • Moving the compressed gas to a smaller diameter combustion cylinder with a geometric compression ratio of approximately 8 improves both the chamber shape and time available at optimum combustion conditions.

  • The Clarke-Brayton engine’s three sequential chambers allow for a large bore compared to stroke enabling a smaller engine size without compromising on combustion chamber shape or friction.

  • With combustion occurring in a smaller diameter cylinder, the required crank diameter is reduced.

  • While the Clarke-Brayton Engine is a four-stroke engine, there is a power stroke every revolution, as with a two-stroke engine. This enables a higher power density.

Motiv suggests that the Clarke-Brayton engine design can deliver brake thermal efficiency of 52%, and the same power as a conventional engine but at 1/4 the size, as well as lower noise levels and less vibration.

Gas transfer. Fundamental to achieving the theoretical benefit, however, is exchanging gas between the cylinders without incurring losses large enough to negate the other benefits. Clarke earlier articulated four requirements for efficient gas exchange:

  • The duration of the transfer process must be long enough and the flow area large enough that the pressure loss arising from the flow should be small relative to the useful pressure differences developed by the engine.

  • Starting the gas exchange between two significant volumes at different pressure, leading to a blow-down event, must be avoided.

  • Stopping the gas exchange should occur at a time when the flow is minimal.

  • The timing of the transfer process itself should not reduce the useful compression and expansion ratios inherent in the engine.

In the CCI engine, gas transfer from the induction to the combustion cylinder starts when the pressures are almost equal and continues for 90 degrees of crank angle during which compression continues with both chambers reducing volume. The end of this transfer occurs when the induction cylinder reaches its minimum volume and the flow stops, so the closing corresponds to a no-flow situation.

Transfer from the combustion cylinder to the exhaust cylinder takes place when the exhaust cylinder volume is minimal. If there is any pressure difference, this can be adjusted by exhaust valve closing time; the flow needed to equalize pressures is very small and the losses accordingly are small.

Transfer occurs during 90 degrees and during this time both cylinders are expanding but because the exhaust cylinder expansion rate is higher most of the exhaust gas leaves during this process.

The end of the transfer occurs when the combustion cylinder is at maximum volume so that flow rate is again small and the loss due to closing the transfer port is minimal.

Resources

  • Clarke, J. and O’Malley, E. (2013) “Analytical Comparison of a Turbocharged Conventional Diesel and a Naturally Aspirated Compact Compression Ignition Engine both Sized for a Highway Truck,” SAE Technical Paper 2013-01-1736 doi: 10.4271/2013-01-1736

  • John Clarke presentation at Engine Stretch Efficiency Colloquium, 2010

  • C. S. Daw, R. L. Graves, R. M. Wagner, J. A. Caton (2010) “Report on the Transportation Combustion Engine Efficiency Colloquium Held at USCAR, March 3–4, 2010” ORNL/TM-2010/265

  • J. M. Clarke and W. G. Berlinger, “A New Compression Ignition Engine Concept for High Power Density,” pp. 45–52 in Proceedings of the 18th Annual Fall Technical Conference of the ASME Internal Combustion Engine Division, J. Caton, Ed., vol. 27-1, book G1011A, 1996

May 8, 2014 in Concept Engines, Engines, Natural Gas | Permalink | Comments (19) | TrackBack (0)

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It's as if someone decided to do Scuderi one better.

Could this possibly be used with something like the OPOC configuration?

EP, With the OPOC engine being piston ported and operating essentially in 2-stroke mode mostly, it strikes me that gas transfer questions might prevent this design being made using such a layout. Even with what they have I would be a little concerned about the durability of the exhaust valves in the combustion cylinders, which seem to have little provision for serious cooling of the valve heads.[use sodium-cooled valves?] Otherwise it seems a very clever re-think of the century-old ICE.

I don't see why much of this can't be achieved, in a single cylinder configuration, with careful valve timing to optimize the expansion ratio, a very high compression ratio and modern controls. Without the excessive piston ring swept area and resulting frictional losses.

Single-cylinder doesn't get you the favorable ratio of combustion chamber area/volume.

Which favorable area/volume ratio do you mean? The comparatively tiny, main combustion cylinder with it's unfavorable area/volume ratio by virtue of it's diminutive size?

Larger engines are generally more efficient, largely due to: a) lower piston ring swept area, b) advantageous area/volume ratio's that reduce combustion heat loss.

The favorable area/volume ratio you get by not needing a tiny, flat squish space to achieve the 54:1 overall compression ratio.

This is a compound pure piston engine in which compression is done in two stages and expansion is also done in two stages.

In contrast, a typical turbo-diesel is also a compound engine having two-stage compression and two-stage expansion but the low-pressure compression and expansion stages are done by centrifugal compressor and turbine expander, leaving the high compression stages for the piston. Since turbocharger and turbocompounder turbines are very small and light for the amount of air that they can move, turbocharged engines have superior power to mass ratio to non-turbocharged engines.

With the above in mind, my first question is:
What is the power density of the Motiv engine?

Obviously, to have adequate power-to-weight and power-to-volume ratio, the Motiv engine must adopt much higher pressures than conventional turbodiesel engine. However, such is not the case! With a 3:1 turbo boost at peak power that is intercooled and a 20:1 CR in the piston part, the overall CR of a HD turbodiesel would be as high as 60:1, while with a 2:1 boost at cruise, the CR would be around 40:1. Obviously, current HD turbodiesels are no slouch when it comes to compression ratios.

The Motiv engine has another trick up its sleeve: Two-stroke cycle! Brilliant. However, as E-P has astutely pointed out, OPOC config. would serve much better for this purpose in term of much better scavenging with uniflow piston-ported arrangement rather than using overhead poppet valve ports, and much better heat transfer efficiency.

However, OPOC gives one large power pulse while a pair of piston-cylinder when fired alternately would give two gentler power pulses. So, back to square one: Why not just keep the current very-well proven 6-cylinder in line config. of HD turbodiesels, but instead of having all 6 firing cylinders, the Motiv engine in l-6 config would need only 2 firing cylinders but firing twice as often as that of a 4-stroke diesel? Of course, the firing cylinders would have piston-ported intake ports on the low end while poppet-valve exhaust ports on the cylinder head as the standard in 2-stroke diesel engines.

The CR of the low-pressure compressor should be no more than 3:1, while the CR of the firing cylinder should be anywhere from 12-15 or higher, in order to keep the exhaust cool enough to avoid melting the valves.

The next issue is the apparent lack of intercooling that is the standard in turbodiesel engines. Peter XX would heavily frown upon this lack of intercooling, and I hope that he would kindly give his expert opinion about this engine design.

In defense of this concept: Clarke (footnote 2) anticipates that his long-borestroke engine would do away with the transmission. This reduces mechanical losses and cuts out a large piece of equipment and weight that needs its own fluid control and deliberate friction. Very good for the commercial vehicles Caterpillar makes. The less fluid, the more control and less choppiness to acceleration.

I suppose that the turbocharger can deal with the acceleration requirements that most gearshift and jakebrake driving anticipate. Nothing precludes an advanced injection-ignition system like Ecoboost to improve particulate and heat management. At some point, such a combination of technologies would make intercooling redundant. And Roger, the two cylinders would not really fire twice as often as a six thanks to slower shaft movement.

We hope that this high compression ratio can be sustained reliably enough to deliver the promised efficiencies under all circumstances. This is the problem all HCCI research anticipates: Changing engine load and crankshaft speed defeat effective HCCI.

I agree that poppet valves can be replaced. But this is apparently for the diesel commercial market and they HATE oil leaks. Metal is more straightforward to some.

How about a radial camshaft and clutch system where the pistons are arranged in parallel, like sticks in a bundle? There were some concepts in the area floated over the years.

Great discussion! A couple points:

1)Surface area to volume ratio: This is not a scalable measurement and therefore cannot be used to compare engines of different sizes. We prefer a dimensionless term of combustion chamber surface area divided by the surface area of a sphere of the same volume. If you compare the Clarke-brayton to a conventional engine of the same power, compare a 159mm bore diesel with a 168mm stroke and a 18:1 compression ratio. How much surface area is there compared to the clarke brayton whose combustion bore is 80mm with an 88mm stroke and about a 12:1 compression ratio (but 56:1 overall)? The clarke-brayton has a lot less surface area for heat loss!

2) Conventional turbocharged diesels do not get anywhere near a 56:1 effective compression ratio. The highest compression diesels out there that I am aware of reach a peak combustion pressure of about 20MPa and the Clarke-Brayton reaches 30MPa.

3)I think there is some confusion about the fuel system being used. The current engine is using a common rail diesel direct injection system. We plan on developing an LNG system in the future. This is not HCCI and it is not spark ignited.

4) Intercooling is of no use on the Clarke-Brayton. Think of the intake cylinder as the compressor on a turbo. The reason you have an intercooler is to reduce the volume of the heated up compressed air so you can get more of it into the cylinder. Our compressed air is forced by the intake piston into the combustion chamber. It must all go in, no matter what (except for a small clearance volume + the volume of the transfer passage). Intercooling would not get any extra air in!

5) We fire every revolution, but it is not a 2-stroke cycle. Thermodynamically it is the same as a 4-stroke cycle(but it takes 5 strokes to work one charge of air all the way through the engine - what should we call this??). It is not a scavenged engine. The transfer ports in and out of the combustion cylinder are valved.

Thanks, Mr. O'Malley, for the clarification and additional info.

Indeed, the whole concept is a brilliant idea for the purpose of compression-ignition of natural gas which requires much higher ignition temperatures. Potential heat loss will indeed be lower due to the smaller combustion chamber surface area.

However, at 56:1 CR ideal adiabatic compression will result in ~1500K temp and 280 bar pressure, starting at 300K ambient.
A turbocharged intercooled engine with 3:1 boost and CR of 20:1 will result in ~66 bar x 3 = 198 bar peak pressure, but much lower temperature of 990 K.

Assuming peak combustion temperature of ~2400 K for both situations, then:
--the Motiv eng will have the gas expanding 2400/1500= 1.6x at 280 bar pressure.
--the turbodiesel will have the gas expanding 2400/990= 2.4x at ~200 bar pressure.
Graph those and take the area under the curve, or do integration math of adiabatic expansion process, and we can see that under ideal condition, the turbodiesel may have a small advantage in work done per cycle for a given air mass.

However, in real life, the Motiv engine may be able to get equal or more work done per given air mass per cycle due to the more efficient adiabatic compression process using less energy, lower heat loss, and more efficient expansion work harnessing process due to the higher efficiency of the large-bore piston.

However, we must expect 2-3x larger piston displacements of the Motiv engine in comparison to the piston displacement of a turbodiesel, due to the 2-3x boost pressures of the turbocharger unit, though the low pressure cylinders and pistons can be built much lighter than those of a typical turbodiesel. So, the Motive engine may not weigh any more than an equivalent-output turbodiesel. In term of bulk, the lack of turbochargers and turbocompounders and all associate pipings and heat exchangers for intercoolers may result in the two engine types having comparable bulk (overall size). The oppose layout is great at reducing bulk and weight from a 1/2 as long a crankshaft and crankcase.

At high loads, the centrifugal compressor and radial turbine expander of the turbocharger are somewhat more lossy in comparison to the large-bore pistons and cylinders. However, at low loads typical of Light-Duty Vehicles (LDV), the 2-3x larger displacement of the Motiv engine in comparison to a turbodiesel may result in higher viscous friction drag, perhaps not too much, more due to the much larger bore than stroke hence low swept surface areas.

According to the video, the central cylinders at BDC are receiving input air from the compression cylinders at top of the video while simultaneously exhausting combusted gas out to the bottom expansion cylinders, so, in effect, scavenging is taking place the same as in a 2-stroke diesel engine. This is how firing can be done at every revolution, while expansion is done twice per revolution.

Piston-ported intake port in order to effect uniflow scavenging is not an issue in the combustion cylinders as far as particulate emission is concerned w/ the use of ashless oil that can burn completely during the lengthy double expansion phases in the both expansion cylinders. DPF can take care of the rest should particulate emission will still be a problem, but I doubt it.

Overall, this is quite a brilliant engine design and appears to be quite feasible and advantageous, and can achieve the 20% efficiency gain over existing turbodiesel, though the expectation of 100% gain over LD petrol engines in LDV in the low-load cruise mode will remain to be seen. It will be quite exciting to follow the continous progress of this Motiv engine through the prototype testing phase.

I can see the Motiv engine with intercooling for 2 reasons:

  1. Reducing the work involved in pumping the intake air into the combustion cylinder.
  2. Reducing the air temperature in the combustion cylinder to limit NOx production.

Regardless, I wish Motive the best of luck.

Good point, E-P.
I forgot to take into account of the higher compression work done to get to 280 bar and 1500 K that must be subtracted from the gross work output in order to get at cycle's net work output. This means that for a given air mass intake, the Motiv engine will have less net work output than a comparable turbodiesel engine. This is in keeping with the principle of Brayton cycle, in which if one raises the pressure ratio while keeping the peak combustion temperature a constant, one will raise efficiency but will lower the net work output per given air mass flow. At the highest possible efficiency point in which the temperature of the compressed air approaches the combustion temperature, almost no net work output will result. A practical Brayton-cycle engine will be a delicate compromise between efficiency and net specific work output, while giving consideration to metallurgy and heat transfer and tribology (lubrication) as well.

Still, the simplicity of the Motiv engine having only 2 injectors for the 6 cylinders and complete absence of turbomachinery as well as absence of intercoolers and the maze of air piping for the degree of efficiency that it can attain would make it quite a worthy pursuit indeed. Undoubtedly, NOx control will need SCR just like other HD turbodiesel due to the equivalent temperature of combustion, so NOx control won't be a problem, while the NG engine can get away with having DPF.

This has been one of the more interesting threads on this site. It's nice to see such interest in alternate designs. I often reside in the past, so please excuse my vantage point. While the energy required to drive a turbocharger is not free, it's exhaust energy, not crankshaft energy. There are some advantages there, especially when compared to crank driven piston compressors. It's been done before and has never been effective on conventional engines. With the possible exception of the Shindawa hybrid 4 engine that used the area under the piston to compress air (even then, it's not a great success). Nor do I see packaging problems of turbochargers as a serious issue. Example: a "hot V" 120 deg V6 with centrally located turbocharger or inline 6 with directly mounted turbo. Charge air cooling can be used, or not. In the discussion above, no mention has been given to increasing the expansion ratio of a conventional compression ignition engine via valve timing and geometric changes. I also have to wonder about heat transfer issues in the main combustion chamber. In the gas turbine engine world, we maintain reasonable temperatures with massive quantities of additional air. How is such a small cylinder going to cope with such high thermal loading?

@cujet,

Please allow me to revise my previous analysis of the specific output of the Motiv engine in comparison to a turbodiesel, and perhaps you will see the advantages of the Motiv design.
Quote from my previous posting:
"Assuming peak combustion temperature of ~2400 K for both situations, then:
--the Motiv eng will have the gas expanding 2400/1500= 1.6x at 280 bar pressure.
--the turbodiesel will have the gas expanding 2400/990= 2.4x at ~200 bar pressure.
At equal mass flow and equal final temperature, the potential for gross work output (W=PV) will be the same. The Motiv engine at 280 bar pressure will have smaller gas volume than the turbodiesel at 200 bar pressure, so the gross work output will be the same. However, the net work output will not be the same because more work input was done to compress the air adiabatically to 280 bar than the work input to compress the air to 200 bar. Subtracting the compression work input (compression stroke) from the gross work output (expansion strokes) will give the NET work output of each engine. We can readily see that the POTENTIAL for net work output of the turbodiesel THEORETICALLY will be a lot higher than that of the Motiv engine.
Note that the 1.6x and the 2.4x may seems to be the ratios of gross energy output over the invested work input, but that is not the whole story. The turbocharger must input work to compress the air mass initially to 3 bar pressure, from 300 K to 410 K, or 110 K temperature gain. Adding this to the 990 K final adiabatic temperature in the cylinder of the turbodiesel will give 1100 K as equivalent of total work input into the air mass before further heat addition. So, dividing 2400 by 1100 = 2.18 as the ratio of gross work output over the work input. So, 1.6x vs 2.18x of gross work output over net work output for Motiv vs turbodiesel.

Graph those and take the area under the curve, or do integration math of adiabatic expansion process, and we can see that THEORETICALLY, the turbodiesel has an advantage in NET work output per cycle for a given air mass.

However, in real life, this is not the case! Most turbodiesels do not have turbocompounder to harness the remaining work output from the exhaust turbine, and the net work potential of the exhaust turbine after powering the compressor is wasted. Only the work done by the piston turning the crankshaft is harnessed as net work output. The pressure in the cylinder of a turbodiesel just before blow down is quite significant, and when multiplied this to the volume of gas at this point in the curve may represent as much as 1/3 of the total potential work output of the combusted gas mass.

See the the graph on page 4 of the following reference:
http://www.imeche.org/docs/default-source/turbocharging-papers/2_Design_validation_and_performance_results_of_a_turbocharged_turbogenerating_biogas_engine_model.pdf?sfvrsn=0

The centrifugal compressor only harness a portion of this energy, and the centrifugal compressor has adiabatic efficiency of only 70-75%, while the intercooler remove a significant chunk of this energy into the ambient air instead of contributing it into the total energy of the air mass intake.

Meanwhile, the radial turbines commercially available for the size of HD turbo-diesel have isentropic efficiency of around 80%. This, in contrast to the large-bore low-pressure piston compressor and expander with very low-tension piston ring and perhaps low-viscousity oil used, in the Motiv engine, may have isentropic efficiency at 90-95%!

So, the Motiv engine may have comparable net specific work output as a HD turbodiesel for a given air mass flow when one considers the losses in the turbocharger and at blow down, while may gain 20% in efficiency in term of fuel burn for a given net output due to lower losses in the low-pressure compressor and expander and no loss in the intercooler since it has none and no loss from exhaust blow down at the typical output power at cruise.

I hope that you can appreciate the reasons for the efficiency gain of the Motiv engine from this analysis, when the entire pressure of the combusted gas is expanded in the large-bore expander piston completely to atmospheric pressure.

@Cujet,

Furthermore, I forgot to address the issue of thermal stress on the small combustion cylinders, as you rightfully pointed out. This is a severe issue and can be a real show stopper. However, I'm confident that Motiv engine designers have already done their homework in determining the upper limit of pressure and temperature of their engine. 280-bar pressure is probably the very upper limit, as selling point for the engine in order to get your attention, however, for more reliable daily operation, perhaps this pressure should be reduce to somewhere closer to 200 bar, in line with current practice. Without intercooling, I hope that 200 bar pressure will be hot enough to ignite natural gas for compression-ignition combustion, since the temperature will be around 1,200 K, while auto-ignition temperature for methane is around 900 K.

One important mitigating factor thermal-wise is the two-stroke cycle of the combustion cylinders. Scavenging of hot exhaust gas with cooler incoming air right at the end of the power stroke will limit the time of exposure of the cylinder chamber to hot gases to about 1/2 of that of a four stroker. This is perhaps the most important factor in reducing heat transfer loss of this design.

However, I would recommend to limit the maximum CR of the compression cylinder to 5 or under, and to raise the CR of the combustion to 10 or higher, in order to reduce thermal stress on the exhaust valves and lubrication film.

If a piston-ported intake valve is used to allow uniflow scavenging as is well-proven in 2-stroke marine diesel engines, then perhaps a dedicated high viscosity high-temperature oil can be used on the combustion cylinder on a total-loss basis to adapt to the higher heat and pressure of the combustion cylinders. This is what actually done on marine 2-stroke diesels that are very durable and very efficient. The remaining low-pressure cylinders can adopt much lower viscosity oils in order to reduce viscous friction drag.

Some say we will all drive electric cars on wireless highways soon, so there should be no need for engines.

Good point SJC.

R-P, thank you for the more detailed thoughts on the Motiv design. It's a good bit of information to chew on. Know that you and I disagree on a number of minor points, such as reciprocating compressor efficiency, turbocharger effectiveness sans intercooler and the required heat transfer of the Motiv combustion chamber (I certainly don't see 2 tiny cylinders doing the work of 4 or 6 larger ones as thermally feasible) Even so, I do see why you have confidence in the Motiv design. And, I agree that with proper design, it's likely to achieve good BSFC numbers.

Quote: "Only the work done by the piston turning the crankshaft is harnessed as net work output." Note: We regularly design turbocharged engines with more intake pressure than exhaust pressure, resulting in recapture of some exhaust energy.

SJC, there will always be a need for energy portability. Being tied to the grid and/or batteries won't work everywhere. Aviation is one example.

@cujet,

Thanks for the interesting discussion, and for sharing your perspective.

Actually it's not just the 2 tiny cylinders doing the work of 4 or 6 larger ones. In the Motiv engine, the two large low-pressure expander cylinders are contributing almost as much torque as the combustion cylinders. Even though they are low in pressures, their very large size compensates for that, and since W=PV, low pressure can be compensated for by large volume to arrive at the same work output.
So, the Motiv engine has 4 working cylinders, each having a power stroke with every revolution, while a 6-cylinder turbodiesel each cylinder produces power once every two revolution so is equal to only 3 cylinders. So, the 2-stroke Motiv engine has more working cylinders per revolution than a 6-cylinder 4-stroke engine.

You're right about "We regularly design turbocharged engines with more intake pressure than exhaust pressure, resulting in recapture of some exhaust energy." but it would take very efficient compressor and turbine pair to do that, may require high manufacturing cost, and the extent of the exhaust pressure recoverable is quite small.

@SJC and Harvey,

A simple and low-cost engine that exceeds 50% thermal efficiency that can use synthetic methane produced from pyrolyzed biomass with simultaneous hydrogenation from renewable H2 would have comparable CO2-sparing impact as a BEV.

BEV advocates should be glad that these modern ICE technology that can run on CO2-neutral fuels will help reduce the scarcity of copper, lithium, and rare-earth metals and will allow continually good profit margins for BEV producers. No doubt many people will prefer BEV's over ICEV's, but heavy duty trucks and taxis and couriers and police and ambulance services will need a sort of vehicles that can be refueled quickly with CO2-neutral fuels. FCV's can be filled up quickly, but they are still EV's that will consume copper and power electronics and rare-earth metals etc...The dual existence of high-efficiency and CO2-neutral ICEV's and EV's will be good thing for our future.

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