Direct injection of H2 is the best approach. No need for throttle plate, control engine power using fuel injection only, like in diesel. Fill up cylinder with unrestricted air to cool the combustion chamber as much as possible, and if needed, overfill it with air to cool it even more, then start to inject H2 at thousands of psi during the compression stroke. The high expansion will cool the compression stroke significantly, hence reducing the work of compression, and reduce the risk of detonation or pre-ignition. Then, more net power will be recuperated in the power stroke. With piezo injector, the rate of injection of H2 can be precisely controlled in order to prevent pre-ignition. Also, with good cooling of combustion chamber with excess air provide by a blower, pre-ignition can also be avoided. Ah, yes, I've predicted efficiency as high as 45% for H2 ICE. Glad someone agrees with me!

"Even so, it is widely acknowledged that pure H2 would permit greater thermodynamic efficiency in an ICE than gasoline, mostly because the compression ratio can be higher (14-15 is optimal) and the combustion is extremely rapid (nearly isochoric)."

I have a question about increasing the compressoin ratio and thermal efficiency that I have not been able to find any good explanations for why the same intake charge will give you higher thermodynamic efficiency as you incease the compression ratio?

I was thinking that the molecular fuel electrons are compressed and then greatly expand during combustion, but this is a kinetic mechanical process not a thermodynamic process.

Could it be that the higher compression adds more enthapy to the fuel to reduce the energy needed for combustion process?

Could it also be that the higher compression minimizes the molecular heat transfer and irreversibility between the fuel molecules and reduces combustion process losses?

Yes, there is a mechanical compression ratio limit due to sealing friction of the chamber.

Furthermore, Barry, higher compression imply more expansion for the product of combustion, hence more work done for the hot gas. Consider an engine with 8:1 compression ratio running at 28" manifold pressure, and another engine using the same fuel at 16:1 compression ratio but at 14" manifold pressure. Each engine will have the same charge concentration at TDC, hence the same combustion temperature and peak pressure after combustion. However, at BDC at the end of the power stroke, the low compression engine will discharge exhaust gas that is only expanded 8 times, thus still has twice the pressure as the exhaust gas of the high compression engine, and at a lot higher temperature than the exhaust gas of the high compression engine. This means that a lot more work is extracted out of the combusted exhaust gas of the higher compression engine, even though the two engines has the same combustion temperature.

U r right, Rafael, regarding injecting of LH2, the thermal stress would be unbearable. Furthermore, energy can be recuperated from the LH2 by turning it into gaseous form at thousands of psi temp using waste heat from the engine, using a high pressure pump similar to those used for pumping feed water in steam engine. The thousand-psi hydrogen gas can then be directly injected into the cylinder using precise piezo injector, and the high pressure hydrogen gas will do more work on the piston as it expand, hence we have a sort of open-cycle Rankine engine on top of an Otto-cycle engine. Thus, the energy used to compress or to liquefy the H2 is partially recuperable.

H2 energy should be compared to other forms as solar to wheel, and not as well to wheel, since it would be wasteful to convert carbon-based fuel to hydrogen.

The short range of vehicle on H2 means that H2 should be used for local commute only, and that Hythane (hydrogen/methane mixture of mostly methane) instead, would be better for occasional long-distance driving since it can give 3 times the range for the same volume in the same hi-pressure tank.

The danger of H2 is well recognized, but it has not stopped a world-wide effort on H2 research into its use as energy currency for the far future. Apparently, the dangers of H2 can be controlled using current technology. Furthermore,we have an analogy in that even though gasoline is a much more flammable fuel and more dangerous than diesel fuel or ethanol, gasoline is still a predominant fuel in comparison to the two safer fuels.

Rafael,you've made an astute assessment and a practical point. Indeed, the "Freedom CAR" initiative by the Bush Adm was mostly of disingenious motive, especially in the cancellation of the PNGV program that was aiming at producing a 80mpg family car. A very efficient liquid-fuel car would have been far more realistic in the short term than the nebulous hydrogen fuel cell vehicle, but if the 80mpg car were produced, then Big Oil would not have reaped the huge record profit as of recently. The 1.8 billions USD given for hydrogen research were given mostly to natural gas and nuclear interests friendly to the Bush Adm.
However, the law of nature is such that for every intended action, there will be unintended results or consequences. The money spent on hydrogen research have resulted in promises of high-efficiency and affordable H2 production. H2 can be produced in one step from solar or wind energy right at the source, and consumed at the source, without requiring large, expensive production plant that will require extensive energy used in transportation, and labor used in material processing at a production plant.
Howver, hydrogen promises may never materialize, or may be years or decades away. So, given the current infrastructure geared toward liquid fuel, xTL processes involving inedible biomass would be of far more immediate practicality rather than waiting for hydrogen to arrive. That is, for the current generation of vehicles. Still, I can't help it but get excited every time progress is made in hydrogen research. Such a clean, clean fuel that requires no catalytic converter, so simple that it can be synthesized in everywhere for immediate consumption or short-term storage...etc. For long term storage or transportation via interstate pipelines, hydrogen can be converted to methane via the introduction of CO2 during the synthesis process. Methane storage is a mature process and requires less space and less extreme condition.

The next debatable issue is which would be cheaper and more efficient to produce from biomass: methane gas, or methanol, or BTL products? Methane is widely used for home heating and can be stored, and has been satisfactorily adapted for used as transportation fuel. A vehicle can be made to run on both methane and hydrogen at high efficiency. So, if more realistic progress is continued to be made in the hydrogen front, then it would be wise to start researching on producing the next generation of vehicles to run on both methane/hydrogen using the same gaseous-fuel-injector and the same high-pressure fuel tank, so that, by the time hydrogen start to come out in large quantity at practical price, then no time will have been wasted waiting for the next generation of vehicles capable of running on hydrogen. Many states with chronic air pollution problem can hardly wait to have hydrogen cars coming on board soon enough, so that's will be where the first batch of hydrogen vehicles will be heading to. Biomass may never be enough to satisfy our ever-increasing transportation energy demand in the future when petroleum will run out.
I can see that most of your points are being accurate and practical. It's just that my aspiration is for longer term and further into the future.

Hi Rafael and Roger,

I've been traveling and didn't have a chance to get back to you and thank you for your comments and insights with the compression ratio characteristics.

Thanks again,
God bless you,

Barry