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The Arguments for Hydrogen Combustion Engines

BMW envisions the development of hydrogen combustion engines that eventually use charged, direct injection to deliver high efficiency. The current Hydrogen 7 is represented by the H2-PFI column. Click to enlarge. Source: BMW

Speaking at the California Air Resources Board Zero Emissions Vehicle (ZEV) Symposium, representatives from Sandia National Laboratories and BMW laid out the rationale and technical strategies for a focus on hydrogen-fueled combustion engines (H2ICE).

Using hydrogen with a combustion engine, according to Dr. Andy Lutz from Sandia, is a pragmatic bridge to a hydrogen economy. The technology is available today and economically viable in the short term, with fewer constraints concerning hydrogen storage compared to fuel cells. Impurities, for example, are a non-issue for a combustion engine (“You burn them right up.”).

PHydrogen engines have demonstrated efficiencies (BTE) in excess of today’s gasoline engines, NOx is the only regulated tailpipe pollutant resulting from hydrogen combustion, and carbon dioxide is a non-issue, at least in terms of the driving (Tank-to-Wheels) component of the lifecycle.

Although current efforts by Ford and BMW reflect early stage development, both BMW and Sandia outlined technology approaches for subsequent generations of H2ICE systems that could deliver significant improvements in fuel economy and emissions reduction, while delivering additional power.

BMW’s projected product pathway. Click to enlarge. Source: BMW.

Dr. Edgar Berger from BMW, in particular, described a future generation H2ICE 4-cylinder engine that could deliver more than 140 kW (188 hp) of power with fuel consumption of 1.4 to 1.6 kg H2/100 km.

One can reach, in fact, 1kg/100km H2—but the price is to reduce vehicle properties and customer benefits.

—Edgar Berger

In terms of its basic combustion properties, hydrogen offers certain benefits and certain challenges compared to gasoline. (See table below.)

Combustion properties of gasoline, CNG and hydrogen. Favorable hydrogen properties are tagged in blue; unfavorable in red. Click to enlarge. Source: Sandia National Laboratories

Its wide flammability range (Φ) supports a much leaner burn mixture—a factor that is important for emissions management strategies. The much higher laminar flame velocity produces stable flames under more extenuating circumstances, and, combined with the higher autoignition temperature, creates a higher research octane number that supports higher compression.

On the downside, hydrogen has a high stoichiometric volume fraction, which affects how much charge passes through the engine in a given displacement, and in turn affects the power of the engine.

It also has a lower minimum ignition energy and hence has a tendency to pre-ignite.

The researchers at Sandia have identified five possible approaches to dealing with the challenges posed by hydrogen combustion.

  1. Continuous ultra-lean (Φ<0.45) operation with improved power densities. This, combined with turbo- or supercharging is the approach Ford is taking with its H2ICE Focus passenger car and E450 shuttle bus. For also has a H2ICE-hybrid research vehicle—the H2RV— that combines a 2.3-liter combustion engine with a 30 hp electric motor. All vehicles deliver SULEV emissions or better.

  2. Operate at stoichiometric conditions (Φ=1) with aftertreatment. Possible routes within this strategy include the use of liquid fueling to prevent preignition if the fuel can be kept cold to the point of injection; direct injection, and the use of Exhaust Gas recirculation.

  3. A multi-mode strategy. This is the approach BMW is taking with its Hydrogen 7, running ultra-lean under partial load to minimize engine-out NOx, and at stoichiometric condition under full load, coupled with the use of a three-way catalyst to handle the resulting NOx. (Earlier post.)

  4. Another variation of the multi-mode strategy uses ultra-lean mixes at low load, pressure boost in the medium range, and then lean NOx traps at high load. Ford is looking into this for the H2ICE Focus.

  5. Sandiah2ice2
    Click to enlarge. Source: Sandia National Laboratories
    Mixture stratification with direct injection. This approach would use a stratified and extremely lean mix at idle. At low-load, it would move to an ultra-lean homogeneous mixture. As load increases, the system would start using stratification with direct injection, and then rely on the lean NOx trap at high load. Sandia concludes that such an approach could theoretically deliver BTE of greater than 45%, with emissions significantly below SULEV.

It’s complicated, but with electronic controls there are a variety of things that can be done.

—Andy Lutz

For its part, BMW outlined an ambitious development plan that it intends to result in mono-fuel hydrogen engines with greatly improved efficiency and reduced fuel consumption that it can apply across its entire model range, from luxury to compact.

Advanced energy management. Click to enlarge. Source: BMW.

Mirroring some of the Sandia work, BMW is ultimately looking toward a charged, direct-injection engine as a future generation platform. Berger also described a hybrid architecture that would combine a small fuel cell with the hydrogen combustion engine to augment electric power for vehicle subsystems and traction power.

A key enabler for this strategy is having sufficient hydrogen on-board to fuel the engine. BMW has already opted for liquid hydrogen storage, with its higher volumetric and gravimetric densities than offered by compressed hydrogen.

Volume and weight of different methods of storing 10 kg of hydrogen, which is equivalent in energy to 38 liters of gasoline. Click to enlarge. Source: BMW.

However, BMW believes that it needs to have 10kg on board hydrogen to met its performance and customer satisfaction objectives. Currently, the Hydrogen 7 stores 8 kg in a 150-liter container.

Accordingly, BMW has work underway to expand the storage density of its liquid hydrogen storage, to decrease the boil-off loss, and to increase the loss-free dormancy time.

Furthermore, for its 5 Series size cars, BMW is developing a shaped storage tank it calls the “double bubble”—a single-tank system providing central storage running down the midline of the car in the tunnel.

BMW’s hydrogen storage roadmap. Click to enlarge. Source: BMW.

Ultimately, it sees using liquid hydrogen in the larger classes (luxury and executive) with 7.5 to 10 kg in a given total package of 250-300 liters. For small to medium-class vehicles, BMW is looking at compressed hydrogen, and possibly some activity with cryo-compressed hydrogen.




> Using hydrogen with a combustion engine, according to Dr. Andy Lutz from Sandia, is a pragmatic bridge to a hydrogen economy.

Yeah - a bridge to nowhere.

Rafael Seidl

BMW's system could be an excellent starting point for short-hop passenger and cargo aviation applications. For vehicles, LH2 makes no economic and little if any ecological sense.


All that talk about engines, and nothing about the big issue:  how to get the hydrogen.


What if you want to leave your car for a month? Do you have to vent the liquid hydrogen tank? There is no way to eliminate boil off of LH2. Finally it takes something like 40% of the available energy to liquify hydrogen which is much worse than the 20% or so needed to compress it to 10000psi.


The recently announced hydrogen BMW 7 series loses (boils off) its fuel in 9 days! Lose one-ninth of your fuel every day; what a deal!! Not ready for prime time.

Paul Dietz

If you're burning the fuel in an IC engine, you might as well stick with hydrocarbon fuels and balance emissions by extracting CO2 from the air or ocean surface waters. The fuel tank would be much smaller (even if you use CNG).

John Baldwin

The planet cannot afford to burn extra gas and coal in power stations to make hydrogen to burn in engines. CO2 on a well to wheel basis is awful.......just about the worst fuel possible.

BMW should burn CNG in their cars, the filling stations exist in Germany, no range issue, can even use bio-methane from landfill to run BMWs...truly sustaianable performance. This is inevitiable.


With an H2 ICE, you need just about as much energy as you would for a gasoline ICE as the efficiency of the H2 ICE is not much better. With a H2 FCV, tank-to-wheels efficiency improves, and you need less onboard storage. This is yet another reason why H2 ICEs are not going to be a very practical bet (despite what BMW says), the main reason being that the well-to-wheels efficiency and emissions are horrible.

Energy storage density is obviously a problem with PHEVs and EVs too, (it would be cool to see a similar graphic for that), but you get 3-5x the tank-to-wheels efficiency, so you need 1/3-1/5 the on-board energy storage, which is not the case with H2 ICEs and even FCVs aren't anywhere near as close (a little more than 2x the efficiency if I remember correctly). Plus you get boil-off if you are using LH2 so you lose fuel if your car sits parked for a while (which I can't imagine consumers would like).

PHEVs are a much better short-term bridge to an electrified vehicle fleet. H2 ICEs are a bridge to more or less nowhere.

Roger Pham

Ah, the argument for H2-ICE is very strong. As I've suspected, the efficiency for H2-ICE can approach 45% using the mixture stratification with direct injection. Of course, all ICE's are most efficient at only at a narrow range between 50%-70% of maximum throttle, meaning that an ICE-electric hybrid will be needed to keep the engine running only in its most efficient range of 45% BTE for most of the time. The gasoline Prius is already calculated by Toyota to have a tank to wheel efficiency of 37%, so 45% efficiency for H2-ICE-electric hybrid is a realistic expectation. In comparison to the Honda's upcoming fuelcell FCX with announced 60% efficiency, this is not so bad, given the lower cost and the higher durability of H2-ICE's in comparison to PEM fuelcells.

To all H2 doubters,

The first thing to remember is that H2 can be produced very easily and very efficiently from almost any type of combustible feedstocks. Meaning that a gasoline station in remote areas can be supplied with crude oil, or natural gas, waste biomass etc. and can produce H2 on the spot. A car running on H2 can vicarously use any type of combustible fuels, and not dependent ONLY on one type, such as gasoline car on gasoline with appropriate octane rating with added ethanol or other kind of additives, or diesel depended on low sulfur fuel with appropriate cetane rating. Gas stations in large cities can have H2 piped-in from a local central H2 reforming plant, with optional CO2 sequestration means.

Given the inefficiency and great expense of H2 production from electrolysis of water at room temp, do not expect this to be the source of H2 for mass consumption. There are other methods of H2 production from renewable energy that can rival the BEV or PHEV in term of total source-to-wheel efficiency. Please look at this link
for my last posting that discuss highly-efficient way to generate H2 that can rival the BEV in term of total efficiency.


I think that BMW plans to use compressed H2 for smaller or lower-cost automobiles.
But, for the high-end autos with heavy weights due to over-abundance of luxurious items, compressed H2 do not offer sufficient volumetric efficiency as LH2 for sufficient range, ergo the use of LH2. The efficiency of LH2 sucks in comparison to compressed H2, but the rich surely can afford it. Those who are wealthy enough to purchase BMW's costing over 100,000 USD do not have to worry about fuel efficiency, only about how far between fill-up, as their time is worth a lot of money. (eg. lawyers at >$150 hourly fee)

LH2 is excellent for commercial aviation jets, and the value of LH2 is not just limited to short hops. The longer the range, the more energy-saving when using LH2. Those who wonders why this is so can look for my previous posting on the same subject.

Roger Pham

...continues from above,
This article left out the most important advantage of H2-ICE-electric hybrid over that of FCV: the ability to combust methane-H2 mixture at any ratio. Methane allows over 3 times the volumetric energy density of H2. Thus, the compressed H2 tank needs not be overly large, but just sufficient for a range of ~120-150 miles. If a range of 360-450 miles is desired, just fill up the tank with a mixture of up to 90% methane and 10% H2 (to improve ignitability), and this is a much more cost-effective solution over that of LH2. Of course, in the future, H2 is expected to be produced cheaper and more energy-efficient than methane from renewable energy sources, so most people will probably choose H2 for daily commute. A fill up every 4-5 days with H2 for daily commute is probably Okay, as it will give one time to grab a cup of coffee, a donut or time to glance at the newstand at a local friendly 7-11 gas station. Those high-dollar doctors or lawyers probably will choose to fill-up with methane instead, and pay a price premium. But, hey, this a free capitalistic country. Different strokes for different folks! Those BEV enthusiasts may choose to miss out on the brief weekly social gatherings at 7-11 gas stations altogether and plug in their vehicles at home instead.


H2 can only be made efficiently from chemical fuels (mostly hydrocarbons, alcohols and the like).  Electricity can be made from all of the above, and much more.  Further, the efficiency of gasification to H2 is around 80%, yielding 36% best-case throughput in a 45%-efficient engine; current batteries can hit 90+% efficiency.  Even if you burn natural gas in a CC turbine to make juice (60%), you can get 54% throughput - 50% better than hydrogen.

Energy security is the other side of the coin.  The fewer the options for the energy supply, the less secure you are.  Hydrogen is inherently less secure than electricity.

Roger Pham

You're getting very close. CC Turbine has an efficiency of 60%, however, transmission to the home socket is only 90% efficient, leaving 54% efficiency from power plant to home socket. BEV is generally accepted as having a grid to wheel efficiency of ~70%, considering all the losses in the charger, battery, power inverter, resistance in the motor, friction in the drive train, etc...So, multiplying 54% by 70% will leave you with 37.8% total efficiency for BEV, which is not too far from a calculated number of 36% overall efficiency for an optimized H2-ICE-electric hybrid vehicle. Now, if your electricity comes from coal in conventional coal power plants having ~40% efficiency, as in 50% of electricity production in the USA, and multiply by 90% efficiency from power plant to home socket, and multiply by 70% efficiency of BEV, you'll have only 25% efficiency for BEV from coal to electricity. Whereas, if you gasify this coal and produce H2, you'll have higher efficiency for H2-car.


Your hydrogen car efficiencies don't include drivetrain losses, so you should omit them from the EV example too.  You also assume that the H2-combustion car will always be running at its best efficiency - it can't, while the EV mostly will.

Roger Pham

The Prius has a tank to wheel efficiency of 37% as posted by Toyota's site. This includes all drive train losses. Due to the higher efficiency of H2 combustion, including higher compression ratio, isochoric combustion process, and ultra-lean burn regime (thanks to high ignitability of H2 at very low concentration) in which less heat rejection into coolant and more heat available to do work, an increase from 37% tank to wheel to 45% tank-to-wheel would be achievable. A full hybrid drive train would ensure that the engine is used only in the most efficient power range. EV's don't run at its highest efficiency in hard acceleration, either, due to higher resistive losses in all circuitries including motor windings, power inverter and batteries. If you race R/C electric cars or fly R/C electric planes, you would realize this. The higher the power setting, more electrical energy will be wasted to heat production.


All those advantages are reversed if you don't start with a fossil fuel.  That's my complaint about hydrogen:  it looks like a scheme to lock in the interests of the fossil-fuel corporations for the next century, and the hell with the environment.

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