Hydrogen is in one way a practically perfect fuel for an internal combustion engine: just burn H2 in oxygen and it produces useful heat and… water vapour. The reaction does not generate carbon dioxide (CO2) or particulates because there is no carbon in the fuel.
In practice a very small amount of CO2 is produced, because some lubricating oil enters the combustion chamber via crankcase ventilation or past the piston rings. But this represents less than 1% of the carbon emissions of a conventionally fuelled engine.
However, only around 21% of our atmosphere is oxygen — 78% of it is nitrogen, and at high combustion temperatures this reacts with oxygen to create polluting nitrogen oxides (NOx). So, much of the effort in designing hydrogen engines has been concentrated on reducing NOx emissions while maintaining power output and thermal efficiency. A conventional catalytic converter may not be necessary: EGR or water injection can be incorporated to reduce combustion temperatures, but the most significant way to eliminate NOx is to run the engine lean — at a λ value of 2 or more. Hydrogen can be burned over a very wide range of air/fuel ratios, and will work at much leaner mixtures than liquid fuels, at λ values of up to 4 or 5. As BorgWarner puts it: “In lean combustion mode, NOx emissions reduce to near-zero levels.”
The auto-ignition temperature of hydrogen is high: around 500°C, compared with less than 280°C for petrol. This allows higher compression ratios to be used, improving thermal efficiency, although that makes conventional compression ignition more difficult, so manufacturers have generally stuck with spark ignition rather than a diesel cycle. They generally also use direct fuel injection, as otherwise the low density of hydrogen prevents enough air from entering the combustion chamber, and (with its fast flame speed and low ignition energy) hydrogen also tends to backfire in the inlet tract.
BMW
BMW built the six-cylinder 745i Turbo – a bi-fuel (petrol or liquid hydrogen) 7-Series saloon – as early as 1984, followed by updated models in 1990 (the 735iL), 1995 (the 728h) and in 2000 the V12 750hL. The Hydrogen 7, introduced in 2007 was a well-developed bi-fuel variant of the 7-Series limousine, with a six-litre V12 developed from its petrol range.
Like the other BMWs, it used hydrogen stored as a liquid (LH2). This looks attractive, as you can store around 75% more hydrogen as a liquid at atmospheric pressure than you can as a gas at 700bar. However, it needs to be kept at -253°C. Even with a vacuum-insulated tank, ambient heat warms and evaporates the fuel, which needs to be bled off to keep the pressure down. For a road car, this is a problem: BMW warned that the Hydrogen 7 should not be parked in an enclosed space, and admitted that it would probably bleed off all its usable fuel in less than a fortnight! (Incidentally, the bleed-off went through a catalytic converter which admitted oxygen to turn the hydrogen into harmless water.)
Since the volumetric energy density of LH2 is still much lower than petrol or diesel — barely a quarter of that of the same volume of petrol or diesel — the BMW’s 170-litre tank only gave it a range of 200km.
The engine used port injection rather than direct injection: this is feasible when using LH2, as the low-temperature fuel cools the charge air and the mixture remains stable until it gets into the cylinder. The firm made the point that the engine delivered the same power using either petrol or liquid hydrogen — but at 256bhp, the specific power output was unimpressive. However, BMW built around 100 cars, making the Hydrogen 7 perhaps the nearest thing to a production hydrogen IC car there has ever been.
In 2009 BMW revealed the experimental H2BVplus engine, with a cylinder head derived from a production diesel. This reportedly achieved a maximum of 42% thermal efficiency — around the same as a top-end turbodiesel. It used high-pressure (300bar) direct injection and “a combination of spark-ignition and diesel combustion systems using surface ignition… followed by a diffusion type of combustion”. BMW said that “the entire characteristic engine map range of a typical passenger car engine can be covered” and “further increases in overall efficiency will be possible… as a result of waste heat utilisation”.
TOYOTA
Toyota has worked longer than most to produce hydrogen-fuelled IC engines: in 2021 it entered a Corolla with a radically modified petrol engine into the Fuji 24-hour race. The three-cylinder 1.6-litre turbocharged spark-ignition engine used direct fuel injection, with hydrogen stored in tanks at 700bar. It was developed from earlier models which required a 50% petrol input to initiate combustion. The car completed over 1,600km, but only 12 hours were spent driving, as the team spent four hours refuelling the car — 35 sessions of seven minutes each — and eight hours in inspection and servicing.
Since then, Toyota has competed in the 24-hour race each year (in 2023 with LH2) and has demonstrated a hydrogen-fuelled version of its GR Yaris rally car. It also revealed the AE86 H2 Concept – a hydrogen-powered ‘restomod’ conversion of its 1980s Corolla coupe.
Toyota is also associated with HySE (Hydrogen Small mobility & Engine technology), a research group set up by Kawasaki, Suzuki, Honda and Yamaha to develop hydrogen-powered engines for ‘small mobility’ — defined as “motorcycles… mini-vehicles, small marine vessels, construction equipment, drones, etc”. Members have already produced off-road four-wheelers with H2 engines, and the consortium will enter the HySE-X1 buggy into the 2024 Dakar Mission 1000 alongside the main race.
MAZDA
In 2006, Mazda combined its classic low-vibration Wankel rotary engine with hydrogen fuel in the RX-8 Hydrogen RE (below). From H2, this twin-rotor bi-fuel car produces 106bhp and 140Nm. An advantage of the Wankel engine burning H2 is its inlet port is effectively a long distance from its combustion chamber (unlike a reciprocating engine) so preignition is less of an issue. A 350bar, 110-litre tank provides 100km range.
Mazda is persisting with the petrol-powered rotary engine, now as a range-extender for its MX-30 hybrid electric car. There are sporadic reports that an H2-powered version is in the works – but no official word yet.
BOX: US RESEARCH
The Hydrogen Opposed-Piston Working Group, which consists of universities and firms such as Mahle Powertrain, Ricardo North America and Shell, is planning an opposed-piston two-stroke (OP2S) hydrogen engine for medium commercial use. The six-piston engine (with three parallel cylinders and two crankshafts) is being developed by Californian firm Achates Power, and uses direct injection.
The group is looking at ‘dual ignition mode combustion’, using spark ignition at cold start and low loads, moving to compression ignition at medium and high loads, giving very high thermal efficiency, it claims.