A 221 Compression Ratio Ammonia-Hydrogen HCCI Engine: Combustion, Load, and Emission Performances

If you wanted to store wind and solar energy for months — not hours, not days, but months — what would you put it in? Not a battery. Something you could ship, stockpile, and burn when the grid needs it. One answer is ammonia, a molecule most people associate with fertilizer, and Pochet, Jeanmart, and Contino built an engine that runs on it. Here's the storage problem they're solving. As wind and solar penetrate the grid, electricity supply and demand swing not just across hours but across seasons — and that mismatch demands very large, long-duration storage with low cost per unit of stored energy. The team cites studies showing that Power-to-Fuel systems, where you convert surplus electricity into a chemical fuel, require roughly three orders of magnitude less capital per unit of stored energy than batteries. The chain works like this: excess renewable electricity drives electrolysis to make hydrogen, and that hydrogen is combined with nitrogen from the air via the Haber-Bosch process to produce ammonia, which you can then store as a liquid. Compressed hydrogen is a poor seasonal storage medium — its volumetric energy density tops out around eight gigajoules per cubic meter even at seven hundred bar, and getting it there costs you forty percent of the energy. Liquid ammonia, by contrast, holds thirteen gigajoules per cubic meter at just nine bar or minus thirty-three degrees Celsius. The tradeoff is real — current ammonia production routes from electricity reach roughly fifty to sixty-two percent efficiency depending on the electrolyser technology — but the energy density advantage is decisive for long-duration storage. And because ammonia contains no carbon, reconverting it to power produces no carbon dioxide. Because the Power-to-Fuel chain carries efficiency losses at every step, the reconversion back to electricity has to be as efficient as possible. That's why Pochet and colleagues focused on Homogeneous-Charge Compression-Ignition engines, or HCCI. In HCCI, a premixed fuel-air charge ignites spontaneously from the heat of compression alone — no spark plug, no diffusion flame. The result is high efficiency and low combustion temperatures, which suppresses the thermal nitrogen oxide formation you get in conventional engines. Hydrogen has already demonstrated HCCI viability, with brake efficiencies above forty percent in small naturally aspirated tests. So the concept is proven. The problem is ammonia. Ammonia resists auto-ignition stubbornly, with slow chemical kinetics that make timely combustion in a compression engine genuinely difficult. Force high intake temperatures to trigger ignition and you reduce the density of the charge entering the cylinder, cutting power output. The team's response was twofold: push the effective compression ratio to twenty-two to one, squeezing the charge into one twenty-second of its original volume to generate intense heat, and blend ammonia with hydrogen, since hydrogen promotes auto-ignition and reduces the intake temperature needed. But hydrogen is reactive enough to cause engine knock at higher loads, so the blend becomes a compromise — and the compromise is exactly what the paper maps. The experimental campaign was thorough by design. Pochet and colleagues varied intake temperature from fifty to two hundred forty degrees Celsius, equivalence ratio from zero point one to zero point sixty-five, and ammonia content from zero up to about ninety-four percent by volume. They also swept Exhaust Gas Air Replacement, which they call EGAR — a synthetic form of exhaust gas recirculation where a fraction of the excess intake air is replaced by emulated exhaust gases at the same molar quantity. All of this ran at a fixed one thousand five hundred revolutions per minute and one bar intake pressure, so the maps reflect changes from those control knobs alone. Misfire was defined as a coefficient of variation of Indicated Mean Effective Pressure above five percent or combustion efficiency below ninety percent, and structural limits capped peak cylinder pressure at one hundred twenty bar and maximum pressure rise rate at twelve bar per crank angle degree. Now here's the performance result that makes this engine interesting. Adding ammonia to a hydrogen HCCI engine increases the work output per cycle by about fifty percent compared to neat hydrogen, while keeping indicated efficiency essentially constant around thirty-seven percent. That sounds backwards — you're adding a fuel that's harder to ignite and slower to burn, and the engine becomes more powerful. The mechanism is counterintuitive but logical. Ammonia's slow kinetics damp combustion intensity. Even at just fifteen to twenty percent ammonia by volume, the peak hydroxyl radical concentration drops by a factor of four, slowing the chain reactions that drive rapid heat release. Combustion duration stretches from about three to seven crank angle degrees, and the maximum pressure rise rate halves for constant fuel energy and timing. Because the burn is less violent, you can load the engine to higher equivalence ratios before hitting the ringing and structural limits that cap neat hydrogen operation. More than three quarters of that Indicated Mean Effective Pressure gain occurs between roughly fifteen and fifty percent ammonia by volume. The blending ratio also becomes the primary control lever for combustion timing. Pochet and colleagues describe it as a faster and more practical way to shift load and timing than varying intake temperature. Too much hydrogen and you get runaway ignition; too much ammonia and combustion stalls toward misfire. But within that range — and the range is wide, extending to ninety-four percent ammonia — the blend gives the operator a real-time dial for combustion control. At the right operating points, combustion efficiencies of ninety-five percent were achieved: at roughly fifty percent ammonia and four point five bar Indicated Mean Effective Pressure, and at roughly ninety percent ammonia and five point zero bar Indicated Mean Effective Pressure. Those numbers represent genuinely clean, efficient combustion from a fuel made entirely from air, water, and surplus electricity. Emissions are where the picture gets complicated, and Pochet and colleagues don't soften it. Ammonia contains nitrogen in the fuel itself, and that fuel-bound nitrogen converts to nitrogen oxides in combustion. For neat hydrogen, measured nitrogen oxides were about fifty parts per million. As soon as ammonia entered the charge, nitrogen oxides rose sharply — roughly linearly with ammonia flow, reaching exhaust concentrations on the order of six thousand parts per million at high ammonia fractions. These are fuel-Nitrogen Oxides emissions, tied to in-cylinder temperatures between about one thousand four hundred and one thousand eight hundred Kelvin, not the thermal nitrogen oxides pathway you'd see in a gas turbine burning air nitrogen. Exhaust Gas Air Replacement was tested as a mitigation strategy, and it partially works. Pushing Exhaust Gas Air Replacement to about eighty percent produced up to a threefold reduction in nitrogen oxides, because reduced oxygen availability favors converting fuel nitrogen to nitrogen rather than nitrogen oxides. But Exhaust Gas Air Replacement simultaneously raised nitrogen dioxide emissions, increased unburned ammonia in the exhaust, and made combustion harder to sustain — stable operation couldn't be maintained above eighty percent Exhaust Gas Air Replacement. So it's a genuine tradeoff, not a solution. Temperature thresholds matter for other emissions too. When maximum combustion temperature falls below about one thousand four hundred Kelvin, nitrous oxide — N2O, a potent greenhouse gas — accumulates and isn't destroyed. The team identified a target range of roughly one thousand seven hundred to one thousand eight hundred Kelvin for bulk ammonia combustion. But even at those temperatures, a floor of about five percent unburned ammonia persists because of fuel trapped in the piston crevice volume, which amounts to nearly five percent of chamber volume at top dead center. That unburned fraction won't disappear without design changes to the engine geometry. For the nitrogen oxides that do form, the paper discusses aftertreatment options: selective catalytic reduction works across a three hundred to four hundred fifty degrees Celsius window, lean nitrogen oxides traps are effective around two hundred fifty degrees Celsius with a roughly two percent fuel-economy penalty, and three-way catalysts require near-stoichiometric conditions above three hundred to three hundred fifty degrees Celsius. No single aftertreatment option solves everything cleanly. What does this all add up to? Pochet, Jeanmart, and Contino demonstrated that an HCCI engine running on ammonia-hydrogen blends is not a theoretical proposition — it's a mapped, experimentally characterized machine with real performance numbers. A fifty percent increase in Indicated Mean Effective Pressure over neat hydrogen, combustion efficiencies approaching ninety-five percent at the right operating points, and a blending ratio that gives operators a practical control lever: those are the results. The honest accounting also includes fuel-Nitrogen Oxides that requires aftertreatment, an unburned ammonia floor tied to crevice geometry, and an Exhaust Gas Air Replacement tradeoff that improves one emission while worsening others. The engineering path forward, as the authors frame it, runs through two changes: boosted intake conditions — turbocharging, to raise fuel load and in-cylinder temperatures — and higher stroke-to-bore ratios, which reduce the surface-to-volume ratio that drives crevice and wall quenching losses. Those are tractable engineering problems. What this paper established is that the operating space exists, the thermochemistry behaves predictably, and the fuel — made from air, water, and whatever electricity the wind and sun overproduce — can drive a real engine with real efficiency. That's the foundation the next generation of ammonia power systems gets to build on. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.