Knocking process in NH3/H2 engine

Florian Hurault, Christine Mounaïm-Rousselle, Fabrice Foucher, Hinata

Moriyama, Toshihiro Fukaya, Youhi Morii, Kaoru Maruta

► To cite this version:

Florian Hurault, Christine Mounaïm-Rousselle, Fabrice Foucher, Hinata Moriyama, Toshihiro Fukaya, et al.. Knocking process in NH3/H2 engine. Twenty-second International Conference on Flow Dynamics (ICFD2025, IFS, tohoku university, Nov 2025, Sendai (Japan), France. hal-05400893

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Knocking process in NH3/H2 engine

Florian HURAULT¹, Fabrice FOUCHER¹, Pierre BREQUIGNY¹, Christine MOUNAİM-ROUSSELLE¹,², Hinata

MORIYAMA³, Toshihiro FUKAYA³, Youhi MORII³, Kaoru MARUTA³

¹Université Orléans, INSA CVL, PRISME, UR4229, Orléans, France

²Institut Universitaire de France, IUF, Paris, France

³Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan

ABSTRACT

As a carbon-free fuel, ammonia is considered as one of the most promising fuels to decarbonize sectors reliant on thermal energy conversion such as internal combustion engines. However, due to its combustion properties, one solution to guarantee engine stability and high performance is not to consider hydrogen as ignition promoter. As a function of operating conditions, the addition of hydrogen can lead to knock occurrence. Experiments have been performed with a single cylinder Diesel light-duty engine equipped with a spark plug. Chemkin simulations were then performed to obtain insight into temperature, kinetic and knock prediction.

1. Introduction

Widely produced for fertilizers, Ammonia (NH₃) is gaining recognition as an efficient hydrogen carrier [1], easily stored at liquid state under mild conditions. As a zero-carbon fuel, it supports decarbonization in power generation, heating, and propulsion. However, its combustion properties—low energy density, low laminar burning velocity, and high auto-ignition temperature— pose challenges [2]. Recent studies have shown that NH₃ can fuel high-compression engines alone [3] or with the help of hydrogen, although this increases the knocking occurrence. Several studies in light and medium duty engines confirm that ammonia/hydrogen mixtures improve stability, flame speed, and efficiency, with optimal hydrogen shares between 10–20 vol.% [4,5]. Such blends mitigate misfires and enhance power output and but also influence combustion efficiencies and NOₓ emissions [6].

To better understand knock limitations, it is essential to investigate the fundamental processes governing ammonia and ammonia/hydrogen combustion under engine-like conditions. Knock phenomenon remains a critical issue as it strongly affects efficiency, stability, and engine durability [7]. Experimental and numerical studies can therefore provide valuable insight into ignition behavior, flame propagation, and end-gas reactions, helping to assess ammonia’s potential as a practical carbon-free fuel in internal combustion engines.

The objective of this study is to investigate the appearance of ammonia/hydrogen knock. For that, experiments were carried out in a single cylinder compression ignition (DW10-PSA) engine, fed by a 90%NH₃/10%H₂ (in volume) at the stoichiometry in the intake port and ignited by a spark. In this study, the operating condition was fixed at a 1.8 bar intake pressure, a 85°C intake temperature at 1000 rpm engine speed. The spark timing was varied to vary cycle by cycle knock occurrence. The combustion development was simulated by using ANSYS Chemkin PRO’s Spark-ignition Engine Zonal Model for Knock Assessment module with Stagni kinetic mechanism [8]. From these simulations, pressure and temperature of fresh gases are extracted and used as a starting point for 2D flame simulations to observe the auto-ignition spot, the propagation of the self-ignited flame front and the interaction of both flame front, self

and spark ignited.

2. Experimental and simulated knock

2.1 Engine experimental Set Up

Knock experiments were conducted in a 0.5 L single-cylinder PSA-DW10 compression ignition engine with a 16.4:1 compression ratio and a flat piston. Engine speed was maintained at 1000 rpm via an electric motor. An NGK J-type spark plug (2 ms coil duration) replaced the diesel injector. Engine specifications are in Table 1, and Figure 1 shows the setup.

<table><caption>Table 1: Engine test bench specifications</caption><thead><tr><td>Engine</td><td>PSA DW10F</td></tr></thead><tbody><tr><td>Displaced volume [cm3]</td><td>499.4</td></tr><tr><td>Stroke [mm]</td><td>88</td></tr><tr><td>Bore [mm]</td><td>85</td></tr><tr><td>Connecting rod length</td><td>141</td></tr><tr><td>Compression ratios</td><td>16.4:1</td></tr><tr><td>Piston head types</td><td>19.7cm³ bowl</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1: Engine test bench diagram</figcaption></figure>

Airflow was regulated using a Coriolis Brooks mass flow meter before heating and filling the intake plenum. Liquid ammonia and hydrogen (99.99% purity, Air Liquide) were supplied, with ammonia vaporized before mixing. Gaseous ammonia and hydrogen amounts were controlled by Brooks thermal flow meters (±1% accuracy) to deliver the intake mixture according to pressure conditions. Intake, exhaust, and in-cylinder pressures were recorded over 100 cycles with 0.1 with 0.1 Crank Angle Degrees (CAD) resolution using a Kübler optical encoder.

Knock intensity, KPP is defined as the peak-to-peak
difference between the in-cylinder pressure signal, P and the non-phased distorting filter of the pressure signal () as shown in equation (1).

$$KPP = \max(P - \bar{P}) - \min(P - \bar{P}) \quad (1)$$

The average knocking intensity, KPP<sub>avg</sub> is obtained by averaging KPP over 100 cycles. Knock phenomenon is assumed to occur when KPP is above one limit, KPP<sub>limit</sub>, determined as the engine speed divided by 1000. In this study, KPP<sub>limit</sub> was fixed to 1 as the engine speed was set to 1000 rpm.

## 2.2 Simulation Set Up

From the in-cylinder pressure data, the heat release rate was calculated through the first thermodynamic principle with the use of the Hohenberg's model [9]. The burnt mass fraction evolution is obtained from the cumulative HRR normalized by the total energy input, to obtain the experimental Burnt Mass Fraction (BMF) profile. To avoid considering the presence of the knock as a combustion acceleration, a fit was performed to remove the burnt mass fraction caused by the knock (Figure 2). This fitted burnt mass fraction curve is then used as an input for the simulation instead of the experimental one.

<figure><img src="image_3.png" /><figcaption>Figure 3: Example of burnt mass fraction curve for knocking case (continuous line) and fitted one (without knock) (dashed line).</figcaption></figure>

The Hohenberg model was also used in the simulation to consider the heat losses. For the simulations, the scaling factor was set so that the simulated pressure and average temperature match the experimental ones until the knock occurs.

## 2.3 Knock experimental results

As the spark timing was advanced as follows: -28, -30, -32 and -33 CAD after Top Dead Center (TDC). Only the earliest spark ignition timing induces a KPP higher than the KPP<sub>limit</sub> (Figure 3). It must be noted the exponential tendency. Figure 4 shows the 100 cycles KPP at Spark timing of -32 CAD. Colored points represent the cycles chosen to perform simulation: High knock (red), mid knock (green) and no knock (blue). The entire spread is due to cycle-to-cycle variation that induces or not favorable conditions for auto-ignition occurrence. When the auto-ignited flame expands, the fuel burns earlier and the combustion phasing is advanced, as can be seen in

<figure><img src="image_4.png" /><figcaption>Figure 2: Evolution of the KPP<sub>avg</sub> as function of the spark timing advance</figcaption></figure>

Figure 2, with BMF evolution. In the case of knock, two zones can be distinguished: before -10 CAD, knock is developing and reaches high knock intensity while after the combustion is stable.

<figure><img src="image_5.png" /><figcaption>Figure 4: KPP for 100 cycles with SIT = -32 CAD. Colored points considered to perform simulation. High knock (red), mid knock (green) and no knock (blue).</figcaption></figure>

## 2.4 Knock simulation results

In Figure 5, both pressure evolutions obtain from experiments and simulations are superimposed during the compression and the auto-ignition.

<figure><img src="image_6.png" /><figcaption>Figure 5: Comparison between experiments and simulations for the three points presented in Figure 4.</figcaption></figure>

When the knock occurs, in experiments, the in-cylinder pressure is stabilized before the expansion stroke make the pressure drops while simulations predict the conitnuity of the combustion. This problem comes from the two zones model considered where all fresh gas amount is considered to burn by the auto-ignited flame so
at the end the entire charge is burned even if in the experiment, the burning efficiency is less than one.

### 3. Conclusions

In this study, engine knock experiments and simulations were conducted using 90.% NH₃/10% H₂ (in volume.) premixed stoichiometric mixture. Following findings were obtained:

* 90 vol.% NH₃/10 vol.% H₂ mixed with air at an equivalence ratio of 1 in a single-cylinder, spark-assisted compression-ignition engine produced knock when the spark timing was advanced beyond -32 CAD aTDC under 1.8 bar intake pressure and 85 °C intake temperature.

* Knock intensity (KPPavg) exhibited an exponential increase with spark advance due to the corresponding exponential rise in end-gas temperature.

In addition to the above, we plan to apply a recently proposed novel knock prediction method to the results of this engine test. The new method was developed based on a fundamental combustion theory and simulations [10,11]. This method's applicability was then demonstrated on experimental engine knock data with primary reference fuels [11,12,13]. We plan to extend this approach to NH₃/H₂ fuels to investigate their knock characteristics.

### References

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[9] G. Hohenberg, SAE Technical Paper, 790825, (1979).

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[12] Y. Yasutake, K. Misono, Y. Suzuki, T. Naiki, M. Watanabe, H. Moriyama, Y. Morii, A. Tsunoda, K. Maruta, SAE Technical Paper, 2025-01-0391, (2025).

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### Acknowledgements

The experimental campaign was partially supported by FVV own funding (FVV 601478). The French researchers acknowledge labex Caprysses (ANR-11-LABX-0006-01) for the financial support.