The Fundamental Challenge
Internal combustion engines remain vital in many applications where electrification is difficult – particularly heavy-duty vehicles, off-road equipment, and industrial machinery. While hydrogen offers a carbon-free alternative to conventional fuels, traditional hydrogen engines face critical limitations:
- Hydrogen’s extremely low ignition energy (just 0.02 mJ versus gasoline’s 0.24 mJ) makes it highly susceptible to knock and pre-ignition
- To avoid these problems, current hydrogen engines typically operate with lean fuel mixtures and lower compression ratios, which reduces both power and efficiency
- Hydrogen’s small quenching distance increases heat losses to cylinder walls
The Innovative Solution: Diesel Principles Meet Spark Ignition
Researchers at the University of Stuttgart and Empa (Swiss Federal Laboratories) developed the “DIH₂jet” concept, which ingeniously combines elements from both diesel and gasoline engines:
- From diesel engines: Using direct fuel injection and diffusion-controlled combustion
- From gasoline engines: Utilizing spark ignition rather than compression ignition
- Unique approach: Positioning the spark plug at the periphery of the hydrogen jet and igniting during active injection
Hydrogen Jet
↓
Cylinder ┌─────┐
Wall ← │ │ → Cylinder
│ │ Wall
│ H │
│ H │
Injector ... │ │
└─────┘
↑
Spark Plug
This approach creates a controlled, stable combustion process that can operate with stoichiometric mixtures (λ=1) while avoiding the abnormal combustion issues that plague conventional hydrogen engines.
The Research Platforms: Complementary Testing Approaches
The researchers employed two complementary test systems, each offering distinct advantages as detailed in Table 1:
Table 1: Research Objectives and Test Rig Suitability
| Research Objective | RCEM | SCE |
|---|---|---|
| Engine-like pressure volume trajectory | Good, but expansion is affected by combustion (free-floating piston) | Excellent (cranktrain) |
| Turbulence | Laminar machine (turbulence only from DI and combustion) | Results from the whole process, including gas exchange |
| Data quality for CFD simulation in 3-D | Very good (known initial condition) | Limited (complex geometry, turbulence) |
| Engine-like pressures and temperatures | Limited due to maximal energy release | Good (pmax = 140 bar) |
| Emission Analysis | Not possible | Very good |
| Quantification of thermodynamic parameters | Excellent | Good |
| Adaptation to different geometries | Good | Limited to no adaption possible |
| Statistical analysis (CoV) | Limited | Excellent |
| Injection | pinjection = 105 bar with single-hole solenoid injector HDEV1 from Bosch | pinjection = 80-190 bar with circular orifice piezo injector HDEV4 from Bosch |
Rapid Compression Expansion Machine (RCEM)
This specialized device simulates a single engine cycle with a free-floating piston and provides excellent optical access for flame visualization.
RCEM Simplified Schematic
┌────────────────────────────┐
│ │
│ ┌──────┐ ┌────────┐ │
│ │Optical│ │Hydraulic││
│ │Window │◄─► │ System │ │
│ └──────┘ └────────┘ │
│ │ │
│ ┌─────┐ │
│ │Free │ │
│ │Piston│ │
│ └─────┘ │
│ │
└────────────────────────────┘
Key features include:
- 790 cm³ displacement volume
- 1.1 bar air filling pressure
- 6.2 compression ratio
- Single-hole solenoid injector at 105 bar
- Operated at λ = 2.9 (lean conditions)
- Offers superior data quality for CFD simulation due to well-defined initial conditions
- Excellent for fundamental combustion visualization and thermodynamic analysis
Single-Cylinder Engine (SCE)
This modified Mercedes-Benz M 254 engine allowed practical performance testing across multiple cycles.
SCE Simplified Schematic
┌────────────────────┐
│ Exhaust Valve │
│ ┌┐ ┌┐ │
│ ┌─────┘└─┘└─────┐ │
│ │ Spark │ │
│ │ Plug │ │
│ │ ● │ │
│ │ │ │
│ │ ▲ │ │
│ │ Injector │ │
│ └──────────────┘ │
│ │ │ │
│ └──────────┘ │
│ Intake Valve │
└────────────────────┘
Key features include:
- 500 cm³ displacement
- Pent-roof tumble design with near-central injector
- Spark plug between exhaust valves
- Circular-orifice piezo injector at 80-190 bar
- Peak pressure capability of 140 bar
- Excellent for emissions analysis and cycle-to-cycle variation assessment
- Represents real-world operating conditions with gas exchange effects
The Combustion Process: A Visual and Quantitative Analysis
Figure 1 provides a fascinating visual record of the jet-guided combustion process in the RCEM through multiple imaging techniques:
Figure 1: Jet-Guided Combustion Test in the RCEM
Time: -1.0 ms -0.6 ms -0.2 ms +4.6 ms
┌────────┐ ┌────────┐ ┌────────┐ ┌────────┐
OH* │ ○ │ │ ○◌ │ │ ○◌◌ │ │ ○◌◌◌ │
Images │ • │ │ •• │ │ ••• │ │ ••••• │
└────────┘ └────────┘ └────────┘ └────────┘
┌────────┐ ┌────────┐ ┌────────┐ ┌────────┐
Schlieren│ ╱╲ │ │ ╱╲╱╲ │ │ ╱╲╱╲╱╲│ │ ╱╲╱╲╱╲╱╲│
Images │ ╱ ╲ │ │ ╱ ╲│ │ ╱ ╲│ │╱ ╲│
└────────┘ └────────┘ └────────┘ └────────┘
Pressure and Heat Release Rate Graph
bar/J/ms│ ┌─┐
25 │ /│ │
│ / │ │Heat Release ↗
20 │ / │ │ _
│ / │ │ / \
15 │ / │ │ / \_
│ / │ │ / \__
10 │ / │ │ / \_
│ / │ │Pressure↗ \_____
5 │_____________/ │ │
└───────────────────────┴─┴───────────────────────▶
-8 -6 -4 -2 0 2 4 6 8 10 12 Time [ms]
OH* Chemiluminescence (Top Row)
These images show the regions of active chemical reactions as bright areas. The sequence reveals:
- At -1.0 ms before TDC: Initial flame kernel formation at the spark location
- By -0.2 ms: Significant flame propagation through premixed hydrogen
- At 4.6 ms: Continued combustion along the periphery of the hydrogen jet
Schlieren Visualization (Middle Row)
This technique visualizes density gradients, clearly showing:
- The developing hydrogen jet
- The flame front propagation
- The interaction between the continuing injection and combustion process
Pressure and Heat Release Data (Bottom Graph)
This correlates perfectly with the visual data, showing:
- Initial cylinder pressure of ~5 bar rising to ~25 bar during combustion
- A characteristic spike in heat release rate (~300 J/ms) during the premixed phase
- The transition to mixing-controlled combustion where heat release follows fuel input
- The mathematical model of fuel power input (black line) matching the heat release rate (red line) during the mixing-controlled phase
The combustion follows this sequence:
- Hydrogen injection begins (~4 ms before spark)
- Spark ignition creates a flame kernel
- A premixed combustion phase produces a spike in heat release rate
- Combustion transitions to a mixing-controlled phase limited by injection rate
- After injection ends, remaining hydrogen undergoes late combustion
Optimizing Injection Parameters
Figure 2 demonstrates how fuel rail pressure and injector needle lift affect combustion in the single-cylinder engine. Three test conditions are shown:
- 80 bar pressure with low injector needle lift (red line)
- 190 bar pressure with low injector needle lift (black line)
- 190 bar pressure with high injector needle lift (green line)
Figure 2: Effect of Injection Parameters (n = 1500 rpm, IMEP = 5 bar, λ = 1)
Cylinder Pressure vs. Crank Angle Heat Release Rate vs. Crank Angle
bar│ J/°CA│
30 │ ╱╲╱╲ 80 │ ╱╲
│ ╱ ╲ │ ╱ ╲
20 │ ╱ ╲ 60 │ ╱ ╲
│ ╱ ╲ │ ╱ ╲
10 │ ╱ ╲ 40 │ ╱ ╲
│ ╱ ╲ │ ╱ ╲
0 │_╱ ╲___ 20 │ ╱ ╲___
└───────────────────────▶ 0 │__╱ ╲____
-20 0 20 40 60 80 °CA └────────────────────▶
-20 0 20 40 60 80 °CA
─── 80 bar - low lift
─── 190 bar - low lift Pressure-Volume Diagram
─── 190 bar - high lift bar│ compression
30 │ ╱╲
Injection Ignition │ ╱ ╲
|←──────→| ● 20 │ ╱ ╲
│ ╱ ╲
10 │╱ ╲ expansion
│ ╲
1 └───────────────────▶
50 400 600 cm³
The diagrams reveal:
Cylinder Pressure Curves (Top Left)
- Higher rail pressure (190 bar) produces more intense combustion
- Peak pressures range from ~18 bar (80 bar fuel pressure) to ~29 bar (190 bar fuel pressure)
- The timing of pressure development varies with injection conditions
Heat Release Rate (Bottom Left)
- All conditions show a characteristic premixed spike followed by mixing-controlled combustion
- Higher injection pressure creates more intense premixed combustion
- The heat release rate during mixing-controlled combustion correlates directly with fuel flow rate
Pressure-Volume Diagram (Right)
- The area enclosed by these curves represents work output
- Higher injection pressure creates greater isochoric (constant-volume) combustion fraction
- This affects both efficiency and mechanical stress on engine components
This data demonstrates how injection parameters can be tuned to optimize the trade-off between:
- Higher efficiency through more rapid combustion
- Lower mechanical stress through controlled pressure rise rates
The Critical Impact of Injection Timing
Figure 3 illustrates how the start of injection (SOI) timing profoundly influences combustion behavior. Three different SOI timings are compared:
- 4 ms before TDC
- 6 ms before TDC
- 9 ms before TDC
Figure 3: Effect of Different SOI Timings on Combustion Characteristics
OH* Images at Premixed Peak Pressure and Heat Release Rate
┌────────┐ ┌────────┐ ┌────────┐ bar/J│ ┌─┐
│ ◌◌ │ │ ◌◌◌ │ │ ◌◌◌◌ │ 30 │ /│ │
│ │ │ │ │ │ │ / │ │
│ │ │ │ │ │ 25 │ / │ │
└────────┘ └────────┘ └────────┘ │ / │ │
tSOI=-4ms tSOI=-6ms tSOI=-9ms 20 │ / │ │ tSOI=-4ms
│ / │ │ tSOI=-6ms
15 │ / │ │ tSOI=-9ms
│ / │ │
10 │________/ │ │
└────────────────────────▶
0 5 10 Time [ms]
Logarithmic pV Diagram
bar│ compression
101│ /
│ /
│ /
│ /
│ / expansion
│ /
100└───────────────────▶
200 400 600 800 cm³
The results, shown through OH* images, pressure/heat release traces, and pV diagrams, reveal:
Earlier SOI (Longer Delay Between SOI and Spark) Results In:
- More extensive premixing of hydrogen and air before ignition
- Larger premixed combustion phase with higher heat release rate peaks (~400 J/ms)
- More extensive flame propagation (larger OH* luminescence area)
- Shorter mixing-controlled combustion phase
- Lower overall combustion duration
The logarithmic pV diagram (right side) shows clear differences in the expansion phase, indicating efficiency impacts from these timing changes.
Achieving Combustion Stability
One of the most critical aspects of any combustion system is cycle-to-cycle stability. Figure 4 presents pressure-volume diagrams for 200 consecutive engine cycles with three different relative ignition timings:
- Spark at 20% of injection duration (left panel)
- Spark at 40% of injection duration (middle panel)
- Spark at 60% of injection duration (right panel)
Figure 4: pV Diagrams of 200 Consecutive Cycles with Different Relative Ignition Timings
Relative Ignition: 20% Relative Ignition: 40% Relative Ignition: 60%
bar│ bar│ bar│
35 │ 35 │ 35 │
│ ╱╲╲╲ │ ╱╲╲ │ ╱╲
30 │ ╱ ╲╲╲ 30 │ ╱ ╲╲ 30 │ ╱ ╲
│╱ ╲╲╲ │╱ ╲╲ │╱ ╲
25 │ ╲╲╲ 25 │ ╲╲ 25 │ ╲
│ ╲╲╲ │ ╲╲ │ ╲
20 │ ╲╲╲ 20 │ ╲╲ 20 │ ╲
│ ╲╲╲ │ ╲╲ │ ╲
15 │ ╲╲╲ 15 │ ╲╲ 15 │ ╲
│ ╲╲╲ │ ╲╲ │ ╲
10 │ ╲╲╲ 10 │ ╲╲ 10 │ ╲
└─────────────────▶ └─────────────────▶ └─────────────────▶
50 100 150 200 cm³ 50 100 150 200 cm³ 50 100 150 200 cm³
─── 200 individual cycles
─── Mean cycle
---- Standard deviation
The figure shows:
- Individual cycle variations (thin gray lines)
- The mean cycle (solid blue line)
- Standard deviation bounds (dashed blue lines)
The data reveals that:
- Later relative ignition timing (60%) produces more consistent combustion
- This improved stability results from:
- More favorable local mixture conditions at the spark plug
- The stabilizing effect of the mixing-controlled phase with constant fuel input
This relationship between ignition timing and stability is crucial for practical engine operation, as excessive cycle-to-cycle variation degrades drivability, emissions control, and efficiency.
Performance and Emissions Trade-offs
Figure 5 presents comprehensive performance and emissions data across different relative ignition timings (20%, 50%, and 80%) for two load conditions:
- IMEP = 5 bar (light gray lines with circles)
- IMEP = 10 bar (dark gray lines with triangles)
Figure 5: Performance and Emissions Measurements at Different Relative Ignition Timings
Indicated Efficiency Pressure Gradient Combustion Stability
[-]│ [bar/°CA]│ [CoVIMEP %]│
0.35 │ ▲ 15 │ ▲ 6 │ ▲
│ / │ / │ /
0.30 │ / 10 │ / 4 │ /
│ ● / │ / │ / ●
0.25 │ / 5 │ ● 2 │ ▲
└─────────────▶ └─────────────▶ └─────────────▶
20 50 80 % 20 50 80 % 20 50 80 %
H₂ at Engine-out NOₓ at Engine-out Exhaust Temperature
[%]│ [ppm]│ [°C]│
1.5 │ 800 │ ▲ 700 │ ▲ ●
│ ● │ / │ /
1.0 │ \ 400 │ / 650 │ /
│ \ │ / │ /
0.5 │ ▲ 0 │ ● 600 │ ▲
└─────────────▶ └─────────────▶ └─────────────▶
20 50 80 % 20 50 80 % 20 50 80 %
○─── IMEP = 5 bar
▲─── IMEP = 10 bar
This data reveals several important trends:
Efficiency (Top Left)
- Indicated efficiency improves with later relative ignition timing
- Higher load (10 bar IMEP) generally yields better efficiency
- Maximum efficiency approaches 0.35 (35%)
Pressure Gradient (Top Center)
- Later ignition timing increases cylinder pressure gradients
- Higher loads produce more aggressive pressure rise rates
- Values range from ~5 bar/°CA to ~15 bar/°CA
Combustion Stability (Top Right)
- CoVIMEP (Coefficient of Variation of Indicated Mean Effective Pressure) shows interesting patterns
- More stable combustion (lower CoV) at medium ignition timing
- Transitions from mixing-controlled to predominantly premixed combustion at 80% timing increases variability
Hydrogen Slip (Bottom Left)
- Unburned hydrogen emissions range from 0.5% to 1.5%
- This represents both an efficiency loss and potential resource for NOx reduction
NOx Emissions (Bottom Center)
- Dramatic increase with later ignition timing
- Higher loads produce significantly more NOx
- Values range from ~200 ppm to ~800 ppm
Exhaust Temperature (Bottom Right)
- Range from 600-700°C
- Pattern follows combustion phasing
- Higher temperatures at higher loads
These measurements highlight the complex trade-offs between efficiency, emissions, and stability that must be balanced in practical applications.
Mathematical Modeling of the Process
The research precisely quantifies the fuel power input using this equation:
Pfuel(t) = (mfuel / Δtinjection) × LHV × [u(t − tSOI) − u(t − tEOI)]
Where:
- u is the unit step function (equals 0 before and 1 after its argument)
- mfuel is the total fuel mass
- LHV is hydrogen’s lower heating value
- tSOI and tEOI are the start and end of injection times
- Δtinjection is the injection duration
Mathematical Model of Fuel Power Input vs. Actual Heat Release
Power│
│ _____________________
│ | |
│ | |
│ | Fuel Power |
│ | Input |
│ | |
│ | |
│ _|_____________________|_
│ /|\ \
│ / | \ \
│ / | \ \
│ / | \ Heat \
│ / | \ Release \
│ / | \ \
│_______/______|______\______________________\______▶
tSOI tEOI Time
This mathematical representation creates a rectangular profile defining the fuel input window, allowing researchers to precisely correlate fuel delivery with heat release during the mixing-controlled phase.
Future Research Directions
This innovative combustion concept opens several promising research avenues:
- Heavy-Duty Applications: Optimizing for larger displacement engines used in trucks, construction equipment, and stationary power generation
- Hydrogen-Specific Injectors: Developing specialized injectors for hydrogen’s unique properties instead of adapting gasoline hardware
- Combustion Optimization: Using computational fluid dynamics to better understand and optimize the jet-ignition interaction
- Emissions Control: Investigating whether unburned hydrogen (0.5-1.5%) could serve as a reducing agent for NOx in the exhaust system
- Efficiency Enhancement: Exploring higher compression ratios and exhaust gas recirculation to further improve thermal efficiency
The Significance of This Innovation
This jet-guided hydrogen combustion approach represents a fundamentally new way to use hydrogen in internal combustion engines. Unlike conventional approaches that struggle with hydrogen’s challenging properties, this method:
- Embraces hydrogen’s characteristics rather than fighting them
- Creates a combustion process with diesel-like stability but zero carbon emissions
- Enables stoichiometric operation for higher power density
- Avoids abnormal combustion phenomena that plague traditional hydrogen engines
This research demonstrates that internal combustion engines still have significant innovation potential, particularly for applications where direct electrification remains challenging. By creatively combining principles from different engine types and adapting them to hydrogen’s unique properties, these researchers have developed a promising path toward carbon-free combustion for the “hard-to-electrify” sectors of our transportation and energy systems.
