How back side illuminated sensor technology supports multi-fuel internal combustion engine research

Increasing our understanding of the interactions between fuel properties and combustion processes can help us develop advanced internal combustion engines (ICE) with better performance and efficiency. To this end, researchers at the University of Wisconsin-Madison’s Engine Research Center (ERC) in the Department of Mechanical Engineering are investigating this interplay by combining sophisticated imaging techniques with high-speed cameras.

At the university’s ERC, Professor David Rothamer and his team are gaining deeper insights into the combustion process in mixing-controlled compression-ignition (MCCI) engines by developing and implementing optical and laser-based diagnostics. They conduct visualization experiments using schlieren and chemiluminescence imaging techniques to design ICEs compatible with various fuels.

Challenges of multi-fuel engines

Multi-fuel combustion engines can use diesel, jet fuel, sustainable aviation fuels, gasoline and bio-derived hydrocarbons like ethanol and methanol. The varied chemical and physical properties of each fuel make developing engines with performance and emissions characteristics similar to conventional ICEs difficult. Rothamer’s team uses optical diagnostics to address these challenges.

Schlieren imaging reveals fuel jet dynamics

For their experiments, the ERC researchers use two Phantom cameras, a v2640 and a v1840. The v2640 captures in-cylinder schlieren imaging. This technique visualizes flows in transparent media by detecting density gradients.

The schlieren method allows researchers to observe density gradients in the fuel jets within the combustion cylinder, showing how fuel penetrates and mixes in the chamber. Inside an optical engine fitted with a fused silica window in the piston and a mirror in the cylinder head, the group gains visual access to the combustion process.

Using a high-powered, collimated blue LED light source, the team reflected light off the piston extension mirror and transmitted it through the piston window. This light passes through the fuel and air mixture, reflects off the head mirror and transmits back out through the piston window. After passing through the piston window, the light reflects off the piston extension mirror a second time and is imaged with the v2640 camera. The v2640 imaged at a resolution of 512 x 176 pixels and a frame rate of 115,200 frames per second (fps). The team recorded sixteen images for every engine crank angle degree.

These measurements show how different fuels mix with air during injection, helping explain variations in ignition characteristics.

Back side illumination optimizes imaging setup

Previously, the researchers employed a Phantom v1840 with an image intensifier for UV-chemiluminescence imaging. This technique captures light emitted by molecules formed during combustion. When combustion occurs, molecules like hydroxyl radical (OH) form in electronically excited states and can emit photons at wavelengths around 310 nanometers.

This approach requires a UV-sensitive camera, UV-transmissive lens and bandpass filter. Finding a suitable high-speed UV-sensitive CMOS camera had been difficult, requiring reliance on intensifiers coupled with visible spectrum cameras. However, intensifiers reduce spatial resolution and produce non-linear responses.

For these experiments, a back side illuminated (BSI) CMOS sensor with fused-silicon cover glass enabled direct UV-chemiluminescence imaging without an intensifier.

The researchers tested a Phantom T3610-UV with UV-extended BSI CMOS sensor technology. They ran optical engine experiments at typical MCCI conditions at 512 × 512 px resolution and 28,800 fps.

The videos showed seven burning plumes corresponding to each nozzle hole. At injection end, the flames recessed to the cylinder center before burnout. The results confirmed that the UV-extended BSI CMOS sensor detected the differences in combustion behavior.

If the UV signal is strong enough, the UV-sensitive technology eliminates the need for image intensifiers, improving spatial resolution at lower system costs. BSI CMOS cameras maintain a linear relationship between incident photons and measured signal, simplifying quantitative diagnostics like planar laser-induced fluorescence and aerosol phosphor thermometry.

Many quantitative diagnostics, such as planar laser-induced fluorescence and aerosol phosphor thermometry, rely on this relationship being linear. As a result, the intensifier must be characterized over a range of conditions, which can be experimentally tedious and result in added measurement uncertainty. With UV-sensitive BSI CMOS cameras like the Phantom T3610-UV, the relationship between incident photons and measured signal is significantly more linear, making advanced quantitative diagnostics more tractable

Kyle D Gilroy, Ph.D. is application development manager at Vision Research.