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All-optical sensing deals with vehicle emissions

03 Jun 2005

Europe is turning to optical technology to monitor combustion efficiency and help tackle the problem of vehicle emissions. James Tyrrell catches up with the latest research.

A combination of optical-temperature sensing, spectroscopy and microwaves could help make Europe's vehicles cleaner and cheaper to run, according to research presented at the Optical Fibre Sensor (OFS-17) conference held in Bruges, Belgium, last month.

The European Union is on a mission to drive down vehicle pollution and is funding research on the topic ahead of its Euro V emissions directive, due to be implemented in 2008/2009. Over the past 13 years, European legislation has reduced acceptable levels of carbon monoxide (CO) from 4.5 to 1.5 g/kWh. Smoke and particulate emissions have also come under scrutiny.

Currently, vehicles rely on an electrochemical oxygen detector, known as a Lambda sensor, to regulate emission. Pollution is minimized when the engine is burning fuel correctly, and the Lambda sensor maintains the optimum fuel:air ratio by measuring the amount of oxygen in exhaust gases.

However, the sensor cannot quantify pollutants such as CO, hydrocarbons and oxides of nitrogen in exhaust gas. And, under the Euro V standard, car-makers will be looking to extract exactly this information. Fortunately, a six-partner European project dubbed Opto-Emi-Sense thinks it has come up with an optical sensor that fits the bill.

"It is a spectroscopy sensing technique - you irradiate the exhaust and monitor the absorption lines of the individual gases," Colin Fitzpatrick, of project partner the University of Limerick, Ireland, told OLE. "The really novel part is that we are actually looking at the mid-infrared [mid-IR], which is where the most sensitive absorption lines for these gases exist." For example, CO has a strong absorption at 4.7 μm, carbon dioxide (CO2) at 4.23 μm and nitrogen monoxide at 5.2 μm.

By detecting individual pollutants as they leave the exhaust, engine-makers will be able to create a highly tuned combustion process that minimizes pollution. This win-win scenario promises cleaner exhaust gas, thanks to more efficient combustion and vehicles that are cheaper to run.

Optical fibre enables the sensor's emitter and detector electronics to be placed a safe distance from the exhaust manifold, protecting the set-up from a highly corrosive environment that can reach temperatures of 500 °C. In the lab, the Limerick team are initially targeting CO2 measurements and will move on to more hazardous gases once the system has been optimized.

"We are almost 18 months into our project and can now detect CO2 using the actual fibres," said Fitzpatrick. "The aim is that, by the end of the 36-month project, we will be testing it on an engine at Fiat." Chalcogenide fibre links the sensor's mid-IR emitter - a hot nichrome filament - to a test cell and then guides the radiation from the measurement site to a pyroelectric detector.

"The biggest challenge has been the fact that the emitters and the detectors are not optimized for use with fibres, so we have had to make our own couplers," explained Fitzpatrick. The team uses a calcium fluoride lens to collimate the emitter's pulsed mid-IR radiation, transmitting the signal across the gas-measurement cell.

A key benefit of the optical technology is its fast response. "The electrochemical [Lambda] sensors have in-built delays, whereas optical techniques are theoretically instantaneous," explained Fitzpatrick. This means that, in principle, an optical sensor could resolve engine performance on a cylinder-by-cylinder basis - a vision held by project partner Centro Ricerche Fiat. Optical sensors are also much less susceptible to cross-sensitivity and cannot be poisoned by pollutants.

However, profiling exhaust-gas species is only part of the combustion-control picture. Exhaust-gas temperature also plays a critical role in Opto-Emi-Sense's diagnostic scheme as it enables the Limerick team to correct for thermal drift when processing optical-absorption spectra. Again, optical sensing is the preferred way of protecting sensitive electronics from a corrosive working environment, giving a long operating life.

City University, London, UK, has been busy developing optical-temperature detectors for the Opti-Emi-Sense project. "We have a couple of [temperature] sensors: one is a fibre-Bragg-grating-based sensor and the other is a fibre-optic fluorescence-decay time-based sensor," Ken Grattan, of City University, told OLE. "The fluorescence sensor measures temperature by monitoring the rate at which fluorescence falls off from a sample."

City University's fibre-Bragg grating works by detecting the shift in peak wavelength as the grating expands with temperature. "We write the grating at 1550 nm and illuminate it with a laser diode," added Grattan. Researchers have taken a dual approach to assess which design stands up best to the tough engine conditions. "It seems that the fibre-Bragg grating is less sensitive to vibration," said Grattan. "[But] we are working on restructuring the fluorescence-based sensor so that it is less vibration-sensitive."

Microwave clean-up

Opto-Emi-Sense's all-optical approach means that the sensor is immune to electromagnetic radiation and is the perfect partner for the microwave-exhaust-gas clean-up technology being developed by Ahmed Al-Shamma'a and colleagues at the University of Liverpool, UK.

"The microwave-plasma system uses exactly the same unit as a microwave oven," said Al-Shamma'a. "The microwaves ionize the exhaust gases, generating a plasma that breaks down the polluting gas species." Unlike catalytic converters, which only operate effectively when exhaust gases reach 300 °C, the microwave unit begins working immediately and also avoids the need for expensive materials such as platinum and palladium.

A big advantage for diesel engines is the way that the microwave unit is able to burn out fine and ultrafine particulates in the exhaust gases. Catalytic converters have to rely on a system of filters, which, as Al-Shamma'a explains, is not ideal. "Current filters only stop particulates larger than 50 μm," he explained. "They are not successful in stopping the ultrafine particulates that can cause lung cancer."

The filters also have to be cleaned, which adds to operating costs. Al-Shamma'a and his colleagues have tested their microwave unit on a commercial fork-lift truck, where they were able to embed the compact device directly into the vehicle's exhaust manifold.

At its halfway stage, Opto-Emi-Sense is already generating promising results, and project partners feel confident that their optical technology will suit the market. "We have stayed away from the types of components that are intrinsically very expensive," commented Fitzpatrick. "To be accepted by the automotive industry, everything has to be scalable down in price with mass production."

The project, which also includes Fiberware and Rostock University of Germany, appears to have found a real need for the optical technology that has spilled over from communications and thermal-imaging sectors. "Bringing us together has been the essential thing," concluded Grattan.

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