The EUREKA E! 3379 CYPRESS project has developed a revolutionary cylinder pressure sensor design making it possible to optimise combustion in diesel engines, reducing harmful particle and nitrogen oxide (NOx) emissions by up to 90%. The sensor is integrated into the glow plug, avoiding any need to modify the engine structure itself, and is capable of withstanding the elevated temperatures and high vibration levels in the cylinders. Engines equipped with such sensors are already being exported to the USA to meet tough EPA regulations. And European car makers are showing great interest to enable their vehicles to meet ever tougher EU standards, particularly EURO 5 from 2009 and EURO 6, due in 2014.

Over seven million diesel-engined cars are sold annually in the European Union. Diesel cars now have a more than 40% market share in the EU-25 and more than 50% in the EU-15 - and these percentages have been growing steadily since 1990. In the USA, around 12% of cars and light trucks are expected to be using diesel engines by 2015, with 25% of such vehicles using diesel engines worldwide by then.

If the sensor worked it would allow an advanced combustion strategy that would enable a very significant reduction in emissions.

Dr Arjan Kölling,
Sensata Technologies, The Netherlands

Diesel engines work on the principal of self ignition. The fuel is injected into the cylinder and undergoes high compression, which causes the air-fuel mixture to ignite. However, in a basic diesel engine, the fuel-air mix is not homogeneous, resulting in lots of soot particles when ignited. Moreover, excess oxygen leads to elevated ignition temperatures causing high levels of NOx that require costly aftertreatment.

However, both the EU and the USA have been enforcing strict controls on exhaust gas emissions since the 1970s, with ever tougher limits on carbon dioxide, hydrocarbons, NOx and particles. EURO 5 will limit NOx to 180 mg/km, while EURO 6 will cut levels to 80 mg/km. And in the USA, tightening of the Environmental Protection Agency regulations from BIN 8 to BIN 5 will cut NOx emissions from 200 mg/mile to 70 mg/mile. Therefore, alternative combustion strategies are required to improve fuel mixing and closed loop combustion control.

The Dutch division of multinational sensor manufacturer Sensata Technologies - formerly Texas Instruments Sensors & Controls - set out with German spark and glow plug manufacturer BERU to develop a glow-plug-integrated cylinder pressure sensor that is a critical enabler for such a combustion strategy. The high risk nature of the project encouraged Sensata Technologies to seek public funding. The co-operation with BERU meant that the project met the pre-conditions for EUREKA labelling - a minimum of two companies from two EUREKA member states. "Moreover we were under significant time pressure and did not have time to look further," says Dr Arjan Kölling of Sensata Technologies and co-ordinator of the CYPRESS project.

Diesel engine manufacturers intended to optimise combustion in situ using a cylinder pressure sensor (CPS) in a closed-loop control system. "The new demanding limits can only be achieved technically if the untreated emissions of diesel engines are already as low as possible," explains Dipl.-Ing. Hans Houben of BERU. "If the sensor worked it would allow an advanced combustion strategy that would enable a very significant reduction in emissions," adds Dr Kölling.

The high temperatures of gases in the diesel engine cylinder - in excess of 900°C - means direct measurement of combustion pressure is a challenge. "Much work has been done on cylinder pressure sensors since the late 1980s but not very successfully," says Dr Kölling. Early sensors were based on the piezo-electric principal that generates a voltage proportionally to pressure and then on an optical approach. But these were stand-alone items that required modifications to the cylinder head for application; modern diesel engines do not have much space left for such possibilities.

"Working with BERU made it possible to develop an integrated sensor," he adds. "Combining the cylinder pressure sensor with the glow plug meant no significant change was necessary to the engine structure - and the glow function does not affect the measurement operation." The glow plug is an essential element of passenger car diesel engines, enabling almost instant cold start.

The new sensor had to be robust and reliable to withstand the harsh temperature and vibration conditions found in the cylinder. And it had to be compact to be integrated into the glow plug. Piezo-resistive technology where electrical resistance varies proportionally to mechanical stress applied was seen as the solution, making it possible to measure both static and dynamic pressure - unlike piezo-electric sensors that measure only dynamic pressure. Piezo-resistive sensors would remain accurate over the lifetime of the engine, unlike piezo-electric units that are susceptible to ageing.

Combining the cylinder pressure sensor with the glow plug meant no significant change was necessary to the engine structure.

Dr Arjan Kölling,
Sensata Technologies, The Netherland

Results of the project have attracted major interest from European car manufacturers and reinforced Europe's global leadership in automotive innovation. The new cylinder pressure sensor is the first of its kind to be sufficiently reliable and low cost to make it suitable for mass production. And integration of the sensor into the glow plug of the diesel engine simplifies engine redesign; the standard glow plug can simply be replaced by the CPS unit that incorporates the glow function.

BERU has already started to market pressure sensor glow plugs incorporating Sensata sensors. Output of the sensor can be relayed continuously to the engine's electronic control system. Depending on the quality of the control system, up to 90% of NOx and particulate emissions can be removed. This success also led to CYPRESS winning the 2008 EUREKA Lillehammer award for projects that improve Europe's environment.

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