Minimizing pollutant emissions using pressure-sensing technology

Minimizing pollutant emissions using pressure-sensing technology

Recall actions in the automobile industry have widespread consequences. Manufacturers have to contend with a huge image loss, as well as higher costs. Vehicle owners, on the other hand, react with anger and uncertainty. A particularly major stir has welled up over the past year with the scandal surrounding manipulated emissions figures. Politics has in turn reacted and signaled towards new testing procedures.

The automobile industry has triggered a true recall-crisis over the last two years. In the USA alone, some 51 million vehicles were ordered for recall during 2015 by the National Highway Traffic Safety Administration (NHTSA). This far exceeds the number actually sold in that same year, even though the vehicles recalled were not all connected to manipulated emissions figures. Some 11 million of these vehicles originate alone from the “Dieselgate” scandal involving the manufacturer Volkswagen. The losses involved are enormous.

Cost pressure and an increasing complexity of the systems built into vehicles are associated with heightened error susceptibilities and their resultant recall actions. This challenge is to be met primarily through improved and even more reliable control systems – on the part of manufacturers and suppliers, as well as government supervisory bodies responsible for legal specifications monitoring. High-grade measurement equipment is therefore needed, which can deliver the most precise of results under varying conditions and thus secure an optimal standards qualification (or post-qualification). A major backlog demand has since opened up in this respect.

The best pressure measurement technology for the best combustion engines

In the development of combustion engines, high-precision pressure transmitters are required, which, during combustion analysis, can facilitate the exact measurement of cylinder pressures, as well as intake and exhaust pressures. Absolute pressure sensors (gas exchange) and high-pressure sensors (injection pressure measurement) must also be of the highest-grade, since, in the latter instance especially, the potential for pollutant minimization is enormous. In this regard, particulates from gasoline engines can be reduced through an increase in injection pressure. Some suppliers are already working on increasing injection pressures to 350 bar or even higher.

Mobile emissions measurement is on the way

The standardized “New European Driving Cycle (NEDC)” is currently being introduced for exhaust and consumption measurements by state regulatory agencies. As we have witnessed, test procedures have given manufacturers all the freedom to influence measurements to their own advantage, since the vehicle is examined only in a test facility rather than under real-world conditions.

Once the manipulations became known, the Committee of Experts from the European Union decided in May 2015 that emissions during type approval are to be tested from late 2017 under practical driving conditions – known as Real Driving Emissions (RDE). The laboratory conditions for conventional checks will be supplemented by a procedure that prevents the use of cut-off devices during testing. The vehicle to be tested will be examined on an open track and thus subjected to variable conditions. Furthermore, random braking and accelerating procedures will also be performed.

Meeting new challenges – using modular pressure-sensor solutions

The RDE procedure obviously poses particular challenges to the measurement technology deployed. In the optimization of emissions figures for combustion engines, initial emphasis falls upon absolute and relative pressure measurement. With the new measurement procedures in mind, these need to perform reliably across a broad temperature range. Whether in the depths of winter or the heights of summer, measured values must be absolutely reliable to reflect a realistic picture of the true exhaust figures. It is also evident that operation at higher pressures can achieve significant reductions. For this reason, higher pressures must also be measurable. The fact that the pressure sensing technology employed operates without failure in mobile applications, given the new procedures, goes largely without saying.

Standard solutions cannot achieve this objective. Even more than that, they are actually part of the problem. Individual challenges require individual solutions. Also required are instruments that are precise as well as flexible enough to perform reliably in differing applications. Only by following this path can cost-efficiency and precisions be reconciled. It is clear that modular systems would be ideal in this context. In coordination with the manufacturer, these can be adapted to individual requirements and thus deliver highly reliable results. This represents a particular advantage in the development of new engines, since the adaptations can be made both straightforwardly and also promptly.

An experience that our customers have been making daily – and for almost 30 years now. As a leading manufacturer of customer-specific, modular measurement systems, we can provide tailored pressure measuring solutions within a short timeframe and in a proficient cooperation with manufacturers. Viewed from a measurements perspective, there exist no obstacles to the development of new, fuel-efficient engines, as well as to their testing under real-world practical conditions.

Mapping boost pressure on downsized turbo engines is the key to success

Mapping boost pressure on downsized turbo engines is the key to success

To meet ever tightening emissions legislation across the world OEMs are turning to downsized Spark Ignition engines. While these smaller engines consume less fuel and produce significantly lower emissions they require forced induction to deliver the performance drivers have come to expect from modern passenger vehicles.

The driveability of these downsized turbo engines must at least equal the performance of their naturally aspirated equivalents. This requires full boost pressure at low engine speeds without running out of steam at high speed, which can only be achieved with a sophisticated boost pressure control system.

The main problem with these forced induction spark ignition engines is the precise control of the air-fuel ratio near stoichiometric values at different boost pressures. At low speeds, these engines are prone to knock under medium to high loads.

Modern pressure control systems

Controlling the turbine-side bypass is the simplest form of boost pressure control.

Once a specific boost pressure is achieved, part of the exhaust gas flow is redirected around the turbine via a bypass. A spring-loaded diaphragm usually operates the wastegate which opens or closes the bypass in response to the boost pressure.

In recent times manufacturers have turned to variable turbine geometry to regulate boost pressure. This variable geometry allows the turbine flow cross-section to be varied to match the engine operating parameters.

At low engine speeds, the flow cross-section is reduced by closing the guide vanes. The boost pressure and hence the engine torque increases as a result of the higher pressure drop between turbine inlet and outlet. During acceleration from low speeds the vanes open and adapt to the corresponding engine requirements.

By regulating the turbine flow cross-section for each operating point the exhaust gas energy can be optimised, and as a result the efficiency of the turbocharger and therefore that of the engine is higher than that achieved with bypass control.

Today, electronic boost pressure regulation systems are increasingly used in modern Spark Ignition petrol engines. When compared with purely pneumatic control, which can only function as a full-load pressure limiter, a flexible boost pressure control allows an optimal part-load boost pressure setting.

The operation of the flap, or vanes, is subjected to a modulated control pressure instead of full boost pressure, using various parameters such as charge temperature, ignition timing advance and fuel quality.

Simulation reduces time to production and development costs

Faced with a plethora of complex variables, manufacturers have turned to simulation during the design and test phase.

A significant hurdle to overcome with downsized turbocharged engines is the narrow range within which the centrifugal compressor operates stably at high boost pressures.

The only way to build an effective simulation model is through extensive real world testing. This testing is mostly carried out on engine dynamometers in climatic chambers.

During wide open, and part throttle, runs the following pressure information is recorded:

  • Intake manifold pressure
  • Boost pressure
  • Barometric pressure

Of course this is all integrated with engine temperatures (Coolant and oil) to gain a picture of engine performance over the full engine speed range.

During this testing it’s important that engineers note any abnormalities in performance, as events such as exhaust pulses at specific engine speed can set up standing waves which can excite the impeller at a critical frequency which will reduce the life of the turbo, or even lead to catastrophic failure.

Therefore the measurement of pressure performance maps of both compressor and turbine is vital for the creation of an accurate extrapolation model for implementation during simulation.

A well-developed simulation tool can save the OEM time and money in dynamometer and road tests, but can only be developed once the pressure maps have been completed.

GDI engines come under pressure to reduce particulate emissions and improve performance

GDI engines come under pressure to reduce particulate emissions and improve performance

With some 40 million gasoline direct-injection (GDI) engines expected to be sold by 2025, it may be surprising to learn that these units emit more hazardous fine particulate matter than a port fuel-injected engine (PFI), or even the latest heavy-duty diesels equipped with a particulate filter.

The potential increase in the market means that GDI particulate emissions, though low compared to those of an unfiltered diesel, are now coming under scrutiny from regulators and manufacturers alike.

To reduce these emissions and improve overall performance engineers are studying new combustion designs and engineering concepts, including increasing the fuel pressure, alternate fuels and exhaust emissions control.

According to Matti Maricq, technical leader in chemical engineering and emissions after treatment at Ford’s Research and Innovation Center in Dearborn, injecting fuel directly into the cylinder enables a clean-burning explosion that wastes little fuel and delivers greater power.

During this process gasoline is sprayed directly where the combustion chamber is the hottest (rather than in the air intake), allowing for a more thorough, even and leaner burn.

Cleaner burning GDIs emit harmful particulates.

But because of incomplete fuel volatilization, partially fuel-rich zones and the “wetting” of piston and cylinder surfaces, GDI engines produce unwanted particulate matter. Most emissions typically occur during cold start and high load transient conditions during the warm-up phase, but this can vary according to load, drive-cycle phase and driver demands.

While “green” critics remain sceptical about so-called “engine management” methods, believing them to be unreliable compared to exhaust filters, most OEMs and component suppliers expect that combustion design and engineering changes will prove more cost-efficient and eventually equally effective.

Current development indicates that higher fuel pressure, possibly touching on 40MPa,together with new ultra-precision injectors will greatly improve future GDI systems. To further optimise the system engineers will also continue to improve injector timing, targeting, metering and atomization.

In a recent study, published by the SAE, it was established that an increase in fuel system pressure improved the homogeneity of the mixture and reduced the tip diffusion flame thereby significantly reducing particulate emissions under homogenous combustion in a GDi engine.

Furthermore, as a result of the enhanced intake charge motion at fuel pressures of between 20 MPa to 40 MPa a further reduction of particulate emissions was achieved.

As indicated by the combustion data, an increase in fuel pressure has a significant impact on the reduction of combustion emissions as well as improving fuel consumption.

However for a GDi system to operate optimally it’s important that, during the design and testing phase the pressure of the fuel in the common rail (CR) is correctly measured so that the ECU can be mapped accordingly.

Measurement of CR fuel pressure is key to lower particulate emissions.

Direct injection pressure is measured with sensors, and the signals are used to determine pump speed and/or volume.

Most direct injection systems use piezo-resistive pressure sensors on the low side of the system. The silicone chip element generates a measurable electrical voltage when pressure is applied, increasing as pressure increases.

On the high-pressure side sensors usually use a metallic membrane on a resistance bridge. When pressure is applied, the bridge generates a change in resistance that results in a change in the applied voltage. The Electronic Control Module (ECM) transforms the voltage into a calculated pressure, generally to within a ±2% accuracy.

To maintain the correct pressure, the ECM pulses the low-pressure pump. The system typically has a regulator and no return lines. Some systems even have integrated temperature sensors in the lines that are used to calculate the density of the fuel so that the fuel trim can be tuned to the amount of energy in the fuel.

To ensure accurate measurement of the line pressure it’s important that high precision pressure transmitters are used to map the pressure within the CR under all engine and load conditions. Any errors in this process can result in incorrect modulation of CR pressure which in turn can result in serious abnormalities, such as cylinder wash which can occur if the CR mean pressure exceeds the injector design pressure when fuel delivery is increased at high loads.

Additionally, with the introduction of the harmonized driving cycle OEMs will be under renewed pressure to meet emissions targets set by regulators, and GDI spark ignition engines will be at the forefront of a new generation of green technologies. However, for this technology to meet upcoming legislation particulate emissions need to be reduced, largely through the accurate control of the CR fuel pressure.

MaP sensors are key to clean engine performance

MaP sensors are key to clean engine performance

In the face of ever tightening Global emissions regulations the motor industry has rapidly adopted clean technologies to reduce harmful Greenhouse gases. Critical to the operation of modern clean-burn engines is the precise control of air-fuel ratios (A/F) to stoichiometric values to attain high catalytic converter efficiency and minimize tailpipe emissions.

In order to optimise the A/F ratio under transient conditions manufacturers employ both closed and open loop systems:

  • A closed-loop system is one in which a signal proportional to the A/F is generated by an exhaust gas oxygen sensor (EGO), also known as a Lambda sensor, located in the exhaust stream.
  • An open-loop, or feedforward system controls injector fuel flow through signals received from an air flow meter.

In both cases signals are fed back through a digital PI controller to regulate the fuel injection pulse width. However these systems have two significant drawbacks:

  • Due to the relatively long delay inherent in the induction-compression-power-exhaust cycle of the engine the feedback, or closed-loop portion of the A/F control system, is only fully effective under steady-state operating conditions.
  • A reliable EGO sensor signal is only available after the sensor has warmed up, and thus closed-loop A/F control is not possible immediately upon starting the engine.

Hence, under transient and cold start conditions, the feed forward portion of the A/F controller is particularly important.

To optimise the A/F under all conditions modern engines are commonly fitted with a Manifold air Pressure (MaP) sensor to measure air pressure in the intake manifold.

The MaP sensor knows just what the engine needs

The MaP sensor continually measures the air pressure, and sends this information to the engine ECU, which inserts the data into a table which is used to control the Injector Pulse Width and Ignition Timing. These pressure readings are relayed to the ECU as output voltage signals.

During the development phase it is critical that the pressures measured in the manifold are accurate. Series production MaP sensors, whilst excellent at feeding signals to the ECU, often have wider tolerances than deemed acceptable for development: Therefore high quality pressure transmitters, such as those produced by STS, are commonly fitted in tandem to the series MaP sensors during the development phase. The readings obtained from these sensors are used to measure any deviations or errors when recording manifold pressures at various throttle openings.

The process is quite complex and requires that output voltages are measured at hundreds of throttle opening points, in order for the engine ECU to create an effective map of the engine’s requirements.

Using the Map sensor to teach the engine ECU

During the development phase, using a calibrated MaP sensor, the manifold pressure is measured at small throttle opening increments and the output voltages recorded against each setting.

At idle, with the throttle partially open, this pressure is measured at around 1/3 of atmospheric pressure, or 0.338Bar in a normally aspirated engine. Since the output voltage of the map sensor is proportional to the increase in pressure, the output voltage at idle will be approximately 5/3 = 1.67V where the nominal full scale output is 5V.

However, in practice, the full scale output of a production map sensor can vary and is usually less than 5V. This is because ofvariations among sensor manufacturers, with the result that a typical full scale voltage is around 4.6V. Because of these variances, during normal operation the map sensor reading will vary between about 1.5 V and 4.5V, with the exception of the vacuum created on the overrun where output voltages of less than 1V can be recorded.

Furthermore, since barometric pressure has a significant impact on the fuel mixture the ECU must also understand what the barometric pressure is. To accomplish this ambient pressure measurements are typically recorded just before the engine is started, just after it is turned off or both.

These measurements are usedto set a baseline condition which corrects the manifold pressure for weather and elevation conditions. In practice this is achieved using ignition-on and engine-off signals. This way, the same sensor that controls the engine while it is running is used for the barometric measurement when the engine is off.

Forced induction turns up the pressure on MaP sensors

When a naturally aspirated engine is converted to forced induction through the addition of a turbo or supercharger, the manifold pressure range has to be extended to include the boost region as well as the vacuum region. In order to cover the full pressure range, a map sensor must be used that covers at least 1.5 bar of pressure or a range that matches the engine design parameters.

Should boost pressures exceed 1.5 bar it’s important that, in order to maintain a full scale reading, a decreasing offset is added to the reading as pressure increases. This has practical importance because in map sensor based engine management systems, it is easy to hit-fuel cut or generate a fault in the ECU if the nominal full scale reading is surpassed. That is why a decreasing offset is mapped when a 2 bar sensor is used to read pressures above the nominal full scale pressure.

Sourcing MaP sensors to effectively meet these wide ranging requirements is not always easy. However, with the MaP sensor playing a crucial role in the effective management of the combustion process it’s important that, in order to accurately record manifold pressures during development, a precisely calibrated high quality MaP sensor is used. And with manufacturers under pressure to further reduce emissions and improve performance, application engineers will continue to demand improvements in the accuracy of the sensors used for development.

The turbocharger succumbs to the pressures of energy conservation

The turbocharger succumbs to the pressures of energy conservation

For many years turbochargers were only found on expensive sports cars and diesel powered engines, but emissions regulations changed the way the world viewed forced induction. Although at the core was still the quest to improve performance, now manufacturers were looking at restoring performance and driveability to downsized fuel-sippingengines. So in the 21stCentury, almost everything from the little 999 cm3 Ford Ecoboost to the latest Ferrari’s all gained shiny new turbo technology.

But almost as soon as the tech came into its own it seems set to become redundant, upstaged by the new eCharger. Already Audi’s fitted this to the series production SQ7 and will be rolling out the technology to future production vehicles as 48 Volt electrification gains traction.

The key advantage to the electrically driven supercharger is that, as with turbochargers, there are no parasitic losses; but unlike most turbo’s there’s no turbo lag either and no need for a wastegate. The powerful electric motor can spool up the impeller to 70,000 rpm in less than a second, which eliminates turbo lag.

This naturally improves driveability and reduces consumption and emissions by between 7 and 20 percent when the device is used on a vehicle equipped with regenerative braking, which captures the car’s kinetic energy and turns it into electricity.

Pressure is key to unlocking the eCharger’s performance

Electronically controlled, the eCharger can be mapped to optimize engine performance while maximizing the energy recovered from the exhaust gas, but in order to achieve this Utopia, engineers need to create a map of the boost the engine requires by measuring manifold pressures at various engine loads and speeds. This can only be done with the aid of top quality pressure sensors.

As with any super/ turbo-charger, it’s important that the unit is matched to the engine’s requirements: Failing to do this, will either starve the engine or result in unnecessary electrical power consumption.

Being a maturing technology, not much research and testing data is available to engineers wishing to explore the boundaries of eCharge superchargers. Although fluid dynamics and electrical engineering can provide good foundations from which to build, it’s still vital that theories are validated under real-world test conditions.

In order to qualify the performance, once the baseline eCharger has been selected, the vehicle is equipped with extremely accurate pressure sensors that are readily calibrated and provide precise readings over a wide range of manifold boost pressures and temperatures. These sensors must also be resistant to vibration and chemical degradation.

Both on the engine dynamometer as well as road testing, throttle position/ engine speed/ Manifold air Pressure and temperatures are continuously recorded to ascertain the interrelationship of these key inputs.

From this information, engineers are able to verify that the correct eCharger configuration has been selected whilst at the same time ensuring that the closed loop engine management controls are able to correctly respond to the key variables.

The result of getting this right delivers a vehicle, such as the SQ7, which has stunning performance, drive ability and fuel consumption whilst still meeting future global emissions regulations.