Reducing Emissions in Gasoline Engines



Colin Pawsey
04/18/2016

How to utilize solutions to achieve the ultimate aim of meeting Euro 6 targets.

Emissions from diesel engines have rightly taken the headlines over the last six months in response to the Volkswagen scandal, but gasoline engines are subject to the same regulations and the reduction of emissions from internal combustion engines presents OEM’s with distinct challenges as they strive to meet EU targets.

  • CO - 1.0 g/km
  • HC - 0.10 g/km
  • NOx - 0.06 g/km
  • PM - 0.005 g/km (Direct Injection only)
  • PM - 6.0x10 ^11/km (Direct Injection only)

Further, the test methods for emissions, which have come under such scrutiny in light of recent events, are expected to be completely revamped by 2017 whereby real world testing will be implemented to garner actual emissions on the road as opposed to results contrived from the lab.

There are various technologies either in place or in development to reduce emissions specifically from gasoline engines, and this article will assess those advances and the steps OEM’s are taking to comply with Euro 6 regulations.

Three Way Cataclyst

As we know, the majority of new gasoline vehicles are now fitted as standard with three-way catalytic converters. They use either a ceramic or metallic substrate to promote the required chemical reactions and therefore reduce emissions. Catalysts need high temperatures to work effectively, which in the past meant that emissions were high during ‘start-up’ periods. By using such materials and mounting the catalyst closer to the engine, these ‘fast light off’ catalysts are able to cut warm-up times so that the system begins working within seconds, thereby reducing emissions.

The term ‘three-way’ refers to the three emissions that the converters reduce: Carbon Monoxide, Hydrocarbons and Nitrogen Oxides. The system typically includes two types of catalyst - a reduction catalyst and an oxidisation catalyst - and an electronic control system. Both catalysts function to enable temperatures whereby the converter can achieve oxidisation, while the electronic control system monitors the exhaust upstream of the converter and uses sensors to gather information to optimize combustion by controlling the air/fuel ratio.

Ceramic Substrates

While three-way catalytic converters are relatively commonplace, the technology used with these systems is constantly evolving. Corning Incorporated is one such company that is developing increasingly efficient ceramic substrates with a focus on reducing emissions on cold start-up.

Up to 70% of regulated emissions can occur during the first 30 seconds after engine start, and according to Corning addressing these ‘cold-start’ emissions is crucial to meeting new standards and regulations. The company produces a range of ‘Celcor’ substrates for both gasoline and diesel engines, and last year launched its ‘FLORA’ substrate with Honda Motor Company. The next-generation substrate will be used on selected Honda models produced in 2016 specifically to improve cold-start emission performance.

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FLORA substrates feature an optimised material micro-structure which reaches operating temperature quicker than standard substrates, enabling catalytic converters to clean exhaust emissions earlier without increased fuel or the use of  additional precious metals. This in turn enables manufacturers to control emissions in a cost-effective way without sacrificing performance. Advantages of the substrates include the flexibility to lower non-methane organic compound emissions, reduce the use of precious metals (found in metallic substrates), and imrpove fuel efficiency.

Exhaust Systems

Particulate filters are commonly associated with diesel engines, but are increasingly being used with direct-injection gasoline engines to reduce particles emissions. In typical applications ‘wall-flow filters’ remove particulate matter from the exhaust via physical filtration using a honeycomb structure - similar to a catalyst substrate but with the channel blocked at both ends. This forces the exhaust gas through the walls between the channels and the particulate matter is deposited as a soot cake on the walls.

Since the continuous flow of soot into the filter would eventually block it, it is necessary to 'regenerate' the filtration properties of the filter by burning off the collected particulate on a regular basis. Trapped particulate burns off at normal exhaust temperatures using the powerful oxidative properties of NO2 and can burn in oxygen when the temperature of the exhaust gas is periodically increased through post-combustion.

Bekaert is one such company that produces gasoline particulate filters which consist of a series of highly porous, 3-dimensional media of sintered metal fibers. The metal fiber media can be shaped into limitless designs enabling manufacturers to easily integrate the technology into exhaust design. The Bekaert filter has some key advantages - high sound absorption eliminates the need for a silencer or muffler, while the low regeneration temperatures used by the filter mean that no complex regeneration strategy is required. The unique structure of Bekaert’s ‘Bekipor’ metal fiber media consists of very fine, lightweight metal fibers (Æ of 17-22-30 μm).

These can be fine-tuned in terms of porosity (up to 88%) and alloy to achieve optimal filter performance and durability. The high porosity of the media can also significantly reduce pressure drop over the filter.

Engine Management

Turbocharging and exhaust gas recirculation are common methods of optimising the combustion process, but manufacturers are also turning to new engine management systems which reduce fuel consumption and therefore reduce emissions.

Valeo is one such supplier that is pushing the boundaries in terms of innovative solutions for the next-generation of vehicles to reduce emissions and support automated mobility. Since the introduction of the Hybrid4All system (pictured above) Valeo has concerted efforts towards reduced fuel consumption through new technologies.

The Hybrid4All system was introduced to provide a better trade-off between price and performance and make hybrid technology affordable and available in entry-level vehicles. It consists of a conventional combustion engine and a low-power electric engine, generating up to 15% reductions in fuel consumption. The company says that its main growth driver beyond 2020 will be autonomous driving, but at present the driving force behind innovation is the desire to reduce emissions through greater efficiency to assist automakers in meeting EU targets.

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These innovations include an electric supercharger which works independently of the air loop. Driven by an electric motor the supercharger responds instantly (within 250 milliseconds) delivering torque on demand and eliminating turbo lag.

The supplier also introduced its EG Efficiency Alternator which increases alternator efficiency by 10% and earned the company a 2015 Automotive News PACE award. Valeo’s focus is on reducing emissions, with an eye on intuitive driving and it is currently running its 2016 Innovation Challenge which invites young engineers to develop ideas for the car of 2030.

Portable Emissions Monitoring Systems

These new technologies will all be subject to new testing requirements that are likely to come into force during 2017. Portable emissions monitoring systems (PEMS) will be used on-road to accurately test vehicles for emissions.

In a speech delivered at the Geneva Motor Show on March 1st 2016, Clean Vehicles Director Greg Archer revealed that the first real world emissions test developed with a car-maker has been jointly developed with PSA. The test procedure T&E, verified by Bureau Veritas, generates a scientifically robust and reproducible figure for the CO2 emissions of a car - representative of the typical driver. The PEMS directly measures the pollution from the exhaust, whilst simultaneously recording speed, acceleration and position of the vehicle to ensure that the car is driven in a style typical of a ‘normal’ driver.

Significantly, there is no electrical connection between the PEMS and the car’s engine management system, so the vehicle is unable to detect that it is being tested and therefore cannot use a ‘defeat device’ to put the car into a ‘low emissions’ mode. This is something that all manufacturers will have to prepare for as new legislation will demand this type of testing over and above typical WLTP cycle testing.

Summary

Emission reduction as a whole is a complex subject, with a variety of systems and technologies in use across a range of different engine models. It is therefore essential that OEM’s take a holistic approach to the problem and utilize a range of solutions to achieve the ultimate aim of meeting Euro 6 targets.

Exhaust gas recirculation, turbo chargers, three-way converters and particulate filters each have an impact on the amount of emissions produced by a vehicle, but further technologies can contribute to reduced fuel consumption to achieve additional results. The surge towards automated driving is in part, whether directly or indirectly, contributing to this aim as hybrid solutions, stop-start technologies and electrification innovations work to significantly improve vehicle efficiency.

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