Pollutants emissions

For automotive engines, the great challenge is not only to conform to future pollutant emissions standards: this is already possible under good conditions. The great challenge is to conform to pollutants emissions standards while responding to CO2 emissions reduction objectives.

Indeed, there is a contradiction between reduction of pollutants emissions, in particular NOx, and reduction of CO2 emissions. This for two reasons:

	a)	Increasing combustion pressure permits improving indicated efficiency, but resulting higher combustion peak-temperature generates a larger amount of NOx;
	b)	If combustion under excess air permits reducing pumping losses to improve the efficiency of vehicles used under continuously variable loads, 3-way catalysts cannot reduce NOx in oxygenated exhaust gases.

The following graph, published by IFP, shows that engine strategies allowing for a significant CO2 reduction also present increased NOx emissions levels:

This situation is well illustrated by the air/fuel ratio range of effectiveness of 3-way catalysts:

It is easy to deduce from this graph why Diesel engines (which operate under excess air) emit less HC and CO and more NOx, and why gasoline engines (which operate under stoichiometric combustion) emit more CO and HC but less NOx.

Emissions standards reflect this situation: they take into account specificities of both gasoline and Diesel engines:

As can be noticed, gasoline engines emit ultra-low amount of particulates. For this reason, particulates emissions are regulated only for Diesel engines. Future regulations represent a real challenge for automotive industry. Indeed, future regulations will ask for the best of the all worlds: ultra low levels of CO, NOx, HC, particulates and CO2.

The good strategy to respond to future regulations is probably not making Diesel engine looking like gasoline engines and gasoline looking like Diesel: not only qualities result from such crossings as both engine types also inherit their respective defects.

As an example, stratified charge Direct Injection makes gasoline engines resembling to Diesel engines while increasing their efficiency on a specific load-speed range. But stratified charge Direct Injection makes gasoline engines inherit two main defects from Diesel ones: NOx after treatment in oxygenated environment (NOx trap becomes necessary), and increased particulates generation (partculates filter could become necessary depending on future regulations).

For SI engines, the ultimate solution will come from the Variable Compression Ratio.

VCR: the ultimate tool for pollutants emissions and after treatment mastering

Highly downsized VCR engines present unique features: whatever Otto or Otto-Atkinson, they provide a high fuel consumption and CO2 reduction while operating under stoichiometric combustion with no need for Direct Injection.

This presents two main advantages:

		3-way catalyst is fully operational to reduce emissions of regulated pollutants (no need for NOx traps and associate defects: extra-costs, sulfur fuel sensitivity and energy consumption for regeneration);
		Avoiding Fuel Direct Injection implementation avoids associate extra costs and particulates emissions.

Concerning this first point, VCR provides both low pollutants emissions levels (CO, HC and NOx), AND low CO2 emissions levels.

But VCR goes further and provides a better control over pollutants generation and after treatment than conventional FCR engines. Indeed, VCR allows controlling main parameters that determine pollutants generation and after treatment devices operation, characteristics and durability.

These parameters are the following:

		Combustion pressure and temperature;
		Expansion ratio and indicated efficiency;
		Combustion chamber volume.

Many relations exist between these 3 main parameters, which allow for several strategies to control pollutants generation and their after treatment.

When the engine is cold:

When the engine is warm:

	a)	At idle speed, VCR engines operate under high compression ratio (about 16:1). As the combustion chamber volume is reduced, the charge contains a lower burnt gases proportion. In addition, on idle operation, VCR peak pressure and temperature is higher than that of Fixed Compression Ratio engines. These features avoid misfiring, and reduce cyclic irregularities and incomplete burning. As a result, engine idle speed can be reduced thanks to a better vibrations level (immediate CO2 emissions reduction), as well as CO and HC generation.
	b)	At part loads, VCR allows a better control over EGR strategies (Exhaust Gases Recirculation). Indeed, combustion chamber volume of VCR engines is smaller at part loads (higher compression ratio) and burnt gases proportion is lower. This naturally extends the burnt gases proportion control range while giving priority to external EGR instead of internal EGR. As a result, recirculated gases are better mixed with the new charge (they pass together through the intake valve), and they can be cooled before beeing introduced in the cylinder. In addition, VCR can compensate for high EGR and restore the combustion speed by increasing end-of-compression-stroke pressure and temperature.
	c)	At max power, VCR avoids extreme charge enrichment which is commonly used on highly supercharged FCR engines to protect their components against excessive thermal constraint (motorhead, exhaust manifold, turbocharger, catalyst...). Such strategies are extremely CO and HC emitting, but thanks to VCR, they can be avoided by reducing exhaust gases temperature at full power thanks to an increased expansion ratio at high speeds (engine knock sensitivity decreases as its speed increases).

Pollutants emissions control, the particular advantages of the MCE-5 technology

If VCR provides several strategies to master pollutants generation and after treatment, the MCE-5 technology permits extending the life time of effectiveness for 3-way catalysts. This is due to the MCE-5 roller-guided piston that highly reduces cylinder wear and distortion as well as wear irregularities (no cylinder ovalization). As a result, oil ring operation is improved during the whole engine life, oil consumption is reduced, and the 3-way catalyst's life time of effectiveness is extended (oil consumption reduces the catalyst effectiveness and life span, and increases its operational temperature)

Conclusion

VCR strategy provides high CO2 reduction under stoichiometric combustion. In addition, both VCR and MCE-5 open the way to new opportunities and strategies to improve pollutants after treatment by means of standards and low-cost 3-way catalysts.