Pumping losses reduction

Compared to Diesel engines, SI engines present high pumping losses at part loads due to load-control by intake-throttling: this penalizes their fuel efficiency. If downsizing permits reducing pumping losses, the latter are not eliminated.

Even if valve throttling instead of butterfly throttling can partly reduce pumping losses (example: BMW Valvetronic), most efficient pumping losses reduction strategies are based on replacing intake-throttling by other load-control strategies. To reduce remaining pumping losses on downsized SI engines, it is interesting to compare future opportunities offered by VCR engines with those of FCR engines. Alternative load-control strategies are two types:

	a)	Non-stoichiometric strategies permitting to control the engine load by air/fuel-ratio. These strategies imply mastering combustion under ultra-lean mixtures: Spark ignited homogeneous lean-burn charge; Spark ignited stratified charge; Compression Ignited homogeneous lean-burn charge (Controlled Auto Ignition).
	b)	Stoichiometric strategies permitting to control the engine load by Late Intake Valve Closing (by back-flow): Adaptive Atkinson cycle

These two strategies families cannot totally replace intake butterfly, but they widely limit the need for intake pressure reduction at part loads (they avoid generating high pumping losses).

Non-stoichiometric strategies (lean-burn)

Whatever homogeneous or stratified, lean-burn combustion presents a big disadvantage: as it occurs under access air, NOx reduction in oxygenated environment cannot be done by a 3-way catalyst.

Two main solutions permit NOx emissions control under lean-mixture combustion:

	a)	Specific NOx after treatment device implementation such as NOx traps that are periodically regenerated. But NOx traps are costly, and their regeneration is fuel-consuming. Other solutions such as SCR (urea) remain difficult to implement on passenger cars;
	b)	Avoid producing NOx, or to produce them at ultra-low levels to conform to emissions standards with no need for NOx after treatment. But there is a strong relation between NOx generation and indicated efficiency: the higher the efficiency, the higher the NOx.

Another major problem for lean-mixture is Spark Ignition. Indeed, combustion initialization and flame propagation widely depend on air/fuel ratio. As a result, misfiring and cyclic irregularities occur at low air/fuel ratios.

These problems can be avoided by replacing Spark Ignition by Compression Ignition. But CAI (Controlled Auto Ignition) is highly difficult to master: it implies to precisely control the ignition timing, which depends on complex chemical and physical phenomena.

On the other hand, if Spark Ignition naturally provides a low pressure gradient due to a non-instantaneous flame propagation, on the opposite, Compression Ignition is based on quasi-instantaneous multiple combustion initializations: this generates noise which is difficult to control.

Whatever Spark or Compression Ignition, VCR allows controlling the most determing factors for lean-mixtures combustion: pressure and temperature.

VCR advantages for lean-mixtures strategies:

Spark ignited homogeneous lean-burn charge:

High air/fuel ratios cannot be reached under homogeneous lean-burn combustion because of flame initialization and propagation limits. VCR permits increasing compression pressure and temperature to restore favorable conditions for combustion process (no misfiring and rapid flame propagation) even under high air/fuel ratios. As a result, VCR permits to push back the bounds of possibility for homogeneous lean-burn combustion while extending its load-speed range of effectiveness.

Stratified charge

Several stratified charge engines are presently commercialized (commercial acronyms: GDI, FSI, HPI...), but as they present several limits and disadvantages, some carmakers decided not to pursue their development:

	a)	They require Fuel Direct Injection: this technology is costly and leads to significant additional particulates emissions compared to Inlet Port Injection;
	b)	As they operate under excess air, the 3-way catalyst cannot reduce NOx. NOx trap is required, which presents several disadvantages (cost, fuel consumption for regeneration, only compatible with ultra-low sulfur fuels);
	c)	They operate under stratified charge on a load-speed range, which is too restricted.

The last point is crucial: benefit of stratified charge for Fuel Consumption reduction is too restricted to particular driving conditions. For this reason, most clients cannot reach Fuel Consumption announced by carmakers. From the commercial point of view, it is diffcult to sell at higher price vehicles whose announced low Fuel Consumption is not confirmed under ordinary driving conditions.

Again, VCR constitutes a major solution for stratified charge: compression pressure and temperature can be increased to guarantee a rapid and complete combustion under ultra-lean mixtures on an extended load-speed range.

Compression ignition

Main challenge for lean-burn Controlled Auto Ignition (CAI) is to precisely set ignition initialization timing in different locations in the combustion chamber. On Fixed Compression Ratio engines, it is necessary to precisely adjust parameters such as intake temperature, burnt gases proportion or even fuel octane number to induce ignition at the required timing.

However, mastering all these parameters is not necessary if Compression Ratio is under control: VCR permits precise setting of ignition timing while taking into account all physical and chemical conditions.

As a result, VCR widely extends the Compression Ignition load-speed range of effectiveness and its positive impact on Fuel Consumption reduction.

Stoichiometric strategies

Load-control by LIVC

Controlling the engine load by Late Intake Valve Closing (LIVC) permits to replace intake throttling by intake back-flow (gases are returned back to the intake manifold until the required charge is obtained inside the cylinder). This widely reduces pumping losses.

However, load-control by LIVC is unsuited to FCR engines: delaying the Intake Valve Closing on a FCR engine reduces the effective Compression Ratio. As a result, compression pressure and temperature are reduced, and combustion conditions are deteriorated (slow flame propagation, reduced effective expansion ratio, high HC and CO generation...).

For this reason, load-control by Late Intake Valve Closing (LIVC) leads to low or nil Fuel Consumption reduction when applied to FCR engines.

VCR advantages for LIVC load-control implementation on SI engines:

VCR permits making LIVC load-control highly effective and with no induced defects.

Indeed, VCR permits to restore the effective Compression Ratio and required combustion conditions whatever the Intake Valve Closing timing: the later the Intake Valve Closing, the higher the engine volumetric ratio (from the geometrical point of view). As a result, the expansion ratio increases as the load decreases.

Thanks to this control, the effective Compression Ratio always remains about 10:1 whatever the LIVC, and the end-of-compression-stroke pressure remains always equivalent to that of a naturally aspirated engine operating at full load.

The resulting thermodynamic cycle has been described by James Atkinson in 1882:

Combining VCR and LIVC to control the engine load presents several advantages:

	a)	Pumping losses are replaced by gas transfer losses (high pumping losses reduction);
	b)	Combustion remains always stoichiometric (only a 3-way catalyst is required for exhaust gases after treatment);
	c)	Expansion ratio rapidly increases as load decreases even on medium-high loads, resulting in an average expansion ratio which is higher than that of an Otto VCR engine;
	d)	Air-fuel mixture is highly homogeneous whatever the load (thanks to back-flow and transfers), and fine-scale turbulence is highly generated thanks to high intake-flow (butterfly always remains wide open).
	e)	Combustion remains rapid and efficient whatever the load: high air-fuel mixture homogeneity, high fine-scale turbulence level, optimum compression pressure and temperature.

Thanks to VCR and LIVC control, the «adaptive» Atkinson cycle highly reduces Fuel Consumption reduction at part loads (fom 10 to 20% FC reduction).

However, implementing the adaptive Atkinson cycle on SI engines not only requires an apprpriate VCR technology, it also requires a Variable Valve Actuation technology to control Late Intake Valve Closing under the best functional conditions.