Experiments, results, prospects

Carmakers references for VCR Fuel Consumption reduction

In March 2000, Saab unveiled its Variable Compression Ratio (VCR) prototype vehicle, which was powered by a 1.6 L supercharged VCR engine named SVC (Saab Variable Compression). The SVC engine delivers 168 kW of power (228 hp) and 305 Nm of torque. Thanks to its SVC engine, Saab claimed to reduce Fuel Consumption of about 30% compared with a conventional naturally aspirated engine of equivalent power. Few months later, FEV Motorentechnik also unveiled its own interpretation of Variable Compression Ratio through an A6 Audi, powered by a 1.8 L VCR engine. In this case, announced Fuel Consumption reduction was about 27%.

Depending on information sources, supercharged VCR engines provide between 7% and 10% additional Fuel Consumption reduction when compared to supercharged Direct Fuel Injection + VVA FCR engines (final Fuel Consumption reduction difference depends on engine max power, vehicle characteristics, and driving cycle). Using DFI + VVA to improve FCR engines’ knock resistance and specific power by charge cooling and charge scavenging is extremely efficient. This has been perfectly demonstrated by the French Petroleum Institute (see SAE 2001-01-736 - G. MONNIER and A. RANINI).

Concerning VCR impact on Fuel Consumption reduction, most carmakers have the following vision (graph presented by Renault in 2002):

FEV Motorentechnik also presents the same analysis in its commercial brochure:

However, as comments for these graphs Fuel Consumption measurement conditions should be specified: Fuel Consumption reduction provided by advanced FCR engines or VCR engines mainly depends on max power, vehicle characteristics, and driving cycle.

Main tendencies for VCR potential fuel saving compared to present naturally aspirated engines can be presented as shown on the following graph: Fuel Consumption reduction potential depends on max power on one hand, and on strategies combined with VCR on the other hand:

Fuel Consumption reduction provided by VCR not only depends on engine max power but on engine power and torque oversizing. In other words, the further the engine power and torque from vehicle’s needs under ordinary driving conditions, the higher the Fuel Consumption reduction provided by downsizing.

As knocking is under control on VCR engines, high supercharging is possible with no need for a low fixed Compression Ratio. As a result, downsizing ratios of about 50% are possible for high powered vehicles. In addition, VCR provides an additional efficiency gain thanks to high expansion ratios at part loads that compensate for pumping losses.

On future Otto-Atkinson VCR engines, high expansion ratios at part loads will permit to benefit from a better efficiency at low loads than at high loads. This is shown on the following graph:

Whatever Otto or Otto-Atkinson VCR engines, as low loads are highly represented on common driving cycle, high expansion ratios at part loads have a significant impact on Fuel Consumption reduction.

Both impact of high downsizing and Compression Ratio optimization on Otto-VCR engine efficiency is shown on the following graph published by Roland Gravel, engine research Program Manager at the American Department of Energy:

But the latter graph doesn’t take into account VCR ultimate benefit: combined with Variable Valve Actuation, Otto-VCR engines will become Otto-Atkinson VCR engines. This will provide significant additional gains in the future: