Fuel Consumption reduction potential

SI engines Fuel Consumption reduction sources

As a reminder, reducing automotive SI engines Fuel Consumption implies increasing positive mechanical work (indicated efficiency) and/or reducing negative mechanical work (all mechanical losses whatever friction or pumping losses) on the entire driving cycle.

Fuel Consumption reduction strategies always result in a new average balance between positive and negative mechanical work:

Only 5 fundamental strategies can be applied on IC engines:

	A	To increase positive mechanical work: 1- Increase of effective expansion ratio, 2- Gas-wall heat transfers reduction.
	B	To decrease negative mechanical work: 3- Pumping losses reduction, 4- Compression losses reduction, 5- Friction losses reduction.

SI engines Fuel Consumption reduction strategies and technologies

The following chart shows main SI engines’ Fuel Consumption strategies:

FC reduction strategies	Sources of effectiveness	Engine strategies
1 - Increased effective expansion ratio	Improved indicated efficiency	High Fixed Compression Ratio (FCR) Variable Compression Ratio
2 - Downsizing	Pumping losses reduction Internal heat transfer surface reduction Flame travel distance reduction Friction losses reduction	High supercharging Increased average BMEP 10:1 Compression Ratio (FCR) From 8:1 to 16:1 Compression Ratio (VCR)
3 - Downspeeding	Pumping losses reduction Friction losses reduction	High supercharging Increased average BMEP
4 - Unthrottled load control	Pumping losses reduction	Load control by air/fuel ratio Load control by Late Intake Valve Closing
5 - Engine mechanical efficiency improvement	Friction losses reduction	Reduction of all friction sources

These strategies can be implemented thanks to different technical approaches and associate technologies:

Technical strategy	Technologies
1 - Direct Fuel Injection	200 bar common rail Direct Injection
2 - Variable Valve Actuation	Cam based Cam phasing Cam phasing + cam changing Cam phasing + continuous valve lift Camless Electromechanical Electrohydraulic Hydromechanical
3 - Variable Compression Ratio	Articulated cylinder head Slide mounted engine block Additional piston in cylinder head Multilinks rod crank mechanism Eccentrics on bearings Hydraulic pistons Gear-based mechanism
4 - Supercharging	Supercharger Turbocharger
5 - Low-friction designs, materials and lubricants	Low-friction coatings Low-tension rings Roller-rockers Reduced rod angle under combustion Reduced diameter bearings
6 - Low-energy consuming peripherals	Electric oil pump Electric water pump

Fuel Consumption reduction provided by VCR compared to FCR

Most of SI engines’ Fuel Consumption strategies can be implemented on both FCR and VCR engines. But the effectiveness of these strategies highly varies depending on whether they are implemented on FCR or VCR engines. Indeed, all that is possible on a Fixed Compression Ratio engine is possible on a Variable Compression Ratio engine, but not the contrary.

In fact, VCR allows controlling parameters that no longer constitute a major limit to Fuel Consumption reduction strategies effectiveness:

	1)	End-of-compression-stroke pressure and temperature
	2)	Combustion peak pressure and temperature
	3)	Expansion ratio and indicated efficiency
	4)	Exhaust gases temperature

As a result VCR improves the efficiency of a wide range of strategies, for which Fixed Compression Ratio constitutes a major limit:

In addition to improving Fuel Consumption strategies effectiveness, VCR avoids implementing sophisticated technologies to compensate for inability of FCR engines to control pressure, temperature and indicated efficiency. This constitutes a significant cost saving source (see: VCR negative costs).