More Performing With Efficiency

Keep in mind our test cycle was not designed to produce the best possible mileage, but intrinsically would be very fuel hungry. We measured the distance traveled for every quart of fuel consumed. Any gain realized would most likely be even greater under steady state cruise conditions such as on the highway. Follow along as HPP lets the pictures tell the story.

Building For Efficiency
Though substantial gains in fuel economy can be realized with external tuning, in essence all that is being done is bringing the engine to its full design potential. No decrease in fuel consumption can occur beyond the inherent thermal efficiency of the original design. Gains beyond that can be huge, but they require a rethinking of the internal engine design, much as is commonly done to make a Pontiac go faster. But, instead, the edict will be mpg instead of mph.

Every internal combustion engine experiences three areas of loss in the potential energy (measured in British thermal units) from the fuel consumed. They are: pumping, frictional, and thermal losses. Even the most powerful Pontiac only uses about 25 percent of the fuel's energy to transmit power to the crankshaft. Approximately a 25 percent loss is assigned to each of the three areas. The easiest and most productive way to increase fuel economy in any engine is to increase the static compression ratio. A higher compression ratio directly impacts the thermodynamic efficiency of the engine, or in simpler terms, reduces the amount of fuel by mass to produce 1 hp.

That is one reason why diesel engines are more fuel efficient than gasoline--the high-compression ratio (22:1 on normally aspirated versions) improves the thermal efficiency. The second reason for a diesel's stingy reputation is the fuel has more Btu's of energy than gasoline.

A higher compression ratio does bring along the need for higher fuel octane, but through good engineering this requirement can also be kept under control. New technologies such as piston and combustion chamber thermal barrier coatings, along with advanced coolants such as the unique Evans NPG+, allow for a higher-than-thought-possible compression ratio with older cylinder head designs.

Another untapped area for decreased fuel consumption is camshaft phase and over-lap. Advanced placement when referenced from the intake lobe centerline will increase volumetric efficiency at small throttle openings and allow the engine to do more work on less fuel. Decreased overlap will limit high-rpm breathing, but will make the engine more fuel efficient at part throttle and light load.

What makes all of this fun for an engine guy like me? Fuel-economy gains are more challenging than going fast. It is an engineering exercise that uses all of the same logic as building a hot rod, only it redirects it to another cause. Who knows, maybe in the near future there will be a spin-off publication of HPP called HEP, or High Efficiency Pontiac. Stranger things have happened.--RTB