Green by Design
Part 4: Engines of Change

With all of the high-profile attention being paid recently to alternative technologies, it can be easy to overlook a revolution taking place as we speak in one of the most unexpected of places, the gasoline engine.

These engine advances are being integrated on a number of vehicles, both conventional and hybrid. One example of the latter is the newly updated Honda Civic Hybrid, which now achieves PZEV and Tier 2 bin 2 emissions levels, and averages a combined city/highway fuel economy of 50 mpg. A quick look at the Civic Hybrid's 1.3-liter i-VTEC engine shows how engineers are incorporating both weight reduction and friction reduction to improve the vehicle's overall efficiency. First, the lightweight aluminum engine block uses a thin-sleeve cylinder wall design to further minimize the amount of metal required. To minimize friction and improve the long-term wear characteristics of the engine, Honda uses a two-stage "plateau honing" machining process that creates an ultra-smooth surface between the cylinders and the pistons. The pistons themselves are made of a lightweight aluminum alloy and have "micro-dimples" on the cylinder walls to provide additional lubrication and reduce friction when the pistons are moving. Furthermore, low-tensile-force piston rings were used to cut friction even further. The connecting rods, which link the pistons to the crankshaft, are made of high-strength forged steel treated with a surface-hardening carbon process. These rods are lighter than traditional connecting rods, yet capable of performing the same job. Finally, Honda employs a hidden engine efficiency trick in using offset cylinder bores. Placement of the crankshaft axis in a more efficient alignment with the cylinder bore axis minimizes the friction resulting from the side thrust of the pistons against the side of the cylinder walls.

Green engine designs go above and beyond mass and friction reduction, however. The Civic Hybrid uses a 3-stage i-VTEC valve control design that alters valve timing depending upon driving conditions: low-speed timing, high-output timing, or a 4-cylinder (full engine) idling functionality known as Variable Cylinder Management (VCM). This is a step beyond the 2005 Civic Hybrid, which used a 2-stage design that operated with conventional timing and 3-cylinder idling. The new design yields an increase in both power and efficiency. When engaged, the high-output valve timing yields a nine percent increase in engine output. Similarly, when the VCM system is engaged, it cuts pumping losses in the engine by 66 percent.

What are pumping losses? Pumping losses are energy losses associated with pumping air in and out of the cylinders. Drivers can feel how strong these losses can be by lifting their foot off the accelerator while staying in gear—a deceleration technique known as "engine braking." In most hybrids, including the Civic Hybrid, these pumping losses limit the amount of mechanical energy the generator receives when producing electricity for the battery pack. To combat this, Honda incorporated VCM to reduce the pumping losses inside the engine. The result of the VCM system is a 70 percent improvement in electrical regeneration capability, as well as permitting the electric motor to power the vehicle under certain driving conditions.

The engine's valvetrain design accommodates a preferred narrow (30 degree) angle between intake and exhaust valves, as well as a more compact combustion chamber. These elements help to evenly distribute the air-fuel mixture in the cylinder, yielding more complete combustion. Further aiding combustion, each cylinder contains twin spark plugs that fire either sequentially or simultaneously, depending upon engine rpm and load conditions, to better control flame propagation in the cylinder. Combined, these factors are an important ingredient in the Civic Hybrid's recipe for remarkably clean PZEV and Tier 2 bin 2 emissions levels.

Continue to Part 5: Tighter Tailpipe Limits