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APPENDIX A: MORE DETAIL ON FUEL ECONOMY TECHNOLOGIES
This section describes in detail each of the fuel economy technologies used in the Civic
VX.
VTEC-E Engine
The Honda Variable Valve Timing and Lift Electronically Controlled (VTEC-E) engine is a
4-valve per cylinder, variable valve engine optimized for fuel economy. It is a completely
different engine than that found in the 1991 and 1992 Civic DX hatchbacks. The VTEC-E
engine has variable valve timing and uses lean burn technology. Additional modifications
to reduce engine friction contribute further to the improved fuel economy of the VTEC-E
engine.
Variable valve timing: Conventional engines use fixed valve timing and lift (height of
intake valve) at all engine speeds. At light loads when lower power is required, fixed valve
timing results in pumping losses, or efficiency losses due to the use of energy to pump air
and fuel into the cylinder and push out the products of combustion. Pumping losses occur
because, in order to reduce power, air and fuel input must be decreased using a throttle that
produces a pressure differential between intake and exhaust valves on the engine cylinder,
resulting in a loss of efficiency. Thus, closing the intake valve at light loads would reduce
the pressure differential and decrease pumping losses. This is the principle on which
variable valve timing operates. In the VTEC-E engine, the valve cycles are electronically
controlled to promote more complete combustion at lower engine speeds, thus reducing
fuel requirements. At low rpm, one of the two intake valves in each cylinder of the VTEC-
E engine is nearly closed. This creates a swirling motion in the combustion chamber. The
mixing and turbulence caused by this motion, combined with central placement of the spark
plug, shorten combustion time and assure more complete combustion. Combustion time is
also lessened by reducing the surface to volume ratio of the combustion chamber. The use
of variable valve timing also enhances low speed torque, allowing gearing modifications
(see below). At higher rpm, when more power output is required, both intake valves are
activated simultaneously with full lift to provide power comparable to engines without
variable valve timing (Honda Motor Co. Ltd 1991). Honda estimates that this feature
produces a 2.5% increase in fuel economy (Harrington 1992).
Lean burn technology: A lean burn engine is operated with air in excess of that required for
complete combustion (a high air/fuel ratio). Fuel economy increases with the addition of
excess air to the combustion chamber, as more air results in more complete combustion of
fuel, improves the efficiency of converting fuel energy into usable work (thermodynamic
efficiency), and decreases pumping losses. The use of variable valve timing and electronic
control enabled Honda to incorporate lean burn technology in the 1992 Civic VX. At low
engine speeds, the VTEC-E engine can operate at air/fuel ratios as high as 25:1, compared
to the stoichiometric ratio of 14.7:1 at which there is just enough air to theoretically achieve
complete combustion. At low rpm, the swirl motion, discussed above, creates turbulence
and results in the formation of a rich mixture (lower air/fuel ratio) that is maintained in a
vortex near the spark plug, while the total air/fuel ratio in the combustion chamber remains
lean. At higher speeds, the ratio decreases to approximately stoichiometric via electronic
control. This return to stoichiometric conditions at high rpm resolves a problem common
to lean burn systems where wide open throttle power decreases because less fuel is burned.
Another problem associated with lean burn is an increase in nitrogen oxide (NO
x
)
emissions due to inability of the 3-way catalysts to control NO
x
at high air/fuel ratios. In
order to meet more stringent NO
x
emission standards in California, Honda modified the
VX to use exhaust gas recirculation (EGR) instead of excess air to dilute a stoichiometric