Ethanol offers significant potential for increasing the
compression ratio of SI engines resulting from its high octane
number and high latent heat of vaporization. A study was conducted
to determine the knock-limited compression ratio of
ethanol-gasoline blends to identify the potential for improved
operating efficiency. To operate an SI engine in a flex fuel
vehicle requires operating strategies that allow operation on a
broad range of fuels from gasoline to E85. Since gasoline or low
ethanol blend operation is inherently limited by knock at high
loads, strategies must be identified which allow operation on these
fuels with minimal fuel economy or power density tradeoffs.
A single-cylinder direct-injection spark-ignited engine with
fully variable hydraulic valve actuation (HVA) is operated at WOT
and other high-load conditions to determine the knock-limited
compression ratio (CR) of ethanol fuel blends. The geometric CR is
varied by changing pistons, producing CR from 9.2 to 12.87. The
effective CR is varied using an electro-hydraulic valve train that
changed the effective trapped displacement using both Early Intake
Valve Closing (EIVC) and Late Intake Valve Closing (LIVC). The EIVC
and LIVC strategies result in effective CR being reduced while
maintaining the geometric expansion ratio.
It was found that at substantially similar engine conditions,
increasing the ethanol content of the fuel results in higher engine
efficiency and higher engine power. These results can be partially
attributed to a charge cooling effect and a higher heating value of
a stoichiometric mixture for ethanol blends (per unit mass of air).
Additional thermodynamic effects on the ratio of specific heats
(\yy) and a mole multiplier are also explored.
It was also found that high CR can increase the efficiency of
ethanol fuel blends, and as a result, the fuel economy penalty
associated with the lower energy content of E85 can be reduced by
about twenty percent. Such operation necessitates that the engine
be operated in a de-rated manner for gasoline, which is knock-prone
at these high CR, in order to maintain compatibility. By using
early and late intake valve closing strategies, good efficiency is
maintained with gasoline, but peak power is about 33% lower than
with E85.