results in pumping losses. These losses are proportional to the intake vacuum, and disappear at
wide open throttle.Lean-burn
Lean-bum is one method to reduce pumping loss. Instead of throttling the air, engine power
can be reduced by reducing the fuel flow so that the air-fuel ratio increases, or becomes leaner. (In
this context, the diesel engine is a lean-bum engine). Most SI engines, however, do not run well at
air: fuel ratios leaner than 18:1, as the combustion quality deteriorates under lean conditions.
Manufacturers provided data on engines constructed to create high swirl and turbulence when the
intake air and fuel are injected into the cylinder that can run well at air: fuel ratios up to 22:1.
Lean-bum engines actually run at high air-fuel ratios only at light loads; they run at stoichiometric
or rich air: fuel ratios at high loads to maximize power. The excess air combustion at light loads
has the added advantage of having a favorable effect on the polytropic coefficient, n, in the
efficiency equation. Modem lean burn engines commercialized recently in Japan do not
completely eliminate throttling loss, but the reduction is sufficient to improve vehicle fuel
economy by 8 to 10 percent. A disadvantage of lean-bum engines, however, is that they cannot
use conventional three-way catalysts to reduce emissions of nitrogen oxides (NOX, and the in-
cylinder NOX emission control from running lean is sometimes insufficient to meet stringent Nox
emissions standards. There are developments in “lean NOX catalysts,” however, that could allow
lean-bum engines to meet the most stringent NOX standards proposed in the future, which will be
discussed later.Variable valve timing
Variable valve timing (VVT) is another method to reduce pumping loss. Instead of using the
butterfly valve to throttle the intake air, the intake valves can be closed early, reducing the time
(and volume) of air intake. The system has some problems at very light load (the short duration of
the intake valve opening leads to weaker in-cylinder gas motion and reduced combustion
stability). Moreover, at high rpm, some throttling losses occur at the valve itself.^47 Hence,
throttling losses can be decreased by 80 percent at light load, low rpm conditions, but by only 40
to 50 percent at high rpm, even with fully VVT.^48
Aside from improved fuel economy, VVT also increases power output over the entire range of
engine rpm. Fully variable valve timing can result in engine output levels of up to 100 brake
horsepower (BHP)/liter at high rpm without the decline in low-speed torque that is characteristic
of four-valve engines with fixed valve timing. In comparison to an engine with fixed valve timing
that offers equal performance, fuel efficiency improvements of 7 to 10 percent are possible. The
principal drawback has historically been the lack of a durable and low cost mechanism to
implement valve timing changes. Honda has commercialized a two stage system in its four-
valve/cylinder engines where, depending on engine speed and load, one of two valve timing and