Environmental Engineering Reference
In-Depth Information
Fig. 4.3 Key processes in residual-affected LTC
exhaust valves open during a portion of the intake stroke. This leads to the
induction of both air and residual (previously exhausted combustion product)
gases from the intake and exhaust manifolds, respectively. Fuel is either introduced
into the intake manifold area prior to induction or via early in-cylinder injection to
assure that it is well mixed with the air and residual prior to combustion. Residual-
affected LTC can also be achieved by retaining some exhaust gas in the cylinder by
closing the exhaust valve early during the exhaust stroke. Again, this is made
possible with a flexible valve system. The specific amounts of air and residual, as
well as the amount of compression (via final valve closure), are varied through
modulation of the intake and exhaust valves. Following the induction process, the
compression of the reactant/residual mixture results in increase of both the in-
cylinder mixture concentrations and temperature. If the reactant concentration
and temperature reach sufficient levels, a uniform autoignition process occurs. A
key characteristic of the autoignition process is that there is no direct trigger for its
initiation. In the conventional SI and diesel strategies, the onset of the combustion
event is triggered with the application of a spark or fuel injection, respectively.
However, in LTC the combustion process has no direct combustion initiator. If
autoignition occurs, the conversion of reactants to combustion products during
the combustion event elevates the in-cylinder gas pressure and temperature. Dur-
ing the expansion stroke this elevated pressure is used to effectively push the piston,
resulting in the extraction of useful work. The exhaust stroke then expels the hot
combustion products into the exhaust manifold. Unlike conventional strategies, a
portion of the exhausted gas is then reinducted or trapped for use during the
subsequent engine cycle. It is this reinduction/trapping process that couples engine
cycles through the exhaust gas temperature.
Despite its efficiency and emissions benefits, LTC exhibits some fundamen-
tal challenges which must be addressed prior to the production-viable imple-
mentation of the process.
4.1.3.2 LTC Challenges
No Direct Initiator of Combustion
As noted previously, residual-affected LTC has no specific initiator of combus-
tion. Ensuring that combustion occurs with acceptable timing, or at all, is more
