Many applications of liquefied petroleum gas (LPG) to commercial
vehicles have used their corresponding diesel engine counterparts
for their basic architecture. Here a review is made of the
application to commercial vehicle operation of a robust 4 L,
light-duty, 6-cylinder in-line engine produced by Ford Australia on
a unique long-term production line. Since 2000 it has had a
dedicated LPG pick-up truck and cab-chassis variant.
A sequence of research programs has focused on optimizing this
engine for low carbon dioxide (CO₂) emissions. Best results (from
steady state engine maps) suggest reductions in CO₂ emissions of
over 30% are possible in New European Drive Cycle (NEDC) light-duty
tests compared with the base gasoline engine counterpart. This has
been achieved through increasing compression ratio to 12, running
lean burn (to λ = 1.6) and careful study (through CFD and bench
tests) of the injected LPG-air mixing system.
The outcome of this work (and others) has stimulated a major
Australian Government investment in gaseous fuels consistent with
increased home consumption of LPG as over half of local LPG
production is exported.
Thus the engine is evaluated in its application to trucks of 15
- 20 tons GVM, needing 170 kW and high torque over the Euro truck
steady state and transient test cycles; the European Stationary
Cycle (ESC) and the European Transient Cycle (ETC) respectively. It
is shown that this engine has the potential to deliver better than
diesel CO₂ emissions with relatively low-cost LPG components and
likely meeting Euro 5 emission standards with only oxidation
catalyst aftertreatment in contrast to the complex aftertreatment
needed for diesel engine compliance with the diesel standard (e.g.,
DPF and SCR using urea for particulates and oxides of nitrogen
(NOx) control).
The paper also comments on the relative merits of liquid-phase,
gaseous-phase and throttle body LPG injection.
Efficiency and emissions performance comparison are made with
the same engine at a higher compression ratio optimized for
compressed natural gas (CNG) and a 6-cylinder Euro 3 diesel engine
of similar power. The data indicates that LPG can perform at a
thermal efficiency equal to that of the diesel engine, but slightly
worse at low NOx levels. Although CNG has highest
efficiency over both ESC and ETC, a significant NOx
control strategy is needed.
