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biogas- and NG-fueled SI engines (Korakianitis et al. 2011 ; Gonz
lez et al. 2013 ).
The advantages of NG or biogas are due to their higher octane number, lower cost,
and reduced GHG and particulate emissions. Thus compared with conventional
gasoline engines, SI engines using natural gas can run at higher compression ratios,
thus producing higher thermal ef
รก
ciencies and lower CO 2 emissions. However, NO x ,
CO, and UHC emissions may be higher, and such engines produce less power than
gasoline-fueled engines due to a combination of factors, including lower volumetric
ef
ciency due to natural-gas injection in the intake manifold, lower stoichiometric
fuel/air ratio of natural gas compared to gasoline, and the lower equivalence ratio at
which these engines may be run in order to reduce NO x emissions. In addition, high-
pressure storage tanks are required to replace the liquid-fuel tank of a conventional
vehicle, and fuel injectors or fuel induction systems need to be modi
ed due to higher
fl
low rates required to overcome the low density of natural gas. Despite these modi-
fications, NGVs cannot operate over the same distances as conventional vehicles due
to the much lower energy content of NG per unit volume.
In summary, more experimental and computational studies are needed to opti-
mize the engine performance and emissions for various operating conditions and
syngas and biogas composition. Research should also focus on examining the use
of syngas and biogas in new engine designs, such as Homogeneous Charge
Compression Ignition (HCCI) and low temperature combustion.
4 Use of Syngas and Biogas in Dual-Fuel Diesel Engines
Numerous studies have examined the viability of a dual-fuel or blended-fuel strategy
using a variety of liquid and gaseous fuels for transportation and power generation.
Such blends include bio/petroleum (Hansen et al. 2005 ), H 2 -gasoline (Changwei and
Shuofeng 2009 ), H 2 -NG (Ma et al. 2007 ; Wang et al. 2008a ; Das et al. 2000 ;
Kahraman et al. 2009 ; Dimopoulos et al. 2008 ; Morrone and Andrea 2009 ), H 2 -CH 4
(Bauer and Forest 2001a , b ), H 2 -diesel (Shirk et al. 2008 ), and syngas-diesel (Bo-
ehman and Le Corre 2008 ; Sahoo et al. 2012 ). The use of NG or biogas in dual-fuel
mode has also been investigated using spark SI (Cho and He 2007 ; Chandra et al.
2011 ), CI (Duc and Wattanavichien 2007 ; Ryu 2013 ), and HCCI (Naber et al. 1994 )
engines. In diesel engines, a gaseous fuel-air mixture is introduced through the intake
valve, while the diesel fuel is injected and compression ignited. Research has focused
on the effect of a gaseous fuel on the performance and emission characteristics of
gasoline or diesel engines. The effects of various parameters, such as fuel composi-
tion, blending ratio, injection timing, EGR, and gaseous fuel-air ratio, have been
characterized at different loads. While a number of aspects have been examined (Liu
et al. 2003 , 2013 ; Shah et al. 2011 ; Sahoo et al. 2009 ), a common observation is that a
dual-fuel strategy with NG-diesel at high loads leads to lower PM, CO 2 , and volatile
organic compounds (VOC) emissions, but somewhat higher CO and UHC emissions,
and that the engine power may be reduced due to lower volumetric energy content of
NG, although similar thermal ef
ciency can be maintained. The bene
ts of dual-fuel
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