Ashless Phosphorus-Containing Lubricating Oil Additives 113
contributor to exhaust “sulfur” content.A debate has therefore arisen regarding the future
level of lubricant sulfur, and diesel engine manufacturers have already expressed interest in
lubricants with a sulfur level as low as 0.2%— considerably below the current value of ∼1%
(Mainwaring, R. Shell Global Solutions, Private Communication, January 2008).
In 1999, the European Union (EU) issued emission requirements for heavy-duty diesels that
anticipated signifi cant reductions in NOx, CO, unburnt hydrocarbons, and particulates over
the period from 2001 to 2008. The greatest challenge was to lower NOx while at the same time
reducing particulates as measures to cor rect the former normally resulted in an increase in the
latter. In addition, in reducing NOx, the effi ciency of the diesel engine would be impaired, and
the result would be an increase in fuel consumption and CO 2 emissions. However, a technique
called selective catalytic reduction (SCR) has now been developed and adopted by several
engine manufacturers in the EU [175]. This involves injecting an aqueous solution of urea
(CO(NH 2 ) 2 ) into the exhaust stream where it degrades to carbon dioxide and ammonia (NH 3 ).
The NH 3 then reduces the NOx to nitrogen (N 2 ) and water on a tungsten/vanadium catalyst. It
is obviously important to avoid NH 3 being released into the atmosphere, and another catalyst
is required to oxidize any residual NH 3 while avoiding oxidation of the nitrogen.
To date, SCR (low NOx but high particulates) has generally been favored over diesel partic-
ulate fi lters (high NOx but low particulates) for reducing emissions by many EU manufactur-
ers. It allows engines to operate more effi ciently—indeed suffi ciently so as to more effi ciently
and more than offsets the cost of the urea. However, although it is an effective technique,
there are concerns about its size, weight, guaranteed availability throughout Europe, and its
effi cacy at the lower temperatures encountered in the exhaust of light-duty applications. These
problems have so far prevented its application to passenger cars. The effects on pollution if
urea is not used in a system designed for its use and how the solution would be made available
to the ordinary motorist are currently the subject of further investigation and debate.3.12 FUELS
- As a result of the concern regarding the direct and indirect impacts of fuel sulfur on
engine emissions, there is pressure to reduce the sulfur content. In the EU, a limit of
50 ppm maximum sulfur in diesel fuel was introduced in 2005, with a further reduction to
10 ppm in 2008 for gasoline engines and in 2009 for diesel engines. At such low levels, it
may be necessary to restore the lubricity of the fuel by additives, and incorporating a small
amount of phosphorus has been shown to be effective [176]. - Considerable concern currently surrounds carbon dioxide emission and its connection with
global warming. Within the EU, the auto producers have reached a voluntary agreement
with the commission to achieve a carbon dioxide emission target of 140 g/km by 2008 with
a further reduction to 130 g/km by 2012. This will encourage a move toward more fuel-
effi cient vehicles, particularly those that are smaller and lighter, and perhaps also to thinner
lubricants requiring even better AW protection.
In the United States, the Senate has ruled that carbon dioxide emission from trucks are a
pollutant rather than a “by-product” of combustion. This is expected to promote reductions
in carbon dioxide levels and hence encourage the use of lower viscosity, more fuel-effi cient
diesel engine oils (Mainwaring, R., Shell Global Solutions, Private Communication, Janu-
ary 2008).
3.13 CONCLUSIONS
Ashless phosphorus-containing additives are available in a wide range of structures and perfor-
mance. Although most are used as AW and EP additives for industrial oils, they can also function as
antioxidants, rust inhibitors, metal passivators, and detergents. In some cases, the multifunctionality