21
Fuels & Lubes International Quarter Two 2016
It’s like having your cake and eating
it too. According to Satoshi Hirano,
project manager in the engine design
and engineering division of Toyota
Motor Corp., popular friction modifiers
that lower viscosity do not contribute
to coking deposit problems potentially
seen with turbocharged, downsized
engines. That is good news, because
both turbocharging and lower viscosity
lubricants are being pursued to boost
vehicle fuel economy while preserving
performance.
Hirano reported on the results of
a Toyota study which showed this at
the recent F+L Week conference in
Singapore, an event organised by F&L
Asia. Not all was good news, however.
The study showed the dangers from
coking, such as oil degradation and
formation of performance-robbing
deposits in the turbocharger.
“Temperature control is very
important from an engine design
perspective. At the same time, from
the viewpoint of engine oil design, im-
provement of the antioxidancy seems
to be an effective way to prevent
deposit formation,” Hirano said.
For gasoline engines, deposit for-
mation begins at temperatures above
180
o
C. Spikes to this temperature and
higher can happen when the following
sequence occurs: a high load engine
operation followed by engine shut
down after short idling. When that
happens, heat from the exhaust sys-
tem flows back into the turbocharger
turbine, shaft and bearing, which can
lead to coking.
It is understood that a turbocharg-
er coking test will need hundreds of
hours of operation to cause the coking
deposit formation. By taking pre-aged
engine oil, the Toyota team success-
fully evaluated each given test condi-
tion in 40 hours and reproduced the
coking deposit formation for certain
test conditions.
The Toyota team also conducted
laboratory tests to look at when coking
happens and under what conditions,
evaluating both used oil and fresh oil.
They found that key was the pres-
ence of insoluble materials, which in
gasoline engine oils, were found to
be polymerised hydrocarbons with
carbonyl and ester functional groups.
Lubricants with even small amounts
of insolubles formed deposits rela-
tively quickly at anything above 180
o
C.
Those free of these external materials
did not coke, even at significantly high-
er temperatures. For instance, fresh
oil free of insolubles started to coke at
280
o
C, a full 100
o
C higher than used
oils with insolubles in them.
“Excessive accumulation of insolu-
bles causes a variety of deposit issues
in the engine,” Hirano said.
The source of insolubles is degrad-
ed fuel molecules, he said. A com-
bustion product, the degraded fuel
molecules end up accumulating in
the engine oil. There they polymerise,
forming long molecular chains. Its
thermal stability is worse than that of
engine oil. When heated sufficiently,
the insolubles clump up into deposits.
A hot tube test was used to investigate
the starting temperature of the coking
phenomenon, by exposing a small
amount of oil to hot air in the heated
glass tube. Used oil collected from the
field, oil oxidised in the lab, as well as
fresh oil were used in the experiment.
Only used oil from the field exhibited
the level of deposit formation that
can lead to coking at the temperature
range related to the turbocharg-
er bearing area. Fresh oil showed
none. The oils that were oxidised in
the lab went through an equipment
called ISOT, which allows the oil to be
degraded without insoluble formation.
Since there was a big difference be-
tween the coking start temperatures
with the used oil and the oil that was
oxidised in-lab (coking temperature
is well above 200° C), Toyota was able
to determine that insolubles are the
main driver of lower coking start tem-
perature. Because the starting point
is degraded fuel that’s been through
combustion, fresh oil doesn’t exhibit
anything like the temperature sensi-
tivity and problem that used oil does.
It should be noted that Toyota tested
both gasoline and diesel oils to investi-
gate the coking start temperature. As
丰田汽车的发动机设计与工程部项目经
理Satoshi Hirano说，常用的摩擦改进剂
可以降低粘度，但与在增压、小尺寸发动机
中可能出现的结焦问题无关。这是好消息，
因为为了提升汽车的燃料经济性同时保持
其性能，人们正在努力追求增压和低粘度
润滑。
在最近由F&L Asia在新加坡举办的F+L
周大会上，Hirano报告了丰田的研究项目，
但并不都是好消息。研究显示了因结焦所带
来的危险，如油品质量下降及在增压器内形
成影响性能的积碳。
“从发动机设计的角度来说，温度控制非
常重要。同时，从发动机油设计的角度来
说，改进抗氧化性似乎是一个防止积碳形成
非常有效的方法，” Hirano说。
对汽油发动机来说，当温度超过180
o
C
时，开始形成积碳。如果出现了如下情况，
可以在瞬间达到并超过这个温度：高负荷运
行后在短时怠速后发动机停转。这时，排气
系统的热量会返回增压器，引起结焦。
我们知道，在结焦测试中，设备需要运行
数百小时后才能形成结焦。通过使用经事先
老化的发动机油，丰田团队成功地在40小
时内对每个给定的测试条件进行了评估，在
其中一些测试条件下重复实现了结焦。
丰田研究小组还进行了实验室测试，针对
废油及新油，研究了何时及在何种条件下会
出现结焦。他们发现关键是不溶性物质，在
汽油机油中是聚合的含羰基和酯功能团的
烃。哪怕只要有少量的不溶物存在，一旦温
度超过180
o
C就会很快形成积碳。没有这
些不溶性杂质的润滑油即使在更高的温度
下也不会出现结焦。例如，不含不溶物的新
鲜油品要在280
o
C下才会结焦，要比含不
溶物的废油高出整整100
o
C。
“不溶物的过量累积会在发动机中导致
各种积碳问题，” Hirano说。
不溶物的来源是降质的燃料分子，他说。
降质的燃料分子是燃烧产物，最终在发动
机油中累积。在那里他们发生聚合反应，形
成长链。其热稳定性较发动机油差，当充分
加热后，这些不溶物形成积碳。
通过热管测试，把少量的油暴露在热玻
璃管中的热空气下，可以用来研究结焦现象
的开始温度。在实验中，使用了从现场收集
的废油样品，在实验室里经氧化的油样，以
及新鲜油品。只有现场采集到的废油会在
增压器轴承区域相关的温度范围内形成足
以结焦的积碳。新鲜油品则无。在实验室中
经氧化的油样经ISOT过程处理，它会使油
