Clean Oil Reduces Engine Fuel Consumption
Jim Fitch, Noria Corporation
Posted 12-13-04
In the July-August issue
of Machinery Lubrication magazine, my column discussed the important role of
lubrication on energy conservation and environmental protection. The more I
delve into this subject, the more I discover the pronounced impact lubrication
has on energy and the environment. A case in point is the impact of clean oil
on fuel consumption and emission in engines.
There are many ways that
a lubricant could fail to deliver fuel-efficient engine performance. Many of
these are due to formulation issues as opposed to transient properties of the
lubricant in service. For instance, there were significant advances in energy
conservation when switching from GF-2 to GF-3 (international quality designation
for gasoline engine lubricants) in 2001 (Figure 1).
Figure 1. GF-3 and GF-2 Comparison Diagram
When a lubricant degrades,
it forms reaction products that become insoluble and corrosive. So too, the
original properties of lubricity and dispersancy can become impaired as the
lubricant ages and additives deplete. Much has been published about the risks
associated with overextended oil drains and the buildup of carbon insolubles
from combustion blow-by.
However, surprisingly little
has been said about the impact of fine abrasives in a lube oil as it relates
to fuel economy over the engine’s life. One can imagine numerous scenarios in
which solid abrasives suspended in the oil could diminish optimum energy performance.
Below is a list of several scenarios:
- Antiwear Additive
Depletion. High soot load of crankcase lubricants has been reported to
impair the performance of ZDDP antiwear additives. Some researchers believe
that soot and dust particles exhibit polar absorbencies, and as such, can
tie-up the AW additive and diminish its ability to control friction in boundary
contacts (cam nose, ring/ liner, etc.).
- Combustion Efficiency
Losses. Sooner or later, wear from abrasive particles and deposits from carbon
and oxide insolubles will interfere with efficient combustion in an engine.
Valve train wear (cams, valve guides, etc.) can impact timing and valve movement.
Wear of rings, pistons and liners influences volumetric compression efficiency
and combustion blow-by resulting in power loss. As has been previously reported
in this magazine, particle-induced wear is greatest when the particle sizes
are in the same range as the oil film thickness (Figure 2). For diesel and
gasoline engines, there are a surprising number of laboratory and field studies
that report the need to control particles below ten microns. One such study
by GM concluded that, “controlling particles in the 3 to 10 micron range had
the greatest impact on wear rates and that engine wear rates correlated directly
to the dust concentration levels in the sump.”1
- Frictional Losses.
When hard clear- ance-size particles disrupt oil films, including boundary
chemical films, increased friction and wear will occur. One researcher reports
that 40 to 50 percent of the friction losses of an engine are attributable
to the ring/cylinder contacts, with two-thirds of the loss assigned to the
upper compression ring.2 It has been documented
that there is an extremely high level of sensitivity at the ring-to-cylinder
zone of the engine to both oil- and air-borne contaminants. Hence, abrasive
wear of the ring/cylinder area of the engine translates directly to increased
friction, blow-by, compression losses and reduced fuel economy.
- Viscosity Churning
Losses. Wear particles contribute to oxidative thickening of aged oil.
High soot load and/or lack of soot dispersancy can also have a large impact
on oil viscosity increases. Viscosity-related internal fluid friction not
only increases fuel consumption but also generates more heat that can lead
to premature degradation of additives and base oil oxidation.
- Stiction Losses.
Deposits in the combustion chamber and valve area can lead to restriction
movements in rings and valve control. When hard particle contamination agglomerates
with soot and sludge to form adherent deposits between valves and guides,
a tenacious interference, called stiction, results. Stiction causes power
loss. It causes the timing of the port openings and closings to vary, leading
to incomplete combustion and risk of backfiring. Advanced phases of this problem
can lead to a burned valve seat.2
Figure 3. Cummins N-14 (430E) Engines
Power
Losses from Wear of Cummins Engines
Figure 3 shows an example of how increased engine wear, in this case due to
overextended oil drains, contributes to power loss in the engine. At 2100 rpm,
the severely worn engine horsepower at the wheels decreased from 365 hp to less
than 300 hp (18 percent). Loss of horsepower translates directly to losses in
fuel economy.
A bus engine fuel consumption
study by G. Andrews, et al. of the University of Leeds (Table 1), provides evidence
of the benefit associated with cleaner oil on fuel economy in an actual road
trial.4 It was noted that the Cummins engine’s fuel efficiency increased
2 percent to 3 percent when a six-micron by-pass filter was used along with
a full flow filter. The study spanned 50,000 miles of service. The fuel consumption
was calculated based on detailed fuelling records from the fleet. In a similar
study reported by the same authors using by-pass filtration, a 5 percent to
8 percent reduction in fuel consumption was achieved on a 1.8 liter Ford passenger
car IDI diesel engine.
A study reported by J. Fodor
and F. Ling of the Research Institute of Automotive Industry-Budapest and published
in Lubrication Engineering magazine (Table 2) found a sharp improvement in fuel
economy in a six-cylinder diesel engine fitted with improved filtration. By
reducing oil contamination by 98 percent, not only was a nearly 5 percent reduction
in fuel consumption achieved but wear and friction were reduced by 93 percent
and 2.9 percent respectively.5
Waste
Stream Emissions
When the engine consumes oil, due primarily to contaminant-induced wear, oil
enters the combustion chamber, burns with the fuel, and is pushed out with exhaust
gases as particles and volatile hydrocarbons. New mineral-based lubricants have
a more volatile light-end fraction and are more prone to hydrocarbon emissions.
The level of exhaust emissions increases considerably over time corresponding
to engine wear and deposit formation in the combustion zone. This leads not
only to greater concentration of exhaust particulates, but also to a higher
percentage that are unburned hydrocarbon, a by-product of oil consumption.
Figure 4. Off-road/tractor Particulate Emissions Predictions
Unlike a new engine, the lubricating oil is a dominant contributor to particulate
matter (PM) emissions in aged engines. The obvious strategy to control/reduce
hydrocarbon emissions is to reduce oil consumption. ...
... This, of course, points
to a strategy of reducing abrasion and wear. According to projections by Barris
of Donaldson Co. (Figure 4), after 12,000 hours of service, an off-road diesel
engine can produce nearly six times more exhaust emissions due to wear associated
with particles and other causes.6
Crankcase
Oil Particle Counts
Good environmental stewardship is everyone’s responsibility. We all benefit
from cleaner air and a safer environment. In addition, the financial impact
that comes from reduced fuel consumption alone can be substantial. Perhaps it’s
time for OEMs and users alike to begin revisiting contamination control practices,
including filtration, associated with internal combustion engines.
If clean oil is important
to control wear, reduce fuel consumption and emissions, perhaps it’s also time
for users to begin asking their laboratories to begin reporting particle counts
and ISO Codes of used crankcase oils. Remember, if it’s important, we measure
it - correctly. What gets measured gets done.
References
- Staley, D.R. (1988).
"Correlating Lube Oil Filtration Efficiencies with Engine Wear". SAE Truck
and Bus Meeting and Exposition (Paper 881825).
- Madhavan, P.V. and Needelman,
W.M. (1988). "Review of Lubricant Contamination and Diesel Engine Wear".
SAE Truck and Bus Meeting and Exposition (Paper 881827).
- McGeehan, J. (2001,
September-October). Uncovering the Problems with Extended Oil Drains. Machinery
Lubrication magazine (www.machinerylubrication.com),
pp. 24-29.
- Andrews, G.E., Li, H.,
Jones, M.H., Hall, J. Rahman, A.A. and Saydali, S. (2000). "The Influence
of an Oil Recycler on Lubricating Oil Quality with Oil Age for a Bus Using
In-Service Testing". SAE 2000 World Congress (Paper 2000-01-0234).
- Foder, J. and Ling,
F.F. (1985, October). Friction Reduction in an IC Engine through Improved
Filtration and a New Lubricant Additive. "Lubrication Engineering". pp. 614-618.
- Barris, M.A. (1995).
"Total Filtration: The Influence of Filter Selection on Engine Wear, Emissions
and Performance". SAE Fuels and Lubricants (Paper 952557).
Jim Fitch, "Clean Oil Reduces Engine Fuel Consumption". Practicing
Oil Analysis Magazine. November 2002 |
