Continued from page 21


Figure 2: The effect of HTHS on fuel economy in the ESC test for Volvo D12D engine (Courtesy P. Klejwegt, Chevron)


The most remarkable – though pretty logical – finding was that lowering HTHS brings about the greatest fuel economy effect under the high speed/low load conditions, which is the Heavy Duty Diesel Engine (HDDE) operation mode, most relevant when an empty truck cruises on a motorway, and therefore, less important for overall fuel economy of fleets. Fig. 3 shows relative importance of different engine operation modes used in the European Stationary Cycle (ESC) cycle.


(valvetrain, reciprocating group, pump, etc.) and then to use some simulation software to evaluate the fuel economy index (FEI) in different test cycles. However, each serious researcher should understand that a simulation result that cannot be properly verified in practice is nothing more than a theoretical prediction, and not a proven fact. For modern highly optimized low-friction powertrains, it is increasingly difficult to reliably assess effects of different friction-reducing technologies on fuel economy. For instance, two identical engines assembled by the same test engineer using the same set of components and assembly lubricants will still reveal subtle differences in engine friction, power output and fuel economy, and the magnitude of this baseline “noise” is often comparable with or even exceeds the effects we need to quantify.


Figure 3: The relative weights of different speed/load modes in the ESC.


The use of an oil of API FA-4 category (HTHS 3.0 cP) brings about only 0.5% FE improvement in the ESC cycle. It is worth noting that under the FIGE (Forschungsinstitut für Geräusche und Erschütterungen) transient cycle, deemed to be more representative of real world experience, the relative weights of points 11, 12, and 13 in Fig. 3 decrease further, and the overall fuel economy drops accordingly, to 0.3% or so. In practice, only 1/3 of the fuel consumed by heavy-duty fleets corresponds to elevated engine speeds and low to moderate loads (BMEP < 6.5 bar).


Another important observation: when we are starting to look into fuel economy numbers below 1%, test rig accuracy becomes a limiting factor. A possible way around is to carry out individual component rig tests


22 LUBE MAGAZINE NO.154 DECEMBER 2019


Another disturbing observation in Fig. 2 is that an increase in oil temperature by 15°C – from 110°C to 125°C – greatly increases the boundary friction contribution for HTHS less than 2.9 cP. As has already been mentioned, the problem is not friction itself, but wear, which may come in the form of piston ring and liner scuffing, rocker pad wear, roller follower wear, crosshead wear, bearing wear, cam lobe wear, and so on. Even though there are some new HDDE designs, eg those from Scania and IVECO, which allow safe deployment of low viscosity oils, such oils should never be used in older engines. This is the reason why FA-4 oils have been so slow in gaining acceptance from fleets. For instance, Scania has recently launched LDF-3 FS 5W-20 engine oil claiming excellent fuel economy when compared with conventional 15W-40 oils. My question is, what is “excellent”, and have you ever been asked for this oil by any of your customers? As a matter of fact, by routine optimization of logistic schemes, one can achieve much better fuel economy without risking engine durability.


It is relatively easy to develop an oil that excels in one particular engine design, and it’s much more difficult to develop an oil that delivers an adequate performance over a broad range of engine designs. Many problems are simply impossible to reproduce in a lab, which presents an immense development challenge. Some lubricant companies start to use the big data approach to gather and analyze field experience, which let them iron out development bugs without even trying to understand the root cause. Artificial intellect tools, when available, will lift this development trend to a totally new level.


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