Energy Efficiency, Emissions, Tribological Challenges and Fluid Requirements of Electrified Passenger Car Vehicles

Round 1

Reviewer 1 Report

This article would be interest to the many readers. 
Please check and make minor revisions to described below.

P.1,Line 18: Tribology: Lubrication:
->Tribology; Lubrication;

General: Please unify the parts written in abbreviations, such as  "full battery electric vehicles (BEVs)", "plug-in hybrid electric vehi-36 cles (PHEVs)"and "EV", to the first appearance after section 1.
 e. g.;P.2 Line 62; "full Battery Electric Vehicle"
       -> P.7 Line 256, P.9 Line 372 P.12 Line 505;"Battery Electric Vehicles"
       P.2 Line 63, P.4 Line 141, P.7 Line 258, P.8 Line 329, P.10 Line 434  and so on; "Plug-In Hybrid Electric Vehicle"
       P.1 Line 42, P.2 Line 48, 51, 52, 54, P.10 Line 386, 391, 395, 402. 421, 425, 426 P.11 Line 487, P.12 Line 492, 493; "electric vehicles" or "electric vehicle" and so on <-> P.6 Line 236, P8 Line 316, 318, 319, P.10 Line 429 ; "EV"

General: Do "hybrid vehicle in P.1 Line 11, P.2 Line 64 P.11 Line 442, 461, 473, 487" and "hybrid electric vehicle in P.2 Line 54, P.10 Line 436 P. 11 Line 441, P.12 Line 505, 507" mean different things?

P.2, Line 54: Please check if "AND" needs to be capitalized.

P.2 Line 71:　Does the sentence "the higher figures will tend to be for heavier vehicles" mean "the heavier vehicles will tend to be for higher values"?

P.2 Line 82: Please confirm the fonts of "at the wheel".

P.2 Line 88-90: Please check the position "." in "wheels [8]. (For a typical driving cycle, 88 such as the New European Driving Cycle (NEDC), the average power required at the 89 wheels, assuming the car mass is 1400 kg, is only about 4 or 5 kW)." 

P.3 Line 121: Check if the reference number "[8]" is correct.

P.4　Line 164-165: How about unifying with either "USA" or "US"? 

P.5 Line 179: Make sure the line breaks are what you intended.

P.7 Equation (1): Does "Fofficial" mean "Foverall"?

P.7 Line 271: Make sure the line breaks are what you intended.

P.9 Line 331-332: Please check the position and number of "(" .

P.9 Line 334-336: Please check the position "." in "the battery can be charged externally. (A very recent review that 334 contains data on the different battery sizes, in kWh, for different vehicles can be found in 335 reference [21])." 

P.9 Line 344, P.11 Line 450, 466: Please check the number of "[21]".

P.10 Line 407:Wouldn't it be more appropriate to write "Kreisel" and "Xing" as "Kreisel Electric" and "XING Mobility" respectively?

P.11 Line 478: Please check if "NOT" needs to be capitalized.

P.13 and P.14: Please check the format of "References" and correct them.
https://www.mdpi.com/journal/lubricants/instructions#submission　

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Author Response

Many thanks for the comprehensive comments. 

I believe I have now used the abbreviations BEV, PHEV and MHEV (for mild hybrid electric vehicles) consistently through the paper now.

There were a couple of times that I used capitalized words - these have now been changed back.

I have clarified in the energy efficiency section that heavier vehicles will tend to need more energy to travel 100 km (i.e. would have higher kWh per 100 km figures for EVs and higher litre per 100 km fuel consumption).

I did intend "at the wheels" to be italicized since a key part of the argument is not so much how much energy is available at the engine (or battery) but how much of this energy actually gets to the wheels to propel the car.

USA has been used consistently now in the paper.

I have changed Fofficial to Foverall (there was some confusion in the original manuscript)

The other typos and references have been checked and formatted in the correct way for the journal.

Reviewer 2 Report

This is a well-written paper, which any reader would enjoy reading however I have two main concerns: firstly, if published, it should be published as a review, not as a research paper; secondly I doubt that the topic of the manuscript fit with the aims and scope of the journal. I would remind of the aims: "to publish experimental, theoretical and computational results that provide new insight and understanding into the scientific and technical basis for lubrication and related phenomenon." As it is the manuscript does not satisfy any of these aims. Yes, there is a section on tribological challenges, but there is no tribology or lubrication science in it. I suggest reject, not because of serious flaws but because the topic of the manuscript is not aligned with that of the journal.

Author Response

Many thanks to the reviewer for their comments.

I am happy for this article to be a review article.

One of the points of this article was to inform readers of the different fluid and tribological requirements of electric vehicles. Although some of the fluids needed are not lubricants (but coolants), traditional lubricants are needed in the EV transmission, in the engine (if it is a hybrid EV) and grease is needed in the electric motors. I have tried to summarise the different tribological challenges that will be encountered in EVs - including for example, the greatly increased number of stop-starts and the possibility of fretting wear. I respect the reviewer's view that this may not fit into the journal's remit, but I would ask them to consider where tribologists and lubrication engineers would go to find out about such information as is contained in the paper?

Reviewer 3 Report

The author gives an interessting and easy to read overview in the sense of a review of current developments in tribology, caused by electrified passenger car vehicles. Even if the results and findings are not new, the reader - especially the newcomer to the topic - will discover various aspects and learn about contexts, so I recommend the publication, taking into account minor additions. My critique is primarily of Chapter 2, and I have minor comments on Sections 3.1, 3.2, and 4.2. The following notes in detail, in order of appearance:

line 25: One bracket misplaced or missing.

lines 70 & 71: "Typically, electric vehicles need 15-25 kWh of energy to travel 100 km (the higher figures will tend to be for heavier vehicles).". This statement is too undifferentiated. The energy consumption is very strongly dependent on the speed. References are missing. Please discuss.

line 77: "[...] between 2-4 times more energy efficient [...]". 4 times seems too optimistic, especially considering that diesel vehicles can tend to use less than 6 l/100 km and the 15 kWh specification for electric vehicles is questionable.

lines 79 & 80: "[...] (i.e. higher than 6 litres/100 km), BEVs could be even more energy efficient than this simple estimate suggests)". The same argument applies to electric vehicles. Real consumption depends very much on the road profile, speed, heating/air conditioning and other consumers. In the case of electric vehicles, consumption is very much dependent on the ambient temperature. Furthermore, in many developed countries, the vehicle has to be heated for typically half a year or more, for which the waste heat of the combustion engine can be usefully utilized. For electric vehicles, this energy requirement must be added! Please discuss.

line 88: Reference 8: A more technical scientific reference is appropriate.

section 2.1, in general: The energy input for electric power generation, transport, infrastructure and battery charging was not taken into account. It must be included in the balance calculation, as it is less favorable than in the case of fuel. "Well-to-Wheel"! Please discuss.

figure 1: The data shown in the diagram cannot be verified and require a more detailed explanation.

comment to line 140: "should help to improve local air quality". I agree, but interestingly, air quality improved little in most urban areas during the Corona pandemic, despite massive reductions in traffic. How can this be explained?

section 2.4, in general: TCO includes the purchase costs. Why aren't this investment costs being discussed here? They are currently considerably higher for electric vehicles than for vehicles with combustion engines.

line 183: "224 g/km". What total mileage (km) was assumed? Please add.

lines 186 & 187: "from [...] manufacturing stage". And what is about maintenance and recycling?

lines 232-234: "this decrease corresponds to the increased use of low energy lighting, and the widespread use of more energy efficient appliances (flat screen TVs, more energy efficient washing machines, spin dryers, dish washers etc.).". Can this statement be proven with references/facts? In developed countries, household electricity consumption is 20-25%. By far the largest share is in industry and commerce. In households, "old" consumers have become more efficient (right), but new consumers have also been added. Couldn't the decline in electricity consumption be due to other effects, such as deindustrialization, especially in the UK? Please discuss and give references.

section 4.2, in general: Some more detailed statements on the formulation of possible new engine oils would be fine in this chapter.

Author Response

Many thanks to their reviewer in taking time to read my draft article and to add their thoughtful comments. On the more substantive technical comments, more detailed response is below: 

1. I have tightened up the section on the energy consumption of electric vehicles. The figures quoted by manufacturers for the vehicle electric consumption will have been measured on standardized driving test cycle (either the older NEDC driving cycle or, more likely, the newer WLTP driving cycle, or their US equivalents). These driving cycles will include a range of speeds and loads. The reviewer is correct that the energy consumption of the vehicle will depend on driving conditions (and acceleration is often more important than the actual speed). I have reworded the section to make this clearer to the reader.
2. I have edited the text so that the energy efficiency comparison now reads 2-3 rather than 2-4. I take the reviewer's point that diesel engines are more efficient than gasoline engines, but it is only really in Europe that passenger car diesel vehicles are used in great numbers, and in recent years the proportion of passenger car diesels being sold has been declining quite rapidly, so the main comparison in the paper has been with gasoline fuelled vehicles.
3. Reference 8 is a document from the UK parliamentary office of science and technology which quotes efficiency figures of 80% for an EV and 20% for a conventional gasoline engine. These figures will have come from a consensus of published data, and I think this is a reasonable reference to include.
4. In section 2.1, I take the reviewer's point that the energy input for electric power generation, transport and infrastructure, and battery charging are not taken into account. On the other hand neither have I included the energy needed to manufacture and transport gasoline. Section 2 was intended to estimate "in-use" emissions only, and so I think it is right to exclude these elements here, although they should be included in a complete life cycle calculation.
5. Figure 1 - I have made clear where the data in Figure 1 has come from
6. Not sure what data the reviewer is referring to when he says air quality has not significantly improved in urban areas during the COVID epidemic. I would have thought NOx and particulate pollution would have decreased due to lower levels of traffic, but I have not seen any data that agrees or disagrees with this.
7. In section 2.4 I have explicitly included purchase costs. I had assumed it was obvious that the total cost of ownership calculation included purchase costs, but I have now made it clear. The point is that, at the moment, it is true that EVs are more costly than conventional vehicles, but since their fuelling and servicing costs are lower, there will come a point at a sufficiently high mileage when the overall costs are lower (the calculations also depend on what you assume for the depreciation of the initial vehicle cost)
8. In Holmberg and Erdemir's work they do not explicitly state that lifetime mileage of vehicles used in their lifecycle calculations but I believe it to be 200,000 km
9. In a total life cycle calculation, maintenance will make a relatively small contribution to the total, but recycling will be very important. The majority of a car nowadays is recycled (particularly the metals and plastics). If batteries can be recycled or used in other applications, this will improve the life cycle calculation for EVs
10. I have reworded the section that speculated why UK electricity demand decreased from 2005 to 2020
11. It is difficult to include more formulation details of passenger car engine oils as in most cases their formulations are proprietary and only known by lubricant manufacturers and/or their lubricant additive suppliers.

 

Round 2

Reviewer 2 Report

In the first review I included an extract from the journal's website showing its objectives.  I do not believe that the reviewed version has addressed my concern that the topic of the manuscript does not align with those objectives. The manuscript could have been greatly improved by not only mentioning tribological challenges in EV but also the existing, or proposed solutions. 

As I mentioned in the first review the paper is well written and there is no doubt that it is a good review of the topic addressed. If the manuscript is to be published as a review, I suggest publication, as it is, even if I believe it is at the fringes of the scope of this journal.