Study of the Miller Cycle on a Turbocharged DI Gasoline Engine Regarding Fuel Economy Improvement at Part Load

Round 1

Reviewer 1 Report

The paper presents an extensive analysis of the effects of a VVT system. The following modifications are required:

- authors must prove that the tested conditions correspond to a Miller cycle (expansion ratio greater than compression ratio) so that the reader can have a deeper understanding. To do that I suggest to add some comparison in logP-logV for some Miller and no-Miller conditions.

- please add such also for the extreme Miller conditions of Figure 7.

- figure 8: descrive how the maximum cylinder temperature was estimated.

- figure 15: use HRR instead of pressure

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper discusses an interesting set of experiments and related analysis/simulation to perform optimization of those results.  As such, the paper fits within the remit of the journal.

However, it requires some modification.  First among this is that many of the scales on the figures are quite unacceptable.  For instance, on Figures 17 to 21 there are either too many decimal places, a totally illogical unit interval, or units with no change (Figure 19).  Many of them have a combination of these.  This all needs fixing.

In relation to this, some of the claims for accuracy for BSFC values are not borne out by the rest of the paper.  One cannot claim BSFC values accurate to 0.01 g/kWh without some explanation of the equipment used to measure torque and fuel flow or by putting error ranges on the measurements involved.  This reviewer spent most of his professional life as an engine development engineer and consequently has some experience of this.  I would suggest that if the authors look at the stack up of accuracy of their measurement devices it would be hard for them to claim accuracy to +/- 0.5 g/kWh if they are using normal equipment.  Similarly, do not make claims for accuracy on BSFC arising from analysis when the model has been validated – approximately – using the test data.  As such some sections need to be rewritten to change this, which will then necessarily have an impact on the improvement claims for efficiency arising from the 1D simulation made later.

Another issue is the claims made on combustion temperature.  A value of 2261.7°C is given at one point, and 2238.8°C at another.  In a manner similar to the above discussion, this seems an indefensible degree of accuracy, and it needs to be changed to “approximately 2260°C” and “approximately 2240°C” or similar.

One other point regards the use of the phrase “retarding the exhaust”.  I found it unclear in places whether this was truly retarding the maximum opening point towards TDC or whether it was in fact going in the opposite direction; “retarding the exhaust” occasionally implied reducing overlap, which is not the case.  I think some kind of timing diagram would help to clarify this.  Specifically, Figure 3 starts the confusion – the arrow with the label “exhaust retardation angle” shows the exhaust advancing…

On line 37, why does delaying the ignition advance angle reduce the effective compression ratio?  Effective CR is only a function of valve timing, not the ignition timing.  Similarly lines 41-43 do not make sense.  The Miller cycle does this by modifying the valve timing so that the effective CR is significantly lower than the geometric CR.

In the literature discussion it is normal to only use the surnames of researchers.  I was surprised not to see any discussion of work by Tuttle or by VW, since they have Miller cycle engines in production, or indeed BMW, because its “Valvetronic” system is effectively a variable early intake valve opening Miller cycle device.  This section does not state that there are two times of Miller cycle – with early or late intake valve timing (the Atkinson cycle requires a specific cranktrain and cannot, as is often claimed, be achieved using valve timings alone).

Specific issues:

In several places there are spaces missing before brackets, e.g. lines 90, 119, 125, 136, 382.

Line 3    “application” not “Application” – also line 121

34          “lightweighting” not “lightweight”

61          insert “the” before “baseline”

90          “at” not “as”

Table 2 – what does “Intake advance angle under” mean?

125        I presume that the relationship at the end of this line is mean to mean the modulus.  If not, this needs to be rewritten as a fuller equation.  I also presume that “baseline” here is supposed to be subscripted.

139        add “the” at the end

141        change “the degradation” to “an increase”

y-axis title of Figure 5 – this is the only place where you have written it over the two panels in a figure; needs to be brought in line with the rest of the figures.

169        insert “the” before “engine’s”

176        change “adjusting” to “is adjusted”

248        “Friction” not “Frictional”

261        “FMEP” not “in-cylinder peak pressure”

266        “are consistent” not “consist”

271        insert “to” before “rise”

272        “correspondingly larger” not “larger correspondingly”

311        insert “an” before “obvious”

Figure 12 – needs second y-axis titles and units.

323        space before “As”

324        remove erroneous extra space before “the intake”

357 and later – when you say “combination of this point” do you mean “settings at this point”?

394        by “charging efficiency” do you mean “volumetric efficiency”?  And if so, is this relative to intake manifold or atmospheric conditions?

404        “determines” not “figures out”

413        “engine” not “machine”

421        remove “slightly”

425        “the variation of two parameters” not “2 parameters variation”

442        “angle” not “angles”

459        provide reference for the Chenn-Flynn formula.

522        I am unsure what the use of the word “analyzing” is for – does something need to be added to the sentence it is in?

540        “result” not “resulting”

548        remove “occur”

554        “owing” not “which owes”

564        change “be avoided to work” to “avoid operating”

Figure 22 – needs something to indicate which data set goes with which y-axis.

577        delete “an” and insert “a relative” before “improvement”

578        change “has been conformed via the experiment” to “was confirmed by a final experiment” – if this is what you mean.

582        change “an” to “the”

587        change “Main” to “The main”

618        change “than the test” to “relative to the test

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The paper was modified taking into account the reviewer's suggetsion