Exhaust Gas Temperature Pulsations of a Gasoline Engine and Its Stabilization Using Thermal Energy Storage System to Reduce Emissions

Round 1

Reviewer 1 Report

The paper has been improved, but quality of graphs are still not adequate. Very poor quality and low readability. 

In most tables or graphs, the decimal digits are too many and probably above of the measurement accuracy of the measurement devices. Revise. On this basis, accuracies, uncertainty analysis is also required.

Author Response

• Graphs and figures were checked and imported in the highest possible quality.
• The decimal digits in tables and graphs were checked. In the table 2, the specific heat capacity was changed to J/kg K (from kJ) to reduce the decimal digits.

Reviewer 2 Report

The manuscript “Exhaust Gas Temperature Pulsations of a Gasoline Engine and Its Stabilization Using Thermal Energy Storage System to Reduce Fuel Consumption and Emissions” aims to introduce a new thermal energy storage system (TESS), based on PCM materials, to improve the TWC efficiency. The proposed topic is interesting and relevant for improving catalyst efficiency and considering even more stringent policies about emissions of internal combustion engines. It is professionally written and logical in the presentation. Accepting the paper after a few minor revisions are made is suggested.

• List of highlights not presented.
• The title “Exhaust Gas Temperature Pulsations of a Gasoline Engine and Its Stabilization Using Thermal Energy Storage System to Reduce Fuel Consumption and Emissions” could be misleading. The manuscript does not discuss the fuel consumption aspect; therefore, it would be preferable to remove it.
• Improve the abstract by adding a few results.
• Authors often use multiple citations. However, it is preferable to use a single reference. Otherwise, the authors would have to summarise the analogy with each of them.
• It is unclear whether the engine and after-treatment models have been validated.
• Lines 133-134: The original base model of 1.5 TSI EVO was combined with the after-treatment model TWC by Ramanathan and Sharma, which uses a published reaction mechanism for a passenger car TWC. It is suggested to add the reference.
• It is suggested to add an overall layout of the TESS+TWC systems
• The authors should improve the engine model section, especially by illustrating the models used, the results obtained and compared with the experimental data.
• Was the heat loss of the exhaust gases during the cold start phase estimated? What disadvantages does it have on the exhaust temperatures and emissions during this phase?
• Was the optimal distance estimated between TESS and TWC?
• Section 3.2 Effect of the TESS. It is not clear the boundaries conditions, particularly the initial temperature of the PCM. Why there are three different initial temperatures? How were estimated?

Author Response

• List of highlights was created

1. TESS with PCM helps to stabilize exhaust gas temperature.
2. Aftertreatment equipped with TESS decreases pollutant emissions.
3. Synergy with other technologies further decreases BSFC, pollutant emissions and CO2.

• Specific numerical results from the simulation were added into the abstract
• The multiple citations were removed. The citations were placed immediately after the information, which is taken from them, unlike the previous mention at the end of the paragraph.
• The simulation model of the combustion engine 1.5 TSI alone was validated with experimental measured data based on engine performance characteristics. These next simulations are made as verification for this specific usage and for design the right geometry of TESS. The experimental validation will be a part of next studies.
• The reference for Ramanathan and Sharma TWC was added into the text.
• The figure of the overall layout of the Engine+TWC+TESS was added into the text
• The figure with principial connections in TESS and simple description was also added into the text (page 7).
• These results were not compared with the experimental data, because this simulation served to design the system itself. Based on these results, it is possible to manufacture and verify this system.
• The exhaust pipe was modelled based on its geometric and material properties and its temperature was calculated in time domain based on initial conditions. Heat losses are therefore solved at the basic level, but they are reflected in the slower charging of the TESS tank, as the entire system is first tempered. During this phase, emissions are higher, like a conventional engine without TESS. This effect has not major effect in global, because it is a relatively short time interval, due to the relatively small amount of material and properties of the steel used in the exhaust pipe, it is a small thermal inertia compared to TESS. However, this effect can be reduced if this system is equipped with, for example, another external power source that will keep TESS charged.
• The distance between TESS and TWC was an effort to keep it as small as possible to avoid further heat loss. There was also an effort to use the current pipeline, and the TESS system would operate as a "plug-and-play" system, replacing only one part of the current pipeline, thus maintaining the position of the catalytic converter. The position of TESS in relation to the turbine is an effort to make it as small as possible to use the highest possible exhaust gas temperature in TESS.
• The initial temperatures of individual materials are different, as these are cases of a fully charged tank (described in the text). Each of these materials has a different melting point, and therefore its initial temperature is different.

Reviewer 3 Report

This article proposes the use of a Phase Change Material thermal storage in the exhaust of a SI engine passenger vehicle. The argument is that variations in temperature of the exhaust gas can negatively impact on the conversion efficiency of the three way catalyst, and the PCM device would mitigate this. 

The paper is generally well written and presented, but there are some minor typos (for example a spare hyphen in the abstract). Overall, the simulation study seems to be accurate, but it does have a few shortcomings.

1) Neither the simulation model nor the data are available for download, so it is impossible to replicate the results.

2) It is not clear whether fuel shot off for engine braking is implemented. It usually has a major impact on exhaust temperature (but also O2 storage in the TCW).

3) It seems that the thermal inertia of the exhaust pipe and the catalyst itself are ignored. These are typically significant for the timescales considered.

4) While the result shows varying conversion efficiency, these need to be correlated with HC and NOx emissions, which tend to be highest during high load. Improving conversion efficiency during low load periods has little impact on total emissions. 

5) Overall, it is not quite clear that PCM is the right solution for this problem, because the amount of energy is not critical, but the ability to exchange it with the exhaust stream is. Maybe a honeycomb design with large surface area would be more appropriate? 

It would be worth commenting on these four concerns. If they are addressed appropriately, the paper should be suitable for publications. 

Author Response

Language was checked and were made some minor corrections. (Mainly definite/ indefinite article)

• The simulation model can be uploaded, however it was created in commerce software, which is not free to download and use. It is recommended to give these models on individual contact with institution Brno University of Technology in which we create this research.
• The fuel supply in the injector settings is solved. Several possible scenarios can occur depending on the crankshaft speed, the accelerator pedal, etc. If the accelerator pedal is depressed, there will first be a short time delay. If there is no re-request to change the engine load, the fuel is cut off.
• The exhaust pipe is entered using the calculated wall temperature element, where the material properties of the pipe, the external conditions acting on the pipe and the initial temperature of the pipe are entered as input values. Its temperature was calculated in time domain based on initial conditions.
• This solution aims directly to the low and partial loads of the drive. During high loads (and full loads) the TWC itself works with very high efficiency. These days, the biggest problem for automotive companies is to reach the emission limits and regulations during homologation in WLTP procedure. This procedure contains dominantly partial loads. In these cases, the TESS system has significant effect.
• This model enters the simulation as a basic comparison model for the function of the TESS with PCM material. This shape (Y) was approached as easier to manufacture and save material. Honeycombs have a larger contact area, and it is really assumed that this shape could have another beneficial effect, but the optimization of the heat exchanger shape is next step and investigation, where the detailed modelling in the Ansys Fluent environment will be used, which will be a part of next article.

Round 2

Reviewer 3 Report

My questions have been answered, and I think the paper is sound. The changes to the paper are minimal, so my rating has changed only slightly. I would have liked to see a few more clarification, but I am now convinced that the paper is sound and suitable for publication.