The Atkinson cycle and improving the ICE’s efficiency

INTRODUCTION

The Internal Combustion Engine (ICE) is one of the most innovative creations the human being has made, in my opinion. The objective of its creation was to be the main source of power to propel vehicles all the way to the modern car as we all know it nowadays.

Ever since the ICE was brought to life, it has operated under the principles of the Otto cycle. This represents the most common thermodynamic cycle found on spark ignition ICE’s. As time passed on, different engine cycles were proposed looking towards higher thermal efficiency such as the Atkinson cycle.

THE ATKINSON CYCLE

The Atkinson cycle can be defined as a modified version of the Otto cycle, designed by James Atkinson in 1882. It is characterised for having a greater expansion stroke than a compression stroke, a fact that is implemented to use more of the energy available in the injected fuel as it is given more time to the mixture to expand and therefore, produce a greater amount of work within the combustion chamber.

In order to achieve its functionality, the Atkinson cycle appeals to the concept of Late Intake Valve Closing (LIVC). This method consists of leaving the intake valves open certain degrees after the piston gets to the Bottom Dead Centre (BDC) with the objective of making sure a correct and uniform air filling is carried out throughout all the cylinders.

PROS

  • Higher thermal efficiency compared to the conventional Otto cycle
  • Emissions reduction
  • Pumping loss reduction
  • Suited to implementation on Hybrid Electric Vehicles (HEV’s) where the lower torque produced can be compensated with an electric motor

CONS

  • Lower torque output
  • Higher complexity

MODELLING THE ATKINSON CYCLE IN DYMOLA

An engine and its components were modelled in Dymola to execute a comparison between the Otto and Atkinson cycles (Fig. 1).

The specifications of the engine are shown in Table 1.

Table 1. BMW Mini Cooper engine specifications
Table 1. BMW Mini Cooper engine specifications
Figure 1. BMW Mini Cooper engine in Dymola

Figure 1. BMW Mini Cooper engine in Dymola

For the Atkinson cycle side, the default timing valve system (used for the Otto cycle) was swapped for a Variable Valve Timing (VVT) system, which allows to adjust the intake valve timing and hence, get the LVIC. Four different case scenarios were simulated as can be seen in Table 2.

Table 2. Atkinson VVT cases
Table 2. Atkinson VVT cases

CYCLE ANALYSIS

The results from the simulations are shown in Figure 2. Each cycle represents a unique cam phasing shift configuration in order to achieve LIVC.

Figure 2. Atkinson cycle LIVC comparison

Figure 2. Atkinson cycle LIVC comparison

COMPARISON BETWEEN THE ATKINSON AND OTTO CYCLE

Finally, a comparison was effectuated between the Atkinson and the Otto cycle to appreciate their difference in performance.

Figure 3. Atkinson cycle vs Otto cycle comparison

Figure 3. Atkinson cycle vs Otto cycle comparison

The next formula was used to calculate the efficiency of the cycles shown in Table 3.

The formula to calculate the efficiency of the cycles shown in Table 3.
Table 3. Efficiency analysis
Table 3. Efficiency analysis

RESULTS ANALYSIS

Figure 2 indicates that the Baseline Atkinson and Atkinson +10° are the cycles with the biggest portion of area under the curve, thus, the cycles that produce the highest amount of work. On the other hand, Atkinson +40° is the cycle producing the lowest amount of work due to too late an inlet valve closure, letting too large an amount of air to escape in the combustion chamber to the intake manifold.

As for Figure 3, the Otto cycle shows the highest peak pressure during the compression stage and the biggest amount of work produced compared to the Atkinson cycles. However, this is a key feature employed by the Atkinson cycle to lower the temperature and speed during the combustion reaction, making better use of the available energy and resources by reducing the mass of the mixture within the combustion chamber and therefore, delivering a reduction in emission levels and fuel consumption.

CONCLUSIONS

Nowadays, the availability of fossil fuels around the world as well as the pollutant emissions emitted are reaching a point of no return. With this in mind, car manufacturers are looking towards green technologies such as hybrid vehicles to reduce their negative impact to the planet. With this vision, this blog post has the aim of clarifying the advantages and disadvantages of the Atkinson cycle using Dymola software as a design, test, and validation tool.

It was proven that the Atkinson cycle offers higher efficiency at part-load conditions compared to the Otto cycle. Making of this, a suitable cycle for HEV’s, a field where the priority is fuel consumption and emission levels reduction. Moreover, these vehicles are designed to be driven most of the time under part-load conditions, an area where the Atkinson cycle shows a compelling advantage. Nevertheless, the Atkinson cycle has the drawback of offering lower torque at low-load conditions. Here is where the benefits of electric motor assist show up. In an HEV both electric and ice motors can work together, thus, at low-load conditions the electric motor will compensate the lack of ICE power, creating a perfect match.

CURRENT & FUTURE APPLICATIONS

Toyota, the well-known Japanese car manufacturer popular for its advances in electric drive technology in cars such as the Prius models, recently unveiled at the 2018 Geneva Motor Show a new 2.0L four-cylinder gasoline engine capable of delivering an outstanding 41% thermal efficiency, considering a typical automotive gasoline ICE operates at around 25%.

Figure 4. New Toyota 2.0L gasoline ICE
(Carney, 2019)
Figure 4. New Toyota 2.0L gasoline ICE
(Carney, 2019)

This was achieved in great measure thanks to the use of a new laser-clad valve seat that shrinks the seat to the absolute minimum of the contact surface with the valve face, improving the intake charge swirl inside the combustion chamber.

Toyota have stated that the engine above achieves 40-41% thermal efficiency depending on the application (conventional vehicle and hybrid variant).

REFERENCES

  1. Ortiz Sanchez, J. (2018). Evaluation of Alternative Cycles for Internal Combustion Engines. Masters. Oxford Brookes University.
  2. Carney, D. (2019). Toyota new gasoline ICEs with 40% thermal efficiency. [online] Sae.org. Available at: https://www.sae.org/news/2018/04/toyota-unveils-more-new-gasoline-ices-with-40-thermal-efficiency.

Written by: Jose Miguel Ortiz Sanchez, Project Engineer

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