Accounting for combustion mode switch dynamics and fuel penalties in drive cycle fuel economy

Significance Statement

In recent article of Nüesch et al. 2016, published in International Journal of Engine Research focused on an engine equipped with the recompression method and establish a methodology that predicts the drive cycle fuel economy that can be achieved whether fuel penalties during mode switches are taken into account.

Reduction of fuel consumption and emissions has been the major concern of gasoline engines which has led to development of advanced combustion engines such as Homogenous charge Compression Ignition (HCCI). This technology offers advantage in increased thermal efficiency and specific ratio, unthrottling at running engine due to lean operation and less emission of NOx due to low peak cylinder temperatures.

At high loads, autoignition causes very high causes very high pressure rise rates which limits the range of recompression HCCI engines to low and medium loads. In addition to this, regime for HCCI combustion is influenced by fuel efficiency gains over Spark/ignition (SI) operation and constraints imposed by emission requirement which induce further reductions in brake specific fuel consumption (BSFC).

Due to delay in mode switches from SI to recompression HCCI, resulting gas together with dilution leads to a change in mixture composition, autoignition, higher combustion efficiency and reduction of exhaust gas temperature compared to standard SI. Hence net benefit in fuel economy can only be realized in HCCI if additional fuel needs are invested during mode switches and duration of HCCI is long enough.

The authors previously studied the interaction between SI/HCCI multimode engine which showed that catalyst oxygen storage fills up and requires depletion and hence rich operation but its resulting lean-rich mixture showed substantial penalties in fuel economy with barely satisfying tailpipe NOx emission constraints. Hence, further study on the research is needed in order to achieve fuel economy whether fuel penalties during mode switches are taken into account.

In order to achieve this, SI/HCCI mode switch was applied to a vehicle simulation and measured engine maps to show analysis of combustion mode switches and results for FTP-75, Highway Fuel Economy Test (HWFET) and US06 driving cycles were taken into consideration. The following notations were taken at every step; time step k, engine torque Te and engine angular velocity ωe lies inside the feasible boundaries of HCCI combustion and the instance is called Visitation of HCCI operating regime.

In order to ascertain the benefits of advanced combustion mode on instantaneous mode switches, the engine regimes on vehicle simulation shows that frequently visited area during the FTP-75 engine operates at lower loads for a significance amount of time of 21%, fuel 19% and distance of 28% where the low temperature doesn’t allow a mode switch to HCCI operating in the SI mode, HWFET time decreases to 19% while fuel decreased to 12%. For US06 cycle, value drops further for time and fuel at 7% and 3% respectively.

From fuel economy results and assuming instantaneous switches, the improvement in miles per gallon (MPG) due to HCCI lies at 3.4% for FTP-75, 1.3% for HWFET and 0.4% for US06 cycles. Hence, FT-75 and HWFET were only considered since improvement in US06 cycle is marginal. Despite 20% of fuel time spent in HCCI during FTP-75, durations of individual visitation of HCCI regime is very short hence, instantaneous mode switches cannot be assumed and mode switch dynamics need to be considered.

From two mode switch developed i.e. SI-HCCI and HCCI-SI, the dynamics of SI-HCCI direction poses a more difficult combustion control while HCCI-SI is challenged by air path control. HCCI-SI is more preferable due to higher fuel penalties than SI-HCCI direction.

Incorporating the assured fuel penalty of mode switches reduces the gain inefficiency due to HCCI combustion significantly for FTP-75 from 3.4% to 2.7% while HWFET reduced from 1.3% to 1%. From the result it was seen that substantial fraction of mode cycles is harmful and penalty ratio (rh) is relatively small since harmful mode switch cycles are short. 6.4% and 3.3% of FTP-75 and HWFET respectively was the difference between instantaneous and penalized cases originate from this harmful switches.

Parametric study on the effect of fuel economy improvement showed that at higher penalties, the penalty contribution of harmful switches are responsible for more than 40% of the reduction in fuel economy when compared to cases where instantaneous mode switches are assumed.

This study the researchers were able to show improvement of up to 3.4% which is significantly below other results.


Journal Reference

Sandro Nuesch1 , Patrick Gorzelic1 , Li Jiang2 , Jeff Sterniak2 , Anna G Stefanopoulou1  Accounting for Combustion Mode switch Dynamics and Fuel Penalties in Drive Cycle Fuel Economy. International Journal of Engine Research, 2016, Volume 17, Issue 4, pp 436-450. 

Show Affiliations
  1. Walter E. Lay Automotive Laboratory, Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA
  2. Robert Bosch LLC, Farmington Hills, Ann Arbor, MI, USA

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