Gasoline Combustion Systems

Gasoline Combustion Systems Beyond 2020

22. October 2014 | Engineering Service

For the next decade the Gasoline engine will remain the primary propulsion source for passenger cars. It has achieved significant improvements in terms of energy efficiency while downsizing in combination with direct injection have resulted in significant improvements in CO2 reduction without sacrifices in terms of “fun-to-drive.” There is further technical solution potential (cf. Fig. 1).

Depending on the vehicle configuration, the upcoming Worldwide harmonized Light vehicles Test Procedures (WLTP) certification values will vary between WLPT low and WLTP high, but on a higher level compared to the NEDC. Significant further fuel consumption reduction can be enabled within the time frame leading up to 2020. Downsizing in combination with downspeeding has put more emphasis on high load operation where further optimization of combustion efficiency is limited due to knocking and preignition effects. Miller cycle, cooled external EGR and VCR represent technologies that have been or will be introduced in the near future in order to improve the efficiency of Gasoline engines at part load as well as during full load operation.

Condensate injection concept offers potential beyond 2020

In search of potential to improve the efficiency of Gasoline combustion even beyond 2020, FEV has developed the condensate injection concept, in which the condensed exhaust gas is fed back into the engine. This offers potential synergies which can be further enabled by exhaust heat recovery. There is also a possible extension of this concept, in which the condensed water from the air conditioning system can be added to a buffer tank from which the condensate is extracted and fed into the engine.

Based upon initial investigations on a TC GDI engine that features PFI injection of the EGR condensate, the concept has been extended in regard to two aspects. Direct injection was considered for the condensate as well as for the Gasoline injection. Moreover the condensation concept was changed to allow utilization of the condensate from the entire exhaust gas stream and not only EGR condensate. This operation principle allows the entire injected condensate to be recycled.

After optimizing the fuel parameters as well as condensate injection, the potential of condensate injection in combination with the Miller cycle and cooled external EGR was evaluated at IMEP = 14.5 bar at n = 2,000 rpm. The engine remained knock limited despite the utilization of both technologies due to the high geometric compression ratio of 13.5.

Using condensate injection via the side injector, MFB 50 decreases linearly with increased water quantity and reaches an optimum of ~ 8° CA ATDC at a water/fuel-ratio of 50 percent (cf. Fig. 2). The best fuel consumption is achieved at a water/fuel-ratio of 37 percent (mass fraction) with an indicated fuel consumption reduction of 3.5 percent in addition to the reduction caused by the efficiency improvements that result from the Miller cycle and cooled EGR.

Further fuel consumption reductions

Similar results are achieved if the engine speed is increased from n = 2,000 rpm to n = 3,000 rpm at almost constant load. Further fuel consumption reductions can be realized with lean burn operation instead of cooled EGR. In such a case, condensate injection can enable direct fuel consumption reduction via knock mitigation and indirect fuel consumption reduction via NOx raw emission reductions.

The water/fuel-ratio variation at n = 2,000 rpm and IMEP = 14.5 bar was also repeated using the central direct injector for water injection and the side direct injector for Gasoline injection (cf. Fig. 3). Slightly higher cyclic fluctuations result in poorer MFB 50 phasing without water injection. Fuel consumption is nearly identical. As a consequence of the higher knock restriction, a higher water/fuel-ratio is required for the setup with the side Gasoline injector to achieve optimal combustion phasing. Condensate injection and Miller cycle are sufficient to enable optimal combustion at medium part load up to ~ 10 bar BMEP even with a 13.5:1 compression ratio.

For an engine concept with variable compression ratio, the combination of condensate injection with Miller cycle and cooled EGR will allow a further increase in efficiency at part load, as well as, since a higher compression ratio can be used, at higher loads.

In further development stages of the Extended Direct Condensate Injection system, material compatibility issues of the water injection system components and other aspects such as freezing protection will be addressed.

Gasoline Combustion Figure 1

Fig. 1: CO2 potential of various technologies at the example
of a C-segment vehicle with 1.0 L 3-cylinder Gasoline
engine

 

Gasoline Combustion Figure 2

Fig. 2: Influence of the injected
water quantity on stoichiometric
combustion with cooled
EGR at n = 2,000 rpm and
IMEP = 14.5 bar

 

Gasoline Combustion Figure 3

Fig. 3: Comparison of side and
central water injection in a water/
fuel-ratio variation with cooled
EGR at n = 2,000 rpm and
IMEP = 14.5 bar

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