Tsinghua study compares two diesel-gasoline combustion modes; both deliver high efficiencies and low emissions
|The effects of gasoline ratio on indicated thermal efficiency of HCII and GDBF modes. Yu et al. Click to enlarge.|
Researchers at Tsinghua University have compared the combustion and emissions characteristics of two dual-fuel (diesel-gasoline) modes intended to integrate the advantages of both fuels to achieve high thermal efficiency and low emission targets. A paper on their results is published in the journal Fuel.
Gasoline Homogeneous Charge Induced Ignition (HCII) by diesel combines the port fuel injection of gasoline to form a homogeneous charge with the direct injection of diesel fuel as an ignition source. (E.g., RCCI, earlier post.) Gasoline/Diesel Blend Fuels (GDBFs) use a premixed blend of diesel and gasoline which is directly injected into the cylinder for combustion. (E.g., dieseline, earlier post.)
Although some researchers have separately studied on HCII mode and GDBF mode, a comparative study of the two combustion modes has not been reported. Comparative study on the combustion characteristics, emissions, thermal efficiency and low-temperature combustion characteristics in the two combustion modes is carried out in a high pressure common-rail single cylinder diesel engine in order to find the most suitable route for gasoline/diesel unified engines.—Yu et al.
HCII is capable of lean combustion at higher compression ratios than conventional SI combustion. The auto-ignition of the direct-injected diesel establishes ignition events in the combustion chamber at multiple sites, and thus initiates rapid heat release close to constant volume combustion, the Tsinghua team noted.
HCII thus avoids several drawbacks that lower the thermal efficiency of SI engines. Additionally, soot emissions can be reduced significantly because the homogeneity of the cylinder charge is significantly enhanced by the port injection of gasoline fuels.
From a controls point of view, HCII can readily change the ratio of gasoline to diesel, which influences the auto-ignition of fuel-air mixtures. However, the researchers noted, having two fuel supply systems increase the cost of the engine.
GDBF mode results in reduced soot emissions because the fuel-air mixing is ameliorated and a more homogeneous fuel-air charge is formed due to the low boiling point and volatile components in the blended fuels.
Multi-point simultaneous compression ignition, achieved due to the high boiling point and low auto-ignition temperature components in the blended fuels, improves thermal efficiency.
Research has shown that dieseline can improve the ignition stability of HCCI and broaden the operating range of that mode. Other work has indicated that GDBF has the characteristics of low cetane-number and high volatility, and can achieve low-temperature combustion at moderate EGR.
Further, as only one fuel supply system is used in GDBF mode, major engine modifications—and cost increases—are avoided.
Meanwhile, if GDBF mode can succeed, it indicates that wide fraction fuels (WFFs) can be used in future engines, which means that crude oil will need not be fractionated to gasoline, kerosene and diesel. This will significantly reduce the costs of vehicle fuel refining and transportation and have great social benefits.—Yu et al.
In the Tsinghua study, the team modified a four-cylinder common rail diesel engine into a single-cylinder research engine. The intake and exhaust systems and the diesel and gasoline fuel supply systems were specially designed for the targeted single-cylinder engine. They compared the combustion and emissions characteristic of HCII and GDBF modes with different EGR rates and gasoline ratios.
In HCII mode, gasoline was port-injected and controlled electronically. The diesel pilot fuel was injected directly into the cylinder, and its injection pressure, injection duration, injection timing and number of injection events could also be adjusted. Gasoline/diesel blends in GDBF mode were directly injected into the cylinder using a high-pressure common rail system.
|Effects of EGR rates on emissions from HCII, GDBF, and diesel. Yu et al. Click to enlarge.|
The researchers found that both HCII and GDBF modes can achieve higher thermal efficiencies than gasoline SI combustion and a similar or even higher thermal efficiency than diesel CI combustion because the the combustion is closer to constant volume combustion.
They also found that both HCII and GDBF modes can achieve low-temperature combustion with extremely low soot and NO emissions when combined with large amounts of EGR. Both modes thus demonstrate an advantage over the diesel CI combustion mode, where the NO emissions decrease and soot emissions increase with the increase of EGR, exhibiting the classical NO-soot trade-off, they noted.
Other findings included:
As the gasoline ratio increased, both HCII and GDBF modes improved the fuel-air mixing, and thus soot emissions reduced significantly, with a maximum reduction of 90% observed during the engine tests. Soot reduction in HCII mode was more significant.
As the gasoline ratio increased, the ignition delay in HCII mode remained almost the same, while the ignition delay in GDBF mode increased significantly. The combustion duration in both HCII and GDBF modes shortened significantly.
As the gasoline ratio increased at low load conditions, the maximum PRR is HCII mode reduced while in GDBF mode it increased. However, the maximum PRR in both the HCII and GDBF modes increased significantly at high load. The increase in HCII mode was greater than GDBF at high load.
As the gasoline ratio increased, the HC emissions in the GDBF mode remained almost unchanged, while the HC emissions in the HCII mode increased significantly. The mechanism of HC emissions is similar to traditional gasoline engines and is mainly from wall and crevice quenching effects of homogeneous gasoline-air mixtures.
As the gasoline ratio increased at low load, the NO emissions reduced in HCII mode, but increased in GDBF mode. The NO emissions in both HCII mode and GDBF modes increased significantly at high load. The NO increase in HCII mode was greater than GDBF at high load.
Chao Yu, Jian-xin Wang, Zhi Wang, Shi-jin Shuai (2013) Comparative study on Gasoline Homogeneous Charge Induced Ignition (HCII) by diesel and Gasoline/Diesel Blend Fuels (GDBF) combustion. Fuel Volume 106 doi: 10.1016/j.fuel.2012.10.068