J-090714-1Engine Performance and Exhaust Characteristics of Direct-injection Diesel Engine Operated with DME (SAE 972973)

1 Because of its low self-ignition temperature (235°C),

2 Compression Ratio 17.7

3 the injection nozzle opening pressure was set at 8.82MPa (90kg/cm2) when the engine ran at 960rpm

4 Note that the recommended opening pressure of the engine for Diesel fuel, on the other hand, was 20.1MPa (205kg/cm2).

5 also in the operating conditions including the fuel feed pressure, which ranged from 1.96 to 2.94MPa (20 to 30kgf/cm2) in other studies.

6 injection time and the engine’s response, it was adjusted to occur at 17bTDC

7 have the opening start at 17bTDC, the actual (dynamic) opening occurred at 13bTDC for DME

8 This exhibited that the opening with DME occurred earlier than gas oil by as much as by four crank angle degrees (CA).

9 Two different injection times were studied, with settings of 17bTDC and 5bTDC (indicated by the manufacturer’s manual), which had one tooth difference between the gears in the engine and injection pump.

10 at the speed of 960rpm and the load to deliver (brake) mean effective pressure (mep) of 0.3MPa (Fig. 7), and mep = 0.6MPa

11 when the load was low (Fig. 7), the rapid pressure rise occurred earlier with DME than with gas oil by about the corresponding amount, although the peak pressure occurred almost at the same CA for both fuels. When the load was high (Fig. 8), the difference in start of pressure rise was even more obvious with DME compared with Diesel fuel. The earlier start of pressure rise in both experiments did not appear to occur due to an easy (chemical) ignition tendency of DME but more the differences in the start of nozzle lift.

12 When the injection time was delayed, the effect of self-ignition tendency of DME seemed to play a predominant role resulting in a far earlier start of pressure rise compared with Diesel operation.

13 The difference in EGT for the two fuels was rather significant, as much as over 50øC,

14 the engine with DME ran relatively quiet by observing that its premixed combustion stage was "milder" than that with the gas oil operation.

Mentioning the mild premixed combustion with DME and its impacts, it is realized that the low soot emission with DME (as shown later in Fig. 22) is not due to a great amount of fuel consumption in its premixed combustion stage, which is the case in the typical gas oil operated
Diesel engine producing a low soot emission.

15 the shorter the ignition delay (Fig.17), the milder the intensity of premixed combustion causing a quiet engine operation,

16 However, a common observation of a short ignition delay in a Diesel engine to produce high soot emissions is no longer true in the DME operated engine.

17 the injection of gas oil completed within a short period of time, particularly when the engine load is low,

18 The continuing combustion for DME at a high load, therefore, is caused by the prolonged injection period.

19 it seemed that the chemical aspect of fuel to produce low soot formation played a dictating role in producing low soot emissions.

20 Note that Glensvig and Sorenson [12] suggested that a DME spray completes the evaporation within a very short period of time after injection. In addition, the high stoichiometric fuel/air ratio will have a greatly diluting effect in a DME spray. Also, the extended injection period means a greater momentum imparted on the DME spray, making it more dispersed. These all are expected help produce a lean local fuel/air ratio, that is, to have lean combustion compared with a Diesel fuel spray.

21 Considerably high NOx emission was observed when the engine was operated by DME under the engine-manufacturer recommended injection time for gas oil, i.e., injection at 17bTDC injection time (Fig. 20). It is noted that this observation is in contrast with low NOx emission reported by others [7]. One of the most probable reasons for the high emission is considered to be the unusually early start of combustion with DME.

22 Note that the dispersed fuel spray formation with DME would help produce low NOx emission.

23 5bTDC, however, the NOx emission was much lower with DME. This finding is explained by the high rate fuel injection (mass of fuel/deg)

24 operation compared with its counter part (Fig. 15) producing high-temperature combustion products with the piston located still near the TDC.

25 The emission of THC from the DME-operated was very low regardless of injection time for a wide range of engine loads. This finding may be explained in terms of several parameters, including the average cylinder temperature (ACT), fuel-air ratio in the spray and chemical
characteristics of the fuel.

26 The dispersed locally lean fuel spray formation expected with DME, as explained earlier, may be a factor consuming the maximum amount of fuel before being wasted in the exhaust.

27 The emission of soot with DME operation was virtually zero (Fig. 22) under the entire experimental conditions investigated in the present study.

28 the fuel is highly volatile, becoming a gaseous jet or attaining a high specific volume as soon as leaving the injector, which transfers a greater amount of momentum during a longer period of injection compared to the gas-oil injection.

29 since DME is an oxygenate like a methanol, which is known to produce low soot in Diesel engines, it may not be difficult to expect a low soot emission by the fuel. Therefore, it seems be
reasonable to consider that the low soot with DME in Diesel engines is either or both the lean fuel/air spray structure and the fuel’s intrinsic tendency of low soot formation in Diesel engines.

30 7. Sorenson, S.C., Mikkelsen, S.E., "Performance and Emissions of a 0.273 Litter Direct Injection Diesel Engine Fueled with Neat Dimethyl Ether," SAE Paper 950064, 1995.