2.4 The Cylinder Liner Environment
2.4.3 Cylinder liner temperatures
Published diesel engine temperature data are usually focused on the high temperature zones which are the cylinder head, piston crown and upper cylinder liner.
Some published liner top temperature data are provided in Table 2-6. The distance from the liner surface to the thermocouple used to record these data has a large effect on the measurement.
TABLE 2-6 REPORTED CYLINDER LINER TEMPERATURES Test engine Fuel
type
Speed (rpm)
Load (bmep, MPa)
Thermo-couple depth (mm)
Max.
Liner Temp.
(qC)
Ref .
Matsui Iron Works
MU323DGS C
Residual Residual
420 1.55 0.5 240-290
287
44 45
Ruston 6 APC
Residual 750 750 750
1.72 0.86 0.43
2-3 2-3 2-3
171-180 141-151 120-128
46
Ruston 6 APC
Distillate 750 750 750
1.72 0.86 0.43
2-3 2-3 2-3
168 139 126
46
A difference in temperature between distillate and residual fuels is also noticeable, the distillate fuel causing a lower liner temperature than the residual fuel. The tabulated temperature is the maximum which is measured near the top of the liner. The temperature is lower in lower parts. This table also show that the liner temperature increases with increasing load.
2.4.3.1 Influence of fuel composition and combustion characteristics on cylinder liner temperature
A relationship between liner temperature and distillate fuel oil composition has not been found in the literature; however a large body of
work on the combustion characteristics of residual fuels is presented in [46]. These data which include one gas oil sample suggests that fuel composition can significantly affect the temperature measured by thermocouples immediately below the surface of the cylinder liner, and hence most certainly also the temperature of the oil film.
This work is done in a 6 cylinder long-stroke medium speed engine rated at 17 bar BMEP. Seven residual test fuels, one reference fuel and one gas oil were tested. Cylinder liner temperatures were recorded amongst other data. The liner temperature was measured with thermocouples positioned 2-3 mm below the liner surface. It was planned in this work to include a test fuel consisting of pure light cycle oil (LCO), however large cycle to cycle pressure variations, high exhaust valve temperature and knocking combustion caused this test to be abandoned.
Recorded cylinder liner temperatures at 750rpm, charge air: 50 C
120 130 140 150 160 170 180
0 25 50 75 100 125
Torque [%]
[C]
Gas oil Ref fuel TF1 TF3
Figure 2-6 Load and fuel dependent maximum cylinder liner temperature Drawn from measurement data obtained in [46]
Data from [46] established that the liner temperature measurement could vary as much as 20qC amongst the residual fuels that were tested, however at maximum load and hence maximum liner temperature, the differences measured were only 8qC. Data for full speed of 750 rpm and
scavenge air temperature of 50qC are provided in Figure 2-6. Only some of the fuels tested in [46] are shown to ease reading.
The largest deviations in surface temperatures were detected at 50% load.
The premixed combustion is largest at this load condition in this engine for the majority of the fuels. Large premixed combustion causes a high rate of pressure rise which is assumed to cause a breakdown of the boundary layer between cylinder air and metal surfaces. Hence the effect of fuel on liner temperature as shown above could equally well be described as the effect of combustion characteristics on liner temperature.
2.4.3.2 Effect of charge air temperature on liner temperature
The charge air temperature was increased from 50 to 70 or 75 degrees in some part load experiments in [46]. The temperature measured within the cylinder liner generally increased, however there were exceptions indicating that the combustion effects in some instances dominates the charge temperature effect. The suggested cause is reduced charge air temperature causing increased premixed combustion which contributes to increased heat transfer. It is thus not possible to generalize the effect of charge air cooling on the cylinder liner.
2.4.3.3 Influence of flame proximity on liner temperature
The temperature on the cylinder liner is higher in line with the fuel spray.
Experiments using heavy fuel in a medium speed engine revealed a difference of 97qC between a thermocouple mounted in the top of a cylinder liner in line with the fuel spray and another mounted at the same height but in-between spray axes [45]. The difference in temperature was attributed to flame proximity. This paper also presented a strong correlation between the combustion duration measured in a fuel ignition apparatus and the cylinder liner temperature in the engine tests and concluded that fuel evaluation should put emphasis not only on ignitability but also combustion speed.
The implicit understanding of these results is that the slower burning fuel burns closer to the cylinder liner. It is interesting to note that lower charge air pressure will increase the speed and penetration of the fuel spray.
Models based upon experimental data presented in [47] indicate an increase in speed and penetration in excess of 30% when the charge density is reduced according to a reduction in charge pressure from 3 to 1 bar and fuel injection pressure is constant. This effect is also documented
by engine tests using constant pressure common rail injection [48]. This study also observed the expected increase in ignition delay with reduced charge air pressure. Hence low load and corresponding low charge air pressure will contribute to an increase in fuel spray speed. The fuel injection pressure, however, is not independent of load in most marine engines.
At constant speed conditions, the fuel pressure increases at a rate which is load independent, however at low load, the injection is aborted before maximum pressure is obtained. The spray in the first part of the injection is thus expected to be significantly longer in the low charge density case;
however the effect on maximum spray length is unknown.
At reduced speed, both pressure increase rate and maximum obtainable pressure are reduced. These interacting and counteracting factors as well as the influence of the ignition delay make prediction of the flame position within the combustion chamber difficult. Additionally, reduced scavenging efficiency will cause a decrease in oxygen concentration in the charge which will cause the flame speed to be reduced. [49]
Experiments using a rapid compression machine suggest that the heat transfer rate increases with the velocity of the flame which contacts the wall surface [50].
The heat transfer from the gas to the cylinder wall has been observed to increase with increasing cylinder wall temperature [51] which is contrary to what might be expected from classic heat transfer theory. This phenomenon has not been completely explained, however it has been suggested that the increased heat transfer is related to the thinner boundary layer formed when the wall temperature increases.
2.4.3.4 Local variations in liner temperature
The local liner temperature is affected by the charge motion within the cylinder. Proximity to the charge air intake will have a cooling effect while the liner surface close to the exhaust outlet may face a higher thermal load. This effect is clearly measurable with thermocouples mounded within the cylinder liner.