An easy way to better diesels? Work at the ISVR
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gives prom is that retarded injection of small droplets could pay off all round
by Paul Brockington
ONE RESEARCH institute, at least, is working on a project which gives promise of both lower pollution and strictly operational advantages, for the Institute of Sound and Vibration Research, Southampton University, has demonstrated that low noise and low NOx are compatible.
This is particularly encouraging because in general the costly develop ment work involved in the quest for cleaner, quieter diesels is likely to be non-productive in terms of km/ h (mpg).
The ISVR's current work gives promise that in due course retarding in jection timing could be combined with the use of a nozzle producing fine fuel droplets to reduce fuel consumption as well as noise; and to give smooth running plus greater top-gear flexibility. Some day it might enable the structural weight of diesel engines to be reduced.
Diesel knock is normally attributed to delay of combustion at the start of in jection and uncontrolled burning of the fuel injected during the delay period at the start of combustion, controlled combustion being burning of the fuel as it leaves the nozzle.
Various devices have been tried that reduce combustion lag, notably pilot injection, which provides for the injec tion of a small quantity of fuel in the form of fine droplets ahead of the bulk of the fuel. Small droplets have a higher surface-to-volume ratio than large ones and are more easily vapor ized in preparation for burning. This re duces the delay period, but because the droplets are finely divided burning is quicker and the rate of pressure rise — other things being equal -is higher.
Combustion lag
The vaporization of large droplets is delayed because a small surface-to volume ratio reduces the amount of fuel that is in contact with the air into which it is injected. While this also increases combustion lag, burning is smoother once it has started. But over a period it is less efficient and emissions are increased.
The problem is to control the burning of small droplets so as to reduce the rate of pressure rise without sacrificing economy. The number of variables are formidable and the type of injection equipment employed is a factor in a complicated equation. Although combustion control is facilitated by turbocharging, of all types of diesel, at higher loads it does not help very much at lower loads. In a drive-by test of an accelerating diesel-engined vehicle the noise produced at lower rpm is typically greater than the noise level at peak rpm on full load. _ According to the ISVR there are two forms of combustion lag. One is a physical delay which is influenced by injection timing and which does not vary to a significant extent with changes in droplet size; the other is chemical delay ...which depends upon the preparedness of fuel for burning.
If fuel is injected as a fine droplet spray and injection timing is suitably retarded, burning starts at a higher cylinder pressure and temperature and preparation for burning is more complete, the probable timing being top dead centre instead of a more normal timing of over 20 deg before tdc. A fairly high rate of pressure rise lat in the cycle is presumably combin with a lower peak temperature. As h been noted by many investigators pollution phenomena, retarding_ injc tion reduces NOx but it normally i curs an output and consumptic penalty and increases smoke emissio Matching the timing of injection tc fine-droplet spray pattern in a prof able way that offsets the disadvantag of late injection without loss of its a vantages could well depend up4 matching the swirl pattern of the inta air to the changed combustic characteristics, and precise control injection at the start is essential. In n opinion this,is not compatible with t use of a jerk-type pump which d charges to the nozzles through loi pipes.
Employing a unit type of pump mig enable regulation of injection to be improved that the fine-dro
let/ matched-swirl/ late-burning teel nique could be developed to give accep able performance/ pollution results. If a very important possibility, partic larly if it provided for an increase in rp and consequently in output_
The ISVR is reticent in making clair for the system other than that it mig make possible the development of a ur
which was cleaner, quieter and lighter (its weight would depend on whether the block were designed to exploit the lower stress levels). But this is exciting news. After all, modification to a standard unit would be simple and readily acceptable to engine manufacturers unless the block were modified. And modifications would he acceptable if there were a very substantial saving in weight.
As mentioned by the ISVR pilot injection is a "small droplet" problem. Whether or not it will play a useful part in the future of the diesel remains to be seen. Experimental work 20 or more years ago proved that it could reduce diesel knock, but it increased fuel consumption and nothing was heard of it for a long time. Work on pilot injection at the ISVR could well make it a feasible proposition
Low-rated fuel
The prospect of the obligatory use of dery having a tower cetane number complicates research work on combustion control. While the octane number of a gasoline is important with regard to the control of pinking and therefore to compression ratio and power output, the cetane rating of diesel fuel has little if any affect. on performance. But a lowrated fuel increases combustion lag, which in turn increases noise and NOx.
Investigations at the ISVR have shown that a reduction in cetane number of 10 increases the noise level of a direct-injection engine by 4dBA. And tests have revealed that reducing droplet size from 40 microns to 30 microns gives a reduction of noise level of 4.5dBA. Variations in the 30/40 micron range can be accommodated without affecting performance or fuel consumption. But it is to be hoped that catering for a tower cetane number will not absorb advances in combustion control that could be applied to improve the operational worth of diesel engines generally.
Compression ratio is another factor that has to be taken into account in designing a low-NOx low-noise diesel; the higher the compression ratio the greater is the rate of heat release and the rate of pressure rise. According to the ISVR an increase of one ratio in the range 16-to1/ 19-to-1 gives an increase in pressure of about 82.8kNisqm (I2psi) per degree of crank angle.
And this information gives support to the argument that a diesel should be designed to operate at its lowest useful compression ratio.