The high-speed cine photography results discussed in the previous chapter su ggested that the injector approach w as, at least, partially successful in its goal o f introducing the fuel into the cylinder in the vicinity o f the spark plug It w as decided that the next logical step w as to attempt to measure the in-cylinder HC concentration at key points and to d o this with a Cambustion H FR 400 fast flame ionisation detector
7.1
Cambustion HFR400 Fast Flame Ionisation Detector
A Cam bustion H FR 400 Fast Flame Ionisation D etector (FFID ) w as used to measure ‘real tim e’ in-cylinder HC concentrations at specific points in the com bustion chamber The high frequency response o f the FFID (2 ms) is obtained by the sample gas being delivered directly to a nozzle exit via short sample lines, w hereupon it is m ixed w ith the fuel gas and burnt (figure 7.1 a). With conventional FID designs the sample is supplied via a pump (figure 7.1 b), and the frequency response is typically 1 second (Cam bustion H FR 400 FFID U ser Manual (V ersion 1 3)).
H am c chamber ■ Ion c o lle c to r __ (electrode) Marne Nozzle Exhaust Sample capillary Air Hydrogen
Exhaust Ion collector
Flame --- Nozzle --- Flame cham ber
Air Fuel gas Sample capillary To vacuum Sample Sample regulator Pump Filter
Figure 7.1 b: Conventional FID arrangement
Under normal flame tem peratures, if the sample gas contains hydrocarbons, negative ions are produced w hen it is burnt It is these ions that are collected at the electrode and used to generate an electrical output which is proportional to the number o f carbon atom s burnt in hydrocarbon form Since the volum e o f the sample gas being burnt at any tim e in the system is dependent on the pressure differential betw een the sample entry point and the constant pressure (C P ) chamber, and b etw een the CP chamber and the flame chamber (figure 7 .2 ), the output is therefore also proportional to the m ass flo w o f the sample gas through the system
It is important to maintain a constant flo w o f sample gas into the n ozzle, regardless o f the pressure fluctuations that may occur at the sample entry point (Cam bustion H FR 400 FFID U ser Manual (version 3 .1 ) and L adom m atos et a l (1 9 9 5 )). The exit from the FID tube must not b ecom e choked (Crawford et a! (1 9 9 6 )). Since the FID tube form s a static pressure tapping on the tee-to p , the flo w through it is independent o f dynam ic pressure effects, and will only vary if the pressure in the CP chamber varies (or m ore precisely if the pressure at point A varies (figure 7.2). A constant pressure in the CP chamber can be maintained by making the CP chamber volum e large in com parison to the sample flow fluctuations and using bleed flo w regulators, together with careful selection o f the FFID tube diameters and the FID flame chamber pressure (Cam bustion H FR 400 FFID U ser
Manual (version 3 .1)) Ladom m atos et al (1 9 9 5 ) dem onstrated the insensitivity o f the FFID output signal to sample pressure, by supplying the end o f the FFID probe with calibration gas o f know n concentration at a pressure o f 1.7 bar, and then rapidly increasing the sample pressure to 44 bar The FFID output w as seen to remain constant although the sample pressure altered.
Ion collector ’FFID head
o o o o o o o o
o o o o o o o o
o o o o o o o o
Flam e cham ber
mm
FID tube H eated transfer sam ple
line (or FFID probe)
AP C onstant pressure
(CP) cham ber
ee - piece Sam ple entry point
Figure 7.2: FFID head and probe
The pressure independence o f the FFID system used in the current work w a s also investigated The engine w as m otored whilst the FFID w as used to sample from the com bustion chamber N o fuel w as injected into the com bustion chamber b ecau se o f the com plexities involved in ensuring that a h o m o gen eou s air fuel mixture existed every cycle in the cylinder B oth the m otored cylinder pressure and the FFID output w ere recorded on an oscillosco p e. A constant output from the FFID w as obtained although the cylinder pressure varied.
This m ethod for determining pressure independence is not com pletely satisfactory in that the m otored cylinder pressure is low er than the cylinder pressure achieved w hen the engine is fired. Additionally, the rate o f increase in pressure in the m otored cylinder pressure trace will not be as great as that encountered under firing conditions (chapter 1,