SISTEMA DE DETECCIÓN Y EXTINCIÓN DE INCENDIOS

In order to obtain a more comprehensive knowledge of both intake port fuel transport in general and specific characteristics of the AAFV, a series of intake and exhaust port HFRFID sampling tests were undertaken on a single-cylinder version of the Ford Zetec engine that was available for test work. A cylinder head was modified such that access into the inlet port was facilitated at three locations, each with the probe being positioned normal to the flow direction.

Test conditions of 800 rev/min idle and 1500 rev/min road load were chosen for their representation of cold-start engine and warm-up load and speeds, and 1650 rev/min WOT to assess the performance of an alternative air supply to the AAFV.

5.2.1 Cambustion high frequency response flame ionisation detector, HFRFID

The Cambustion instrument is an FID-based device designed to measure unbumed hydrocarbon concentrations on a very short time scale to the extent that a crank angle resolved characteristic UHC signature is obtained.

Significant ion formation occurs when a hydrocarbon fuel is burned, the number of ions being very nearly proportional to the number of carbon atoms burned. The quantity of ions produced in the flame of a non-hydrocarbon such as hydrogen is limited to thermal ionisation and as such is very low at normal flame temperatures. The method of production in the hydrocarbon flame is associated with a process called chemi-ionisation. Conventional FID design uses the physical properties of the two ionisation processes by introducing a representative sample of the measured flow into a hydrogen flame. An ion collector is used to measure the concentration of the mass flow of hydrocarbons within the sample. In a conventional FID the necessary gas flows are controlled by capillary tubes and pressure regulators, and the

instmments generally have a response time of greater than one second. The

fuel gas at the nozzle exit, see figure 5.1, drawn directly into the FID by maintaining the flame chamber pressure below that o f the sample pressure. Consequently, the

chamber normally operates below atmospheric conditions. The accuracy of the

measured concentration depends upon the control o f the FID chamber vacuum which determines the mass flow rate and pressure fluctuations o f sample gas. A typical 10- 90% response time o f the HFRFID is approximately 4 ms whilst the transport delay, or time constant, is dependent on the capillary tube bore and length used to supply to sample as well as on the pressure differential maintained between the measurement location and the flame chamber.

Figure 5.1

Comparison of a conventional FID and the Cambustion HFRFID

F la m e C h a m b e r 1°“ C o lle c to r N o z z le I— Air S a m p le C a p illa ry F u e l G as To V a c u u m S a m p le R e g u l a to r F i lt e r F la m e C h a m b e r Io n C o l le c t o r A S a m p le C a p illa ry —- H y d ro g e n C O N V EN TIO N A L FID A R R A N G EM EN T C A M B U ST IO N F R F ID A R R A N G E M E N T

Predictions of the time constant are made using software supplied by the manufacturers. The theoretical estimate takes no account o f processes occurring within the flame itself. One-dimensional air flow is assumed at all times and only an approximate correction for compressibility effects is made. A more complete analysis o f the equations used within the software may be found within the HFR400 Quick Users Guide.

If pressure fluctuations within the sampling location are more than ± 0.75 Pa, an additional damping chamber is recommended by the manufacturer in order to

minimise oscillation of the flow rate and therefore pressure changes in the flame

chamber. Pressure changes over a long time scale are removed by bleed flow

regulators, see figure 5 .2, and short time scale fluctuations are reduced by making the volume o f the damping or constant pressure chamber large by comparison to the fluctuations o f the sample pressure The minimum sample pressure is set by both the limits o f the vacuum pump and having an acceptable pressure differential between the

HFRFID vacuum chamber and the sample location. The pressure differential is

determined by flow requirements of the instrument for accurate measurement and the need to reduce the time delay. For inlet port sampling, the vacuum pump presents a limit to the minimum manifold pressure that can be used, which in practice was found to be approximately 47 kPa (absolute).

Figure 5.2

Sectioned view of the Cambustion HFRFID

14 15 1. R e m o v a b l e inozzle 2. F l a m e 3. C o l l e c t o r e l e c t r o d e 4. Lid 5. Glow p l u g 6. T h e r m o c o u p l e 7. C o l l e c t o r i n s u l a t o r a s s e m b l y 8. E l e c t r i c a l c o n n e c t o r 9. FID b l e e d 10. Vac s u p p l y 11. Air 12. CP b l e e d 13. F u e l 14. B u l k h e a d 15. B a s e p l a t e 16. R e m o v e a b l e n o z z l e 17. I n s p e c t i o n h a t c h 18. 0 r in g

A dynamic calibration unit, DCU, allows the sample probe to remain in situ during span gas calibration. This has the benefit o f ensuring that all measurements are taken

from the same location. The sample line is heated to a controlled temperature,

5.2.2 Design of cylinder head sample sites

A Ford Zetec cylinder head was used throughout the duration o f the HFRFID tests, adapted to fit an existing single-cylinder, cylinder block and crank case assembly. Bore and stroke were matched to the production engine The cylinder head was a cut-down version of a standard Ford Zetec cylinder head containing a single operating combustion chamber. The head was designed for this project and considerable work was required to develop a method of lubricating the camshafts. The intake manifold was modified to suit with the internal volume reduced by approximately one third compared to the four cylinder item. HRFFID probe sites were placed in the intake

manifold, inlet port riser and exhaust port. Measurements o f cylinder pressure,

cylinder head temperature, inlet manifold pressure just upstream o f the HFRFID sample location, and an AFR meter sensor in the exhaust, see figures 5.3 & 5.4 The option to operate a cylinder pressure bleed tapping for the AAFV was also included, see section 5.3.2. Mention should also be made that the cylinder block and crankcase assembly o f the engine had been modified to provide optical access for a previous project. This involved the use of an extended piston with a quartz piston crown. The optical access was not used for the AAFV investigations, but the piston did feature a single low-mounted compression ring. This should not have affected the intake port sampling results, but did provide an unnaturally large crevice volume for the exhaust port sampling tests. This is discussed further in the relevant section. The specification o f the test engine is contained in Appendix A5.

Figure 5.3

View of the intake port showing the HFRFID probe sites B & C

Figure 5.4

Overall view of the engine test bed

The intake HFRFID sample sites were located 30, 60 & 120 mm upstream o f the intake valve and are identified in the text as locations B, C & D. Location ‘A’ refers to the exhaust port probe. Site D was positioned 20 mm upstream o f the injector location All intake probes were positioned to align with the central axis o f the port normal to the flow direction.