PANELES SOLARES FOTOVOLTAICOS

Singlepoint injection systems require the injector to be positioned centrally, or slightly off centre, above a throttle-plate [41, 78]. The tim ing of the injection pulses is usually once every 180* crank angle (CA). This reduces the variation in AFR betw een cylinders to a m inim um , as show n by Knapp and Lembke [41], and Takeda et al [78]. Takeda et al found injection at TDC of each cylinder to give the best phasing of the injectors, w hilst K napp and Lem bke sim ply used the ignition signal for each cylinder to provide the injector triggering.

M u ltipoint system s m ay be m uch m ore flexible. Typically, how ever, the injectors are positioned so th at they fire on the back of the inlet valve. The reason for this is to utilise the hot surface to evaporate as much of the im pinging fuel as possible. As pointed o u t by Nogi et al [64], electronic m ultipoint injection has usually in the p ast been phased sim ultaneous injection, w hereby all the injectors are fired at once, d u rin g some optim um w indow ' of crank angle. This gives problem s in trying to phase the injectors w ith respect to each inlet valve event. Increasingly complex ECU's often allow sequential injection, w hereby each injector

operates separately at some defined angle relative to TDC of the appropriate cylinder. This angle m ay be 'm apped' for various engine operating conditions or tem peratures.

Nogi et al [64] carried o u t some tests in o rd er to establish a relationship betw een fuel injection tim ing and HC emissions. They also found th at (regardless of engine speed and m anifold depression) the HC em issions w ere alw ays m uch higher w hen injection occurred d u rin g the FVO period. They considered this to be d u e to a lack of com plete evaporation of the fuel at the inlet valve, allow ing th at fuel into the cylinder and onto the cylinder walls.

Bandel et al [7] m ade extensive com parisons of injection tim ing o n a single cylinder, two valve, high com pression fast b u m (HCFB) e ig in e . They found that w h en using sequential injection, injection early in the FVO period caused increased IMEP variation, HC emissions and ISFC, w ith a reduction in NOx. This backs the findings a n d conclusions of Nogi et al. H ow ever, Bandel et al found that injecting com paratively late in the intake stroke had the opposite effect. They recorded reduced IMEP variation and reduced ISFC, w ith increased NOx emissions, and HC emissions around the sam e level as for injection onto a closed valve. They related these results to im proved com bustion d u e to a reduced ignition delay and, therefore, a shorter combustion duration.

They felt th at these effects w ere d u e to charge stratification, w hich is d e p e n d an t on injection tim ing. It w as postulated th at w ith swirl assisted com bustion system s, if fuel is injected w ith the inlet valve closed, o r early in the intake stroke, th en the- charge is stratified, w ith rich m ixtures near the piston, a n d lean m ixtures n e a r the spark p lug (a com puter m odel was used which backed up this theory).

Injection late in the intake stroke leads to richer m ixtures in the vicinity of the spark plug, im proving com bustion stability and reducing fuel consum ption. In a d d itio n , air intake velocities are also high, w hich can im prove the m ixture prep aratio n and contribute to the favourable charge stratification.

Bandel et al next tried to com pare different injector types a n d positions, w ith injection tim ing optim ized for lowest HC emissions. Four injector configurations w ere used.

1) Injector w ith air assist, m ounted in intake manifold. 2) Injector w ith air assist, m ounted in intake port. 3) Pencil spray injector, m ounted in intake port. 4) Cone spray injector, m ounted in intake port.

In general, the air assisted injectors perform ed the best (particularly the one m ounted in the m anifold), w ith m ost stable com bustion, highest lean lim its and low est ISFC. This w ould point to the desirability of small droplets, in order to give the best perform ance. W orthy of note though, is the decreasing influence of injector type w ith increasing speed.

Nogi et al [64] also attem pted to com pare various injectors, in this case to m inim ise HC emissions caused by injection d uring the FVO period. The m ost successful results w ere obtained using a pintle injector aim ed at an im paction target. They believed this type to produce small evenly distrib u ted droplets. The results in this case show ed very little difference in HC levels betw een injection during the FVO and FVC periods. Transient changes w ere m ade to the

AFR of the m ixture supplied to the engine, and the target im pact injector again gave the best results, w ith both lowest HC emissions and shortest response time. Again this points to the desirability of sm all droplets.

Bandel et al [7] investigated transient operation on a four cylinder engine. They noted that w hilst u n d e r steady state conditions, sequential injection d u rin g the FVO period prim arily im proved fuel consum ption, injection tim ed for m inim um storage time in the ports gave overall em ission advantages. How ever, d u rin g transients, injection d u rin g the IVO period avoided excursions to overlean m ixtures, thus m inim ising em ission deterioration. Some benefits (in steady state) w ere seen w ith dual sequential injection, fitted to a single cylinder engine. This used fuel storage in the intake p o rt to produce a very lean hom ogeneous initial charge, followed by second injection late in the IVO period, to produce a distinct charge stratification. This im proved com bustion stability, NOx emissions and ISFC, com pared to results achievable w ith single sequential injection.

Shayler [75] investigated the effect on emissions, of changing injector position and injection tim ing, on a single cylinder engine u n d er steady state conditions. His w ork w as based on a series of theories and hypotheses, backed up by some experim ental w ork. H e postulated that changes in em ission trends could be predicted by considering the variatio n s in charge hom ogeneity, cmd associated combustion effects d ue to these changes.

W hen the injector w as m oved further upstream , trends of decreasing NO x and HC, w ith increasing CO w ere observed. The changes in HC and CO w ere suggested as being d u e to the increased residence time of the m ixture. W hen injection occurred d u rin g the inlet valve open (IVO) period, the values of HC and CO tended to be high, w ith low values of NOx. It was su g g ested th a t these trends m ay have been d u e to large d ro p le ts a n d b u lk charge inhom ogeneity, causing charge cooling and incom plete com bustion. W hen injection occurred during the inlet valve closed (FVC) period, then these trends reversed.

An in terestin g p o in t to note is th at Shayler felt th at as the p o in t of injection m oved upstream (and thus residence tim es increased), the em issions becam e less sensitive to both injection tim ing and m ixture quality. Evidence w as found of the former, how ever, m oving the injector upstream will alm ost certainly degrade transient performance.