UBICACIÓN Y MONTAJE DE LOS ELEMENTOS

The need for swirl and turbulence in the charge entering the cylinder has been dem onstrated. It w as also m ade clear that increasing these w ill usually resu lt in a loss of volum etric efficiency.

W ith sin g lep o in t system s, the m easures available for tu n in g are norm ally lim ited to varying the length of tiie intake runners a n d /o r using balance pipes betw een branches.

W ith m u ltip o in t injection, how ever, th ere is a m uch g re a te r flexibility. V arious arrangem ents of intake pipe lengths, plenum cham bers, balance pipes and throttles are possible. Lenz [45] believed that tuned intake system s could be used to im prove not only m axim um perform ance, b u t also low speed torque. H e stated that, on average, SFC could be reduced by approxim ately 5% by appropriately designed system s although he d id not specify u n der w h a t operating conditions. This is an im portant benefit of m ultipoint injection not yet considered in this w ork.

Long valve overlaps are being used to increase high speed charging efficiency, w ith a consequent increase in residual gases entering the intake system at low speeds. This is a topic

w hich increasingly needs to be addressed, w ith valve o r p o rt th ro ttlin g being a possible solution.

W hat is em erging is the use of cylinder heads w ith 4 valves per cylinder and m ultipoint injection. These can then take advantage of the large p o rt areas for gas exchange, w ith the flexibility to allow complex manifolds to gain the m axim um benefits.

l J t l 2 Four valves per cylinder - a special case

The use of 4 valves per cylinder is becom ing increasingly w idespread in the search for high perform ance fast b u m engines w ith low emissions. Four valves p er cylinder is not a new concept, b u t it is only recently th at it has become econom ically attractive. The increase in com ponents (obviously double the n um ber of valves, often n e ed in g tw o cam shafts) and increased complexity, is being offset by better d esign/m anufacturing, and m arket dem and to im prove engine performance beyond the limits of 2 valve per cylinder designs. Benefits can be gained from the increased port area available, and also from optim ized pent-roof com bustion cham bers, w ith centrally located spark plugs.

Fraidl, Mikvlic and Quissek [25] considered developm ent strategies for such engines. They singled out three key features of 4 valve engines.

1. Flat torque characteristics w ith favourable low end behaviour and h ig h pow er output. 2. Low engine-out em issions (that is em issions p rio r to passing th ro u g h any catalytic converter, etc), u n d er steady state and transient operating conditions.

3. Part load fuel economy im provem ent potential of stoichiometric tuned engines.

They p ointed o u t the m ain areas being developed to gain im provem ents in the above features.

1. Variable gas exchange systems.

2. Combustion systems for improved fuel economy. 3. Engine m anagem ent system advances.

The paper w as aim ed at showing the potential of a 4 valve, controlled b u m rate combustion system . O ne of the m ain features of the paper w as to outline the use of variable intake system s to give high pow er at high speeds, and also high torque a t low speeds, by im proved volum etric efficiency. Examples of the system s p u t forw ard are: variable ram pipe length, v ariable valve tim ing and d u al intake run n ers w ith p o rt deactivation. The last option involves having an intake runner to each of the two inlet valves, w ith one ru n n er being shut off, using a butterfly valve, as necessary.

M ore im portantly (for this work), Fraidl Mikvlic and Q uissek pointed o u t a serious problem th at can exist w ith the m ixture preparation of 4 valve engines. D ue to the large intake p ort areas the gas velocities at part load can be very low (if no port deactivation devices, etc are fitted). This m akes these engines particularly susceptible to poor initial m ixture quality, as there is little assistance from the gas m otion at these conditions.

This problem w as considered by Fujii, Aoyama and M anabu [27]. Their approach w as to use tw in intake runners, w ith a port deactivation (DEAC) valve in the secondary of these two

runners. The object of this w as to increase the velocity of the m ixture (thus im proving its m ixing and turbulence), by effectively shutting off the secondary inlet valve at low engine speeds. At the sam e tim e they m oved the injector from its previous position (spraying at the partition w all betw een the valves), to a new location, central in th e prim ary intake port, spraying at the valve itself. They carried o u t steady flow rig tests, w hich indicated swirl resulting from the use of the DEAC valve. In addition, d u rin g engine tests, the lean lim it was increased by 2 AFR's, witii im proved com bustion stability a n d a shorter com bustion period. It w as felt th at the DEAC valve also reduced the backflow of exhaust gases into the intake system, which w ould have a beneficial influence.

W hen the DEAC unit w as progressively opened, the H C em issions w ere seen to increase. They suggested that this was d u e to the low ering of the gas velocity on the prim ary side, back to its previous value, although it is possible th at charge stratification occurred (w ith fuel entering only one port). It w as found, how ever, th at HC em ission levels could be reduced (with the DEAC valve open), by using a w ider spray injector w ith a sm aller fuel drop sizes. This perhaps suggests that high air velocity is less im portant w ith sm aller initial drop sizes.

In a d d itio n to the results already noted, Fujii, A oyam a and M anabu used h igh speed photography to observe the com bustion process. W ith the DEAC valve closed, very little fluctuation in flam e p ro p agation w as seen. H ow ever, w ith the valve open, diffusion com bustion w as occasionally noted, and w ith the original injection arrangem ents diffusion com bustion alw ays occurred. The reason given for diffusion com bustion w as that m uch of the fuel w as able to a d h e re to th e w alls (p a rtic u la rly to the p a rtitio n in th e original configuration), allowing unmixed fuel to flow into tiie com bustion chamber.

This increase in w etted surface area, due to the use of one injector for tw o ports, aim ed at the central partition, has been recognised as another problem w ith 4 valve engines. The result of this configuration is often poor transient response at low speeds.

Iw ata, Furuhashi and Ujihashi [40] described the developm ent of a tw in spray injector to overcom e this problem . This allows one spray to be aim ed at the back of each valve, giving a sim ilar arran g em en t to that found on 2 valve engines. Prior to a d o p tin g this approach, how ever, they attem pted to use a single sp ra y air assisted injector. In this particular application the reduced fuel droplet sizes produced d id n ot give a significant im provem ent in transient response, over the single spray pintle type injector. The low gas velocities present (as no DEAC valve, etc, was fitted), m ay have been p a rtly to blam e, by still allow ing significant deposition of fuel on the w alls, despite the sm all dro p sizes, as the injector was aim ed at the partition wall. H igher air velocities m ay have m ore readily entrained these droplets.

Fraidl, M ikvlic and Q uissek [25] used tw in sp ray injectors to o p tim ize the m ixture preparation to their 4 valve engines (this is also reported by Bandel et al [7 ], the w ork being from the same program m e). These authors combined the two spray injection, w ith a system of p o rt deactiv atio n . They aim ed to tran sfer som e of the beneficial aspects of charge stratification, observed on 2 valve HCFB engines [7] (outlined in section 1.7.9), to 4 valve

HCFB engines. They used one neutral port (with air m otion directed d ow nw ard) a n d one tangential port (which created swirl) for each cylinder. The airflow through the n eutral p ort w as then deactivated w hen high swirl w as required. This created charge stratification by causing a rich m ixture to pass through the neutral port, w ith a lean m ixture th ro u g h the tangential port. Favourable results w ere produced, w ith a high tolerance to EGR (up to 20% w ith less th an 5% IMEP variations). This gave a low em ission engine w ith very stable combustion.