Cuestionario de Daniele Grasso (El Confidencial)

1.10.3.1

Intake Only

At part load conditions, the camshaft is advanced so that the inlet valve opens earlier during the exhaust stroke and the valve overlap period is thus increased causing m ore o f the burnt g a ses to flow into the inlet port to be recycled in the follow in g cycle. The inlet valve also clo ses earlier in the com pression stroke, thus decreasing the amount o f fresh charge pushed into the inlet port by the upward m ovem ent o f the piston.

S t a n d a r d T D C 1 0 10 KVO v \ c 30 Advance In creased Overlap TD C BDC Kariy In ta k e V alve O p en in g BDC

E arly In ta k e \ alve C losing

E x h a u s t - 40

i-

o

E xhaust gas backflow into intake p o rt is d raw n back into the cylinder during the intake stroke

N O , IS red u ced (internal E G R ) H C is reduced (u n b u m e d HC )

L ow er M A P is required for a given load therefore intake stroke p u m ping w ork is increased

Figure 1.23 a: Inlet only camshaft phasing [Stein et al (1996)1

1.10.3.2

Exhaust Only

The exhaust camshaft is significantly retarded at part load conditions to increase the valve overlap and hence the amount o f internal EGR. The late opening o f the exhaust valve allow s an increase in the expansion w ork (figure 1.23 b).

1.10.3.3

Dual Equal

The cam shafts are significantly retarded at part load conditions. The objectives o f this are: delayed valve overlap for increased residual dilution, late intake valve closin g for pumping w ork reduction and late exhaust valve opening for increased expansion (Stein et a l (1 9 9 6 )) (figure 1.23 c).

standard TDC 10 10 IV O E V C 30 Retard TDC Increased ovcriap I y L a teE V O Eate Exhaust Valve Closing

- 4

BDC

Late Exhaust Valve Opening

— BIX’

E xhaust gas is draw n back into the cylinder:

N O x is reduced (internal E G R ) H C is reduced (u n b u m ed HC r e -b u rn e d ) Intake pum p in g w o ik is reduced

E xpansion w ork is increased H C is red u ced due to additional oxidation during expansion

Figure 1.23 b: Exhaust only camshaft phasing [Stein et a! (1996)]

S ta n d a rd TlX R e ta r d T tK E V C IVO IVC E VO BDC BDC

t ale Kxliausi Valve O pening Delayed O v erlap Late Intake Valve Closing

E sp n isio n wolV is increased E xhnist gas U drawn back into the cyhnder

at exhaut* pressure N O , is reduced (intenial EOR) H C is reduced (uobum ed HC re-burned ) Intake p u m ping work is reduced

H i^ e r M A P is required for a given load and so intake pum ping w ork is reduced

oxidiAion during cxpanaon

Figure 1.23 c: Dual inlet camshaft phasing [Stein et al (1996)[

1.10.3.4

Dual Independent

This metho(i is essentially a refinement o f dual equal retard. B oth cam shafts are significantly retarded but the overlap is variable rather than fixed

The effects o f valve timing on em issions and pumping work at lo w speed and low/part load are summarised in table 1 7

Event Effect _B(£lanatiOTi_

E arly E \ O In creased HC in c re a se d N O .

Increased fuel consu m p tio n C O u n c h an g ed

L ate E \ O S lig h t in c re a se in N O . N o effect on H C an d C O E a rly E \ ( D ecreased HC

D ecreased N O .

C O & fuel c o n su m p tio n un ch an g ed L .a teF A C D ecreased HC

In cre a se d N O , (fo r m odest retard ) C O & friel co n su m p tio n un ch an g ed E a rly IV O D ecreased HC

D ecreased N O .

C O & fiiel c o n su m p tio n u nchanged L ate IS O In c re a se d N O .

H C , C O & FC luichanged E arK IV C ' HC N O , C O & FC un ch an g ed L ate l \ C ’ H C N O C O & FC un ch an g ed

P rem a tu re release o f c o m b u stio n p ro d u c ts

H ig h er p eak cy lin d er te m p c ra tm e &

p re s su re

W id e r tiiro ltle op en in g to m ain tain p o w er

R eten tio n o f H C -ric h s c ro ll-o ff g a s In c re a se d resid u al g as fractio n R e -in tro d u c tio n o f H C -ric h s c ro ll-o ff g as

L o w er resid u al g as frac tio n R etention o f H C -ric h s c ro ll-o ff g a s b y re-in tro d u ctio n

In creased re sid u al g a s fractio n L ow er re sid u al g as fractio n

O n l\ sm all d ev ia tio n s fro m s ta n d a rd tim in g s w ere in v e stig a te d N O . m ig h t be ex p e cted to d ec re ase s h g h tb

As I . fuel co n s u m p tio n sh o u ld also fall b ec au se o f re d u ce d p u m p in g losses

Table 1.7: Effects of valve timing on emissions and pumping work at low speed and low/part load [Seabrook (1995))

1.10.4

Lean Burn Engines

Lean burn engines normally u se com pact com bustion chamber designs (figure 1 2 4 ) and require a sufficient amount o f in-cylinder turbulence to allow the leaner mixtures to be burnt before com bustion quality deteriorates to unacceptable levels and em ission quantities increase. The com pact com bustion chamber design shortens the flame propagation distance and, coupled with high turbulence, the ignition delay and com bustion duration are reduced

•

r

m

Figure 1.24: Compact combustion chamber design |H orie et al (1992)]

Lean burn engines have the capacity to decrease em ission levels as w ell as being m ore efficient This is partly b ecause leaner mixtures require less throttling since the lean AFRs are realised not by reducing the amount o f fuel, but rather by increasing the amount o f air The con seq u en ce is a reduction in pumping lo sses In addition, m ore favourable values o f y (ratio o f principal specific heats) are realised and, together with the higher com pression ratios which may be utilised, lead to an increase in cycle efficiency as predicted by equation 1.2.

Lean burn engines require the charge to be as h om ogen eou s as possible ‘in order to gain the full benefit o f lean com bustion in terms o f N O x reductions and g o o d driveability’ (Hardalupas et a l (1 9 9 5 )). Unfortunately the w idespread use o f the lean burn engine has been inhibited due to difficulties in achieving legislative limits. The TW C cannot be used (section 1 .9.3) and it is proving difficult to develop an alternative catalyst for operation with lean mixtures.

In the absence o f a lean N O x catalyst, the ob viou s w ay o f achieving N O x and fuel econom y levels is to d evelop a com bustion system with far greater E G R tolerance than at present, operating at stochiom etric and hence allow ing the u se o f a TW C to m eet em ission levels One possible means is to stratify the EG R (S to k es et a l (1 9 9 4 )). A general introduction to stratified charge engines is given before this is discussed.

1.10.5 stratified Charge Engines

An alternative to the conventional spark ignition engine is the stratified charge (S C ) engine H ere the fuel distribution within the cylinder is non-uniform , w ith a rich but ignitable mixture in the vicinity o f the spark plug and a leaner non-ignitable mixture o f air or exhaust gas in the remainder o f the com bustion chamber S ton e (1 9 9 4 ) lists the follow in g advantages o f a stratified charge engine:

1 ) L ow er exhaust em issions than conventional SI engines due to lo w N O x levels at lean mixtures (figure 1.7).

2) Improved efficiency since throttling lo sses are reduced