New emissions testing to revert engine downsizing trend...

[Tommy Cookers]

In the context we are discussing (over-fuelling in downsized+boosted engines to avoid knock?), I am not following how the AFR/dissociation relationship would affect anything, given that the end-gas is essentially uncombusted charge and the dissociation occurs during actual combustion?

dissociation (before combustion of end-gas) makes the end-gas cooler before its combustion than it would have been without dissociation ?
so the detonation-limit boost would be higher than without dissociation ?
(ok it seems unreasonable if I have said that by chemical effects richness actually reduces dissociation)

aviation type fuel ratings summarise the gain in limit-boost realisable by using richer mixture but a high and fixed charge temperature
typical gain of the usual (high-aromatic) Avgas is 30% ie 100/130 rating
'default' motor fuel ie without significant aromatics shows eg 9% gain (that's really what the 80/87 Avgas spec is)

my point - fuel richness chemical effects on limit-boost are significant, the benefits of richness should not be attributed only to temperature


it would be very interesting to have more technical information on the reported emissions issue of these downsized engines
the easy suggestion is to lose the richness and a few of the peak bhp, and to comply with the intended more realistic emissions tests

imo
the downsized engines have torque/rpm characteristics inadequate for pleasant and good controllability at typical speed eg around 30 mph
this and the deficient transient response cannot be fully cancelled by the software-shaped relationship between accelerator and throttle positions
part-time electric supercharging is the necessary next step, if downsizing is not abandoned

these long-stroke, highly-boosted downsized jobs seem a bit new and shocking, though I have as above criticised the Ecotec in the past
can eg the cheap Dacia buyer still choose between a 90 bhp downsized 3 cylinder or an older-design 4 cylinder engine ?
the more conservative 4 might be the better choice

[pgfpro]

Here is what I think.

It's not only about full load enrichment.

I'll talk about gasoline engines but the Diesel factors are similar.

The downsized boosted engines get favourable certified CO2 and emissions from the fact that NEDC is a much lighter load cycle than the real world.

Downsized boosting gives most benefits at light vehicle demand loads, where the friction+pumping benefit of a small engine has the biggest impact on BSFC.

Even before it hits any f/l enrichment a downsized+boosted engine's BSFC benefit will fall with load.

The CR is lower than N/A and could possibly need to be even lower if some enrichment is presently being used to mitigate knock, off-cycle.

In the real world a highly loaded downsized-boosted engine the peak combustion pressures could well be higher than N/A, for a given vehicle load demand, which gives higher BSNOx.

Here is an article which shows the real world vs. cycle fuel economy effect.

http://emissionsanalytics.com/beware-th ... ownsizing/

That's not super-coherent but it is more or less correct, I think.

That said - the downsized, boosted engine is not dead by a long way.

Brian I think your spot on.

The CR is lower than N/A and could possibly need to be even lower if some enrichment is presently being used to mitigate knock, off-cycle.

I think manufactures will be lowering their CR in the future and using the newer advance turbos to make up the difference in power. There should be no reason for the manufactures to have to have fuel enrichment in a 1.5L or larger size engine.