New emissions testing to revert engine downsizing trend...

[rjsa]

http://mobile.reuters.com/article/idUSK ... ce=twitter

[Cold Fussion]

So why does a larger swept capacity allow for tighter emissions control under this testing regime? Tangentially, is there a study out there somewhere there that analyses the emissions reductions of targeted emissions reduction policies of ICE's vs a reduction in emissions of increasing engine BSFC/ increased overall vehicle fuel economy? I imagine designing engines for emissions control intimately hampers their fuel efficiency.

[rjsa]

VW is replacing its 1.4 liter three-cylinder diesel with a four-cylinder 1.6 for cars like the Polo, they said, while Renault is planning a near-10 percent enlargement to its 1.6 liter R9M diesel, which had replaced a 1.9-litre model in 2011.

In real-driving conditions, the French carmaker's 0.9-litre gasoline H4Bt injects excess fuel to prevent overheating, resulting in high emissions of unburned hydrocarbons, fine particles and carbon monoxide.

Cleaning that up with exhaust technology would be too expensive, sources say, so the three-cylinder will be dropped for a larger successor developing more torque at lower regimes to stay cool.

From the article.

[Cold Fussion]

That's still very vague. It's not unusual for an engine to run richer than stoichiometric.

[Tommy Cookers]

the article seems very interesting
one can believe that manufacturers improve driving appeal in 'off-test' running states without generating a 'defeat device' VW-type legal liability

surely the legislators are satisfied with their corporate-average approach to fuel/CO2 economy ?
judging by their enforced fraudulent presentation of overall emissions and economy of PHEV/EVs etc (boosting corporate average)
these will escape the air quality researchers debunking (of vehicular tailpipe emission rules)


but, to the point in hand .....
so-called 3 way catalysis means running in a standard rapid cycle between slightly lean and 4% rich
ie traditionally the cat is sized to oxidize a quantity of CO and HC that is quite closely proportionate to the air massflow

when the engine is run significantly richer than this (at highish massflow) there will be much additional CO and HC .......
to oxidise this the cat would generate more heat and so need to be oversize to avoid overheating and damaging itself
conventionally sized it will ultimately allow the additional pollutants to go to atmosphere
coping for a while on oxygen stored from standard cycle running, but failing once this stored oxygen is depleted (by rich running)
(though NOx reduction adds stored oxygen (and cools the cat ??) the engine is not always generating NOx)

rich running allows higher boost by deterring detonation via a small benefit to combustion cooling and benefit to combustion chemistry
though particulates increase disproportionately with richness, and will pass through the cat

[Brian Coat]

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.

[Andres125sx]

Interesting article, thanks

Some questions from an ignorant:


What´s the reason for the overheating of the smaller engines wich results in injecting more fuel only for cooling purposes?

There´s no other way to cool down the engine?

Any reason going to twin cilynder does not solve it as the cilynders will be bigger?

[Brian Coat]

I'll try to answer your cooling question.

It mostly relates to Gasoline engines.

The combustion cooling referred to above is direct cooling of the air/fuel mixture using the latent heat of vapourisation of the fuel itself. This is done by using a 'rich' mixture; that is to say by injecting more fuel than is required for complete combustion of the air.

The benefit gained is a reduction in the temperature of the as-yet unburned fuel air mixture, sitting behind the flame front proceeding from the vicinity of the spark plug, across the combustion chamber. This mixture is referred to as the end gas, because it gets burned at the end.

Direct end-gas cooling is beneficial becuase it can prevent the on-set of knock, which is spontaneous auto-ignition of the end gas, which occurs above certain end-gas temperatures and pressures. Knock is undesirable because it can damage the engine - terminally.

The temperatures and pressures in the end-gas are higher at higher torque levels, so engine makers tend to run a stoichiometric air fuel ratio at low to medium loads (in order to allow the catalyst to work) but a rich mixture at high loads (to prevent knock).

The downside to the fuel cooling trick is that it uses a lot of fuel and the catalyst does not work when the mixture is rich so the tailpipe emissions go through the roof.

This phenomenon is not unique to downsized and boosted engines but (1) the fact that they are boosted tends to give them higher end-gas temperatures even when the charge intake air has been cooled by a heat exhanger. (2) For a given vehicle acceleration, the smaller boosted engine will be running at a higher specific torque; so the engine is perhaps more likely to get into the rich mixture zone than a bigger 'lazier' unboosted engine.

The rich mixture also lowers the exhaust gas temperature and the catalyst temperature, which improves catalyst durability. Sometimes on a boosted engine, the WOT AFR will be determined by the need to manage the catalyst temperature to a level low enough to achieve emissions control system durability - the catalyst is often very close to the engine, to promote reaching operating temperature as soon as possible after cold start.

Your cylinder number question: Whether or not reducing the number of cylinders, for a given engine size would make matters better/worse, would depend on how you did it; but in general, things which delay complete combustion have the potential to make things worse knock-wise. It's not really a fix, I fear.

Having said all that, high load enrichment is only one reason downsized boosted engines are not so great in the real world as on the euro test cycle.

[Tommy Cookers]

well at the moment I think that ..........
the cooling effect of rich mixture may be overestimated in some people's minds - since all the fuel cools, not just the 'rich' few %

dissociation is a bad thing, it is strongly driven by combustion temperature, and so maxes at stoichiometric fuelling (of a boosted engine)
richening helps in part by cooling but strongly by chemical effects on combustion and deterrence of the suplus fuel to the dissociation
these effects were established with aviation fuel rating in the 1930s and are apparent in motor fuels tested using aviation (supercharge) procedures
so must to some extent apply to our subject engines here

richening reducing dissociation gives a cooler exhaust (so a cooler catalyst) because more heat has been converted to work

btw
dissociation means some carbon dioxide partway through the stroke changes to carbon monoxide and oxygen and absorbs much heat....
then later in the stroke as expansion causes further cooling, reassociation changes these back into carbon dioxide and emits the heat

[Andres125sx]

Thank you both but Brian, I didn´t understand this

Your cylinder number question: Whether or not reducing the number of cylinders, for a given engine size would make matters better/worse, would depend on how you did it; but in general, things which delay complete combustion have the potential to make things worse knock-wise. It's not really a fix, I fear.

You mean readucing number of cylinders delay complete combustion? If so, what´s the reason?


I was thinking about a very basic example, if a four cylinder 2.0 engine works ok, any reason a twin cylinder 1.0 don´t work as well? Cylinders are exactly the same

[Brian Coat]

Apologies, we had our wires crossed - I thought you meant huge cylinders and same total swept volume.

[Andres125sx]

No need to apologize

So a 1.0 twin cylinder should be ok?


Some time ago someone at F1T told me F1 engines can´t be downsized much more as there´s a limit, I wonder if this limit is related to the one being dicussed here wich causes increased emissions

[Brian Coat]

well at the moment I think that ..........
the cooling effect of rich mixture may be overestimated in some people's minds - since all the fuel cools, not just the 'rich' few %

dissociation is a bad thing, it is strongly driven by combustion temperature, and so maxes at stoichiometric fuelling (of a boosted engine)
richening helps in part by cooling but strongly by chemical effects on combustion and deterrence of the suplus fuel to the dissociation
these effects were established with aviation fuel rating in the 1930s and are apparent in motor fuels tested using aviation (supercharge) procedures
so must to some extent apply to our subject engines here

richening reducing dissociation gives a cooler exhaust (so a cooler catalyst) because more heat has been converted to work

btw
dissociation means some carbon dioxide partway through the stroke changes to carbon monoxide and oxygen and absorbs much heat....
then later in the stroke as expansion causes further cooling, reassociation changes these back into carbon dioxide and emits the heat

I think I'm missing something here. 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?

I may have wrong end of stick about what you meant.

[Brian Coat]

Andres125sx:"Some time ago someone at F1T told me F1 engines can´t be downsized much more as there´s a limit"

Absent concerns about rules, regulations, efficiency-at-small-scale, emissions, etc., one could argue that there is no limit at all ... by saying that highest power density engine will have zero bore and stroke and is called a gas turbine.

[J.A.W.]

Trouble is, small gas-turbines suffer from poor efficiency due to scale, opposite to SI piston engines,
& as potential F1 mills, thus cannot compete on the currently needful BSFC/fuel limitation factor.

F1 itself, while now concerned with engines eking out maximum power from every drop of juice,
are not intended to (& certainly do not) - comply with road-reg EPA-type emissions protocols,
- which are in themselves, more of a political/industry choice, than being solidly based on scientific harm reduction grounds...