Author Topic: 1.5 chain or timing belt  (Read 755 times)

culzean

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Re: 1.5 chain or timing belt
« Reply #15 on: March 10, 2020, 08:50:57 AM »
I recall the other problem with turbo engines is that the real life emissions tend to be higher than those during the controlled testing.
Also naturally aspirated engines have this "problem", even if less than turbocharged.

Isn't it the higher combustion temperature of forced induction engines ( due to higher CR and leaner mixture ) that increases the Nox emissions though ?
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

jazzaro

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Re: 1.5 chain or timing belt
« Reply #16 on: March 10, 2020, 11:37:53 AM »
The matter is complex.
NOx come from very high temperatures, this is true.
Diesel engines have a great NOx production, even if the average temperature in the diesel cycle is low (they take a lot of time for warming up, this means that cylinder walls and the head are not in contact for a long time with hot gas); the problem comes from the type of combustion because the fuel burns  in the same time in the whole cylinder space, so you have a very short moment when temperatures are very high (nox production) and a long period when temps are lower (expansion). This is also why diesel engines produce particulate matter: fuel starts burning during the knock, quite all fuel burns completely  except from bigger drops that stop burning because temperatures fall down. Common rail try to solve this problems using multiple injections, no more one or two big knocks but more little knocks, the idea is to have four or five small peaks and no more one or two big peaks, and smaller size fuel spray.
Petrol engines produces a very lower amount of nox, except from ones in the 80ies without catalytic converter. These small engines, produced by Fiat and Peugeot (Fire and TU series) used a lean burn mixture (18-20:1) to gain good economy, but they had to abandon this strategy when the european commission decided to use fuel injection and catalytic converters, that need a stoichmetric mixture 14,7:1. Thats why that kind of engines were quite bad with electronic fuel injection and cats: fluid flux, manifolds and sparkplug position were ok for lean burn, not for stoichometric run mode, so engeneers had to understand how to re-arrange the engine settings.
Petrol engines have a different combustion process from diesel, the combustion spreads from the spark to the farest part of the cylinder, average temperatures are much higher than diesel but peak temperatures are lower. NOx production is lower because only small part of time and area reach so high temperatures  to combine nitrogenum with oxigen.  This is true in low power conditions, but if you push the gas pedal then temperatures would rise quickly, too much heat for pistons, valves, manifolds, turbo and cats, so engeneers have to cool the combustion process adding fuel to the mixture; it's quite a paradox but the air-fuel mixture does not heat so much during compression thanks to the liquid fuel evaporation, this process needs energy. And considering that there is low oxygen inside the cylinder, the next combustion process will not produce more heat than a stoichometric combustion. Some turbocharged engines (as BMW) use water spray  injection inside the inlet manifold: water evaporates before and during compression and helps keeping temperatures low.
Without enriching the AF ratio,  petrol engines would break by wear because pistons, piston rings, valves and valve rings would run over their temperature limits; so part of the fuel, when you push the gas pedal, goes to to wet cylinder walls, the piston head and valves and so to cool them. This happens in every petrol engine, naturally aspirated or turbocharged, and you can check it with a simple EOBD tool following  real time the AFR number; you will see that in cruising mode the AFR will be 14,7:1 (or 1, depending by how the software shows it), but this number will fall every time you push the gas pedal more than half path (is path correct?). Clearly, if an engine runs out of the stoichometric condition, the catalytic converter will run poor because it needs a precise balance of elements, some parts of residual oxygen, some of Nox, some of HC and CO, and the 14,7:1 is the best for it; enriching the AFR, the cat will get less oxygen and more HC and CO, so emissions will raise, especially carbon oxides and unburned fuel.
That's why a 100hp 1.0t Ford has bigger real consumption than a 100hp 1.3 NA Honda; when you push the gas, the Ford engine gives more torque, but it must enrich more than the Honda; supercharged engines can run leaner only in stratified charge (direct fuel injection), but in average they run with a richer AFR.
Anyway, both naturally aspirated and  forced induction engines enrich the AFR, even if NA do it much less; the only way to limit this problem is to reduce  specific power and specific torque, and now we understand why american engines usually have big displacement and low power: a 2500cc NA engine will have less AFR enrichment than a 1.0turbo giving 200nm torque and 140hp... If USA rules want to strongly push the gas pedal during the test, a 2500 NA will be much better than a 1.0T for emissions. And this is why  hybrid vehicles like toyotas and I-MMD hondas use underpowered atkinson engines: when they run, they often do it with a widely open butterfly valve to reduce pumping losses, and since they have to run with low emissions and this needs low enrichment, they will have low specific torque and power to raise reliability and reduce emissions.
« Last Edit: March 10, 2020, 02:20:04 PM by jazzaro »

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