I'd like to see some evidence of that, because I find the claim highly dubious. Even if we accept that petrol engines "always" wear quicker than diesel engines, by what mechanism does that "wear" increase NOx production to or beyond diesel levels ? NOx production difference between petrol and diesel engines is largely due to the fundamental operating condition differences of petrol and diesel.Paul-R wrote:Me too. I remember some ten or fifteen years ago that it was reported that diesel engines had higher NOx emissions than petrol when the engine was new but that, because a petrol engine wears quicker than a diesel one, over the whole lifetime of an engine a petrol one emitted more NOx than a diesel.white exec wrote:Do wonder, too, about the 'whole life' emissions of these systems, i.e. total output resulting from normal driving and regeneration/cleaning.
NOx production favours high temperatures, high pressures, and abundant oxygen supply. A diesel engine combustion chamber runs at higher temperatures than petrol, much higher pressures, and runs very lean any time you don't have your foot planted to the floor. Air flow is not throttled on a diesel so a light throttle means a very lean mixture with loads of oxygen and nitrogen present in the cylinder.
Add the high pressure and temperature and you get a lot of NOx produced. This is a fundamental limitation of the diesel combustion cycle - you can only lower NOx by lowering cylinder temperatures and oxygen/nitrogen availability (EGR) or filtering it out post combustion using something like SCR. The EGR valve blanking that everyone seems to love to do will cause the NOx emissions to go way up in a diesel as it is primarily there for NOx reduction.
A petrol engine on the other hand has a throttle plate to restrict air flow, (thus reducing oxygen and nitrogen availability when only a small amount of fuel needs to be burnt) never runs lean (not on purpose anyway) and either runs at stoichometric when cruising, (carefully fine tuned with the oxygen sensor) rich when accelerating, or with no fuel injected at all when coasting on the overrun. You can see typical NOx production on a petrol engine as a function of mixture here:
This graph shows the raw output of the engine before the 3 way cat. The higher levels of NOx produced by a lean mixture don't occur in practice because the engine doesn't operate in this region.
Because petrol engines don't produce anywhere near as much NOx as a diesel engine. No special emissions control equipment needs to be fitted to a petrol engine to easily out perform a diesel in NOx emissions, even a diesel with NOx reduction emissions equipment.I wonder whether that is still true and, if so, why aren't petrol engines being chased to fit AdBlue systems as well.
Just look at the Euro standards for NOx for passenger petrol and diesel engines:
https://www.dieselnet.com/standards/eu/ld.php
Although Euro 6 requirements are very tight for both petrol and diesel, look back to for example Euro 3, set in the year 2000 - the limits are 0.5 g/km for diesel and 0.15 g/km for petrol. Why ? Because a petrol engine of the day could easily meet 0.15 without any emissions control equipment (beyond the standard 3 way cat and oxygen sensor) while a diesel engine of that era could not feasibly meet 0.15 without loads of added emissions equipment that wasn't standard fit at the time.
Diesels are catching up to petrols for NOx performance but only by adding loads of additional emissions controls systems, which until recently people saw fit to remove/disable to save themselves a few quid...
Now the chickens are coming home to roost as the law starts to clamp down on such activities.While a petrol engine typically produces more CO2 (which is an unavoidable product of hydrocarbon combustion) than a diesel due to being less efficient in terms of MPG, there is no question that the exhaust of a petrol engine with a 3 way cat and an oxygen sensor is "cleaner" in almost every other single regard than a diesel, both because the raw output of the engine before emissions devices is so much cleaner, and also because its possible to use a 3 way cat on a petrol engine while a diesel is forced to use a 2 way cat (due to running lean all the time) and a conglomeration of other emissions devices and techniques. Those that think otherwise are deluding themselves.
There is no "hidden cost" in emissions control equipment of a petrol engine like there is with a DPF etc on a diesel that need periodic repair or replacement - in theory a 3 way cat in a petrol engine can last forever unless the engine has a major fault that causes lots of raw unburnt fuel to be dumped into the exhaust overheating the cat - but these days quality of cats is sufficient and ECU's are smart enough to detect problems and shut off fuel supply to a faulty cylinder that this is a rare occurrence and would only happen if a driver knowingly drove the car hard and long with a severe misfire.
