Emissions Tuning Fundamentals: Tuning For Emissions

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Tuning For Emissions

09.55

00:00	Because we need to know when we could expect the clearance to be at the minimum and check under these conditions.
00:06	If we take another look at the valve timing events as the engine moves through the cycle, it should be obvious that the potential danger zone is when the piston is near TDC and the inlet valves and exhaust valves are both open.
00:19	Usually for the exhaust valves, the danger area is approximately 10° before top dead centre as the piston is chasing the valves closed.
00:28	Anything that results in the valves being opened further at this point in the engine cycle will reduce that clearance.
00:34	It hopefully should make sense that retarding the exhaust cam will reduce this clearance and therefore we'd want to check the clearance with the cam in its most retarded position.
00:44	The inverse is true for the intake valves though and the danger area here occurs around about 10° after top dead centre.
00:52	Here the valves are opening and now they're chasing the piston down the bore.
00:56	As with the exhaust valves, anything that results in more valve opening at this point will reduce the clearance and advancing the intake cam will have this result.
01:05	For this reason, we want to have the intake cam at full advance while confirming clearance.
01:11	While it's hard to give absolutes, in this module we're going to cover lots of specific tips and practices valuable in real world tuning scenarios when you're trying to produce a clean calibration.
01:22	We've talked about EGR, reducing combustion temperature and with that in mind, remember it should be used in conjunction with leaner air fuel ratios that would otherwise create unwanted NOx emissions.
01:33	Typically this will occur during light to moderate load operation.
01:36	Next, under all conditions, be mindful to avoid cat overheating.
01:40	We've already mentioned that this can be catastrophic but how do we stop it? On a diesel engine, this involves reducing fuel mass or increasing air mass to run leaner which will then cool the exhaust gas temperatures.
01:52	On a gas engine, this typically involves applying additional enrichment but that's not the full story though.
01:58	While enrichment can help cool a catalyst, if there's enough oxygen available, adding additional fuel results in additional catalysation which actually creates more heat inside the cat.
02:09	At high airflow and engine load, we've seen cat core temperature and exhaust pressure increase significantly due to additional enrichment.
02:17	Then after the excess oxygen in the exhaust is consumed, the additional enrichment may have the cat cooling effect that most people would traditionally expect.
02:26	We've talked about using EGD probes and as a reminder, they are critical in monitoring the cooling impact of EGR as well as keeping the cats alive.
02:35	On the opposite end of the spectrum are cold start and warm up conditions.
02:40	Here we want to heat cats up as quickly as reasonably possible in order to get them working efficiently.
02:46	One way to achieve this is with stratified injection which is a tool employed on direct injection systems to satisfy normal engine combustion first, then exhaust combustion with a second fuel pulse.
02:59	This all happens in the same stroke and the second smaller shot of fuel occurs quite late in the cycle, causing rapid heating and expansion in the exhaust system.
03:08	Many gasoline DI systems today aren't capable of stratified injection while driving because the injection pressure isn't high enough to overcome the combustion chamber pressure that late in the compression stroke.
03:19	At idle though, the chamber pressure is much lower and the fuel pressure can therefore overcome it.
03:24	Stratified fuel injection is often combined with lots of airflow and retarded ignition timing.
03:29	Having more oxygen available and moving primary combustion later in the cycle helps to increase exhaust temperature and mass flow in order to really get the cat cooking.
03:39	This is usually audible during the first 10-20 seconds after cold start where the exhaust has a louder, more aggressive note and the idle speed is held higher before everything returns to a quiet normal idle.
03:51	On some vehicles, an air pump system is also employed at this time, providing additional oxygen straight into the exhaust to increase the potential for oxidation in the cats, which again increases cat core temperature.
04:03	Be warned though, lambda readings taken during air pump operation will appear extra lean but this doesn't reflect the real result of combustion.
04:11	The sensor is just seeing combustion results plus the additional oxygen supplied into the exhaust by the air pump system.
04:19	It's like reviewing wideband data when you have a big exhaust leak near the sensor.
04:23	The reading looks super lean but that's not what's actually happening inside the combustion chamber.
04:28	If you have the option, you can briefly disable the air pump, dial in fueling to hit targets, then re enable the air pump and trust that the OE air pump corrections will handle the rest.
04:38	With the next emissions control strategy, we've already touched on how the EVAP system injects stored fuel vapours into the intake track and they then become part of combustion.
04:48	While this extra fuel is being provided, the ECU attempts to quantify its fuel mass, often based on charcoal canister pressure and other factors.
04:56	In order to avoid overfueling the engine, while this additional fuel is being provided, the ECU reduces the fuel mass being delivered by the injectors in proportion.
05:05	This is often handled by systems that you won't have access to in reflash software, and these should be left alone anyway.
05:12	The OE calibrators spend a lot of time on this and there's no need for us to reinvent the wheel.
05:17	That said, you may notice fuel trimming acting over a broad range and less predictably while the EVAP system is purging.
05:24	On any system where you can datalog EVAP system behaviour, we'd suggest doing so and that'll help you best understand the system operation and optimise your tune.
05:33	Next, as you've likely noticed, modern vehicles are more likely to exhibit something called rev hang, where the drop in engine speed is delayed after coming off the throttle.
05:42	The problem is, transitioning from combustion to overrun in a manner that's optimal for economy and emissions isn't necessarily ideal for drivability.
05:52	While some reduction in airflow during this time may be possible without a negative emissions impact, be mindful of the ignition timing during this process and during deceleration.
06:01	It's not uncommon to see ignition values that appear very out of place during this time but you may find knocks increases dramatically if you alter them so tread carefully here.
06:11	Next up, variable valve timing has become far more common, not only for its performance benefits but also its emissions benefits.
06:18	By increasing intake to exhaust overlap, EGR can be simulated to some extent.
06:23	This can be done instead of actual EGR hardware being fitted and sometimes VVT simulated EGR and separate EGR systems are utilised together.
06:34	Variable valve timing is also used to approximate something called the Atkinson cycle on some engines.
06:40	This is a tweak on the conventional gas engine Otto cycle and it sacrifices torque output for engine efficiency.
06:48	This is achieved by shortening the compression in comparison to the expansion or power stroke which reduces cylinder filling but improves thermal and fuel efficiency.
06:58	While you probably won't end up performance tuning an engine fully optimised to run the Atkinson cycle at all times, versions of this are found in some hybrids like the Prius and a few other vehicles that we wouldn't necessarily call performance orientated.
07:12	Achieving Atkinson cycle mechanically is understandably tricky as we effectively need the piston to travel a shorter distance during the intake and compression strokes than it does on the power and exhaust strokes.
07:23	What's becoming more common though, and you may encounter during performance tuning, is an engine designed to run an Otto cycle for peak performance, that uses variable valve timing to give a degree of the Atkinson cycle effect when driving at cruise or part throttle.
07:38	Much like some of the effect of a full EGR system can be achieved through variable valve timing, VVT can shorten or lengthen the compression stroke for Atkinson cycle purposes as well.
07:49	EGR can be induced by retarding the exhaust cam, advancing the intake cam or the combination of both.
07:57	This increases the period of overlap between exhaust and induction phases of the four stroke cycle.
08:02	On the other hand, if you retard the intake cam, induction will continue further into the compression stroke, shortening the compression phase.
08:10	In this condition, the fresh intake charge starts to be forced back out of the intake valve at the start of the compression stroke before the valves have closed.
08:18	This also causes the induction phase to intrude less on the exhaust phase, thus extending the exhaust phase and making the engine operate more Atkinson like.
08:27	Engines that allow independent phasing of the intake and exhaust cams give the greatest range of overlap adjustment and possibilities but phasing only the intake or only the exhaust cam can still have a pronounced impact.
08:39	Next we're going to cover some practical advice on specific emissions readings.
08:44	On a gas engine, you can ruin carbon monoxide or carbon dioxide while your hydrocarbons and NOx values pass so you have to keep a close eye on everything.
08:54	During over enrichment, hydrocarbons will increase but if you aren't comparing it to modal baseline data, it may not be obvious that it's become worse.
09:03	Carbon monoxide or carbon dioxide will often spike though and reduction of the over enrichment is often the solution here.
09:10	After you've reduced enrichment, you'll usually find hydrocarbons went down too.
09:15	If reducing enrichment causes an increase in NOx, you've gone too far in the other direction.
09:20	At that point you'll want to add some of that fuel back in until you find the right balance.
09:25	It's not always that simple but this has been enough to make a great deal of tunes run clean in the lab and the outliers are simply that.
09:33	If you end up in a situation where these simple practices aren't getting you to success, remember that achieving emissions compliance is a far higher level of calibration than simply making an engine make more power.
09:44	It won't always be easy but with patience and perseverance, your projects will either achieve success or perhaps you'll find that a part or parts combination simply isn't appropriate within the emissions constraints.
09:56	We've run through a lot of practical tips in this module so let's recap some of the key points.
10:01	Catalysts have optimal operating ranges so we want to heat them up quickly, then avoid overheating them.
10:07	Warm up techniques include air injection, retarded combustion, multi pulse injection and cam phasing.
10:13	Catalyst over temp protection typically involves additional enrichment on gas engines and reduced fuelling on diesels but avoiding overheating the cat in the first place should be your goal.
10:23	EGT probes are vital to monitoring catalyst conditions.
10:27	You generally won't be adjusting EGR or EVAP behaviour but be aware that EGR might alter knock behaviour and EVAP can make fuel trimming go outside of your desired range.
10:38	Variable valve timing can be used to effectively shorten or lengthen each stroke, allowing for EGR and Atkinson like effects.
10:46	At this point I'd like to share a practical example from the emissions lab.
10:50	Variable valve timing can have a pronounced impact on engine operation and even the sound of the engine coming out of the exhaust.
10:57	Without testing though, you don't really know what the emissions impact will be.
11:00	Sometimes it's large, sometimes none at all and that's why we test.